PRESSURE MEASUREMENTINC 331

TABLE OF CONTENTS

A. INTRODUCTION

B. MANOMETER

C. ELASTIC-ELEMENT MECHANICAL PRESSURE GAGES

D. ELECTRONIC PRESSURE SENSORS

E. DIFFERENTIAL PRESSURE SENSORS

F. HIGH PRESSURE MEASUREMENT

G. LOW-PRESSURE (VACUUM) MEASUREMENT

INTRODUCTION

PRESSUREPressure is defined as force per unit area but force = mass xacceleration (Newton’s second law of motion) and acceleration is rate of change of velocity. Thus if pressure is force/area, it equates to (mass x rate of change of velocity)/area. This gives pressure the dimensions of mass x length/(time2 length2) which simplifies to mass/(length x time2) or M.L-1.T-2. Thus, from the definition it can be shown that pressure is derived from three base quantities; mass, length and time.The relationship between pressure (p), force (F) and area (A) is given by:

INTRODUCTION

If a vessel were to contain no molecules whatsoever, the pressure would be zero. Pressures measured on the scale which uses this zero value as its reference point are said to be absolute pressures. Atmospheric pressure at the surface of the earth varies but is approximately 105 Pa (1 000 mbar); this is 105 Pa absolute pressure because it is expressed with respect to zero pressure that is no molecules at all.

Atmospheric pressure is the force exerted on a surface of unit area caused bythe earth’s gravitational attraction of the air vertically above that area. It istransmitted equally in all directions within the air and may be measured by avariety of techniques. The density of the air above the surface of the earth isrelated to changes in temperature and global weather patterns, causingvariations in the downward force and hence pressure. We are all familiar witheffect of changes in atmospheric pressure: high pressure systems are linked toclear skies, low pressure areas to rain and storms.Atmospheric pressure decreases with increasing altitude. At the top of amountain, the remaining column of air above us is smaller and the accelerationdue to gravity is less.

Atmospheric pressure

Gage PressureSuch a gauge is designed to measure pressure values expressed with respect to atmospheric pressure and thus indicates zero when its measurement port ‘merely’ contains molecules at atmospheric pressure. These measurements are commonly known as gauge-mode pressure measurements. Thus the difference between an absolute pressure value and a gauge pressure value is the variable value of atmospheric pressure:

absolute pressure = gauge pressure + atmospheric pressure

Vacuum Pressure

Vacuum definition is not precise but it is commonly taken to mean pressures below, and often considerably below, atmospheric pressure. It does not have separate units and we do not say that “vacuum equals force per unit area”.

INTRODUCTION :Standards and Calibration

Deadweight testing

Pressure is defined as force per unit area. Dead-Weight Pressure Testers use these measurements of force and area to produce a pressure to calibrate instrument with great accuracy. The force is derived from weights and the area is that of a piston in a cylinder

Pressure Calibrator

Secondary pressure testing

Liquid column instruments• Barometer• A mercury barometer has a glass tube 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 known as Torricellian vacuum. 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.

Evangelista Torricelli; (1608–1647) was an Italian physicist and mathematician, best known for his invention of the barometer,

Manometer

• A manometer is a device similar to a barometer that can be used to measure the pressure of a gas trapped in a container. A closed-end manometer is a U-shaped tube with one closed arm, one arm that connects to the gas to be measured, and a nonvolatile liquid (usually mercury) in between. As with a barometer, the distance between the liquid levels in the two arms of the tube (h in the diagram) is proportional to the pressure of the gas in the container. An open-end manometer (Figure) is the same as a closed-end manometer, but one of its arms is open to the atmosphere. In this case, the distance between the liquid levels corresponds to the difference in pressure between the gas in the container and the atmosphere

MANOMETERS

MANOMETERS

ELASTIC TRANSDUCERS

ELASTIC TRANSDUCERS : Bourdon tubes

ELASTIC TRANSDUCERS : Bellows pressure gage

ELASTIC TRANSDUCERS : Diaphragm pressure gage

Diaphragm pressure gage

A diaphragm is a sheet of a semi-flexible material anchored at its periphery and most often round in shape. It serves either as a barrier between two chambers, moving slightly up into one chamber or down into the other depending on differences in pressure, or as a device that vibrates when certain frequencies are applied to it

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Strain-Gauge

What is Strain?• Strain is the amount of deformation of a body due to an

applied force. More specifically, strain (ε) is defined as the fractional change in length, as shown in Figure below.

Definition of Strain

Strain can be positive (tensile) or negative (compressive).

Shearing Strain• Shearing strain,γ, is defined as the angular changein radians between two line segments that were orthogonal in the undeformed state.

If we had a thick book sitting on a table top and we applied a force parallel to the covers, we could see the shear strain by observing the edges of the pages.

Stress and Strain• Stress is the force an object

generates inside by responding to an applied external force, P. If an object receives an external force from the top, it internally generates a repelling force to maintain the original shape. The repelling force is called internal force and the internal force divided by the cross-sectional area of the object is called stress, which is expressed as a unit of Pa (Pascal) or N/m2.

Bonded metallic strain gauge.

Strain gauge is a device whose electrical resistance varies in proportion to the amount of strain in the device. For example, the piezoresistive strain gauge is a semiconductor device whose resistance varies nonlinearly with strain. The most widely used gauge is the bonded metallic strain gauge.

Gage Factor

The gage factor, K, differs depending on the metallic materials. The copper-nickel alloy (Advance) provides a gage factor around 2. Thus, a strain gage using this alloy for the sensing element enables conversion of mechanical strain to a corresponding electrical resistance change.

Major types of resistance strain gages• Bonded foil strain gages•The first bonded, metallic wire-type strain gage was developed in 1938. The metallic foil-type strain gage consists of a grid of wire filament (a resistor) of approximately 0.001 in. (0.025 mm) thickness, bonded directly to the strained surface by a thin layer of epoxy resin. When a load is applied to the surface, the resulting change in surface length is communicated to the resistor and the corresponding strain is measured in terms of the electrical resistance of the foil wire, which varies linearly with strain.

ELECTRONIC PRESSURE SENSORS : Strain gauge

Bonded foil strain gages

INC 336 Industrial process measurement, 2005

ELECTRONIC PRESSURE SENSORS : Strain gauge

Bonded foil strain-gage pressure transducer

ELECTRONIC PRESSURE SENSORS : Strain gauge

Strain gauge transducer with diaphragm element

Wheatstone circuit

ELECTRONIC PRESSURE SENSORS : Strain gauge

Diaphragm-type strain-gage pressure pickup

Boned semiconductor strain gagesA further improvement is the thin-film strain gage that eliminates the need for adhesive bonding. The gage is produced by first depositing an electrical insulation (typically a ceramic) onto the stressed metal surface, and then depositing the strain gage onto this insulation layer. Vacuum deposition or sputtering techniques are used to bond the materials molecularly.

ELECTRONIC PRESSURE SENSORS : Diffused sensor

Diffused sensor transducers and auto reference techniques

Thin film strain gages

In the case of strain gauges, thin film sensor deposition directly onto the stressed substrate is an option to foil strain gauges epoxied to the substrate, resistive strain gauges and silicon strain gauges that eliminates many of the problems of these later technologies.

ELECTRONIC PRESSURE SENSORS : Strain gauge

• Sensors to be deposited directly onto conductive and non-conductive components. No mounting cements or adhesives needed.

• Minimal process intrusion. Structural modifications of components typically not necessary. Will not disrupt gas flows.

• Fast time response. Adds neglible amount of mass to system and made with thermally conductive materials.

• Harsh environment capability. Can be made to withstand temperatures in excess of 1000oC.

• Sensors to be placed on 3 dimensional parts. Thin film conductive traces can route leads around corners and edges.

Thin-film strain gages

ELECTRONIC PRESSURE SENSORS : Capacitive

ELECTRONIC PRESSURE SENSORS : Potentiometric

ELECTRONIC PRESSURE SENSORS : Resonant wire

A wire is gripped by a static member at one end, and by the sensing diaphragm at the other. An oscillator circuit causes the wire to oscillate at its resonant frequency. A change in process pressure changes the wire tension, which in turn changes the resonant frequency of the wire.

A digital counter circuit detects the shift. Because this change in frequency can be detected quite precisely, this type of transducer can be used for low differential pressure applications as well as to detect absolute and gauge pressures.

ELECTRONIC PRESSURE SENSORS : Piezoelectric

Piezoelectric crystal circuit Piezoelectric Sensor

Quartz crystal Oscillator crystal

ELECTRONIC PRESSURE SENSORS : Piezoelectric

Linear variable differential transformer

Linear reluctance transducer

Inductance is that property of an electric circuit that expresses the amount of electromotive force (emf) induced by a given rate of change of current flow in the circuit. Reluctance is resistance to magnetic flow, the opposition offered by a magnetic substance to magnetic flux. In these sensors, a change in pressure produces a movement, which in turn changes the inductance or reluctance of an electric circuit.

ELECTRONIC PRESSURE SENSORS : Electro-Optical

Optical pressure transducers detect the effects of minute motions due to changes in process pressure and generate a corresponding electronic output signal . A light emitting diode (LED) is used as the light source, and a vane blocks some of the light as it is moved by the diaphragm.

DIFERENTIAL PRESSURE TRANSDUCERS

DIFERENTIAL PRESSURE TRANSDUCERS

DIFERENTIAL PRESSURE TRANSDUCERS

Diffused strain-gage differential-pressureTransmitter (Honeywell Inc.)

HIGH PRESSURE MEASUREMENT

Very-high-pressure transducer

LOW PRESSURE MEASUREMENT

Vacuum gage types and ranges

LOW PRESSURE MEASUREMENT

McLeod gauge

The McLeod Gage is considered the standard for low-pressure (vacuum) measurements, where the pressure is below 10-4 torr* (10-4 mmHg, 1.33×10-2 Pa, 1.93×10-6 psi). A McLeod Gage compresses a sample of low pressure gas to a sufficiently high pressure, obtains the compressed pressure from a standard manometer, and then calculates the original low pressure through Boyle's law. The compression is passed through a dense, nearly-incompressible, low vapor pressure fluid, such as mercury. *1 Torr; ≈ 1 mmHg

LOW PRESSURE MEASUREMENT

McLeod gage

A McLeod gauge is a scientific instrument to measure very low pressures, down to 10-7 Torr.The torr (symbol: Torr), defined as 1/760 atm.

1 Torr; ≈ 1 mmHg

Today, these gauges have largely been replaced by electronic vacuum gauges. McLeod gauges continue to be used as a calibration standard for electronic gauges

LOW PRESSURE MEASUREMENT

Pirani gageThermocouple gage

Thermal Conductivity gauge rely on the fact that the ability of a gas to conduct heat decreaseswith pressure*. 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.

*If the gas pressure is reduced, the cooling effect will decrease

The simplest form of an ionization gauge. The grid is a loosely wound spiral of wire surrounding the filament, and exerts little control on the electron stream. With a constant high current of electrons to the anode, positive ions from the remaining gas are attracted to the grid and the resulting grid current is measured and taken as proportional to gas pressure.

Ionization gauge

DYNAMIC EFFECTS OF VOLUMES AND TUBING

Transducer installation types

DYNAMIC EFFECTS OF VOLUMES AND TUBING

Fill Fluid : The fill fluid will affect the safety of process, accuracy and response time of the measurement. The following four points need to be considered:

• Temperature limitations• Expansion coefficient• Process compatibilty• Viscoscity

Temperature Effects : Three variables can be changed to reduce the errors caused by temperature effects

• Diameter of capillary• Choice of fill fluid• Diaphragm size

ACCESSORIES

Gauge cock and gauge valves are applied to the temporary stoppage of measuring fluid for the purposes of pressure gauge maintenance,inspection and repair

These joints are applied to the connection between diipe diameters.

This dampener is of variable type being able to adjust pressure amplitude and applied to the decrease in pressure pulsation.

Gauge cock Gauge valve

Connecting joint

Dampener

ACCESSORIES

This pipe siphon is applied to measuring steam pressure or the case where the temperature of measuring fluid is high

This tank siphon is applied to the sulstitution measuring fluid and the case where oil is forbidden.

SMART PRESSURE TRANSMITTERS

SUMMARY