These formulas allow you to calculate how many psi of air you consume on the surface per minute. Knowing how to apply these formulas will allow you to moniter and improve you air consumption, as well as how to roughly predict the maximum amount of time you can dive with your tank.
When finding your SAC in psi you need to first know several variables; DEPTH of the dive(in ATA), TIME, and the PRESSURE of tank going in/comming out.
It is easiest to make a list of the numbers as follows:
Depth (ft):.........................In (pressure of tank):
Depth (ATA):......................Out (pressure of tank):
Time:.............................. TC (Total Consumption):
-To find your depth in ATA (Absolute Total Atmospheres): Devide the depth in ft by 33 for saltwater and 34 for fresh, then add 1.
-To find your TC: Subtract the pressure in the tank comming out of the water from the pressure going in.
The Formulas:
1st) TC / time = BAC (Bottom Air Consumption)
2nd) BAC / ATA = SAC in PSI
Example:
Depth: 25 ft (saltwater).........In: 2800 psi
Depth: 1.7575 ATA ..............Out: 2000 psi
Time: 24 min.....................TC: 800 psi
1) 25ft/33(saltwater)=.7575
.7575+1=1.7575ATA
2) 2800psi-2000psi=800psi TC
(1st) 800psi/24min= 33.33psi per min
(2nd) 33.33/1.7575ATA= 18.97psi per min
SAC: 18.97 psi per min
Whew! we made it
If you monitor your air consumption using these formulas for a while you will be able to average your totals to find an accurate SAC. Once you know your average you can use it to calculate the total amount of time you can dive with a tank. All you need to do is multiply your SAC by the ATA of the divesite to find your BAC, then simply divide that number from the psi in your tank to find the total number of min's you can dive.
Happy Diving!