TITLE.PM5

38 ENGINEERING THERMODYNAMICS

dharm
M-therm/th2-1.pm5

5. Their sensitivity can be changed by changing manometric fluids.


6. Particularly suitable to low pressures and low differential pressures.
   Disadvantages. The disadvantages of manometers are as follows :


7. Unsuitable for recording.


8. Generally large, bulky and fragile.


9. Their calibration is affected by changes in gravitational force and density of fluids and
   their calibration changes with altitude and temperature.


10. Surface tension of manometric fluid creates a capillary effect and possible hysteresis.


11. A particular manometer can be used only for measurement of a particular fluid/fluids.


12. Meniscus height has to be determined by accurate means to ensure improved accuracy.
    Elastic pressure elements
    Elastic pressure elements or mechanical type of transducers are used for measurement of
    very high pressures upto about 700 MN/m^2. There are three main types of pressure elements.
    (a) Bourdon tube
    (b) Diaphragms
    (c) Bellows.
    The action of these mechanical transducers depends upon the displacement caused by the
    pressure. The displacement produced may actuate a pointer whose deflection may be direct meas-
    ure of the pressure applied or the displacement is measured with the help of a secondary trans-
    ducer which is electrical in nature. The output of the secondary transducer which is electrical in
    format is a measure of the pressure applied.
    (a) Bourdon tubes/elements :
    Fig. 2.20 shows Bourdon tube configurations.
    Advantages. The Bourdon tube element has the following advantages :


13. Simple in construction and cheap.


14. Available in several different ranges.


15. Capability to measure gauge, absolute and differential pressures.


16. The sensitivities of Bourdon tube may be changed by changes in their dimensions.


17. Excellent sensitivity.


18. Simple and straight forward calibration with dead weight tester.


19. Easily adapted to strain, capacitance, magnetic and other electrical transducers.
    Disadvantages :


20. Susceptibility to shock and vibration.


21. Inherent hysteresis and slow response to pressure changes.


22. Unsuitable for low pressure applications.

Axis

Small

angular

displacement

Applied

pressure, p

p

Fixed

pivot

point

p

Scale

()a ()b ()c

Scale

Fig. 2.20. (a) Twisted tube, (b) Helical, (c) Spiral.