Units derived from the bar include themegabar (symbol:Mbar),kilobar (symbol:kbar),decibar (symbol:dbar),centibar (symbol:cbar), andmillibar (symbol:mbar).
The wordbar has its origin in theAncient Greek wordβάρος (baros), meaningweight. The unit's official symbol isbar;[citation needed] the earlier symbolb is now deprecated and conflicts with the uses ofb denoting the unitbarn orbit, but it is still encountered, especially asmb (rather than the propermbar) to denote the millibar. Between 1793 and 1795, the wordbar was used for a unit of mass (equal to the moderntonne) in an early version of the metric system.[9]
Atmospheric air pressure wherestandard atmospheric pressure is defined as 1013.25 mbar, 101.325 kPa,1.01325 bar, which is about 14.7pounds per square inch. Despite the millibar not being anSI 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 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, theMeteorological Service of Canada uses kilopascals and hectopascals on their weather maps.[10][11] In contrast, Americans are familiar with the use of the millibar in US reports ofhurricanes and other cyclonic storms.[12][13]
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 an increase in depth of1.019716 m. In sea water with respect to the gravity variation, the latitude and thegeopotential anomaly the pressure can be converted into metres' depth according to an empirical formula (UNESCO Tech. Paper 44, p. 25).[14] 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 ofvacuum and invacuum engineering, residual pressures are typically given in millibar, althoughtorr ormillimetre of mercury (mmHg) were historically common.
Pressures resulting fromdeflagrations are often expressed in units of bar.[15]
In the automotive field,turbocharger boost is often described in bar outside the United States.Tire pressure is often specified in bar. Inhydraulic 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" (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.[2][16]
^International Astronomical Union Style Manual. Comm. 5 in IAU Transactions XXB, 1989, Table 6
^Allen, William H. (1965-01-01)."Dictionary of Technical Terms for Aerospace Use".NASA Special Publication.bar. … Some writers have used bar as equivalent tobarye (1 dyne per square centimeter). …barye. … Sometimes calledbar ormicrobar. …microbar (abbr μb). … In British literature the termbarye has been used. … Unfortunately, the bar was once used in acoustics to mean 1 dyne per square centimeter, but this is no longer correct.
