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Less than .34 is enough to breath. I have about 25 dives between 0.3 and 0.35. My normal range is 0.35 to 0.45.
The formula is:
SCR = (1-P2/P1) x Vf/Aa/Tm
P1 = pressure in psi at beginning of test dive
P2 = pressure in psi at end of test dive
Vf = actual volume of air in tank in cu. ft.
Aa = pressure in absolute atmospheres
Tm = time of dive in minutes
Virtually all my dives are between 0.4 and 0.45cfm. Warm clear water leisurely sightseeing dives. This is definitely towards the low end of the scale.
It was 0.75cfm before 50 dives. About 0.6cfm around dive 75. 0.5cfm after dive #130, gradually going down to a very repeatable 0.4 to 0.43cfm around dive #200.
Right now, my SAC is about 0.35cfm in a no motion drift, about 0.5cfm at a transit speed of 1kt (100'/minute), and 0.6 or 0.7cfm while swimming hard. Aerobic fitness level mostly affects how fast my air consumption goes up with increasing workload. If I'm out of shape, then swimming hard requires a lot more air.
In addition to aerobic conditioning and stress levels, there are a number of less easily controlled factors that greatly influence this number: workloads, body mass and water temps.
Perhaps the biggest thing influencing SAC rates as reported by most folks is the bul**hit factor, however. Like stories about golf and fishing, never trust someone who volunteers to talk about their gas consumption.
Besides, I don't think the above formula (That I saw) takes into account the descent and ascent plus S/S @ 15ft. As we all know, if you take your TRT as "Bottom Time" and use the Max Depth as the constant, you're not very accurate with the math. (Unless the "Test Dive" is "part" of a dive, meaning the Depth IS constant, and the time accurate)> But then, you're concentrating on the numbers then, "to get a good score".
Then again, we're not all diving Caymans either....