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Table of Content

RTD Pyrometer Bimetallic Strip Thermostate Thermometer Thermocouple Thermopile Thermistor

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Resistance thermometer

Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material, platinum, nickel or copper. The material has a predictable change in resistance as the temperature changes; it is this predictable change that is used to determine temperature.

They are slowly replacing the use of thermocouples in many industrial applications below 600 °C, due to higher accuracy and repeatability.[1][2]

R vs T relationship of various metals :

Common RTD sensing elements constructed of platinum, copper or nickel have a unique, and repeatable and predictable resistance versus temperature relationship (R vs T) and operating temperature range. The R vs T relationship is defined as the amount of resistance change of the sensor per degree of temperature change.[3] The relative change in resistance (temperature coefficient of resistance) varies only slightly over the useful range of the sensor.

Platinum is a noble metal and has the most stable resistance-temperature relationship over the largest temperature range. Nickel elements have a limited temperature range because the amount of change in resistance per degree of change in temperature becomes very non-linear at temperatures over 572 °F (300 °C). Copper has a very linear resistance-temperature relationship, however copper oxidizes at moderate temperatures and cannot be used over 302°F (150°C).

Platinum is the best metal for RTDs because it follows a very linear resistance-temperature relationship and it follows the R vs T relationship in a highly repeatable manner over a wide temperature range. The unique properties of platinum make it the material of choice for temperature standards over the range of -272.5 °C to 961.78 °C, and is used in the sensors that define the International Temperature Standard, ITS-90. Platinum is chosen also because of its chemical inertness.

The significant characteristic of metals used as resistive elements is the linear approximation of the resistance versus temperature relationship between 0 and 100 °C. This temperature coefficient of resistance is called alpha, α. The equation below defines α; its units are ohm/ohm/°C.

the resistance of the sensor at 0°C

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the resistance of the sensor at 100°C

Pure platinum has an alpha of 0.003925 ohm/ohm/°C and is used in the construction of laboratory grade RTDs. Conversely two widely recognized standards for industrial RTDs IEC 6075 and ASTM E-1137 specify an alpha of 0.00385 ohms/ohm/°C. Before these standards were widely adopted several different alpha values were used. It is still possible to find older probes that are made with platinum that have alpha values of 0.003916 ohms/ohm/°C and 0.003902 ohms/ohm/°C.

These different alpha values for platinum are achieved by doping; basically carefully introducing impurities into the platinum. The impurities introduced during doping become embedded in the lattice structure of the platinum and result in a different R vs.T curve and hence alpha value.[4]

Calibration

To characterize the R vs T relationship of any RTD over a temperature range that represents the planned range of use, calibration must be performed at temperatures other than 0°C and 100°C. Two common calibration methods are the fixed point method and the comparison method.[5]

Fixed point calibration, used for the highest accuracy calibrations, uses the triple point, freezing point or melting point of pure substances such as water, zinc, tin, and argon to generate a known and repeatable temperature. These cells allow the user to reproduce actual conditions of the ITS-90 temperature scale. Fixed point calibrations provide extremely accurate calibrations (within ±0.001°C) A common fixed point calibration method for industrial-grade probes is the ice bath. The equipment is inexpensive, easy to use, and can accommodate several sensors at once. The ice point is designated as a secondary standard because its accuracy is ±0.005°C (±0.009°F), compared to ±0.001°C (±0.0018°F) for primary fixed points.

Comparison calibrations, commonly used with secondary SPRTs and industrial RTDs, the thermometers being calibrated are compared to calibrated thermometers by means of a bath whose temperature is uniformly stable Unlike fixed point calibrations, comparisons can be made at any temperature between –100°C and 500°C (–148°F to 932°F). This method might be more cost-effective since several sensors can be calibrated simultaneously with automated equipment. These, electrically heated and well-stirred baths, use silicone oils and molten salts as the medium for the various calibration temperatures.

Element types

There are three main categories of RTD sensors; Thin Film, Wire-Wound, and Coiled Elements. While these types are the ones most widely used in industry there are some places where other more exotic shapes are used, for example carbon resistors are used at ultra low temperatures (-173 °C to -273 °C).[6]

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Carbon resistor elements are widely available and are very inexpensive. They have very reproducible results at low temperatures. They are the most reliable form at extremely low temperatures. They generally do not suffer from significant hysteresis or strain gauge effects.

Strain free elements use a wire coil minimally supported within a sealed housing filled with an inert gas. These sensors are used up to 961.78 °C and are used in the SPRT’s that define ITS-90. They consisted of platinum wire loosely coiled over a support structure so the element is free to expand and contract with temperature, but it is very susceptible to shock and vibration as the loops of platinum can sway back and forth causing deformation.

Thin film elements have a sensing element that is formed by depositing a very thin layer of resistive material, normal platinum, on a ceramic substrate; This layer is usually just 10 to 100 angstroms (1 to 10 nanometers) thick.[7] This film is then coated with an epoxy or glass that helps protect the deposited film and also acts as a strain relief for the external lead-wires. Disadvantages of this type are that they are not as stable as their wire wound or coiled counterparts. They also can only be used over a limited temperature range due to the different expansion rates of the substrate and resistive deposited giving a "strain gauge" effect that can be seen in the resistive temperature coefficient. These elements work with temperatures to 300 °C.

Wire-wound elements can have greater accuracy, especially for wide temperature ranges. The coil diameter provides a compromise between mechanical stability and allowing expansion of the wire to minimize strain and consequential drift. The sensing wire is wrapped around an insulating mandrel or core. The winding core can be round or flat, but must be an electrical insulator. The coefficient of thermal expansion of the winding core material is matched to the sensing wire to minimize any mechanical strain. This strain on

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the element wire will result in a thermal measurement error. The sensing wire is connected to a larger wire, usually referred to as the element lead or wire. This wire is selected to be compatible with the sensing wire so that the combination does not generate an emf that would distort the thermal measurement. These elements work with temperatures to 660 °C.

Coiled elements have largely replaced wire-wound elements in industry. This design has a wire coil which can expand freely over temperature, held in place by some mechanical support which lets the coil keep its shape. This “strain free” design allows the sensing wire to expand and contract free of influence from other materials; in this respect it is similar to the SPRT, the primary standard upon which ITS-90 is based, while providing the durability necessary for industrial use. The basis of the sensing element is a small coil of platinum sensing wire. This coil resembles a filament in an incandescent light bulb. The housing or mandrel is a hard fired ceramic oxide tube with equally spaced bores that run transverse to the axes. The coil is inserted in the bores of the mandrel and then packed with a very finely ground ceramic powder. This permits the sensing wire to move while still remaining in good thermal contact with the process. These Elements works with temperatures to 850 °C.

The current international standard which specifies tolerance, and the temperature-to-electrical resistance relationship for platinum resistance thermometers is IEC 60751:2008, ASTM E1137 is also used in the United States. By far the most common devices used in industry have a nominal resistance of 100 ohms at 0 °C, and are called Pt100 sensors ('Pt' is the symbol for platinum).

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The sensitivity of a standard 100 ohm sensor is a nominal 0.385 ohm/°C. RTDs with a sensitivity of 0.375 and 0.392 ohm/°C as well as a variety of others are also available.

Function

Resistance thermometers are constructed in a number of forms and offer greater stability, accuracy and repeatability in some cases than thermocouples. While thermocouples use the Seebeck effect to generate a voltage, resistance thermometers use electrical resistance and require a power source to operate. The resistance ideally varies linearly with temperature.

The platinum detecting wire needs to be kept free of contamination to remain stable. A platinum wire or film is supported on a former in such a way that it gets minimal differential expansion or other strains from its former, yet is reasonably resistant to vibration. RTD assemblies made from iron or copper are also used in some applications. Commercial platinum grades are produced which exhibit a temperature coefficient of resistance 0.00385/°C (0.385%/°C) (European Fundamental Interval).[8] The sensor is usually made to have a resistance of 100 Ω at 0 °C. This is defined in BS EN 60751:1996 (taken from IEC 60751:1995). The American Fundamental Interval is 0.00392/°C,[9] based on using a purer grade of platinum than the European standard. The American standard is from the Scientific Apparatus Manufacturers Association (SAMA), who are no longer in this standards field. As a result the "American standard" is hardly the standard even in the US.

Measurement of resistance requires a small current to be passed through the device under test. This can cause resistive heating, causing significant loss of accuracy if manufacturers' limits are not respected, or the design does not properly consider the heat path. Mechanical strain on the resistance thermometer can also cause inaccuracy. Lead wire resistance can also be a factor; adopting three- and four-wire, instead of two-wire, connections can eliminate connection lead resistance effects from measurements (see below); three-wire connection is sufficient for most purposes and almost universal industrial practice. Four-wire connections are used for the most precise applications.

Advantages and limitations

The advantages of platinum resistance thermometers include:

High accuracy Low drift Wide operating range Suitability for precision applications.

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Limitations: RTDs in industrial applications are rarely used above 660 °C. At temperatures above 660 °C it becomes increasingly difficult to prevent the platinum from becoming contaminated by impurities from the metal sheath of the thermometer. This is why laboratory standard thermometers replace the metal sheath with a glass construction. At very low temperatures, say below -270 °C (or 3 K), because there are very few phonons, the resistance of an RTD is mainly determined by impurities and boundary scattering and thus basically independent of temperature. As a result, the sensitivity of the RTD is essentially zero and therefore not useful.

Compared to thermistors, platinum RTDs are less sensitive to small temperature changes and have a slower response time. However, thermistors have a smaller temperature range and stability.

Sources of error:

The common error sources of a PRT are:

Interchangeability: the “closeness of agreement” between the specific PRT's Resistance vs. Temperature relationship and a predefined Resistance vs. Temperature relationship, commonly defined by IEC 60751.[10]

Insulation Resistance: Error caused by the inability to measure the actual resistance of element. Current leaks into or out of the circuit through the sheath, between the element leads, or the elements.[11]

Stability: Ability to maintain R vs T over time as a result of thermal exposure.[12]

Repeatability: Ability to maintain R vs T under the same conditions after experiencing thermal cycling throughout a specified temperature range.[13]

Hysteresis : Change in the characteristics of the materials from which the RTD is built due to exposures to varying temperatures.[14]

Stem Conduction: Error that results from the PRT sheath conducting heat into or out of the process.

Calibration/Interpolation: Errors that occur due to calibration uncertainty at the cal points, or between cal point due to propagation of uncertainty or curve fit errors.

Lead Wire: Errors that occur because a 4 wire or 3 wire measurement is not used, this is greatly increased by higher gauge wire.

o 2 wire connection adds lead resistance in series with PRT element.o 3 wire connection relies on all 3 leads having equal resistance.

Self Heating: Error produced by the heating of the PRT element due to the power applied. Time Response: Errors are produced during temperature transients because the PRT

cannot respond to changes fast enough. Thermal EMF: Thermal EMF errors are produced by the EMF adding to or subtracting

from the applied sensing voltage, primarily in DC systems.

RTDs vs thermocouples

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The two most common ways of measuring industrial temperatures are with resistance temperature detectors (RTDs) and thermocouples. Choice between them is usually determined by four factors.

temperature: If process temperatures are between -200 to 500 °C (-328 to 932 °F), an industrial RTD is the preferred option. Thermocouples have a range of -180 to 2,320 °C (-292 to 4,208 °F),[15] so for temperatures above 500 °C (932 °F) they are the only contact temperature measurement device.

response time: If the process requires a very fast response to temperature changes—fractions of a second as opposed to seconds (e.g. 2.5 to 10 s)—then a thermocouple is the best choice. Time response is measured by immersing the sensor in water moving at 1 m/s (3 ft/s) with a 63.2% step change.

size : A standard RTD sheath is 3.175 to 6.35 mm (0.1250 to 0.250 in) in diameter; sheath diameters for thermocouples can be less than 1.6 mm (0.063 in).

accuracy and stability requirements: If a tolerance of 2 °C is acceptable and the highest level of repeatability is not required, a thermocouple will serve. RTDs are capable of higher accuracy and can maintain stability for many years, while thermocouples can drift within the first few hours of use.

Construction

These elements nearly always require insulated leads attached. At temperatures below about 250 °C PVC, silicon rubber or PTFE insulators are used. Above this, glass fibre or ceramic are used. The

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measuring point, and usually most of the leads, require a housing or protective sleeve, often made of a metal alloy which is chemically inert to the process being monitored. Selecting and designing protection sheaths can require more care than the actual sensor, as the sheath must withstand chemical or physical attack and provide convenient attachment points.

Wiring configurations

Two-wire configuration

The simplest resistance thermometer configuration uses two wires. It is only used when high accuracy is not required, as the resistance of the connecting wires is added to that of the sensor, leading to errors of measurement. This configuration allows use of 100 meters of cable. This applies equally to balanced bridge and fixed bridge system.

Three-wire configuration

In order to minimize the effects of the lead resistances, a three-wire configuration can be used. Using this method the two leads to the sensor are on adjoining arms. There is a lead resistance in each arm of the bridge so that the resistance is cancelled out, so long as the two lead resistances are accurately the same. This configuration allows up to 600 meters of cable.

Four-wire configuration

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The four-wire resistance thermometer configuration increases the accuracy and reliability of the resistance being measured: the resistance error due to lead wire resistance is zero. In the diagram above a standard two-terminal RTD is used with another pair of wires to form an additional loop that cancels out the lead resistance. The above Wheatstone bridge method uses a little more copper wire and is not a perfect solution. Below is a better configuration, four-wire Kelvin connection. It provides full cancellation of spurious effects; cable resistance of up to 15 Ω can be handled.

Classifications of RTDs

The highest accuracy of all PRTs is the Standard platinum Resistance Thermometers (SPRTs). This accuracy is achieved at the expense of durability and cost. The SPRTs elements are wound from reference grade platinum wire. Internal lead wires are usually made from platinum while internal supports are made from quartz or fuse silica. The sheaths are usually made from quartz or sometimes Inconel depending on temperature range. Larger diameter platinum wire is used, which drives up the cost and results in a lower resistance for the probe (typically 25.5 ohms). SPRTs have a wide temperature range (-200°C to 1000°C) and approximately accurate to ±0.001°C over the temperature range. SPRTs are only appropriate for laboratory use.

Another classification of laboratory PRTs is Secondary Standard platinum Resistance Thermometers (Secondary SPRTs). They are constructed like the SPRT, but the materials are more cost-effective. SPRTs commonly use reference grade, high purity smaller diameter platinum wire, metal sheaths and ceramic type insulators. Internal lead wires are usually a nickel based alloy. Secondary SPRTs are limited in temperature range (-200°C to 500°C) and are approximately accurate to ±0.03°C over the temperature range.

Industrial PRTs are designed to withstand industrial environments. They can be almost as durable as a thermocouple. Depending on the application industrial PRTs can use thin film

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elements or coil wound elements. The internal lead wires can range from PTFE insulated stranded nickel plated copper to silver wire, depending on the sensor size and application. Sheath material is typically stainless steel; higher temperature applications may demand Inconel. Other materials are used for specialized applications.

Applications

Sensor assemblies can be categorized into two groups by how they are installed or interface with the process: immersion or surface mounted.

Immersion sensors take the form of an SS tube and some type of process connection fitting. They are installed into the process with sufficient immersion length to ensure good contact with the process medium and reduce external influences.[16] A variation of this style includes a separate thermowell that provides additional protection for the sensor.[17] These styles are used to measure fluid or gas temperatures in pipes and tanks. Most sensors have the sensing element located at the tip of the stainless steel tube. An averaging style RTD however, can measure an average temperature of air in a large duct.[18] This style of immersion RTD has the sensing element distributed along the entire probe length and provides an average temperature. Lengths range from 3 to 60 feet.

Surface mounted sensors are used when immersion into a process fluid is not possible due to configuration of the piping or tank, or the fluid properties may not allow an immersion style sensor. Configurations range from tiny cylinders[19] to large blocks which are mounted by clamps,[20] adhesives, or bolted into place. Most require the addition of insulation to isolate them from cooling or heating effects of the ambient conditions to insure accuracy.

Other applications may require special water proofing or pressure seals. A heavy-duty underwater temperature sensor is designed for complete submersion under rivers, cooling ponds, or sewers. Steam autoclaves require a sensor that is sealed from intrusion by steam during the vacuum cycle process.

Immersion sensors generally have the best measurement accuracy because they are in direct contact with the process fluid. Surface mounted sensors are measuring the pipe surface as a close approximation of the internal process fluid.

Pyrometer

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pyrometer is a non-contacting device that intercepts and measures thermal radiation, a process known as pyrometry. This device can be used to determine the temperature of an object's surface.

The word pyrometer comes from the Greek word for fire, "πυρ" (pyro), and meter, meaning to measure. Pyrometer was originally coined to denote a device capable of measuring temperatures of objects above incandescence (i.e. objects bright to the human eye).

Principle of operation

A pyrometer has an optical system and a detector. The optical system focuses the thermal radiation onto the detector. The output signal of the detector (temperature T) is related to the thermal radiation or irradiance j* of the target object through the Stefan–Boltzmann law, the constant of proportionality σ, called the Stefan-Boltzmann constant and the emissivity ε of the object.

This output is used to infer the object's temperature. Thus, there is no need for direct contact between the pyrometer and the object, as there is with thermocouples and resistance temperature detectors (RTDs).

History

The potter Josiah Wedgwood invented the first pyrometer to measure the temperature in his kilns.[1] Modern pyrometers became available when the first disappearing filament pyrometer was built by L. Holborn and F. Kurlbaum in 1901.[2] This device superimposed a thin, heated filament over the object to be measured and relied on the operator’s eye to detect when the filament vanished.[3] The object temperature was then read from a scale on the pyrometer.

The temperature returned by the vanishing filament pyrometer and others of its kind, called brightness pyrometers, is dependent on the emissivity of the object. With greater use of brightness pyrometers, it became obvious that problems existed with relying on knowledge of the value of emissivity. Emissivity was found to change, often drastically, with surface roughness, bulk and surface composition, and even the temperature itself.[4]

To get around these difficulties, the ratio or two-color pyrometer was developed. They rely on the fact that Planck's law, which relates temperature to the intensity of radiation emitted at individual wavelengths, can be solved for temperature if Planck’s statement of the intensities at two different wavelengths is divided. This solution assumes that the emissivity is the same at both wavelengths [3] and cancels out in the division. This is known as the gray body assumption. Ratio pyrometers are essentially two brightness pyrometers in a single instrument. The operational principles of the ratio pyrometers were developed in the 1920s and 1930s, and they were commercially available in 1939.[2]

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As the ratio pyrometer came into popular use, it was determined that many materials, of which metals are an example, do not have the same emissivity at two wavelengths.[5] For these materials, the emissivity does not cancel out and the temperature measurement is in error. The amount of error depends on the emissivities and the wavelengths where the measurements are taken.[3] Two-color ratio pyrometers cannot measure whether a material’s emissivity is wavelength dependent.

To more accurately measure the temperature of real objects with unknown or changing emissivities, multiwavelength pyrometers were envisioned at the US National Institute of Standards and Technology and described in 1992.[2] Multiwavelength pyrometers use three or more wavelengths and mathematical manipulation of the results to attempt to achieve accurate temperature measurement even when the emissivity is unknown, changing, and different at all wavelengths.[3][4][5]

Applications

Pyrometers are suited especially to the measurement of moving objects or any surfaces that can not be reached or can not be touched.

Smelter Industry

Temperature is a fundamental parameter in metallurgical furnace operations. Reliable and continuous measurement of the melt temperature is essential for effective control of the operation. Smelting rates can be maximized, slag can be produced at the optimum temperature, fuel consumption is minimized and refractory life may also be lengthened. Thermocouples were the traditional devices used for this purpose, but they are unsuitable for continuous measurement because they rapidly dissolve.

Over-the-bath Pyrometer

Salt bath furnaces operate at temperatures up to 1300 °C and are used for heat treatment. At very high working temperatures with intense heat transfer between the molten salt and the steel being treated, precision is maintained by measuring the temperature of the molten salt. Most errors are caused by slag on the surface which is cooler than the salt bath.[6]

Tuyère Pyrometer

The Tuyère Pyrometer is an optical instrument for temperature measurement through the tuyeres which are normally used for feeding air or reactants into the bath of the furnace.

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(1) Display.(2) Optical.(3) Fibre optic cable and Periscope. (4) Pyrometer tuyère adapter having:i. Bustle pipe connection. ii. Tuyère clamp iii. Clamp washer iv. Clamp stud c/w and fastening hardware v. Gasket vi. Noranda Tuyère Silencer vii. valve seat viii. ball (5) Pneumatic Cylinder: i. Smart Cylinder Assembly with Internal proximity switch ii. Guard Plate Assembly iii. Temporary Flange Cover Plate used to cover periscope entry hole on tuyère adapter when no cylinder is installed on the tuyère. (6) Operator station panel (7) Pyrometer light station (8) Limit switches (9) 4 conductor cab tire (10) Ball Valve (11) Periscope Air pressure switch. (12) Bustle Pipe Air pressure switch. (13) Airline filter/regulator (14) Directional control valve, Sub-plate, silencer and speed control mufflers. (15) 2" nom. low pressure air hose, 40m length

Steam boilers

A steam boiler may be fitted with a pyrometer to measure the steam temperature in the superheater.

Hot Air Balloons

A hot air balloon is equipped with a pyrometer for measuring the temperature at the top of the envelope in order to prevent overheating of the fabric.

Pyrometry of gases

Pyrometry of gases presents difficulties. These are most commonly overcome by using thin filament pyrometry or soot pyrometry. Both techniques involve small solids in contact with hot gases.

Bimetallic stripFrom Wikipedia, the free encyclopediaJump to: navigation, search

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This article is about the temperature-sensitive mechanical device. For metals composed of a mixture of two or more chemical elements, see alloy.

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (February 2012)

Diagram of a bimetallic strip showing how the difference in thermal expansion in the two metals leads to a much larger sideways displacement of the strip

A bimetallic coil from a thermometer reacts to the heat from a lighter, by uncoiling and then coiling back up when the lighter is removed.

A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases brass instead of copper. The strips are joined together throughout their length by riveting, brazing or welding. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.

The sideways displacement of the strip is much larger than the small lengthways expansion in either of the two metals. This effect is used in a range of mechanical and electrical devices. In some applications the bimetal strip is used in the flat form. In others, it is wrapped into a coil for compactness. The greater length of the coiled version gives improved sensitivity.

Contents

1 History 2 Applications

o 2.1 Clocks o 2.2 Thermostats o 2.3 Thermometers o 2.4 Heat engines

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o 2.5 Electrical devices 3 Calculations 4 See also 5 External links 6 Notes

History

John Harrison's Memorial in Westminster Abbey, London

The earliest surviving bimetallic strip was made by the eighteenth-century clockmaker John Harrison who is generally credited with its invention. He made it for his third marine chronometer (H3) of 1759 to compensate for temperature-induced changes in the balance spring.[1] It should not be confused with his bimetallic mechanism for correcting for thermal expansion in the gridiron pendulum. His earliest examples had two individual metal strips joined by rivets but he also invented the later technique of directly fusing molten brass onto a steel substrate. A strip of this type was fitted to his last timekeeper, H5. Harrison's invention is recognized in the memorial to him in Westminster Abbey, England.

Applications

Clocks

Mechanical clock mechanisms are sensitive to temperature changes which lead to errors in time keeping. A bimetallic strip is used to compensate for this in some mechanisms. The most common method is to use a bimetallic construction for the circular rim of the balance wheel. As the spring controlling the balance becomes weaker with increasing temperature, so the balance becomes smaller in diameter to keep the period of oscillation (and hence timekeeping) constant.

Thermostats

See also: Tipping points and 'popping disc' bimetal thermostats

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In the regulation of heating and cooling, thermostats that operate over a wide range of temperatures are used. In these, one end of the bimetal strip is mechanically fixed and attached to an electrical power source, while the other (moving) end carries an electrical contact. In adjustable thermostats another contact is positioned with a regulating knob or lever. The position so set controls the regulated temperature, called the set point.

Thermostat with bimetal coil at (2)

Some thermostats use a mercury switch connected to both electrical leads. The angle of the entire mechanism is adjustable to control the set point of the thermostat.

Depending upon the application, a higher temperature may open a contact (as in a heater control) or it may close a contact (as in a refrigerator or air conditioner).

The electrical contacts may control the power directly (as in a household iron) or indirectly, switching electrical power through a relay or the supply of natural gas or fuel oil through an electrically operated valve. In some natural gas heaters the power may be provided with a thermocouple that is heated by a pilot light (a small, continuously burning, flame). In devices without pilot lights for ignition (as in most modern gas clothes dryers and some natural gas heaters and decorative fireplaces) the power for the contacts is provided by reduced household electrical power that operates a relay controlling an electronic ignitor, either a resistance heater or an electrically powered spark generating device.

Thermometers

A direct indicating dial thermometer (such as a patio thermometer or a meat thermometer) uses a bimetallic strip wrapped into a coil. One end of the coil is fixed to the housing of the device and the other drives an indicating needle. A bimetallic strip is also used in a recording thermometer.

Heat engines

Simple toys have been built which demonstrate how the principle can be used to drive a heat engine.[citation needed]

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Electrical devices

Bimetal strips are used in miniature circuit breakers to protect circuits from excess current. A coil of wire is used to heat a bimetal strip, which bends and operates a linkage that unlatches a spring-operated contact. This interrupts the circuit and can be reset when the bimetal strip has cooled down.

Bimetal strips are also used in time-delay relays, lamp flashers, and fluorescent lamp starters. In some devices the current running directly through the bimetal strip is sufficient to heat it and operate contacts directly.

Calculations

Curvature of a Bimetallic Beam:

Where and are the Young's Modulus and height of Material One and and are the Young's Modulus and height of Material Two. is the misfit strain, calculated by:

Where α1 is the Coefficient of Thermal Expansion of Material One and α2 is the Coefficient of Thermal Expansion of Material Two. ΔT is the current temperature minus the reference temperature (the temperature where the beam has no flexure).[2][3]

ThermostatFrom Wikipedia, the free encyclopedia

Jump to: navigation, search

This article is about the temperature regulating device. For the French cooking oven temperature scale, see Gas Mark#Other cooking temperature scales.

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (March 2009)

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Honeywell's iconic "The Round" model T87 thermostat, one of which is in the Smithsonian.

A Honeywell electronic thermostat in a retail store

A thermostat is a component of a control system which senses the temperature of a system so that the system's temperature is maintained near a desired setpoint. The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer fluid as needed, to maintain the correct temperature. The name is derived from the Greek words thermos "hot" and statos "a standing".

A thermostat may be a control unit for a heating or cooling system or a component part of a heater or air conditioner. Thermostats can be constructed in many ways and may use a variety of sensors to measure the temperature. The output of the sensor then controls the heating or cooling apparatus. A Thermostat may switch on and off at temperatures either side of the setpoint the extent of the difference is known as hysteresis and prevents too frequent switching of the controlled equipment.

The first electric room thermostat was invented in 1883 by Warren S. Johnson.[1][2] Early technologies included mercury thermometers with electrodes inserted directly through the glass, so that when a certain (fixed) temperature was reached the contacts would be closed by the mercury. These were accurate to within a degree of temperature.

Common sensor technologies in use today include:

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Bimetallic mechanical or electrical sensors Expanding wax pellets Electronic thermistors and semiconductor devices Electrical thermocouples

These may then control the heating or cooling apparatus using:

Direct mechanical control Electrical signals Pneumatic signals

Contents

1 Mechanical o 1.1 Bimetal o 1.2 Wax pellet

1.2.1 Automotive 1.2.2 Shower and other hot water controls

o 1.3 Gas expansion o 1.4 Pneumatic

2 Electrical o 2.1 Bimetallic switching thermostats o 2.2 Simple two wire thermostats

2.2.1 Millivolt thermostats 2.2.2 24 volt thermostats 2.2.3 Ignition sequences in modern systems 2.2.4 Line voltage thermostats

o 2.3 Combination heating/cooling regulation 3 Heat pump regulation

o 3.1 Digital o 3.2 Household thermostat location

4 Dummy thermostats 5 See also 6 References 7 External links

Mechanical

This covers only devices which both sense and control using purely mechanical means.

Bimetal

Domestic water and steam based central heating systems have traditionally been controlled by bi-metallic strip thermostats, and this is dealt with later in this article. Purely mechanical control

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has been localised steam or hot-water radiator bi-metallic thermostats which regulated the individual flow. However, Thermostatic Radiator Valves (TRV) are now being widely used.

Purely mechanical thermostats are used to regulate dampers in some rooftop turbine vents, reducing building heat loss in cool or cold periods.

Some automobile passenger heating systems have a thermostatically controlled valve to regulate the water flow and temperature to an adjustable level. In older vehicles the thermostat controls the application of engine vacuum to actuators that control water valves and flappers to direct the flow of air. In modern vehicles, the vacuum actuators may be operated by small solenoids under the control of a central computer.

Wax pellet

AutomotiveMain article: Wax thermostatic element

Car engine thermostat

Perhaps the most common example of purely mechanical thermostat technology in use today is the internal combustion engine cooling system thermostat, used to maintain the engine near its optimum operating temperature by regulating the flow of coolant to an air-cooled radiator. This

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type of thermostat operates using a sealed chamber containing a wax pellet that melts and expands at a set temperature. The expansion of the chamber operates a rod which opens a valve when the operating temperature is exceeded. The operating temperature is determined by the composition of the wax. Once the operating temperature is reached, the thermostat progressively increases or decreases its opening in response to temperature changes, dynamically balancing the coolant recirculation flow and coolant flow to the radiator to maintain the engine temperature in the optimum range.

On many automobile engines, including all General Motors products, the thermostat does not restrict flow to the heater core. The passenger side tank of the radiator is used as a bypass to the thermostat, flowing through the heater core. This prevents formation of steam pockets before the thermostat opens, and allows the heater to function before the thermostat opens. Another benefit is that there is still some flow through the radiator if the thermostat fails.

Shower and other hot water controls

A thermostatic mixing valve uses a wax pellet to control the mixing of hot and cold water. A common application is to permit operation of an electric water heater at a temperature hot enough to kill Legionella bacteria (above 60C/140F), while the output of the valve produces water that is cool enough to not immediately scald (49C/120F).

Gas expansion

Thermostats are sometimes used to regulate gas ovens. It consists of a gas-filled bulb connected to the control unit by a slender copper tube. The bulb is normally located at the top of the oven. The tube ends in a chamber sealed by a diaphragm. As the thermostat heats up, the gas expands applying pressure to the diaphragm which reduces the flow of gas to the burner.

Pneumatic

A pneumatic thermostat is a thermostat that controls a heating or cooling system via a series of air-filled control tubes. This "control air" system responds to the pressure changes (due to temperature) in the control tube to activate heating or cooling when required. The control air typically is maintained on "mains" at 15-18psi (although usually operable up to 20psi). Pneumatic thermostats typically provide output/ branch/ post-restrictor(for single-pipe operation) pressures of 3-15psi which is piped to the end device (valve/ damper actuator/ Pneumatic-Electric switch, etc.) [3]

The pneumatic thermostat was invented by Warren Johnson in 1895 soon after he invented the electric thermostat. In 2009, Harry Sim was awarded a patent for a pneumatic-to-digital interface that allows pneumatically controlled buildings to be integrated with building automation systems to provide similar benefits as DDC.

Electrical

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Bimetallic switching thermostats

Bimetallic thermostat for buildings.

Water and steam based central heating systems have traditionally had overall control by wall-mounted bi-metallic strip thermostats. These sense the air temperature using the differential expansion of two metals to actuate an on/off switch. Typically the central system would be switched on when the temperature drops below the set point on the thermostat, and switched off when it rises above, with a few degrees of hysteresis to prevent excessive switching. Bi-metallic sensing is now being superseded by electronic sensors. A principal use of the bi-metallic thermostat today is in individual electric convection heaters, where control is on/off, based on the local air temperature and the set point desired by the user. These are also used on air-conditioners, where local control is required.

Simple two wire thermostats

Milivolt thermostat mechanism

The illustration is the interior of a common two wire heat-only household thermostat, used to regulate a gas-fired heater via an electric gas valve. Similar mechanisms may also be used to

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control oil furnaces, boilers, boiler zone valves, electric attic fans, electric furnaces, electric baseboard heaters, and household appliances such as refrigerators, coffee pots, and hair dryers. The power through the thermostat is provided by the heating device and may range from millivolts to 240 volts in common North American construction, and is used to control the heating system either directly (electric baseboard heaters and some electric furnaces) or indirectly (all gas, oil and forced hot water systems). Due to the variety of possible voltages and currents available at the thermostat, caution must be taken when selecting a replacement device.

1. Set point control lever. This is moved to the right for a higher temperature. The round indicator pin in the center of the second slot shows through a numbered slot in the outer case.

2. Bimetallic strip wound into a coil. The center of the coil is attached to a rotating post attached to lever (1). As the coil gets colder the moving end — carrying (4) — moves clockwise.

3. Flexible wire. The left side is connected via one wire of a pair to the heater control valve.4. Moving contact attached to the bimetal coil.thence to the heater's controller.5. Magnet . This ensures a good contact when the contact closes. It also provides hysteresis to

prevent short heating cycles, as the temperature must be raised several degrees before the contacts will open. As an alternative, some thermostats instead use a mercury switch on the end of the bimetal coil. The weight of the mercury on the end of the coil tends to keep it there, also preventing short heating cycles. However, this type of thermostat is banned in many countries due to its highly and permanently toxic nature if broken. When replacing these thermostats they must be regarded as chemical waste.

6. Fixed contact screw. This is adjusted by the manufacturer. It is connected electrically by a second wire of the pair to the thermocouple and the heater's electrically operated gas valve.

Not shown in the illustration is a separate bimetal thermometer on the outer case to show the actual temperature at the thermostat.

Millivolt thermostats

As illustrated in the use of the thermostat above, the power is provided by a powerpile, which is a combination of many thermocouples, heated by the pilot light. This produces little power and so the system must use a low power valve to control the gas. This type of device is generally considered obsolete as pilot lights waste a surprising amount of gas (in the same way a dripping faucet can waste a large amount of water over an extended period), and are also no longer used on stoves, but are still to be found in many gas water heaters and gas fireplaces. (Their poor efficiency is acceptable in water heaters, since most of the energy "wasted" on the pilot light is still being coupled to the water and therefore helping to keep the tank warm). It also makes it unnecessary for an electrical circuit to be run to the water heater. For tankless (on demand) water heaters, pilot ignition is preferable because it is faster than hot-surface ignition and more reliable than spark ignition.)

Some programmable thermostats will control these systems.

24 volt thermostats

The majority of modern heating/cooling/heat pump thermostats operate on low voltage (typically 24 volts AC) control circuits. The source of the 24 volt AC power is a control transformer

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installed as part of the heating/cooling equipment. The advantage of the low voltage control system is the ability to operate multiple electromechanical switching devices such as relays, contactors, and sequencers using inherently safe voltage and current levels.[4] Built into the thermostat is a provision for enhanced temperature control using anticipation. A heat anticipator generates a small amount of additional heat to the sensing element while the heating appliance is operating. This opens the heating contacts slightly early to prevent the space temperature from greatly overshooting the thermostat setting. A mechanical heat anticipator is generally adjustable and should be set to the current flowing in the heating control circuit when the system is operating. A cooling anticipator generates a small amount of additional heat to the sensing element while the cooling appliance is not operating. This causes the contacts to energize the cooling equipment slightly early, preventing the space temperature from climbing excessively. Cooling anticipators are generally non-adjustable.Electromechanical thermostats use resistance elements as anticipators. Most electronic thermostats use either thermistor devices or integrated logic elements for the anticipation function. In some electronic thermostats, the thermistor anticipator may be located outdoors, providing a variable anticipation depending on the outdoor temperature. Thermostat enhancements include outdoor temperature display, programmability, and system fault indication. While such 24 volt thermostats are incapable of operating a furnace when the mains power fails, most such furnaces require mains power for heated air fans (and often also hot-surface or electronic spark ignition) so no functionality is lost. In other circumstances such as piloted wall and "gravity" (fanless) floor and central heaters the low voltage system described previously may be capable of remaining functional when electrical power is unavailable.

Terminal codes and colors

Terminal Code Color Description

R Red 24 volt

RH / RC Red 24 volt HEAT / COOL load

C / X 24 volt Common

W / W1 White Heat

W2 White Backup Heat

Y / Y1 Yellow Cool

G Green Fan

O / OB Orange Reversing valve (Heat Pump)

E Emergency Heat (Heat Pump)

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Ignition sequences in modern systems

Gas

1. Start drafting fan (if the furnace is relatively recent) to create a column of air flowing up the chimney

2. Heat ignitor or start spark-ignition system3. Open gas valve to ignite main burners4. Wait (if furnace is relatively recent) until the heat exchanger is at proper operating

temperature before starting main blower fan or circulator pump

Oil

1. Similar to gas, except rather than opening a valve, the furnace will start an oil pump to inject oil into the burner

Electric

1. The blower fan or circulator pump will be started, and a large electromechanical relay or TRIAC will turn on the heating elements

Coal (including grains such as corn, wheat, and barley, or pellets made of wood, bark, or cardboard)

1. Generally rare today (though grains and pellets are increasing in popularity); similar to gas, except rather than opening a valve, the furnace will start a screw to drive coal/grain/pellets into the firebox

With non-zoned (typical residential, one thermostat for the whole house) systems, when the thermostat's R (or Rh) and W terminals are connected, the furnace will go through its startup rituals and produce heat.

With zoned systems (some residential, many commercial systems — several thermostats controlling different "zones" in the building), the thermostat will cause small electric motors to open valves or dampers and start the furnace or boiler if it's not already running.

Most programmable thermostats will control these systems.

Line voltage thermostats

Line voltage thermostats are most commonly used for electric space heaters such as a baseboard heater or a direct-wired electric furnace. If a line voltage thermostat is used, system power (in the United States, 120 or 240 volts) is directly switched by the thermostat. With switching current often exceeding 40 amperes, using a low voltage thermostat on a line voltage circuit will result at least in the failure of the thermostat and possibly a fire. Line voltage thermostats are sometimes used in other applications, such as the control of fan-coil (fan powered from line voltage blowing through a coil of tubing which is either heated or cooled by a larger system) units in large

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systems using centralized boilers and chillers, or to control circulation pumps in hydronic heating applications.

Some programmable thermostats are available to control line-voltage systems. Baseboard heaters will especially benefit from a programmable thermostat which is capable of continuous control (as are at least some Honeywell models), effectively controlling the heater like a lamp dimmer, and gradually increasing and decreasing heating to ensure an extremely constant room temperature (continuous control rather than relying on the averaging effects of hysteresis). Systems which include a fan (electric furnaces, wall heaters, etc.) must typically use simple on/off controls.

Combination heating/cooling regulation

Depending on what is being controlled, a forced-air air conditioning thermostat generally has an external switch for heat/off/cool, and another on/auto to turn the blower fan on constantly or only when heating and cooling are running. Four wires come to the centrally-located thermostat from the main heating/cooling unit (usually located in a closet, basement, or occasionally in the attic): One wire usually red, supplies 24 volts AC power to the thermostat, while the other three supply control signals from the thermostat, usually white for heat, yellow for cooling, and green to turn on the blower fan. The power is supplied by a transformer, and when the thermostat makes contact between the 24 volt power and one or two of the other wires, a relay back at the heating/cooling unit activates the corresponding heat/fan/cool function of the unit(s).

A thermostat, when set to "cool", will only turn on when the ambient temperature of the surrounding room is above the set temperature. Thus, if the controlled space has a temperature normally above the desired setting when the heating/cooling system is off, it would be wise to keep the thermostat set to "cool", despite what the temperature is outside. On the other hand, if the temperature of the controlled area falls below the desired degree, then it is advisable to turn the thermostat to "heat".

Heat pump regulation

Thermostat design

The heat pump is a refrigeration based appliance which reverses refrigerant flow between the indoor and outdoor coils. This is done by energizing a reversing valve (also known as a "4-way"

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or "change-over" valve). During cooling, the indoor coil is an evaporator removing heat from the indoor air and transferring it to the outdoor coil where it is rejected to the outdoor air. During heating, the outdoor coil becomes the evaporator and heat is removed from the outdoor air and transferred to the indoor air through the indoor coil. The reversing valve, controlled by the thermostat, causes the change-over from heat to cool. Residential heat pump thermostats generally have an "O" terminal to energize the reversing valve in cooling. Some residential and many commercial heat pump thermostats use a "B" terminal to energize the reversing valve in heating. The heating capacity of a heat pump decreases as outdoor temperatures fall. At some outdoor temperature (called the balance point) the ability of the refrigeration system to transfer heat into the building falls below the heating needs of the building. A typical heat pump is fitted with electric heating elements to supplement the refrigeration heat when the outdoor temperature is below this balance point. Operation of the supplemental heat is controlled by a second stage heating contact in the heat pump thermostat. During heating, the outdoor coil is operating at a temperature below the outdoor temperature and condensation on the coil may take place. This condensation may then freeze onto the coil, reducing its heat transfer capacity. Heat pumps therefore have a provision for occasional defrost of the outdoor coil. This is done by reversing the cycle to the cooling mode, shutting off the outdoor fan, and energizing the electric heating elements. The electric heat in defrost mode is needed to keep the system from blowing cold air inside the building. The elements are then used in the "reheat" function. Although the thermostat may indicate the system is in defrost and electric heat is activated, the defrost function is not controlled by the thermostat. Since the heat pump has electric heat elements for supplemental and reheats, the heat pump thermostat provides for use of the electric heat elements should the refrigeration system fail. This function is normally activated by an "E" terminal on the thermostat. When in emergency heat, the thermostat makes no attempt to operate the compressor or outdoor fan.

Digital

See also: Programmable thermostat

Residential digital thermostat

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Lux Products' Model TX900TS Touch Screen Thermostat.

Newer digital thermostats have no moving parts to measure temperature and instead rely on thermistors or other semiconductor devices such as a resistance thermometer (resistance temperature detector). Typically one or more regular batteries must be installed to operate it, although some so-called "power stealing" digital thermostats use the common 24 volt AC circuits as a power source, but will not operate on thermopile powered "millivolt" circuits used in some furnaces. Each has an LCD screen showing the current temperature, and the current setting. Most also have a clock, and time-of-day and even day-of-week settings for the temperature, used for comfort and energy conservation. Some advanced models have touch screens, or the ability to work with home automation or building automation systems.

Digital thermostats use either a relay or a semiconductor device such as triac to act as switch to control the HVAC unit. Units with relays will operate millivolt systems, but often make an audible "click" noise when switching on or off.

More expensive models have a built-in PID controller, so that the thermostat knows ahead how the system will react to its commands. For instance, setting it up that temperature in the morning at 7 a.m. should be 21°C, makes sure that at that time the temperature will be 21°C, where a conventional thermostat would just start working at that time. The PID controller decides at what time the system should be activated in order to reach the desired temperature at the desired time. It also makes sure that the temperature is very stable (for instance, by reducing overshoots[citation

needed]).

Most digital thermostats in common residential use in North America and Europe are programmable thermostats, which will typically provide a 30% energy savings if left with their default programs; adjustments to these defaults may increase or reduce energy savings.[citation needed]

The programmable thermostat article provides basic information on the operation, selection and installation of such a thermostat.

Household thermostat location

The thermostat should be located away from the room's cooling or heating vents or device, yet exposed to general airflow from the room(s) to be regulated. An open hallway may be most appropriate for a single zone system, where living rooms and bedrooms are operated as a single zone. If the hallway may be closed by doors from the regulated spaces then these should be left

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open when the system is in use. If the thermostat is too close to the source controlled then the system will tend to "short cycle", and numerous starts and stops can be annoying and in some cases shorten equipment life. A multiple zoned system can save considerable energy by regulating individual spaces, allowing unused rooms to vary in temperature by turning off the heating and cooling.

Dummy thermostats

It has been reported that many thermostats in office buildings are non-functional dummy devices, installed to give tenants' employees an illusion of control.[5][6] These dummy thermostats are in effect a type of placebo button. However, these thermostats are often fully functional in the sense that they are used to detect the temperature in the zone, even though their controls are disabled and not used in lieu of the environmental controls of the building. This function is often referred to as "lockout".[7]

ThermometerFrom Wikipedia, the free encyclopediaJump to: navigation, search

Mercury laboratory thermometer

A thermometer (from the Greek θερμός, thermos, meaning "hot" and μἐτρον, metron, "measure") is a device that measures temperature or temperature gradient using a variety of different principles.[1] A thermometer has two important elements: the temperature sensor (e.g. the bulb on a mercury-in-glass thermometer) in which some physical change occurs with temperature, plus some means of converting this physical change into a numerical value (e.g. the visible scale that is marked on a mercury-in-glass thermometer).

There are many types and many uses for thermometers, as detailed below in sections of this article.

Contents

1 Temperature 2 Development 3 Physical principles of thermometry

o 3.1 Thermometric materials o 3.2 Constant volume thermometry o 3.3 Radiometric thermometry

4 Primary and secondary thermometers 5 Calibration 6 Precision, accuracy, and reproducibility 7 Nanothermometry

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8 Uses 9 Various types of thermometer 10 See also 11 References 12 Further reading 13 External links

Temperature

While an individual thermometer is able to measure degrees of hotness, the readings on two thermometers cannot be compared unless they conform to an agreed scale. There is today an absolute thermodynamic temperature scale. Internationally agreed temperature scales are designed to approximate this closely, based on fixed points and interpolating thermometers. The most recent official temperature scale is the International Temperature Scale of 1990. It extends from 0.65 K (−272.5 °C; −458.5 °F) to approximately 1,358 K (1,085 °C; 1,985 °F).

Development

Various authors have credited the invention of the thermometer to Cornelis Drebbel, Robert Fludd, Galileo Galilei or Santorio Santorio. The thermometer was not a single invention, however, but a development.

Philo of Byzantium and Hero of Alexandria knew of the principle that certain substances, notably air, expand and contract and described a demonstration in which a closed tube partially filled with air had its end in a container of water.[2] The expansion and contraction of the air caused the position of the water/air interface to move along the tube.

Such a mechanism was later used to show the hotness and coldness of the air with a tube in which the water level is controlled by the expansion and contraction of the air. These devices were developed by several European scientists in the 16th and 17th centuries, notably Galileo Galilei.[3] As a result, devices were shown to produce this effect reliably, and the term thermoscope was adopted because it reflected the changes in sensible heat (the concept of temperature was yet to arise).[3] The difference between a thermoscope and a thermometer is that the latter has a scale.[4] Though Galileo is often said to be the inventor of the thermometer, what he produced were thermoscopes.

The first clear diagram of a thermoscope was published in 1617 by Giuseppe Biancani: the first showing a scale and thus constituting a thermometer was by Robert Fludd in 1638. This was a vertical tube, closed by a bulb of air at the top, with the lower end opening into a vessel of water. The water level in the tube is controlled by the expansion and contraction of the air, so it is what we would now call an air thermometer.[5]

The first person to put a scale on a thermoscope is variously said to be Francesco Sagredo [6] or Santorio Santorio[7] in about 1611 to 1613.

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The word thermometer (in its French form) first appeared in 1624 in La Récréation Mathématique by J. Leurechon, who describes one with a scale of 8 degrees.[8]

The above instruments suffered from the disadvantage that they were also barometers, i.e. sensitive to air pressure. In about 1654 Ferdinando II de' Medici, Grand Duke of Tuscany, made sealed tubes part filled with alcohol, with a bulb and stem, the first modern-style thermometer, depending on the expansion of a liquid, and independent of air pressure.[8] Many other scientists experimented with various liquids and designs of thermometer.

However, each inventor and each thermometer was unique—there was no standard scale. In 1665 Christiaan Huygens suggested using the melting and boiling points of water as standards, and in 1694 Carlo Renaldini proposed using them as fixed points on a universal scale. In 1701 Isaac Newton proposed a scale of 12 degrees between the melting point of ice and body temperature. Finally in 1724 Daniel Gabriel Fahrenheit produced a temperature scale which now (slightly adjusted) bears his name. He could do this because he manufactured thermometers, using mercury (which has a high coefficient of expansion) for the first time and the quality of his production could provide a finer scale and greater reproducibility, leading to its general adoption. In 1742 Anders Celsius proposed a scale with zero at the boiling point and 100 degrees at the freezing point of water,[9] though the scale which now bears his name has them the other way around.[10]

In 1866 Sir Thomas Clifford Allbutt invented a clinical thermometer that produced a body temperature reading in five minutes as opposed to twenty.[11] In 1999 Dr. Francesco Pompei of the Exergen Corporation introduced the world's first temporal artery thermometer, a non-invasive temperature sensor which scans the forehead in about two seconds and provides a medically accurate body temperature.[12][13]

Old thermometers were all non-registering thermometers. That is, the thermometer did not hold the temperature after it was moved to a place with a different temperature. Determining the temperature of a pot of hot liquid required the user to leave the thermometer in the hot liquid until after reading it. If the non-registering thermometer was removed from the hot liquid, then the temperature indicated on the thermometer would immediately begin changing to reflect the temperature of its new conditions (in this case, the air temperature). Registering thermometers are designed to hold the temperature indefinitely, so that the thermometer can be removed and read at a later time or in a more convenient place. The first registering thermometer was designed and built by James Six in 1782, and the design, known as Six's thermometer is still in wide use today. Mechanical registering thermometers hold either the highest or lowest temperature recorded, until manually re-set, e.g., by shaking down a mercury-in-glass thermometer, or until an even more extreme temperature is experienced. Electronic registering thermometers may be designed to remember the highest or lowest temperature, or to remember whatever temperature was present at a specified point in time.

Thermometers increasingly use electronic means to provide a digital display or input to a computer.

Physical principles of thermometry

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Various thermometers from the 19th century.

Comparison of the Celsius and Fahrenheit scales

Thermometers may be described as empirical or absolute. Absolute thermometers are calibrated numerically by the thermodynamic absolute temperature scale. Empirical thermometers are not in general necessarily in exact agreement with absolute thermometers as to their numerical scale readings, but to qualify as thermometers at all they must agree with absolute thermometers and with each other in the following way: given any two bodies isolated in their separate respective thermodynamic equilibrium states, all thermometers agree as to which of the two has the higher temperature, or that the two have equal temperatures.[14] For any two empirical thermometers, this does not require that the relation between their numerical scale readings be linear, but it does require that relation to be strictly monotonic.[15] This is a fundamental character of temperature and thermometers.[16][17][18]

As it is customarily stated in textbooks, taken alone, the so-called "zeroth law of thermodynamics" fails to deliver this information, but the statement of the zeroth law of

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thermodynamics by James Serrin in 1977, though rather mathematically abstract, is more informative for thermometry: "Zeroth Law – There exists a topological line which serves as a coordinate manifold of material behaviour. The points of the manifold are called 'hotness levels', and is called the 'universal hotness manifold'."[19] To this information there needs to be added a sense of greater hotness; this sense can be had, independently of calorimetry, of thermodynamics, and of properties of particular materials, from Wien's displacement law of thermal radiation: the temperature of a bath of thermal radiation is proportional, by a universal constant, to the frequency of the maximum of its frequency spectrum; this frequency is always positive, but can have values that tend to zero.

There are several principles on which empirical thermometers are built, as listed in the section of this article entitled "Primary and secondary thermometers". Several such principles are essentially based on the constitutive relation between the state of a suitably selected particular material and its temperature. Only some materials are suitable for this purpose, and they may be considered as "thermometric materials". Radiometric thermometry, in contrast, can be only very slightly dependent on the constitutive relations of materials. In a sense then, radiometric thermometry might be thought of as "universal". This is because it rests mainly on a universality character of thermodynamic equilibrium, that it has the universal property of producing blackbody radiation.

Thermometric materials

Bi-metallic stem thermometers used to measure the temperature of steamed milk

Bi-metallic thermometer for cooking and baking in an oven

There are various kinds of empirical thermometer based on material properties.

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Many empirical thermometers rely on the constitutive relation between pressure, volume and temperature of their thermometric material. For example, mercury expands when heated.

If it is used for its relation between pressure and volume and temperature, a thermometric material must have three properties:

(1) its heating and cooling must be rapid. That is to say, when a quantity of heat enters or leaves a body of the material, the material must expand or contract to its final volume or reach its final pressure and must reach its final temperature with practically no delay; some of the heat that enters can be considered to change the volume of the body at constant temperature, and is called the latent heat of expansion at constant temperature; and the rest of it can be considered to change the temperature of the body at constant volume, and is called the specific heat at constant volume. Some materials do not have this property, and take some time to distribute the heat between temperature and volume change.[20]

(2) its heating and cooling must be reversible. That is to say, the material must be able to be heated and cooled indefinitely often by the same increment and decrement of heat, and still return to its original pressure, volume and temperature every time. Some plastics do not have this property;[21]

(3) its heating and cooling must be monotonic.[15][22] That is to say, throughout the range of temperatures for which it is intended to work, (a) at a given fixed pressure, either (α) the volume increases when the temperature increases, or else (β) the volume decreases when the temperature increases; not (α) for some temperatures and (β) for others; or (b) at a given fixed volume, either (α) the pressure increases when the temperature increases, or else (β) the pressure decreases when the temperature increases; not (α) for some temperatures and (β) for others.

At temperatures around about 4 °C, water does not have the property (3), and is said to behave anomalously in this respect; thus water cannot be used as a material for this kind of thermometry for temperature ranges near 4 °C.[17][23][24][25][26]

Gases, on the other hand, all have the properties (1), (2), and (3)(a)(α) and (3)(b)(α). Consequently, they are suitable thermometric materials, and that is why they were important in the development of thermometry.[27]

Constant volume thermometry

According to Preston (1894/1904), Regnault found constant pressure air thermometers unsatisfactory, because they needed troublesome corrections. He therefore built a constant volume air thermometer.[28] Constant volume thermometers do not provide a way to avoid the problem of anomalous behaviour like that of water at approximately 4 °C.[26]

Radiometric thermometry

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A clinical mercury-in-glass thermometer

Planck's law very accurately quantitatively describes the power spectral density of electromagnetic radiation, inside a rigid walled cavity in a body made of material that is completely opaque and poorly reflective, when it has reached thermodynamic equilibrium, as a function of absolute thermodynamic temperature alone. A small enough hole in the wall of the cavity emits near enough blackbody radiation of which the spectral radiance can be precisely measured. The walls of the cavity, provided they are completely opaque and poorly reflective, can be of any material indifferently. This provides a well-reproducible absolute thermometer over a very wide range of temperatures, able to measure the absolute temperature of a body inside the cavity.

Primary and secondary thermometers

Thermometers can be divided into two separate groups according to the level of knowledge about the physical basis of the underlying thermodynamic laws and quantities. For primary thermometers the measured property of matter is known so well that temperature can be calculated without any unknown quantities. Examples of these are thermometers based on the equation of state of a gas, on the velocity of sound in a gas, on the thermal noise (see Johnson–Nyquist noise), voltage or current of an electrical resistor, on blackbody radiation, and on the angular anisotropy of gamma ray emission of certain radioactive nuclei in a magnetic field. Primary thermometers are relatively complex.

Secondary thermometers are most widely used because of their convenience. Also, they are often much more sensitive than primary ones. For secondary thermometers knowledge of the measured property is not sufficient to allow direct calculation of temperature. They have to be calibrated against a primary thermometer at least at one temperature or at a number of fixed temperatures. Such fixed points, for example, triple points and superconducting transitions, occur reproducibly at the same temperature.

Calibration

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Mercury-in-glass thermometer

Thermometers can be calibrated either by comparing them with other calibrated thermometers or by checking them against known fixed points on the temperature scale. The best known of these fixed points are the melting and boiling points of pure water. (Note that the boiling point of water varies with pressure, so this must be controlled.)

The traditional method of putting a scale on a liquid-in-glass or liquid-in-metal thermometer was in three stages:

1. Immerse the sensing portion in a stirred mixture of pure ice and water at 1 Standard atmosphere (101.325 kPa; 760.0 mmHg) and mark the point indicated when it had come to thermal equilibrium.

2. Immerse the sensing portion in a steam bath at 1 Standard atmosphere (101.325 kPa; 760.0 mmHg) and again mark the point indicated.

3. Divide the distance between these marks into equal portions according to the temperature scale being used.

Other fixed points used in the past are the body temperature (of a healthy adult male) which was originally used by Fahrenheit as his upper fixed point (96 °F (36 °C) to be a number divisible by 12) and the lowest temperature given by a mixture of salt and ice, which was originally the definition of 0 °F (−18 °C).[29] (This is an example of a Frigorific mixture). As body temperature varies, the Fahrenheit scale was later changed to use an upper fixed point of boiling water at 212 °F (100 °C).[30]

These have now been replaced by the defining points in the International Temperature Scale of 1990, though in practice the melting point of water is more commonly used than its triple point, the latter being more difficult to manage and thus restricted to critical standard measurement. Nowadays manufacturers will often use a thermostat bath or solid block where the temperature is held constant relative to a calibrated thermometer. Other thermometers to be calibrated are put into the same bath or block and allowed to come to equilibrium, then the scale marked, or any deviation from the instrument scale recorded.[31] For many modern devices calibration will be stating some value to be used in processing an electronic signal to convert it to a temperature.

Precision, accuracy, and reproducibility

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The "Boyce MotoMeter" radiator cap on a 1913 Car-Nation automobile, used to measure temperature of vapor in 1910s and 1920s cars.

The precision or resolution of a thermometer is simply to what fraction of a degree it is possible to make a reading. For high temperature work it may only be possible to measure to the nearest 10 °C or more. Clinical thermometers and many electronic thermometers are usually readable to 0.1 °C. Special instruments can give readings to one thousandth of a degree. However, this precision does not mean the reading is true or accurate.

Thermometers which are calibrated to known fixed points (e.g. 0 and 100 °C) will be accurate (i.e. will give a true reading) at those points. Most thermometers are originally calibrated to a constant-volume gas thermometer.[citation needed] In between a process of interpolation is used, generally a linear one.[31] This may give significant differences between different types of thermometer at points far away from the fixed points. For example the expansion of mercury in a glass thermometer is slightly different from the change in resistance of a platinum resistance of the thermometer, so these will disagree slightly at around 50 °C.[32] There may be other causes due to imperfections in the instrument, e.g. in a liquid-in-glass thermometer if the capillary tube varies in diameter.[32]

For many purposes reproducibility is important. That is, does the same thermometer give the same reading for the same temperature (or do replacement or multiple thermometers give the same reading)? Reproducible temperature measurement means that comparisons are valid in scientific experiments and industrial processes are consistent. Thus if the same type of thermometer is calibrated in the same way its readings will be valid even if it is slightly inaccurate compared to the absolute scale.

An example of a reference thermometer used to check others to industrial standards would be a platinum resistance thermometer with a digital display to 0.1 °C (its precision) which has been calibrated at 5 points against national standards (−18, 0, 40, 70, 100 °C) and which is certified to an accuracy of ±0.2 °C.[33]

According to British Standards, correctly calibrated, used and maintained liquid-in-glass thermometers can achieve a measurement uncertainty of ±0.01 °C in the range 0 to 100 °C, and a

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larger uncertainty outside this range: ±0.05 °C up to 200 or down to −40 °C, ±0.2 °C up to 450 or down to −80 °C.[34]

Nanothermometry

Nanothermometry is an emergent research field that deals with the knowledge of temperature in the sub-micrometric scale. When the spatial scale of the object to measure temperature decreases bellow a micrometer, conventional thermometers are not suitable and new methods and materials have to be used. Usually the nanothermometrs are classified as luminescent thermometers (if use light to measure temperature) and non-luminescent thermometers (systems of nano-size and molecular dimensions, in which thermometric property is not directly related to luminescence).[35]

Uses

Outdoor display thermometer in Ashgabat

Thermometers have been built which utilize a range of physical effects to measure temperature. Temperature sensors are used in a wide variety of scientific and engineering applications, especially measurement systems. Temperature systems are primarily either electrical or mechanical, occasionally inseparable from the system which they control (as in the case of a mercury-in-glass thermometer). Thermometers are used within roadways in cold weather climates to help determine if icing conditions exist. Indoors, thermistors are used in climate control systems such as air conditioners, freezers, heaters, refrigerators, and water heaters.[36] Galileo thermometers are used to measure indoor air temperature, due to their limited measurement range.

Alcohol thermometers, infrared thermometers, mercury-in-glass thermometers, recording thermometers, thermistors, and Six's thermometers are used outside in areas which are well-exposed to the elements at various levels of the Earth's atmosphere and within the Earth's oceans is necessary within the fields of meteorology and climatology. Aircraft use thermometers and hygrometers to determine if atmospheric icing conditions exist along their flight path, and these measurements are used to initialize weather forecast models. Thermometers are used within roadways in cold weather climates to help determine if icing conditions exist and indoors within climate control systems.

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Bi-metallic stemmed thermometers, thermocouples, infrared thermometers, and thermistors are handy during cooking in order to know if meat has been properly cooked. Temperature of food is important because if it sits within environments with a temperature between 5 and 57 °C (41 and 135 °F) for four hours or more, bacteria can multiply leading to foodborne illnesses.[36] Thermometers are used in the production of candy.

Medical thermometers such as mercury-in-glass thermometers,[37] infrared thermometers,[38] pill thermometers, and liquid crystal thermometers are used within health care to determine if individuals have a fever or are hypothermic.

Thermochromic liquid crystals are also used in mood rings and in thermometers used to measure the temperature of water in fish tanks.

Fiber Bragg grating temperature sensors are used within nuclear power facilities to monitor reactor core temperatures and avoid the possibility of nuclear meltdowns.[39]

A thermometer constructed for probing stored food is also sometimes called a "temperature wand".[40]

Various types of thermometer

Alcohol thermometer Balco alloy Beckmann differential thermometer Bi-metal mechanical thermometer Coulomb blockade thermometer Galileo thermometer Heat meter Infrared thermometer

Liquid crystal thermometer Phosphor thermometry Pyrometer Quartz thermometer Rectal thermometry

ThermocoupleFrom Wikipedia, the free encyclopediaJump to: navigation, search

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Thermocouple connected to a multimeter displaying room temperature in °C.

A thermocouple consists of two conductors of different materials (usually metal alloys) that produce a voltage in the vicinity of the point where the two conductors are in contact. The voltage produced is dependent on, but not necessarily proportional to, the difference of temperature of the junction to other parts of those conductors. Thermocouples are a widely used type of temperature sensor for measurement and control[1] and can also be used to convert a temperature gradient into electricity. Commercial thermocouples are inexpensive,[2] interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self powered and require no external form of excitation. The main limitation with thermocouples is accuracy; system errors of less than one degree Celsius (C) can be difficult to achieve.[3]

Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.

Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.

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A thermocouple measuring circuit with a heat source, cold junction and a measuring instrument.

Contents

1 Principle of operation o 1.1 Properties of thermocouple circuits

2 Practical use o 2.1 Voltage–temperature relationship o

o 2.2 Cold junction compensation 3 Grades 4 Types

o 4.1 K o 4.2 E o 4.3 J o 4.4 N o 4.5 Platinum types B, R, and S o 4.6 T o 4.7 C o 4.8 M o 4.9 Chromel-gold/iron

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5 Aging of thermocouples 6 Thermocouple comparison 7 Applications

o 7.1 Steel industry o 7.2 Heating appliance safety o 7.3 Thermopile radiation sensors o 7.4 Manufacturing o 7.5 Power production o 7.6 Thermoelectric cooling o 7.7 Process plants

8 See also 9 References 10 External links

Principle of operation

Main article: Seebeck effect

In 1821, the German–Estonian physicist Thomas Johann Seebeck discovered that when any conductor is subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect or Seebeck effect. Any attempt to measure this voltage necessarily involves connecting another conductor to the "hot" end. This additional conductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose the original. Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate different voltages, leaving a small difference in voltage available for measurement. That difference increases with temperature, and is between 1 and 70 microvolts per degree Celsius (µV/°C) for standard metal combinations.

The voltage is not generated at the junction of the two metals of the thermocouple but rather along that portion of the length of the two dissimilar metals that is subjected to a temperature gradient. Because both lengths of dissimilar metals experience the same temperature gradient, the end result is a measurement of the difference in temperature between the thermocouple junction and the reference junction.

Properties of thermocouple circuits

The behavior of thermoelectric junctions with varying temperatures and compositions can be summarized in three properties:[4]

Homogeneous material—a thermoelectric current cannot be sustained in a circuit of a single homogeneous material by the application of heat alone, regardless of how it might vary in cross section. In other words, temperature changes in the wiring between the input

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and output do not affect the output voltage, provided all wires are made of the same materials as the thermocouple.

Intermediate materials—the algebraic sum of the thermoelectric EMFs in a circuit composed of any number of dissimilar materials is zero if all of the junctions are at a uniform temperature. So if a third metal is inserted in either wire and if the two new junctions are at the same temperature, there will be no net voltage generated by the new metal.

Successive or intermediate temperatures—if two dissimilar homogeneous materials produce thermal EMF1 when the junctions are at T1 and T2 and produce thermal EMF2 when the junctions are at T2 and T3, the EMF generated when the junctions are at T1 and T3 will be EMF1 + EMF2, provided T1<T2<T3.

Practical use

Voltage–temperature relationship

Polynomial Coefficients 0-500 °C[5]

(for Type K)1 25.083552 7.860106x10−2

3 −2.503131x10−1

4 8.315270x10−2

5 −1.228034x10−2

6 9.804036x10−4

7 −4.413030x10−5

8 1.057734x10−6

9 −1.052755x10−8

For typical metals used in thermocouples, the output voltage increases almost linearly with the temperature difference (ΔT) over a bounded range of temperatures. For precise measurements or measurements outside of the linear temperature range, non-linearity must be corrected. The nonlinear relationship between the temperature difference (ΔT) and the output voltage ( a few mV) of a thermocouple can be approximated by a polynomial:

The coefficients an are given for n from 0 to between 5 and 13 depending upon the metals. In some cases better accuracy is obtained with additional non-polynomial terms.[5] A database of voltage as a function of temperature, and coefficients for computation of temperature from voltage and vice-versa for many types of thermocouple is available online.[5]

In modern equipment the equation is usually implemented in a digital controller or stored in a look-up table;[6] older devices use analog circuits.

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Piece-wise linear approximations are an alternative to polynomial corrections.[7]

Cold junction compensation

Thermocouples measure the temperature difference between two points, not absolute temperature. To measure a single temperature one of the junctions—normally the cold junction—is maintained at a known reference temperature, and the other junction is at the temperature to be sensed.[8]

Having a junction of known temperature, while useful for laboratory calibration, is not convenient for most measurement and control applications. Instead, they incorporate an artificial cold junction using a thermally sensitive device such as a resistance thermometer, thermistor or diode to measure the temperature of the input connections at the instrument, with special care being taken to minimize any temperature gradient between terminals. Hence, the voltage from a known cold junction can be simulated, and the appropriate correction applied. This is known as cold junction compensation. Some integrated circuits are designed for cold junction temperature compensation for specific thermocouple types.

Grades

Thermocouple wire is available in several different metallurgical formulations per type, typically, in decreasing levels of accuracy and cost: special limits of error, standard, and extension grades.

Extension grade wires made of the same metals as a higher-grade thermocouple are used to connect it to a measuring instrument some distance away without introducing additional junctions between dissimilar materials which would generate unwanted voltages; the connections to the extension wires, being of like metals, do not generate a voltage.

In the case of platinum thermocouples, extension wire is a copper alloy, since it would be prohibitively expensive to use platinum for extension wires. The extension wire is specified to have a very similar thermal coefficient of EMF to the thermocouple, but only over a narrow range of temperatures; this reduces the cost significantly.

The temperature-measuring instrument must have high input impedance to prevent any significant current draw from the thermocouple, to prevent a resistive voltage drop across the wire. Changes in metallurgy along the length of the thermocouple (such as termination strips or changes in thermocouple type wire) will introduce another thermocouple junction which affects measurement accuracy.

Types

Certain combinations of alloys have become popular as industry standards. Selection of the combination is driven by cost, availability, convenience, melting point, chemical properties, stability, and output. Different types are best suited for different applications. They are usually

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selected on the basis of the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria include the inertness of the thermocouple material, and whether it is magnetic or not. Standard thermocouple types are listed below with the positive electrode first, followed by the negative electrode.

K

Type K (chromel {90% nickel and 10% chromium}—alumel {95% nickel, 2% manganese, 2% aluminium and 1% silicon}) is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/°C, chromel positive relative to alumel.[9] It is inexpensive, and a wide variety of probes are available in its −200 °C to +1250 °C / -330 °F to +2460 °F range. Type K was specified at a time when metallurgy was less advanced than it is today, and consequently characteristics may vary considerably between samples. One of the constituent metals, nickel, is magnetic; a characteristic of thermocouples made with magnetic material is that they undergo a deviation in output when the material reaches its Curie point; this occurs for type K thermocouples at around 350 °C . Wire color standard is yellow (+) and red (-).

E

Type E (chromel–constantan)[6] has a high output (68 µV/°C) which makes it well suited to cryogenic use. Additionally, it is non-magnetic. Wide range is −50 to 740 °C and Narrow range is −110 to 140 °C. Wire color standard is purple (+) and red (-).

J

Type J (iron–constantan) has a more restricted range than type K (−40 to +750 °C), but higher sensitivity of about 55 µV/°C.[2] The Curie point of the iron (770 °C)[10] causes an abrupt change in the characteristic, which determines the upper temperature limit.

N

Type N (Nicrosil–Nisil) (nickel-chromium-silicon/nickel-silicon) thermocouples are suitable for use between −270 °C and 1300 °C owing to its stability and oxidation resistance. Sensitivity is about 39 µV/°C at 900 °C, slightly lower compared to type K.

Designed at the Defence Science and Technology Organisation (DSTO), Australia, by Noel A Burley, type N thermocouples overcome the three principal characteristic types and causes of thermoelectric instability in the standard base-metal thermoelement materials:[11]

1. A gradual and generally cumulative drift in thermal EMF on long exposure at elevated temperatures. This is observed in all base-metal thermoelement materials and is mainly due to compositional changes caused by oxidation, carburization or neutron irradiation that can produce transmutation in nuclear reactor environments. In the case of type K, manganese and aluminium elements from the KN (negative) wire migrate to the KP (positive) wire resulting in a down-scale drift due to chemical contamination. This effect is cumulative and irreversible.

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2. A short-term cyclic change in thermal EMF on heating in the temperature range ca. 250–650 °C, which occurs in types K, J, T and E thermocouples. This kind of EMF instability is associated with structural changes like magnetic short range order.

3. A time-independent perturbation in thermal EMF in specific temperature ranges. This is due to composition-dependent magnetic transformations that perturb the thermal EMFs in type K thermocouples in the range ca. 25-225 °C, and in type J above 730 °C.

Nicrosil and Nisil thermocouple alloys show greatly enhanced thermoelectric stability relative to the other standard base-metal thermocouple alloys because their compositions substantially reduces the thermoelectric instability described above. This is achieved primarily by increasing component solute concentrations (chromium and silicon) in a base of nickel above those required to cause a transition from internal to external modes of oxidation, and by selecting solutes (silicon and magnesium) that preferentially oxidize to form a diffusion-barrier, and hence oxidation inhibiting films.[12]

Platinum types B, R, and S

Types B, R, and S thermocouples use platinum or a platinum–rhodium alloy for each conductor. These are among the most stable thermocouples, but have lower sensitivity than other types, approximately 10 µV/°C. Type B, R, and S thermocouples are usually used only for high temperature measurements due to their high cost and low sensitivity.

B

Type B thermocouples use a platinum–rhodium alloy for each conductor. One conductor contains 30% rhodium while the other conductor contains 6% rhodium. These thermocouples are suited for use at up to 1800 °C. Type B thermocouples produce the same output at 0 °C and 42 °C, limiting their use below about 50 °C.

R

Type R thermocouples use a platinum–rhodium alloy containing 13% rhodium for one conductor and pure platinum for the other conductor. Type R thermocouples are used up to 1600 °C.

S

Type S thermocouples are constructed using one wire of 90% Platinum and 10% Rhodium (the positive or "+" wire) and a second wire of 100% platinum (the negative or "-" wire). Like type R, type S thermocouples are used up to 1600 °C. In particular, type S is used as the standard of calibration for the melting point of gold (1064.43 °C).

T

Type T (copper – constantan) thermocouples are suited for measurements in the −200 to 350 °C range. Often used as a differential measurement since only copper wire touches the probes. Since

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both conductors are non-magnetic, there is no Curie point and thus no abrupt change in characteristics. Type T thermocouples have a sensitivity of about 43 µV/°C.

C

Type C (tungsten 5% rhenium – tungsten 26% rhenium) thermocouples are suited for measurements in the 0 °C to 2320 °C range. This thermocouple is well-suited for vacuum furnaces at extremely high temperatures. It must never be used in the presence of oxygen at temperatures above 260 °C.

M

Type M thermocouples use a nickel alloy for each wire. The positive wire (20 Alloy) contains 18% molybdenum while the negative wire (19 Alloy) contains 0.8% cobalt. These thermocouples are used in vacuum furnaces for the same reasons as with type C. Upper temperature is limited to 1400 °C. It is less commonly used than other types.

Chromel-gold/iron

In chromel-gold/iron thermocouples, the positive wire is chromel and the negative wire is gold with a small fraction (0.03–0.15 atom percent) of iron. It can be used for cryogenic applications (1.2–300 K and even up to 600 K). Both the sensitivity and the temperature range depends on the iron concentration. The sensitivity is typically around 15 µV/K at low temperatures and the lowest usable temperature varies between 1.2 and 4.2 K.

Aging of thermocouples

Thermoelements are often used at high temperatures and in reactive furnace atmospheres. In this case the practical lifetime is limited by aging. The thermoelectric coefficients of the wires in a thermocouple that is used to measure very high temperatures change with time, and the measurement voltage accordingly drops. The simple relationship between the temperature difference of the joints and the measurement voltage is only correct if each wire is homogeneous. As thermocouples age in a process their conductors can lose homogeneity due to chemical and metallurgical changes caused by extreme or prolonged exposure to high temperatures. If the inhomogeneous section of the thermocouple circuit is exposed to a temperature gradient the measured voltage will differ resulting in error. For this reason, aged thermocouples cannot be taken out of their installed location and recalibrated in a bath or test furnace to determine error. This also explains why error can sometimes be observed when an aged thermocouple is pulled partly out of a furnace—as the sensor is pulled back, inhomogenous sections may see exposure to increased temperature gradients from hot to cold as the inhomogeneous section now passes through the cooler refractory area, contributing significant error to the measurement. Likewise, an aged thermocouple that is pushed deeper into the furnace might sometimes provide a more accurate reading if being pushed further into the furnace causes the area of inhomogeneity to be located in an area of the furnace where it is no longer exposed to a temperature gradient.[13]

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Thermocouple comparison

The table below describes properties of several different thermocouple types. Within the tolerance columns, T represents the temperature of the hot junction, in degrees Celsius. For example, a thermocouple with a tolerance of ±0.0025×T would have a tolerance of ±2.5 °C at 1000 °C.

TypeTemperature

range °C (continuous)

Temperature range °C

(short term)

Tolerance class one

(°C)

Tolerance class two

(°C)

IEC Color code

BS Color code

ANSI Color code

K 0 to +1100−180 to +1300

±1.5 between −40 °C and 375 °C±0.004×T between 375 °C and 1000 °C

±2.5 between −40 °C and 333 °C±0.0075×T between 333 °C and 1200 °C

J 0 to +750 −180 to +800

±1.5 between −40 °C and 375 °C±0.004×T between 375 °C and 750 °C

±2.5 between −40 °C and 333 °C±0.0075×T between 333 °C and 750 °C

N 0 to +1100−270 to +1300

±1.5 between −40 °C and 375 °C±0.004×T between 375 °C and 1000 °C

±2.5 between −40 °C and 333 °C±0.0075×T between 333 °C and 1200 °C

R 0 to +1600 −50 to +1700 ±1.0 between 0 °C and 1100 °C±[1 +

±1.5 between 0 °C and 600 °C±0.0025×T

Not defined.

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0.003×(T − 1100)] between 1100 °C and 1600 °C

between 600 °C and 1600 °C

S 0 to 1600 −50 to +1750

±1.0 between 0 °C and 1100 °C±[1 + 0.003×(T − 1100)] between 1100 °C and 1600 °C

±1.5 between 0 °C and 600 °C±0.0025×T between 600 °C and 1600 °C

Not defined.

B+200 to +1700

0 to +1820Not Available

±0.0025×T between 600 °C and 1700 °C

No standard use copper wire

No standard use copper wire

Not defined.

T −185 to +300 −250 to +400

±0.5 between −40 °C and 125 °C±0.004×T between 125 °C and 350 °C

±1.0 between −40 °C and 133 °C±0.0075×T between 133 °C and 350 °C

E 0 to +800 −40 to +900

±1.5 between −40 °C and 375 °C±0.004×T between 375 °C and 800 °C

±2.5 between −40 °C and 333 °C±0.0075×T between 333 °C and 900 °C

Chromel/AuFe −272 to +300 n/a

Reproducibility 0.2% of the voltage; each sensor needs individual calibration.

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Applications

Thermocouples are suitable for measuring over a large temperature range, up to 2300 °C. Applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes. They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such applications thermistors, silicon bandgap temperature sensors and resistance temperature detectors are more suitable.

Steel industry

Type B, S, R and K thermocouples are used extensively in the steel and iron industries to monitor temperatures and chemistry throughout the steel making process. Disposable, immersible, type S thermocouples are regularly used in the electric arc furnace process to accurately measure the temperature of steel before tapping. The cooling curve of a small steel sample can be analyzed and used to estimate the carbon content of molten steel.

Heating appliance safety

Many gas-fed heating appliances such as ovens and water heaters make use of a pilot flame to ignite the main gas burner when required. If it goes out, gas may be released, which is a fire risk and a health hazard. To prevent this, some appliances use a thermocouple in a fail-safe circuit to sense when the pilot light is burning. The tip of the thermocouple is placed in the pilot flame, generating a voltage which operates the supply valve which feeds gas to the pilot. So long as the pilot flame remains lit, the thermocouple remains hot, and the pilot gas valve is held open. If the pilot light goes out, the thermocouple temperature falls, causing the voltage across the thermocouple to drop and the valve to close. Some combined main burner and pilot gas valves (mainly by Honeywell) reduce the power demand to within the range of a single universal thermocouple heated by a pilot (25 mV open circuit falling by half with the coil connected to a 10–12 mV, 0.2–0.25 A source, typically) by sizing the coil to be able to hold the valve open against a light spring, only after the initial turning-on force is provided by the user pressing and holding a knob to compress the spring during first lighting. These systems are identifiable by the 'press and hold for x minutes' in the pilot lighting instructions. (The holding current requirement of such a valve is much less than a bigger solenoid designed for pulling the valve in from closed would require.) Special test sets are made to confirm the valve let-go and holding currents as an ordinary milliameter cannot be used as it introduces more resistance than the gas valve coil. Apart from testing the open circuit voltage of the thermocouple, and the near short-circuit DC continuity through the thermocouple gas valve coil, the easiest non-specialist test is substitution of a known good gas valve.

Some systems, known as millivolt control systems, extend the thermocouple concept to both open and close the main gas valve as well. Not only does the voltage created by the pilot thermocouple activate the pilot gas valve, it is also routed through a thermostat to power the main gas valve as well. Here, a larger voltage is needed than in a pilot flame safety system

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described above, and a thermopile is used rather than a single thermocouple. Such a system requires no external source of electricity for its operation and so can operate during a power failure, provided all the related system components allow for this. Note that this excludes common forced air furnaces because external power is required to operate the blower motor, but this feature is especially useful for un-powered convection heaters. A similar gas shut-off safety mechanism using a thermocouple is sometimes employed to ensure that the main burner ignites within a certain time period, shutting off the main burner gas supply valve should that not happen.

Out of concern for energy wasted by the standing pilot, designers of many newer appliances have switched to an electronically controlled pilot-less ignition, also called intermittent ignition. With no standing pilot flame, there is no risk of gas buildup should the flame go out, so these appliances do not need thermocouple-based pilot safety switches. As these designs lose the benefit of operation without a continuous source of electricity, standing pilots are still used in some appliances. The exception is later model instantaneous (aka "tankless") water heaters that use the flow of water to generate the current required to ignite the gas burner, in conjunction with a thermocouple as a safety cut-off device in the event the gas fails to ignite, or the flame is extinguished.

Thermopile radiation sensors

See also: bolometer

Thermopiles are used for measuring the intensity of incident radiation, typically visible or infrared light, which heats the hot junctions, while the cold junctions are on a heat sink. It is possible to measure radiative intensities of only a few μW/cm2 with commercially available thermopile sensors. For example, some laser power meters are based on such sensors.

Manufacturing

Thermocouples can generally be used in the testing of prototype electrical and mechanical apparatus. For example, switchgear under test for its current carrying capacity may have thermocouples installed and monitored during a heat run test, to confirm that the temperature rise at rated current does not exceed designed limits.

Power production

Main article: Thermoelectric generator

A thermocouple can produce current to drive some processes directly, without the need for extra circuitry and power sources. For example, the power from a thermocouple can activate a valve when a temperature difference arises. The electrical energy generated by a thermocouple is converted from the heat which must be supplied to the hot side to maintain the electric potential. A continuous transfer of heat is necessary because the current flowing through the thermocouple tends to cause the hot side to cool down and the cold side to heat up (the Peltier effect).

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Thermocouples can be connected in series to form a thermopile, where all the hot junctions are exposed to a higher temperature and all the cold junctions to a lower temperature. The output is the sum of the voltages across the individual junctions, giving larger voltage and power output. In a radioisotope thermoelectric generator, the radioactive decay of transuranic elements as a heat source has been used to power spacecraft on missions too far from the Sun to use solar power.

Thermopiles heated by kerosene lamps were used to run batteryless radio receivers in isolated areas.[14] There are commercially produced lanterns that use the heat from a candle to run several light-emitting diodes, and thermoelectrically-powered fans to improve air circulation and heat distribution in wood stoves.

Thermoelectric cooling

Main article: Thermoelectric cooling

The Peltier effect can be used for cooling, in the reverse process to a thermoelectric generator. Instead of generating electric power, the thermocouple consumes it, working as a heat pump.

Process plants

Chemical production and petroleum refineries will usually employ computers for logging and limit testing the many temperatures associated with a process, typically numbering in the hundreds. For such cases a number of thermocouple leads will be brought to a common reference block (a large block of copper) containing the second thermocouple of each circuit. The temperature of the block is in turn measured by a thermistor. Simple computations are used to determine the temperature at each measured location.

ThermistorFrom Wikipedia, the free encyclopediaJump to: navigation, search

Negative temperature coefficient (NTC) thermistor, bead type, insulated wires

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A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.

Thermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range, typically −90 °C to 130 °C.[1]

Contents

1 Basic operation 2 Steinhart–Hart equation 3 B or β parameter equation 4 Conduction model 5 Self-heating effects 6 Applications 7 History 8 See also 9 References 10 External links

Basic operation

Thermistor symbol

Assuming, as a first-order approximation, that the relationship between resistance and temperature is linear, then:

where

= change in resistance= change in temperature

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= first-order temperature coefficient of resistance

Thermistors can be classified into two types, depending on the sign of . If is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, or posistor. If is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. Resistors that are not thermistors are designed to have a as close to zero as possible, so that their resistance remains nearly constant over a wide temperature range.

Instead of the temperature coefficient k, sometimes the temperature coefficient of resistance (alpha sub T) is used. It is defined as[2]

This coefficient should not be confused with the parameter below.

Steinhart–Hart equation

In practice, the linear approximation (above) works only over a small temperature range. For accurate temperature measurements, the resistance/temperature curve of the device must be described in more detail. The Steinhart–Hart equation is a widely used third-order approximation:

where a, b and c are called the Steinhart–Hart parameters, and must be specified for each device. T is the temperature in kelvin and R is the resistance in ohms. To give resistance as a function of temperature, the above can be rearranged into:

where

and

The error in the Steinhart–Hart equation is generally less than 0.02 °C in the measurement of temperature over a 200 °C range.[3] As an example, typical values for a thermistor with a resistance of 3000 Ω at room temperature (25 °C = 298.15 K) are:

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B or β parameter equation

NTC thermistors can also be characterised with the B (or β) parameter equation, which is

essentially the Steinhart Hart equation with , and ,

Where the temperatures are in kelvins and R0 is the resistance at temperature T0 (25 °C = 298.15 K). Solving for R yields:

or, alternatively,

where .

This can be solved for the temperature:

The B-parameter equation can also be written as . This can be used to convert the function of resistance vs. temperature of a thermistor into a linear function of

vs. . The average slope of this function will then yield an estimate of the value of the B parameter.

Conduction model

Many NTC thermistors are made from a pressed disc or cast chip of a semiconductor such as a sintered metal oxide. They work because raising the temperature of a semiconductor increases the number of electrons able to move about and carry charge - it promotes them into the conduction band. The more charge carriers that are available, the more current a material can conduct. This is described in the formula:

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= electric current (amperes)= density of charge carriers (count/m³)= cross-sectional area of the material (m²)

= velocity of charge carriers (m/s)

= charge of an electron ( coulomb)

The current is measured using an ammeter. Over large changes in temperature, calibration is necessary. Over small changes in temperature, if the right semiconductor is used, the resistance of the material is linearly proportional to the temperature. There are many different semiconducting thermistors with a range from about 0.01 kelvin to 2,000 kelvins (−273.14 °C to 1,700 °C).

Most PTC thermistors are of the "switching" type, which means that their resistance rises suddenly at a certain critical temperature. The devices are made of a doped polycrystalline ceramic containing barium titanate (BaTiO3) and other compounds. The dielectric constant of this ferroelectric material varies with temperature. Below the Curie point temperature, the high dielectric constant prevents the formation of potential barriers between the crystal grains, leading to a low resistance. In this region the device has a small negative temperature coefficient. At the Curie point temperature, the dielectric constant drops sufficiently to allow the formation of potential barriers at the grain boundaries, and the resistance increases sharply. At even higher temperatures, the material reverts to NTC behaviour. The equations used for modeling this behaviour were derived by W. Heywang and G. H. Jonker in the 1960s.

Another type of PTC thermistor is the polymer PTC, which is sold under brand names such as "Polyswitch" "Semifuse", and "Multifuse". This consists of a slice of plastic with carbon grains embedded in it. When the plastic is cool, the carbon grains are all in contact with each other, forming a conductive path through the device. When the plastic heats up, it expands, forcing the carbon grains apart, and causing the resistance of the device to rise rapidly. Like the BaTiO3 thermistor, this device has a highly nonlinear resistance/temperature response and is used for switching, not for proportional temperature measurement.

Yet another type of thermistor is a silistor, a thermally sensitive silicon resistor. Silistors are similarly constructed and operate on the same principles as other thermistors, but employ silicon as the semiconductive component material.

Self-heating effects

When a current flows through a thermistor, it will generate heat which will raise the temperature of the thermistor above that of its environment. If the thermistor is being used to measure the temperature of the environment, this electrical heating may introduce a significant error if a correction is not made. Alternatively, this effect itself can be exploited. It can, for example, make a sensitive air-flow device employed in a sailplane rate-of-climb instrument, the electronic variometer, or serve as a timer for a relay as was formerly done in telephone exchanges.

The electrical power input to the thermistor is just:

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where I is current and V is the voltage drop across the thermistor. This power is converted to heat, and this heat energy is transferred to the surrounding environment. The rate of transfer is well described by Newton's law of cooling:

where T(R) is the temperature of the thermistor as a function of its resistance R, is the temperature of the surroundings, and K is the dissipation constant, usually expressed in units of milliwatts per degree Celsius. At equilibrium, the two rates must be equal.

The current and voltage across the thermistor will depend on the particular circuit configuration. As a simple example, if the voltage across the thermistor is held fixed, then by Ohm's Law we

have and the equilibrium equation can be solved for the ambient temperature as a function of the measured resistance of the thermistor:

The dissipation constant is a measure of the thermal connection of the thermistor to its surroundings. It is generally given for the thermistor in still air, and in well-stirred oil. Typical values for a small glass bead thermistor are 1.5 mW/°C in still air and 6.0 mW/°C in stirred oil. If the temperature of the environment is known beforehand, then a thermistor may be used to measure the value of the dissipation constant. For example, the thermistor may be used as a flow rate sensor, since the dissipation constant increases with the rate of flow of a fluid past the thermistor.

The power dissipated in a thermistor is typically maintained at a very low level to ensure insignificant temperature measurement error due to self heating. However, some thermistor applications depend upon significant "self heating" to raise the body temperature of the thermistor well above the ambient temperature so the sensor then detects even subtle changes in the thermal conductivity of the environment. Some of these applications include liquid level detection, liquid flow measurement and air flow measurement.[4]

Applications

PTC thermistors can be used as current-limiting devices for circuit protection, as replacements for fuses. Current through the device causes a small amount of resistive heating. If the current is large enough to generate more heat than the device can lose to its surroundings, the device heats up, causing its resistance to increase, and therefore causing even more heating. This creates a self-reinforcing effect that drives the resistance upwards, reducing the current and voltage available to the device.

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PTC thermistors were used as timers in the degaussing coil circuit of most CRT displays. When the display unit is initially switched on, current flows through the thermistor and degaussing coil. The coil and thermistor are intentionally sized so that the current flow will heat the thermistor to the point that the degaussing coil shuts off in under a second. For effective degaussing, it is necessary that the magnitude of the alternating magnetic field produced by the degaussing coil decreases smoothly and continuously, rather than sharply switching off or decreasing in steps; the PTC thermistor accomplishes this naturally as it heats up. A degaussing circuit using a PTC thermistor is simple, reliable (for its simplicity), and inexpensive.

NTC thermistors are used as resistance thermometers in low-temperature measurements of the order of 10 K.

NTC thermistors can be used as inrush-current limiting devices in power supply circuits. They present a higher resistance initially which prevents large currents from flowing at turn-on, and then heat up and become much lower resistance to allow higher current flow during normal operation. These thermistors are usually much larger than measuring type thermistors, and are purposely designed for this application.

NTC thermistors are regularly used in automotive applications. For example, they monitor things like coolant temperature and/or oil temperature inside the engine and provide data to the ECU and, indirectly, to the dashboard.

NTC thermistors can be also used to monitor the temperature of an incubator. Thermistors are also commonly used in modern digital thermostats and to monitor the

temperature of battery packs while charging. Thermistors are also used in the hot ends of 3D printers, they produce heat and keep a

constant temperature for melting the plastic filament. NTC Thermistors are used in the Food Handling and Processing industry, especially for

food storage systems and food preparation. Maintaining the correct temperature is critical to prevent food borne illness.

NTC Thermistors are used throughout the Consumer Appliance industry for measuring temperature. Toasters, coffee makers, refrigerators, freezers, hair dryers, etc. all rely on thermistors for proper temperature control.

ThermopileFrom Wikipedia, the free encyclopediaJump to: navigation, search Not to be confused with Thermopylae.

Thermoelectric effect

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Principles[show]

Applications[hide]

Thermoelectric materials

Thermocouple

Thermopile

Thermoelectric cooling

Thermoelectric generator

Radioisotope thermoelectric generator

Automotive thermoelectric generator

v

t

e

A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel.

Thermopiles do not respond to absolute temperature, but generate an output voltage proportional to a local temperature difference or temperature gradient.

Thermopiles are used to provide an output in response to temperature as part of a temperature measuring device, such as the infrared thermometers widely used by medical professionals to measure body temperature. They are also used widely in heat flux sensors (such as the Moll thermopile and Eppley pyrheliometer)[1][2][3] and gas burner safety controls. The output of a thermopile is usually in the range of tens or hundreds of millivolts.[4] As well as increasing the signal level, the device may be used to provide spatial temperature averaging.[5]

Thermopiles are also used to generate electrical energy from, for instance, heat from electrical components.

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Peltier effect

The Peltier effect is the presence of heat at an electrified junction of two different metals and is named for French physicist Jean-Charles Peltier, who discovered it in 1834. When a current is made to flow through a junction composed of materials A and B, heat is generated at the upper

junction at , and absorbed at the lower junction at . The Peltier heat absorbed by the lower junction per unit time is equal to

where is the Peltier coefficient for the thermocouple composed of materials A and B and ( ) is the Peltier coefficient of material A (B). varies with the material's temperature and

its specific composition: p-type silicon typically has a positive Peltier coefficient below ~550 K, but n-type silicon is typically negative.

The Peltier coefficients represent how much heat current is carried per unit charge through a given material. Since charge current must be continuous across a junction, the associated heat flow will develop a discontinuity if and are different. Depending on the magnitude of the current, heat must accumulate or deplete at the junction due to a non-zero divergence there caused by the carriers attempting to return to the equilibrium that existed before the current was applied by transferring energy from one connector to another. Individual couples can be connected in series to enhance the effect. Thermoelectric heat pumps exploit this phenomenon, as do thermoelectric cooling devices found in refrigerators.[c

Bimetallic stripFrom Wikipedia, the free encyclopediaJump to: navigation, search This article is about the temperature-sensitive mechanical device. For metals composed of a mixture of two or more chemical elements, see alloy.

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (February 2012)

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Diagram of a bimetallic strip showing how the difference in thermal expansion in the two metals leads to a much larger sideways displacement of the strip

A bimetallic coil from a thermometer reacts to the heat from a lighter, by uncoiling and then coiling back up when the lighter is removed.

A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases brass instead of copper. The strips are joined together throughout their length by riveting, brazing or welding. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.

The sideways displacement of the strip is much larger than the small lengthways expansion in either of the two metals. This effect is used in a range of mechanical and electrical devices. In some applications the bimetal strip is used in the flat form. In others, it is wrapped into a coil for compactness. The greater length of the coiled version gives improved sensitivity.

Contents

1 History 2 Applications

o 2.1 Clocks o 2.2 Thermostats o 2.3 Thermometers o 2.4 Heat engines o 2.5 Electrical devices

3 Calculations 4 See also 5 External links 6 Notes

History

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John Harrison's Memorial in Westminster Abbey, London

The earliest surviving bimetallic strip was made by the eighteenth-century clockmaker John Harrison who is generally credited with its invention. He made it for his third marine chronometer (H3) of 1759 to compensate for temperature-induced changes in the balance spring.[1] It should not be confused with his bimetallic mechanism for correcting for thermal expansion in the gridiron pendulum. His earliest examples had two individual metal strips joined by rivets but he also invented the later technique of directly fusing molten brass onto a steel substrate. A strip of this type was fitted to his last timekeeper, H5. Harrison's invention is recognized in the memorial to him in Westminster Abbey, England.

Applications

Clocks

Mechanical clock mechanisms are sensitive to temperature changes which lead to errors in time keeping. A bimetallic strip is used to compensate for this in some mechanisms. The most common method is to use a bimetallic construction for the circular rim of the balance wheel. As the spring controlling the balance becomes weaker with increasing temperature, so the balance becomes smaller in diameter to keep the period of oscillation (and hence timekeeping) constant.

Thermostats

See also: Tipping points and 'popping disc' bimetal thermostats

In the regulation of heating and cooling, thermostats that operate over a wide range of temperatures are used. In these, one end of the bimetal strip is mechanically fixed and attached to an electrical power source, while the other (moving) end carries an electrical contact. In adjustable thermostats another contact is positioned with a regulating knob or lever. The position so set controls the regulated temperature, called the set point.

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Thermostat with bimetal coil at (2)

Some thermostats use a mercury switch connected to both electrical leads. The angle of the entire mechanism is adjustable to control the set point of the thermostat.

Depending upon the application, a higher temperature may open a contact (as in a heater control) or it may close a contact (as in a refrigerator or air conditioner).

The electrical contacts may control the power directly (as in a household iron) or indirectly, switching electrical power through a relay or the supply of natural gas or fuel oil through an electrically operated valve. In some natural gas heaters the power may be provided with a thermocouple that is heated by a pilot light (a small, continuously burning, flame). In devices without pilot lights for ignition (as in most modern gas clothes dryers and some natural gas heaters and decorative fireplaces) the power for the contacts is provided by reduced household electrical power that operates a relay controlling an electronic ignitor, either a resistance heater or an electrically powered spark generating device.

Thermometers

A direct indicating dial thermometer (such as a patio thermometer or a meat thermometer) uses a bimetallic strip wrapped into a coil. One end of the coil is fixed to the housing of the device and the other drives an indicating needle. A bimetallic strip is also used in a recording thermometer.

Heat engines

Simple toys have been built which demonstrate how the principle can be used to drive a heat engine.[citation needed]

Electrical devices

Bimetal strips are used in miniature circuit breakers to protect circuits from excess current. A coil of wire is used to heat a bimetal strip, which bends and operates a linkage that unlatches a spring-operated contact. This interrupts the circuit and can be reset when the bimetal strip has cooled down.

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Bimetal strips are also used in time-delay relays, lamp flashers, and fluorescent lamp starters. In some devices the current running directly through the bimetal strip is sufficient to heat it and operate contacts directly.

Calculations

Curvature of a Bimetallic Beam:

Where and are the Young's Modulus and height of Material One and and are the Young's Modulus and height of Material Two. is the misfit strain, calculated by:

Where α1 is the Coefficient of Thermal Expansion of Material One and α2 is the Coefficient of Thermal Expansion of Material Two. ΔT is the current temperature minus the reference temperature (the temperature where the beam has no flexure).[2][3]

Venturi effectFrom Wikipedia, the free encyclopedia

Jump to: navigation, search

The pressure in the first measuring tube (1) is higher than at the second (2), and the fluid speed at "1" is lower than at "2", because the cross-sectional area at "1" is greater than at "2".

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A flow of air through a venturi meter, showing the columns connected in a U-shape (a manometer) and partially filled with water. The meter is "read" as a differential pressure head in cm or inches of water.

Flow in a Venturi tube

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The Venturi effect is named after Giovanni Battista Venturi (1746–1822), an Italian physicist.

Contents

1 Background 2 Experimental apparatus

o 2.1 Venturi tubes o 2.2 Orifice plate

3 Instrumentation and measurement o 3.1 Flow rate o 3.2 Differential Pressure

4 Examples 5 See also 6 References 7 External links

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Background

The Venturi effect is a jet effect; as with a funnel the velocity of the fluid increases as the cross sectional area decreases, with the static pressure correspondingly decreasing. According to the laws governing fluid dynamics, a fluid's velocity must increase as it passes through a constriction to satisfy the principle of continuity, while its pressure must decrease to satisfy the principle of conservation of mechanical energy. Thus any gain in kinetic energy a fluid may accrue due to its increased velocity through a constriction is negated by a drop in pressure. An equation for the drop in pressure due to the Venturi effect may be derived from a combination of Bernoulli's principle and the continuity equation.

The limiting case of the Venturi effect is when a fluid reaches the state of choked flow, where the fluid velocity approaches the local speed of sound. In choked flow the mass flow rate will not increase with a further decrease in the downstream pressure environment.

However, mass flow rate for a compressible fluid can increase with increased upstream pressure, which will increase the density of the fluid through the constriction (though the velocity will remain constant). This is the principle of operation of a de Laval nozzle. Increasing source temperature will also increase the local sonic velocity, thus allowing for increased mass flow rate.

Referring to the diagram to the right, using Bernoulli's equation in the special case of incompressible flows (such as the flow of water or other liquid, or low speed flow of gas), the theoretical pressure drop at the constriction is given by:

where is the density of the fluid, is the (slower) fluid velocity where the pipe is

wider, is the (faster) fluid velocity where the pipe is narrower (as seen in the figure). This assumes the flowing fluid (or other substance) is not significantly compressible - even though pressure varies, the density is assumed to remain approximately constant.

Experimental apparatus

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Venturi tube demonstration apparatus built out of PVC pipe and operated with a vacuum pump

Venturi tubes

The simplest apparatus, as shown in the photograph and diagram, is a tubular setup known as a Venturi tube or simply a venturi. Fluid flows through a length of pipe of varying diameter. To avoid undue drag, a Venturi tube typically has an entry cone of 30 degrees and an exit cone of 5 degrees.

Orifice plate

Venturi tubes are more expensive to construct than a simple orifice plate which uses the same principle as a tubular scheme, but the orifice plate causes significantly more permanent energy loss.[1]

Instrumentation and measurement

Venturis are used in industrial and in scientific laboratories for measuring the flow of liquids.

Flow rate

A venturi can be used to measure the volumetric flow rate, .

Since

then

A venturi can also be used to mix a liquid with a gas. If a pump forces the liquid through a tube connected to a system consisting of a venturi to increase the liquid speed (the diameter decreases), a short piece of tube with a small hole in it, and last a venturi that decreases speed (so the pipe gets wider again), the gas will be sucked in through the small hole because of changes in pressure. At the end of the system, a mixture of liquid and gas will appear. See aspirator and pressure head for discussion of this type of siphon.

Differential Pressure

Main article: Pressure head

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As fluid flows through a venturi, the expansion and compression of the fluids cause the pressure inside the venturi to change. This principle can be used in metrology for gauges calibrated for differential pressures. This type of pressure measurement may be more convenient, for example, to measure fuel or combustion pressures in jet or rocket engines. The first large-scale Venturi meters to measure liquid flows were developed by Clemens Herschel who used them to measure small and large flows of water and wastewater beginning at the end of the 19th century.[2]

Examples

The Venturi effect may be observed or used in the following:

Cargo eductors on oil product and chemical ship tankers Inspirators that mix air and flammable gas in grills, gas stoves, Bunsen burners and airbrushes Water aspirators that produce a partial vacuum using the kinetic energy from the faucet water

pressure Steam siphons using the kinetic energy from the steam pressure to create a partial vacuum Atomizers that disperse perfume or spray paint (i.e. from a spray gun). Foam firefighting nozzles and extinguishers Carburetors that use the effect to suck gasoline into an engine's intake air stream Wine aerators , used to infuse air into wine as it is poured into a glass The capillaries of the human circulatory system, where it indicates aortic regurgitation Aortic insufficiency is a chronic heart condition that occurs when the aortic valve's initial large

stroke volume is released and the Venturi effect draws the walls together, which obstructs blood flow, which leads to a Pulsus Bisferiens.

Protein skimmers (filtration devices for saltwater aquaria) In automated pool cleaners that use pressure-side water flow to collect sediment and debris The barrel of the modern-day clarinet, which uses a reverse taper to speed the air down the

tube, enabling better tone, response and intonation Compressed air operated industrial vacuum cleaners Venturi scrubbers used to clean flue gas emissions Injectors (also called ejectors) used to add chlorine gas to water treatment chlorination systems Steam injectors use the Venturi effect and the latent heat of evaporation to deliver feed water

to a steam locomotive boiler. Sand blasters used to draw fine sand in and mix it with air Emptying bilge water from a moving boat through a small waste gate in the hull—the air

pressure inside the moving boat is greater than the water sliding by beneath A scuba diving regulator to assist the flow of air once it starts flowing Modern vaporizers to optimize efficiency In Venturi masks used in medical oxygen therapy In recoilless rifles to decrease the recoil of firing Ventilators The diffuser on an automobile Large cities where wind is forced between buildings In windy mountain passes, resulting in erroneous pressure altimeter readings[3]

The leadpipe of a trombone, affecting the timbre Foam proportioners used to induct fire fighting foam concentrate into fire protection systems

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The Bernoulli Principle and its corollary, the Venturi effect, are essential to aerodynamic as well as hydrodynamic design concepts. Airfoil and hydrofoil designs to lift and steer air and water vessels (airplanes, ships and submarines) are derived from applications of the Bernouoli Principle and the Venturi effect, as are the instruments that measure rate of movement through the air or water (velocity indicators). Stability indication and control mechanisms such as gyroscopic attitude indicators and fuel metering devices, such as carburetors, function as a result of gas or fluid pressure differentials that create suction as demonstrated and measurable by gas/fluid pressure and velocity equations derived from the Bernoulli Principle and the Venturi Effect.

A simple way to demonstrate the Venturi effect is to squeeze and release a flexible hose in which fluid is flowing: the partial vacuum produced in the constriction is sufficient to keep the hose collapsed.

Venturi tubes are also used to measure the speed of a fluid, by measuring pressure changes at different segments of the device. Placing a liquid in a U-shaped tube and connecting the ends of the tubes to both ends of a Venturi is all that is needed. When the fluid flows though the Venturi the pressure in the two ends of the tube will differ, forcing the liquid to the "low pressure" side. The amount of that move can be calibrated to the speed of the fluid flow.[1]

Orifice plateFrom Wikipedia, the free encyclopedia

Jump to: navigation, search

Flat-plate, sharp-edge orifice

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ISO 5167 Orifice Plate

An orifice plate is a device used for measuring the flow rate. Either a volumetric or mass flow rate may be determined, depending on the calculation associated with the orifice plate. It uses the same principle as a Venturi nozzle, namely Bernoulli's principle which states that there is a relationship between the pressure of the fluid and the velocity of the fluid. When the velocity increases, the pressure decreases and vice versa.

Contents

1 Description 2 Uses 3 Incompressible flow through an orifice 4 Flow of gases through an orifice

o 4.1 Calculation of expansion factor 5 Permanent pressure drop for incompressible fluids 6 See also 7 References 8 External links

Description

An orifice plate is a thin plate with a hole in the middle. It is usually placed in a pipe in which fluid flows. When the fluid reaches the orifice plate, the fluid is forced to converge to go through the small hole; the point of maximum convergence actually occurs shortly downstream of the physical orifice, at the so-called vena contracta point (see drawing to the right). As it does so, the velocity and the pressure changes. Beyond the vena contracta, the fluid expands and the velocity and pressure change once again. By measuring the difference in fluid pressure between the normal pipe section and at the vena contracta, the volumetric and mass flow rates can be obtained from Bernoulli's equation.

Uses

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Orifice plates are most commonly used for continuous measurement of fluid flow in pipes. They are also used in some small river systems to measure flow rates at locations where the river passes through a culvert or drain. Only a small number of rivers are appropriate for the use of the technology since the plate must remain completely immersed i.e. the approach pipe must be full, and the river must be substantially free of debris.

A restrictive flow orifice, a type of orifice plate, is a safety device to control maximum flow from a compressed gas cylinder.[1]

In the natural environment, large orifice plates are used to control onward flow in flood relief dams. In these structures a low dam is placed across a river and in normal operation the water flows through the orifice plate unimpeded as the orifice is substantially larger than the normal flow cross section. However, in floods, the flow rate rises and floods out the orifice plate which can then only pass a flow determined by the physical dimensions of the orifice. Flow is then held back behind the low dam in a temporary reservoir which is slowly discharged through the orifice when the flood subsides.

Incompressible flow through an orifice

By assuming steady-state, incompressible (constant fluid density), inviscid, laminar flow in a horizontal pipe (no change in elevation) with negligible frictional losses, Bernoulli's equation reduces to an equation relating the conservation of energy between two points on the same streamline:

or:

By continuity equation:

  or   and  :

Solving for :

and:

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The above expression for gives the theoretical volume flow rate. Introducing the beta

factor as well as the coefficient of discharge :

And finally introducing the meter coefficient which is defined as to obtain the final equation for the volumetric flow of the fluid through the orifice:

Multiplying by the density of the fluid to obtain the equation for the mass flow rate at any section in the pipe:[2][3][4][5]

where:

= volumetric flow rate (at any cross-section), m³/s

= mass flow rate (at any cross-section), kg/s

= coefficient of discharge, dimensionless

= orifice flow coefficient, dimensionless

= cross-sectional area of the pipe, m²

= cross-sectional area of the orifice hole, m²

= diameter of the pipe, m

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= diameter of the orifice hole, m

= ratio of orifice hole diameter to pipe diameter, dimensionless

= upstream fluid velocity, m/s

= fluid velocity through the orifice hole, m/s

= fluid upstream pressure, Pa with dimensions of kg/(m·s² )

= fluid downstream pressure, Pa with dimensions of kg/(m·s² )

= fluid density, kg/m³

Deriving the above equations used the cross-section of the orifice opening and is not as realistic as using the minimum cross-section at the vena contracta. In addition, frictional losses may not be negligible and viscosity and turbulence effects may be present. For that reason, the coefficient

of discharge is introduced. Methods exist for determining the coefficient of discharge as a function of the Reynolds number.[3]

The parameter is often referred to as the velocity of approach factor[2] and dividing the coefficient of discharge by that parameter (as was done above) produces the flow coefficient

. Methods also exist for determining the flow coefficient as a function of the beta

function and the location of the downstream pressure sensing tap. For rough approximations, the flow coefficient may be assumed to be between 0.60 and 0.75. For a first approximation, a flow coefficient of 0.62 can be used as this approximates to fully developed flow.

An orifice only works well when supplied with a fully developed flow profile. This is achieved by a long upstream length (20 to 40 pipe diameters, depending on Reynolds number) or the use of a flow conditioner. Orifice plates are small and inexpensive but do not recover the pressure drop as well as a venturi nozzle does. If space permits, a venturi meter is more efficient than an orifice plate.

Flow of gases through an orifice

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In general, equation (2) is applicable only for incompressible flows. It can be modified by

introducing the expansion factor to account for the compressibility of gases.

is 1.0 for incompressible fluids and it can be calculated for compressible gases.[3]

Calculation of expansion factor

The expansion factor , which allows for the change in the density of an ideal gas as it expands isentropically, is given by:[3]

For values of less than 0.25, approaches 0 and the last bracketed term in the above equation approaches 1. Thus, for the large majority of orifice plate installations:

where:

= Expansion factor, dimensionless

=

= specific heat ratio ( ), dimensionless

Substituting equation (4) into the mass flow rate equation (3):

and:

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and thus, the final equation for the non-choked (i.e., sub-sonic) flow of ideal gases through an orifice for values of β less than 0.25:

Using the ideal gas law and the compressibility factor (which corrects for non-ideal gases), a practical equation is obtained for the non-choked flow of real gases through an orifice for values of β less than 0.25:[4][5][6]

Remembering that and (ideal gas law and the compressibility factor)

where:

= specific heat ratio ( ), dimensionless

= mass flow rate at any section, kg/s

= upstream real gas flow rate, m³/s

= orifice flow coefficient, dimensionless

= cross-sectional area of the orifice hole, m²

= upstream real gas density, kg/m³

= upstream gas pressure, Pa with dimensions of kg/(m·s²)

= downstream pressure, Pa with dimensions of kg/(m·s²)

= the gas molecular mass, kg/mol (also known as the molecular weight)

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= the Universal Gas Law Constant = 8.3145 J/(mol·K)

= absolute upstream gas temperature, K

= the gas compressibility factor at and , dimensionless

A detailed explanation of choked and non-choked flow of gases, as well as the equation for the choked flow of gases through restriction orifices, is available at Choked flow.

The flow of real gases through thin-plate orifices never becomes fully choked. Cunningham (1951) first drew attention to the fact that choked flow will not occur across a standard, thin, square-edged orifice.[7] The mass flow rate through the orifice continues to increase as the downstream pressure is lowered to a perfect vacuum, though the mass flow rate increases slowly as the downstream pressure is reduced below the critical pressure.[8]

Permanent pressure drop for incompressible fluids

For a square-edge orifice plate with flange taps[9]:

where:

= permanent pressure drop

= indicated pressure drop at the flange taps

And rearranging the formula near the top of this article:

RotameterFrom Wikipedia, the free encyclopedia

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Jump to: navigation, search

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (October

2011)

A rotameter is a device that measures the flow rate of liquid or gas in a closed tube.

It belongs to a class of meters called variable area meters, which measure flow rate by allowing the cross-sectional area the fluid travels through to vary, causing some measurable effect. [1]

Contents

1 History 2 Implementation 3 Advantages 4 Disadvantages 5 Railway rotameter

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6 References 7 External links

History

The first variable area meter with rotating float was invented by Karl Kueppers in Aachen in 1908. This is described in the German patent 215225. Felix Meyer found the first industrial company "Deutsche Rotawerke GmbH" in Aachen recognizing the fundamental importance of this invention. They improved this invention with new shapes of the float and of the glass tube. Kueppers invented the special shape for the inside of the glass tube that realized a symmetrical flow scale.

The brand name Rotameter was registered by the British company GEC Rotameter Co, in Crawley, and still exists, having been passed down through the acquisition chain: KDG Instruments, Solartron Mobrey, and Emerson Process Management (Brooks Instrument). Rota with their "Rotamesser" are now owned by Yokogawa Electric Corp.

Implementation

A rotameter consists of a tapered tube, typically made of glass with a 'float', actually a shaped weight, inside that is pushed up by the drag force of the flow and pulled down by gravity. Drag force for a given fluid and float cross section is a function of flow speed squared only, see drag equation.

A higher volumetric flow rate through a given area increases flow speed and drag force, so the float will be pushed upwards. However, as the inside of the rotameter is cone shaped (widens), the area around the float through which the medium flows increases, the flow speed and drag force decrease until there is mechanical equilibrium with the float's weight.

Floats are made in many different shapes, with spheres and ellipsoids being the most common. The float may be diagonally grooved and partially colored so that it rotates axially as the fluid passes. This shows if the float is stuck since it will only rotate if it is free. Readings are usually taken at the top of the widest part of the float; the center for an ellipsoid, or the top for a cylinder. Some manufacturers use a different standard.

The "float" must not float in the fluid: it has to have a higher density than the fluid, otherwise it will float to the top even if there is no flow.

Advantages

A rotameter requires no external power or fuel, it uses only the inherent properties of the fluid, along with gravity, to measure flow rate.

A rotameter is also a relatively simple device that can be mass manufactured out of cheap materials, allowing for its widespread use.

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Since the area of the flow passage increases as the float moves up the tube, the scale is approximately linear. [1]

Disadvantages

Due to its use of gravity, a rotameter must always be vertically oriented and right way up, with the fluid flowing upward.

Due to its reliance on the ability of the fluid or gas to displace the float, graduations on a given rotameter will only be accurate for a given substance at a given temperature. The main property of importance is the density of the fluid; however, viscosity may also be significant. Floats are ideally designed to be insensitive to viscosity; however, this is seldom verifiable from manufacturers' specifications. Either separate rotameters for different densities and viscosities may be used, or multiple scales on the same rotameter can be used.

Due to the direct flow indication the resolution is relatively poor compared to other measurement principles. Readout uncertainty gets worse near the bottom of the scale. Oscillations of the float and parallax may further increase the uncertainty of the measurement.

Rotameters normally require the use of glass (or other transparent material), otherwise the user cannot see the float. This limits their use in many industries to benign fluids, such as water.

Rotameters are not easily adapted for reading by machine; although magnetic floats that drive a follower outside the tube are available.

Usually rotameters aren't made in very large sizes (more than 4 inches/100 mm), but bypass designs are sometimes used on very large pipes. [1]

Clear glass is used which is highly resistant to thermal shock and chemical action.

Railway rotameter

NSWGR Rotameter

The New South Wales Government Railways constructed in 1903 a device for measuring the length of its lines of railway. That authority named the machine a Rotameter. It consisted of a four-wheel trolley with an additional large fifth wheel which traveled alo

Nozzle

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From Wikipedia, the free encyclopedia

Jump to: navigation, search

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (March 2009)

A water nozzle

Center pivot irrigation sprinkler nozzles, used in crop irrigation

Rotator style pivot applicator sprinkler

End Gun style pivot applicator sprinkler

A nozzle is a device designed to control the direction or characteristics of a fluid flow (especially to increase velocity) as it exits (or enters) an enclosed chamber or pipe via an orifice.

A nozzle is often a pipe or tube of varying cross sectional area, and it can be used to direct or modify the flow of a fluid (liquid or gas). Nozzles are frequently used to control the rate of flow, speed, direction, mass, shape, and/or the pressure of the stream that emerges from them.

Contents

1 Types

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o 1.1 Jet o 1.2 High velocity o 1.3 Propelling o 1.4 Magnetic o 1.5 Spray o 1.6 Vacuum o 1.7 Shaping

2 See also 3 References 4 External links

Types

Jet

A gas jet, fluid jet, or hydro jet is a nozzle intended to eject gas or fluid in a coherent stream into a surrounding medium. Gas jets are commonly found in gas stoves, ovens, or barbecues. Gas jets were commonly used for light before the development of electric light. Other types of fluid jets are found in carburetors, where smooth calibrated orifices are used to regulate the flow of fuel into an engine, and in jacuzzis or spas.

Another specialized jet is the laminar jet. This is a water jet that contains devices to smooth out the pressure and flow, and gives laminar flow, as its name suggests. This gives better results for fountains.

Nozzles used for feeding hot blast into a blast furnace or forge are called tuyeres.

High velocity

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A rocket nozzle

Frequently the goal is to increase the kinetic energy of the flowing medium at the expense of its pressure and internal energy.

Nozzles can be described as convergent (narrowing down from a wide diameter to a smaller diameter in the direction of the flow) or divergent (expanding from a smaller diameter to a larger one). A de Laval nozzle has a convergent section followed by a divergent section and is often called a convergent-divergent nozzle ("con-di nozzle").

Convergent nozzles accelerate subsonic fluids. If the nozzle pressure ratio is high enough the flow will reach sonic velocity at the narrowest point (i.e. the nozzle throat). In this situation, the nozzle is said to be choked.

Increasing the nozzle pressure ratio further will not increase the throat Mach number beyond unity. Downstream (i.e. external to the nozzle) the flow is free to expand to supersonic velocities. Note that the Mach 1 can be a very high speed for a hot gas; since the speed of sound varies as the square root of absolute temperature. Thus the speed reached at a nozzle throat can be far higher than the speed of sound at sea level. This fact is used extensively in rocketry where hypersonic flows are required, and where propellant mixtures are deliberately chosen to further increase the sonic speed.

Divergent nozzles slow fluids, if the flow is subsonic, but accelerate sonic or supersonic fluids.

Convergent-divergent nozzles can therefore accelerate fluids that have choked in the convergent section to supersonic speeds. This CD process is more efficient than allowing a convergent nozzle to expand supersonically externally. The shape of the divergent section also ensures that the direction of the escaping gases is directly backwards, as any sideways component would not contribute to thrust.

Propelling

Main article: Propelling nozzle

A jet exhaust produces a net thrust from the energy obtained from combusting fuel which is added to the inducted air. This hot air is passed through a high speed nozzle, a propelling nozzle which enormously increases its kinetic energy.[1]

For a given mass flow, greater thrust is obtained with a higher exhaust velocity, but the best energy efficiency is obtained when the exhaust speed is well matched with the airspeed. However, no jet aircraft can maintain velocity while exceeding its exhaust jet speed, due to momentum considerations. Supersonic jet engines, like those employed in fighters and SST aircraft (e.g. Concorde), need high exhaust speeds. Therefore supersonic aircraft very typically use a CD nozzle despite weight and cost penalties. Subsonic jet engines employ relatively low,

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subsonic, exhaust velocities. They thus employ simple convergent nozzles. In addition, bypass nozzles are employed giving even lower speeds.

Rocket motors use convergent-divergent nozzles with very large area ratios so as to maximise thrust and exhaust velocity and thus extremely high nozzle pressure ratios are employed. Mass flow is at a premium since all the propulsive mass is carried with vehicle, and very high exhaust speeds are desirable.

Magnetic

Magnetic nozzles have also been proposed for some types of propulsion, such as VASIMR, in which the flow of plasma is directed by magnetic fields instead of walls made of solid matter.

Spray

Main article: Spray nozzle

Many nozzles produce a very fine spray of liquids.

Atomizer nozzles are used for spray painting, perfumes, carburettors for internal combustion engines, spray on deodorants, antiperspirants and many other uses.

Air-Aspirating Nozzle-uses an opening in the cone shaped nozzle to inject air into a stream of water based foam (CAFS/AFFF/FFFP) to make the concentrate "foam up". Most commonly found on foam extinguishers and foam handlines.

Swirl nozzles inject the liquid in tangentially, and it spirals into the center and then exits through the central hole. Due to the vortexing this causes the spray to come out in a cone shape.

Vacuum

Vacuum cleaner nozzles come in several different shapes.

Shaping

Some nozzles are shaped to produce a stream that is of a particular shape. For example extrusion molding is a way of producing lengths of metals or plastics or other materials with a particular cross-section. This nozzle is typically referred to as a die.

In a flow metering device based on the Bernoulli Equation the downstream pressure after an obstruction will be lower than the upstream pressure before. To understand orifice, nozzle and venturi meters it's therefore necessary to explore the Bernoulli Equation.

The Bernoulli Equation

Assuming a horizontal flow (neglecting minor elevation differences between measuring points) the Bernoulli Equation can be modified to:

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p1 + 1/2 ρ v12 = p2 + 1/2 ρ v2

2         (1)

where

p = pressure

ρ = density

v = flow velocity

The equation can be adapted to vertical flow by adding elevation heights h1 and h2.

Assuming uniform velocity profiles in the upstream and downstream flow - the Continuity Equation can be expressed as

q = v1 A1 = v2 A2         (2)

where

q = flow rate

A = flow area

Combining (1) and (2), assuming A2 < A1, gives the "ideal" equation:

q = A2 [ 2(p1 - p2) / ρ(1 - (A2 / A1)2) ]1/2         (3)

For a given geometry (A), the flow rate can be determined by measuring the pressure difference p1 - p2.

The theoretical flow rate q will in practice be smaller (2 - 40%) due to geometrical conditions.

The ideal equation (3) can be modified with a discharge coefficient:

q = cd A2 [ 2(p1 - p2) / ρ(1 - (A2 / A1)2) ]1/2         (3b)

where

cd = discharge coefficient

The discharge coefficient cd is a function of the jet size - or orifice opening - the

area ratio = Avc / A2

where

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Avc = area in "vena contracta"

"Vena Contracta" is the minimum jet area that appears just downstream of the restriction. The viscous effect is usually expressed in terms of the nondimensional parameter Reynolds Number - Re.

Due to the Benoulli and Continuity Equation the velocity of the fluid will be at it's highest and the pressure at the lowest in "Vena Contracta". After the metering device the velocity will decrease to the same level as before the obstruction. The pressure recover to a pressure level lower than the pressure before the obstruction and adds a head loss to the flow.

Equation (3) can be modified with diameters to:

q = cd π/4 D22 [ 2(p1 - p2) / ρ(1 - d4) ]1/2         (4)

where

D2 = orifice, venturi or nozzle inside diameter

D1 = upstream and downstream pipe diameter

d = D2 / D1 diameter ratio

π = 3.14

Equation (4) can be modified to mass flow for fluids by simply multiplying with the density:

m = cd π/4 D22 ρ [ 2(p1 - p2) / ρ(1 - d4) ]1/2         (5)

When measuring the mass flow in gases, its necessary to considerate the pressure reduction and change in density of the fluid. The formula above can be used with limitations for applications with relatively small changes in pressure and density.

The Orifice Plate

The orifice meter consists of a flat orifice plate with a circular hole drilled in it. There is a pressure tap upstream from the orifice plate and another just downstream. There are in general three methods of placing the taps. The coefficient of the meter depends upon the position of taps.

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Flange location - Tap location 1 inch upstream and 1 inch downstream from face of orifice "Vena Contracta" location - Tap location 1 pipe diameter (actual inside) upstream and 0.3 to 0.8

pipe diameter downstream from face of orifice Pipe location - Tap location 2.5 times nominal pipe diameter upstream and 8 times nominal pipe

diameter downstream from face of orifice

The discharge coefficient - cd - varies considerably with changes in area ratio and the Reynolds number. A discharge coefficient cd = 0.60 may be taken as standard, but the value varies noticeably at low values of the Reynolds number.

Discharge Coefficient - cd

Diameter Ratio

d = D2 / D1

Reynolds Number - Re

104 105 106 107

0.2 0.60 0.595 0.594 0.594

0.4 0.61 0.603 0.598 0.598

0.5 0.62 0.608 0.603 0.603

0.6 0.63 0.61 0.608 0.608

0.7 0.64 0.614 0.609 0.609

The pressure recovery is limited for an orifice plate and the permanent pressure loss depends primarily on the area ratio. For an area ratio of 0.5, the head loss is about 70 - 75% of the orifice differential.

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The orifice meter is recommended for clean and dirty liquids and some slurry services. The rangeability is 4 to 1 The pressure loss is medium Typical accuracy is 2 to 4% of full scale The required upstream diameter is 10 to 30 The viscosity effect is high The relative cost is low

References

American Society of Mechanical Engineers (ASME). 2001. Measurement of fluid flow using small bore precision orifice meters. ASME MFC-14M-2001.

International Organization of Standards (ISO 5167-1:2003). Measurement of fluid flow by means of pressure differential devices, Part 1: Orifice plates, nozzles, and Venturi tubes inserted in circular cross-section conduits running full. Reference number: ISO 5167-1:2003.

International Organization of Standards (ISO 5167-1) Amendment 1. 1998. Measurement of fluid flow by means of pressure differential devices, Part 1: Orifice plates, nozzles, and Venturi tubes inserted in circular cross-section conduits running full. Reference number: ISO 5167-1:1991/Amd.1:1998(E).

American Society of Mechanical Engineers (ASME). B16.36 - 1996 - Orifice Flanges

The Venturi Meter

In the venturi meter the fluid is accelerated through a converging cone of angle 15-20o and the pressure difference between the upstream side of the cone and the throat is measured and provides a signal for the rate of flow.

The fluid slows down in a cone with smaller angle (5 - 7o) where most of the kinetic energy is converted back to pressure energy. Because of the cone and the gradual reduction in the area there is no "Vena Contracta". The flow area is at a minimum at the throat.

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High pressure and energy recovery makes the venturi meter suitable where only small pressure heads are available.

A discharge coefficient cd = 0.975 can be indicated as standard, but the value varies noticeably at low values of the Reynolds number.

The pressure recovery is much better for the venturi meter than for the orifice plate.

The venturi tube is suitable for clean, dirty and viscous liquid and some slurry services. The rangeability is 4 to 1 Pressure loss is low Typical accuracy is 1% of full range Required upstream pipe length 5 to 20 diameters Viscosity effect is high Relative cost is medium

References

International Organization of Standards - ISO 5167-1:2003 Measurement of fluid flow by means of pressure differential devices, Part 1: Orifice plates, nozzles, and Venturi tubes inserted in circular cross-section conduits running full. Reference number: ISO 5167-1:2003.

American Society of Mechanical Engineers ASME FED 01-Jan-1971. Fluid Meters Their Theory And Application- Sixth Edition

The Nozzle

Nozzles used for determining fluid's flowrate through pipes can be in three different types:

The ISA 1932 nozzle - developed in 1932 by the International Organization for Standardization or ISO. The ISA 1932 nozzle is common outside USA.

The long radius nozzle is a variation of the ISA 1932 nozzle. The venturi nozzle is a hybrid having a convergent section similar to the ISA 1932 nozzle and a

divergent section similar to a venturi tube flowmeter.

Discharge Coefficient - cd

Diameter Ratio

d = D2 / D1

Reynolds Number - Re

104 105 106 107

0.2 0.968 0.988 0.994 0.995

0.4 0.957 0.984 0.993 0.995

0.6 0.95 0.981 0.992 0.995

0.8 0.94 0.978 0.991 0.995

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Discharge Coefficient - cd

Diameter Ratio

d = D2 / D1

Reynolds Number - Re

104 105 106 107

The flow nozzle is recommended for both clean and dirty liquids The rangeability is 4 to 1 The relative pressure loss is medium Typical accuracy is 1-2% of full range Required upstream pipe length is 10 to 30 diameters The viscosity effect high The relative is medium

References

American Society of Mechanical Engineers ASME FED 01-Jan-1971. Fluid Meters Their Theory And Application- Sixth Edition

International Organization of Standards - ISO 5167-1:2003 Measurement of fluid flow by means of pressure differential devices, Part 1: Orifice plates, nozzles, and Venturi tubes inserted in circular cross-section conduits running full. Reference number: ISO 5167-1:2003.

Example - Kerosene Flow Through a Venturi Meter

The pressure difference dp = p1 - p2 between upstream and downstream is 100 kPa (1 105 N/m2). The specific gravity of kerosene is 0.82.

Upstream diameter is 0.1 m and downstream diameter is 0.06 m.

Density of kerosene can be calculated as:

ρ = 0.82 (1000 kg/m3)

    = 820 (kg/m3)

Density, Specific Weight and Specific Gravity - An introduction and definition of density, specific weight and specific gravity. Formulas with examples.

Upstream and downstream area can be calculated as:

A1 = π ((0.1 m)/2)2

    = 0.00785 (m2)

A2 = π  ((0.06 m)/2)2

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    = 0.002826 (m2)

Theoretical flow can be calculated from (3):

q = A2 [ 2(p1 - p2) / ρ(1 - (A2/A1)2) ]1/2

q = (0.002826 m2) [ 2 (105 N/m2) / (820 kg/m3)(1 - ( (0.002826 m2) / (0.00785 m2) )2) ]1/2

    = 0.047 (m3/s)

For a pressure difference of 1 kPa (0,01x105 N/m2) - the theoretical flow can be calculated:

q = (0.002826 m2) [ 2 (0.01 105 N/m2) / (820 kg/m3)(1 - ( (0.002826 m2) / (0.00785 m2) )2) ]1/2

    = 0.0047 (m3/s)

The mass flow can be calculated as:

m = q ρ

    = (0.0047 m3/s) (820 kg/m3)

    = 3.85 (kg/s)

Flow Rate and Change in Pressure Difference

Note! - The flow rate varies with the square root of the pressure difference.

From the example above:

a tenfold increase in the flow rate requires a one hundredfold increase in the pressure difference!

Transmitters and Control System

The nonlinear relationship have impact on the pressure transmitters operating range and requires that the electronic pressure transmitters have the capability to linearizing the signal before transmitting it to the control system.

Pitot tubeFrom Wikipedia, the free encyclopedia

Jump to: navigation, search

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Aircraft use pitot tubes to measure airspeed. The example, from an Airbus A380, combines a pitot tube (pencil point shape) with a static port and an angle-of-attack vane (black). Air-flow, relative to this device, is right to left.

Types of pitot tubes

A pitot (pron.: / ̍ p iː t oʊ / ) tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot in the early 18th century[1] and was modified to its modern form in the mid-19th century by French scientist Henry Darcy.[2] It is widely used to determine the airspeed of an aircraft and to measure air and gas velocities in industrial applications. The pitot tube is used to measure the local velocity at a given point in the flow stream and not the average velocity in the pipe or conduit.[3]

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Contents

1 Theory of operation 2 Operation 3 Industry applications 4 See also 5 References 6 External links

Theory of operation

The basic pitot tube consists of a tube pointing directly into the fluid flow. As this tube contains fluid, a pressure can be measured; the moving fluid is brought to rest (stagnates) as there is no outlet to allow flow to continue. This pressure is the stagnation pressure of the fluid, also known as the total pressure or (particularly in aviation) the pitot pressure.

The measured stagnation pressure cannot itself be used to determine the fluid velocity (airspeed in aviation). However, Bernoulli's equation states:

Stagnation pressure = static pressure + dynamic pressure

Which can also be written

Solving that for velocity we get:

NOTE: The above equation applies ONLY to fluids that can be treated as incompressible. Liquids are treated as incompressible under almost all conditions. Gases under certain conditions can be approximated as incompressible. See Compressibility.

where:

is fluid velocity;

is stagnation or total pressure;

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is static pressure;

and is fluid density.

The value for the pressure drop – or due to , the reading on the manometer:

Where:

is the density of the fluid in the manometer

is the manometer reading

The dynamic pressure, then, is the difference between the stagnation pressure and the static pressure. The static pressure is generally measured using the static ports on the side of the fuselage. The dynamic pressure is then determined using a diaphragm inside an enclosed container. If the air on one side of the diaphragm is at the static pressure, and the other at the stagnation pressure, then the deflection of the diaphragm is proportional to the dynamic pressure, which can then be used to determine the indicated airspeed of the aircraft. The diaphragm arrangement is typically contained within the airspeed indicator, which converts the dynamic pressure to an airspeed reading by means of mechanical levers.

Instead of separate pitot and static ports, a pitot-static tube (also called a Prandtl tube) may be employed, which has a second tube coaxial with the pitot tube with holes on the sides, outside the direct airflow, to measure the static pressure.[4]

Operation

Pitot tubes on aircraft commonly have heating elements called pitot heat to prevent the tube from becoming clogged with ice. The failure of these systems can have catastrophic consequences, as in the case of Austral Líneas Aéreas Flight 2553, Birgenair Flight 301 (investigators suspected that some kind of insect could have created a nest inside the pitot tube: the prime suspect is a species called the black and yellow mud dauber wasp), Northwest Airlines Flight 6231, Aeroperú Flight 603 (blocked static port), and of one X-31.[5] The French air safety authority BEA said that pitot tube icing was a contributing factor in the crash of Air France Flight 447 from high altitude into the Atlantic Ocean.[6]

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Industry applications

Pitot tube from a F/A-18

In industry, the velocities being measured are often those flowing in ducts and tubing where measurements by an anemometer would be difficult to obtain. In these kinds of measurements, the most practical instrument to use is the pitot tube. The pitot tube can be inserted through a small hole in the duct with the pitot connected to a U-tube water gauge or some other differential pressure gauge (alnor) for determining the velocity inside the ducted wind tunnel. One use of this technique is to determine the volume of air that is being delivered to a conditioned space.

The fluid flow rate in a duct can then be estimated from:

Volume flow rate (cubic feet per minute) = duct area (square feet) × velocity (feet per minute)

Volume flow rate (cubic meters per second) = duct area (square meters) × velocity (meters per second)

In aviation, airspeed is typically measured in knots.

See also

Air data boom Altimeter Annubar Anti-icing Atmospheric icing Calibrated airspeed

Deicing Flow measurement Gyrocompass Icing conditions Kiel probe Mach number

Piezometer Pitot-static system Position error Stagnation pressure True airspeed

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References

Notes

1. ̂ Pitot, Henri (1732). "Description d'une machine pour mesurer la vitesse des eaux courantes et le sillage des vaisseaux" (PDF). Histoire de l'Académie royale des sciences avec les mémoires de mathématique et de physique tirés des registres de cette Académie: 363–376. Retrieved 2009-06-19.

2. ̂ Darcy, Henry (1858). "Note relative à quelques modifications à introduire dans le tube de Pitot" (PDF). Annales des Ponts et Chaussées: 351–359. Retrieved 2009-07-31.

3. ̂ Geankoplis, C.J. (2003). Transport processes and separation process principles (includes unit operations) (4th ed.). New Jersey: Prentice Hall.

4. ̂ "How Aircraft Instruments Work." Popular Science, March 1944, pp. 116.5. ̂ NASA Dryden news releases. (1995)6. ̂ "Training flaws exposed in Rio-Paris crash report". Reuters. 5 July, 2012. Retrieved 5 October

2012.

Bibliography

Kermode, A.C. (1996) [1972]. Mechanics of Flight. Barnard, R.H. (Ed.) and Philpott, D.R. (Ed.) (10th ed.). Prentice Hall. pp. 63–67. ISBN 0-582-23740-8.

Pratt, Jeremy M. (2005) [1997]. The Private Pilot's Licence Course: Principles of Flight, Aircraft General Knowledge, Flight Performance and Planning (3rd ed.). gen108-gen111. ISBN 1-874783-23-3.

Tietjens, O.G. (1934). Applied Hudro- and Aeromechanics, based on lectures of L. Prandtl, Ph.D. Dove Publications, Inc.. pp. 226–239. ISBN 0-486-60375-X.

Saleh, J.M. (2002). Fluid Flow Handbook. McGraw-Hill Professional.

Elbow Flow Meters Clifford A. Pugh, Hydraulic Investigations and Laboratory Services Group

Elbow meters are based on the principle of "conservation of momentum."  Momentum conservation requires that the momentum flux (momentum per unit time) remain unchanged as steady flow occurs through an isolated system of fluid.  Since momentum is a vector quantity, a change in direction of the flow causes a reduction of momentum in the original direction which is offset by an increase in the new direction.  In an elbow, such as the mitred elbow shown in figure 1,  the momentum in the horizontal direction is changed by the pipe turning down.  This change in direction causes the flow to exert a force on the pipe elbow.

F = Q (V2-V1)      [Momentum Equation]     (1) Where, F = Force

 = the fluid densityQ = the discharge (flow)V = the velocity vector

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Figure 1 - Forty-five degree mitred bend with pressure taps on the inside and outside of

the bend.  The pressure differential is related to the square of the velocity. This force results in an increased pressure on the outside of the bend and a decreased

pressure on the inside.  The pressure difference is proportional to the square of the velocity. The general form of the equation would be :

          (2) Figure 1 shows recommended pressure tap locations for a mitred bend according to ID

Tech, inc.  ID Tech sells an "Electronic Flow Calculator" based on an elbow meter. The coefficents of discharge (Cd) for mitred bends (determined empirically by ID Tech) are proprietary. Their toll free phone number is 888-782-0498.

Figure 2 shows a multiple level outlet at Beltzville Dam in Pennsylvania. Differential pressures across opposing pressure taps (P1 and P2) and stream gage measurements were used to develop the rating curve and equation shown in figure 3 (Hart and Pugh, 1975).

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Figure 2 - Water Quality Control outlet at Beltzville Dam (Hart and Pugh, 1975).  The

pressure differential between P1 and P2 was empirically calibrated to obtain a discharge relationship. The elbow meter is used to set desired outlet flows for normal operations.

Figure 3 - Elbow Meter Calibration, Beltzville Dam. Similar empirical relationships could be developed for pipe bends in the field by using a

strap on acoustic flowmeter (or another flow measurement method) to obtain the data and develop the equation. One differential pressure transducer would be connected to the high

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and low pressure taps to measure the differential and obtain flow. This is a simple and relatively accurate device if the pressure taps are properly installed and the "burrs" are cleaned from the inside of the tap. A slight rounding of the edge of the taps helps to improve their performance.

As a practical matter, the lower limit of an elbow meter is about 2 ft/s.  Pressure differences and discharge measurement accuracy are very low below this velocity.  Flow Tech recommends an upper velocity limit of 10 ft/s, this is probably due to seperation at the sharp bend in the mitred elbow. Higher velocities may be allowable for elbows with a constant radius such as the example in figure 2 and 3.

 Pressure gauges are devices used for measuring the pressure of a gas or liquid. These gauges have been designed to withstand the most demanding environments & are offered in a wide assortment of dial sizes, case styles and wetted parts materials.

   Bourdon Type Pressure Gauge  

   These are used for measurement of pressure and vacuum and are suitable for all clean and non-clogging liquid and gaseous media. The Bourdon Tube is a thin walled tube of oval cross section which may be of ‘C’ form or spirally wound. This tube expands when pressure is applied internally; this expansion is converted into rotation of a concentric pointer with a gear movement. C-type Bourdon tubes are used for low pressure ranges and helical / spiral tubes for higher pressure ranges.

 

   

Technical Specification

Dial Size50 mm, 63 mm, 100 mm, 150 mm, 200 mm, 250 mm only

Ranges0 to 1, 1.6, 2, 2.5, 4, 6, 10, 16 25, 40, 60, 100,

160, 250, 400 & 600 kg/cm

Standard Accuracy  ±1% of F.S.D

Sensing Element Bourdon Tube

Bourdon Tube Material Stainless Steel (AISI 316 SS)

Connection Screwed

Connection size (Male)½ BSP, NPT, BSPT, API; M 20 x 1.5 (1/4" NPT (M) for 50 mm dial)

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Connection Material Stainless Steel (AISI 316 SS)

Joint Between Bourdon Tube Connection Argon Arc welded

Movement Stainless Steel (AISI 304 SS)

Window Toughened Glass, Plain Glass, Acrylic Glass

MountingsDirect with bottom entry Surface with bottom entry Flush panel with back entry

Momentary over pressure allowed As per IS 3624 – 1979

Case/Enclosure AISI 304 SS/ Aluminium

   

   Capsule Type Pressure Gauge  

   Capsule Pressure gauges are designed for measuring low/very low pressures and vacuum. The pressure medium needs to be gaseous. The capsule consists of two diaphragms joined and sealed around their circumference. Several such capsules may be combined to enable measurement of very low pressures.

 

   

Technical Specification

Dial size 63 mm/100 mm/150 mm

Accuracy ±1% of Range Span

Sensing Element Capsule

Capsule Material AISI 316 SS

Movement Maters AISI 304 SS

EnclosureCast Aluminum weather proof stove enameled black AISI 304 SS weather proof self colour finish

Mounting Direct with bottom entry

Direct Panel Panel Mounting back entry (cut out = 167 mm)

Capsule Chamber Cast aluminum stove enameled black

Connection Material AISI 316 SS

Connection Size ½" BSP (M) ½" NPT (M). M 20 x 1.5 or Equivalent

Range 0 to 250 mm Wc  to 0 to 10000 mm Wc

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Pressure gauges & flow metersPressure and flow readings provide a means of assessing the performance of hydraulic and pneumatic systems and aid troubleshooting when malfunctions occur.

Jan. 1, 2012 Hydraulics & Pneumatics

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Pressure and flow readings provide a means of assessing the performance of hydraulic and pneumatic systems and aid troubleshooting when malfunctions occur.

The majority of gauges for measuring pressure have one characteristic in common: the pressure being measured is the only source of energy required to provide a visual indication of static pressure. Some form of elastic chamber inside the gauge case converts the pressure to motion, which is translated through suitable links, levers, and gearing into movement of a pointer across an indicating scale. Three types of elastic chambers are commonly used in gauges for fluid power systems:

C-shaped, spiral, and helical Bourdon tubes

bellows, and single- and multi-capsule stacks.

Bourdon-tube designs

Since the invention of the Bourdon-tube gauge more than a century ago, pressure

Fig. 1. Cutaway view of C-shaped Bourdon-tube pressure gage. Pressure-induced strain in the Bourdon tube causes it to deform. Transmitting this deformation to a pointer through a movement linkage provides a visual indication of pressure.

Fig. 2. Simplified view of spiral Bourdon-tube pressure gage and movement.

Fig. 3. Simplified view of helical Bourdon-tube pressure gage and movement.

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gauge manufacturers have been developing different types of gauges to meet specific needs without ever changing the basic principle of the Bourdon tube's operation. Bourdon-tube gauges, Figure 1, are now commonly available to measure a wide range of gauge, absolute, sealed, and differential pressures, plus vacuum.

They are manufactured to an accuracy as high as 0.1% of span and in dial diameters from 1-1/2 to 16 in. A variety of accessories can extend their performance and usefulness. For example, snubbers and gauge isolators can be installed to protect the sensitive internal workings of the gauge from pressure spikes. The availability of Bourdon-tube pressure gauges to meet specific needs, coupled with their inherent ruggedness, simplicity, and low cost has resulted in their wide use in many applications.

Gauges using C-shaped Bourdon tubes as the elastic chamber - the type shown in Figure 1 - are by far the most common. Pressurized fluid enters the stem at the bottom (which is sometimes center-back-mounted instead) and passes into the Bourdon tube. The tube has a flattened cross section and is sealed at its tip. Any pressure in the tube in excess of the external pressure (usually atmospheric) causes the Bourdon tube to elastically change its shape to a more circular cross section.

This change in shape of the cross section tends to straighten the C-shape of the Bourdon tube. With the bottom stem end fixed, the straightening causes the tip at the opposite end to move a short distance - 1/16 to 1/2 in., depending on the size of the tube. A mechanical movement then transmits this tip motion to a gear train that rotates an indicating pointer over a graduated scale to display the applied pressure. Often, a movement is incorporated to provide mechanical advantage to multiply the relatively short movement of the tube tip.

Spiral and helical Bourdons

Bourdon tubes also may be made in the form of a spiral, Figure 2, or a helix, Figure 3. Each uses a long length of flattened tubing to provide increased tip travel. This does not change the operating principle of the Bourdon tube, but produces tip motion equal to the sum of the individual motions that would result from each part of the spiral or helix considered as a C-shape. Small-diameter spirals and helices can be manufactured to provide enough motion to drive the indicating pointer directly through an arc up to 270° without having to use a multiplying movement. Alternatively, they may be manufactured to be used in conjunction with a multiplying movement. In this case, the required motion is distributed over several turns, resulting in lower stress in the Bourdon material. This improves fatigue life when compared to a C-shaped Bourdon tube in the same pressure range.

Bellows and diaphragms

Low-pressure applications do not generate enough force in the Bourdon tube to operate the multiplying mechanism; therefore, Bourdon-tube gauges are not generally used for pressure spans under 12 psi. For these ranges, some other form of elastic chamber must be used, a metallic bellows, Figure 4, for example. These bellows generally are made by forming thin-wall tubing. However, to obtain a reasonable fatigue life and motion that is more linear with pressure,

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a coil spring supplements the inherent spring rate of the bellows. These spring-loaded bellows gauges generally are used in pressure ranges having spans to 100 psi and to 1 in. Hg.

Metallic diaphragms also are used as the elastic chamber in low-pressure gauges. A diaphragm plate is formed from thin sheet metal into a shallow cup having concentric corrugations. To make an element with a low spring rate that generates substantial deflection from a small change in pressure, two plates can be soft soldered, brazed, or welded at their periphery to form a capsule, and additional capsules can be joined at their centers to form a stack, Figure 5.

Generally, the measured pressure is applied to the interior of the element and no supplemental coil springs are used. A 2-in. diameter capsule (two plates) will provide about 0.060 in. of motion without exceeding the elastic limit of the material. This is usually enough to operate a high-ratio multiplying movement because diaphragm deflection can transmit high force.

Diaphragm elements often are used in gauges to indicate absolute pressure. In this form, the diaphragm element is evacuated. sealed, and mounted within a closed chamber. The pressure to be measured is admitted to the closed chamber and surrounds the diaphragm element. Changes in the measured pressure cause the element to deflect, but because atmospheric pressure is excluded and has no effect on the indication, the gauge may be calibrated in terms of absolute pressure. If the applied pressure is atmospheric pressure, the gauge is known as a barometer.

Diaphragm elements also may be used in an opposing arrangement. By evacuating one side of the assembly, the gauge can indicate absolute pressure. If a pressure is applied to one side of the assembly, and a second pressure is applied to the other side, then the differential pressure will be indicated. The differential pressure is limited with respect to the static pressure that can be applied. That is, the gauge may be suitable to indicate between 10 psi and 12 psi, but not be suitable to indicate between 100 psi and 102 psi. Also, the consequence of inadvertently applying full pressure to one side of the element and no pressure to the other side of the element must be considered.

Selection

Specifying a pressure gauge involves a number of considerations:

connection size - nominal size of the port or fitting into which the gauge will be threaded, male or female, and thread size

mounting configuration - bottom or back-center stem mounted or panel mounted dial size - large enough to be seen clearly from a distance but small enough to prevent taking up

excessive space units of measure - determine whether the dial should be calibrated in psi, bar, kPa, etc. Many

manufacturers offer gauges with dual-dimensioned scales materials of construction - gauges may have a glass or plastic crystal, metal or plastic case, and

usually a brass connection. Ensure that materials are compatible with the environment and fluid dry or liquid filled - liquid-filled gauges generally contain glycerin to dampen effects of shock and

vibration, and provide continuous lubrication of the movement to extend life, and

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pressure range - as a rule of thumb, select a gauge with a maximum pressure reading twice that of the anticipated measured pressure. This provides a safety margin to prevent temporary high-pressure pulsations or spikes from damaging the gauge.

Options and accessories

A variety of options and accessories are available to enhance life and operation of gauges. Digital readout is accomplished by mounting a strain gauge to the sensing element and using on-board electronics to convert the strain induced by pressure into digital readout on an LED or LCD panel. Digital gauges require a power source - generally a long-life battery - and may use a switch so that power is consumed only when a button is pushed to read the pressure.

A gauge isolator, mounted between the gauge and circuit, prevents the gauge from being exposed to fluid pressure unless a button is pushed. In this manner, the gauge is not exposed to pressure spikes and pulsations unless they occur when pressure is being read.

Orifices or snubbers protect gauges by smoothing out pressure fluctuations seen by the gauge. Snubbers may cause gauges to respond sluggishly, but can extend life by damping rapid pressure fluctuations. To help protect the gauge from external physical shock, case protectors can be used, which encapsulate the gauge in rubber.

A wide variety of other useful options - such as an integral adjustable pressure switch - are available from manufacturers to make pressure gauges even more versatile.

Flow meters

Unlike pressure gauges, which have been permanently mounted on the vast majority of hydraulic and pneumatic systems for decades, flow meters continue to be used primarily for testing to assess the performance of a system, Figure 6. Systems requiring continuous monitoring of flow generally use electronic flow sensors rather than flow meters, which require no power.

Electronic flow sensors use a variety of sensing elements (turbines, positive-displacement chambers, differential-pressure measurement, etc.) to generate an electronic signal proportional to or otherwise representative of flow. This signal is then routed to an electronic display panel or control circuit. However, flow sensors produce no visual indication of flow by themselves, and they need some source of external power to transmit a signal to an analog or digital display.

Self-contained flow meters, on the other hand, rely on the dynamics of flow to provide a visual indication of flow. Although design details differ from one manufacturer to another, flow meters operate on the principle of dynamic pressure. The main components are a tapered shaft and spring-loaded piston, Figure 7.

With no fluid flow, the actuating spring pushes the piston to its left-most position. As fluid enters from the left side, pressure acts against the spring and builds to open the orifice formed between the ID of the piston and OD of the tapered shaft by pushing the piston to the right. As the piston is pushed farther to the right, the orifice area increases because the effective area of the tapered

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shaft decreases. Eventually, the orifice area will be large enough so that dynamic pressure from flow equals the opposing spring force. The position of the piston in equilibrium, then, provides an indication of flow.

For some applications, flow can be measured directly by comparing the piston position to a calibrated scale marked on the flow meter's transparent outer case. For most hydraulic applications, however, the piston usually has a magnet embedded that moves a follower collar. Position of the collar can then be compared to a calibrated scale.

Because flow indication depends on fluid dynamics, changes in a fluid's physical properties can affect readings. This is because a flow meter is calibrated to a fluid having a certain specific gravity within a range of viscosities. A wide deviation in temperature can change a hydraulic fluid's specific gravity and viscosity, so if a flow meter is used when the fluid is very hot or very cold, flow readings may not conform to manufacturers' specifications. However, because most equipment is tested under operating conditions, readings generally should fall within manufacturers' specifications for accuracy.

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BarometerFrom Wikipedia, the free encyclopedia

Jump to: navigation, search

Schematic drawing of a simple mercury barometer with vertical mercury column and reservoir at base

Continuum mechanics

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Laws[show]

Solid mechanics [show]

Fluid mechanics [show]

Rheology [show]

Scientists[show]

v

t

e

Old barometers from the Musée des Arts et Métiers, Paris

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Goethe's device

Barometer from 1890s

A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used within surface weather analysis to help find surface troughs, high pressure systems, and frontal boundaries.

Contents

1 History 2 Types

o 2.1 Water-based barometers o 2.2 Mercury barometers o 2.3 Aneroid barometers o 2.4 Barographs

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o 2.5 More unusual barometers 3 Applications 4 Compensations

o 4.1 Temperature o 4.2 Altitude

5 Patents 6 See also 7 References 8 Further reading

History

Although Evangelista Torricelli is universally credited with inventing the barometer in 1643,[1][2]

[3] historical documentation also suggests Gasparo Berti, an Italian mathematician and astronomer, unintentionally built a water barometer sometime between 1640 and 1643.[1][4] French scientist and philosopher René Descartes described the design of an experiment to determine atmospheric pressure as early as 1631, but there is no evidence that he built a working barometer at that time.[1]

On July 27, 1630, Giovanni Battista Baliani wrote a letter to Galileo Galilei explaining an experiment he had made in which a siphon, led over a hill about twenty-one meters high, failed to work. Galileo responded with an explanation of the phenomenon: he proposed that it was the power of a vacuum that held the water up, and at a certain height the amount of water simply became too much and the force could not hold any more, like a cord that can support only so much weight.[5][6] This was a restatement of the theory of horror vacui ("nature abhors a vacuum"), which dates to Aristotle, and which Galileo restated as resistenza del vacuo.

Galileo's ideas reached Rome in December 1638 in his Discorsi. Raffaele Magiotti and Gasparo Berti were excited by these ideas, and decided to seek a better way to attempt to produce a vacuum than with a siphon. Magiotti devised such an experiment, and sometime between 1639 and 1641, Berti (with Magiotti, Athanasius Kircher and Niccolò Zucchi present) carried it out.[6]

Four accounts of Berti's experiment exist, but a simple model of his experiment consisted of filling with water a long tube that had both ends plugged, then standing the tube in a basin already full of water. The bottom end of the tube was opened, and water that had been inside of it poured out into the basin. However, only part of the water in the tube flowed out, and the level of the water inside the tube stayed at an exact level, which happened to be 10.3 m, the same height Baliani and Galileo had observed that was limited by the siphon. What was most important about this experiment was that the lowering water had left a space above it in the tube which had had no intermediate contact with air to fill it up. This seemed to suggest the possibility of a vacuum existing in the space above the water.[6]

Torricelli, a friend and student of Galileo, dared to look at the entire problem from a different angle. In a letter to Michelangelo Ricci in 1644 concerning the experiments with the water barometer, he wrote:

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Many have said that a vacuum does not exist, others that it does exist in spite of the repugnance of nature and with difficulty; I know of no one who has said that it exists without difficulty and without a resistance from nature. I argued thus: If there can be found a manifest cause from which the resistance can be derived which is felt if we try to make a vacuum, it seems to me foolish to try to attribute to vacuum those operations which follow evidently from some other cause; and so by making some very easy calculations, I found that the cause assigned by me (that is, the weight of the atmosphere) ought by itself alone to offer a greater resistance than it does when we try to produce a vacuum.[7]

It was traditionally thought (especially by the Aristotelians) that the air did not have lateral weight: that is, that the kilometers of air above the surface did not exert any weight above bodies. Even Galileo had accepted the weightlessness of air as a simple truth. Torricelli questioned that assumption, and instead proposed that air had weight, and that it was the latter (not the attracting force of the vacuum) which held (or rather, pushed) up the column of water. He thought that the level the water stayed at (c. 10.3 m) was reflective of the force of the air's weight pushing on it (specifically, pushing on the water in the basin and thus limiting how much water can fall from the tube into it). In other words, he viewed the barometer as a balance, an instrument for measurement (as opposed to merely being an instrument to create a vacuum), and because he was the first to view it this way, he is traditionally considered the inventor of the barometer (in the sense in which we use the term now).[6]

Because of rumors circulating in Torricelli's gossipy Italian neighborhood, which included that he was engaged in some form of sorcery or witchcraft, Torricelli realized he had to keep his experiment secret to avoid the risk of being arrested. He needed to use a liquid that was heavier than water, and from his previous association and suggestions by Galileo, he deduced by using mercury, a shorter tube could be used. With mercury, then called "quicksilver", which is about 14 times heavier than water, a tube only 80 cm was now needed, not 10.5 m.[8]

In 1646, Blaise Pascal along with Pierre Petit, had repeated and perfected Torricelli's experiment after hearing about it from Marin Mersenne, who himself had been shown the experiment by Torricelli toward the end of 1644. Pascal further devised an experiment to test the Aristotelian proposition that it was vapors from the liquid that filled the space in a barometer. His experiment compared water with wine, and since the latter was considered more "spiritous", the Aristotelians expected the wine to stand lower (since more vapors would mean more pushing down on the liquid column). Pascal performed the experiment publicly, inviting the Aristotelians to predict the outcome beforehand. The Aristotelians predicted the wine would stand lower. It did not.[6]

However, Pascal went even further to test the mechanical theory. If, as suspected by mechanical philosophers like Torricelli and Pascal, air had lateral weight, the weight of the air would be less at higher altitudes. Therefore, Pascal wrote to his brother-in-law, Florin Perier, who lived near a mountain called the Puy de Dome, asking him to perform a crucial experiment. Perier was to take a barometer up the Puy de Dome and make measurements along the way of the height of the column of mercury. He was then to compare it to measurements taken at the foot of the mountain to see if those measurements taken higher up were in fact smaller. In September 1648, Perier carefully and meticulously carried out the experiment, and found that Pascal's predictions had been correct. The mercury barometer stood lower the higher one went.[6]

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Types

Water-based barometers

The concept that decreasing atmospheric pressure predicts stormy weather, postulated by Lucien Vidie, provides the theoretical basis for a weather prediction device called a "storm glass" or a "Goethe barometer" (named for Johann Wolfgang Von Goethe, the renowned German writer and polymath who developed a simple but effective weather ball barometer using the principles developed by Torricelli).

The weather ball barometer consists of a glass container with a sealed body, half filled with water. A narrow spout connects to the body below the water level and rises above the water level. The narrow spout is open to the atmosphere. When the air pressure is lower than it was at the time the body was sealed, the water level in the spout will rise above the water level in the body; when the air pressure is higher, the water level in the spout will drop below the water level in the body. A variation of this type of barometer can be easily made at home.[9]

Mercury barometers

A mercury barometer has a glass tube with a height of at least 84 cm, closed at one end, with an open mercury-filled reservoir at the base. The weight of the mercury creates a vacuum in the top of the tube. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir. Since higher temperature at the instrument will reduce the density of the mercury, the scale for reading the height of the mercury is adjusted to compensate for this effect.

Torricelli documented that the height of the mercury in a barometer changed slightly each day and concluded that this was due to the changing pressure in the atmosphere.[1] He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight"[source?].

The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters or feet (torr): the pressure is quoted as the level of the mercury's height in the vertical column. Typically, atmospheric pressure is measured between 26.5 to 31.5 inHg. One atmosphere (1 atm) is equivalent to 760 millimeters of mercury.

Design changes to make the instrument more sensitive, simpler to read, and easier to transport resulted in variations such as the basin, siphon, wheel, cistern, Fortin, multiple folded, stereometric, and balance barometers. Fitzroy barometers combine the standard mercury barometer with a thermometer, as well as a guide of how to interpret pressure changes. Fortin barometers use a variable displacement mercury cistern, usually constructed with a thumbscrew pressing on a leather diaphragm bottom. This compensates for displacement of mercury in the column with varying pressure. To use a Fortin barometer, the level of mercury is set to the zero level before the pressure is read on the column. Some models also employ a valve for closing the

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cistern, enabling the mercury column to be forced to the top of the column for transport. This prevents water-hammer damage to the column in transit.

On June 5, 2007, a European Union directive was enacted to restrict the sale of mercury, thus effectively ending the production of new mercury barometers in Europe.

Aneroid barometers

See also: Barograph

Old aneroid barometer

Modern aneroid barometer

An aneroid barometer, invented in 1843 by French scientist Lucien Vidie uses a small, flexible metal box called an aneroid cell (capsule), which is made from an alloy of beryllium and copper.[10] The evacuated capsule (or usually more capsules) is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Many models include a manually set needle which is used to mark the current measurement so a change can be seen. In

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addition, the mechanism is made deliberately "stiff" so that tapping the barometer reveals whether the pressure is rising or falling as the pointer moves.

Barographs

A barograph, which records a graph of some atmospheric pressure, uses an aneroid barometer mechanism to move a needle on a smoked foil or to move a pen upon paper, both of which are attached to a drum moved by clockwork.[11]

More unusual barometers

The Galaxy Nexus has a built-in barometer[12]

There are many other more unusual types of barometer. From variations on the storm barometer, such as the Collins Patent Table Barometer, to more traditional looking designs such as Hooke's Otheometer and the Ross Sympiesometer. Some, such as the Shark Oil barometer,[13] work only in a certain temperature range, achieved in warmer climates

An barometer can also be found in the new Samsung Galaxy Nexus smartphone,[14] which is included to provide a faster GPS lock.[15]

Applications

See also: Surface weather analysis and Weather forecasting

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Digital graphing barometer.

Barograph using five stacked aneroid barometer cells.

Using barometric pressure and the pressure tendency (the change of pressure over time) has been used in weather forecasting since the late 19th century.[16] When used in combination with wind observations, reasonably accurate short-term forecasts can be made.[17] Simultaneous barometric readings from across a network of weather stations allow maps of air pressure to be produced, which were the first form of the modern weather map when created in the 19th century. Isobars, lines of equal pressure, when drawn on such a map, gives a contour map showing areas of high and low pressure.[18] Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Atmospheric lift caused by low-level wind convergence into the surface low brings clouds and potentially precipitation.[19] The larger the change in pressure, especially if more than 3.5 hPa, the larger the change in weather can be expected. If the pressure drop is rapid, a low pressure system is approaching, and there is a greater chance of rain . Rapid pressure rises, such as in the wake of a cold front, are associated with improving weather conditions, such as clearing skies.[20]

Compensations

Temperature

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The density of mercury will change with temperature, so a reading must be adjusted for the temperature of the instrument. For this purpose a mercury thermometer is usually mounted on the instrument. Temperature compensation of an aneroid barometer is accomplished by including a bi-metal element in the mechanical linkages. Aneroid barometers sold for domestic use typically have no compensation.

Altitude

As the air pressure will be decreased at altitudes above sea level (and increased below sea level) the actual reading of the instrument will be dependent upon its location. This pressure is then converted to an equivalent sea-level pressure for purposes of reporting and for adjusting aircraft altimeters (as aircraft may fly between regions of varying normalized atmospheric pressure owing to the presence of weather systems). Aneroid barometers have a mechanical adjustment for altitude that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude.

Patents

Table of Pneumaticks, 1728 Cyclopaedia

US 2194624 , G. A. Titterington, Jr, "Diaphragm pressure gauge having temperature compensating means", issued 1940-03-26, assigned to Bendix Aviat Corp

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U.S. Patent 2,472,735 : C. J. Ulrich : "Barometric instrument" U.S. Patent 2,691,305 : H. J. Frank : Barometric altimeter" U.S. Patent 3,273,398 : D. C. W. T. Sharp : "Aneroid barometer" U.S. Patent 3,397,578 : H. A. Klumb : "Motion amplifying mechanism for pressure responsive

instrument movement" U.S. Patent 3,643,510 : F. Lissau : "Fluid displacement pressure gauges" U.S. Patent 4,106,342 : O. S. Sormunen : "Pressure measuring instrument" U.S. Patent 4,238,958 : H. Dostmann : "Barometer" U.S. Patent 4,327,583 : T. Fijimoto : "Weather forecasting device"

Pressure measurementFrom Wikipedia, the free encyclopedia

Jump to: navigation, search

The construction of a bourdon tube gauge. Construction elements are made of brass

Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges.

A manometer could also refer to a pressure measuring instrument, usually limited to measuring pressures near to atmospheric. The term manometer is often used to refer specifically to liquid column hydrostatic instruments.

A vacuum gauge is used to measure the pressure in a vacuum—which is further divided into two subcategories, high and low vacuum (and sometimes ultra-high vacuum). The applicable pressure range of many of the techniques used to measure vacuums have an overlap. Hence, by combining several different types of gauge, it is possible to measure system pressure continuously from 10 mbar down to 10−11 mbar.

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Contents

1 Absolute, gauge and differential pressures - zero reference 2 Units 3 Static and dynamic pressure

o 3.1 Applications 4 Instruments

o 4.1 Hydrostatic 4.1.1 Piston 4.1.2 Liquid column 4.1.3 McLeod gauge

o 4.2 Aneroid 4.2.1 Bourdon

4.2.1.1 Mechanical details 4.2.2 Diaphragm 4.2.3 Bellows

5 Electronic pressure sensors o 5.1 Thermal conductivity

5.1.1 Two-wire 5.1.2 Pirani (one wire)

o 5.2 Ionization gauge 5.2.1 Hot cathode 5.2.2 Cold cathode

6 Calibration 7 Dynamic transients 8 History 9 European (CEN) Standard 10 US (ASME) Standards 11 See also 12 References 13 External links

Absolute, gauge and differential pressures - zero reference

Everyday pressure measurements, such as for tire pressure, are usually made relative to ambient air pressure. In other cases measurements are made relative to a vacuum or to some other ad hoc reference. When distinguishing between these zero references, the following terms are used:

Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.

Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. To distinguish a negative pressure, the value may be appended with the word "vacuum" or the gauge may be labeled a "vacuum gauge."

Differential pressure is the difference in pressure between two points.

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The zero reference in use is usually implied by context, and these words are added only when clarification is needed. Tire pressure and blood pressure are gauge pressures by convention, while atmospheric pressures, deep vacuum pressures, and altimeter pressures must be absolute.

For most working fluids where a fluid exists in a closed system, gauge pressure measurement prevails. Pressure instruments connected to the system will indicate pressures relative to the current atmospheric pressure. The situation changes when extreme vacuum pressures are measured; absolute pressures are typically used instead.

Differential pressures are commonly used in industrial process systems. Differential pressure gauges have two inlet ports, each connected to one of the volumes whose pressure is to be monitored. In effect, such a gauge performs the mathematical operation of subtraction through mechanical means, obviating the need for an operator or control system to watch two separate gauges and determine the difference in readings.

Moderate vacuum pressure readings can be ambiguous without the proper context, as they may represent absolute pressure or gauge pressure without a negative sign. Thus a vacuum of 26 inHg gauge is equivalent to an absolute pressure of 30 inHg (typical atmospheric pressure) − 26 inHg = 4 inHg.

Atmospheric pressure is typically about 100 kPa at sea level, but is variable with altitude and weather. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes. For example, when a car drives up a mountain, the (gauge) tire pressure goes up because atmospheric pressure goes down. The absolute pressure in the tire is essentially unchanged.

Using atmospheric pressure as reference is usually signified by a g for gauge after the pressure unit, e.g. 70 psig, which means that the pressure measured is the total pressure minus atmospheric pressure. There are two types of gauge reference pressure: vented gauge (vg) and sealed gauge (sg).

A vented gauge pressure transmitter for example allows the outside air pressure to be exposed to the negative side of the pressure sensing diaphragm, via a vented cable or a hole on the side of the device, so that it always measures the pressure referred to ambient barometric pressure. Thus a vented gauge reference pressure sensor should always read zero pressure when the process pressure connection is held open to the air.

A sealed gauge reference is very similar except that atmospheric pressure is sealed on the negative side of the diaphragm. This is usually adopted on high pressure ranges such as hydraulics where atmospheric pressure changes will have a negligible effect on the accuracy of the reading, so venting is not necessary. This also allows some manufacturers to provide secondary pressure containment as an extra precaution for pressure equipment safety if the burst pressure of the primary pressure sensing diaphragm is exceeded.

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There is another way of creating a sealed gauge reference and this is to seal a high vacuum on the reverse side of the sensing diaphragm. Then the output signal is offset so the pressure sensor reads close to zero when measuring atmospheric pressure.

A sealed gauge reference pressure transducer will never read exactly zero because atmospheric pressure is always changing and the reference in this case is fixed at 1 bar.

An absolute pressure measurement is one that is referred to absolute vacuum. The best example of an absolute referenced pressure is atmospheric or barometric pressure.

To produce an absolute pressure sensor the manufacturer will seal a high vacuum behind the sensing diaphragm. If the process pressure connection of an absolute pressure transmitter is open to the air, it will read the actual barometric pressure.

Units

Pressure units

v t e

pascal bartechnical

atmospherestandard

atmospheretorr

pounds per square inch

Pa bar at atm Torr psi

1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 9.8692×10−6 7.5006×10−3 1.450377×10−4

1 bar 105 ≡ 106 dyn/cm2 1.0197 0.98692 750.06 14.50377

1 at 0.980665 ×105 0.980665 ≡ 1 kp/cm2 0.9678411 735.5592 14.22334

1 atm 1.01325 ×105 1.01325 1.0332 ≡ p0 ≡ 760 14.69595

1 Torr 133.3224 1.333224×10−3 1.359551×10−3 1.315789×10−3 ≈ 1 mmHg 1.933678×10−2

1 psi 6.8948×103 6.8948×10−2 7.03069×10−2 6.8046×10−2 51.71493 ≡ 1 lbF/in2

The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m−2 or kg·m−1·s−2). This special name for the unit was added in 1971; before that, pressure in SI was expressed in units such as N·m−2. When indicated, the zero reference is stated in parenthesis following the unit, for example 101 kPa (abs). The pound per square inch (psi) is still in widespread use in the US and Canada, for measuring, for instance, tire pressure. A letter is often appended to the psi unit to indicate the measurement's zero reference; psia for absolute, psig for gauge, psid for differential, although this practice is discouraged by the NIST.[1]

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Because pressure was once commonly measured by its ability to displace a column of liquid in a manometer, pressures are often expressed as a depth of a particular fluid (e.g., inches of water). Manometric measurement is the subject of pressure head calculations. The most common choices for a manometer's fluid are mercury (Hg) and water; water is nontoxic and readily available, while mercury's density allows for a shorter column (and so a smaller manometer) to measure a given pressure. The abbreviation "W.C." or the words "water column" are often printed on gauges and measurements that use water for the manometer.

Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely. So measurements in "millimetres of mercury" or "inches of mercury" can be converted to SI units as long as attention is paid to the local factors of fluid density and gravity. Temperature fluctuations change the value of fluid density, while location can affect gravity.

Although no longer preferred, these manometric units are still encountered in many fields. Blood pressure is measured in millimetres of mercury (see torr) in most of the world, and lung pressures in centimeters of water are still common, as in settings for CPAP machines. Natural gas pipeline pressures are measured in inches of water, expressed as "inches W.C." Scuba divers often use a manometric rule of thumb: the pressure exerted by ten meters depth of water is approximately equal to one atmosphere. In vacuum systems, the units torr, micrometre of mercury (micron), and inch of mercury (inHg) are most commonly used. Torr and micron usually indicates an absolute pressure, while inHg usually indicates a gauge pressure.

Atmospheric pressures are usually stated using kilopascal (kPa), or atmospheres (atm), except in American meteorology where the hectopascal (hPa) and millibar (mbar) are preferred. In American and Canadian engineering, stress is often measured in kip. Note that stress is not a true pressure since it is not scalar. In the cgs system the unit of pressure was the barye (ba), equal to 1 dyn·cm−2. In the mts system, the unit of pressure was the pieze, equal to 1 sthene per square metre.

Many other hybrid units are used such as mmHg/cm² or grams-force/cm² (sometimes as kg/cm² without properly identifying the force units). Using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as a unit of force is prohibited in SI; the unit of force in SI is the newton (N).

Static and dynamic pressure

Static pressure is uniform in all directions, so pressure measurements are independent of direction in an immovable (static) fluid. Flow, however, applies additional pressure on surfaces perpendicular to the flow direction, while having little impact on surfaces parallel to the flow direction. This directional component of pressure in a moving (dynamic) fluid is called dynamic pressure. An instrument facing the flow direction measures the sum of the static and dynamic pressures; this measurement is called the total pressure or stagnation pressure. Since dynamic pressure is referenced to static pressure, it is neither gauge nor absolute; it is a differential pressure.

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While static gauge pressure is of primary importance to determining net loads on pipe walls, dynamic pressure is used to measure flow rates and airspeed. Dynamic pressure can be measured by taking the differential pressure between instruments parallel and perpendicular to the flow. Pitot-static tubes, for example perform this measurement on airplanes to determine airspeed. The presence of the measuring instrument inevitably acts to divert flow and create turbulence, so its shape is critical to accuracy and the calibration curves are often non-linear.

Applications

Altimeter Barometer MAP sensor Pitot tube Sphygmomanometer

Instruments

Many instruments have been invented to measure pressure, with different advantages and disadvantages. Pressure range, sensitivity, dynamic response and cost all vary by several orders of magnitude from one instrument design to the next. The oldest type is the liquid column (a vertical tube filled with mercury) manometer invented by Evangelista Torricelli in 1643. The U-Tube was invented by Christian Huygens in 1661.

Hydrostatic

Hydrostatic gauges (such as the mercury column manometer) compare pressure to the hydrostatic force per unit area at the base of a column of fluid. Hydrostatic gauge measurements are independent of the type of gas being measured, and can be designed to have a very linear calibration. They have poor dynamic response.

Piston

Piston-type gauges counterbalance the pressure of a fluid with a spring (for example tire-pressure gauges of comparatively low accuracy) or a solid weight, in which case it is known as a deadweight tester and may be used for calibration of other gauges.

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Liquid column

The difference in fluid height in a liquid column manometer is proportional to the pressure difference.

Liquid column gauges consist of a vertical column of liquid in a tube that has ends which are exposed to different pressures. The column will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube. A very simple version is a U-shaped tube half-full of liquid, one side of which is connected to the region of interest while the reference pressure (which might be the atmospheric pressure or a vacuum) is applied to the other. The difference in liquid level represents the applied pressure. The pressure exerted by a column of fluid of height h and density ρ is given by the hydrostatic pressure equation, P = hgρ. Therefore the pressure difference between the applied pressure Pa and the reference pressure P0 in a U-tube manometer can be found by solving Pa − P0 = hgρ. In other words, the pressure on either end of the liquid (shown in blue in the figure to the right) must be balanced (since the liquid is static) and so Pa = P0 + hgρ. If the fluid being measured is significantly dense, hydrostatic corrections may have to be made for the height between the moving surface of the manometer working fluid and the location where the pressure measurement is desired except when measuring differential pressure of a fluid (for example across an orifice plate or venturi), in which case the density ρ should be corrected by subtracting the density of the fluid being measured.[2]

Although any fluid can be used, mercury is preferred for its high density (13.534 g/cm3) and low vapour pressure. For low pressure differences well above the vapour pressure of water, water is commonly used (and "inches of water" is a common pressure unit). Liquid-column pressure

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gauges are independent of the type of gas being measured and have a highly linear calibration. They have poor dynamic response. When measuring vacuum, the working liquid may evaporate and contaminate the vacuum if its vapor pressure is too high. When measuring liquid pressure, a loop filled with gas or a light fluid can isolate the liquids to prevent them from mixing but this can be unnecessary, for example when mercury is used as the manometer fluid to measure differential pressure of a fluid such as water. Simple hydrostatic gauges can measure pressures ranging from a few Torr (a few 100 Pa) to a few atmospheres. (Approximately 1,000,000 Pa)

A single-limb liquid-column manometer has a larger reservoir instead of one side of the U-tube and has a scale beside the narrower column. The column may be inclined to further amplify the liquid movement. Based on the use and structure following type of manometers are used[3]

1. Simple Manometer2. Micromanometer3. Differential manometer4. Inverted differential manometer

A McLeod gauge, drained of mercury

McLeod gauge

A McLeod gauge isolates a sample of gas and compresses it in a modified mercury manometer until the pressure is a few mmHg. The gas must be well-behaved during its compression (it must not condense, for example). The technique is slow and unsuited to continual monitoring, but is capable of good accuracy.

Useful range: above 10-4 torr [4] (roughly 10-2 Pa) as high as 10−6 Torr (0.1 mPa),

0.1 mPa is the lowest direct measurement of pressure that is possible with current technology. Other vacuum gauges can measure lower pressures, but only indirectly by measurement of other

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pressure-controlled properties. These indirect measurements must be calibrated to SI units via a direct measurement, most commonly a McLeod gauge.[5]

Aneroid

Aneroid gauges are based on a metallic pressure sensing element that flexes elastically under the effect of a pressure difference across the element. "Aneroid" means "without fluid," and the term originally distinguished these gauges from the hydrostatic gauges described above. However, aneroid gauges can be used to measure the pressure of a liquid as well as a gas, and they are not the only type of gauge that can operate without fluid. For this reason, they are often called mechanical gauges in modern language. Aneroid gauges are not dependent on the type of gas being measured, unlike thermal and ionization gauges, and are less likely to contaminate the system than hydrostatic gauges. The pressure sensing element may be a Bourdon tube, a diaphragm, a capsule, or a set of bellows, which will change shape in response to the pressure of the region in question. The deflection of the pressure sensing element may be read by a linkage connected to a needle, or it may be read by a secondary transducer. The most common secondary transducers in modern vacuum gauges measure a change in capacitance due to the mechanical deflection. Gauges that rely on a change in capacitances are often referred to as Baratron gauges.

Bourdon

Membrane-type manometer

The Bourdon pressure gauge uses the principle that a flattened tube tends to straighten or regain its circular form in cross-section when pressurized. Although this change in cross-section may be hardly noticeable, and thus involving moderate stresses within the elastic range of easily workable materials, the strain of the material of the tube is magnified by forming the tube into a C shape or even a helix, such that the entire tube tends to straighten out or uncoil, elastically, as it is pressurized. Eugene Bourdon patented his gauge in France in 1849, and it was widely adopted because of its superior sensitivity, linearity, and accuracy; Edward Ashcroft purchased Bourdon's American patent rights in 1852 and became a major manufacturer of gauges. Also in 1849, Bernard Schaeffer in Magdeburg, Germany patented a successful diaphragm (see below) pressure gauge, which, together with the Bourdon gauge, revolutionized pressure measurement

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in industry.[6] But in 1875 after Bourdon's patents expired, his company Schaeffer and Budenberg also manufactured Bourdon tube gauges.

In practice, a flattened thin-wall, closed-end tube is connected at the hollow end to a fixed pipe containing the fluid pressure to be measured. As the pressure increases, the closed end moves in an arc, and this motion is converted into the rotation of a (segment of a) gear by a connecting link that is usually adjustable. A small-diameter pinion gear is on the pointer shaft, so the motion is magnified further by the gear ratio. The positioning of the indicator card behind the pointer, the initial pointer shaft position, the linkage length and initial position, all provide means to calibrate the pointer to indicate the desired range of pressure for variations in the behaviour of the Bourdon tube itself. Differential pressure can be measured by gauges containing two different Bourdon tubes, with connecting linkages.

Bourdon tubes measure gauge pressure, relative to ambient atmospheric pressure, as opposed to absolute pressure; vacuum is sensed as a reverse motion. Some aneroid barometers use Bourdon tubes closed at both ends (but most use diaphragms or capsules, see below). When the measured pressure is rapidly pulsing, such as when the gauge is near a reciprocating pump, an orifice restriction in the connecting pipe is frequently used to avoid unnecessary wear on the gears and provide an average reading; when the whole gauge is subject to mechanical vibration, the entire case including the pointer and indicator card can be filled with an oil or glycerin. Tapping on the face of the gauge is not recommended as it will tend to falsify actual readings initially presented by the gauge.The Bourdon tube is separate from the face of the gauge and thus has no effect on the actual reading of pressure. Typical high-quality modern gauges provide an accuracy of ±2% of span, and a special high-precision gauge can be as accurate as 0.1% of full scale.[7]

In the following illustrations the transparent cover face of the pictured combination pressure and vacuum gauge has been removed and the mechanism removed from the case. This particular gauge is a combination vacuum and pressure gauge used for automotive diagnosis:

Indicator side with card and dial

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Mechanical side with Bourdon tube

the left side of the face, used for measuring manifold vacuum, is calibrated in centimetres of mercury on its inner scale and inches of mercury on its outer scale.

the right portion of the face is used to measure fuel pump pressure and is calibrated in fractions of 1 kgf/cm² on its inner scale and pounds per square inch on its outer scale.

Mechanical details

Mechanical details

Stationary parts:

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A: Receiver block. This joins the inlet pipe to the fixed end of the Bourdon tube (1) and secures the chassis plate (B). The two holes receive screws that secure the case.

B: Chassis plate. The face card is attached to this. It contains bearing holes for the axles. C: Secondary chassis plate. It supports the outer ends of the axles. D: Posts to join and space the two chassis plates.

Moving Parts:

1. Stationary end of Bourdon tube. This communicates with the inlet pipe through the receiver block.

2. Moving end of Bourdon tube. This end is sealed.3. Pivot and pivot pin.4. Link joining pivot pin to lever (5) with pins to allow joint rotation.5. Lever. This is an extension of the sector gear (7).6. Sector gear axle pin.7. Sector gear.8. Indicator needle axle. This has a spur gear that engages the sector gear (7) and extends through

the face to drive the indicator needle. Due to the short distance between the lever arm link boss and the pivot pin and the difference between the effective radius of the sector gear and that of the spur gear, any motion of the Bourdon tube is greatly amplified. A small motion of the tube results in a large motion of the indicator needle.

9. Hair spring to preload the gear train to eliminate gear lash and hysteresis.

Diaphragm

A pile of pressure capsules with corrugated diaphragms in an aneroid barograph.

A second type of aneroid gauge uses the deflection of a flexible membrane that separates regions of different pressure. The amount of deflection is repeatable for known pressures so the pressure can be determined by using calibration. The deformation of a thin diaphragm is dependent on the difference in pressure between its two faces. The reference face can be open to atmosphere to measure gauge pressure, open to a second port to measure differential pressure, or can be sealed against a vacuum or other fixed reference pressure to measure absolute pressure. The deformation can be measured using mechanical, optical or capacitive techniques. Ceramic and metallic diaphragms are used.

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Useful range: above 10-2 Torr [8] (roughly 1 Pa)

For absolute measurements, welded pressure capsules with diaphragms on either side are often used.

shape:

Flat corrugated flattened tube capsule

Bellows

In gauges intended to sense small pressures or pressure differences, or require that an absolute pressure be measured, the gear train and needle may be driven by an enclosed and sealed bellows chamber, called an aneroid, which means "without liquid". (Early barometers used a column of liquid such as water or the liquid metal mercury suspended by a vacuum.) This bellows configuration is used in aneroid barometers (barometers with an indicating needle and dial card), altimeters, altitude recording barographs, and the altitude telemetry instruments used in weather balloon radiosondes. These devices use the sealed chamber as a reference pressure and are driven by the external pressure. Other sensitive aircraft instruments such as air speed indicators and rate of climb indicators (variometers) have connections both to the internal part of the aneroid chamber and to an external enclosing chamber.

Electronic pressure sensors

Main article: Pressure sensor

Piezoresistive Strain Gage

Uses the piezoresistive effect of bonded or formed strain gauges to detect strain due to applied pressure.

Capacitive

Uses a diaphragm and pressure cavity to create a variable capacitor to detect strain due to applied pressure.

Magnetic

Measures the displacement of a diaphragm by means of changes in inductance (reluctance), LVDT, Hall Effect, or by eddy current principal.

Piezoelectric

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Uses the piezoelectric effect in certain materials such as quartz to measure the strain upon the sensing mechanism due to pressure.

Optical

Uses the physical change of an optical fiber to detect strain due applied pressure.

Potentiometric

Uses the motion of a wiper along a resistive mechanism to detect the strain caused by applied pressure.

Resonant

Uses the changes in resonant frequency in a sensing mechanism to measure stress, or changes in gas density, caused by applied pressure.

Thermal conductivity

Generally, as a real gas increases in density -which may indicate an increase in pressure- its ability to conduct heat increases. In this type of gauge, a wire filament is heated by running current through it. A thermocouple or Resistance Temperature Detector (RTD) can then be used to measure the temperature of the filament. This temperature is dependent on the rate at which the filament loses heat to the surrounding gas, and therefore on the thermal conductivity. A common variant is the Pirani gauge, which uses a single platinum filament as both the heated element and RTD. These gauges are accurate from 10 Torr to 10−3 Torr, but they are sensitive to the chemical composition of the gases being measured.

Two-wire

One wire coil is used as a heater, and the other is used to measure nearby temperature due to convection.

Pirani (one wire)

A Pirani gauge consist of a metal wire open to the pressure being measured. The wire is heated by a current flowing through it and cooled by the gas surrounding it. If the gas pressure is reduced, the cooling effect will decrease, hence the equilibrium temperature of the wire will increase. The resistance of the wire is a function of its temperature: by measuring the voltage across the wire and the current flowing through it, the resistance (and so the gas pressure) can be determined. This type of gauge was invented by Marcello Pirani.

Thermocouple gauges and thermistor gauges work in a similar manner, except a thermocouple or thermistor is used to measure the temperature of the wire.

Useful range: 10-3 - 10 Torr [9] (roughly 10-1 - 1000 Pa)

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Ionization gauge

Ionization gauges are the most sensitive gauges for very low pressures (also referred to as hard or high vacuum). They sense pressure indirectly by measuring the electrical ions produced when the gas is bombarded with electrons. Fewer ions will be produced by lower density gases. The calibration of an ion gauge is unstable and dependent on the nature of the gases being measured, which is not always known. They can be calibrated against a McLeod gauge which is much more stable and independent of gas chemistry.

Thermionic emission generate electrons, which collide with gas atoms and generate positive ions. The ions are attracted to a suitably biased electrode known as the collector. The current in the collector is proportional to the rate of ionization, which is a function of the pressure in the system. Hence, measuring the collector current gives the gas pressure. There are several sub-types of ionization gauge.

Useful range: 10-10 - 10-3 torr (roughly 10-8 - 10-1 Pa)

Most ion gauges come in two types: hot cathode and cold cathode. A third type that is more sensitive and expensive known as a spinning rotor gauge exists, but is not discussed here. In the hot cathode version, an electrically heated filament produces an electron beam. The electrons travel through the gauge and ionize gas molecules around them. The resulting ions are collected at a negative electrode. The current depends on the number of ions, which depends on the pressure in the gauge. Hot cathode gauges are accurate from 10−3 Torr to 10−10 Torr. The principle behind cold cathode version is the same, except that electrons are produced in the discharge of a high voltage. Cold Cathode gauges are accurate from 10−2 Torr to 10−9 Torr. Ionization gauge calibration is very sensitive to construction geometry, chemical composition of gases being measured, corrosion and surface deposits. Their calibration can be invalidated by activation at atmospheric pressure or low vacuum. The composition of gases at high vacuums will usually be unpredictable, so a mass spectrometer must be used in conjunction with the ionization gauge for accurate measurement.[10]

Hot cathode

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Bayard-Alpert hot-cathode ionization gauge

A hot-cathode ionization gauge is composed mainly of three electrodes acting together as a triode, wherein the cathode is the filament. The three electrodes are a collector or plate, a filament, and a grid. The collector current is measured in picoamps by an electrometer. The filament voltage to ground is usually at a potential of 30 volts, while the grid voltage at 180–210 volts DC, unless there is an optional electron bombardment feature, by heating the grid, which may have a high potential of approximately 565 volts. The most common ion gauge is the hot-cathode Bayard-Alpert gauge, with a small ion collector inside the grid. A glass envelope with an opening to the vacuum can surround the electrodes, but usually the Nude Gauge is inserted in the vacuum chamber directly, the pins being fed through a ceramic plate in the wall of the chamber. Hot-cathode gauges can be damaged or lose their calibration if they are exposed to atmospheric pressure or even low vacuum while hot. The measurements of a hot-cathode ionization gauge are always logarithmic.

Electrons emitted from the filament move several times in back and forth movements around the grid before finally entering the grid. During these movements, some electrons collide with a gaseous molecule to form a pair of an ion and an electron (Electron ionization). The number of these ions is proportional to the gaseous molecule density multiplied by the electron current emitted from the filament, and these ions pour into the collector to form an ion current. Since the gaseous molecule density is proportional to the pressure, the pressure is estimated by measuring the ion current.

The low-pressure sensitivity of hot-cathode gauges is limited by the photoelectric effect. Electrons hitting the grid produce x-rays that produce photoelectric noise in the ion collector. This limits the range of older hot-cathode gauges to 10−8 Torr and the Bayard-Alpert to about 10−10 Torr. Additional wires at cathode potential in the line of sight between the ion collector and the grid prevent this effect. In the extraction type the ions are not attracted by a wire, but by an open cone. As the ions cannot decide which part of the cone to hit, they pass through the hole and form an ion beam. This ion beam can be passed on to a:

Faraday cup Microchannel plate detector with Faraday cup Quadrupole mass analyzer with Faraday cup Quadrupole mass analyzer with Microchannel plate detector Faraday cup ion lens and acceleration voltage and directed at a target to form a sputter gun. In this case a

valve lets gas into the grid-cage.

See also: Electron ionization

Cold cathode

There are two subtypes of cold-cathode ionization gauges: the Penning gauge (invented by Frans Michel Penning), and the Inverted magnetron, also called a Redhead gauge. The major difference between the two is the position of the anode with respect to the cathode. Neither has a

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filament, and each may require a DC potential of about 4 kV for operation. Inverted magnetrons can measure down to 1x10−12 Torr.

Likewise, cold-cathode gauges may be reluctant to start at very low pressures, in that the near-absence of a gas makes it difficult to establish an electrode current - in particular in Penning gauges, which use an axially symmetric magnetic field to create path lengths for electrons that are of the order of metres. In ambient air, suitable ion-pairs are ubiquitously formed by cosmic radiation; in a Penning gauge, design features are used to ease the set-up of a discharge path. For example, the electrode of a Penning gauge is usually finely tapered to facilitate the field emission of electrons.

Maintenance cycles of cold cathode gauges are, in general, measured in years, depending on the gas type and pressure that they are operated in. Using a cold cathode gauge in gases with substantial organic components, such as pump oil fractions, can result in the growth of delicate carbon films and shards within the gauge that eventually either short-circuit the electrodes of the gauge or impede the generation of a discharge path.

Calibration

Pressure gauges are either direct- or indirect-reading. Hydrostatic and elastic gauges measure pressure are directly influenced by force exerted on the surface by incident particle flux, and are called direct reading gauges. Thermal and ionization gauges read pressure indirectly by measuring a gas property that changes in a predictable manner with gas density. Indirect measurements are susceptible to more errors than direct measurements.

Dead-weight tester McLeod mass spec + ionization

Dynamic transients

When fluid flows are not in equilibrium, local pressures may be higher or lower than the average pressure in a medium. These disturbances propagate from their source as longitudinal pressure variations along the path of propagation. This is also called sound. Sound pressure is the instantaneous local pressure deviation from the average pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water. The effective sound pressure is the root mean square of the instantaneous sound pressure over a given interval of time. Sound pressures are normally small and are often expressed in units of microbar.

frequency response of pressure sensors resonance

History

Further information: Timeline of temperature and pressure measurement technology

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European (CEN) Standard

EN 472 : Pressure gauge - Vocabulary. EN 837-1 : Pressure gauges. Bourdon tube pressure gauges. Dimensions, metrology,

requirements and testing. EN 837-2 : Pressure gauges. Selection and installation recommendations for pressure gauges. EN 837-3 : Pressure gauges. Diaphragm and capsule pressure gauges. Dimensions, metrology,

requirements, and testing.

US (ASME) Standards

B40.100-2005: Pressure gauges and Gauge attachments. PTC 19.2-2010 : Performance test code for pressure measurement.

Manometers A Manometer is a device to measure pressures. A common simple manometer consists of

a U shaped tube of glass filled with some liquid. Typically the liquid is mercury because of its high density.

Case 1

In the figure to the right we show such a U shaped tube filled with a liquid. Note that both ends of the tube are open to the atmosphere. Thus both points A and B are at atmospheric pressure. The two points also have the same vertical height.

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Case 2

Now the top of the tube on the left has been closed. We imagine that there is a sample of gas in the closed end of the tube.

The right side of the tube remains open to the atmosphere. The point A, then, is at atmospheric pressure.

The point C is at the pressure of the gas in the closed end of the tube.

The point B has a pressure greater than atmospheric pressure due to the weight of the column of liquid of height h.

The point C is at the same height as B, so it has the same pressure as B. And we have already seen that this is equal to the pressure of the gas in the closed end of the tube.

Thus, in this case the pressure of the gas that is trapped in the closed end of the tube is greater than atmospheric pressure by the amount of pressure exerted by the column of liquid of height h.

Case 3

Now we show another possible arrangement of the manometer with the top of the left side of the tube closed. Perhaps the closed end of the tube contains a sample of gas as before, or perhaps it contains a vacuum.

The point A is at atmospheric pressure.

The point C is at whatever pressure the gas in the closed end of the tube has, or if the closed end contains a vacuum the pressure is zero.

Since the point B is at the same height as point A, it must be at atmospheric pressure too. But the pressure at B is also the sum of the pressure at C plus the pressure exerted by the weight of the column of liquid of height h in the tube.

We conclude that pressure at C, then, is less than atmospheric pressure by the amount of pressure exerted by the column of liquid of height h.

If the closed end of the tube contains a vacuum, then the pressure

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at point C is zero, and atmospheric pressure is equal to the pressure exerted by the weight of the column of liquid of height h. In this case, the manometer can be used as a barometer to measure atmospheric pressure.

We conclude with a discussion of the units for pressure measurements. Recall that

pressure is defined as the force per area. The SI unit for pressure is the pascal, which is one newton per square meter.

For example, atmospheric pressure varies with the weather and is usually about 100 kilopascals. Another common unit for measuring atmospheric pressure is mm of mercury, whose value is usually about 760 mm. Put another way, if the closed end of the tube in Case 3 above contains a vacuum, the height h is about 760 mm.

In many situations, measuring pressures in units of length of the liquid in the manometer is perfectly adequate. The remainder of this document discusses how to convert from those units to pascals.

The figure to the right shows a cylinder of liquid of height h and area A.

The weight of the cylinder is its mass m times the acceleration due to gravity g. This is the force exerted by the cylinder of liquid on whatever is just below it:

F = m g

The pressure p is this force divided by the area A of the face of the cylinder.

p = F/A

The mass of the cylinder is the density of the liquid times the volume V.

m = V

The volume is the area A of the face of the cylinder times its height h.

V = A h

So, the pressure p is:

p  = F / A

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  = mg / A

 = V g / A

 = Ah g / A

 = hg

Thus, if , h and g are measured in SI units, the pressure p will be in pascals. Note that the value is independent of the area of the cylinder.

Pressure measuring devices using liquid columns in vertical or inclined tubes are called manometers. One of the most common is the water filled u-tube manometer used to measure pressure difference in pitot or orifices located in the airflow in air handling or ventilation system.

Vertical U-Tube Manometer

The pressure difference in a vertical U-Tube manometer can be expressed as p d = γ h      = ρ g h        (1) where p d = pressure γ = specific weight of the fluid in the tube (kN/m3 , lb/ft3 ) ρ = density (kg/m3 , lb/ft3 ) g = acceleration of gravity (9.81 m/s2 , 32.174 ft/s2 ) h = liquid height (m, ft)

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The specific weight of water, which is the most commonly used fluid in u-tube manometers, is 9.81 kN/m3 or 62.4 lb/ft3 .

Example - Differential Pressure Measurement in an Orifice

A water manometer connects the upstream and downstream of an orifice located in an air flow. The difference height of the water column is 10 mm.

The pressure difference head can then be expressed as: p d = 9.8 (kN/m3 ) 103  (N/kN) 10 (mm) 10-3  (m/mm)      = 98 N/m2 (Pa) where 9.8 (kN/m3 ) is the specific weight of water in SI-units.

Inclined U-Tube Manometer

Common problems when measuring pressure differences in low velocity systems as air ventilation system are the low column heights and satisfying accurately.

The pressure difference in a inclined u-tube can be expressed as p d = γ h sin(θ)         (2) where θ = angle of column relative the horizontal plane Inclining the tube manometer will increase the accuracy of the measurement.

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Example - Differential Pressure Measurement with an Inclined U-Tube manometer

We use the same data as in the example above, except that the U-Tube is inclined to 45o . The pressure difference head can then be expressed as: p d = 9.8 (kN/m3 ) 103  (N/kN) 10 (mm) 10-3  (m/mm) sin(45)      = 69.3 N/m2 (Pa)