Bar (unit)

Thebaris ametric unitofpressuredefined as 100,000Pa(100 kPa), though not part of theInternational System of Units(SI). A pressure of 1 bar is slightly less than the current averageatmospheric pressureon Earth atsea level(approximately 1.013 bar).By thebarometric formula, 1 bar is roughly the atmospheric pressure on Earth at an altitude of 111 metres at 15 °C.
The bar and the millibar were introduced by the Norwegian meteorologistVilhelm Bjerknes, who was a founder of the modern practice ofweather forecasting, with the bar defined as onemegadyneper squarecentimeter.
TheSI brochure, despite previously mentioning the bar,now omits any mention of it.The bar has been legally recognised in countries of theEuropean Unionsince 2004.The USNational Institute of Standards and Technology(NIST)deprecatesits use except for "limited use inmeteorology" and lists it as one of several units that "must not be introduced in fields where they are not presently used".TheInternational Astronomical Union(IAU) also lists it under "Non-SI units and symbols whose continued use is deprecated".
Units derived from the bar include themegabar(symbol:Mbar),kilobar(symbol:kbar),decibar(symbol:dbar),centibar(symbol:cbar), andmillibar(symbol:mbar).
The bar is defined using theSI derived unit,pascal:1 bar≡ 100,000 Pa ≡ 100,000 N/m.
Thus,1 baris equal to:
and 1 bar is approximately equal to:
1millibar(mbar) is equal to:
The wordbarhas its origin in theAncient Greekwordβάρος(baros), meaningweight. The unit's official symbol isbar;the earlier symbolbis now deprecated and conflicts with the uses ofbdenoting the unitbarnorbit, but it is still encountered, especially asmb(rather than the propermbar) to denote the millibar. Between 1793 and 1795, the wordbarwas used for a unit of mass (equal to the moderntonne) in an early version of the metric system.
Atmospheric air pressurewherestandard atmospheric pressureis defined as 1013.25 mbar, 101.325kPa, 1.01325 bar, which is about 14.7pounds per square inch.  Despite the millibar not being anSIunit, 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 usinghectopascals(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 ofEnvironment Canadauses kilopascals and hectopascals on their weather maps.In contrast, Americans are familiar with the use of the millibar in US reports ofhurricanesand other cyclonic storms.
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 inmetres. Specifically, an increase of 1 decibar occurs for every 1.019716 m increase in depth. In sea water with respect to the gravity variation, the latitude and thegeopotential anomalythe pressure can be converted into metres' depth according to an empirical formula (UNESCO Tech. Paper 44, p. 25).As a result, decibar is commonly used inoceanography.
In scuba diving, bar is also the most widely used unit to express pressure, e.g. 200 bar being a full standard scuba tank, and depth increments of 10 metre of seawater being equivalent to 1 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 ofvacuumand invacuum engineering, residual pressures are typically given in millibar, althoughtorrormillimeter of mercury(mmHg) were historically common.
Pressures resulting fromdeflagrationsare often expressed in units of bar.
In the automotive field,turbochargerboost is often described in bar outside the United States.Tire pressureis often specified in bar. Inhydraulic machinerycomponents 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.
Unicodehas characters for "mb" (U+33D4㏔SQUARE MB SMALL), "bar" (U+3374㍴SQUARE BAR) andミリバール(U+334A㍊SQUARE MIRIBAARU; "millibar" spelt inkatakana), 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 MPa, is commonly used in geological systems, particularly in experimentalpetrology.
The abbreviations "bar(a)" and "bara" are sometimes used to indicateabsolute pressures, and "bar(g)" and "barg" forgauge 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.