The standard atmosphere (symbol: atm) is a
unit
Unit may refer to:
Arts and entertainment
* UNIT, a fictional military organization in the science fiction television series ''Doctor Who''
* Unit of action, a discrete piece of action (or beat) in a theatrical presentation
Music
* ''Unit'' (alb ...
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 e ...
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 surfa ...
's average
atmospheric pressure
Atmospheric pressure, also known as 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 1013.25 millibars, 7 ...
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 standardised g ...
.
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
dyne
The dyne (symbol: dyn; ) is a derived unit of force specified in the centimetre–gram–second (CGS) system of units, a predecessor of the modern SI.
History
The name dyne was first proposed as a CGS unit of force in 1873 by a Committee of ...
s 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
Temperature measurement (also known as thermometry) describes the process of measuring a current local temperature for immediate or later evaluation. Datasets consisting of repeated standardized measurements can be used to assess temperature tren ...
."
[
In ]chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
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 defined ...
(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 (u ...
:≈ kgf KGF or ''kgf'' may refer to:
*Keratinocyte growth factor
*King George's Fields, UK, recreation grounds
*Kolar Gold Fields
*The IATA code for Sary-Arka Airport, Karaganda, Kazakhstan
* ''K.G.F'' (film series), Indian Kannada-language film series
* ...
/cm2
:≈ technical atmosphere
Technical may refer to:
* Technical (vehicle), an improvised fighting vehicle
* Technical analysis, a discipline for forecasting the future direction of prices through the study of past market data
* Technical drawing, showing how something is co ...
:≈ 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
Vienna Standard Mean Ocean Water (VSMOW) is an isotopic standard for water. Despite the name, VSMOW is pure water with no salt or other chemicals found in the oceans. The VSMOW standard was promulgated by the International Atomic Energy Agency ( ...
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
The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (). Thus one torr is exactly (≈ ).
Historically, one torr was intended to be the same as one " millimeter of merc ...
(Torr)[Torr and mm-Hg, 0°C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable.]
:≈ inHg
Inch of mercury (inHg and ″Hg) 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 heigh ...
, 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 °C
:≈ pounds-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 t ...
(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. 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.
Notes
See also
* Standard temperature and pressure
*Atmospheric pressure
Atmospheric pressure, also known as 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 1013.25 millibars, 7 ...
*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 a ...
References
{{reflist
Units of pressure
Atmosphere