Conversion from Atmospheres to Megapounds per square inch

Information about the Atmosphere (atm)

The atmosphere is a unit of pressure defined as exactly 101,325 pascals. Its symbol is atm. It is not a measurement of the air at any particular place or moment but a fixed reference value, chosen in 1954 by the General Conference on Weights and Measures to sit close to the average pressure at sea level in temperate latitudes.

Its usefulness is that it turns pressure into a ratio. Saying that a vessel holds gas at six atmospheres says immediately that the pressure inside is six times what is outside, which is the fact that determines whether a container will burst, how much gas it holds and how fast it will empty. No other pressure unit carries that meaning in the number itself.

Chemistry built its early framework on it. Gas laws were written with pressures in atmospheres, the gas constant had a value tailored to litres and atmospheres, and standard conditions for tabulating thermodynamic data were defined at one atmosphere. In 1982 the International Union of Pure and Applied Chemistry changed the standard state to exactly one bar, so modern tables use bar and older ones use atmospheres, and the difference of 1.3 per cent matters in careful work.

Diving and hyperbaric medicine still count in it. Pressure underwater is described in atmospheres absolute, a diver at thirty metres experiencing about four, and hyperbaric oxygen therapy is delivered at two to three atmospheres absolute in a sealed chamber. Because gas volume and gas uptake by the body both scale with absolute pressure, the unit is doing real physiological work rather than merely reporting a reading.

The atmosphere also fixes the older mercury scales. One atmosphere is exactly 760 millimetres of mercury by definition, which is what ties the torr and the millimetre of mercury to the metric system, and it is 29.92 inches of mercury, the setting an American pilot dials into an altimeter as standard.

A related unit, the technical atmosphere of one kilogram-force per square centimetre, is close but not equal at 98,066.5 pascals. Documents from Japan, Korea and eastern Europe sometimes use it, and mistaking one for the other introduces an error of about three per cent.

One atmosphere equals 101,325 pascals, 1.01325 bar, 760 millimetres of mercury, or about 14.696 pounds per square inch.

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.