Standard atmosphere (unit)


The standard atmosphere is a unit of pressure defined as. It is sometimes used as a reference or standard pressure. It is approximately equal to the atmospheric pressure at sea level.

History

It was originally defined as the pressure exerted by 760 mm of mercury at 0 °C and standard gravity. It was used as a reference condition for physical and chemical properties, and was implicit in the definition of the centigrade scale of temperature by defining 100 °C as being the boiling point of water at this pressure. In 1954, the 10th General Conference on Weights and Measures 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 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 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.

Pressure units and equivalencies

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 the pressure of the medium vs vacuum only. For example, underwater pressure of 3 ata would mean that pressure includes 1 atm of air above water and also 2 atm of water itself.