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The standard atmosphere (symbol: atm) is a unit of pressure defined as Pa. It is sometimes used as a ''reference pressure'' or ''standard pressure''. It is approximately equal to Earth's average atmospheric pressure at sea level.


History

The standard atmosphere was originally defined as the pressure exerted by a 760 mm column of mercury at and standard gravity (''g''n = ). It was used as a reference condition for physical and chemical properties, and the definition of the centigrade temperature scale set 100 °C as the boiling point of water at this pressure. In 1954, the 10th General Conference on Weights and Measures (CGPM) adopted ''standard atmosphere'' for general use and affirmed its definition of being precisely equal to
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s per square centimetre (). This defined pressure in a way that is independent of the properties of any particular substance. In addition, the CGPM noted that there had been some misapprehension that the previous definition (from the 9th CGPM) "led some physicists to believe that this definition of the standard atmosphere was valid only for accurate work in thermometry." In
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and in various industries, the reference pressure referred to in '' standard temperature and pressure'' was commonly prior to 1982, but standards have since diverged; in 1982, the International Union of Pure and Applied Chemistry recommended that for the purposes of specifying the physical properties of substances, ''standard pressure'' should be precisely .


Pressure units and equivalencies

A pressure of 1 atm can also be stated as: :≈ kgf/cm2 :≈ m H2OAs a unit of measurement, the conventional metre of water (mH2O) is defined as an ideal column of water with density of 1000 kg/m3 under standard gravity gn of 9.80665 m/s2 i.e. 1 m × 1000 kg/m3 × 9.80665 m/s2 = 9806.65 Pa (though in practice the density of pure water is always less). 1 cmH2O = 0.01 mH2O and 1 inH2O = 0.0254 mH2O. :≈ mmHgAs a unit of measurement, the conventional millimetre of mercury (mmHg) is defined as an ideal column of mercury with density of under standard gravity gn of 9.80665 m/s2 i.e. 0.001 m × × 9.80665 m/s2 ≈ 133.322 Pa. 1 inHg = 25.4 mmHg. : :≈ inHg :≈ in H2O :≈ pounds-force per square foot (lbf/ft2) The notation ata has been used to indicate an absolute pressure measured in either standard atmospheres (atm) or technical atmospheres (at).


See also

* International Standard Atmosphere


References

{{reflist Units of pressure Atmospheric pressure