Switching back to HE based backgas after deco?

[CD_in_Chitown]

During the discussion on Rick Inman's recent Trimix dive it seemed like a couple of posters suggested returning to backgas when the symptoms presented. Specifically, not to pick on Rick M

The only post-dive critique I have for you is the same as what your instructor suggested, in that any physiological wierdness should dictate a change of gasses if you have another suitable one handy.

Is this a qualification statement "if you have another suitable one" that would indicate switching to an HE based gas after deco obligations are cleared is bad mojo?

That is the impression I received in my Ad Nitrox/Deco class, yes I'm a tadpole in the tech realm that only knows the theories regarding Helium that leak out into the common body of knowledge, so help me understand would that be a viable option in a situation where you had cause to question the integrity of your deco gas, or is the other "suitable" gas to buddy-breathe your buddy's deco cylinder? Is the never switch back to He mix a hard-fast rule that is not to be tampered with? Or are their risks associated that must be addressed upon surfacing but wouldn't be as catastrophic as say breathing CO tainted or otherwise unfit deco gas?

This was offered as a tid-bit of knowledge that I would learn more about in Trimix training, so if its totally misunderstood and not applicable here feel free to flame away, I can take it. Like I said I'm just wondering in reference to some of the responses in Rick I's thread. I'm not executing with He for the foreseeable future and at least one class from now, just looking for clarification on the theory. Which I understand is based on the speed at which He is on-gassed.

Thanks for any input,
Chris

 

[in_cavediver]

Is this a qualification statement "if you have another suitable one" that would indicate switching to an HE based gas after deco obligations are cleared is bad mojo?

I'll throw a different light on the subject. When I have done deeper/longer dives with more than 15 minutes at 20' on 02, I take 'air' breaks which has involved switching back to a trimix blend for a couple minutes. This is more to address the issues with long exposures to 02 at high PO2's but it has on a couple occasions meant I switched back to He after some deco, but not all.

In Rick's case, I think the overall message is that if you think you have a bad gas, get to a known good gas, then sort out the problem. Staying on a suspect gas when you have another, viable by PO2 gas to breathe, is insanity.

If I suspected contaminated deco gas, I would solve the immediate gas problem however is easiest (read backgas), then execute my lost deco gas contingency. (you do have one right?)

 

[Rick Murchison]

Except in the case of an extreme hypoxic mix (in which case you'll have a travel gas to go to) your back gas is suitable as a breathing gas regardless of your deco status. You must always be able to complete deco with loss of a deco bottle, and one possible loss would be the decision to abandon it because you start feeling weird while breathing it. As in_cavediver mentioned, you just fall back on your contingency deco schedule if you haven't already completed it. No big deal.
As for switching back to a gas with He in it after deco's complete, I have no concerns any more than switching back to it for air breaks on a long deco schedule or breathing it on the next dive. Perhaps there's some confusion w/r/t isobaric conterdiffusion here, but I don't really see how.
Rick