| Kilopascals (kPa) | Nanopascals (nPa) |
|---|---|
| 1 Kilopascal | 1000000000000 nPa |
| 2 Kilopascals | 2000000000000 nPa |
| 3 Kilopascals | 3000000000000 nPa |
| 4 Kilopascals | 4000000000000 nPa |
| 5 Kilopascals | 5000000000000 nPa |
| 10 Kilopascals | 10000000000000 nPa |
| 20 Kilopascals | 20000000000000 nPa |
| 25 Kilopascals | 25000000000000 nPa |
| 50 Kilopascals | 50000000000000 nPa |
| 100 Kilopascals | 100000000000000 nPa |
| Reference | Kilopascals (kPa) | Nanopascals (nPa) |
|---|---|---|
| Atmospheric pressure at sea level | 101.325 kPa | 1.01325 × 1014 nPa |
| Healthy blood pressure (120 mmHg) | 16 kPa | 1.6 × 1013 nPa |
| A car tyre | 220 kPa | 2.2 × 1014 nPa |
| A racing bicycle tyre | 600 kPa | 6 × 1014 nPa |
The kilopascal is a unit of pressure equal to one thousand pascals. Its symbol is kPa. It is a hundredth of a bar and ten hectopascals, and it is the coherent SI unit that engineering was meant to adopt when the pascal proved too small to write comfortably. Standard atmospheric pressure is 101.325 kilopascals.
Tyres are where most people meet it. Canada, Australia, New Zealand and much of Asia state recommended tyre pressures in kilopascals, so a car door placard reads 220 or 240 rather than 32 or 35 pounds per square inch. The two scales sit side by side on most modern gauges, and the conversion is close to seven kilopascals to the pound per square inch.
Civil engineering uses it for loads on ground and structure. The bearing capacity of a soil is quoted in kilopascals, a firm clay allowing perhaps 150 and a soft one much less, and wind loading on a facade is calculated in the same unit. Because a kilopascal is also a kilonewton per square metre, structural calculations move between force and pressure without a conversion factor.
Medicine uses it in one important place. Arterial blood gases are reported in kilopascals in Britain, Ireland and much of Europe, where a healthy oxygen partial pressure is eleven to thirteen, while the United States reports the same measurement in millimetres of mercury as eighty to a hundred. The two numbers describe identical blood, and a clinician reading a foreign chart must know which convention it follows.
Vacuum work counts downward in it. A rough vacuum is a few kilopascals absolute, a domestic vacuum cleaner pulls perhaps twenty kilopascals below atmospheric, and freeze-drying operates well under one. Stating vacuum as an absolute pressure in kilopascals avoids the ambiguity of describing it as a negative gauge pressure.
Its awkwardness is only rhetorical. A hundred kilopascals is exactly one bar and reads less neatly, which is why the bar has held on in Europe despite decades of official preference for the pascal. Where the kilopascal has won, as on Canadian and Australian tyre placards, it has done so because the number is printed rather than spoken.
One kilopascal equals 1000 pascals, 0.01 bar, 10 hectopascals, or about 0.145 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.