It was originally defined as the pressure exerted by 760 mm of mercury at 0 °C and standard gravity (g = 9.80665 m/s2). It was used as a reference condition for physical and chemical properties, and was implicit in the definition of the Centigrade (later Celsius) scale of temperature by defining 100 °C as being the boiling point of water at this pressure. In 1954, the 10th Conférence Générale des Poids et Mesures (CGPM) adopted standard atmosphere for general use and affirmed its definition of being precisely equal to 1013250dynes per square centimetre (101325Pa). This defined both temperature and pressure independent of the properties of particular substance. In addition (the CGPM noted) there had been some misapprehension that it "led some physicists to believe that this definition of the standard atmosphere was valid only for accurate work in thermometry."
= 1 ata (atmosphere absolute). The ata unit is used in place of atm to indicate that the pressure shown is the total ambient pressure, compared to vacuum, of the system being calculated or measured. For example, for underwater pressures, a pressure of 3.1 ata would mean that the 1 atm of the air above water is included in this value and the pressure due to water would total 2.1 atm.
^Torr and mm-Hg, 0°C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable.
^ abcThis is the customarily accepted value for cm–H2O, 4 °C. It is precisely the product of 1 kg-force per square centimeter (one technical atmosphere) times 1.013 25 (bar/atmosphere) divided by 0.980 665 (one gram-force). It is not accepted practice to define the value for water column based on a true physical realization of water (which would be 99.997 495% of this value because the true maximum density of Vienna Standard Mean Ocean Water is 0.999 974 95 kg/l at 3.984 °C). Also, this “physical realization” would still ignore the 8.285 cm–H2O reduction that would actually occur in a true physical realization due to the vapor pressure over water at 3.984 °C.
^ abNIST value of 13.595 078(5) g/ml assumed for the density of Hg at 0 °C
Scuba divers and others use the word atmosphere and "atm" in relation to pressures that are relative to mean atmospheric pressure at sea level (1.013 bar). For example, a partial pressure of oxygen is calibrated typically using air at sea level, so is expressed in units of atm.
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