To determine how long a scuba tank should last while scuba diving, follow the steps below:
1. Determine your SAC Rate
with the easy-to-use calculator on DiveBuddy. The SAC Rate is the PSI/min used.
Example: If I dive to 33 feet for 60 minutes and use 2500 PSI of an 80cu/ft tank that originally had 3000 PSI...my SAC Rate would be 20.8 PSI per minute. So at the surface, 2500 PSI would last me about 120 minutes (ie: 2500 / 20.8).
2. Determine your air consumption rate at your planned depth.
Example: If I know my SAC Rate is 20.8 and I plan to dive to 66 feet (ie: 3 atmospheres of pressure), I would multiply 20.8 by 3 to get an air consumption rate of 62.4 PSI/min at 66 feet.
3. Determine how long your scuba tank will last at your planned depth.
Example: If I dive with a scuba tank that holds 3000 PSI, and I want to leave 500 PSI in the tank when I’m done...that leaves 2500 PSI of usable air. Divide 2500 by 62.4 and you get 40 mins.
So...given my SAC Rate, my planned depth, my available tank air...I should be able to dive to 66 feet for about 40 mins.
Summary (given sample data above):
Tank air time at surface: 120 mins
Tank air time at 33 feet: 60 mins
Tank air time at 66 feet: 40 mins