#2935
mo - 7/03/2008 10:55 AM
Calculating Air Consumption:


That first factor depth, means that to measure our air consumption we need to do it in relation to the surface or we are comparing apples with oranges for every dive we do. If we did two identical dives one at 10 meters and one at 20 meters then we should expect to use twice the amount of air for the 20 meter dive. By converting our air consumption to an equivalent value as if we were standing on the surface, we can compare different dives and see if we are generally improving or not.


The calculations for metric divers are fairly straight forward. Take a 20 minute dive to 20 meters where you used 140 bar and you were diving with a 10 litre tank:


140 bar x 10 litre cylinder = 1400 litres consumed during the dive.


1400 litres / 20 minutes = 70 litres / min air consumption rate at 20 meter depth.


At 20 meters, the air is 3 times denser than at the surface so we divide by 3 to get a 23.3 litres /min Surface Air Consumption Rate.


For imperial divers, the calculations get a little more complex with the addition of the way tanks are described. In this case, an 80 cubic foot tank is the volume of gas it contains at it’s working pressure. Not it’s true internal volume at normal atmospheric pressure which is how metric tanks are defined.


So lets take a 20 minute dive to 66 feet where you used 2000 psi and were diving with an 80 cubic foot tank rated to 3000 psi working pressure.


80 cubic feet x 2000 psi / 3000 psi = 53.3 cubic feet of air used.


53.3 cubic feet / 20 minutes = 2.6 cubic feet / minute at 66 feet depth.


At 66 feet the air is 3 times denser than at the surface and so we divide by 3 to get a 0.89 cubic feet per minute Surface Air Consumption Rate.