Dil switch off helium during deco

[beldridg]

Same, I am never carrying 32% along on a deep trimix dive.

If you flush with 50% at the end of the 70ft stop (let's assume you actually get it up to 1.6) your ppO2 will drop to 1.4 after you move up to 60ft.

If you only manage to flush your 70ft stop up to 1.5 you'll be at 1.35 after coming up to 60ft.

Paraphrasing Simon from past posts

The helium penalty doesn't seem to be supported by empirical data but there are other (ill defined physiological) factors which increase risk as you go deeper. So by following the helium penalty you at least partially addressing the rising risk of DCS for other reasons.

Good point. I could always do the Nx50 flush near the end of my 70' stop.

In the big picture, it probably doesn't matter either way, it is more about flushing out the Helium quicker than you would with the Dil gas (rich in helium) in the loop and that will have an impact on runtime. Whether I do that at the start of the 70' stop or the end likely doesn't have a big change in runtime.

But, doing the Nx50 dil flush at some point during the 70' stop does.

As you know, managing runtime and exposure in cold Pacific waters is definitely a consideration on these dives and doing a dil switch/flush to help reduce runtime and off-gas the helium faster might be a good idea.

I really believe in "there is no free lunch" so I'm sure there is a tradeoff with that plan.

- brett

 

[rjack321]

Good point. I could always do the Nx50 flush near the end of my 70' stop.

In the big picture, it probably doesn't matter either way, it is more about flushing out the Helium quicker than you would with the Dil gas (rich in helium) in the loop and that will have an impact on runtime. Whether I do that at the start of the 70' stop or the end likely doesn't have a big change in runtime.

But, doing the Nx50 dil flush at some point during the 70' stop does.

As you know, managing runtime and exposure in cold Pacific waters is definitely a consideration on these dives and doing a dil switch/flush to help reduce runtime and off-gas the helium faster might be a good idea.

I really believe in "there is no free lunch" so I'm sure there is a tradeoff with that plan.

- brett

Eh I don't bother. My coldest dives are either north Vancouver Island caves (down to 4C/38F). Or trips to Lake Superior (same temps on the bottom but with a thermocline). Puget Sound is at the very least 44F and that's in March. I consider that only modestly cold now.

I suppose whenever I get back into my cave projects (covid willing) where I actually have 2 deco gasses, I might try doing a 50% flush. I probably won't reset my dil though, just use it as an extra layer of conservatism. The problem is I'm not exactly sure having nitrogen as my loop inert instead of helium is more conservative.

 

[Wibble]

I run 50/80 but I'm considering changing to 50/75 because it more closely reflects what I actually exit the water with. I always check my SurfGF before exiting and it is 70-75 on most dives.

There's a move to reduce GF-hi downwards from 80, certainly spoken about by Dr Simon Mitchell.

 

[tbone1004]

There's a move to reduce GF-hi downwards from 80, certainly spoken about by Dr Simon Mitchell.

I wouldn't say it's a "move" though you can ask @Dr Simon Mitchell directly on here. From what I've understood is that he is personally running a lower gf-hi but I would hardly call that a "movement"

 

[Dr Simon Mitchell]

Hello,

tbone's interpretation of my 'position' on GF is correct. I frequently find myself in the position of being the only diving physician in remote locations and would prefer not to get sick. So I choose conservative decompression settings which often includes a GF Hi of 70. That is definitely not to say that 80 is too high, but is rather a reflection of my context sensitive 'risk thermostat' in particular situations. Having said that, I would say that I never dive with a HF Hi greater than 80 these days.

On the original topic of this thread, I have attached a paper on decompression from technical dives written by David and myself in 2013. Most of the concepts discussed remain current even though it is 9 years old. It directly addresses the issues being discussed here and interested thread participants should read it.

To briefly summarise, there are doubts over whether helium and nitrogen kinetics really do differ in the tissues of most relevance to decompression from typical technical bounce dives. Thus there are doubts over whether the helium penalty should exist in this sort of diving, but its existence in decompression algorithms may have resulted in the right amount of decompression but potentially for the wrong reasons so lying to your computer about it may not be a smart idea.

This uncertainty also makes it uncertain whether a helium to nitrogen switch during decompression actually has any benefit. There is some evidence (see the paper) that it doesn't. However, my sense is that if you want to minimise the helium penalty, actually doing the switch is probably better than lying to your computer. This would be increasingly true of more extreme technical dives that are approaching what one might call 'sub-saturation' dives (eg Harry and Craig's recent 16 hour decompression in the Pearse Resurgence), because as we explain in the paper whole body helium washout is indeed measurably faster than for nitrogen in saturation diving. So the more extreme the dive the more likely it is that slower tissues where helium-nitrogen kinetics actually are different would be loaded, and the more likely it would become that a switch could result in a 'counter-diffusion exchange where loss of helium is faster than uptake of nitrogen, and therefore less tissue supersaturation. However, switches should not be made too deep at a stage when the inner ear remains supersaturated because that is the one tissue that can be adversely affected by counter diffusion in a helium to nitrogen switch during decompression.

Happy to answer any questions that come out of reading the paper.

Simon M

 

[vitaly]

Hello @Dr Simon Mitchell,

Thank you for your reply. I do 200'-270' CCR dives a few times a month and dive below 300' occasionally. I use 50/70GF and never had DCS symptoms so far. Would you recommend to use O-Dive to control the decompression quality?

Thank you,
Vitaly

 

[rjack321]

Hi Simon
So if the helium penalty is "the right idea but for the wrong reasons" what are the reasons or the mechanism for that extra time? Do we know or have some educated guesses?

 

[macado]

Thank you @Dr Simon Mitchell for responding with your input and others for this thread. I'm very happy it has remained civil and has not devolved into a complete flame fest that usually happens with these conversations.

 

[Wibble]

Would you recommend to use O-Dive to control the decompression quality?

What is "O-Dive"?

 

[GF99/99]

Dil switches for optimizing decompression is a controversial topic. As it seems that most who have already chimed in are doing them, I too do them however I have been doing multiple dill switches for example typical 130m dive would be dil switch at 60m to a 17/40 a 17% at 60m is 1.2 so good way to do a cell verification and also at this point I would bump up set point to 1.2 (bottom phase I dive at a 1.0) then next dill switch typically I do at 18m to a 50% a 50% at 18m is 1.4, once again good cell verifications and also at this point bump set point up to 1.4. Then lastly at 6m do an 02 flush and again good cell verification, then finish deco at 1.4 (you would be surprised on long dives how much CNS you an reduce by staying at 1.4 vs. 1.6 and you only add 3-6min of deco)
Or for a 180m dive I would add in another switch at 100m to 11/65, cell verification and bump up setpoint to 1.2 and then same switch at 60m also.

For deeper dives I dive fairly conservative (or at least to my perception) 50/70 GF and the last time I used an O'Dive I had a 95% quality rating. I am 35, about 180lbs, 31" waist, work out, healthy eater, non smoker, non drinker, and I always hydrate the day be for a big dive, also drink electrolyte drink on deco.

I think dil switches is a very personalized thing especially based on our body types, personal experiences, dive conditions, who your mentors were, etc. What I do some might think is stupid but it works for me and what works for others may not work for me. Due to the lack of definitive scientific data, honestly I think dil switches is a lot of personal trial and error and seeing how your body feels after the dive in theory I think they make good sense but in reality I think it is very person dependent kind of like choosing a GF.

I would be very curious to hear from anyone that is very opposed to dil switches, in the past I have heard from some that are very against them and say they are very dangerous.

Also since there was some mention about MCCR's and ECCR's I have done dil switches on both no issues even when flushing through to 1.6. I have even have done dil switches on an MCCR and ECCR at the same time while diving bail out rebreather.