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        Charles’ Law.  Charles’ Law states that at constant pressure, the volume of a gas is directly proportional to the temperature (Kelvin).  The English version:  If you increase the temperature of a gas, it expands. If you cool it down, it contracts.

         Charles’ Law is often cited in scuba texts to explain why the pressure in the scuba tank goes up when the tank gets hot (or why the pressure drops when the tank cools down). This is a gross misapplication of Charles’ Law.  Notice that the first part of the law is “at constant pressure”. If the pressure is going up or down, it isn’t constant and Charles’ Law does not apply. The scuba industry seems to have made a bit of a leap of logic and deduced that if the volume of a gas varies directly with temperature then if you put a gas in a rigid container (like a scuba tank) and heat it up, since it can’t expand, the pressure must increase. That is true, but it isn’t Charles’ Law. It’s Amonton’s Law which states that the pressure of a gas is directly proportional to the temperature (Kelvin) at constant volume.

         So, when you’re discussing tank pressure in relation to temperature, you’re talking about Amonton’s Law, not Charles’ Law.  More important than whose law it actually was is how much pressure change you get as temperature changes. It’s fairly simple to work out a rough estimate. Just assume about 5 psi per degree Fahrenheit of temperature change.  A tank filled at 800F to 3000 psi then put in the trunk of a car at 1500F will contain about 3350 psi.  (1500F - 800F = 700F | 700F X 5 psi/0F = 350 psi | 3000 psi + 350 psi = 3350 psi) Going the other direction, if you got a hot fill to 3000 psi at 1000F and your tank cooled down to 600F once you got in the water, the tank pressure would drop to about 2800 psi.  (1000F - 600F = 400F | 400F X 5 psi/0F = 200 psi | 3000 psi - 200 psi = 2800 psi)