Pounds per square inch (PSI) is a measurement of pressure in the Imperial system of measurement.
PSI is commonly used to measure the pressure of gasses (pneumatic pressure) or liquids (hydraulic pressure). PSI is also used as a measure of tensile strength, defined as resistance to pulling forces, and elastic modulus strength, defined as resistance to deformation, which controls the stiffness of materials.
In pneumatic and hydraulic pressure, PSI expresses the force exerted on its containing vessel by the two relative fluids. In fluid pressure measurement, the use of PSI is generally relative to atmosphere. This is because PSI as measured by PSI gauge (PSIG) is inherently measured as a differential balance against atmospheric pressure, generally being immersed in it. PSI measurement may also be absolute: relative to a vacuum or PSI absolute (PSIA).
PSI is used to measure tensile strength in materials where thousands of PSI (Kpsi) are common and to measure elastic modulus of materials where millions of PSI (Mpsi) are common. Gauges measuring pressure are used in all manner of instrumentation for vehicles, pneumatic and hydraulic machines, as well as industrial and safety systems. In compressed air power, a PSI gauge is the equivalent to a fuel gauge.
Equivalent terms in other measurement systems are:
- Metrics pascal (Pa) = 6.8948 xPSI.
- Multiples of atmospheric pressure (ATM) = 6.8046 xPSI.
- Technical atmospheres: 1 kilogram of force per centimeter squared (AT) = 7.03069 x PSI.
- Torr, the measurement used by the inventor of the barometer = 51.71493 PSI.
- Bar, a rounding off of atmosphere based on 100 000 Pa = 6.8948 x PSI.