Conversion from Decapascals to Megapascals

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 Megapascal (MPa)

The megapascal is a unit of pressure equal to one million pascals, written MPa. It is ten bar, and it is exactly one newton per square millimetre. That last identity is the reason the unit dominates engineering: a stress calculated from a force in newtons and an area in square millimetres comes out in megapascals with no conversion at all.

Materials strength is stated in it almost universally. A common structural steel yields at 355 megapascals, high-strength alloy steels at over a thousand, aluminium alloys between 200 and 500, and structural concrete at 25 to 50 in compression. Because those numbers are what a designer compares against a calculated stress, the megapascal is the unit in which most of the built world is specified.

Structural drawings often write N/mm² instead, and the two are the same. Some national codes prefer one form and some the other, but a plate marked 355 N/mm² and one marked 355 MPa carry identical steel. Knowing that they are the same unit removes what looks like a discrepancy between drawings from different countries.

Hydraulics works at these pressures too. Construction machinery runs at twenty to thirty-five megapascals, industrial presses higher, and water-jet cutting reaches four hundred, at which pressure a stream of water a fraction of a millimetre across will cut steel. Pressure vessels, pipelines and hydrogen storage are all designed and tested against figures in this range.

Above the megapascal, the gigapascal takes over for stiffness rather than strength. Young's modulus for steel is about 200 gigapascals, for aluminium 70 and for concrete 30, and those numbers describe how much a material deflects rather than when it breaks. Keeping strength in megapascals and stiffness in gigapascals is a convention that makes the two easy to tell apart at a glance.

For conversion, one megapascal is ten bar, 145 pounds per square inch, and just under ten atmospheres. The last comparison is worth holding: a pressure of one megapascal is about ten times what the air outside exerts, which puts even modest industrial pressures in a useful human perspective.

One megapascal equals 1,000,000 pascals, 10 bar, 1 newton per square millimetre, or about 145 pounds per square inch.