| Decapascals (daPa) | Nanopascals (nPa) |
|---|---|
| 1 Decapascal | 10000000000 nPa |
| 2 Decapascals | 20000000000 nPa |
| 3 Decapascals | 30000000000 nPa |
| 4 Decapascals | 40000000000 nPa |
| 5 Decapascals | 50000000000 nPa |
| 10 Decapascals | 100000000000 nPa |
| 20 Decapascals | 200000000000 nPa |
| 25 Decapascals | 250000000000 nPa |
| 50 Decapascals | 500000000000 nPa |
| 100 Decapascals | 1000000000000 nPa |
| Reference | Decapascals (daPa) | Nanopascals (nPa) |
|---|---|---|
| Atmospheric pressure at sea level | 10132.5 daPa | 1.01325 × 1014 nPa |
| Healthy blood pressure (120 mmHg) | 1600 daPa | 1.6 × 1013 nPa |
| A car tyre | 22000 daPa | 2.2 × 1014 nPa |
| A racing bicycle tyre | 60000 daPa | 6 × 1014 nPa |
The decapascal is a unit of pressure equal to ten pascals. Its symbol is daPa. Deca is the least used prefix in the metric system almost everywhere, but the decapascal is an exception, because one clinical discipline adopted it and has kept it: audiology measures the pressure inside the middle ear in decapascals.
Tympanometry is the test in question. A probe seals the ear canal and varies the air pressure in it while measuring how much sound the eardrum reflects, and the resulting curve is plotted against pressure in decapascals. The sweep runs from about plus 200 to minus 400, and the position of the peak shows the pressure at which the eardrum moves most freely, which is normally the pressure of the middle ear itself.
That single graph carries a great deal of clinical information. A peak near zero means the middle ear is at the same pressure as the room, which is the healthy state. A peak displaced towards minus 200 or beyond suggests the Eustachian tube is not equalising properly, and a flat curve with no peak at all suggests fluid behind the eardrum. Each of those readings is a position on a decapascal scale.
The unit was chosen for the same reason the hectopascal was chosen in meteorology. Middle-ear pressure had long been recorded in millimetres of water, and one millimetre of water is 0.98 decapascals, so switching to the metric unit left every clinical number and every published reference range almost unchanged. A change of unit that shifts the figures by two per cent is one that practitioners will actually accept.
Outside audiology the decapascal is essentially unused. Ventilation and building airtightness work at similar pressures, an airtightness test being run at fifty pascals, but those figures are written in pascals, and everything above them moves to hectopascals or kilopascals. The gap between one and a hundred pascals is otherwise unclaimed.
For scale, ten pascals is the pressure a light breeze exerts on a wall, or the difference between the two ends of a well-sealed corridor when a door opens. It is also roughly the pressure change from riding a lift up one floor, which is why ears sometimes need clearing in a tall building.
One decapascal equals 10 pascals, 0.1 hectopascals, about 1.02 millimetres of water, or about 0.00145 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.