Download - Pressure Measurement
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4.3 Pressure Measurement
● Represented as a force per unit area● Discussion is restricted to fluid● Term used:
➢Absolute pressure: refers to the absolute value of the force per unit area exerted on the containing wall by the fluid.➢Gauge pressure: represent the difference between the absolute pressure and the local atmospheric pressure➢Vacuum: represents the amount by which the atmospheric exceeds the absolute pressure.• Pressure SI unit: Newton per square meter(N/m2) or
pascal (Pa)
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4.3.1 Types of Pressure Transducers
● There are many types of pressure transducers● Common used :
1) Mechanical Pressure-Measurement Devices (Manometer)2) Dead-Weight Tester3) Bourdon-Tube Pressure Gauge4) Diaphragm and Bellows Gauges
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1) Mechanical Pressure-Measurement Devices (Manometer)
● Offer simplest means for pressure measurement● The fluid manometer is a widely used device for measurement of fluid pressures under steady-state and laboratory conditions [refer figure 4.6]
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Figure 4.6 a) U-tube Manometer b) Well-type Manometer
(a) (b)
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2)Dead-Weight Tester
● A device used for balancing a fluid pressure with a known weight● Typically used for static calibration of pressure gauge and seldom employed for actual measurement. [refer figure 4.7]● Architecture and operation: - The apparatus set-up for calibration of the pressure gauge G. - The chamber and cylinderof the tester are filled with a clean oil by first moving the plunger to its most forward position and then slowly withdrawing it while the oil is poured in through the opening for the piston.
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Figure 4.7 Dead Weight Tester
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- The gauge to be tested is installed and the piston is inserted in the cylinder. - The pressure exerted on the fluid by the piston is now transmitted to the gauge when valve is opened. - This pressure may be varied by adding weights to the piston or by using different piston-cylinder combinations of varying areas. - The viscous friction between the piston and the cylinder in the axial direction may be substantially reduced by rotating the piston-weight assembly while measurement is taken
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3) Bourdon-Tube Pressure
● Used in many applications because of its consistency,● And inexpensive measurement of static pressure measurement.● Commercially available in many sizes (1-to-16 in diameter) and accuracies.● The heise gauge is an extremely accurate bourdon-tube gauge with an accuracy of 0.1% of full-scale reading and it is employ as a secondary pressure standard in laboratory work.
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● The construction of the bourdon-tube gauge is shown in figure 4.8● The bourdon-tube itself is usually an elliptical cross-sectional tube having “C” shape configuration● Operation: - When the pressure is applied to the inside of the tube an elastic deformation results which ideally is proportional to the pressure. - The degree of linearity depends on the quality of the gauge. - The end of gauge is connected to a spring-loaded linkage which amplifies the displacement and transform it to an angular rotation of the pointer.
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Figure 4.8 Bourdon Tube
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- The linkage is constructed so that the mechanism may be adjusted for optimum linearity and minimum hysteresis as well as compensate for wear which may develop over a period of time. - An electrical-resistance strain gauge may also be installed on the bourdon-tube to sense thr elastic deformation.
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4) Diaphragm and Bellows Gauges
● Represent similar types of elastic deformation devices useful for pressure measurement applications.● Architecture and operation: Diaphragm gauge: - Consider first the flat diaphragm subjected to the differential pressure p1-p2 as shown in figure 4.9. - The diaphragm will be deflected in accordance with this pressure differential and the deflection sensed an appropriate displacement transducer. - Various types of diaphragm gauge are shown figure 4.10
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Figure 4.9 (a) Diaphragm and (b) Bellows
(a) (b)
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Bellows Gauge: - The bellows gauge is shown in figure 4.9(b). - A differential gauge pressure force causes displacement of the bellows, which may be converted to an electrical signal or undergo a mechanical amplification to permit display of the output on an indicator dial. - Figure 4.10 shows various types of bellows gauges. ● The bellows gauge is generally unsuitable for transient measurements because of the larger relative motion and mass involved.● The diaphragm gauge which may be quite stiff, involves rather small displacements and is suit for high frequency pressure measurement.
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Figure 4.10 Various types of pressure gauges