Figure 1. Relationship of pressure terms to one another

Question:
What is the difference between psia and psig?

Answer:
In the English system of units, measurements relating the pressure of a system to a reference pressure are accomplished by specifying the pressure in terms of pounds per square inch absolute (psia) or gauge (psig).

Pressure measurement is divided into the following three (3) different categories:

Absolute pressure – the absolute value of the force per unit area exerted on a surface (typically by a fluid). A “perfect” vacuum, for example, is zero.

Gauge pressure – the measurement of the difference between the absolute pressure and local atmospheric pressure (which can vary due to such factors as temperature, altitude and relative humidity). Negative signs are usually omitted.

Differential pressure – the difference in pressure between two points.

Examples of absolute pressure include atmospheric pressure and vacuum level while gauge pressure includes such items as tire pressure and blood pressure.

Most of us in heat-treat shops are familiar with pressure gauges. They are found in two basic types:

A vented gauge, for example, allows the outside air pressure to be exposed to the negative side of the pressure-sensing diaphragm, via a vented cable or a hole on the side of the device, so that it always measures the pressure referred to barometric pressure at ambient temperature.

A sealed gauge, although similar, has atmospheric pressure sealed on the negative side of the diaphragm. Examples are hydraulic gauges where atmospheric-pressure changes will have a negligible effect on the accuracy of the reading, so venting is not necessary. This also allows some manufacturers to provide secondary pressure containment as an extra safety precaution.