Calculating Sac Rate in Excel

[Capt Jim Wyatt]

Spectre accurately stated...
Jim is talking about removing the ascent and descent from the calculation... which is _definately_ correct. If you leave in the resting portions of the dive, you end up with a SAC rate that gives you something smattered between your resting and your working rate. You're working rate will be higher, your resting rate will be lower.

That smattering between your working and resting rates is what Jim was [unfortunately not effectively] stating to be less accurate.

Generally I do not need people to interpret what I say..however in this case clearly I needed spectre to do so.



:drain:

 

[Silverback]

MAK52580 once bubbled...
Hello everyone. I had a question and thought that it might be best posed to tek divers. ...

Marc

Marc.. did all this answer your question...

 

[Charlie99]

Silverback once bubbled...
Marc.. did all this answer your question...

I doubt it! If Marc is like most divers, he is simply logging the maximum depth, not the average depth that is needed to calculate SAC.

SAC is pretty simple if you just go back to the basics and do it step by step. Here's an example of a 80' max, 43' average depth dive for 35 minutes that uses 1900psi from an AL80:

1. Convert psi used to cubic feet used. 1900psi from AL80 (77.4 cu ft at 3000psi) is 1900*77.4/3000 = 49 cu ft.

2. That is an average of 49 cu ft/ 35 minutes = 1.4 cfm actual usage rate.

3. Adjust for depth to get SAC ( or RMV if you prefer that term):
Ignore the 80' max depth. That is irrelevant.
43' AVERAGE depth is 43+33=76 fsw absolute, or 76/33= 2.3ata.

Divide the actual 1.4cfm by 2.3 to get 0.61 cfm SAC.

If you understand the logic of the above sequence, then you can regenerate the equations whenever needed without having to look anything up.

 

[Diver0001]

but..use the max depth to get the best, most accurate number.

Your thinking only applies if your max depth is deeper than your average depth. If you make a deep bounce the result is likely to be biased too low.

R..

 

[Soggy]

Your thinking only applies if your max depth is deeper than your average depth. If you make a deep bounce the result is likely to be biased too low.

Your max depth will always be deeper than your average depth.

 

[rjens]

I have also been putting together a dive planner using Excel. If anyone would like to try it out and give me some feedback I would really appreciate it. If you want a copy send me an email at rob.jensen@mckesson.com . The model is actually my attempt to pull together several threads re to RMV/SAC, multilevel diving without a computer and planning a dive. The model allows you to select your tanks, and either calculate your SAC after a multilevel dive or plan a dive including air needed for a multilevel dive (and taking into account your ending pressure group. I would love some feedback if anyone is interested...

Thanks
RJ

 

[NEWreckDiver]

Wow, This is a lot to chew on.

SAC rates are used for gas planning. You should have a working SAC rate and rest (deco stops) SAC rate. There really isn't any other reason for them. It should be calculated with a constant depth for a set about of time noting the about of air consumed. This is to give you a conservative number.

Remember if you run into trouble during a dive, more than likely you will need more time to resolve the problem. What gives you more time?

You really can check your rate on any dive. You do not need to make a special dive to calculate them.

Good Luck and Safe Diving