View source for Bar (unit)

{{Short description|Unit of pressure equal to 100,000 Pa}}
{{other uses|Bar (disambiguation)}}
{{Infobox unit
| bgcolor      = 
| name         = bar
| image        = Aluminium cylinder.jpg
| caption      = An aluminium cylinder of wall thickness {{convert|5|mm}} after an external [[pressure]] of 700&nbsp;bar was applied to it
| standard     = [[metric system]]
| quantity     = [[pressure]]
| symbol       = bar
| symbol2      = 
| namedafter   = 
| extralabel   = 
| extradata    = 
| units1       = [[SI units]]
| inunits1     = {{convert|1|bar|disp=out|lk=on}}
| units2       = [[CGS units]]
| inunits2     = {{val|e=6|ul=Ba}}
| units3       = [[US customary units]]
| inunits3     = {{convert|1.000000|bar|psi|disp=out|lk=on|comma=gaps}}
| units4       = [[Atmospheric pressure|Atmospheres]]
| inunits4     = {{convert|1.000000|bar|atm|disp=out|lk=on|comma=gaps}}
}}

The '''bar''' is a [[List of metric units|metric unit]] of [[pressure]] defined as 100,000&nbsp;[[Pascal (unit)|Pa]] (100&nbsp;kPa), though not part of the [[International System of Units]] (SI). A pressure of 1&nbsp;bar is slightly less than the current average [[atmospheric pressure]] on Earth at [[sea level]] (approximately 1.013&nbsp;bar).<ref name="BIPMSI" /><ref name=BSI /> By the [[barometric formula]], 1&nbsp;bar is roughly the atmospheric pressure on Earth at an altitude of 111 metres at 15&nbsp;°C.

The bar and the millibar were introduced by the Norwegian meteorologist [[Vilhelm Bjerknes]], who was a founder of the modern practice of [[weather forecasting]], with the bar defined as one [[mega-|mega]][[dyne]] per square [[centimetre]].<ref>{{cite web |url=http://docs.lib.noaa.gov/rescue/mwr/046/mwr-046-02-0073.pdf |title=Nomenclature of the unit of absolute pressure, Charles F. Marvin, 1918 |website=noaa.gov |access-date=6 May 2018 |url-status=live |archive-url=https://web.archive.org/web/20170429123958/https://docs.lib.noaa.gov/rescue/mwr/046/mwr-046-02-0073.pdf |archive-date=29 April 2017 }}</ref>

The [[SI brochure]], despite previously mentioning the bar,<ref>Resolution 6 of the 9th CGPM, 1948, lists the bar (symbol bar) in a table   illustrating rriting and printing of unit symbols</ref> now omits any mention of it.<ref name="BIPMSI">{{SIbrochure9th}}.</ref> The bar has been legally recognised in countries of the [[European Union]] since 2004.<ref name=BSI>[[British Standard]] BS 350:2004 ''Conversion Factors for Units''.</ref> The US [[National Institute of Standards and Technology]] (NIST) [[deprecated|deprecates]] its use except for "limited use in [[meteorology]]" and lists it as one of several units that "must not be introduced in fields where they are not presently used".<ref>[https://www.nist.gov/pml/wmd/metric/upload/SP1038.pdf NIST Special Publication 1038] {{webarchive |url=https://web.archive.org/web/20160319083656/http://www.nist.gov/pml/wmd/metric/upload/SP1038.pdf |date=2016-03-19 }}, Sec. 4.3.2; [http://physics.nist.gov/cuu/pdf/sp811.pdf NIST Special Publication 811, 2008 edition] {{webarchive 
 |url=https://web.archive.org/web/20160603203340/http://physics.nist.gov/cuu/pdf/sp811.pdf |date=2016-06-03 }}, Sec. 5.2</ref> The [[International Astronomical Union]] (IAU) also lists it under "Non-SI units and symbols whose continued use is deprecated".<ref>International Astronomical Union Style Manual.  Comm. 5 in IAU Transactions XXB, 1989, Table 6</ref>

Units derived from the bar include the '''megabar''' (symbol: '''Mbar'''), '''kilobar''' (symbol: '''kbar'''), '''decibar''' (symbol: '''dbar'''), '''centibar''' (symbol: '''cbar'''), and '''millibar''' (symbol: '''mbar''').

== Definition and conversion ==
The bar is defined using the [[SI derived unit]], [[Pascal (unit)|pascal]]: {{val|1|u=bar}} ≡ {{val|100000|u=Pa}} ≡ {{val|100000|u=N/m<sup>2</sup>}}.

Thus, {{val|1|u=bar}} is equal to:

* {{val|1000000|u=Ba}} ([[barye]]) (in [[CGS units]]);
and 1 bar is approximately equal to:
* {{convert|1|bar|atm|sigfig=8|disp=out|lk=on|comma=gaps}}
* {{convert|1|bar|psi|sigfig=8|disp=out|lk=on|comma=gaps}}
* {{convert|1|bar|inHg|sigfig=8|disp=out|lk=on|comma=gaps}}
* {{convert|1|bar|mmHg|sigfig=8|disp=out|lk=on|comma=gaps}}
* {{convert|1|bar|Torr|sigfig=8|disp=out|lk=on|comma=gaps}}
* {{val|1019.716}} centimetres of water (cmH<sub>2</sub>O) (1 bar approximately corresponds to the [[Pressure measurement#Gauge|gauge pressure]] of water at a depth of 10 metres).

1 [[#Usage|millibar]] ('''mbar''') is equal to:
* {{val|0.001|u=bar}}
* {{convert|1|mbar|Pa|disp=out|lk=off|comma=gaps}}.

{{Pressure Units}}

== Origin ==
The word ''bar'' has its origin in the [[Ancient Greek]] word {{lang|grc|βάρος}} ({{Transliteration|grc|baros}}), meaning [[weight]]. The unit's official symbol is '''bar''';{{citation needed|date=December 2023}} the earlier symbol '''b''' is now deprecated and conflicts with the uses of '''b''' denoting the unit [[barn (unit)|barn]] or [[bit]], but it is still encountered, especially as '''mb''' (rather than the proper '''mbar''') to denote the millibar. Between 1793 and 1795, the word ''bar'' was used for a unit of mass (equal to the modern [[tonne]]) in an early version of the metric system.<ref>{{Cite web |url=https://books.google.com/books?id=QwEOAAAAQAAJ&dq=centigravet+decimetre&pg=PA54 |title=Instructions abrégée sur les mesures déduites de la grandeur de la terre et sur les calculs relatifs à leur division décimale, 1793: gravet, bar |year=1793 |access-date=2016-05-06 |archive-date=2023-01-15 |archive-url=https://web.archive.org/web/20230115082642/https://books.google.nl/books?id=QwEOAAAAQAAJ&pg=PA54&dq=centigravet+decimetre&hl=en&sa=X&ved=0CEMQ6AEwBWoVChMI26XQ6Kb5xwIVgossCh3Isgs1#v=onepage&q=centigravet%20decimetre&f=false |url-status=live }}</ref>

== Usage ==
[[File:Day5pressureforecast.png|thumb|Map showing atmospheric pressure in millibar, or hectopascals]]
[[File:ReifendruckPruefen.jpg|right|thumb|A [[tire-pressure gauge]] displaying bar (outside) and [[pounds per square inch]] (inside)]]

[[Atmospheric pressure|Atmospheric air pressure]] where [[standard atmospheric pressure]] is defined as 1013.25&nbsp;mbar, 101.325&nbsp;[[pascal (unit)|kPa]], {{val|1.01325|u=bar}}, which is about {{convert|1.00|atm|psi|disp=out|abbr=off|lk=on}}.  Despite the millibar not being an [[SI]] unit, meteorologists and weather reporters worldwide have long measured air pressure in millibar as the values are convenient. After the advent of SI units, some meteorologists began using [[hectopascals]] (symbol hPa) which are numerically equivalent to millibar; for the same reason, the hectopascal is now the standard unit used to express barometric pressures in aviation in most countries. For example, the weather office of [[Environment Canada]] uses kilopascals and hectopascals on their weather maps.<ref>{{cite web|url=http://www.weatheroffice.gc.ca/jet_stream/index_e.html|title=Canadian Weather at a Glance - Environment Canada|first=Environment|last=Canada|date=2013-04-16|website=www.weatheroffice.gc.ca|access-date=6 May 2018|url-status=live|archive-url=https://web.archive.org/web/20180102181014/http://weatheroffice.gc.ca/jet_stream/index_e.html|archive-date=2 January 2018}}</ref><ref>{{cite web|url=http://www.weatheroffice.gc.ca/canada_e.html|title=Canadian Weather - Environment Canada|first=Environment|last=Canada|date=2013-04-16|website=www.weatheroffice.gc.ca|access-date=6 May 2018|url-status=live|archive-url=https://web.archive.org/web/20180102180938/http://weatheroffice.gc.ca/canada_e.html|archive-date=2 January 2018}}</ref> In contrast, Americans are familiar with the use of the millibar in US reports of [[hurricane]]s and other cyclonic storms.<ref>[https://www.wpc.ncep.noaa.gov/sfc/90fwbg.gif US government atmospheric pressure map]</ref><ref>[https://s.w-x.co/util/image/map/WEB_Current_Weather_Map_1280x720.jpg?v=ap&w=1280&h=720&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 The Weather Channel]</ref>

In fresh water, there is an approximate numerical equivalence between the change in pressure in decibar and the change in depth from the water surface in [[metre]]s. Specifically, an increase of 1 decibar occurs for an increase in depth of {{val|1.019716|u=m}}. In sea water with respect to the gravity variation, the latitude and the [[gravity anomaly|geopotential anomaly]] the pressure can be converted into metres' depth according to an empirical formula (UNESCO Tech. Paper 44, p.&nbsp;25).<ref>{{cite web |url=http://unesdoc.unesco.org/images/0005/000598/059832eb.pdf |access-date=2014-05-11 |url-status=live |archive-url=https://web.archive.org/web/20150412051004/http://unesdoc.unesco.org/images/0005/000598/059832eb.pdf |archive-date=2015-04-12 |title=Algorithms for computation of fundamental properties of seawater |author=Scientific Committee on Oceanic Research |date=1983 }}</ref>  As a result, decibar is commonly used in [[oceanography]].

In scuba diving, bar is also the most widely used unit to express pressure, e.g. 200&nbsp;bar being a full standard scuba tank, and depth increments of 10 metre of seawater being equivalent to 1&nbsp;bar of pressure.

Many engineers worldwide use the bar as a unit of pressure because, in much of their work, using pascals would involve using very large numbers. In measurement of [[vacuum]] and in [[vacuum engineering]], residual pressures are typically given in millibar, although [[torr]] or [[millimetre of mercury]] (mmHg) were historically common.

Pressures resulting from [[deflagration]]s are often expressed in units of bar.<ref>{{cite book |title=NFPA 68 Standard on Explosion Protection by Deflagration Venting |edition=2023 |ref=C.3.3}}</ref>

In the automotive field, [[turbocharger]] boost is often described in bar outside the United States. [[Cold inflation pressure|Tire pressure]] is often specified in bar. In [[hydraulic machinery]] components are rated to the maximum system oil pressure, which is typically in hundreds of bar. For example, 300 bar is common for industrial fixed machinery.

In the maritime ship industries, pressures in piping systems, such as cooling water systems, is often measured in bar.

[[Unicode]] has characters for "mb" ({{unichar|33D4|SQUARE MB SMALL}}), "bar" ({{unichar|3374|SQUARE BAR}}) and {{lang|ja|ミリバール}} ({{unichar|334A}}; "millibar" spelt in [[katakana]]), but they exist only for compatibility with legacy Asian encodings and are not intended to be used in new documents.

The kilobar, equivalent to 100&nbsp;MPa, is commonly used in geological systems, particularly in experimental [[petrology]].

The abbreviations "bar(a)" and "bara" are sometimes used to indicate [[Pressure measurement#Absolute, gauge and differential|absolute pressures]], and "bar(g)" and "barg" for [[Pressure measurement#Absolute, gauge and differential|gauge pressures]]. The usage is deprecated but still prevails in the oil industry (often by capitalized "BarG" and "BarA"). As gauge pressure is relative to the current ambient pressure, which may vary in absolute terms by about 50 mbar, "BarG" and "BarA" are not interconvertible. Fuller descriptions such as "gauge pressure of 2 bars" or "2-bar gauge" are recommended.<ref name="BSI" /><ref>{{cite web|url=http://www.npl.co.uk/reference/faqs/what-do-the-letters-'g'-and-'a'-denote-after-a-pressure-unit-(faq-pressure)|title=What do the letters 'g' and 'a' denote after a pressure unit? (FAQ - Pressure) : FAQs : Reference : National Physical Laboratory|access-date=7 February 2016|url-status=live|archive-url=https://web.archive.org/web/20160407204714/http://www.npl.co.uk/reference/faqs/what-do-the-letters-'g'-and-'a'-denote-after-a-pressure-unit-(faq-pressure)|archive-date=7 April 2016}}</ref>

== See also ==
* [[Centimetre or millimetre of water]]
* {{section link|Conversion of units|Pressure or mechanical stress}}
* [[List of metric units]]
* [[Metric prefix]]
* {{section link|Orders of magnitude (pressure)|100kPa}}

== References ==
: {{citizendium|title=Bar (unit)}}
{{reflist|30em}}

== External links ==
* [http://www.bipm.org/en/publications/si-brochure/table8.html Official SI website: Table 8. Non-SI units accepted for use with the SI]
* [https://www.wpc.ncep.noaa.gov/sfc/90fwbg.gif US government atmospheric pressure map showing atmospheric pressure in mbar]

[[Category:Units of pressure]]
[[Category:Non-SI metric units]]