Pressure Measurement

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)

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

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]

Figure 4.6 a) U-tube Manometer b) Well-type Manometer

(a) (b)

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.

Figure 4.7 Dead Weight Tester

- 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

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.

● 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.

Figure 4.8 Bourdon Tube

- 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.

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

Figure 4.9 (a) Diaphragm and (b) Bellows

(a) (b)

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.

Figure 4.10 Various types of pressure gauges