| Nanopascals (nPa) | Megapascals (MPa) |
|---|---|
| 1 Nanopascal | 1 × 10-15 MPa |
| 2 Nanopascals | 2 × 10-15 MPa |
| 3 Nanopascals | 3 × 10-15 MPa |
| 4 Nanopascals | 4 × 10-15 MPa |
| 5 Nanopascals | 5 × 10-15 MPa |
| 10 Nanopascals | 1 × 10-14 MPa |
| 20 Nanopascals | 2 × 10-14 MPa |
| 25 Nanopascals | 2.5 × 10-14 MPa |
| 50 Nanopascals | 5 × 10-14 MPa |
| 100 Nanopascals | 1 × 10-13 MPa |
| Reference | Nanopascals (nPa) | Megapascals (MPa) |
|---|---|---|
| Atmospheric pressure at sea level | 1.01325 × 1014 nPa | 0.101325 MPa |
| Healthy blood pressure (120 mmHg) | 1.6 × 1013 nPa | 0.016 MPa |
| A car tyre | 2.2 × 1014 nPa | 0.22 MPa |
| A racing bicycle tyre | 6 × 1014 nPa | 0.6 MPa |
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 megapascal is a unit of pressure equal to one million pascals, written MPa. It is ten bar, and it is exactly one newton per square millimetre. That last identity is the reason the unit dominates engineering: a stress calculated from a force in newtons and an area in square millimetres comes out in megapascals with no conversion at all.
Materials strength is stated in it almost universally. A common structural steel yields at 355 megapascals, high-strength alloy steels at over a thousand, aluminium alloys between 200 and 500, and structural concrete at 25 to 50 in compression. Because those numbers are what a designer compares against a calculated stress, the megapascal is the unit in which most of the built world is specified.
Structural drawings often write N/mm² instead, and the two are the same. Some national codes prefer one form and some the other, but a plate marked 355 N/mm² and one marked 355 MPa carry identical steel. Knowing that they are the same unit removes what looks like a discrepancy between drawings from different countries.
Hydraulics works at these pressures too. Construction machinery runs at twenty to thirty-five megapascals, industrial presses higher, and water-jet cutting reaches four hundred, at which pressure a stream of water a fraction of a millimetre across will cut steel. Pressure vessels, pipelines and hydrogen storage are all designed and tested against figures in this range.
Above the megapascal, the gigapascal takes over for stiffness rather than strength. Young's modulus for steel is about 200 gigapascals, for aluminium 70 and for concrete 30, and those numbers describe how much a material deflects rather than when it breaks. Keeping strength in megapascals and stiffness in gigapascals is a convention that makes the two easy to tell apart at a glance.
For conversion, one megapascal is ten bar, 145 pounds per square inch, and just under ten atmospheres. The last comparison is worth holding: a pressure of one megapascal is about ten times what the air outside exerts, which puts even modest industrial pressures in a useful human perspective.
One megapascal equals 1,000,000 pascals, 10 bar, 1 newton per square millimetre, or about 145 pounds per square inch.