Encyclopedia > Manometric pressure unit
Pressure
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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 (unit)[?].
Pressure is sometimes measured not as an absolute pressure[?], but as the pressures above atmospheric pressure, also 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: <math>\mathbf{f} = \nabla \mathbf{P} </math>; if it concerns the gravitational force, the force density is the specific weight.
| 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 | 1.103 | 0.1013 | 10330 | 1.033 | 1 | 760 |
| 1 Torr = | 133 | 0.00133 | 1.33×10-4 | 13.6 | 0.00132 | 0.00132 | 1 |
See also:
- Conversion Calculator for Units of Pressure (http://www.ex.ac.uk/cimt/dictunit/ccpress.htm)
- http://www.npl.co.uk/pressure/punits
- An Exercise in Air Pressure (http://avc.comm.nsdlib.org/cgi-bin/wiki_grade_interface.pl?An_Exercise_In_Air_Pressure) from the National Science Digital Library.
Pressure can also be psychological, political, etc.; see also peer pressure.
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