Calculating Ideal Gas Mix?
- Thread starter Kingpatzer
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For sure. The implied meaning of "best mix" as used in the TDI manuals is "the mix that enables you to push your ppO2 as close to 1.4 for the bottom portion of the dive to minimise inert gas loading", but that is just a single decision variable for what is actually a multi-variable decision. And so that mix isn't always the best mix for the actual dive.....
Who are these "most of us" you speak of, DIR divers or tech divers in general? I find best mix is the mix that will give me a reasonable po2 for the planned depth. Standard mixes have certain advantages but are sub-optimal for most depths from a decompression standpoint. Sure they are easy to mix but so is air if that's all you're concerned about.
Slamfire
Contributor
One of the points that I find very attractive about rebreathers is the "permanent" best mix you have throughout the dive, irrespective of depth. I find that the open circuit best mix logic gives you a more compatible transition into rebreather setpoint thinking -- at least from a diving philosophy point of view. Not that transitioning from a standard gas philosophy to rebreathers needs to be particularly difficult.
Also, rjack, I'm not that far away from you and I commonly do planned deco dives with 25% and 28%. For me they work well for dives were 25/25 would be the std mix to use.
Also, rjack, I'm not that far away from you and I commonly do planned deco dives with 25% and 28%. For me they work well for dives were 25/25 would be the std mix to use.
UCFKnightDiver
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Except that your constant "best" actually becomes counterproductive below about 220ft-ish. Ie. you end up with more deco than on OC
Why bother? A 3% difference is pretty trival. Your software might give you some precise difference in deco but its cut with an axe.
I'm not afraid of deco, I'll take the "penalty" for diving a mix which might be considered "weak" on O2 if it keeps my site options open.
Aka "best" is not necessarily ppO2 1.4 Although most texts are written as if you should hit 1.4 at exactly 152ft or whatever (as Andy pointed out)
ucfdiver
Contributor
I definitely agree best mix should not always mean 1.4. To me best mix means selecting a PO2 and END applicable to the diving conditions and even logistics of the dive, Best mix for 150ft off Cozumel is not the best mix for a cold high flow cave dive to 150ft. I tend not to think in absolutes and adapt to my conditions.
Slamfire
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How so? Can you expand on this? I'll be the 1st to admit I don't know much about rebreathers. I'm just starting to read operator manuals of the Cis Lunar MkIV and the Evolutions. I'm guessing your observation has something to do with helium loading? Aren't rebreathers flexible enough to be programed to deliver OC type depth-variable ppO2 so that you don't incurr the longer deco you are referring to?
I don't bother about chosing one over the other. I meant that I'll use either 25% or 28%, whichever I have available in my tanks for, say, a 130' dive. For instance, I may have a residual one third of my tanks with 32% when I go to top them up with air, so that I end up with 25% after they're filled. I'll then use them to go to 130' even though the best mix should be 28%. As you pointed out, the 3% difference only adds a few mins of extra deco and you're with a slightly safer ppO2. That said if I happened to know at the fill up time that my next dive will not be deeper than 130', then I'll fill them up with 28%.
If you compare a rebreather diver using 1.2 throughout the dive and an OC diver using 1.4 for bottom gas and well selected 1.6 PO2 deco gases then the OC diver will probably incur less deco on dives in the 200ft+ range. If that same RB diver used 1.4 thought the dive he would probably regain that advantage but high constant PO2s on CCR are not a good idea.
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