Bar (unit)

Bar (unit)

The bar is a non-SI unit of pressure, defined by the IUPAC as exactly equal to 100,000 Pa. It is about equal to the atmospheric pressure on Earth at sea level, and since 1982 the IUPAC has recommended that the standard for atmospheric pressure should be harmonized to 100,000 Pa = 1 bar ≈ 750.0616827 Torr. The same definition is used in the compressor and the pneumatic tool industries (ISO 2787).

The bar and the millibar were introduced by the British meteorologist William Napier Shaw in 1909, while he was the director of the Meteorological Office in London.

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 or cgs units, but they are accepted for use with the SI. The bar is legally recognized in countries of the European Union.

Bar(g) is a unit of gauge pressure, i.e., pressure in bars above ambient or atmospheric pressure; see absolute pressure and gauge pressure below.