# Atmosphere (unit)

The **standard atmosphere** (symbol: **atm**) is a unit of pressure defined as 101325 Pa (1.01325 bar). It is sometimes used as a *reference* or *standard pressure*.

## History

In 1954 the 10th Conférence Générale des Poids et Mesures (CGPM) adopted *standard atmosphere* for general use and affirmed its definition of being precisely equal to 1,013,250 dynes per square centimetre (101325 Pa).^{[1]} This value was intended to represent the mean atmospheric pressure at mean sea level at the latitude of Paris, France, and does reflect the mean sea level pressure for many industrialized nations that are at broadly similar latitudes.

In chemistry and in various industries, the reference pressure referred to in “Standard Temperature and Pressure” (STP) was commonly 1 atm (101.325 kPa) but standards have since diverged; in 1982, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of specifying the physical properties of substances, “*standard pressure*” should be precisely 100 kPa (1 bar).^{[2]}

## Pressure units and equivalencies

Pascal | Bar | Technical atmosphere | Standard atmosphere | Torr | Pounds per square inch | |
---|---|---|---|---|---|---|

(Pa) | (bar) | (at) | (atm) | (Torr) | (psi) | |

1 Pa | ≡ 1 N/m^{2} |
10^{−5} |
×10^{−5} 1.0197 |
×10^{−6} 9.8692 |
×10^{−3} 7.5006 |
377×10^{−4} 1.450 |

1 bar | 10^{5} |
≡ 100 kPa
≡ 10 |
1.0197 | 92 0.986 | 750.06 | 77 14.503 |

1 at | 65×10^{4} 9.806 |
665 0.980 | ≡ 1 kp/cm^{2} |
8411 0.967 | 735.5592 | 34 14.223 |

1 atm | 25×10^{5} 1.013 |
25 1.013 | 1.0332 | 1 | ≡ 760 | 95 14.695 |

1 Torr | 133.3224 | 224×10^{−3} 1.333 |
551×10^{−3} 1.359 |
≡ 1/760 ≈ 789×10^{−3} 1.315 |
≡ 1 Torr
≈ 1 mm |
678×10^{−2} 1.933 |

1 psi | ×10^{3} 6.8948 |
×10^{−2} 6.8948 |
69×10^{−2} 7.030 |
×10^{−2} 6.8046 |
93 51.714 | ≡ 1 lb_{F} /in^{2} |

A pressure of 1 atm can also be stated as:

- ≡ 1.01325 bar
- ≡ 101325 pascal (Pa) or 101.325 kilopascal (kPa)
- ≡ 1013.25 millibars (mbar, also mb)
- ≡ 760 torr
^{[n 1]} - ≈ 760.001 mm-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available
^{[n 2]}^{[n 3]} - ≈ 29.9213 in-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available
^{[n 3]} - ≈ 1.033 227 452 799 886 kgf/cm²
- ≈ 1.033 227 452 799 886 technical atmosphere
- ≈ 1033.227 452 799 886 cm–H
_{2}O, 4 °C^{[n 2]} - ≈ 406.782 461 732 2385 in–H
_{2}O, 4 °C^{[n 2]} - ≈ 14.695 948 775 5134 pounds-force per square inch (psi)
- ≈ 2116.216 623 673 94 pounds-force per square foot (psf)
- = 1 ata. The ata unit is used in place of atm to indicate that the pressure shown is the total ambient pressure, compared to vacuum, of the system being calculated or measured.
^{[3]}For example, for underwater pressures, a pressure of 3.1 ata would mean that the 1 atm of the air above water is included in this value and the pressure due to water would total 2.1 atm.

- Notes:

- ↑ Torr and mm-Hg, 0°C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable.
- 1 2 3 This is the customarily accepted value for cm–H
_{2}O, 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–H_{2}O reduction that would actually occur in a true physical realization due to the vapor pressure over water at 3.984 °C. - 1 2 NIST value of 13.595 078(5) g/ml assumed for the density of Hg at 0 °C

## Other applications

Scuba divers and others use the word *atmosphere* and "atm" in relation to pressures that are relative to mean atmospheric pressure at sea level (1.013 bar). For example, a partial pressure of oxygen is calibrated typically using air at sea level, so is expressed in units of atm.

The old European unit technical atmosphere (at) is equal to 1 kilogram-force per square centimetre (kgf/cm^{2}), 066.5 98 Pa.

## See also

## References

- ↑ BIPM Definition of the standard atmosphere
- ↑ IUPAC.org, Gold Book,
*Standard Pressure* - ↑ Scuba Diving & Other Fun Activities,
*The Difference Between An ATM & An ATA*