Standard atmosphere (unit)

4 min readApr 30, 2021

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.

History[edit]

The standard atmosphere was originally defined as the pressure exerted by 760 mm of mercury at 0 °C and standard gravity (gn = 9.80665 m/s2).[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[edit]

Pressure units

Pascal Bar Technical atmosphere Standard atmosphere Torr Pound per square inch(Pa)(bar)(at)(atm)(Torr)(lbf/in2)1 Pa1 Pa ≡ 1 Pa1 Pa = 10−5 bar1 Pa = 1.0197×10−5 at1 Pa = 9.8692×10−6 atm1 Pa = 7.5006×10−3 Torr1 Pa = 0.000 145 037 737 730 lbf/in21 bar105≡ 100 kPa

≡ 106 dyn/cm2

= 1.0197= 0.98692= 750.06= 14.503 773 773 0221 at98066.50.980665≡ 1 kgf/cm20.967 841 105 354 1735.559 240 114.223 343 307 120 31 atm≡ 101325≡ 1.013251.0332176014.695 948 775 514 21 Torr133.322 368 4210.001 333 2240.001 359 511/760 ≈ 0.001 315 7891 Torr

≈ 1 mmHg

0.019 336 7751 lbf/in26894.757 293 1680.068 947 5730.070 306 9580.068 045 96451.714 932 572≡ 1 lbf/in2

A pressure of 1 atm can also be stated as:

≡ 101325 pascals (Pa)≡ 1.01325 bar≈ 1.033 kgf/cm2≈ 1.033 technical atmosphere≈ 10.33 m H2O, 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 H2O, 4 °C[n 1]≈ 14.6959 pounds-force per square inch (lbf/in2)≈ 2116.22 pounds-force per square foot (lbf/ft2)= 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[edit]

^ Jump up to: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.
^ Jump up to:a b 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.

References[edit]

^ Resnick, Robert; Halliday, David (1960). Physics for Students of Science and Engineering Part 1. New York: Wiley. p. 364.
^ Jump up to:a b “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.

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Standard atmosphere (unit)

Contents

History[edit]

Pressure units and equivalencies[edit]

Notes[edit]

See also[edit]

References[edit]

Written by Ar Rabble