Standard atmosphere (unit)
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The standard atmosphere (symbol: atm) is a unit of pressure defined as Template:Val 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.[1]
History
The standard atmosphere was originally defined as the pressure exerted by a 760 mm column of mercury at Template:Convert and standard gravity (gn = Template:Val).[2] 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 Template:Val dynes per square centimetre (Template:Val).[3] 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."[3]
In chemistry and in various industries, the reference pressure referred to in standard temperature and pressure was commonly Template:Convert 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 Template:Convert.[4]
Pressure units and equivalencies
| Script error: No such module "Navbar". | Pascal | Bar | Technical atmosphere | Standard atmosphere | Torr | Pound per square inch |
|---|---|---|---|---|---|---|
| (Pa) | (bar) | (at) | (atm) | (Torr) | (psi) | |
| 1 Pa | Template:N/A | Template:Val | Template:Val | Template:Val | Template:Val | Template:Val |
| 1 bar | Template:Val | Template:N/A | = Template:Val | = Template:Val | = Template:Val | = Template:Val |
| 1 at | Template:Val | Template:Val | Template:N/A | Template:Val | Template:Val | Template:Val |
| 1 atm | ≡ Template:Val | ≡ Template:Val | Template:Val | Template:N/A | 760 | Template:Val |
| 1 Torr | Template:Val | Template:Val | Template:Val | Template:Sfrac ≈ Template:Val | Template:N/A | Template:Val |
| 1 psi | Template:Val | Template:Val | Template:Val | Template:Val | Template:Val | Template:N/A |
A pressure of 1 atm can also be stated as:
- ≈ Template:Val kgf/cm2
- ≈ Template:Val m H2O[5]
- ≈ Template:Val mmHg[6]
- ≈ Template:Val inHg[6]
- ≈ Template:Val in H2O[5]
- ≈ Template:Val pounds-force per square foot (lbf/ft2)
The notation ata has been used to indicate an absolute pressure measured in either standard atmospheres (atm)[7]Template:Better source needed or technical atmospheres (at).[8]
See also
References
- ↑ Script error: No such module "citation/CS1".
- ↑ Script error: No such module "citation/CS1".
- ↑ a b Script error: No such module "citation/CS1".
- ↑ IUPAC.org, Gold Book, Standard Pressure
- ↑ a b 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. Script error: No such module "citation/CS1".
- ↑ a b As a unit of measurement, the conventional millimetre of mercury (mmHg) is defined as an ideal column of mercury with density of Template:Val under standard gravity gn of 9.80665 m/s2 i.e. 0.001 m × Template:Val × 9.80665 m/s2 ≈ 133.322 Pa. 1 inHg = 25.4 mmHg. Script error: No such module "citation/CS1".
- ↑ Script error: No such module "citation/CS1".
- ↑ Script error: No such module "citation/CS1".