How's your SAC?

[Daryl Morse]

The SAC can be computed as follows:

SAC =
(((Pb-Pe)/Pb)x((Pb/Pw)xV)/T)/((D/33)+1)

where:
Pb = Beginning pressure
Pe = Ending pressure
Pw = Nominal working pressure of the tank
V = Volume of tank in cubic feet
T = Time of dive in minutes
D = Average depth of dive in feet

One thing you can do to get an even more accurate computation of your SAC is to check the pressure of your tank after you get into the water to adjust for psi fluctuations due to indicated pressure differences between a warm and cool tank.

You equation can be simplified because Pb cancels out. I write it like this:

SAC = (Pb-Pe)x(V/Pw)/((D/33)+1)/T

If you think about it, the first part of the equation is to calculate air used in PSI, the second part is to convert PSI into volume (i.e., PSI x Volume/PSI = PSI), the third part is to normalize the volume to surface pressure, the last part is convert volume into rate of consumption.

As to your point about measuring pressure in the water, that usually makes a two or three hundred PSI difference, which can be a difference of 10% or more.

 

[derwoodwithasherwood]

Don't sweat it Mike. It's not part of the PADI curriculum for AOW so you may not have taught it. Nowadays everyone wants to be a "tech" diver so these calculation have been adapted from tech programs. I had never heard of it either until I began tech training in 2000 but today some instructors are bringing technical disciplines into recreational programs.

Interesting assumption. It was a part of the basic OW program of the old NASDS. We were taught to include our SAC as an integral part of dive planning process.