Conversion from 3 Decapascals to Gigapascals

Information about the Decapascal (daPa)

The decapascal is a unit of pressure equal to ten pascals. Its symbol is daPa. Deca is the least used prefix in the metric system almost everywhere, but the decapascal is an exception, because one clinical discipline adopted it and has kept it: audiology measures the pressure inside the middle ear in decapascals.

Tympanometry is the test in question. A probe seals the ear canal and varies the air pressure in it while measuring how much sound the eardrum reflects, and the resulting curve is plotted against pressure in decapascals. The sweep runs from about plus 200 to minus 400, and the position of the peak shows the pressure at which the eardrum moves most freely, which is normally the pressure of the middle ear itself.

That single graph carries a great deal of clinical information. A peak near zero means the middle ear is at the same pressure as the room, which is the healthy state. A peak displaced towards minus 200 or beyond suggests the Eustachian tube is not equalising properly, and a flat curve with no peak at all suggests fluid behind the eardrum. Each of those readings is a position on a decapascal scale.

The unit was chosen for the same reason the hectopascal was chosen in meteorology. Middle-ear pressure had long been recorded in millimetres of water, and one millimetre of water is 0.98 decapascals, so switching to the metric unit left every clinical number and every published reference range almost unchanged. A change of unit that shifts the figures by two per cent is one that practitioners will actually accept.

Outside audiology the decapascal is essentially unused. Ventilation and building airtightness work at similar pressures, an airtightness test being run at fifty pascals, but those figures are written in pascals, and everything above them moves to hectopascals or kilopascals. The gap between one and a hundred pascals is otherwise unclaimed.

For scale, ten pascals is the pressure a light breeze exerts on a wall, or the difference between the two ends of a well-sealed corridor when a door opens. It is also roughly the pressure change from riding a lift up one floor, which is why ears sometimes need clearing in a tall building.

One decapascal equals 10 pascals, 0.1 hectopascals, about 1.02 millimetres of water, or about 0.00145 pounds per square inch.

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.