Diving Hypoxic Diluent on a Normoxic Dive

Depth-junkie

How is possible???

If you are on a set piont of say 1.3 at 20m on a dil of 8/65 you are on a fraction of 43% O2, no diffrent than 1.3 with an air dil. Dill is irelavent

ATB
Gareth

dave t

just to go back to the origional point 10/50 is NOT an allowable gas for normoxic trimix course, if you read the standards (carefully) it says dives must be conducted on normoxic trimix diluent. This means breathable at the surface,

10/50 is of course not breathable at the surface

decoweenie

Dave,

Thanks for confirming it!

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clivethediver2

Hi all.
My IANTD CCR normoxic trimix c-card has on the bottom of it the following-
"Qualified to dive to 60m using trimix diluent containing a minimum of 16% O2 and bailout gas containing a minimum of 20% O2."

Cheers Clive

silent running

Hi Gareth, yes of course you're right that sitting at the same depth with both dils, the FO2 is no different. But I'm speaking of during the actual ascent, with loop volume expanding and venting out both the dil and the newly added O2, and this in addition to the small amount of inert gas coming out of the body back into the loop. It will take longer for the PO2 to stabilize during ascent with a hypoxic dil, than with a dill that has a higher O2 content, like air. That's all I'm saying, that's what I noticed in my limited, recent experience with hypoxic dil. -Andy

Steve

I think someting else might of been happening mate. Absoloutely no dil should be being added during ascent so the dil is irrelivant. The amount you are offgassing between injections would be really really small and I doubt it would be measurable by anyone.

If I could be so bold as to make a guess I'd say your loop volume was quite full (possibly at max) and the solenoid wasnt able to keep up with your ascent speed.

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silent running

Hi Steve, no I certainly wasn't adding any dil. But there certainly is still dil in the loop during ascent and if it's hypoxic and expanding and the PO2 is dropping, then one would notice a bit more O2 injections than the same ascent with a non-hypoxic dil, right? And any venting will still take away some of the injected O2 as well, right?

While it may be possible that my loop volume was too high, I'm usually pretty good at min vol. The twins always remind me when the loop is too full via crotch strap and I don't remember any complaints from those 2 on my first hypoxic dil dive...

But you may have stumbled on to another possible explanation. The injection period/amount on the Prism may be less than other units. This would also explain why my Inspo buddies have more spiking issues on descents than I do, which unless I stop, is never. -Andy

Steve

Without getting cute with gas laws; wouldn't all the gasses in the loop be expanding at the same amount? With no injections the fO2 wont change. The pO2 will drop due to decreasing pressure. This has nothing to do with hypoxic V's normoxic dil.

Venting will remove added O2 that's for sure, but it will be the same amount when using hypoxic or normoxic dil if you have the same pO2 and are at the same depth on 2 seperate dives.

I run my loop really low on ascent and breathe a breath out my nose as I leave each stop. This seems to help with the ammount of dilution the injected O2 has to deal with and my pO2 stays very close to where I want it too.

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dteubner

But there isn't any dil in the loop. There is only oxygen, helium and nitrogen. At the start of the ascent it doesnt matter at all where the gasses came from.

If you take a simple example. Imagine a 30m dive on Heliox with a setpoint of 1.2, with 2 dils, say Heliox 10 and heliox 30. With the hypoxic dil, more of the oxygen in the loop has come out of the O2 tank but, when you start the ascent you have 1.2 ATA of O2 and 2.8 ATA of He in both cases. It just doesn't matter where the O2 comes from. If you don't add any more dil the oxygen will fall at exactly the same rate on the ascent and offgassing will be identical.

The only difference in offgassing would occur if the He/N2 ratio were different between the two dils. The O2 content of the dil is completely irrelevant on a direct ascent.

Dave T

silent running

Dave, Steve, I think I understand what you are saying now. But just to make sure I'm being clear, I'm not talking about switching dil during ascent. The comparison is btw different dives with different dil, but the same shallow portion of the ascent. I think I should have stayed with my original air dil vs. 8/65 tmix shallow ascent comparison. When I asked another Prism diver about this, they said thay also noticed an increase in the injections during ascent btw using hypoxic tmix and air dils. I guess it's possible we are imagining it, and I haven't done enough hypoxic dil dives yet to be sure...

But wouldn't a high He hypoxic mix say 10/90, being less dense over all than an N2 based mix-like air-expand more as the pressure drops and keep the PPO2 lower than when using an N2 based mix and thereby need more O2 injected to keep the same PPO2? And what about that He coming out of the body during the ascent? I know it's not much, but it's coming out faster than any N2, right? Wouldn't it also affect the volume and at least cause more frequent venting of the loop and thus increase the injects?

Steve, good tip about letting out some gas after a stop, I think I usually do, but I'll try to make it a habit. -Andy