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Instrumentation Device Components

Semester 2nd

Chapter 5

Instrumentation Components, Indicators, Recorders and encoders

Prof Z D Mehta Prof U P Soni

Instrumentation and Control department Govt Polytechnic Ahmedabad

Prof. Z. D. Mehta , Prof. U.P.Soni, GOVT.POLYTECHNIC AHMEDABAD Page 2

Describe the working of various types of indicators:

• Indicators:-A device which monitors and displays the condition of a process variable without exerting any control action. Indicators may be equipped with alarms or other auxiliary outputs.

• Indicating Instruments:-

1. Analog instruments:- The analog instruments display a quantity in terms of deflection of a pointer (i.e. analog displacement) or by an angle corresponding to the quantity under measurements.

The examples of analog instruments are Electronic voltmeter, analog multi meter, CR0 etc.

2. Digital Instruments:- These are the instruments which indicate measurement in a decimal display.

The examples of digital Instruments are : Digital voltmeter, digital multi meter counter, logic probes etc.

• Advantages of analog instruments. 1. The analog instruments are simple in construction. 2. Where accuracy is not of much consideration, analog instruments are still used. 3. The analog instruments are cheaper. At present digital instruments are very costly but with advancement of technology they are being cheaper day by day. 4. The digital instruments are very complex in construction and their repair if not impossible is very costly.

• Advantages of digital instruments are:- 1. They display directly into decimal, so human errors are eliminated. 2. The reading may be taken to any no. of significant figures by shifting the decimal point, thus it enhances to the accuracy 3. The digital output of the digital instrument can be fed into computer, memories, or any data storage device 4. The power consumption of the digital instruments is negligible.

• Compare digital and analog instrument:-

Sr. no

Digital instruments Analog instruments

Prof. Z. D. Mehta , Prof. U.P.Soni, GOVT.POLYTECHNIC AHMEDABAD Page 3

1. These instruments display a quantity in decimal number.

These display a quantity in terms of deflection of a pointer on a calibrated scale

2. They have much greater accuracy.

They have comparatively poor accuracy.

3. Their resolution is very high.

Their resolution is comparatively poor.

4. They consume negligible power during measurement.

They consume larger power

5. They do not load the circuit. They load the circuit under measurement.

6. They are of complex construction. They are simple in construction. 7. They are affected greatly by

environment. They are little affected.

8. They are free from human errors.

They suffer from parallax and other human errors.

• Plotters:-

1. One who plots. 2. A computer output device that draws graphs or pictures, usually by moving a pen.

• X–Y plotter:-

An X–Y plotter is a plotter that operates in two axes of motion ("X" and "Y") in order to draw continuous vector graphics. The term was used to differentiate it from standard plotters which had control only of the "y" axis, the "x" axis being continuously fed to provide a plot of some variable with time. Plotters differ from Inkjet and Laser printers in that a plotter draws a continuous line, much like a pen on paper, while inkjet and laser printers use a very fine matrix of dots to form images, such that while a line may appear continuous to the naked eye, it in fact is a discrete set of points.

• Recorder:- The output device which gives output for permanent record is called a recorder

• Type of recorders:- 1. Analog recorders.

2.Digital recorders.

Prof. Z. D. Mehta , Prof. U.P.Soni, GOVT.POLYTECHNIC AHMEDABAD Page 4

• functions of recorders:-

The recorders perform the following functions: (i) Recording : They can record or store information. (ii) Replaying: They can relay or reproduce the stored information whenever desired. (iii) Erasing: They can erase or delete the unwanted information. Only for Knowledge:

• classification of recorders:-

Classify analog recorders. (i) Graphic recorders (ii) Magnetic tape recorders (iii) Oscilloscope recorders. (1) Graphic recorders:- The graphic recorders display and store a “pen and ink” record of some physical event. The basic elements of this recorder are (z) Chart for displaying and storing the recorded information, (u) Stylus moving w r t paper by suitable means of inter-connection to couple stylus to the source of information (ii) Magnetic tape recorders:- The recorders described till now are low frequency recorders The magnetic tape recorders are used at high frequencies it is sometimes desirable to record data m such a way that the same an be produced This is possible by magnetic tape recorders.

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Due to their used at higher frequencies, the magnetic type recorders are extensively used in instrumentation. (iii) Oscilloscope recorders. (1)strip chart (graphic) recorders. (2) -Y Recorders (1) Strip chart recorders. This recorder records one more variables w. r. t. times therefore it is also called X-t recorder the various systems of the recorder are described below 1 Paper drive system

This system consists of a long roll of graph paper (called chart) moving vertically driven by a synchronous motor with speed control mechanism. 2 Marketing mechanism The various marking mechanism are described below (1) Pen and Ink There are various types of pens like Bucket pen, V and Ball point pen Similarly, there are many types of capillary ink feeding systems Ball point pen in the most suitable for higher speeds, usually red ink is used though others colours are also used.

(2)X—Y recorders. A strip chart recorder records the variation of a quantity w. r. t. time while the X-Y recorder records the relationship between two variables X and Y. In X-Y recorders an emf is plotted versus other emf. This is done by having a self balancing potentiometer, which controls the movement of the graph paper while another self balancing potentiometer controls the position of the pen (or stylus) A X-Y recorder consists of a pair of servo system, driving a recording pen in two axes with reference to the stationary graph paper. Attenuations are used to bring the input signals acceptable to the recorder. The fig. 22 shows the block diagram of an X—Y recorder. A signal enters each of the two channels. The signals are attenuated to about 0.5 mV the range acceptable to the recorder. It is then passed to a balance circuit, where it is compared with a reference voltage. The difference between the signal and the reference voltage (i.e. “error signal”) is fed to .‘ chopper “that converts d. c. signal into a. c. signal. The output of the chopper is now amplified to a level that will actuate a servo motor, which drives the pen arm. The action described above takes place in both the axes simultaneously and we get a record of a variable X w.r.t. to another variable Y. An X-Y recorder has a speed of 1.5 m/s. The used chart size is 25 x 18 cm.

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The X — Y recorders are very useful such as for drawing characteristics of motor vacuum tubes, rectifier, transistor etc. and for plotting stress/strain curve, hysteresis curve etc.

The pointer moves on the paper and indicates the displacement under measurement. The instantaneous value of the displacement can be read from the calibrated scale if provided.

____________________________________________________________________

• Valve:- A valve is a device that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically valves fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure.

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The simplest, and very ancient, valve is simply a freely hinged flap which drops to obstruct fluid (gas or liquid) flow in one direction, but is pushed open by flow in the opposite direction. This is called a check valve, as it prevents or "checks" the flow in one direction.

Valves have many uses, including controlling water for Irrigation, industrial uses for controlling processes, residential uses such as on / off & pressure control to dish and clothes washers & taps in the home. Even aerosols have a tiny valve built in. Valves are also used in the military & transport sectors.

• Valve actuator:- Actuators are used for the automation of industrial valves and can be found in all kinds of technical process plants: they are used in waste water treatment plants, power plants and even refineries. This is where they play a major part in automating process control. The valves to be automated vary both in design and dimension. The diameters of the valves range from a few inches to a few meters.

Prof. Z. D. Mehta , Prof. U.P.Soni, GOVT.POLYTECHNIC AHMEDABAD Page 8

Valve actuator

• Valve positioner: - A positioner is a device put into a valve to ensure that it is at a correct position of opening as per the control signal. An I/P converter only sends the opening/closing request to valve but cannot confirm its position. Positioner senses the valve opening through a position feedback link connected to valve stem which is its input signal. I/P converter output is its set point input. The difference between these two is the error signal based on which the positioner positions the valve to correct position to reduce error to zero. Hence positioner is nothing but a pneumatic feedback controller. Controlled external supply air to positioner provides power to positioner to position a valve. Also positioner is used in a valve when valve operating signal range is different from I/P converter output range.

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• Filled system:- Filled-system temperature measurement methods depend upon three well-known physical phenomena:

• A liquid will expand or contract in proportion to its temperature and in accordance to the liquid’s coefficient of thermal/volumetric expansion.

• An enclosed liquid will create a definite vapor pressure in proportion to its temperature if the liquid only partially occupies the enclosed space.

The pressure of a gas is directly proportional to its temperature in accordance with the basic principle of the universal/perfect gas law: PV = nRT where P = absolute pressure, V = volume, T = absolute temperature, R = universal gas constant and n = number of gas particles (moles).

• Relay:- A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and most have double throw (changeover) switch contacts as shown in the diagram.

Ladder diagram symbol

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Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical.

• Relay circuit

The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification.

• Bourdon tube:-

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.

Prof. Z. D. Mehta , Prof. U.P.Soni, GOVT.POLYTECHNIC AHMEDABAD Page 11

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.

Prof. Z. D. Mehta , Prof. U.P.Soni, GOVT.POLYTECHNIC AHMEDABAD Page 12

Working:-

Diaphragm:-

Prof. Z. D. Mehta , Prof. U.P.Soni, GOVT.POLYTECHNIC AHMEDABAD Page 13

Prof. Z. D. Mehta , Prof. U.P.Soni, GOVT.POLYTECHNIC AHMEDABAD Page 14

Theory:-

Capsule:-

Fig.

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Swirl: - When the flowing medium passes a flow straightened all velocity components perpendicular to the straightened direction are revoked. The flow straightened is held in position by bearings at which the related energy can be detected directly and with high accuracy as reaction force. For this method no further requirements concerning the velocity distribution in the flow field are necessary to obtain exact results for the embedded angular momentum.

De swirl:-Pressure sensors capable of operating at higher temperatures have long been sought for testing of aircraft engines, since each improvement in sensor durability opens new areas of the engine to the straightforward measurement of rapidly varying pressures. A silicon carbide pressure sensor has been developed which can operate at 500°C, a temperature at which the mechanical properties of silicon, the most commonly used pressure sensor material, are severely degraded

Proving ring:- The proving ring is a device used to measure force. It consists of an elastic ring of known diameter with a measuring device located in the center of the ring. Proving rings come in a variety of sizes. They are made of a steel alloy. Manufacturing consists of rough machining from annealed forgings, heat treatment, and precision grinding to final size and finish.

Proving ring

The proving ring consists of two main elements, the ring itself and the diameter-measuring system, shown on the right in the exploded view of a proving ring. Forces are applied to the ring through the external bosses. The resulting change in diameter, referred to as the deflection of the ring, is measured with a micrometer screw and the vibrating reed mounted diametrically within the ring.

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Piston cylinder:- in mechanical engineering, sliding cylinder with a closed head (the piston) that is moved reciprocally in a slightly larger cylindrical chamber (the cylinder) by or against pressure of a fluid, as in an engine or pump. The cylinder of a steam engine is closed by plates at both ends, with provision for the piston rod, which is rigidly attached to the piston, to pass through one of the end cover plates by means of a gland and stuffing box (steam-tight joint). The cylinder of an internal-combustion engine is closed at one end by a plate called the head and open at the other end to permit free oscillation of the connecting rod, which joins the piston to the crankshaft. The cylinder head contains the spark plugs on spark-ignition (gasoline) engines and usually the fuel nozzle on compression-ignition (diesel) engines; on most engines the valves that control the admission of fresh air–fuel mixtures and the escape of burned fuel are also located in the head.

Piston cylinder diagram

Regulators:- In automatic control, a regulator is a device which has the function of maintaining a designated characteristic. It performs the activity of managing or maintaining a range of values in a machine. The measurable property of a device is managed closely by specified conditions or an advance set value; or it can be a variable according to a predetermined arrangement scheme. It can be used generally to connote any set of various controls or devices for regulating or controlling items or objects.

Examples are a voltage regulator (which can be a transformer whose voltage ratio of transformation can be adjusted, or an electronic circuit that produces a defined voltage), a pressure regulator, such as a diving regulator, which maintains its output at a fixed pressure lower than its input, and a fuel regulator (which controls the supply of fuel).

(1)Voltage regulator:- A voltage regulator is designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.

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(2) Pressure regulator:- A pressure regulator is a valve that automatically cuts off the flow of a liquid or gas at a certain pressure. Regulators are used to allow high-pressure fluid supply lines or tanks to be reduced to safe and/or usable pressures for various applications. Gas pressure regulators are used to regulate the gas pressure and are not appropriate for measuring flow rates.

Application:-

• Low current consumption • Overvoltage/ Short-circuit protection • Reverse polarity protection • Over temperature protection • Load protection

Lubricators:- Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each other, by interposing a substance called lubricant between the surfaces to carry or to help carry the load (pressure generated) between the opposing surfaces. The interposed lubricant film can be a solid, a solid/liquid dispersion, a liquid, a liquid-liquid dispersion or, exceptionally, a gas.

Application:- Mechanical devices to supply lubricants are called lubricators. A simple form of lubricator is a container mounted over a bearing or other part and provided with a hole or an adjustable valve through which the lubricant is gravity-fed at the desired rate of flow. Wick-feed oilers are placed under moving parts, and by pressing against them they feed oil by capillary action. Horizontal bearings are frequently oiled by a rotating ring or chain that carries oil from a reservoir in the bearing housing and distributes it along the bearing through grooves or channels. Bath oiling is useful where an oil-tight reservoir can be provided in which the bearing journal may be submerged; the pool of oil helps to carry away heat from contact surfaces. Splash-oiling devices are used where gears, bearings, or other parts contained in housings have moving parts that dip into the lubricant and splash it on the bearings or into distribution channels. Centralized oiling systems usually consist of a reservoir, pump, and tubes through which oil is circulated, while heaters or coolers may be introduced to change the viscosity of the lubricant for various parts of the system. Many oiling operations are automatically synchronized to start and stop with the machinery.

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Thermo wells:- thermo wells are among the simplest yet least well publicized accessories used in industrial temperature measurement applications. There are many variations of two basic kinds; low pressure and high pressure. They are used to provide an isolation between a temperature sensor and the environment, either liquid, gas or slurry. A thermo well allows the temperature sensor to be removed and replaced without compromising either the ambient region or the process. The price paid for such luxury is made up of:

1. Added purchase and installation costs, 2. Slower temporal response to temperature changes and, 3. Increased temperature measurement error, due mostly to stem heat loss down the length of the thermo well.

Applications:-

thermo well’s main purpose is to protect the temperature sensor to insure long life and accurate measurement. Maintenance is aided by allowing the sensor to be removed without having to drain the pipe or tank. Sensor changes can be done on the fly.

Floats:- a device that rides on the surface of the fluid or solid within the storage vessel. The float itself must be of substantially lesser density than the substance of interest, and it must not corrode or otherwise react with the substance.

Plum bobs:- A plumb-bob or a plummet is a weight, usually with a pointed tip on the bottom, that is suspended from a string and used as a vertical reference line, or plumb-line.

Plum bobs

Displacer:-

The displacer is a special-purpose piston, used in Beta and Gamma type Stifling engines, to move the working gas back and forth between the hot and cold heat exchangers. Depending on the type of engine design, the displacer may or may not be sealed to the cylinder, i.e. it may be a loose fit within the cylinder, allowing the working gas to pass around it as it moves to occupy the part of the cylinder beyond.

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Elbow taps:- A differential pressure exists when a flowing fluid changes direction due to a pipe turn or elbow, as shown in Figure 3 below. The pressure difference results from the centrifugal force. Since pipe elbows exist in plants, the cost for these meters is very low. However, the accuracy is very poor; there are only applied when reproducibility is sufficient and other flow measurements would be very costly.

Orifice plates: _

The orifice plate Flow Detector is the simplest of the flow-path restrictions used in flow detection, as well as the most economical. Orifice plates are flat plates 1/16 to 1/4 inch thick. They are normally mounted between a pair of flanges and are installed in a straight run of smooth pipe to avoid disturbance of flow patterns from fittings and valves.

Three kinds of orifice plates are used: concentric, eccentric, and segmental (as shown in Figure F1).

Figure F1

The concentric orifice plate is the most common of the three types. As shown, the orifice is equidistant (concentric) to the inside diameter of the pipe. Flow through a sharp-edged orifice plate is characterized by a change in velocity. As the fluid passes through the orifice, the fluid converges, and the velocity of the fluid increases to a maximum value. At this point, the pressure

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is at a minimum value. As the fluid diverges to fill the entire pipe area, the velocity decreases back to the original value. The pressure increases to about 60% to 80% of the original input value. The pressure loss is irrecoverable; therefore, the output pressure will always be less than the input pressure. The pressures on both sides of the orifice are measured, resulting in a differential pressure which is proportional to the flow rate.

Segmental and eccentric orifice plates are functionally identical to the concentric orifice. The circular section of the segmental orifice is concentric with the pipe. The segmental portion of the orifice eliminates damming of foreign materials on the upstream side of the orifice when mounted in a horizontal pipe. Depending on the type of fluid, the segmental section is placed on either the top or bottom of the horizontal pipe to increase the accuracy of the measurement.

Eccentric orifice plates shift the edge of the orifice to the inside of the pipe wall. This design also prevents upstream damming and is used in the same way as the segmental orifice plate.

Application:-

• High differential pressure generated • Exhaustive data available • Low purchase price and installation cost • Easy replacement • Low cost • Available in many material. • They can be used with differential pressure devices.

Venture tube:- Due to simplicity and dependability, the Venturi tube flow meter is often used in applications where it's necessary with higher Turn Down Rates, or lower pressure drops, than the orifice plate can provide.

In the Venturi Tube the fluid flow rate is measured by reducing the cross sectional flow area in the flow path, generating a pressure difference. After the constricted area, the fluid is passes through a pressure recovery exit section, where up to 80% of the differential pressure generated at the constricted area, is recovered.

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With proper instrumentation and flow calibrating, the Venturi Tube flow rate can be reduced to about 10% of its full scale range with proper accuracy. This provides a Turndown Rate 10:1

Applications:-

For applications where high permanent pressure loss is not tolerable, a venturi tube can be used. Because of its gradually curved inlet and outlet cones, almost no permanent pressure drop occurs.

Pitot tube:- The pitot tube are one the most used (and cheapest) ways to measure fluid flow, especially in air applications as ventilation and HVAC systems, even used in airplanes for the speed measurement.

Application:- 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 pitot tube measures the fluid flow velocity by converting the kinetic energy of the flow into potential energy.

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The use of the pitot tube is restricted to point measuring. With the "annubar", or multi-orifice pitot probe, the dynamic pressure can be measured across the velocity profile, and the annubar obtains an averaging effect.

Flow Nozzle: A flow nozzle consists of a restriction with an elliptical contour approach section that terminates in a cylindrical throat section. Pressure drop between the locations one pipe diameter upstream and one-half pipe diameter downstream is measured. Flow nozzles provide an intermediate pressure drop between orifice plates and venturi tubes; also, they are applicable to some slurry systems.

Applications:-

Flow nozzles are widely used for flow measurements at high velocities. They are more rugged and more resistant to erosion than the sharp-edged orifice plate. An example use of flow nozzles are the measurement of flow in the feed and bleed lines of the PHT system.

Flapper nozzle:- Most pneumatic instruments use a simple but highly sensitive mechanism for converting mechanical motion into variable air pressure: the baffle-and-nozzle assembly (sometimes referred to as a flapper- and-nozzle assembly). A baffle is nothing more than a flat object obstructing the flow of air out of a small nozzle by close proximity:

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Applications:- the nozzle flapper mechanism has widely been used as a transducer which converts displacement of the flapper into pneumatic pressure. In this study, the nozzle flapper is utilized for a pneumatic servo system which converts the displacement of the flapper into displacement of an object. From this displacement, for example, electrical signal will be obtainable by using a differential transformer. For the servo control system, this device may be applicable as a highly sensitive null detector. Back pressure of the nozzle flapper which is caused by movement of the flapper is supplied to a pneumatic cylinder, and its nozzle is attached to a piston of that cylinder. The piston and nozzle, therefore, move with the flapper.

Thermocouple:- When two wires composed of dissimilar metals arejoined at both ends and one of the ends is heated, there is a continuous current which flows in the thermoelectric circuit.

A thermocouple consists of two dissimilar conductors in contact, which produce a voltage when heated. The size of the voltage is dependent on the difference of temperature of the junction to other parts of the circuit. 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.

Use of thermocouple:-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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Color code of thermocouple:-

Types of thermo-couple

Color code

Alloy combination

J Black Iron-copper(constantan)

K Yellow Chromium-nickel T Blue Copper-constantan E Purple Nickel -copper N Orange Nicrosil-nisil R Green Rhodium(13%)-platinum S Green Rhodium(10%)-platinum U Green Copper-low nickel B White Rhodium(30%%)-

platinum(6%)_ G Red Tungsten-rhenium(26%) C Red Tungsten-rhenium(5%) D Red Tungsten-rhenium(5%)

• Proximity sensor:- A very common application for a sensor in the industrial sector, a proximity sensor simply detects when something comes into its proximity. Such a sensor, which gives a switched output on detecting something coming into proximity, is called a proximity switch.

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Proximity

• Types of Proximity Switches:- A critical element of sensing, automation & control, Proximity Switches are available in Inductive, Capacitive & Optical,

• Applications:-

(1)Inductive:-

Machine Tools/Robotics. Bulk Material Handling. For Counting Position Sensing.

(2)capacitive:-

Pharmaceuticals/Food Processing. Plastics. For level Sensing.

(3)optical:-

Robotics/Automobile/Foundry/Plastic Pharma/Food Processing/Engg. For Position/Level sensing/counting Automation Safety Instruments.

Limit switch:-

limit switch is a switch operated by the motion of a machine part or presence of an object. They are used for control of a machine, as safety interlocks, or to count objects passing a point. Standardized limit switches are industrial control components manufactured with a variety of operator types, including lever, roller plunger, and whisker type. Limit switches may be directly mechanically operated by the motion of the operating lever. A reed switch may be used to

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indicate proximity of a magnet mounted on some moving part. The class of proximity switches operates by the disturbance of an electromagnetic field, by capacitance, or by sensing a magnetic field

Safety switches:-

Limit switch is an electronic device specially developed for sensing the level of an approaching stockpile on stacker-reclaimed or material in a hopper, in a silo and to provide an output signal which can be used to initiate control actions. (1)level switch:- Liquid level switches are used to detect and limit the amount of liquid in a storage tank. Limit switches are also used to control the amount of liquid flowing through pipes to control fluid pressure inside the pipe. In continuous level measurement, the level of a medium in a tank or silo is detected and converted into an electronic signal. The level signal is either displayed directly on site or incorporated into a process control or management system. The measured media are liquids, pastes, powders or bulk solids. Yet another style of electronic level switch uses ultrasonic sound waves to detect the presence of process material (either solid or liquid) at one point:

(2)flow switch:- A Flow switch is normally a simple device that monitors flow and sends a trip signal to another device such as a pump to protect it. Some flow switch applications are 1. Pump Protection 2. Cooling circuit protection 3. High and low flow rate alarm and general flow monitoring. Flow switch types are used on air, steam and liquid vent-captor air flow-monitors are used to monitor air flow in a wide variety of industrial applications for air or gaseous medium e. g. air conditioning, ventilation, air filter monitoring, extraction fans, blowers, damper regulators and controlling air flow rates in energy conservation systems. The vent-captor is also ideal for monitoring air flow in thyristor cabinets, motor/generators and shipping containers.

“paddles” as the flow-sensing element, the motion of which actuates one or more switch contacts. The “normal” status of a switch is the condition of minimum stimulus. A flow switch will be in its “normal” status when it senses minimum flow (i.e. no fluid moving through the pipe). A flow switch is one detecting the flow of some fluid through a pipe. Flow switches often use

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(3) displacement switches:- Excellent stability for general use in oils and water. With large float displacement, switch withstands rough service; suitable for high viscosity liquids. Intermediate in size, LS-1800 switches provide long life and dependability to meet a broad range of requirements.

(4)pressure switches:-

Pressure switch is a form of switch that closes an electrical contact when a certain set pressure has been reached on its input. The switch may be designed to make contact either on pressure rise or on pressure fall. Another type of pressure switch detects mechanical force; for example, a pressure-sensitive mat is used to automatically open doors on commercial buildings.

A pressure switch is one detecting the presence of fluid pressure. Pressure switches often use Diaphragms or bellows as the pressure-sensing element, the motion of which actuates one or more switch contacts. The “normal” status of a switch is the condition of minimum stimulus. A pressure switch will be in its “normal” status when it senses minimum pressure (e.g. n applied pressure, or in some cases a vacuum condition).

Uses of pneumatic pressure switches include:

Switch a household well water pump automatically when water is drawn from the pressure tank.

Switching off an electrically driven gas compressor when a set pressure is achieved in the reservoir

Switching off a gas compressor, whenever there is no feed in the suction stage. in-cell charge control in a battery

Prof. Z. D. Mehta , Prof. U.P.Soni, GOVT.POLYTECHNIC AHMEDABAD Page 28

Switching on/off an alarm light in the cockpit of an aircraft if cabin pressure (based on altitude) is critically low

Hydraulic pressure switches have various uses in automobiles, for example:

To switch on a warning light if engine oil pressure falls below a safe level In dust control systems (bag filter), a pressure switch is mounted on the header which will

raise an alarm when air pressure in the header is less than necessary to gain or decline energy beyond the set value

To control automatic transmission torque converter lock-up.

(5)temperature switches:-

A temperature switch is a switch that is responsive to temperature changes. Temperature switches generally are provided with a temperature responsive element which will open or close a switch when a predetermined minimum pressure or temperature is sensed by the responsive element. For protection against thermal overload, semiconductor switches are provided with integrated temperature sensors. The temperature sensors acquire the temperature of the power switch and convert this into a temperature-dependent, analog signal which then can be interpreted in a circuit. Temperature sensitive switches, such as a thermostat, typically comprise a temperature sensor which is used to open or close electrical contacts at specified temperatures. A bimetal strip of dissimilar metals is used as the sensing element for temperature sensitive switches.

A temperature switch is one detecting the temperature of an object. Temperature switches often use bimetallic strips as the temperature-sensing element, the motion of which actuates one or more switch contacts. An alternative design uses a metal bulb filled with a fluid that expands with temperature, causing the switch mechanism to actuate based on the pressure this fluid exerts against a diaphragm or bellows. This latter temperature switch design is really a pressure switch, whose pressure is a direct function of process temperature by virtue of the physics of the entrapped fluid inside the sensing bulb. The “normal” status of a switch is the condition of minimum stimulus. A temperature switch will be in its “normal” status when it senses minimum temperature (i.e. cold in some cases a condition colder than ambient).

(6)speed:-

Prof. Z. D. Mehta , Prof. U.P.Soni, GOVT.POLYTECHNIC AHMEDABAD Page 29

• use of speed switches is: Indicate broken or slipping drive belts Detect over speed or under speed conditions of mechanical equipment Indicate mechanical failure Protect mechanical equipment Reduce down time