THE GENERAL GAS LAW
The general gas law is a combination of Boyle’s law, Charles’ law, and Gay-
Lussac’s law, and is used to predict the behavior of a given quantity of gas when
pressure, volume, or temperature changes.
The formula for expressing the general gas law is:
The following points should be noted when using the
general gas law:
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There can be only one unknown value.
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If it is known that a value remains unchanged (such as the volume of a tank) or
that the change in one of the variables will be of little consequence, cancel the
value out of both sides of the equation to simplify the computations.
Sample Problem 1. A bank of cylinders having an internal volume of 20 cubic
feet is to be charged with helium and oxygen to a final pressure of 2,200 psig to
provide mixed gas for a dive. The cylinders are rapidly charged from a large
premixed supply, and the gas temperature in the cylinders rises to 160°F by the
time final pressure is reached. The temperature in the cylinder bank compartment
is 75°F. Determine the final cylinder pressure when the gas has cooled.
The pressure when the cylinder cools will be 1896.3
psig.
Sample Problem 2. Using the same scenario as in Sample Problem 1, determine
the volume of gas at standard temperature and pressure (STP = 70°F @ 14.7 psia)
resulting from rapid charging.
Sample Problem 3. Determine the volume of the gas at STP resulting from slow
charging (maintaining 70°F temperature to 2,200 psig).
Sample Problem 4. A 100-cubic-foot salvage bag is to be used to lift a 3,200-
pound torpedo from the sea floor at a depth of 231 fsw. An air compressor with a
suction of 120 cfm at 60°F and a discharge temperature of 140°F is to be used to
inflate the bag. Water temperature at depth is 55°F. To calculate the amount of time
required before the torpedo starts to rise (neglecting torpedo displacement,
breakout forces, compressor efficiency and the weight of the salvage bag), the
displacement of the bag required to lift the torpedo is computed as follows:
(Note that the 140°F compressor
discharge temperature is an intermediate temperature and does not enter into
the problem.)
