Dil switch off helium during deco

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moose_grunt

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For deep (300 ft +) ccr trimix dives, what's the current thinking about a dil swap and flush to a non-helium mix during deco, to reduce the overall deco requirements (ostensibly somehow circumventing the helium penalty in the process)?

I seem to remember this being a thing like 10-15 years ago, along with lying to your computer about helium content, but thought that kinda died out with the overall move to more helium and more conservative deco for a lot of people. IIRC, the topic was usually buried in discussions of ICD and deep stops, and usually ended up getting derailed by the latter.

The helium penalty article that Shearwater widely shared a few years ago seems to have rekindled this idea, and despite the preface, it seems that many people in my circle have adopted a trimix to nitrox dil swap and flush on the way up from deep dives. They are quite happy with the substantial deco savings you'll get from deco planning software (for example, 30 min at 300 ft with 12/65 at 1.3 gives ~185 min deco at 50/70, while the same parameters with a dil swap and flush to 32% at 100 ft results in 150 min deco, a savings of 30 min. It gets more substantial around 400-450 ft, taking an hour or more off), and the aforementioned article has been interpreted by them to mean that the ICD concern is largely overblown.

What're the current thoughts on this approach, especially in regards to the supposed greater deco efficiency (shown by a shorter deco time)?
 

rjack321

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Nobody actually knows. Sometimes it works, sometimes it doesn't. 15 years ago 300ft was a huge dive. Now people are doing 450ft (a whopping 50% deeper) and returning to the "huge dive" bag of tricks all over again. Beware unreporting of DCS which is probably less than in the distant past when we used language like "provocative profile" or "deserved hit". But nevertheless the numerator of DCS impacted dives is still almost assuredly larger than we actually know (e.g. DCS hits / total dives 300+ft with dil switches).

IBCD is almost certainly a myth if you keep your ENDs reasonable. ie switching to air dil at 200ft is going to be a problem, switching to air dil at 100ft is not likely a problem, switching to air or nitrox at 70ft is definitely no big deal with thousands upon thousands of IBCD free switches, The fHe to fN2 alarms you get in some software are primitive and ignore depth which seems to be a significant influence.
 

macado

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I'm pretty curious about this too and would love to hear more opinions about what the actual science says.

I don't really have any answers beyond what I've experienced myself. I admit to having done it more than a few times on 300ft dives but I'm not entirely convinced of the benefit vs. fudging the helium percentage in my computer (another very controversial subject in itself).

All things considered - I do know that on dives running same profile, same exact gradient factors, and same diluent (10/70) my friends that flushed with 32% on deco ended up getting out of the water about ~30 minutes faster than me on a 4.5 hour dive when I didn't change diluent and flush. None of us got bent but that's hardly a qualifier or an endorsement for doing this. The question is could I have simply gotten out of the water 30 minutes earlier with them? I don't really know.

All I know is I've gotten wildly different opinions about doing this.
 
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moose_grunt

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Thanks for the responses so far, keep them coming!

Personally, I wonder whether the time savings is an actual reduction of decompression requirements (I.e. you truly do need less deco), or whether it's just an alteration of the algorithm that results in an unclear acceptance of more risk (but people get away with it because they start with conservative settings anyways). For example, I can shave the same time off by diving a 60/85 with constant dil vs doing 50/70 with a dil swap. There's a clear assumption of greater deco risk when doing so, and I can probably get away with it without getting bent. I wonder if dil swaps are similar--less an actual benefit, and more of a finagling of the algorithm that gives a perceived benefit at the cost of greater risk.
 

beldridg

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I agree, this is a great topic and timely conversation for some of the dives I'm planning.

I'm not a doctor or a specialist in this area, but I think the main thing that a Dil swap gives you is a faster off-gassing of Helium in the slow tissues due to an increased pressure gradient differential.

When you are on CCR and you ascend, the partial pressure of Helium slowly decreases with depth as you add more O2 to maintain a constant PO2. However, the remaining gas still has a lot of Helium and so it doesn't off-gas quickly.

If you change the diluent to a lower Helium percentage, then it inherently has a greater partial pressure difference and therefore off-gasses quicker.

- brett
 

rjack321

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I agree, this is a great topic and timely conversation for some of the dives I'm planning.

I'm not a doctor or a specialist in this area, but I think the main thing that a Dil swap gives you is a faster off-gassing of Helium in the slow tissues due to an increased pressure gradient differential.

When you are on CCR and you ascend, the partial pressure of Helium slowly decreases with depth as you add more O2 to maintain a constant PO2. However, the remaining gas still has a lot of Helium and so it doesn't off-gas quickly.

If you change the diluent to a lower Helium percentage, then it inherently has a greater partial pressure difference and therefore off-gasses quicker.

- brett
It's the reason why OC deco is faster than CCR deco as well.
 

JonG1

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Its referenced here and discussed by the author on ccrx fb page, with demonstrable results in terms of bubble score, although other changes to ascent habits were also incorporated.
 

beldridg

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Its referenced here and discussed by the author on ccrx fb page, with demonstrable results in terms of bubble score, although other changes to ascent habits were also incorporated.

Thanks for the link.

It is interesting to note that he flushed out the helium with Nx32 but did NOT change the dil on his computer. So, essentially, he off-gassed the helium faster than his computer thought he did so he paid the "helium penalty" and did extra deco.

It would be cool to see what would have happened with his O-Dive scores if he did change his computer.

- brett
 

rjack321

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Thanks for the link.

It is interesting to note that he flushed out the helium with Nx32 but did NOT change the dil on his computer. So, essentially, he off-gassed the helium faster than his computer thought he did so he paid the "helium penalty" and did extra deco.

It would be cool to see what would have happened with his O-Dive scores if he did change his computer.

- brett
Yeah and he also had an extra 5mins (I think) on top of the 50/75 GFs and the attempt to breath 100% at 6m and up stops.
 

beldridg

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Yeah and he also had an extra 5mins (I think) on top of the 50/75 GFs and the attempt to breath 100% at 6m and up stops.

I also extend my deco on deeper dives. I usually add an extra 3-5 minutes of deco and then spend 3-5 minutes slowly coming up from the last stop (in my case, usually 6m/20'). However, I don't breathe O2, I just stay on the loop at 1.3.

I remember there was an interesting thread a while back about padding deco for long dives. One viewpoint was that it makes a lot of sense and the other viewpoint was that you've already done enough deco and if anything, you should pad shorter deco dives. I don't want to stray too far off the original topic though.

I am considering using a Dil flush on my next deep dive but need to think about it more.

I almost always carry Nx50 on those dives (and not Nx32). Any thoughts about flushing with Nx50 at 60' (I don't want the high PO2 hit at 70') instead of Nx32 at 100+'?

- brett
 

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