User:Milton Beychok/Sandbox: Difference between revisions

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An '''atmosphere ''' (symbol: atm) is a unit of [[pressure]] defined as 101,325 [[Pascal (unit)|Pa]].<ref name=BIPM>[http://www.bipm.org/jsp/en/ViewCGPMResolution.jsp?CGPM=10&RES=4 BIPM Definition of the standard atmosphere]</ref> For practical purposes, it is often replaced by the [[Bar (unit)|bar]], defined as 100,000 Pa.<ref name=IUPAC>IUPAC Gold Book, [http://goldbook.iupac.org/S05921.html Standard Pressure]</ref>  The difference between an atm and a bar, which is 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]]s per [[square centimeter]] (101,325 [[Pascal (unit)|Pa]]).<ref name=BIPM/> This value was intended to represent the mean atmospheric pressure at mean sea level at the latitude of [[Paris|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).
In [[chemistry]], the original definition of "Standard Temperature and Pressure" was a reference temperature of 0 [[Celsius|°C]] (273.15 [[kelvin|K]]) and pressure of 101.325 kPa (1 atm). However, in 1982, the [[International Union of Pure and Applied Chemistry]] (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).<ref name=IUPAC/> However, the atm continues to be used quite often as a unit of pressure.
==Pressure units and equivalencies ==
{{Pressure}}
A pressure of 1 atm can also be stated as:
:&equiv; 1013.25 [[hectopascal]] (hPa)
:&equiv; 1013.25 [[millibar]]s (mbar, also mb)
:&equiv; 760 [[torr]]<sup> [B]</sup>
:&asymp; 760.001 mm-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available<sup> [B, C]</sup>
:&asymp; 29.9213 in-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available<sup> [C]</sup>
:&asymp; 1033.2275 cm-H<sub>2</sub>O, 4 °C<sup> [A]</sup>
:&asymp; 406.7825 in-H<sub>2</sub>O, 4 °C<sup> [A]</sup>
:&asymp; 2116.2166 pounds-force per square foot (psf)
:Notes:
:<sup>A</sup> This is the customarily accepted value for cm-H<sub>2</sub>O, 4 °C and in-H<sub>2</sub>O, 4 °C
:<sup>B</sup> Torr and mm-Hg, 0°C are often taken to be identical.  For most practical purposes (to 5 significant digits), they are interchangeable.
:<sup>C</sup> [[NIST]] value of 13.595 078(5) g/ml assumed for the density of Hg at 0 °C.
== References ==
{{reflist}}
---------------------
Related Articles <br/>
*[[Reference conditions of gas temperature and pressure]]
*[[Atmospheric pressure]]
*[[Bar (unit)]]

Revision as of 13:32, 14 August 2009

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