| Gigapascals (GPa) | Micropascals (µPa) |
|---|---|
| 1 Gigapascal | 1 × 1015 µPa |
| 2 Gigapascals | 2 × 1015 µPa |
| 3 Gigapascals | 3 × 1015 µPa |
| 4 Gigapascals | 4 × 1015 µPa |
| 5 Gigapascals | 5 × 1015 µPa |
| 10 Gigapascals | 1 × 1016 µPa |
| 20 Gigapascals | 2 × 1016 µPa |
| 25 Gigapascals | 2.5 × 1016 µPa |
| 50 Gigapascals | 5 × 1016 µPa |
| 100 Gigapascals | 1 × 1017 µPa |
| Reference | Gigapascals (GPa) | Micropascals (µPa) |
|---|---|---|
| Atmospheric pressure at sea level | 0.000101325 GPa | 1.01325 × 1011 µPa |
| Healthy blood pressure (120 mmHg) | 0.000016 GPa | 1.6 × 1010 µPa |
| A car tyre | 0.00022 GPa | 2.2 × 1011 µPa |
| A racing bicycle tyre | 0.0006 GPa | 6 × 1011 µPa |
The gigapascal is a unit of pressure equal to a billion pascals, written GPa. It is a thousand megapascals and ten thousand bar. It is the unit in which materials science states stiffness rather than strength, and in which the pressures inside planets and inside high-pressure laboratories are described.
Elastic modulus is its principal use. Steel has a Young's modulus of about 200 gigapascals, copper 117, aluminium 70, ordinary glass 70, concrete around 30, timber along the grain 10 to 15, and rubber less than a tenth of one. Those numbers describe how much a material stretches under load, not when it breaks, and they explain why a steel beam of the same strength as an aluminium one still deflects less.
Diamond marks the top of the ordinary scale. Its modulus of roughly 1200 gigapascals is the highest of any natural material, which is why it is used to make anvils for high-pressure work: nothing else can push that hard without deforming first. Synthetic diamond is made industrially at about five gigapascals and high temperature, in presses built specifically to hold that pressure.
The interior of the earth is described in the same unit. Pressure at the base of the crust is around one gigapascal, at the boundary between mantle and core about 136, and at the centre of the planet roughly 360. Laboratory diamond anvil cells now reach and exceed the central value, which allows the behaviour of iron and silicates under planetary conditions to be studied directly rather than inferred.
For contrast, the deepest point in the ocean exerts only about 0.11 gigapascals. That comparison is worth keeping, because it shows how much larger geological pressures are than anything associated with water: the bottom of the Mariana Trench is a thousandth of the pressure at the centre of the earth.
Below the gigapascal, the megapascal describes strength, and above it there is little except stellar and theoretical physics. The terapascal appears mainly in the elastic modulus of carbon nanotubes and in the interiors of giant planets, so the gigapascal is effectively the top of the range that laboratories and engineers work in.
One gigapascal equals 1,000,000,000 pascals, 1000 megapascals, 10,000 bar, or about 145,000 pounds per square inch.
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.