Thestandard atmosphere (symbol:atm) is aunit ofpressure defined as101325Pa. It is sometimes used as areference pressure orstandard pressure. It is approximately equal toEarth's averageatmospheric pressure atsea level.[1]
The standard atmosphere was originally defined as the pressure exerted by a 760 mm column ofmercury at 0 °C (32 °F) and standard gravity (gn =9.80665 m/s2).[2] It was used as a reference condition for physical and chemical properties, and the definition of thecentigrade temperature scale set 100 °C as the boiling point of water at this pressure. In 1954, the 10thGeneral Conference on Weights and Measures (CGPM) adoptedstandard atmosphere for general use and affirmed its definition of being precisely equal to1013250dynes persquare centimetre (101325Pa).[3] 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 inthermometry."[3]
^abAs 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.BS 350:Part 1:1974 Conversion factors and tables, Part 1. Basis of tables. Conversion factors. British Standards Institution. 1974. p. 49.
^abAs a unit of measurement, the conventional millimetre of mercury (mmHg) is defined as an ideal column of mercury with density of13595.1 kg/m3 under standard gravity gn of 9.80665 m/s2 i.e. 0.001 m ×13595.1 kg/m3 × 9.80665 m/s2 ≈ 133.322 Pa. 1 inHg = 25.4 mmHg.BS 350:Part 1:1974 Conversion factors and tables, Part 1. Basis of tables. Conversion factors. British Standards Institution. 1974. p. 49.
