MacDive Forums

Hi Nick,

Something strange happened to my MACDIVE... The software usually gives me a SAC rate calculation based on the Max Depth of a dive. However, last Sunday I imported a dive and all the SAC rates changed. I did some math ( ! ) and figured that it was using Average Depth instead of Max Depth to calculate the SAC number. This is strange since I had previously downloaded dives from this same Gekko to my Mac with nothing changing, even after upgrading to 2.0.5.

Is this normal? I'm used to calculating SAC from Max Depth and my planning is done accordingly. I'd love to have this "fixed" or perhaps you could add another column for SAC (Max Depth) vs SAC (Avg Depth)? Or (fingers crossed) I hope my database isn't corrupt...

HELP!

Thanks!
Max

SAC has always been based on the average depth.

nick wrote:SAC has always been based on the average depth.

Average or weighted average? If I do a 60 minute dive and 40 minutes of it is spent at 60 feet and the balance on descent, ascent and safety stop, I wouldn't expect that to be calculated as all being at 30 feet for SAC purposes. Weighted average, based on sample counts at depth, when available, right?

--Richard

No, the overall average.

Calculating SAC with the maximum depth makes no sense at all. Imaging a touch and go dive where you spend very little time at the maximum depth, and the majority of time at shallow depth. The resulting value would be overestimated compared to the real value. Using the average depth gives a much more realistic value.

rlmalisz wrote:Average or weighted average?

In practice there is no difference between the two, because most dive computers have a constant samplerate throughout the dive and hence the weights (which would be equal to the length of each sample) are constant and drop out of the equation.

nick wrote:No, the overall average.

I'm pretty sure you use a real average (weighted or not), and not something stupid like maxdepth/2. I think this is what rlmalisz was asking for.