| Atmospheres (atm) | Micropascals (µPa) |
|---|---|
| 1 Atmosphere | 101325000000 µPa |
| 2 Atmospheres | 202650000000 µPa |
| 3 Atmospheres | 303975000000 µPa |
| 4 Atmospheres | 405300000000 µPa |
| 5 Atmospheres | 506625000000 µPa |
| 10 Atmospheres | 1013250000000 µPa |
| 20 Atmospheres | 2026500000000 µPa |
| 25 Atmospheres | 2533125000000 µPa |
| 50 Atmospheres | 5066250000000 µPa |
| 100 Atmospheres | 10132500000000 µPa |
| Reference | Atmospheres (atm) | Micropascals (µPa) |
|---|---|---|
| Atmospheric pressure at sea level | 1 atm | 1.01325 × 1011 µPa |
| Healthy blood pressure (120 mmHg) | 0.157908 atm | 1.6 × 1010 µPa |
| A car tyre | 2.17123 atm | 2.2 × 1011 µPa |
| A racing bicycle tyre | 5.92154 atm | 6 × 1011 µPa |
The atmosphere is a unit of pressure defined as exactly 101,325 pascals. Its symbol is atm. It is not a measurement of the air at any particular place or moment but a fixed reference value, chosen in 1954 by the General Conference on Weights and Measures to sit close to the average pressure at sea level in temperate latitudes.
Its usefulness is that it turns pressure into a ratio. Saying that a vessel holds gas at six atmospheres says immediately that the pressure inside is six times what is outside, which is the fact that determines whether a container will burst, how much gas it holds and how fast it will empty. No other pressure unit carries that meaning in the number itself.
Chemistry built its early framework on it. Gas laws were written with pressures in atmospheres, the gas constant had a value tailored to litres and atmospheres, and standard conditions for tabulating thermodynamic data were defined at one atmosphere. In 1982 the International Union of Pure and Applied Chemistry changed the standard state to exactly one bar, so modern tables use bar and older ones use atmospheres, and the difference of 1.3 per cent matters in careful work.
Diving and hyperbaric medicine still count in it. Pressure underwater is described in atmospheres absolute, a diver at thirty metres experiencing about four, and hyperbaric oxygen therapy is delivered at two to three atmospheres absolute in a sealed chamber. Because gas volume and gas uptake by the body both scale with absolute pressure, the unit is doing real physiological work rather than merely reporting a reading.
The atmosphere also fixes the older mercury scales. One atmosphere is exactly 760 millimetres of mercury by definition, which is what ties the torr and the millimetre of mercury to the metric system, and it is 29.92 inches of mercury, the setting an American pilot dials into an altimeter as standard.
A related unit, the technical atmosphere of one kilogram-force per square centimetre, is close but not equal at 98,066.5 pascals. Documents from Japan, Korea and eastern Europe sometimes use it, and mistaking one for the other introduces an error of about three per cent.
One atmosphere equals 101,325 pascals, 1.01325 bar, 760 millimetres of mercury, or about 14.696 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.