Atmosphere (unit)

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Standard atmosphere (symbol: atm) is a unit of pressure. Standard atmosphere is a non-SI unit that is internationally recognized. The 10th Conférence Générale des Poids et Mesures (10th CGPM) adopted standard atmosphere for general use and affirmed its definition of being precisely equal to 1,013,250 dynes per cm2 (101,325 Pa). This value is 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).

Equivalencies

A pressure of 1 atm can also be stated as:

≡1.013 25 bar
≡101.325 kPa
≡1013.25 millibars (mbar, also mb) or hectopascals (hPa)
≡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]
≈1.033 227 452 799 886 kg-force/cm²
≈1.033 227 452 799 886 technical atmosphere
≈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)
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

Applications

In chemistry, standard temperature and pressure (STP) is defined as a reference temperature of 0°C (273.15 K) and pressure of 101.325 kPa (1 atm). In 1990, the standard pressure was changed from 1atm to an absolute pressure of 100kPa.

In the United Kingdom, scuba divers and others often use the word atmosphere loosely to mean 1 bar (1000 millibars, or 100 000 Pa).

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

Pressure units
Pascal Bar Technical atmosphere Standard atmosphere Torr Pounds 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