Bar (unit)
From Freepedia
A bar (symbol bar) is a unit of pressure. It is not an SI unit. It is accepted (although discouraged) for use with the SI. The bar is still widely used in descriptions of pressure because it is about the same as atmospheric pressure.
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Definition
The bar is defined as:
- 1 bar = 100 000 pascals (Pa) = 100 kPa = 1 000 000 dynes/square centimetre
Conversions
- 1 bar = 100 kN/m²
Origin
The word bar finds its origin in the Greek word báros, meaning weight. Its official symbol is "bar"; the earlier "b" is now deprecated, but still often seen especially as "mb" rather than the proper "mbar" for millibars.
Discussion
Atmospheric air pressure is often given in millibars where "standard" sea level pressure is defined as 1013.25 mbar (hPa), equal to (1.01325 bar). The millibar is also not an SI unit of measure; however, it is still used locally in meteorology when describing atmospheric pressure. The SI unit is the pascal (Pa), with 1 mbar = 100 Pa = 1 hPa = 0.1 kPa. Meteorologists worldwide have long measured air pressure in millibars. After the introduction of SI units, many preferred to preserve the customary pressure figures. Therefore, some continue to use millibars under their own name, while others use hPa (which are equivalent to millibars) so they could stick to the same numeric scale. Similar pressures are given in kilopascals in practically all other fields where the hecto prefix is hardly ever used. In Canadian weather reports, the normal is kPa.
In everyday use, pressure is often measured with reference to atmospheric pressure. This is gauge pressure and denoted by barg, often written with no spaces, spoken "bar gauge", and sometimes using symbols such as 'bar(g)'. For example, if someone says that their car tires are pressured up to 2.3 bar they actually mean bars gauge: the pressure in the tire is really 3.3 bar, but only 2.3 bar above atmospheric, which is the scale a tire gauge would read. When absolute pressure is desired, it is sometimes denoted 'bara' or 'bar(a)' for "bar absolute". The alteration of the symbols of units of measure for this purpose is now deprecated.
