The standard atmosphere (symbol: atm) is a
unit
Unit may refer to:
General measurement
* Unit of measurement, a definite magnitude of a physical quantity, defined and adopted by convention or by law
**International System of Units (SI), modern form of the metric system
**English units, histo ...
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
The General Conference on Weights and Measures (abbreviated CGPM from the ) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in 1875 under the terms of the Metre C ...
(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
The square metre ( international spelling as used by the International Bureau of Weights and Measures) or square meter (American spelling) is the unit of area in the International System of Units (SI) with symbol m2. It is the area of a square w ...
().
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
Temperature measurement (also known as thermometry) describes the process of measuring a current temperature for immediate or later evaluation. Datasets consisting of repeated standardized measurements can be used to assess temperature trends.
...
."
[
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
A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high. Currently, it is defined as exactly , or approximately 1 torr = atmosphere = &nb ...
[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. ]
:
:≈ inHg
Inch of mercury (inHg, ″Hg, or in) is a non- SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States.
It is the pressure exerted by a column of mercury in h ...
absolute 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
The International Standard Atmosphere (ISA) is a static atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes or elevations. It has been established to provide ...
References
{{reflist
Units of pressure
Atmospheric pressure