Conversion from 2 Gigapascals to Bars

Information about the Gigapascal (GPa)

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.

Information about the Bar (bar)

The bar is a unit of pressure equal to exactly 100,000 pascals, or 100 kilopascals. Its symbol is bar. It is not part of the International System of Units, but it is tolerated alongside it and it is used throughout European engineering, because it happens to sit almost exactly on standard atmospheric pressure: one atmosphere is 1.01325 bar.

That coincidence is the whole reason for its success. A pressure quoted in bar can be read as very nearly a number of atmospheres, so a reading of six bar in a compressed air line means about six times the pressure outside, which is a fact an engineer can use without arithmetic. The name comes from the Greek word for weight, and the unit was proposed by the Norwegian meteorologist Vilhelm Bjerknes around 1909.

Diving uses it because the sea obliges. Every ten metres of seawater adds almost exactly one bar, so a diver at twenty metres is under three bar of absolute pressure, counting the atmosphere above. Decompression tables, gas consumption and cylinder pressures all follow from that one relationship, and the whole practice of diving is easier to teach in bar than in any other unit.

Everyday machinery works at a few bar. Car tyres are inflated to between two and two and a half, workshop compressed air runs at six to eight, mains water arrives at three to six, and an espresso machine extracts at nine, a figure so standard that it appears on the front of the machine. Watches carry water resistance ratings in bar, where ten bar corresponds nominally to a hundred metres of depth.

The distinction between gauge and absolute pressure matters more here than anywhere. A tyre gauge reading 2.2 bar means 2.2 bar above the surrounding air, so the absolute pressure inside is 3.2. Industrial practice writes barg for gauge and bara for absolute, and mixing the two is a common source of error in specifications and in safety calculations.

Standards bodies have discouraged the bar for decades in favour of the pascal, without much effect. The kilopascal is the coherent alternative and is used in Canada, Australia and parts of Asia, but a hundred kilopascals reads less naturally than one bar, and habit has proved stronger than the recommendation.

One bar equals 100,000 pascals, 100 kilopascals, 1000 millibars, or about 14.5 pounds per square inch.