Bar (unit)

The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at 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 cgs units, but they are accepted for use with the SI. The bar is widely used in descriptions of pressure because it is only about 1% smaller than the atmosphere, and is legally recognized in countries of the European Union.

Except for the power of ten, the definition of bar fits in the sequence of SI pressure units (Pa, kPa, MPa), namely, 1 bar ≡ 100,000 Pa = 100 kPa = 0.1 MPa. This is in contrast to the well-known unit of pressure, atmosphere, which now is defined to be 1.01325 bar exactly.

The bar and the millibar were introduced by the British meteorologist William Napier Shaw in 1909. He was the director of the Meteorological Office in London from 1907 to 1920.

Barg is a unit of gauge pressure, i.e., pressure in bars above ambient or atmospheric pressure; see Absolute pressure and gauge pressure below.

Definition

1 atm (atmosphere) = 1.01325 bar

1 kbar = 1000 bar = 100,000 kPa = 1,000,000,000 dyn/cm² = 100 MPa = 0.1 GPa

1 bar = 100 kPa = 1,000,000 dynes per square centimeter (baryes) = 0.987 atm (atmospheres) = 14.5038 psi = 29.53 inHg = 750.06 torr

1 dbar = 0.1 bar = 10 kPa = 100,000 dyn/cm²

1 cbar = 0.01 bar = 1 kPa

1 mbar = 0.001 bar = 0.1 kPa = 1 hPa (hectopascal) = 1,000 dyn/cm²

1 Torr = 1.33 mbar = 1mm Hg = 133.3pa

Example conversion: 1 atm pressure = 1.01325 bar = 1.01325 x 10⁵ Pa = 1.01325 x 10⁵ N/m²

Origin

The word bar has its origin in the 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.

The bar and millibar were introduced by Sir Napier Shaw in 1909 and internationally adopted in 1929. one Torr is equal to 1.33 mbar(Torr =1.33mbar)

Usage

Atmospheric air pressure is often given in millibars where "standard" sea level pressure (1 atm) is defined as 1013.25 mbar (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 Environment Canada uses kilopascals and hectopascals on their weather maps. In contrast, Americans are familiar with the use of the millibar in US reports of hurricanes 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 metres. Specifically, an increase of 1 decibar occurs for every 1.019716 metre increase in depth close to the surface. 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 the United Kingdom in terms of the bar.

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

The kilobar is commonly used in geological systems, particularly in experimental Petrology.

Absolute pressure and gauge pressure

Bourdon tube pressure gauges, 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 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 bar".

See also

Conversion of units

Orders of magnitude (pressure)

References

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External links

Official SI website: Table 8. Non-SI units accepted for use with the SI

Conversion factors from bar to various pressure units

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