Discover

The 'pound per square inch' or, more accurately, 'pound-force per square inch' (symbol: 'psi' or 'lbf/sq in') is a unit of pressure based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied to an area of one square inch:
1 lbf/sq in ≈ 6,894.757 Pa (the SI unit of pressure).

Contents

Relation to other measures

Psig

KSI and Kip

Magnitude

Conversions

Notes

See also

External links

Relation to other measures

Other abbreviations are used that append a modifier to "psi". However, the US National Institute of Standards and Technology recommends that, to avoid confusion, any modifiers be instead applied to the quantity being measured rather than the unit of measure[1] For example, "''P''_g = 100 psi" rather than "''P'' = 100 psig".

★ psia (pounds-force per square inch absolute) - gauge pressure plus local atmospheric pressure. Replace "''x'' psia" with "''P''_a = ''x'' psi".

★ psid (psi difference) - difference between two pressures. Replace "''x'' psid" with "Δ''P'' = ''x'' psi".

★ psig (pounds-force per square inch gauge). Replace "''x'' psig" with "''P''_g = ''x'' psi".

★ psivg (psi vented gauge) - difference between the measuring point and the local pressure. Replace "''x'' psivg" with "''P''_vg = ''x'' psi".

★ psisg (psi sealed gauge) - difference between a chamber of air sealed at atmospheric pressure and the pressure at the measuring point. Replace "''x'' psisg" with "''P''_sg = ''x'' psi".

Psig

Psig (pound-force per square inch gauge) is a unit of pressure relative to atmospheric pressure at sea level. By contrast, psi measures pressure relative to a vacuum (such as that in space). Most pressure gauges, such as tire gauges, are calibrated to read zero at sea level, because most applications require the difference of pressure.
At sea level, Earth's atmosphere actually exerts a pressure of 14.7 psi (see below). Humans do not feel this pressure because internal pressure of liquid in their bodies matches the external pressure. If a pressure gauge is calibrated to read zero in space, then at sea level on Earth it would read 14.7 psi. Thus a reading of 30 psig on a tire gauge, represents an absolute pressure of 44.7 psi.
Psi is often used incorrectly instead of psig.^[1]

KSI and Kip

The ksi (kip, "kilo-pound[-force] per square inch") is 1000 psi, combining the prefix kilo with the psi abbreviation. It is occasionally used in materials science and mechanical engineering to specify stress and Young's modulus.

Magnitude

★ Atmospheric pressure at sea level: ''P''_a = 14.7 psi

★ Automobile tire overpressure: ''P''_g = 32 psi

★ Bike tire overpressure: ''P''_g = 65 psi

★ Air brake reservoir overpressure: 90 psi ≤ ''P''_g ≤ 120 psi

★ Full scuba tank overpressure: ''P''_g = 3,000 psi

Conversions

Notes

1. TurnFast Glossary www.turnfast.com/refc_glossary/glossary.shtml, psi and psig

change from (atm. presure at sea level (1 Pa = 14.5 psi) to (1 bar = 14.5 psi)

See also

★ Conversion of units

★ Pressure

External links

★ Pressure measurement primer