# Standard 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 pressure* or *standard pressure*. It is approximately equal to Earth's atmospheric pressure at sea level.

Atmosphere | |
---|---|

Unit of | Pressure |

Symbol | atm |

Conversions | |

1 atm in ... | ... is equal to ... |

SI units | 101.325 kPa |

US customary units | 14.69595 psi |

## History

The standard atmosphere was originally defined as the pressure exerted by 760 mm of mercury at 0 °C and standard gravity (*g*_{n} = 9.80665 m/s^{2}).[1] It was used as a reference condition for physical and chemical properties, and was implicit in the definition of the centigrade scale of temperature, which defined 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 1013250 dynes per square centimetre (101325 Pa).[2] 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."[2]

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).[3]

## Pressure units and equivalencies

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

(Pa) | (bar) | (at) | (atm) | (Torr) | (lbf/in^{2}) | |

1 Pa | 1 Pa ≡ 1 Pa | 1 Pa = 10^{−5} bar |
1 Pa = 1.0197×10^{−5} at |
1 Pa = 9.8692×10^{−6} atm |
1 Pa = 7.5006×10^{−3} Torr |
1 Pa = 0.000 145 037 737 730 lbf/in^{2} |

1 bar | 10^{5} |
≡ 100 kPa
≡ 10 |
= 1.0197 | = 0.98692 | = 750.06 | = 14.503 773 773 022 |

1 at | 98066.5 | 0.980665 | ≡ 1 kgf/cm^{2} |
0.967 841 105 354 1 | 735.559 240 1 | 14.223 343 307 120 3 |

1 atm | ≡ 101325 | ≡ 1.01325 | 1.0332 | 1 | 760 | 14.695 948 775 514 2 |

1 Torr | 133.322 368 421 | 0.001 333 224 | 0.001 359 51 | 1/760 ≈ 0.001 315 789 | 1 Torr
≈ 1 mmHg |
0.019 336 775 |

1 lbf/in^{2} |
6894.757 293 168 | 0.068 947 573 | 0.070 306 958 | 0.068 045 964 | 51.714 932 572 | ≡ 1 lbf/in^{2} |

A pressure of 1 atm can also be stated as:

- ≡ 101325 pascals (Pa)
- ≡ 1.01325 bar
- ≈ 1.033 kgf/cm
^{2} - ≈ 1.033 technical atmosphere
- ≈ 10.33 m H
_{2}O, 4 °C[n 1]

- ≈ 760 mmHg, 0 °C, subject to revision as more precise measurements of mercury's density become available[n 1][n 2]
- ≡ 760 torr (Torr)[n 3]
- ≈ 29.92 inHg, 0 °C, subject to revision as more precise measurements of mercury's density become available[n 2]

- ≈ 406.782 in H
_{2}O, 4 °C[n 1] - ≈ 14.6959 pounds-force per square inch (lbf/in
^{2}) - ≈ 2116.22 pounds-force per square foot (lbf/ft
^{2}) - = 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.[4] For example, 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

- 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. - NIST value of 13.595 078(5) g/ml assumed for the density of Hg at 0 °C
- Torr and mm-Hg, 0°C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable.

## See also

- Standard conditions for temperature and pressure
- Atmospheric pressure
- International Standard Atmosphere

## References

- Resnick, Robert; Halliday, David (1960).
*Physics for Students of Science and Engineering Part 1*. New York: Wiley. p. 364. - "BIPM - Resolution 4 of the 10th CGPM".
*www.bipm.org*. - IUPAC.org, Gold Book,
*Standard Pressure* - "The Difference Between An ATM & An ATA".
*Scuba Diving & Other Fun Activities*. March 2, 2008.