Bar unit: Encyclopedia - Bar unit

Bar (unit)

The bar (symbol bar) and the millibar (symbol mbar, also mb) are units of pressure. They are not SI units, but 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.

Bar unit - Definition

The bar and millibar are defined as:

• 1 bar = 100 000 pascals (Pa) = 1 000 000 dynes/square centimetre
• 1 mbar = 0.001 bar = 100 Pa = 1 000 dyn/cm²

Bar unit - 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.

The bar and millibar were introduced by Sir Napier Shaw in 1909 and internationally adopted in 1929.

Bar unit - 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). Despite millibars' not being an SI unit they are 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 hectopascals (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 particular, Canadian weather reports use kilopascals.

Americans are familiar with the millibar in US reports of hurricanes and other cyclonic storms, where lower central pressure generally means higher winds and a stronger storm.

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 units of measure for this purpose 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".

Unicode has a character for "mb": (㏔), but exists only for compatibility with legacy Asian encodings; it should not be used in new texts.

See also

• Conversion of units

Category: Units of pressure

Adapted from the Wikipedia article "Bar unit", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki