Size is not the only thing that matters.OK, first you need to know a few crucial bits of info to assess whether you can complete a dive for a given amount of time to a certain depth. You'll need to know:
A. Your tank's current capacity
B. The total dive time including safety stops, ascent, descent and bottom time.
C. The maximum depth of the dive
D. Your breathing rate or air consumption per minute.
Firstly, let's work out how much air you've got in your tank.You need to know the tanks internal volume at the surface (one atmosphere of pressure) and the pressure of its fill in atmospheres (atm).Internal volume is known from those strange markings stamped on the neck of your tank. When the tank is tested it is filled with water and pressurised. There is a marking WC (Water Capacity) stamped on the neck which states how much water the tank will hold internally for its test. Make sure this is in kilos not imperial measures! As 1 litre of water weighs 1 kilo the WC can be converted directly into internal volume in litres of atmospheric pressure. Next read your pressure gauge to see the gauge or fill pressure (GP). Convert this to atmospheres. 1 atm = 1.013 bar or 1.033 kg/cm2 or 14.7lbs/in2.
Now you can work out the capacity of your particular tank at any gauge pressure by using the formula: