Conversion from Megapounds per square inch to Decipascals

Information about the Megapound per square inch (Mpsi)

The megapound per square inch is a unit of pressure equal to one million pounds per square inch, or 6.895 gigapascals. Its symbol is Mpsi. Nothing operates at this pressure; the unit exists solely to state elastic modulus, which is the quantity that describes how stiff a material is rather than how strong.

Two numbers in it are memorised by every American mechanical engineer. Steel has a Young's modulus of 29 million pounds per square inch and aluminium of 10 million, and that ratio of roughly three to one governs how the two behave in every structure. A steel part and an aluminium part of identical shape and identical strength will not deflect identically, and the difference is exactly that factor.

The rest of the common metals fill in around them. Titanium sits at about 16.5 million pounds per square inch, copper at 17, magnesium at 6.5, and cast iron between 12 and 20 depending on grade. Because modulus barely changes with heat treatment or alloying within a metal family, these figures are far more stable than strength figures and can be relied on across grades.

Composites are where the unit does its most interesting work. Standard carbon fibre reaches about 33 million pounds per square inch along the fibre, high-modulus grades 55 to 85, and glass fibre only about 10, the same as aluminium. Since a composite laminate has a different modulus in every direction, designers work with a set of values rather than one, and the unit appears many times on a single datasheet.

Its metric equivalent is the gigapascal, and the conversion is 6.895 gigapascals to the megapound per square inch. Steel at 29 million pounds per square inch is 200 gigapascals, and aluminium at 10 million is 69, which are the two most quoted stiffness figures in metric engineering as well. The two traditions are describing exactly the same materials.

Distinguishing modulus from strength matters more here than anywhere. A steel with a modulus of 29 million pounds per square inch might yield at 50 thousand, a difference of nearly six hundred times, and the two properties are independent: heat treatment can double the strength of a steel while leaving its stiffness untouched.

One megapound per square inch equals 1,000,000 pounds per square inch, about 6.895 gigapascals, about 68,950 bar, or 1000 kilopounds per square inch.

Information about the 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.