Conversion from Atmospheres to Terapascals

Information about the Atmosphere (atm)

The atmosphere is a unit of pressure defined as exactly 101,325 pascals. Its symbol is atm. It is not a measurement of the air at any particular place or moment but a fixed reference value, chosen in 1954 by the General Conference on Weights and Measures to sit close to the average pressure at sea level in temperate latitudes.

Its usefulness is that it turns pressure into a ratio. Saying that a vessel holds gas at six atmospheres says immediately that the pressure inside is six times what is outside, which is the fact that determines whether a container will burst, how much gas it holds and how fast it will empty. No other pressure unit carries that meaning in the number itself.

Chemistry built its early framework on it. Gas laws were written with pressures in atmospheres, the gas constant had a value tailored to litres and atmospheres, and standard conditions for tabulating thermodynamic data were defined at one atmosphere. In 1982 the International Union of Pure and Applied Chemistry changed the standard state to exactly one bar, so modern tables use bar and older ones use atmospheres, and the difference of 1.3 per cent matters in careful work.

Diving and hyperbaric medicine still count in it. Pressure underwater is described in atmospheres absolute, a diver at thirty metres experiencing about four, and hyperbaric oxygen therapy is delivered at two to three atmospheres absolute in a sealed chamber. Because gas volume and gas uptake by the body both scale with absolute pressure, the unit is doing real physiological work rather than merely reporting a reading.

The atmosphere also fixes the older mercury scales. One atmosphere is exactly 760 millimetres of mercury by definition, which is what ties the torr and the millimetre of mercury to the metric system, and it is 29.92 inches of mercury, the setting an American pilot dials into an altimeter as standard.

A related unit, the technical atmosphere of one kilogram-force per square centimetre, is close but not equal at 98,066.5 pascals. Documents from Japan, Korea and eastern Europe sometimes use it, and mistaking one for the other introduces an error of about three per cent.

One atmosphere equals 101,325 pascals, 1.01325 bar, 760 millimetres of mercury, or about 14.696 pounds per square inch.

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.