CHARLES'LAW: For any gas at a constant pressure the volume of the gas will vary directly with the absolute temperature.

Simply stated: that for every change in temperature plus or minus, there will be a corresponding change in pressure of approximately 5 psi plus or minus. An aluminum Scuba tank with a working pressure of 3000 psi has a blowout plug set for 3500 psi. When a tank is filled, it is hot due to the compression of the air, (this heat can cause damage the metal so tanks should be filled underwater). It is normally filled to around 3200 psi and then allowed to cool. If the diver takes this tank, and places it out in the sun, without cooling, the blowout plug may blow, due to the heat of the sun. Whenever tanks are left in direct sunlight, the pressure inside will increase (Charles' Law). Likewise the tank pressure will decrease as it cools in the water. It is this cooling that accounts for most divers' air consumption at the beginning of a dive.

When keeping records of air consumption, it is a good idea to place your tank in the water you will be diving in about ten minutes prior to the dive. Doing this will allow the tank's temperature to get in equilibrium with the surrounding water and a more accurate air consumption rate can be made.

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