The Ideal Gas Law
The ideal gas law states the relationship between the volume, pressure and temperature of an ideal gas are related by the equation pV = nRT, where p is the pressure, V is the volume, n is the number of moles of gas, R is the gas constant and T is the temperature.
Air as an Ideal Gas
Under the conditions normally associated with filling a SCUBA tank, air behaves as an ideal gas.
Pressure and Temperature
When a SCUBA tank is filled, the pressure inside the SCUBA tank is increasing as more air is being added. Since the cylinder has a nearly fixed volume, the increasing pressure produces a corresponding increase in temperature.
Submerging the Cylinder
The heat produced inside the cylinder is transferred to the walls of the cylinder. Submersing the tank in water allows the heat dissipate into the environment more quickly than air.
Cooling the Cylinder
After the tank has been filled, a drop in the temperature of the tank will result in a drop of the pressure of the air inside the tank. In practical terms, a warm tank at 3,000 psi has less air than a cool tank at 3,000 psi.
Maximum Fill Rate
This transfer of heat also allows the tank to be filled quicker. However, increasing the fill rate may not permit enough time for the heat built up in the air and walls of the cylinder to dissipate into the water, resulting in a drop in pressure once the air and walls cool.
Article Written By David Chandler
David Chandler has been a freelance writer since 2006 whose work has appeared in various print and online publications. A former reconnaissance Marine, he is an active hiker, diver, kayaker, sailor and angler. He has traveled extensively and holds a bachelor's degree from the University of South Florida where he was educated in international studies and microbiology.