Pressure Measurement
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Pressure Measurement

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Pressure Measurement Pressure Measurement Presentation Transcript

  • 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/m 2 ) 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 Tester
    • 3) Bourdon-Tube Pressure Gauge
    • 4) 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