National Physical Laboratory

Are all pressure units equally valid? (FAQ - Pressure)

No, they are not.

The internationally recognised SI unit for pressure is the pascal, abbreviated to Pa, and this is the unit realised by the primary measurement standards in the world's national metrology institutes to provide traceability for pressure measurements.

There are many other pressure units in service - indeed pressure has more units than most subjects and it is usually assumed that to convert from one to another all that is needed is the right conversion factor. For example 1 mbar equals 100 pascals; 1 bar equals 100 000 pascals or 1 kilogram-force per square centimetre (kgf/cm2) equals the not-so-round-number of 98 066.5 pascals. The conversion factors in these examples are EXACT - no amount of computing will produce another decimal place. But not all conversions are or can be expressed exactly because their definitions are incompatible with the definition of the pascal. Use of certain pressure units guarantees the introduction of conversion errors and increases the chance of confusion-induced mistakes.

The problematical units include those known as manometric units - which define pressure in terms of the length of liquid column at a specified (but non-realisable) density - and include all those using two of the words: inches, feet, millimetres, centimetres, mercury, water, oil or their abbreviations, for example millimetres of mercury (mmHg). In short, it is the need to assume fixed and exact but ultimately incorrect values of liquid density and acceleration due to gravity that inherently limits knowledge of the relationship between these units and the pascal. By contrast, the magnitude of pressure values expressed in the SI pressure unit, the pascal, can flex (albeit not by much) to take account of technological improvements in the underlying definitions of mass, length and time - the SI base quantities from which pressure is derived.

Also see notes on the Pressure units page.

Last Updated: 25 Mar 2010
Created: 9 Aug 2007

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