SAC or surface air consumption is defined in psi/min used at the surface by some folks and others as cf/min... which still others call RMV.rakjar once bubbled...
Although I read all the information on these posts, I can not remember or better yet, I don't think I was ever taught SAC.
Don't get side tracked by that debate... but know this, SAC figured in psi/minute works for a single size of tank while SAC figured in cf/minute works for all sizes of tanks but must be converted back to psi/minute for the specific size tank you are using.
To find out what your psi/minute SAC was during a dive you can take the average depth reading from your cobra (if it gives average depth) and use this formula:
PSI used/((Average depth/33) + 1) X dive time in minutes
For example:
2000 PSI used on a 30 minute dive that averaged 45'
2000/ ((45/33)+1) X 30 = 2000/~71 = ~ 28 psi minute
Which for planning purposes I would round up to 30 psi/minute.
If the tank you were using happened to be an AL80 then you could figure ~ 2.6 cubic feet per 100 psi so you would have used roughly 52 cubic feet on this sample dive (20X2.6=52).
The number 71 in the above example is ATA minutes... that is atmosphere absolute minutes... the equivalent amount of time at the surface.
I you wanted to figure your SAC in cubic feet per minute you could divide 52 by 71 to get your approximate SAC of .73 cf/minute.