Decipascal (dPa)

The decipascal is a unit of pressure equal to one tenth of a pascal. Its symbol is dPa. It belongs to the group of metric prefixes that exist because the system is regular rather than because anyone asked for them: deci, centi, deca and hecto fill the gaps between the thousand-step prefixes, and of those four only hecto found a permanent home in pressure, in the hectopascal of weather reports.

That regularity is the point of the metric system rather than an accident of it. Every prefix from yocto to yotta applies to every unit, so the decipascal is defined whether or not anyone writes it. The alternative would be a system in which some combinations are legal and others are not, and users would then have to memorise a table of exceptions instead of a single rule.

In practice pressure work skips from the pascal to the hectopascal or kilopascal, and the four intermediate prefixes go unused. There is a reason for that beyond habit. The thousand-step prefixes — milli, kilo, mega — line up with how numbers are grouped in writing, so a value in kilopascals is read off a value in pascals by moving the digit grouping, not by counting decimal places. Deci and centi break that alignment.

The decipascal does have a natural size, though. A tenth of a pascal is roughly the pressure of a single sheet of thin paper laid on a table, or the difference in air pressure over about a centimetre of height. It is the pressure a moth exerts on a windowpane, and about a hundredth of the pressure difference across a closed interior door in a ventilated building.

Where such pressures need writing down, the convention is to use the pascal with a decimal: an anemometer calibration might record 0.4 pascals rather than 4 decipascals, and a ventilation specification 50 pascals rather than 500 decipascals. The number is the same and the pascal keeps the page consistent with every other reading.

The unit is still perfectly valid, and a converter has to handle it, because it turns up in older instrument manuals, in occasional national standards, and wherever an author has decided that a figure reads better with one digit before the point than with three after it.

One decipascal equals 0.1 pascals, 100 millipascals, 0.001 hectopascals, or about 0.0000145 pounds per square inch.

Decipascal conversion table