Conversion from 3 Decapascals to Kilopascals

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 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.