Conversion from Terapascals to Kilopascals

Information about the Terapascal (TPa)

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.

Information about the Kilopascal (kPa)

The kilopascal is a unit of pressure equal to one thousand pascals. Its symbol is kPa. It is a hundredth of a bar and ten hectopascals, and it is the coherent SI unit that engineering was meant to adopt when the pascal proved too small to write comfortably. Standard atmospheric pressure is 101.325 kilopascals.

Tyres are where most people meet it. Canada, Australia, New Zealand and much of Asia state recommended tyre pressures in kilopascals, so a car door placard reads 220 or 240 rather than 32 or 35 pounds per square inch. The two scales sit side by side on most modern gauges, and the conversion is close to seven kilopascals to the pound per square inch.

Civil engineering uses it for loads on ground and structure. The bearing capacity of a soil is quoted in kilopascals, a firm clay allowing perhaps 150 and a soft one much less, and wind loading on a facade is calculated in the same unit. Because a kilopascal is also a kilonewton per square metre, structural calculations move between force and pressure without a conversion factor.

Medicine uses it in one important place. Arterial blood gases are reported in kilopascals in Britain, Ireland and much of Europe, where a healthy oxygen partial pressure is eleven to thirteen, while the United States reports the same measurement in millimetres of mercury as eighty to a hundred. The two numbers describe identical blood, and a clinician reading a foreign chart must know which convention it follows.

Vacuum work counts downward in it. A rough vacuum is a few kilopascals absolute, a domestic vacuum cleaner pulls perhaps twenty kilopascals below atmospheric, and freeze-drying operates well under one. Stating vacuum as an absolute pressure in kilopascals avoids the ambiguity of describing it as a negative gauge pressure.

Its awkwardness is only rhetorical. A hundred kilopascals is exactly one bar and reads less neatly, which is why the bar has held on in Europe despite decades of official preference for the pascal. Where the kilopascal has won, as on Canadian and Australian tyre placards, it has done so because the number is printed rather than spoken.

One kilopascal equals 1000 pascals, 0.01 bar, 10 hectopascals, or about 0.145 pounds per square inch.