sumguy,
Air consumption is a personal aspect of diving. In other words, it varies from diver to diver. Even more, it varies for the same diver from dive to dive. Factors which will affect your air consumption rate are: work load, physical fitness, recent illnesses, water temperature, depth and comfort level. A useful method of calculating your personal air consumption rate involves finding your Respiratory Minute Volume (RMV). This is more useful than other methods because you can use it with any size tank. To find your RMV, descend to a comfortable depth. Relax, note your tank's pressure and sit quietly for 10 minutes. Again note your tank's pressure. Next, repeat the process while swimming normally for 10 minutes. At this point you know how many PSI/10 minutes you use at a specific depth for a specific tank both swimming and at rest. This is easily converted to cubic feet. Divide the total volume of the tank by the working pressure of the tank. This results in a tank factor expressed as cu ft/1 PSI for this particular tank. Multiply the tank factor by the PSI used then divide by 10 for each of the figures, PSI used resting and PSI used swimming. This is your RMV for that depth. Now convert it to a surface figure. Take your RMV at depth and divide by the ATMA of your depth. (D+33)/33=ATMA. RMV at depth/ATMA = RMV. Do this for both resting and swimming. You now have two baseline figures. To figure your air consumption at any depth, merely take your surface RMV and multiply by the ATMA of the depth you are planning.
Pounds per square inch (PSI)
Depth (D)
Atmosphere absolute (ATMA) ATMA = (D+33)/33
Tank factor is different for each type of tank (F)
Total tank volume (V)
Working pressure of the tank (W)
PSI used (P)
Respiratory minute volume (RMV)
V/W=F
(F*P)/10 = RMV at depth
RMV at depth/ATMA = RMV
