| Reference | Bars (bar) | Kilopascals (kPa) |
|---|---|---|
| Atmospheric pressure at sea level | 1.01325 bar | 101.325 kPa |
| Healthy blood pressure (120 mmHg) | 0.16 bar | 16 kPa |
| A car tyre | 2.2 bar | 220 kPa |
| A racing bicycle tyre | 6 bar | 600 kPa |
The bar is a unit of pressure equal to exactly 100,000 pascals, or 100 kilopascals. Its symbol is bar. It is not part of the International System of Units, but it is tolerated alongside it and it is used throughout European engineering, because it happens to sit almost exactly on standard atmospheric pressure: one atmosphere is 1.01325 bar.
That coincidence is the whole reason for its success. A pressure quoted in bar can be read as very nearly a number of atmospheres, so a reading of six bar in a compressed air line means about six times the pressure outside, which is a fact an engineer can use without arithmetic. The name comes from the Greek word for weight, and the unit was proposed by the Norwegian meteorologist Vilhelm Bjerknes around 1909.
Diving uses it because the sea obliges. Every ten metres of seawater adds almost exactly one bar, so a diver at twenty metres is under three bar of absolute pressure, counting the atmosphere above. Decompression tables, gas consumption and cylinder pressures all follow from that one relationship, and the whole practice of diving is easier to teach in bar than in any other unit.
Everyday machinery works at a few bar. Car tyres are inflated to between two and two and a half, workshop compressed air runs at six to eight, mains water arrives at three to six, and an espresso machine extracts at nine, a figure so standard that it appears on the front of the machine. Watches carry water resistance ratings in bar, where ten bar corresponds nominally to a hundred metres of depth.
The distinction between gauge and absolute pressure matters more here than anywhere. A tyre gauge reading 2.2 bar means 2.2 bar above the surrounding air, so the absolute pressure inside is 3.2. Industrial practice writes barg for gauge and bara for absolute, and mixing the two is a common source of error in specifications and in safety calculations.
Standards bodies have discouraged the bar for decades in favour of the pascal, without much effect. The kilopascal is the coherent alternative and is used in Canada, Australia and parts of Asia, but a hundred kilopascals reads less naturally than one bar, and habit has proved stronger than the recommendation.
One bar equals 100,000 pascals, 100 kilopascals, 1000 millibars, or about 14.5 pounds per square inch.
The kilopascal is a unit of pressure equal to one thousand pascals. Its symbol is kPa. It is a hundredth of a bar and ten hectopascals, and it is the coherent SI unit that engineering was meant to adopt when the pascal proved too small to write comfortably. Standard atmospheric pressure is 101.325 kilopascals.
Tyres are where most people meet it. Canada, Australia, New Zealand and much of Asia state recommended tyre pressures in kilopascals, so a car door placard reads 220 or 240 rather than 32 or 35 pounds per square inch. The two scales sit side by side on most modern gauges, and the conversion is close to seven kilopascals to the pound per square inch.
Civil engineering uses it for loads on ground and structure. The bearing capacity of a soil is quoted in kilopascals, a firm clay allowing perhaps 150 and a soft one much less, and wind loading on a facade is calculated in the same unit. Because a kilopascal is also a kilonewton per square metre, structural calculations move between force and pressure without a conversion factor.
Medicine uses it in one important place. Arterial blood gases are reported in kilopascals in Britain, Ireland and much of Europe, where a healthy oxygen partial pressure is eleven to thirteen, while the United States reports the same measurement in millimetres of mercury as eighty to a hundred. The two numbers describe identical blood, and a clinician reading a foreign chart must know which convention it follows.
Vacuum work counts downward in it. A rough vacuum is a few kilopascals absolute, a domestic vacuum cleaner pulls perhaps twenty kilopascals below atmospheric, and freeze-drying operates well under one. Stating vacuum as an absolute pressure in kilopascals avoids the ambiguity of describing it as a negative gauge pressure.
Its awkwardness is only rhetorical. A hundred kilopascals is exactly one bar and reads less neatly, which is why the bar has held on in Europe despite decades of official preference for the pascal. Where the kilopascal has won, as on Canadian and Australian tyre placards, it has done so because the number is printed rather than spoken.
One kilopascal equals 1000 pascals, 0.01 bar, 10 hectopascals, or about 0.145 pounds per square inch.