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 1,013,250 dynes per square centimetre (101325 Pa). 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."
In chemistry and in various industries, the reference pressure referred to in “Standard Temperature and Pressure” (STP) was commonly 1 atm (101.325 kPa) 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 100 kPa (1 bar).
|Pascal||Bar||Technical atmosphere||Standard atmosphere||Torr||Pounds per square inch|
|1 Pa||≡ 1 N/m2||10−5||×10−51.0197||×10−69.8692||×10−37.5006||377×10−41.450|
|1 bar||105||≡ 100 kPa
≡ 106 dyn/cm2
|1 at||65×1049.806||6650.980||≡ 1 kgf/cm2||84110.967||735.5592||3414.223|
|1 atm||25×1051.013||251.013||1.0332||1||≡ 760||9514.695|
|1 Torr||133.3224||224×10−31.333||551×10−31.359||≡ 1/760 ≈ 789×10−31.315||≡ 1 Torr
≈ 1 mmHg
|1 lbf/in2||×1036.8948||×10−26.8948||69×10−27.030||×10−26.8046||9351.714||≡ 1 lbf /in2|
A pressure of 1 atm can also be stated as:
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.