| Decipascals (dPa) | Terapascals (TPa) |
|---|---|
| 1 Decipascal | 1 × 10-13 TPa |
| 2 Decipascals | 2 × 10-13 TPa |
| 3 Decipascals | 3 × 10-13 TPa |
| 4 Decipascals | 4 × 10-13 TPa |
| 5 Decipascals | 5 × 10-13 TPa |
| 10 Decipascals | 1 × 10-12 TPa |
| 20 Decipascals | 2 × 10-12 TPa |
| 25 Decipascals | 2.5 × 10-12 TPa |
| 50 Decipascals | 5 × 10-12 TPa |
| 100 Decipascals | 1 × 10-11 TPa |
| Reference | Decipascals (dPa) | Terapascals (TPa) |
|---|---|---|
| Atmospheric pressure at sea level | 1013250 dPa | 0.000000101325 TPa |
| Healthy blood pressure (120 mmHg) | 160000 dPa | 0.000000016 TPa |
| A car tyre | 2200000 dPa | 0.00000022 TPa |
| A racing bicycle tyre | 6000000 dPa | 0.0000006 TPa |
The decipascal is a unit of pressure equal to one tenth of a pascal. Its symbol is dPa. It belongs to the group of metric prefixes that exist because the system is regular rather than because anyone asked for them: deci, centi, deca and hecto fill the gaps between the thousand-step prefixes, and of those four only hecto found a permanent home in pressure, in the hectopascal of weather reports.
That regularity is the point of the metric system rather than an accident of it. Every prefix from yocto to yotta applies to every unit, so the decipascal is defined whether or not anyone writes it. The alternative would be a system in which some combinations are legal and others are not, and users would then have to memorise a table of exceptions instead of a single rule.
In practice pressure work skips from the pascal to the hectopascal or kilopascal, and the four intermediate prefixes go unused. There is a reason for that beyond habit. The thousand-step prefixes — milli, kilo, mega — line up with how numbers are grouped in writing, so a value in kilopascals is read off a value in pascals by moving the digit grouping, not by counting decimal places. Deci and centi break that alignment.
The decipascal does have a natural size, though. A tenth of a pascal is roughly the pressure of a single sheet of thin paper laid on a table, or the difference in air pressure over about a centimetre of height. It is the pressure a moth exerts on a windowpane, and about a hundredth of the pressure difference across a closed interior door in a ventilated building.
Where such pressures need writing down, the convention is to use the pascal with a decimal: an anemometer calibration might record 0.4 pascals rather than 4 decipascals, and a ventilation specification 50 pascals rather than 500 decipascals. The number is the same and the pascal keeps the page consistent with every other reading.
The unit is still perfectly valid, and a converter has to handle it, because it turns up in older instrument manuals, in occasional national standards, and wherever an author has decided that a figure reads better with one digit before the point than with three after it.
One decipascal equals 0.1 pascals, 100 millipascals, 0.001 hectopascals, or about 0.0000145 pounds per square inch.
The terapascal is a unit of pressure equal to a thousand gigapascals, written TPa. It is ten million bar. Nothing built by engineers operates at this pressure, and the unit belongs instead to two quite separate corners of science: the stiffness of the strongest materials known, and the interiors of large planets.
Carbon nanostructures put it on the map. A single-walled carbon nanotube has a Young's modulus close to one terapascal, and a sheet of graphene the same, which makes them the stiffest materials ever measured relative to their weight. Diamond, long the benchmark, comes in at 1.2 terapascals. Those three numbers are the reason the unit appears at all in materials science.
Stiffness at this level is not the same as strength. A nanotube resists stretching enormously, but a real fibre made of many of them fails at a far lower stress because the tubes slide past one another. Confusing a terapascal modulus with a terapascal breaking strength is one of the commonest errors in popular accounts of these materials.
Planetary interiors reach genuine terapascal pressures. The centre of Jupiter is estimated at three to four terapascals, and the cores of larger gas giants beyond that. Under such conditions hydrogen behaves as a metal, which is what generates the planet's magnetic field, so the unit describes a state of matter rather than a load on a structure.
Laboratories can now reach it briefly. Laser-driven shock compression and pulsed magnetic techniques drive samples into the terapascal range for nanoseconds at a time, long enough to record how a material's density and structure respond. Those experiments are the only direct evidence available about matter under the conditions inside giant planets.
For scale, one terapascal is ten million times atmospheric pressure and about a thousand times the pressure at the centre of the earth divided by three. The number stops being something a person can feel and becomes a description of what atoms do when they are pushed close enough together to change their chemistry.
One terapascal equals 1,000,000,000,000 pascals, 1000 gigapascals, 10,000,000 bar, or about 145 million pounds per square inch.