##### From Maintenance of Instruments & Systems, 2nd Edition

## Pressure Transmitters

### Theory of Operation

Pressure is defined as force per unit area. If a weight of 100 pounds acts on an area of one square inch, the pressure would be stated as 100 pounds/1 square inch or 100 pounds per square inch (psi). However, this definition of psi by itself is meaningless because reference conditions are not defined.

Pressure can have one of two reference points: atmospheric pressure or absolute zero pressure. A pressure measurement using atmospheric pressure as the reference point is known as gage pressure (psig). The pressure measurement referencing absolute zero pressure is known as absolute pressure (psia).

Recall that atmospheric pressure at sea level is 14.7 psi. Gage pressure uses this atmospheric pressure as a reference and calls it 0 psig. Absolute pressure, as its name would suggest, uses absolute zero as a reference; therefore, ?14.7 psig equals 0 psia. Figure 6-1 shows the relationship between gage and absolute pressure measurements.

Figure 6-1: Absolute to Gage Conversions.

Of course, there are other units of measure besides pounds per square inch. U.S. process engineers still talk about pressure above atmospheric in terms of psi, but at lower pressures (below 10 psig) they frequently use inches of water (in. H _{2}O) instead. And (to add confusion), below atmospheric pressure, millimeters of mercury (mm Hg) are commonly used. European engineers generally use kilograms per square centimeter or millimeters of water for lower pressures and bars or millibars for the higher-pressure measurements. Figure 6-2 shows conversions between...

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