Bar (unit)

The bar is a measurement unit of pressure, equal to ten newtons per square centimetre. The word bar is of Greek origin, báros meaning weight.

The preferred SI unit of pressure is the pascal (Pa), which is one newton per square metre. 1 bar is equal to 100,000 Pa. The bar is still widely used by the general public and in industry because 1 bar is so close to atmospheric pressure and for everyday purposes can be taken as equivalent.

Atmospheric air pressure is often given in millibars, where "standard" sea level pressure is equal to 1013.2 millibars (1.0132 bar). If the measure is given in SI units, the hectoPascal (hPa) gives the same number as the millibar measure, i.e. 1000 mb = 1000 hPa. Thus the millibar and the hectopascal are directly interchangeable. This follows from the definition of the units. In SI units the preferred multipliers are 1000's, hence the preferred unit should be the kiloPascal (kPa; 1 bar = 100 kPa) and it is occasionally used in meteorological publications. However hPa remains the most common, for historical reasons.

In everyday use pressure is often measured with reference to atmospheric pressure: this is gauge pressure and denoted by barg, spoken "bar gauge". For example if someone says that their car tyres are pressured up to 2.3 bar they actually mean barg: the pressure in the tyre is really 3.3 bar, but only 2.3 bar above atmospheric! When absolute pressure is desired then it's denoted bara, or "bar absolute".

The Boyle-Mariotte Law defines that an air filled balloon has a pressure of 1 bar when at sea level. Submerging the balloon will increase the air pressure one bar every 10 metres; at 10 metres = 2 bar, 20 metres = 3 bar, 30 metres = 4 bar.

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