Pressure Measurements

About pressure measurements

Pressure measurement is a very common requirement for most industrial process control systems and many different types of pressure-sensing and pressure measurement systems are available. Before considering these in detail, however, it is important to explain some terms used in pressure measurement and to define clearly the difference between absolute pressure, gauge pressure and differential pressure. The absolute pressure of a fluid defines the difference between the pressure of the fluid and the absolute zero of pressure, whereas gauge pressure describes the difference between the pressure of a fluid and atmospheric pressure. Absolute and gauge pressure are therefore related by the expression:
 

absolute pressure = gauge pressure + atmospheric pressure

The term differential pressure is used to describe the difference between two absolute pressure values, such as the pressures at two different points within the same fluid (often between the two sides of a flow restrictor in a system measuring volume flow rate).

The range of pressures for which measurement is commonly required is the span from 1.013 bar to 7000 bar (1 atmosphere to 6910 atmospheres). Apart from certain types of diaphragm-type instrument which use either a piezoelectric crystal or a capacitive transducer to measure the pressure-induced displacement, all the instruments listed below are restricted to measuring only static pressures. Instruments and techniques for measuring dynamic pressures have not been included because their measurement is a very specialized area which is not of general interest.

1. Measurement of Mid-Range Pressures

2. U-Tube Manometer

3. Dead-Weight Gauge

4. Well-Type Manometer (Cistern Manometer)

5. Diaphragm

6. Bellows

7. Bourdon Tube

8. Resonant-Wire Devices

9. Fiber Optic Pressure Sensors

10. Low Pressure Measurement

11. Thermocouple Gauge

12. Pirani Gauge

13. Intelligent Pressure Transducers

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