Conversion from 3 Pounds per square inch to Kilopounds per square inch

Information about the Pound per square inch (psi)

The pound per square inch is a unit of pressure equal to 6894.76 pascals. Its symbol is psi. It is the force of one pound spread over one square inch, and it is the working pressure unit of the United States and of a great deal of equipment sold worldwide, because so much hydraulic and pneumatic machinery is designed to American specifications.

Its most familiar appearance is on tyres. A car is inflated to between 30 and 35 pounds per square inch, a bicycle tyre to 80 or more, and a heavy truck to around 100. Because the pressure and the contact patch together carry the vehicle, a small change in the figure changes handling, fuel consumption and tyre life, which is why it is printed inside the door of every car.

Compressed systems use the same scale. A workshop air line runs at 90 to 120 pounds per square inch, a domestic water supply arrives at 40 to 80, hydraulic machinery works at around 3000, and a full scuba cylinder holds 3000 or more. The convenient coincidence is that atmospheric pressure is 14.7 pounds per square inch, so a reading of 30 means roughly three times atmospheric in absolute terms.

That absolute distinction is written into the symbol. Engineers write psig for gauge pressure, measured above the surrounding air, and psia for absolute pressure, measured from vacuum. A tyre at 32 psig is at 46.7 psia, and confusing the two in a calculation of gas quantity or vessel strength produces an error of one atmosphere, which at low pressures is most of the answer.

Aviation uses it for cabin differential. An airliner maintains a pressure difference of about eight to nine pounds per square inch between cabin and outside air at cruising altitude, and the fuselage is designed and fatigue-tested against that repeated loading. The figure sets the effective cabin altitude and therefore how passengers feel at the end of a long flight.

Converting to metric is exact but not memorable, since a pound per square inch is 6894.76 pascals. The two rules of thumb worth carrying are that one bar is about 14.5 pounds per square inch and that one pound per square inch is about seven kilopascals; both are close enough for judging a gauge and neither is close enough for a design calculation.

One pound per square inch equals 6894.76 pascals, about 0.0689 bar, about 6.89 kilopascals, or about 0.068 atmospheres.

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.