Bar (unit)

"Kbar" redirects here. For the knife, see KA-BAR.
An aluminium cylinder
(5 mm or 0.197 in thickness) after 700 bar (10 ksi) pressure.

The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is exactly equal to 100000 Pa and is slightly less than the average atmospheric pressure on Earth at sea level.[1][2]

The bar and the millibar were introduced by the Norwegian meteorologist Bjerknes, who was a founder of the modern practice of weather forecasting.[3]

Use of the bar is deprecated by some bodies in some fields. The BIPM lists it as one of the "non-SI units [that authors] should have the freedom to use" but does not include it among the "Non-SI units accepted for use with the SI",.[1] The NIST deprecates its use in the US except for "limited use in metrology" and lists it as one of several units that "must not be introduced in fields where they are not presently used."[4] The IAU also lists it under "Non-SI units and symbols whose continued use is deprecated."[5] As of 2004, the bar is legally recognized in countries of the European Union.[2]

Units derived from the bar include the megabar (symbol: Mbar), kilobar (symbol: kbar), decibar (symbol: dbar), centibar (symbol: cbar), and millibar (symbol: mbar or mb). The notation bar(g), though deprecated by various bodies,[2] represents gauge pressure, i.e., pressure in bars above ambient or atmospheric pressure.

Definition and conversion

The bar is defined using the SI unit pascal, namely: 1 bar100000 Pa. 1 bar is therefore equal to:

• 100 kPa (in SI units)
• 1×105 N/m2 (alternative representation in SI units)
• 1000000 Ba (barye) (in cgs units)

and approximately equal to

Origin

The word bar has its origin in the Greek word βάρος (baros), meaning weight. The unit's official symbol is bar; the earlier symbol b is now deprecated and conflicts with the use of b as a unit symbol to denote the barn, but it is still encountered, especially as mb (rather than the proper mbar) to denote the millibar. The word bar had already been used as a unit name between 1793 and 1795, in an early version of the metric system, for a unit of weight.[6]

Usage

Atmospheric air pressure is often given in millibars where standard sea level pressure is defined as 1013 mbar, 101.3 (kPa), or 1.013 bar. Despite the millibar not being an SI unit, meteorologists and weather reporters worldwide have long measured air pressure in millibars as the values are convenient. After the advent of SI units, some meteorologists began using hectopascals (symbol hPa) which are numerically equivalent to millibars; 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.[7][8] In contrast, Americans are familiar with the use of the millibar in US reports of hurricanes and other cyclonic storms.

In fresh water, there is an approximate numerical equivalence between the change in pressure in decibars and the change in depth from the water surface in metres. 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 the geopotential anomaly the pressure can be converted into meters depth according to an empirical formula (UNESCO Tech. Paper 44, p. 25).[9] As a result, decibars are commonly used in 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, turbocharger boost is often described in bars in the metric part of the world (i.e. outside the USA).

Unicode has characters for "mb" (㏔, U+33D4) and "bar" (㍴, U+3374), 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 experimental petrology.

"Bar(a)" and "bara" are sometimes used to indicate absolute pressures and "bar(g)" and "barg" for gauge pressures. This usage is deprecated and fuller descriptions such as "gauge pressure of 2 bar" or "2 bar gauge" are recommended.[2][10]

References

Pressure units
Pascal Bar Technical atmosphere Standard atmosphere Torr Pounds per square inch
(Pa) (bar) (at) (atm) (Torr) (psi)
1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 9.8692×10−6 7.5006×10−3 1.450377×10−4
1 bar 105 ≡ 100 kPa

≡ 106 dyn/cm2

1.0197 0.98692 750.06 14.50377
1 at 9.80665×104 0.980665 ≡ 1 kp/cm2 0.9678411 735.5592 14.22334
1 atm 1.01325×105 1.01325 1.0332 1 760 14.69595
1 Torr 133.3224 1.333224×10−3 1.359551×10−3 ≡ 1/760 ≈ 1.315789×10−3 ≡ 1 Torr

≈ 1 mmHg

1.933678×10−2
1 psi 6.8948×103 6.8948×10−2 7.03069×10−2 6.8046×10−2 51.71493 ≡ 1 lbF /in2