Atmospheres Unit: History and Usage

What is Atmospheres Unit?


Atmosphere (symbol: atm) is the pressure exerted by the weight of air in the atmosphere acting on 1 square centimeter and is defined as being equal to 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.

For practical purposes it has been replaced by the bar which is 105 Pa. The difference of about 1% is not significant for many applications, and is within the error range of common pressure gauges.

1 atmosphere =  101.325 kPa
1 atmosphere = 14.69595 psi
1 atmosphere = 1.013250 bar



Other name
The standard atmosphere 



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 (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).



Atmospheres are most commonly used when the precise air pressure is unimportant. For example, I might say that I heated my sample in dry air at 1 atm. In this case I mean that I didn’t measure the air pressure, but I assumed it was basically the pressure of the room, which is more or less 1 atm.

Atmospheres might also be used in scientific fields where the reference to the real atmosphere is important, or when trying to communicate to the general public. Atmospheres are an intuitive, coarse unit of pressure.

Atmosphere 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 as the boiling point of water at this pressure.
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.

The old European unit technical atmosphere (at) is roughly equal to the gauge pressure under 10 m of water; 1 at = 98066.5 Pa.