| Reference | Bars (bar) | Micropascals (µPa) |
|---|---|---|
| Atmospheric pressure at sea level | 1.01325 bar | 1.01325 × 1011 µPa |
| Healthy blood pressure (120 mmHg) | 0.16 bar | 1.6 × 1010 µPa |
| A car tyre | 2.2 bar | 2.2 × 1011 µPa |
| A racing bicycle tyre | 6 bar | 6 × 1011 µPa |
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 micropascal is a unit of pressure equal to one millionth of a pascal. Its symbol is µPa. It is one of the smallest pressure units in practical use, and it owes its place to a single number: 20 micropascals, the reference pressure against which every sound level in air is measured.
That figure is the threshold of hearing — roughly the quietest sound a healthy young ear can detect at a frequency of about 1,000 hertz. When acousticians defined the decibel scale for airborne sound they needed a fixed pressure to compare against, and they chose that threshold. Zero decibels means a sound pressure of 20 micropascals; every decibel figure quoted for a road, a machine or a concert is a ratio to that number.
The scale that follows is steep. Twenty decibels is ten times the reference pressure, 200 micropascals. Sixty decibels, ordinary conversation, is a thousand times it, or 20 millipascals. A hundred and twenty decibels, the threshold of pain, is a million times it: 20 pascals. The ear covers a range of a million to one in pressure, and the micropascal sits at the bottom of it.
Underwater acoustics uses the same unit but a different reference. Sound in water is referred to 1 micropascal rather than 20, because the threshold of human hearing is meaningless in the sea. That difference matters enormously: a level quoted in decibels underwater is not comparable to one in air, and the gap between the two conventions is about 26 decibels before any other correction. Sonar figures, whale-song measurements and shipping-noise studies all carry the re 1 µPa qualifier for that reason.
Outside acoustics the micropascal appears in vacuum science. A good high vacuum is around 100 micropascals, and ultra-high vacuum, the regime used for surface physics and particle accelerators, runs from a few micropascals down to nanopascals. Pumping a chamber to that level takes hours of baking to drive adsorbed gas off the walls.
The unit also shows up in radiation pressure and in the very small pressure differences that laboratory instruments resolve. Anything measured in micropascals is a measurement rather than a force anyone would feel, which is exactly what makes the reference-level convention so useful.
One micropascal equals 0.000001 pascals, 0.001 millipascals, 1,000 nanopascals, or about 0.000000000145 pounds per square inch.