Conversion from Bars to Megapounds per square inch

Information about the Bar (bar)

The bar is a unit of pressure equal to exactly 100,000 pascals, or 100 kilopascals. Its symbol is bar. It is not part of the International System of Units, but it is tolerated alongside it and it is used throughout European engineering, because it happens to sit almost exactly on standard atmospheric pressure: one atmosphere is 1.01325 bar.

That coincidence is the whole reason for its success. A pressure quoted in bar can be read as very nearly a number of atmospheres, so a reading of six bar in a compressed air line means about six times the pressure outside, which is a fact an engineer can use without arithmetic. The name comes from the Greek word for weight, and the unit was proposed by the Norwegian meteorologist Vilhelm Bjerknes around 1909.

Diving uses it because the sea obliges. Every ten metres of seawater adds almost exactly one bar, so a diver at twenty metres is under three bar of absolute pressure, counting the atmosphere above. Decompression tables, gas consumption and cylinder pressures all follow from that one relationship, and the whole practice of diving is easier to teach in bar than in any other unit.

Everyday machinery works at a few bar. Car tyres are inflated to between two and two and a half, workshop compressed air runs at six to eight, mains water arrives at three to six, and an espresso machine extracts at nine, a figure so standard that it appears on the front of the machine. Watches carry water resistance ratings in bar, where ten bar corresponds nominally to a hundred metres of depth.

The distinction between gauge and absolute pressure matters more here than anywhere. A tyre gauge reading 2.2 bar means 2.2 bar above the surrounding air, so the absolute pressure inside is 3.2. Industrial practice writes barg for gauge and bara for absolute, and mixing the two is a common source of error in specifications and in safety calculations.

Standards bodies have discouraged the bar for decades in favour of the pascal, without much effect. The kilopascal is the coherent alternative and is used in Canada, Australia and parts of Asia, but a hundred kilopascals reads less naturally than one bar, and habit has proved stronger than the recommendation.

One bar equals 100,000 pascals, 100 kilopascals, 1000 millibars, or about 14.5 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.