Kharon
Contributor
I guess I'm not sure I understand what you are saying. I believe over sufficient dives an established SAC should be a good number for gas planning... but of course you should build in a buffer as conditions, temp, current, etc will obviously effect your average SAC.
What I am trying to say is that the calculation of the actual rate of gas used during a dive is more valuable (at least to me) than a calculated SAC that has to have fudge factors re-applied re. planned depth, etc.
As an example, it looks like the median SAC based on the dive plots you've shown is around .75, that seems like a good starting point for gas planning and then if you know there will be lower temp or more workload you can add more buffer or if you know it's warm water no current you can add less buffer.
Actually, with a mean of 1.3 cf/min of actual gas usage, using 1.3 cf/min is a much more accurate (for me) starting point. If I use 0.75 and expect to get 86 minutes out of 77.4 CF (with 500 psi remaining) I'm going to be disappointed. If I use 1.3 cf/min I'll most likely be fairly close to 49.6 minutes (leaving 500 psi in the tank). Actual dive times for this data set are mean 50 min, mode 51 min, and median 52 min - no where near 86 minutes.
the tight grouping of all your other points looks like fairly consistent SAC over different dives so unless all of those dives were identical conditions (temp, current, finning) I'd say that's a fairly consistent SAC you have
Actually, almost all the dives were under virtually identical conditions (the high outliers were abnormally high current - don't know what the low outlier was). The only variable was dive length, max depth, and average depth. The first two plots were done to eliminate any of that as a possible controlling factor. The third plot, if SAC was a good indicator of actual amount of gas used, should show a correlation very close to 1::1 - it does not.
My point is that if you want to get a good estimate of how much gas you use on average, calculating the actual amount of gas used on a dive is far more accurate than the artificial SAC/RMV. Don't take my word for it. Go back and do the calculations on your dives and see if the amount of gas actually used matches your SAC. I'll bet it won't.
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