National Physical Laboratory

What do the letters 'g' and 'a' denote after a pressure unit? (FAQ - Pressure)

The letters are meant to stand for gauge-mode and absolute-mode respectively - where a gauge-mode pressure is one normally measured with respect to ambient pressure and an absolute-mode pressure is one measured with respect to zero pressure (a more detailed explanation of pressure modes).

Despite appearing on many pressure instruments however, abbreviations such as barg, psig, kPaA, kPaG, bara, psia etcetera are not legitimate and such formatting conflicts with international written standards and good practice, and should not be used. Similarly the letters NG, ng, D or d should not be used to denote negative gauge-mode or differential-mode pressures. Where needed, the distinction should come instead through the context of use, essentially meaning that the words absolute and gauge should be printed in full next to the pressure unit - for example: bar absolute and kPa gauge. This method of formatting leaves the underlying unit intact (bar, kPa in the previous examples) and avoids implying that the unit is in some way altered by the suffix.

Part of the reason for this seemingly pedantic rule is that pressure is a quantity derived from knowledge of force and area (pressure = force/area) and none of the fundamental pressure unit definitions take the operational mode directly into account. The meanings of gauge- and absolute-mode cannot be defined as clearly and accurately as the underlying unit - for example does gauge mean 'with respect to ambient atmospheric pressure' or perhaps 'a pressure close to but not necessarily exactly at prevailing atmospheric pressure'? Is the reference pressure in an absolute mode measurement 1 000 Pa (that is about 1% of nominal atmospheric pressure - a 'good' vacuum to many people), 1 Pa, 0.0001 Pa or some other pressure? (See definition of a vacuum.) Thus there are no legitimate pressure units ending in 'a' or 'g' etc and there are never likely to be any.

Whatever pressure units you are using, if you want to follow internationally agreed and recognised conventions (recommended!) a descriptive word should be used to denote the pressure mode and not a letter.

Last Updated: 25 Mar 2010
Created: 9 Aug 2007


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