Standard 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 Earth's atmospheric pressure at sea level.

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

History

It 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 (later Celsius) scale of temperature by defining 100 °C as being 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) 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
Pressure units
Pascal Bar Technical atmosphere Standard atmosphere Torr Pound 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:

≡ 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 the pressure of the medium vs vacuum only.[4] For example, underwater pressure of 3 ata would mean that pressure includes 1 atm of air above water and also 2 atm of water itself.

Notes
  1. 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.
  2. NIST value of 13.595 078(5) g/ml assumed for the density of Hg at 0 °C
  3. Torr and mm-Hg, 0°C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable.
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See also

References
  1. Resnick, Robert; Halliday, David (1960). Physics for Students of Science and Engineering Part 1. New York: Wiley. p. 364.
  2. "BIPM - Resolution 4 of the 10th CGPM". www.bipm.org.
  3. IUPAC.org, Gold Book, Standard Pressure
  4. "The Difference Between An ATM & An ATA". Scuba Diving & Other Fun Activities. March 2, 2008.
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