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Atmosphere (unit)

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

Atmosphere
Unit ofPressure
Symbolatm 
Conversions
1 atm in...... is equal to...
SI units101.325 kPa
U.S. customary units14.69595 psi

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).[4] 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).[5] 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."[5]

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).[6]

Pressure units and equivalencies

Pressure units
    		PascalBarTechnical atmosphereStandard atmosphereTorrPounds per square inch
    (Pa)(bar)(at)(atm)(Torr)(lbf/in2)
    1 Pa≡ 1N/m210−51.0197×10−59.8692×10−67.5006×10−30.000 145 037 737 730
    1 bar105≡ 100kPa≡ 106dyn/cm21.01970.98692750.0614.503 773 773 022
    1 at98066.50.980665≡ 1kgf/cm20.967 841 105 354 1735.559 240 114.223 343 307 120 3
    1 atm1013251.013251.0332176014.695 948 775 514 2
    1 Torr133.322 368 4210.001 333 2240.001 359 511/760≈ 0.001 315 7891 Torr ≈ 1mmHg0.019 336 775
    1 lbf/in26894.757 293 1680.068 947 5730.070 306 9580.068 045 96451.714 932 572≡ 1lbf/in2

    A pressure of 1 atm can also be stated as:

    ≡≡(Pa) orkilopascals (kPa)≡(mbar or mb)≡(Torr)[1]≈, 0 °C, subject to revision as more precise measurements of mercury’s density become available[2][3]≈, 0 °C, subject to revision as more precise measurements of mercury’s density become available[3]≈/cm²≈≈, 4 °C[2]≈, 4 °C[2]≈(psi)≈(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.[7]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:

    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 (at) is equal to 1 kilogram-force per square centimetre (kgf/cm2), 98066.5 Pa.

    See also

    References

    [1]
    Citation Linkopenlibrary.orgTorr and mm-Hg, 0°C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable.
    Sep 24, 2019, 8:16 PM
    [2]
    Citation Linkopenlibrary.orgThis 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.
    Sep 24, 2019, 8:16 PM
    [3]
    Citation Linkopenlibrary.orgNIST value of 13.595 078(5) g/ml assumed for the density of Hg at 0 °C
    Sep 24, 2019, 8:16 PM
    [4]
    Citation Linkopenlibrary.orgResnick, Robert; Halliday, David (1960). Physics for Students of Science and Engineering Part 1. New York: Wiley. p. 364.
    Sep 24, 2019, 8:16 PM
    [5]
    Citation Linkwww.bipm.org"BIPM - Resolution 4 of the 10th CGPM". www.bipm.org.
    Sep 24, 2019, 8:16 PM
    [6]
    Citation Linkgoldbook.iupac.orgIUPAC.org, Gold Book, Standard Pressure
    Sep 24, 2019, 8:16 PM
    [7]
    Citation Linkscubadoobydoo.blogspot.com.br"The Difference Between An ATM & An ATA". Scuba Diving & Other Fun Activities. March 2, 2008.
    Sep 24, 2019, 8:16 PM
    [11]
    Citation Linken.wikipedia.orgThe original version of this page is from Wikipedia, you can edit the page right here on Everipedia.Text is available under the Creative Commons Attribution-ShareAlike License.Additional terms may apply.See everipedia.org/everipedia-termsfor further details.Images/media credited individually (click the icon for details).
    Sep 24, 2019, 8:16 PM