Keeping Helium in your deco gas

[Waskey]

Was just reading something, and I've also been in a few discussions about this rescently.

What do people think about the theory of keeping a bit of helium in your travel mix, and sometimes in your deco mix, to prevent a helium spike when ascending? Does anyone actually do it? Or is it more a case of the longer the run, the more helium you put in?

[Janos]

I looked into a while back. I think that it's a good think for intermediate mixes, but I'd always want a Helium-free final deco mix.

Janos

[Okeanos]

Originally Posted by Waskey

to prevent a helium spike when ascending?

It's actually to prevent a nitrogen spike. I run Helium in my bailout mixes, I have a 30/40 and an 80/10 and also 100% O2. You should try and never increase you % of nitrogen, small amounts are OK, a lot depends on the dive and the diver.

[NotDeadYet]

I have helium in my dil for all my diving so all the deco is done with helium in the mix, even <40m dives. I must admit, I feel a hell of a lot better post-dive since I started doing it, not fatigue at all and no niggles. At 6m I go to pure O2 though, I don't see any advantage (and a lot of disadvantage) in having helium in your last stop.

Cheers,

Stuart

[nigelH]

Originally Posted by NotDeadYet

I have helium in my dil for all my diving so all the deco is done with helium in the mix, even <40m dives. I must admit, I feel a hell of a lot better post-dive since I started doing it, not fatigue at all and no niggles. At 6m I go to pure O2 though, I don't see any advantage (and a lot of disadvantage) in having helium in your last stop.

I run a medium helium DIL and although I'll flush it a bit to get the O2 up at 6m it's never 100% so I'm always breathing some Helium until I'm back on the boat and switching to tea.

I can see a case for not going back onto a high helium mix once you are off it but I'm not sure the plan for getting off helium if you are only substituting nitrogen is so strong.

[NotDeadYet]

Originally Posted by nigelH

I run a medium helium DIL and although I'll flush it a bit to get the O2 up at 6m it's never 100% so I'm always breathing some Helium until I'm back on the boat and switching to tea.

I can see a case for not going back onto a high helium mix once you are off it but I'm not sure the plan for getting off helium if you are only substituting nitrogen is so strong.

I suppose what I mean is, I don't see any point in having any inerts in the final stop gas. It's your longest stop, why choose a gas that's optimal only for the shorter stops.

If I shut the ADV off and put a bit of effort into flushing then I can get it to 1.6 and have it stay 1.5+. Obviously you'll have inerts from offgassing causing the ppO2 to decay over time, on short stops it's irrelevant but on longer stops then when I do breaks I do a flush when I go back on the loop which gets it back up again. It's a bit wasteful on O2 but it's still cheaper than OC.

I've often wondered if we'll one day see deco tracking based on monitoring what volume of inerts are offgased. The system knows the ppO2, an electronic rebreather knows how much O2 is added, by keeping a track on ADV/dump valve activity then it could be interpreted from that. I guess if you dump through your nose then your buggered. Or alternatively, watching pO2 decay on a pure O2 loop at 6m might also give an indication of when deco is finished.

[Janos]

Originally Posted by NotDeadYet

If I shut the ADV off and put a bit of effort into flushing then I can get it to 1.6 and have it stay 1.5+. Obviously you'll have inerts from offgassing causing the ppO2 to decay over time, on short stops it's irrelevant but on longer stops then when I do breaks I do a flush when I go back on the loop which gets it back up again. It's a bit wasteful on O2 but it's still cheaper than OC.

How high can you get it on the KISS? I can only really get mine as high as 1.53, which I attribute to calibrating when the loop is dry and it's saturated at the end of the dive.

Also what ppO2 do you go down to on air breaks?

Originally Posted by NDY

Or alternatively, watching pO2 decay on a pure O2 loop at 6m might also give an indication of when deco is finished.

I think it's possible, but it would depend on your profile. On a longer dive where the slower tissues become controlling then you might not have the precision.

Janos

[nigelH]

Originally Posted by NotDeadYet

I suppose what I mean is, I don't see any point in having any inerts in the final stop gas. It's your longest stop, why choose a gas that's optimal only for the shorter stops.

I'll join you on that one. I also like O2 because 6m is still and 3m is always in surge and I don't want to be seasick on a stop with time to do.

Originally Posted by Janos

How high can you get it on the KISS? I can only really get mine as high as 1.53, which I attribute to calibrating when the loop is dry and it's saturated at the end of the dive.

The ppH2O coming out of you is satuated so 0.0627bar. It is temperature dependent but I suspect the loop is pretty even when settled so I think you win.