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G, Thanks for the followup. I've got multiple questions, but please bear in mind I'm trying to learn here not to doubt the veracity of your statements. First question, in a private e-mail someone said the ideal gas setup for the type of dive we're talking about here (<=150m, <=30min), would be hyperoxic trimix with 50/50 deco. Why would you want hyperoxic? At 150m, a normoxic mix is already a PO2 of 1.26, why push it further? I'd think with as "benign" as He seems to be, it would be better to keep the PO2 low and add more He (to keep END <= 100FSW). The remaining questions have to do with choosing 50/50 over O2. As I said before, I had thought O2 would be better for most of the reasons listed in the Baker's Dozen against 80/20. In particular these seem to stand out: "5) The 80/20 mix is in fact totally useless and contraindicated as a deco gas. At thirty feet it is only a 1.52 ppo2 ( the real 1.6 ppo2 gas would be 84/16) and as such does not either provide the right oxygen window, nor does it does it work as well as pure oxygen without an inert gas at any depth." The 50/50 is has an even lower PO2 at the shallower stops and has even more inert gas. Similarly: " 8) Any perceived decompression benefit of using a higher ppo2 at 30 feet with 80/20 is then given back by the lowered ppo2 at 20 feet, not to mention the fact that the presence of the inert gas in the breathing mixture defeats the purpose of using oxygen in the first place ( see the Physiology and Medicine of Diving) . The ppo2 of 80/20 at 20 feet is 1.28, not much of an oxygen window, and at 10 feet it is 1.04 - useless for deco. To make matters worse, you can not get out from your 30 foot stop in an emergency ( not doing the other stops) on the 80/20 mix without really risking a type 2 hit." Again, wouldn't the same logic apply to 50/50? The benefit of the higher PPO2 at 70 feet is given back at shallower stops. And definitely: "9) This is a dangerous method to achieve a greater total volume of gas for the bad breathers (another obvious reason the gas is in vogue), who should not be incurring these decos, and even that benefit of having more gas is lost since it is breathed at 30 feet, and then has to last for the other stops." If you start the 50/50 at 70FSW, you're definitely going to need more gas than the O2 at 20FSW. And: "11) In an emergency situation, getting onto the pure O2 for 20 minutes or so (for long dives something approximating the bottom time or a any decent interval) would give you a real good shot at getting out of the water having missed the rest of your deco and living through it with pain hits only." And similarly: " 12) If there is some problem with your deco or you otherwise develop symptoms and need oxygen either on the surface or back in the water, it is silly to have not had it there all along." So what I'm wondering is, is the following explanation the reason why 50/50 is better than O2: By using 50/50 you open your O2 window much earlier and use the gradient to help clear much of your deco earlier. And the reason why the Baker's Dozen doesn't really apply is that the previous mentioned benefit outweighs the "penalties" whereas the depth you'd start 80/20 is not significantly different enough from where you'd start O2 for it to outweigh all of the penalties. Oh, and can you elaborate a little more about what you mean when you say "50 is better as you take less beating". What beating do you mean? The longer O2 exposure? Thanks for any insights you can provide. Regards, Dave Trey wrote: > > Yes, but in practice for a fit diver, 50 is better as you take less > beating if you can deco from the gradient successfuly, which I can. I > use no deco gas if possible at all. > > David Chamberlin wrote: > > > > I'd also be interested in hearing some comments on this. A while ago, > > when asked about single gas usage, I could've sworn JJ responded that > > for shorter dives in the <50m range, O2 would be the choice. I always > > found that curious since JJ and G seem to try and clear out their deco > > as early as possible, which would indicate 50/50 would be the better > > choice. Then when trying to find the message I was thinking of, I > > stumbled across this post from JJ: > > -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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