P1v1 P2v2

The formula for expressing the general gas law is: -----— = -----— Where:

 Pi = initial pressure (absolute) Vi = initial volume Ti = initial temperature (absolute) P2 = final pressure (absolute) V2 = final volume T2 = final temperature (absolute)

Two simple rules must be kept in mind when working with the general gas law: There can be only one unknown value.

The equation can be simplified if it is known that a value remains unchanged (such as the volume of an air cylinder) or that the change in one of the variables is of little consequence. In either case, cancel the value out of both sides of the equation to simplify the computations.

Sample Problem 1. Your ship has been assigned to salvage a sunken LCM landing craft located in 130 fsw. An exploratory dive, using scuba, is planned to survey the wreckage. The scuba cylinders are charged to 2,250 psig, which raises the temperature in the tanks to 140 °F. From experience in these waters, you know that the temperature at the operating depth will be about 40°F. Apply the general gas law to find what the gauge reading will be when you first reach the bottom. (Assume no loss of air due to breathing.)

1. Simplify the equation by eliminating the variables that will not change. The volume of the tank will not change, so V1 and V2 can be eliminated from the formula in this problem: