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Pressure Measurement

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