Lake County -- IF you dive with a computer or bottom timer, you should be able to get a readout from it that gives you the "avg depth" of the dive. This "avg depth" is usually a "time weighted" avg and so is useful for determining the avg SAC rate for the dive. If you don't have that readout, then one way to get a "Time Weighted Avg" is to just mark down your depth every five minutes, totally it up and divide by the number of markings to get a time weighted avg depth. (Or you can just try to keep a running total in your head -- 85'=9; 71'=7 -- 7+9=16/2 -- avg 80' which is good enough.)
Charlie -- you responded while I was writing -- the "avg depth" works really well ONLY IF it is time weighted as opposed to saying -- "I was at 90 and then 70 and then 45" and expecting a useful number by just averging those three numbers since the time at each depth is what is really important.
Charlie -- you responded while I was writing -- the "avg depth" works really well ONLY IF it is time weighted as opposed to saying -- "I was at 90 and then 70 and then 45" and expecting a useful number by just averging those three numbers since the time at each depth is what is really important.