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Pound per square inch

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The pound per square inch or, more accurately, pound-force per square inch (symbol: psi or lbf/in2) is a unit of pressure in the U.S. customary units. It is the pressure exerted by a force of one pound-force applied to an area of one square inch.

1 psi approximately equals 6,894.757 Pa, where pascal (Pa) is the SI unit of pressure.

Other pressure units and equivalents

Pressure Units
  pascal
(Pa)
bar
(bar)
atmosphere
(atm)
torr
(torr)
pound-force
per square inch

(psi)
kilogram-force
per square centimeter

(kgf/cm2)
1 Pa ≡ 1 N/m2 10−5 9.8692×10−6 7.5006×10−3 145.04×10−6 1.01972×10−5
1 bar 100,000 ≡ 106 dyn/cm2 0.98692 750.06 14.504 1.01972
1 atm 101,325 1.01325 ≡ 1 atm 760 14.696 1.03323
1 torr 133.322 1.3332×10−3 1.3158×10−3 ≡ 1 torr
≈ 1 mmHg
19.337×10−3 1.35951×10−3
1 psi 6,894.76 68.948×10−3 68.046×10−3 51.715 ≡ 1 lbf/in2 7.03059×10−2
1 kgf/cm2 98,066.5 0.980665 0.967838 735.5576 14.22357 ≡ 1 kgf/cm2

Example reading:  1 Pa = 1 N/m2  = 10−5 bar  = 9.8692×10−6 atm  = 7.5006×10−3 torr, etc.
Note: mmHg is an abbreviation for millimetre of mercury
About the torr: There is no consensus in the technical literature about whether the name of the torr should be "Torr" or "torr". Nor is there any consensus about whether the symbol for that unit of pressure should be "Torr" or "torr". Both the United Kingdom's National Physical Laboratory (see Pressure Units) and New Zealand's Measurement Standards Laboratory (see Barometric Pressure Units) use "torr" as the name and as the symbol. An extensive search of the website of the U.S. National Institute of Standards and Technology found no such clear-cut definitions. Therefore, this table uses "torr" as both the name and the symbol.

Absolute pressure versus gauge pressure

Bourdon tube pressure gauges, vehicle tire gauges and many other types of pressure gauges are zero referenced to atmospheric pressure, which means that they measure the pressure above atmospheric pressure. However, absolute pressures are zero referenced to a complete vacuum. Thus, the absolute pressure of any system is the gauge pressure of the system plus the local atmospheric or ambient pressure.

An example of the difference is between gauge and absolute pressure is the air pressure in a vehicle tire. A tire pressure gauge might read 32 psi (220 kPa) as the gauge pressure, but that means the pressure is 32 psi (220 kPa) above atmospheric pressure. Since atmospheric pressure at sea level is about 14.7 psi (101) kPa, the absolute pressure in the tire is therefore about 46.7 psi (321 kPa).

In the U.S. customary units, gauge pressure and absolute pressure are very commonly abbreviated as psig and psia respectively. In the above example, the tire pressure would commonly be written as 32 psig or 46.7 psia.

In technical writing, using the SI metric system of units, the use of kPa(g) or kPa(a) is not recommended. Instead, for the example above, it is recommended to write a gauge pressure of 220 kPa or an absolute pressure of 321 kPa. Where space is limited, such as on pressure gauge dials, table headings or graph labels, the use of a modifier, such as kPa (gauge) and kPa (absolute) or kPa (gauge) and kPa (absolute), is strongly encouraged.[1][2] This discussion of pressure unit modifiers also applies to any other pressure units as well.

References

  1. Search Results 1 and 2 (from the website of the National Physics Laboratory, United Kingdom)
  2. Arnold Ivan Jones and Cornelius Wandmacher (2007). Metric Units in Engineering:Going SI, Revised Edition. American Society of Civil Engineers, page 147. ISBN 0-7844-0070-9.