| Atmospheres (atm) | Nanopascals (nPa) |
|---|---|
| 1 Atmosphere | 101325000000000 nPa |
| 2 Atmospheres | 202650000000000 nPa |
| 3 Atmospheres | 303975000000000 nPa |
| 4 Atmospheres | 405300000000000 nPa |
| 5 Atmospheres | 506625000000000 nPa |
| 10 Atmospheres | 1.01325 × 1015 nPa |
| 20 Atmospheres | 2.0265 × 1015 nPa |
| 25 Atmospheres | 2.533125 × 1015 nPa |
| 50 Atmospheres | 5.06625 × 1015 nPa |
| 100 Atmospheres | 1.01325 × 1016 nPa |
| Reference | Atmospheres (atm) | Nanopascals (nPa) |
|---|---|---|
| Atmospheric pressure at sea level | 1 atm | 1.01325 × 1014 nPa |
| Healthy blood pressure (120 mmHg) | 0.157908 atm | 1.6 × 1013 nPa |
| A car tyre | 2.17123 atm | 2.2 × 1014 nPa |
| A racing bicycle tyre | 5.92154 atm | 6 × 1014 nPa |
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 nanopascal is a unit of pressure equal to one billionth of a pascal. Its symbol is nPa. It marks the far end of the pressure scale, the region where the idea of pressure as a push on a surface stops being useful and becomes a statement about how few particles are present.
The clearest home for the unit is extreme high vacuum. Ordinary laboratory vacuum reaches millipascals; ultra-high vacuum, used for surface physics and for the beam pipes of particle accelerators, reaches micropascals. Below that lies extreme high vacuum at nanopascals and lower, and reaching it takes a sealed chamber, hours of baking at two hundred degrees to drive gas out of the metal itself, and pumps that trap molecules rather than push them.
At those pressures a chamber is not empty. A nanopascal still contains something like a quarter of a million molecules per cubic centimetre, which sounds like a great many until you compare it with the twenty-five billion billion in the same volume of room air. What matters is not the count but the mean free path: a molecule now travels thousands of kilometres before striking another, so it hits the walls long before it meets a neighbour.
That is exactly the point. Surface science needs a sample to stay clean for the length of an experiment, and at ordinary pressures a fresh surface is covered by a layer of adsorbed gas in about a nanosecond. At a nanopascal the same surface stays clean for days. The vacuum is not there to remove air but to buy time.
Space provides the natural comparison. Low Earth orbit is around a micropascal, still dense enough that the atmosphere drags on satellites and eventually pulls them down. Interplanetary space is nanopascals. Interstellar space is far lower still, roughly a femtopascal, which no terrestrial pump has ever matched — the best laboratory vacuums are still denser than the space between the stars.
Radiation pressure lands in similar territory. Sunlight falling on a perfectly absorbing surface at Earth's distance exerts about 4.5 micropascals, and at the distance of the outer planets it falls to nanopascals. Solar sails work with these numbers, which is why they must be enormous and light to gather a usable force.
One nanopascal equals 0.000000001 pascals, 0.001 micropascals, 1,000 picopascals, or about 0.000000000000145 pounds per square inch.