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


The standard atmosphere was originally defined as the pressure exerted by 760 mm of mercury at 0 °C and standard gravity (''g''n = ). It was used as a reference condition for physical and chemical properties, and was implicit in the definition of the centigrade scale of temperature, which defined 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  dynes per square centimetre (). This defined both temperature and pressure independent of the properties of particular substance. In addition, the CGPM noted that 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." In chemistry and in various industries, the reference pressure referred to in ''standard temperature and pressure'' (STP) was commonly but standards have since diverged; in 1982, the International Union of Pure and Applied Chemistry (IUPAC) 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: :≡ pascals (Pa) :≡ bar :≈ kgf/cm2 :≈ technical atmosphere :≈ m H2O, 4 °CThis 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. :≈ mmHg, 0 °C, subject to revision as more precise measurements of mercury's density become available :≡ torr (Torr)Torr and mm-Hg, 0°C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable. :≈ inHg, 0 °C, subject to revision as more precise measurements of mercury's density become availableNIST value of 13.595 078(5) g/ml assumed for the density of Hg at 0 °C :≈ in H2O, 4 °C :≈ pounds-force per square inch (lbf/in2) :≈ pounds-force per square foot (lbf/ft2) := 1 ata (atmosphere absolute). The ata unit is used in place of atm to indicate the total pressure of the system, compared to a vacuum. For example, underwater pressure of 3 ata would mean that this pressure includes 1 atm of air pressure and thus 2 atm due to the water.


See also

*Standard conditions for temperature and pressure *Atmospheric pressure *International Standard Atmosphere


{{reflist Category:Units of pressure Category:Atmosphere