The standard atmosphere (symbol: atm) is a unit of pressure defined as 101325 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.
|1 atm in ...||... is equal to ...|
|SI units||101.325 kPa|
|US customary units||14.69595 psi|
|other metric units||1.013250 bar|
The standard atmosphere was originally defined as the pressure exerted by 760 mm of mercury at 0 °C (32 °F) and standard gravity (gn = 9.80665 m/s2). It was used as a reference condition for physical and chemical properties, and was implicit in the definition of the Celsius temperature scale, which defined 100 °C (212 °F) 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 1013250 dynes per square centimetre (101325 Pa). 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 was commonly 1 atm (101.325 kPa) 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 100 kPa (1 bar).
|Pascal||Bar||Technical atmosphere||Standard atmosphere||Torr||Pound per square inch|
|1 Pa||—||1 Pa = 10−5 bar||1 Pa = 1.0197×10−5 at||1 Pa = 9.8692×10−6 atm||1 Pa = 7.5006×10−3 Torr||1 Pa = 0.000145037737730 lbf/in2|
|1 bar||105||—||= 1.0197||= 0.98692||= 750.06||= 14.503773773022|
|1 atm||≡ 101325||≡ 1.01325||1.0332||—||760||14.6959487755142|
|1 Torr||133.322368421||0.001333224||0.00135951||1/760 ≈ 0.001315789||—||0.019336775|
A pressure of 1 atm can also be stated as:
The ata unit is used in place of atm to indicate the total pressure of the system, compared to a vacuum. For example, an underwater pressure of 3 ata would mean that this pressure includes 1 atm of air pressure and thus 2 atm due to the water.