Standard atmosphere (unit)

The standard atmosphere (symbol: atm) is a unit of pressure defined as $101,325$ 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.

History
The standard atmosphere was originally defined as the pressure exerted by a 760 mm column of mercury at 0 C and standard gravity (gn = $9.807 m/s2$). 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 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 $1,013,250$ dynes per square centimetre ($101,325 Pa$). This defined pressure in a way that is independent of the properties of any particular substance. In addition, the CGPM noted that there had been some misapprehension that the previous definition (from the 9th CGPM) "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 101.325 kPa prior to 1982, 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.

Pressure units and equivalencies
A pressure of 1 atm can also be stated as:


 * ≡ $101,325$ pascals (Pa)
 * ≡ $1.013$ bar
 * ≈ $1.033$ kgf/cm2
 * ≈ $1.033$ technical atmosphere
 * ≈ $10.33$ m H2O, 4 °C


 * ≈ $760$ mmHg, 0 °C, subject to revision as more precise measurements of mercury's density become available
 * ≡ $760$ torr (Torr)
 * ≈ $29.92$ inHg, 0 °C, subject to revision as more precise measurements of mercury's density become available


 * ≈ $406.782$ in H2O, 4 °C
 * ≈ $14.696$ pounds-force per square inch (lbf/in2)
 * ≈ $2,116.22$ 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, 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.