Conversion from Atmospheres to Pounds 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 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.