Bar
A non-SI unit · quantity: pressure
01 · Definition
The Norwegian meteorologist Vilhelm Bjerknes proposed the bar in 1909 so that air pressure would come out as a number near one. The reckoning proved exact: the atmosphere at the surface of the Earth is 1.013 bar.
One bar equals a hundred thousand pascals exactly — that is a definition, not a measurement. The unit is not part of the SI, yet it is tolerated everywhere: gauges, tyres, hydraulics, compressors and the whole of European engineering are marked in bars. The reason is simple: the pascal is too small, and the pressure in a tyre would have to be written as 250 000 Pa instead of 2.5 bar. Meteorologists use the thousandth part — the millibar — and in newer reports the same number as hectopascals: 1013 hPa and 1013 mbar are one and the same.
A ten-metre layer of water presses exactly as hard as the atmosphere. Hence the diver’s rule: at twenty metres the pressure is three times that at the surface, and the air in the cylinder is used up three times as fast.
02 · Conversion
Enter a pressure — the sheet converts it
Pressure has the thickest zoo of units of all: the bar, the atmosphere, the technical atmosphere, psi, millimetres of mercury and of water, the torr. Not one of the factors is a round number — except the bar’s, and that is precisely why it drove the atmosphere out of engineering.
A gauge shows pressure above atmospheric by default: zero on the dial means the atmosphere. So «2.2 bar in the tyre» is 3.2 bar absolute, and in calculations that difference must not be skipped.
| Unit | Name | Value | Where it is met |
|---|---|---|---|
| bar | bar | 1 | gauges, tyres, hydraulics |
| mbar | millibar | 1000 | weather reports, vacuum equipment |
| kbar | kilobar | 0.001 | geophysics, diamond synthesis |
| Pa | pascal | 100000 | the SI unit |
| kPa | kilopascal | 100 | structural loads |
| MPa | megapascal | 0.1 | strength of materials |
The gauge shows pressure above atmospheric. The absolute value is greater by exactly one bar: 2.2 bar in the tyre means 3.2 bar inside.
03 · Orders of magnitude
from vacuum to the centre of the Earththe atmosphere and low-pressure equipment
04 · Measuring instruments
What measures bars
A curved tube straightens under pressure and moves the pointer. Cheap, reliable, and still the main instrument reading in bars on any shop floor.
A corrugated box is squeezed by atmospheric pressure. Weather reports read it in hectopascals, which are numerically the same as millibars.
A silicon diaphragm changes its resistance as it deforms. Fitted in cars, compressors and industrial controllers.
It measures absolute pressure and turns it into depth: every bar above atmospheric is ten metres of water. Decompression is reckoned from the same figure.
05 · Writing rules
bar in lower case, and always say gauge or absolute
The symbol is written in three lower-case letters. Only small and large prefixes are allowed: mbar, kbar; «decabar» is not used. The main rule is not about spelling: without the note «gauge» or «absolute» a figure in bars is ambiguous, and English practice even has separate marks for it, barg and bara.
The bar and the atmosphere differ by 1.3 % — negligible for a tyre, but for a gas cylinder or hydraulics at three hundred bar the difference is already four bar. In documentation one must not be put for the other.
06 · Neighbouring units
The bar is merely a multiple of the pascal written short, so the whole family of pressure stands nearby: the newton per square metre in the SI, psi in British practice, millimetres of mercury in medicine. Meteorology chose the hectopascal precisely because it turned out numerically equal to the familiar millibar.
In the Mariana Trench the pressure reaches 1086 bar — more than a tonne on every square centimetre of the vessel’s hull.
07 · Historical section
What pressure was measured in before the bar
The pressure of a mercury column 760 mm high at zero degrees. Rounding to the bar proved handier, and engineering went over to bars.
The kilogram-force per square centimetre. Gauges in the Soviet tradition were graduated exactly so, hence the habit of saying «atmospheres» for bars.
The first barometer was a mercury one, and medicine and vacuum technology still count in millimetres of mercury.
The dyne per square centimetre — the original unit of the CGS system, from which Bjerknes counted off a million and got the bar.
A unit invented to make weather charts easy to read
Bjerknes chose exactly a hundred thousand pascals not for any physical reason but so that synoptic charts would read faster: the isobars on them are labelled with three-digit numbers around a thousand, and any departure catches the eye at once. When the SI brought in the pascal, meteorologists faced a choice — redraw every chart or find a way round. The way found was elegant: the hectopascal is numerically equal to the millibar, so the reports did not change by a single digit; only the name of the unit did. The bar itself is excluded from the SI, yet it goes on working in engineering, where the pascal is awkward in scale.
Catalogue · units of measurement
A passport for every quantity
Seven SI base units, twenty-two derived ones with names of their own, and the non-SI units that neither engineering nor daily life can do without. Each gets its own sheet: definition, conversion, instruments, writing rules. In 36 languages.
Non-SI units
open the bar sheetSI base units
in terracotta — those that make up the barSI derived units with names of their own
twenty-two sheetsPassport language
36 languages. The symbol bar and the formulae are not translated — they are the same in every locale; the prose, the examples and the writing rules of each script are.