Editing Bar

{{Wikipedia|Bar (unit)}}


The '''bar''' is a [[Unit of measurement|unit]] of [[pressure]] defined as 100 [[kilopascal]]s. It is about equal to the atmospheric pressure on Earth at {{WPlink|sea level}}.

Other units derived from the bar are the '''megabar''' (symbol: '''Mbar'''), '''kilobar''' (symbol: '''kbar'''), '''decibar''' (symbol: '''dbar'''), '''centibar''' (symbol: '''cbar'''), and '''millibar''' (symbol: '''mbar''' or '''mb'''). They are not [[SI]] units, nor are they [[Absolute centimeter-gram-second system|cgs]] units, but they are accepted for use with the [[SI]].<ref>{{SIbrochure8th|page=127}}.</ref> The bar is widely used in descriptions of pressure because it is only about 1% smaller than the [[atmosphere]] (symbol: atm), whicH now is defined to be {{val|1.01325|u=bar}} exactly. The bar is legally recognized in countries of the {{WPlink|European Union}}.<ref name=BSI>{{WPlink|British Standard}} BS 350:2004 ''Conversion Factors for Units''</ref> 

In 1985 the {{WPlink|IUPAC}} recommended that the standard for atmospheric pressure should be harmonized to 100,000&nbsp;Pa = 1&nbsp;[[bar]] = 750&nbsp;[[Torr]]. The same definition is used in the compressor and the pneumatic tool industries (ISO 2787).

The bar and the millibar were introduced by the British {{WPlink|meteorologist}} {{WPlink|William Napier Shaw}} in 1909. He was the director of the {{WPlink|Meteorological Office}} in {{WPlink|London}} from 1907 to 1920.<ref>[http://www.encyclopedia.jrank.org/Cambridge/entries/061/Sir-William-Napier-Shaw.html Sir William Napier Shaw]</ref>

'''Barg''' is a unit of [[gauge pressure]], i.e., pressure in bars above ambient or atmospheric pressure; see [[#Absolute pressure and gauge pressure|absolute pressure and gauge pressure]] below.

==Definition and conversion==
The bar is defined using the SI unit [[pascal]], namely: {{val|1|u=bar}} ≡ {{val|100000|ul=Pa}}.
{{val|1|u=bar}} is therefore equal to:
* {{val|100|u=kPa}}
* {{val|1000000|u=[[dyne|dyn]]/[[centimeter|cm]]<sup>2</sup>}} ([[barye]]s)
* {{val|0.987|u=atm}}
* {{val|14.5038|ul=psi}}
* {{val|29.53|u=[[Inches of Mercury|inHg]]}}
* {{val|750.06|u=[[torr]]}}
* {{val|1|e=5|u=[[N]]/m<sup>2</sup>}}

==Origin==
The word ''bar'' has its origin in the {{WPlink|Greek language|Greek}} word ''βάρος'' (baros), meaning [[weight]]. Its official symbol is "bar"; the earlier "b" is now deprecated, but still often seen especially in "mb" rather than the proper "mbar" for millibars.

== Usage ==
[[Atmospheric pressure|Atmospheric air pressure]] is often given in millibars where "standard" {{WPlink|sea level}} pressure (1 atm) is defined as 1013.25 [[Millibar|mbar]] ([[hectopascal|hPa]]), equal to 1.01325 bar. Despite millibars not being an [[SI]] unit, meteorologists and weather reporters worldwide have long measured air pressure in millibars. After the advent of SI units, some meteorologists began using hectopascals (symbol hPa) which are numerically equivalent to millibars. For example, the weather office of {{WPlink|Environment Canada}} uses kilopascals and hectopascals on their weather maps.<ref>[http://www.weatheroffice.gc.ca/jet_stream/index_e.html Environment Canada Weather Map]</ref><ref>[http://www.weatheroffice.gc.ca/canada_e.html Weather - Environment Canada]</ref> In contrast, Americans are familiar with the use of the millibar in US reports of {{WPlink|hurricane}}s and other cyclonic storms.

In water, there is an approximate numerical equivalence between the change in pressure in decibars and the change in depth from the sea surface in [[meter]]s.  Specifically, an increase of 1 decibar occurs for every 1.019716 meter increase in depth close to the surface. As a result, decibars are commonly used in {{WPlink|oceanography}}.

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 the automotive field, {{WPlink|turbocharger}} boost is often described in bars in the metric part of the world (i.e. Europe)

{{WPlink|Unicode}} has a character for "mb": {{unicode|㏔}}, U+33D4, but it exists only for compatibility with legacy Asian encodings. There is also a character "bar": {{unicode|㍴}}, U+3374.

The kilobar is commonly used in geological systems, particularly in experimental {{WPlink|petrology}}.

== Absolute pressure and gauge pressure ==
{{WPlink|Bourdon tube}} [[pressure gauge]]s, vehicle tire gauges, and many other types of pressure gauges are zero-referenced to atmospheric pressure, which means that they measure the pressure above atmospheric pressure (which is around 1 bar); this is ''gauge'' pressure and is often referred to in writing as ''barg'' (spoken "bar gauge"). In contrast, absolute pressures are zero referenced to a complete vacuum and when expressed in bar are often referred to as ''bara''. Thus, the absolute pressure of any system is the gauge pressure of the system plus atmospheric pressure. The usage of bara and barg is now deprecated, with qualification of the physical property being preferred, e.g., "The gauge pressure is 2.3 bar; the absolute pressure is 3.3 SAM bar".<ref name=BSI/>

In the {{WPlink|United States}}, where pressures are still often expressed in pounds per square inch (symbol ''psi''), gauge pressures are referred to as ''psig'' and absolute pressures are referred to as ''psia''. Gauge pressure is also sometimes spelled as ''gage pressure''.

Sometimes, the context in which the word pressure is used helps to identify it as meaning either the absolute or gauge pressure. However, in truth, whenever a pressure is expressed in any units (bar, Pa, psi, atm, etc.), it should be denoted in some manner as being either absolute or gauge pressure to avoid any possible misunderstanding. One recommended way of doing so is to spell out what is meant, for example as ''bar gauge'' or ''kPa absolute''.<ref>[http://www.npl.co.uk/server.php?show=ConWebDoc.1160 FAQ] (from the website of the National Physics Laboratory, United Kingdom)</ref>

== See also ==
* [[Conversion of units#Pressure_or_mechanical_stress|Conversion of units]]
* [[Orders of magnitude (pressure)#100kPa|Orders of magnitude (pressure)]]

== References ==
:{{citizendium|title1=Bar_(unit)|title2=Bar (unit)}}
{{reflist}}

== External lin[http://www.bipm.org/en/si/si_brochure/chapter4/table8.html Official SI website: Table 8. Non-SI units accepted for use with the SI] ==
*[http://www.convert-me.com/en/convert/units/pressure/pressure.bar.en.html Conversion factors from bar to various pressure units]
*[https://unitconverter.net/pressure-conversion Pressure Conversion]
{{Pressure Units}}
[[Category:Units of pressure or stress]]
[[Category:Non-SI metric units]]