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Pressure Measurement

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Pressure Measurement

  1. 1. 4.3 Pressure Measurement <ul><li>Represented as a force per unit area </li></ul><ul><li>Discussion is restricted to fluid </li></ul><ul><li>Term used: </li></ul><ul><ul><li>Absolute pressure: refers to the absolute value of the force </li></ul></ul><ul><ul><li>per unit area exerted on the containing wall by the fluid. </li></ul></ul><ul><ul><li>Gauge pressure: represent the difference between the </li></ul></ul><ul><ul><li>absolute pressure and the local atmospheric pressure </li></ul></ul><ul><ul><li>Vacuum: represents the amount by which the atmospheric </li></ul></ul><ul><ul><li>exceeds the absolute pressure. </li></ul></ul><ul><ul><li>Pressure SI unit: Newton per square meter(N/m 2 ) or </li></ul></ul><ul><ul><li> pascal (Pa) </li></ul></ul>
  2. 2. 4.3.1 Types of Pressure Transducers <ul><li>There are many types of pressure transducers </li></ul><ul><li>Common used : </li></ul><ul><li>1) Mechanical Pressure-Measurement Devices (Manometer) </li></ul><ul><li>2) Dead-Weight Tester </li></ul><ul><li>3) Bourdon-Tube Pressure Gauge </li></ul><ul><li>4) Diaphragm and Bellows Gauges </li></ul>
  3. 3. 1) Mechanical Pressure-Measurement Devices (Manometer) <ul><li>Offer simplest means for pressure measurement </li></ul><ul><li>The fluid manometer is a widely used device for </li></ul><ul><li>measurement of fluid pressures under steady-state </li></ul><ul><li>and laboratory conditions [refer figure 4.6] </li></ul>
  4. 4. Figure 4.6 a) U-tube Manometer b) Well-type Manometer (a) (b)
  5. 5. 2)Dead-Weight Tester <ul><li>A device used for balancing a fluid pressure with </li></ul><ul><li>a known weight </li></ul><ul><li>Typically used for static calibration of pressure gauge </li></ul><ul><li>and seldom employed for actual measurement. </li></ul><ul><li>[refer figure 4.7] </li></ul><ul><li>Architecture and operation: </li></ul><ul><li>- The apparatus set-up for calibration of the pressure </li></ul><ul><li>gauge G. </li></ul><ul><li>- The chamber and cylinderof the tester are filled with </li></ul><ul><li>a clean oil by first moving the plunger to its most </li></ul><ul><li>forward position and then slowly withdrawing it while </li></ul><ul><li>the oil is poured in through the opening for the piston. </li></ul>
  6. 6. Figure 4.7 Dead Weight Tester
  7. 7. - 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
  8. 8. 3) Bourdon-Tube Pressure <ul><li>Used in many applications because of its consistency, </li></ul><ul><li>And inexpensive measurement of static pressure </li></ul><ul><li>measurement. </li></ul><ul><li>Commercially available in many sizes (1-to-16 in </li></ul><ul><li>diameter) and accuracies. </li></ul><ul><li>The heise gauge is an extremely accurate bourdon-tube </li></ul><ul><li>gauge with an accuracy of 0.1% of full-scale reading </li></ul><ul><li>and it is employ as a secondary pressure standard in </li></ul><ul><li>laboratory work. </li></ul>
  9. 9. <ul><li>The construction of the bourdon-tube gauge is shown </li></ul><ul><li>in figure 4.8 </li></ul><ul><li>The bourdon-tube itself is usually an elliptical </li></ul><ul><li>cross-sectional tube having “C” shape configuration </li></ul><ul><li>Operation: </li></ul><ul><li>- When the pressure is applied to the inside of the tube </li></ul><ul><li>an elastic deformation results which ideally is </li></ul><ul><li>proportional to the pressure. </li></ul><ul><li>- The degree of linearity depends on the quality of the </li></ul><ul><li>gauge. </li></ul><ul><li>- The end of gauge is connected to a spring-loaded </li></ul><ul><li>linkage which amplifies the displacement and transform </li></ul><ul><li>it to an angular rotation of the pointer. </li></ul>
  10. 10. Figure 4.8 Bourdon Tube
  11. 11. - 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.
  12. 12. 4) Diaphragm and Bellows Gauges <ul><li>Represent similar types of elastic deformation </li></ul><ul><li>devices useful for pressure measurement applications. </li></ul><ul><li>Architecture and operation: </li></ul><ul><li>Diaphragm gauge: </li></ul><ul><li>- Consider first the flat diaphragm subjected </li></ul><ul><li>to the differential pressure p1-p2 as shown in </li></ul><ul><li>figure 4.9. </li></ul><ul><li>- The diaphragm will be deflected in accordance with </li></ul><ul><li>this pressure differential and the deflection sensed </li></ul><ul><li>an appropriate displacement transducer. </li></ul><ul><li>- Various types of diaphragm gauge are shown figure 4.10 </li></ul>
  13. 13. Figure 4.9 (a) Diaphragm and (b) Bellows (a) (b)
  14. 14. <ul><li>Bellows Gauge: </li></ul><ul><li>- The bellows gauge is shown in figure 4.9(b). </li></ul><ul><li>- A differential gauge pressure force causes displacement </li></ul><ul><li>of the bellows, which may be converted to an electrical </li></ul><ul><li>signal or undergo a mechanical amplification to </li></ul><ul><li>permit display of the output on an indicator dial. </li></ul><ul><li>- Figure 4.10 shows various types of bellows gauges. </li></ul><ul><li>The bellows gauge is generally unsuitable for transient </li></ul><ul><li>measurements because of the larger relative motion and </li></ul><ul><li>mass involved. </li></ul><ul><li>The diaphragm gauge which may be quite stiff, involves </li></ul><ul><li>rather small displacements and is suit for high frequency </li></ul><ul><li>pressure measurement. </li></ul>
  15. 15. Figure 4.10 Various types of pressure gauges