SAC rate calculations

[samcantrell]

How is SAC calculated?
I get SAC of 0.44 for a dive with an average depth of 16.5 feet, max 23, delta air 1160 and a dive time of 17.6 min.
MacDive shows a SAC of 0.15. (I wish it was that low!)

How is this calculated? What's going on?

Thanks

[nick]

It is calculated based on your tank size (and working pressure if you are set to imperial units), average depth, duration, and air used.

[samcantrell]

I don't believe that the calculation is correct (or at least, I'm interpenetrating the data wrong).

I'm using the formula:

(psi used ÷ working pressure) X tot. cyl. cap.
____________________________________ ÷ min. = Cubic feet per min SAC
(depth in feet + 33) ÷ 33

For a dive with psi used = 1450.37 psi and a working pressure of 3277.85 psi, a cyl cap of 77.8 CF,
a depth of 17.11 ft and a duration of 19.3 min gives me a SAC of 1.17. The same dive in MacDive
shows a SAC of 0.16 CF/min.

I'm not sure what the disconnect is; 0.16 is far too low a number. What am I doing wrong?

Thanks

[nick]

What is your depth? The formatting seems a bit weird there, is that min depth? It should be average if that is the case.

[samcantrell]

I was at an average depth (per the Cobra) of 17.11 feet with a max of 23.

[nick]

Email me your MacDive.sqlite file - I'm not at home right now, so I can't check, but I'll take a look after work.

[nick]

Ok I checked your data. In your Tank, you have the Tank Size set to 10. That should be 77.8 according to your calculations. Fix that and your SAC rate is much higher. Anything else is likely rounding.