| Centipascals (cPa) | Nanopascals (nPa) |
|---|---|
| 1 Centipascal | 10000000 nPa |
| 2 Centipascals | 20000000 nPa |
| 3 Centipascals | 30000000 nPa |
| 4 Centipascals | 40000000 nPa |
| 5 Centipascals | 50000000 nPa |
| 10 Centipascals | 100000000 nPa |
| 20 Centipascals | 200000000 nPa |
| 25 Centipascals | 250000000 nPa |
| 50 Centipascals | 500000000 nPa |
| 100 Centipascals | 1000000000 nPa |
| Reference | Centipascals (cPa) | Nanopascals (nPa) |
|---|---|---|
| Atmospheric pressure at sea level | 10132500 cPa | 1.01325 × 1014 nPa |
| Healthy blood pressure (120 mmHg) | 1600000 cPa | 1.6 × 1013 nPa |
| A car tyre | 22000000 cPa | 2.2 × 1014 nPa |
| A racing bicycle tyre | 60000000 cPa | 6 × 1014 nPa |
The centipascal is a unit of pressure equal to one hundredth of a pascal. Its symbol is cPa. It is a legitimate metric combination that almost nobody writes, and understanding why it is unused says more about how pressure is actually recorded than a list of its applications ever could.
The centi prefix has one great success behind it, the centimetre, and that success came from the human scale of the metre. A metre is too long for the width of a hand and a millimetre too short, so the centimetre filled a real gap. Pressure has no comparable gap. The pascal is already so small that dividing it further gives numbers no instrument in ordinary use can resolve.
A centipascal is one hundredth of the pressure of a sheet of paper on a table. It is the pressure difference across a millimetre of air, or roughly what a grain of sand exerts spread over a fingernail. Nothing in engineering, meteorology or building practice needs that resolution: airtightness testing works at fifty pascals, ventilation at a few hundred, and weather at tens of thousands.
Where pressures smaller than a pascal do matter — acoustics and vacuum work — the convention jumps straight to the millipascal and the micropascal, because those fit the thousand-step ladder that the rest of the measurement system uses. A sound pressure of 20 millipascals is written that way rather than as 2 centipascals, even though the two are the same, because everything around it on the page is in thousand-step units.
The unit is nevertheless well defined, and that is the metric system working as designed. Every prefix combines with every unit without exception, so a reader who has never seen cPa can still decode it on sight from the prefix alone. A system with gaps would require a table; a system without them requires only the rule.
Occasional appearances do occur. Older instrument manuals sometimes use it, some national standards mention it in passing, and a few specialist fields adopt it locally when their numbers happen to fall between one and a hundred in that unit. A converter has to handle it for exactly those cases.
One centipascal equals 0.01 pascals, 10 millipascals, 0.1 decipascals, or about 0.00000145 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.