| Micropascals (µPa) | Megapascals (MPa) |
|---|---|
| 1 Micropascal | 1 × 10-12 MPa |
| 2 Micropascals | 2 × 10-12 MPa |
| 3 Micropascals | 3 × 10-12 MPa |
| 4 Micropascals | 4 × 10-12 MPa |
| 5 Micropascals | 5 × 10-12 MPa |
| 10 Micropascals | 1 × 10-11 MPa |
| 20 Micropascals | 2 × 10-11 MPa |
| 25 Micropascals | 2.5 × 10-11 MPa |
| 50 Micropascals | 5 × 10-11 MPa |
| 100 Micropascals | 1 × 10-10 MPa |
| Reference | Micropascals (µPa) | Megapascals (MPa) |
|---|---|---|
| Atmospheric pressure at sea level | 1.01325 × 1011 µPa | 0.101325 MPa |
| Healthy blood pressure (120 mmHg) | 1.6 × 1010 µPa | 0.016 MPa |
| A car tyre | 2.2 × 1011 µPa | 0.22 MPa |
| A racing bicycle tyre | 6 × 1011 µPa | 0.6 MPa |
The micropascal is a unit of pressure equal to one millionth of a pascal. Its symbol is µPa. It is one of the smallest pressure units in practical use, and it owes its place to a single number: 20 micropascals, the reference pressure against which every sound level in air is measured.
That figure is the threshold of hearing — roughly the quietest sound a healthy young ear can detect at a frequency of about 1,000 hertz. When acousticians defined the decibel scale for airborne sound they needed a fixed pressure to compare against, and they chose that threshold. Zero decibels means a sound pressure of 20 micropascals; every decibel figure quoted for a road, a machine or a concert is a ratio to that number.
The scale that follows is steep. Twenty decibels is ten times the reference pressure, 200 micropascals. Sixty decibels, ordinary conversation, is a thousand times it, or 20 millipascals. A hundred and twenty decibels, the threshold of pain, is a million times it: 20 pascals. The ear covers a range of a million to one in pressure, and the micropascal sits at the bottom of it.
Underwater acoustics uses the same unit but a different reference. Sound in water is referred to 1 micropascal rather than 20, because the threshold of human hearing is meaningless in the sea. That difference matters enormously: a level quoted in decibels underwater is not comparable to one in air, and the gap between the two conventions is about 26 decibels before any other correction. Sonar figures, whale-song measurements and shipping-noise studies all carry the re 1 µPa qualifier for that reason.
Outside acoustics the micropascal appears in vacuum science. A good high vacuum is around 100 micropascals, and ultra-high vacuum, the regime used for surface physics and particle accelerators, runs from a few micropascals down to nanopascals. Pumping a chamber to that level takes hours of baking to drive adsorbed gas off the walls.
The unit also shows up in radiation pressure and in the very small pressure differences that laboratory instruments resolve. Anything measured in micropascals is a measurement rather than a force anyone would feel, which is exactly what makes the reference-level convention so useful.
One micropascal equals 0.000001 pascals, 0.001 millipascals, 1,000 nanopascals, or about 0.000000000145 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.