 x Atmosphere (unit) Encyclopedia
The standard atmosphere (symbol: atm) is an international reference pressure defined as 101325 Pa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

and formerly used as unit of pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

. For practical purposes it has been replaced by the bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

which is 105 Pa. The difference of 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 dyne
Dyne
In physics, the dyne is a unit of force specified in the centimetre-gram-second system of units, a predecessor of the modern SI. One dyne is equal to exactly 10 µN...

s per square centimeter (101 325 Pa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

). This value was intended to represent the mean atmospheric pressure at mean sea level at the latitude of Paris, France
Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...

, 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
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, the original definition of “Standard Temperature and Pressure” (STP
Standard conditions for temperature and pressure
Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...

) was a reference temperature of 0 °C (273.15 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

) and pressure of 101.325 kPa (1 atm). However, in 1982, the International Union of Pure and Applied Chemistry
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science . The international headquarters of IUPAC is located in Zürich,...

(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).

## Pressure units and equivalencies

A pressure of 1 atm can also be stated as:
≡1.013 25 bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

≡ 1013.25 hectopascal (hPa)
≡ 1013.25 millibars (mbar, also mb)
≡ 760 torr
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...

[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]
≈ 1.033 227 452 799 886 kgf
KGF
KGF may refer to:*Keratinocyte Growth Factor*King George's Fields A UK set of 471 memorial playing fields and recreation grounds*Kolar Gold Fields*The IATA code for Sary-Arka Airport, Karaganda, Kazakhstan...

/cm²
≈ 1.033 227 452 799 886 technical atmosphere
Technical atmosphere
A technical atmosphere is a non-SI unit of pressure equal to one kilogram-force per square centimeter.The symbol "at" clashes with that of the katal , the SI unit of catalytic activity; a kilotechnical atmosphere would have the symbol "kat", indistinguishable from the symbol for the katal...

≈ 1033.227 452 799 886 cm–H2O, 4 °C [A]
≈ 406.782 461 732 2385 in–H2O, 4 °C [A]
≈ 14.695 948 775 5134 pounds-force per square inch (psi)
≈ 2116.216 623 673 94 pounds-force per square foot (psf)

Notes:
A 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.
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

## 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
Technical atmosphere
A technical atmosphere is a non-SI unit of pressure equal to one kilogram-force per square centimeter.The symbol "at" clashes with that of the katal , the SI unit of catalytic activity; a kilotechnical atmosphere would have the symbol "kat", indistinguishable from the symbol for the katal...

(at) is roughly equal to the gauge pressure under 10 m of water; 1 at = 98066.5 Pa.

• Standard conditions for temperature and pressure
Standard conditions for temperature and pressure
Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...

• Atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

• Bar (unit)
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar... 