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## What are absolute, gauge and differential pressures 'modes'? (FAQ - Pressure)

To avoid ambiguity, when referring to a pressure value it is important to specify its mode. What are pressure modes?

If a vessel were to contain no molecules whatsoever, the pressure would be zero. Pressures measured on a scale which uses this zero value as their reference point are said to be absolute pressures. Atmospheric pressure at the surface of the earth varies but is approximately 105 Pa (1 000 mbar, 1 000 hPa); this is 105 Pa absolute pressure because it is expressed with respect to zero pressure - that is no molecules at all.

In everyday life, however, many applications of pressure are not so much dependent on the absolute value of a pressure as the difference between it and the pressure of the atmosphere. A punctured car tyre is said to have 'no air in it' and a connected tyre pressure gauge would read 'zero' whilst obviously still containing atmospheric air. Such gauges are designed to measure pressure values that are expressed with respect to atmospheric pressure and thus indicate zero when the measurement port 'merely' contains molecules at atmospheric pressure. These measurements are known as gauge pressure measurements. Thus the difference between an absolute pressure value and a gauge pressure value is the variable value of atmospheric pressure:

Absolute pressure = gauge pressure + atmospheric pressure

In some cases - for example when measuring the pressure reduction in an engine manifold - it is the value of the pressure reduction below the external 'reference' atmospheric pressure which is required, rather than the absolute value of the pressure. This is sometimes known as a negative gauge pressure measurement but it should be appreciated that the concept of a negative absolute pressure is meaningless.

In other applications, where knowledge of the pressure difference between two places or systems is needed, the reference pressure may not necessarily be either zero or atmospheric pressure but some other value. These are known as differential pressures. For example, the flow of gas along a pipeline depends on the pressure difference between the ends of the pipe and in practice both ends are usually at comparatively high pressures.

If serious errors are to be avoided, it is important when making pressure measurements to be clear which mode of measurement is being employed: absolute, gauge (positive or negative) or differential.

Pressure modes are illustrated in the diagram; note that the reference line for gauge-mode measurements is not straight, illustrating the changeable nature of atmospheric pressure.

(Text adapted from the Guide to the Measurement of Pressure and Vacuum)

Last Updated: 16 Mar 2015
Created: 9 Aug 2007

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