From the original definition, other units can be substituted (g for kg cm or mm for m) to produce a whole range of combinations such as gf/m², kgf/cm², and gf/mm². For specifying industrial pressures, the kilopascal is used when SI units are preferred (1000 kPa = 145 psi). Similar suffixes and notes are sometimes applied to SI units, for example 101 kPa (abs).Īs the pascal is a very small unit, it is commonly quoted in vacuum applications. This distinguishes it from an absolute pressure measurement (lba/in 2, psia), which is relative to vacuum. For example, in the USCS system, lbf/in 2 (the ‘f’ stands for force) or psig (the ‘g’ stands for gauge) shows that the value is relative to ambient atmospheric pressure. Sometimes, pressure units are appended with letters to show how the value has been measured.
#Psi meaning preassure plus
Most pressure measurements (gauge pressures) are made relative to ambient air pressure – the gauge shows a zero reading when exposed to atmospheric pressure.Īn absolute pressure is referenced against a perfect vacuum, using an absolute scale, so it is equal to gauge pressure plus atmospheric pressure (Torr is an absolute unit).Ī differential pressure is the difference in pressure between two places in a system (Head values are differential pressures). Pressure values can be stated in three ways: Pressure units of the SI and USCS systems Measurement System The standard unit of pressure in this system is pound per square inch (PSI): the pressure resulting from a force of one pound applied to an area of one square inch. This is based on Imperial units such as the pound (lb) and inch (in) or foot (ft).
In North America, however, the US Customary System is preferred. The basic unit of pressure is the pascal, defined as the pressure exerted by a force of one newton perpendicularly upon an area of one square metre.
It is widely accepted and used across the world.
The SI system of units is the International System of Units (Système International) derived from the metric system and is based on the kilogram and the metre. This wide variation is partly down to historical or cultural differences, or a particular method of defining and measuring pressure is more convenient, intuitive and useful in some applications but not in others. Many terms, abbreviations and acronyms are used to describe pressure and values can be quoted in a host of different units.
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