Atmosphere (unit)

An atmosphere (symbol: atm) is a unit of pressure measurement defined as 101,325 Pa. For practical purposes, it is often replaced by the bar, defined as 100,000 Pa. The difference between an atm and a bar, which is about 1%, is not significant for many applications, and is within the error range of common pressure gauges.

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 centimeter (101,325 Pa). This value was intended to represent the mean atmospheric pressure at mean sea level at the latitude of Paris, France, and as a practical matter, truly reflects the mean sea level pressure for many of the industrialized nations (those with latitudes similar to Paris).

In chemistry, the original definition of "Standard Temperature and Pressure" was a reference temperature of 0 °C (273.15 K) and pressure of 101.325 kPa (1 atm). However, in 1982, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of specifying the physical properties of substances, the "standard pressure" should be defined as precisely 100 kPa (exactly 1 bar). However, the atm continues to be used quite often as a unit of pressure.

Pressure units and equivalencies

Pressure Units

	pascal (Pa)	bar (bar)	atmosphere (atm)	torr (torr)	pound-force per square inch (psi)	kilogram-force per square centimeter (kgf/cm2)
1 Pa	≡ 1 N/m2	10−5	9.8692×10−6	7.5006×10−3	145.04×10−6	1.01972×10−5
1 bar	100,000	≡ 106 dyn/cm2	0.98692	750.06	14.504	1.01972
1 atm	101,325	1.01325	≡ 1 atm	760	14.696	1.03323
1 torr	133.322	1.3332×10−3	1.3158×10−3	≡ 1 torr ≈ 1 mmHg	19.337×10−3	1.35951×10−3
1 psi	6,894.76	68.948×10−3	68.046×10−3	51.715	≡ 1 lbf/in2	7.03059×10−2
1 kgf/cm2	98,066.5	0.980665	0.967838	735.5576	14.22357	≡ 1 kgf/cm2

Example reading:  1 Pa = 1 N/m²  = 10⁻⁵ bar  = 9.8692×10⁻⁶ atm  = 7.5006×10⁻³ torr, etc.
Note: mmHg is an abbreviation for millimetre of mercury

A pressure of 1 atm can also be stated as:

≡ 1013.25 hectopascal (hPa)

≡ 1013.25 millibars (mbar, also mb)

≡ 760 torr ^[B]

≈ 760.001 mm-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available ^{[B, C]}

≈ 29.9213 in-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available ^[C]

≈ 1033.2275 cm-H₂O, 4 °C ^[A]

≈ 406.7825 in-H₂O, 4 °C ^[A]

≈ 2116.2166 pounds-force per square foot (psf)

Notes:

^A This is the customarily accepted value for cm-H₂O, 4 °C and in-H₂O, 4 °C

^B Torr and mm-Hg, 0°C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable.

^C NIST value of 13.595 078(5) g/ml assumed for the density of Hg at 0 °C.

Absolute pressure and gauge pressure

Bourdon tube pressure gauges, vehicle tire gauges and many other types of pressure gauges are zero referenced to atmospheric pressure, which means that they measure the pressure above atmospheric pressure. However, absolute pressures are zero referenced to a complete vacuum. Thus, the absolute pressure of any system is the gauge pressure of the system plus atmospheric pressure.

In the United States, where pressures are still often expressed in pounds per square inch (symbol psi), gauge pressures are referred to as psig and absolute pressures are referred to as psia. Gauge pressure is also sometimes spelled as as gage pressure.

Sometimes, the context in which the word pressure is used helps to identify it as meaning either the absolute or gauge pressure. However, in truth, whenever a pressure is expressed in any units (atm, bar, Pa, psi, etc.), it should be denoted in some manner as being either absolute or gauge pressure to avoid any possible misunderstanding. One recommended way of doing so is to spell out what is meant, for example as atm absolute, kPa absolute or bar gauge.