The standard atmosphere (symbol: atm) is a
unit of
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
defined as
Pa. It is sometimes used as a ''reference pressure'' or ''standard pressure''. It is approximately equal to
Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
's average
atmospheric pressure
Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1,013. ...
at
sea level
Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...
.
History
The standard atmosphere was originally defined as the pressure exerted by a 760 mm column of
mercury at and standard gravity (''g''
n = ).
It was used as a reference condition for physical and chemical properties, and the definition of the
centigrade temperature scale set 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
dyne
The dyne (symbol: dyn; ) is a derived units of measurement, unit of force (physics), force specified in the centimetre–gram–second system of units, centimetre–gram–second (CGS) system of units, a predecessor of the modern International S ...
s per
square centimetre ().
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
Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
and in various industries, the reference pressure referred to in ''standard temperature and pressure
Standard temperature and pressure (STP) or standard conditions for temperature and pressure are various standard sets of conditions for experimental measurements used to allow comparisons to be made between different sets of data. The most used ...
'' was commonly prior to 1982, but standards have since diverged; in 1982, the International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
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:
:≈ kgf/cm2
:≈ m H2O[As a unit of measurement, the conventional metre of water (mH2O) is defined as an ideal column of water with density of 1000 kg/m3 under standard gravity gn of 9.80665 m/s2 i.e. 1 m × 1000 kg/m3 × 9.80665 m/s2 = 9806.65 Pa (though in practice the density of pure water is always less). 1 cmH2O = 0.01 mH2O and 1 inH2O = 0.0254 mH2O. ]
:≈ mmHg[As a unit of measurement, the conventional millimetre of mercury (mmHg) is defined as an ideal column of mercury with density of under standard gravity gn of 9.80665 m/s2 i.e. 0.001 m × × 9.80665 m/s2 ≈ 133.322 Pa. 1 inHg = 25.4 mmHg. ]
:
:≈ inHgabsolute pressure
Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in unit of measurement, units of force per unit of surface area. Many techniques have been developed for the me ...
measured in either standard atmospheres (atm) or technical atmosphere
Technical may refer to:
* Technical (vehicle), an improvised fighting vehicle
* Technical area, an area which a manager, other coaching personnel, and substitutes are allowed to occupy during a football match
* Technical advisor, a person who ad ...
s (at).
See also
* International Standard Atmosphere
References
{{reflist
Units of pressure
Atmospheric pressure