National Physical Laboratory

My pressure gauge is scaled in 'inches' - what does this mean? (FAQ - Pressure)

There are no pressure units called just inches but there are inches of mercury - properly abbreviated to inHg (with only the 'H' a capital letter) and inches of water. If the instrument is designed for measuring atmospheric pressure - essentially a barometer - the scale probably ranges from about 28 inHg to 31 inHg, pressure values that roughly equate to 950 hPa and 1050 hPa (see pressure units), a range that includes the minimum and maximum values of atmospheric pressure encountered in the UK (see historical barometric data). If the instrument is measuring inches of water it is almost certainly not for measuring atmospheric pressure but possibly for measuring small pressure differences (see pressure modes) across an orifice, pipeline or shaft etc. Either way please see the cautionary note about these so-called manometric units as neither of the above units can be exactly converted to the internationally recognised pressure unit - the pascal - and the several conflicting definitions of inches of water are so vague and misused that their use can lead to significant errors.

Last Updated: 25 Mar 2010
Created: 9 Aug 2007

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