# Atmosphere (unit)

Atmosphere
Unit ofPressure
Symbolatm
Conversions
1 atm in ...... is equal to ...
SI units   101.325 kPa
U.S. customary units   14.69595 psi

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. It is approximately equal to the atmospheric pressure at sea level.

## History

It was originally defined as the pressure exerted by 760 mm of mercury at 0 °C and standard gravity (g = 9.80665 m/s2). It was used as a reference condition for physical and chemical properties, and was implicit in the definition of the Centigrade (later Celsius) scale of temperature by defining 100 °C as being the boiling point of water at this pressure. 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 1013250 dynes per square centimetre (101325 Pa). This defined both temperature and pressure independent of the properties of particular substance. In addition (the CGPM noted) 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 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).

## Pressure units and equivalencies

Pressure units
Pascal Bar Technical atmosphere Standard atmosphere Torr Pounds per square inch
(Pa) (bar) (at) (atm) (Torr) (lbf/in2)
1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 9.8692×10−6 7.5006×10−3 0.000 145 037 737 730
1 bar 105 ≡ 100 kPa

≡ 106 dyn/cm2

1.0197 0.98692 750.06 14.503 773 773 022
1 at 98066.5 0.980665 ≡ 1 kgf/cm2 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/in2 6894.757 293 168 0.068 947 573 0.070 306 958 0.068 045 964 51.714 932 572 ≡ 1 lbf/in2

A pressure of 1 atm can also be stated as:

1.01325 bar
101325 pascals (Pa) or 101.325 kilopascals (kPa)
1013.25 millibars (mbar or mb)
760 torr (Torr)[n 1]
760.001 mmHg, 0 °C, subject to revision as more precise measurements of mercury’s density become available[n 2][n 3]
29.9213 inHg, 0 °C, subject to revision as more precise measurements of mercury’s density become available[n 3]
1.033227452799886 kgf/cm²
1.033227452799886 technical atmosphere
1033.227452799886 cm H2O, 4 °C[n 2]
406.7824617322385 in H2O, 4 °C[n 2]
14.6959487755134 pounds-force per square inch (psi)
2116.21662367394 pounds-force per square foot (psf)
= 1 ata (atmosphere absolute). 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. 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:
1. ^ Torr and mm-Hg, 0°C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable.
2. ^ a b 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.
3. ^ a b 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).[citation needed] 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/cm2), 98066.5 Pa.[citation needed]