SAC Calculations

[DiveGeek]

Me again... This time I'm curious about the SAC Rate on dives:

For instance, here's the rate on the Gases tab... http://glui.me/?i=w3hd741lm68j932/2014- ... 48_AM.png/
and here's SAC Rate for the same record on the summary tab... http://glui.me/?i=u7j5d3zkan6d238/2014- ... 49_AM.png/

Why is there a discrepancy?

[nick]

Because your tank is set to 17 seconds of usage.

The per-tank and per-dive SAC rates are calculated differently. IE, the total duration of the dive vs the total duration of the tank. This way, all of the tanks together get a weighted average applied to calculate the full SAC rate.

Thank You
MacDive Support

[DiveGeek]

Gah... okay, then this is another area where entering the duration in HH:MM:SS doesn't seem to work properly.

Thanks for the explanation.

[nick]

From memory, only the surface interval uses hours, the rest do not. It was done for convenience, because who cares about seconds in a surface interval.

Really this is only an issue for manually entered dives, and even then, you're making it harder than it needs to be. Leave the surface intervals blank. Add all your dives.

Then when you're done, select them all, right click on them and choose "Renumber Dives". MacDive will calculate the repetitive dive numbers and surface intervals for you automatically. Any dives you download from a computer will also have this applied. You never really need to do it manually.

Thanks

[DiveGeek]

Great to know, thanks. Yes, I agree we don't need seconds on the surface time entries but consistently using HH:MM:SS format everywhere would be far less confusing. As it is now, trying to enter 1 hour and 3 minutes as 01:03 gets calculated as 1 minute, 3 seconds everywhere but surface interval.

Also, I did some research on calculating SAC (I was curious) and figured out that MacDive is actually calculating RMV (Respiratory Minute Volume) and not SAC (Surface Air Consumption), per se.

I used this site (http://www.divenerd.com/sac-rmv/) to figure out what my dives SAC and RMV were, and MacDive matches the RMV values precisly.

Fun stuff!