TITLE.PM5

BASIC CONCEPTS OF THERMODYNAMICS 39

dharm
M-therm/th2-1.pm5

(b) Diaphragm elements :

Fig. 2.21 shows basic diaphragm types.

Fig. 2.21. Basic diaphragm types.

Advantages :

1. Small in size and moderately priced.


2. Wide linear range.


3. Can withstand high over pressures and under pressures.


4. Small hysteresis.


5. Can be used for measurement of absolute and differential pressures as also vacuum.
   Disadvantages :


6. Need protection against shocks and vibrations.


7. Cannot be used to measure high pressures.


8. Difficult to repair.
   (c) Bellow gauges/elements :
   Fig. 2.22 shows some bellow gauges.

Pointer Scale

p (Pressure)

Bellows

To pointer

Bellows

Spring

(a) Bellows pressure gauge (b) Spring loaded bellows gauge

Fig. 2.22. Bellow gauges.

Advantages :

1. Simple and rugged construction.


2. Useful for measurement of low and medium pressures.


3. Moderate cost.


4. Can be used for measurement of absolute, gauge and differential pressures.
   Disadvantages :


5. Need spring for accurate characterisation.


6. Greater hysteresis and zero drift problems.


7. Unsuitable for transient measurements due to longer relative motion and mass.


8. Requires compensation for ambient temperature changes.