| Nanopascals (nPa) | Pounds per square inch (psi) |
|---|---|
| 1 Nanopascal | 1.4503773773 × 10-13 psi |
| 2 Nanopascals | 2.9007547546 × 10-13 psi |
| 3 Nanopascals | 4.35113213191 × 10-13 psi |
| 4 Nanopascals | 5.80150950921 × 10-13 psi |
| 5 Nanopascals | 7.25188688651 × 10-13 psi |
| 10 Nanopascals | 1.4503773773 × 10-12 psi |
| 20 Nanopascals | 2.9007547546 × 10-12 psi |
| 25 Nanopascals | 3.62594344326 × 10-12 psi |
| 50 Nanopascals | 7.25188688651 × 10-12 psi |
| 100 Nanopascals | 1.4503773773 × 10-11 psi |
| Reference | Nanopascals (nPa) | Pounds per square inch (psi) |
|---|---|---|
| Atmospheric pressure at sea level | 1.01325 × 1014 nPa | 14.6959 psi |
| Healthy blood pressure (120 mmHg) | 1.6 × 1013 nPa | 2.3206 psi |
| A car tyre | 2.2 × 1014 nPa | 31.9083 psi |
| A racing bicycle tyre | 6 × 1014 nPa | 87.0226 psi |
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.
The pound per square inch is a unit of pressure equal to 6894.76 pascals. Its symbol is psi. It is the force of one pound spread over one square inch, and it is the working pressure unit of the United States and of a great deal of equipment sold worldwide, because so much hydraulic and pneumatic machinery is designed to American specifications.
Its most familiar appearance is on tyres. A car is inflated to between 30 and 35 pounds per square inch, a bicycle tyre to 80 or more, and a heavy truck to around 100. Because the pressure and the contact patch together carry the vehicle, a small change in the figure changes handling, fuel consumption and tyre life, which is why it is printed inside the door of every car.
Compressed systems use the same scale. A workshop air line runs at 90 to 120 pounds per square inch, a domestic water supply arrives at 40 to 80, hydraulic machinery works at around 3000, and a full scuba cylinder holds 3000 or more. The convenient coincidence is that atmospheric pressure is 14.7 pounds per square inch, so a reading of 30 means roughly three times atmospheric in absolute terms.
That absolute distinction is written into the symbol. Engineers write psig for gauge pressure, measured above the surrounding air, and psia for absolute pressure, measured from vacuum. A tyre at 32 psig is at 46.7 psia, and confusing the two in a calculation of gas quantity or vessel strength produces an error of one atmosphere, which at low pressures is most of the answer.
Aviation uses it for cabin differential. An airliner maintains a pressure difference of about eight to nine pounds per square inch between cabin and outside air at cruising altitude, and the fuselage is designed and fatigue-tested against that repeated loading. The figure sets the effective cabin altitude and therefore how passengers feel at the end of a long flight.
Converting to metric is exact but not memorable, since a pound per square inch is 6894.76 pascals. The two rules of thumb worth carrying are that one bar is about 14.5 pounds per square inch and that one pound per square inch is about seven kilopascals; both are close enough for judging a gauge and neither is close enough for a design calculation.
One pound per square inch equals 6894.76 pascals, about 0.0689 bar, about 6.89 kilopascals, or about 0.068 atmospheres.