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# Pressure

Pressure is a measure of force per unit area.

The SI unit for pressure is the pascal, equal to one newton per square metre or kg•s-2•m-1. Non-SI measures (still in use in some parts of the world) include the PSI or pound-force per square inch and the bar. In the United States air pressure is often measured in inches of mercury (as in the mercury barometer.

Again,

P = F ÷ A

''where P equals pressure, F equals force, and A equals area.

Pressure is sometimes measured not as an absolute pressure, but as the excess of that pressure above atmospheric pressure, sometimes called gauge pressure. An example of this is the air pressure in a tire of a car, which might be said to be "thirty PSI", but is actually thirty PSI above atmospheric pressure. In technical work, this would be written as "30 PSIG" or, more commonly, "30 psig".

The standard atmosphere or atmosphere (abbreviation atm) is a curious unit of pressure, defined to six figures of precision to approximate a reality that varies constantly from place to place and moment to moment. It is approximately equal to typical air pressures at sea level and defined to be

1 atm = 101 325 pascal

Obsolete manometric units of pressure such as inches of water or millimeters of mercury are based on the pressure exerted by the weight of some "standard" fluid under some "standard" gravity. They are effectively attempts to define a unit for expressing the readings of a manometer.

Manometric pressure units should no longer be used for scientific or engineering purposes, due to the lack of repeatability inherent in their definitions.

The force density f (= ∂F/∂V) is equal to the gradient of the pressure: ; if it concerns the gravitational force, the force density is the specific weight.

Pressure units and conversion factors
Pascal bar N/mm2 kp/m2 kp/cm2 (=1 at) atm Torr
1 Pa (N/m2)= 1 10-5 10-6 0.102 0.102×10-4 0.987×10-5 0.0075
1 bar (daN/cm2) = 100.000 1 0.1 10200 1.02 0.987 750
1 N/mm2 = 105 10 1 1.02×105 10.2 9.87 7500
1 kp/m2 = 9.81 9.81×10-5 9.81×10-6 1 10-4 0.968×10-4 0.0736
1 kp/cm2 (1 at) = 98100 0.981 0.0981 10000 1 0.968 736
1 atm (760 Torr) = 101325 1.013 0.1013 10330 1.033 1 760
1 Torr = 133 0.00133 1.33×10-4 13.6 0.00132 0.00132 1