Conversion from Kilopounds per square inch to Bars

Information about the Kilopound per square inch (ksi)

The kilopound per square inch is a unit of pressure equal to one thousand pounds per square inch, or 6.895 megapascals. Its symbol is ksi. It is not a pressure unit in ordinary use but a stress unit, and it is how American engineering states the strength of structural materials.

Structural steel is specified in it. The grade long known as A36 yields at 36 kilopounds per square inch, and the wide-flange steel that replaced it as the American standard yields at 50. Reinforcing bar of grade 60 yields at 60, and high-strength bolts are rated at 120 in tension. Those numbers are the vocabulary of American structural design in the way that 355 and 500 are in Europe.

Concrete uses the same scale downward. A common structural mix reaches four kilopounds per square inch in compression at twenty-eight days, high-performance mixes reach eight or more, and the specified figure appears on drawings, in the mix design and on the test cylinder report. Aluminium alloys sit between the two, a common heat-treated grade yielding at about 35.

Fracture mechanics extends the unit in an unusual direction. Fracture toughness is quoted in kilopounds per square inch times the square root of an inch, a compound that looks strange but is dimensionally correct, and it describes how large a crack a material will tolerate before it runs. The metric equivalent is megapascals times the square root of a metre, and converting between them requires care.

Its relationship to the metric world is simple in principle. One kilopound per square inch is 6.895 megapascals, so a fifty ksi steel yields at 345 megapascals and a sixty ksi bar at 414. Anyone reading drawings from both traditions ends up learning that factor, because the two systems name the same steels with different numbers.

The name is sometimes misread. A kilopound here is a thousand pounds of force, not the kilopound of mass sometimes called a kip in shipping, and the pressure it names has nothing to do with any weight in a container. Reading ksi as a stress rather than as a load avoids the confusion.

One kilopound per square inch equals 1000 pounds per square inch, about 6.895 megapascals, about 68.95 bar, or about 68 atmospheres.

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.