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 a ...
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 harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
's average
atmospheric pressure at
sea level
Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardis ...
.
History
The standard atmosphere was originally defined as the pressure exerted by 760 mm of
mercury at and standard gravity (''g''
n = ).
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 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
dynes per
square centimetre ().
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
Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
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
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:
:≡ pascals
The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI), and is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is defin ...
(Pa)
:≡ bar
Bar or BAR may refer to:
Food and drink
* Bar (establishment), selling alcoholic beverages
* Candy bar
* Chocolate bar
Science and technology
* Bar (river morphology), a deposit of sediment
* Bar (tropical cyclone), a layer of cloud
* Bar ( ...
:≈ kgf/cm2
:≈ technical atmosphere
:≈ m H2O, 4 °C[This is the customarily accepted value for cm–H2O, 4 °C. It is precisely the product of 1 kg-force per square centimeter (one technical atmosphere) times 1.013 25 (bar/atmosphere) divided by 0.980 665 (one gram-force). It is not accepted practice to define the value for water column based on a true physical realization of water (which would be 99.997 495% of this value because the true maximum density of Vienna Standard Mean Ocean Water is 0.999 974 95 kg/L at 3.984 °C). Also, this "physical realization" would ''still'' ignore the 8.285 cm–H2O reduction that would actually occur in a true physical realization due to the vapor pressure over water at 3.984 °C.]
:≈ mmHg, 0 °C, subject to revision as more precise measurements of mercury's density become available[Torr and mm-Hg, 0°C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable.]
:≈ inHg, 0 °C, subject to revision as more precise measurements of mercury's density become available[ NIST value of 13.595 078(5) g/mL assumed for the density of Hg at 0 °C]
:≈ in H2O, 4 °Cpounds-force per square inch
The pound per square inch or, more accurately, pound-force per square inch (symbol: lbf/in2; abbreviation: psi) is a unit of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied ...
(lbf/in2)
:≈ 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.
Notes
See also
* Standard temperature and pressure
* Atmospheric pressure
* International Standard Atmosphere
References
{{reflist
Units of pressure
Atmosphere