# Bar (unit)

Physical unit
Unit name bar
Unit symbol ${\ displaystyle \ mathrm {bar}}$, earlier: ${\ displaystyle \ mathrm {b}}$
Physical quantity (s) pressure
Formula symbol ${\ displaystyle p}$
dimension ${\ displaystyle {\ mathsf {M \; L ^ {- 1} \; T ^ {- 2}}}}$
In SI units ${\ displaystyle \ mathrm {1 \, bar = 1 \ cdot 10 ^ {5} \, {\ frac {kg} {m \ cdot s ^ {2}}}}}$ ${\ displaystyle \ mathrm {{} = 1 \ cdot 10 ^ {5} \, Pa}}$
Named after ancient Greek βαρύς barýs , German ‚difficult '

The bar (from ancient Greek βαρύς barýs , German 'heavy' ) is a legal unit for pressure in physics and technology (derived from the International System of Units SI ) . As a rule of thumb : 1 bar is roughly the air pressure on the earth's surface or the pressure of a water column 10 meters high. A millibar is about the water pressure by 1 cm of water column or 10 mm H 2 O .

On January 1, 1978, the bar replaced the units of measurement physical atmosphere ( atm for short ) and technical atmosphere ( at for short ) in the Federal Republic of Germany and Austria . The widespread abbreviation atü (atmosphere overpressure) was replaced by "bar overpressure". The International System of Units uses the Pascal as the unit of measurement for pressure. On January 1, 1980, the pascal was introduced as a binding unit of measurement for pressure in the GDR and the old units kp / cm² , mWS and at (atmosphere) or atm were no longer permitted. According to the EU directive 80/181 / EEC (unit directive ), the bar unit can still be used.

## Relation to other pressure units

The bar is defined as:

${\ displaystyle 1 \, \ mathrm {bar} = 10 ^ {5} \, \ mathrm {\ frac {kg} {m \ cdot s ^ {2}}} = 10 ^ {5} \, \ mathrm {\ frac {N} {m ^ {2}}} = 10 ^ {5} \, \ mathrm {Pa}}$

It differs from the “ Pascal ”, the pressure unit in the International System of Units (SI), by a factor of exactly 10 5 . However, the bar itself does not belong to the International System of Units (SI). At the 9th  General Conference on Weights and Measures (CGPM), prior to the creation of the SI, it was included in the list of spellings. In the 1st edition of the SI brochure (1970) up to the 6th edition (1991) it was listed as a "temporarily approved unit"; in the 7th edition (1997) and the 8th edition (2006) it is listed under Other non-SI units that are used for special purposes. It is no longer mentioned in the 9th edition (2019).

The normal pressure ( physical atmosphere , atm) is very close to 1 bar:

atm = 1.01325 bar = 1013.25 mbar.

The air pressure was previously given in millibars (mbar). Because of

1 mbar = 1 hPa = 100 Pa

Today the hectopascal (hPa) is often used.

## Absolute and relative pressure

While in the technical atmosphere a distinction was made between absolute pressure (unit ata) and overpressure (unit atü) in the unit symbol, the unit bar is used according to the SI guidelines for both absolute pressure and pressure differences (overpressure). If the tire pressure measuring device at the petrol station shows a pressure of 2.3 bar, this means the overpressure compared to the atmospheric pressure of approx. 1 bar. The pressure in the car tire is therefore 2.3 bar above atmospheric pressure. The absolute pressure is thus around 3.3 bar.