Trimix gas switching

[saturated]

Great add,

you are the man! That is what I've been beating my head trying to remember. Just could'nt quite grab the words.

Even if this can be difficult to explain with isobaric counterdiffusion principles, it is possible that the variations in gas density and pulmonary dynamics may slow down the elimination of helium.

Abstracted from: C. Edmonds, C Lowry, J Pennefather. Diving and Subaquatic Medicine ( 3rd Edition). Chapter 11, Historical and physiological concepts of decompression. Butterworth Heinemann, Oxford, 1992.

There we go, so the answer may have been incorrect after all, well I've survived on Heliox for along time, so I guess I'll stick to what works.

PS: There are plenty of DIR goats out there

 

[madmole]

Yep, but the DIR goats are keeling over far more than the original Buhlman ones

 

[Tigerscuba]

Hey Sat I think all those years living in a bubble melted your brain.
in fact are'nt you in a bubble now.

Anyway Madmole is correct; however the jury is still out on that one. Mathematically it is possible to justify the switch to air, with current models. While I do know the "rigs" have done a great deal of research with helium and hydrogen the nitrogen research is in its infancy. From reports I've read though the arguments are indeed strong that a isobaric counterdiffusion between He and N2 may be a great risk for the Vestibular bends. Not a good way to spend your weekend.

Thu jury is still out so like sat, I'll stick to heliox.

 

[fins wake]

Anyway Madmole is correct;

Not quite as simple as that!

I've been talking today to someone involved in producing deco plans for 250 - 300m dives and they are amasing a load of evidence that their divers are getting bent if they swap back to Nitrox or air at 40m or so, Staying on the He doesn't do this

It would be good if "someone" had these results published for peer review in a hyperbaric physiology journal, since it runs counter to a general concensus.

Nitrogen and He do interact, its a thing called isobaric counterdiffusion

Yes, they interact. Deep tissue counterdiffusion is a very possible outcome of switching from nitrogen to helium , particularly at a 'deeper' depth, as at shallower depths the results were less severe (Harvey 1977, Hamilton et al 1982). There are similar results in complex COMEX experiments when switching from hydreliox to heliox, BTW.

However, it is generally accepted that divers may switch the inert gas from helium to nitrogen (Hamilton 1976, Lambertsen 1989). I note that Madmole mentions that this

causes such nasties as Vestibular bends (a bend almost exclusive to folks who swap from He to Nitrogen mixes)

Yes, this is the controversial part. Perhaps this might have happened in some instances. Alternatively, it could be the result of the use of an inadequate decompression table (Hamilton&Thalmann in Bennett&Elliot 2003) or perhaps even of incorrect decompression procedures?

Now, there are reports dating back to WWII of adverse effects when switching to air at 50 metres (Momsen 1942) and possible vestibular or inner ear DCS or counterdiffusion sickness when using rapid shifts deeper than 33 metres (Hamilton 1976). Hamilton & Thalmann (2003) also mention the possibility of this being the abrupt exposure to nitrogen narcosis ...

Interestingly I've been talking today to someone involved in producing deco plans for 250 - 300m dives

At dives to these extreme depths, I certainly believe a deep gas switch to air (nitrogen) may create problems. As I've previously suggested, if nothing else for the effects of nitrogen narcosis.

From the above any budding technical diver attempting a nitrogen-rich switch should contemplate doing that at shallower depths, say from 20 metres. This is consistent with my first post, BTW:

The disadvantage, as I see it, is that if you do the switch too deep, you will end up heavily narked,

(I'm consistent you, know, there is method to the madness ... )

Alternatively, of course, one runs with Thalmann's successful findings on heliox decompression (1985) and stays on the helium-based 'bottom mix'. Nobody actually suggests otherwise. But then preferably by paying the added decompression penalty. It's a free choice.

I'm sorry this is such a long post, but there is a tendency on many forums to run with very simple opinions and solutions decompression-wise and although I think Madmole is a fantastic resource on Inspiration diving and all-around great guy, I don't really agree offhand with all the decompression opinions expressed in his post.

Its to do with the larger nitrogen moleclules attaching to the He bubble seeds. A small He bubble is mobile and diffuses quickly, once the N attaches it alters the physical properties of the bubble.

An almost inert gas attaching to a truly inert gas molecule? Hmmmm ... Nope, sorry, can't buy that offhand ...

There we go, so the answer may have been incorrect after all, well I've survived on Heliox for along time, so I guess I'll stick to what works.

There is this somewhat exasperating tendency to believe (particularly on US boards) that there is a 'right' and a 'wrong' ... This post hopefully shows it's much more complicated than that. But as a conclusion, yes, if you're happy with heliox decompression, stick with it. But do try and pay the decompression price.

 

[Tigerscuba]

Without a switch we do pay the price in Min's but ultimately there is no argument involved with staying on. Just more time on deco.

 

[saturated]

Hey guys,

first my appologies. I've read some of what I wrote and it is alot of babbling.

Tigerscuba-No I am not in a bubble at the moment, I am on my 2 month surface interval. 2years to retirement....

Finswake-Congrats you've hit the nail on the head. There are no absolutes when dealing with theory. As stated in your post the pressure gradient of the gas switch probably is the greatest contributor to isobaric counterdiffusion(if it happens at all). We do run into problems when we accept theory as fact. So while gas switching would reduce my deco according to known models, I choose not to. While I may spend more time in the water, it least my profiles are based on what I consider to be fact. So I feel pretty safe in staying with the Heliox. I would note that I cannot however argue with anyone doing the switch, we may both be right.......Cheers and thanks for all your info(well researched).....

 

[rbdave]

Hi Fins Wake,

How is the ppO2 of your 'bottom mix' markedly lower than in your diluent? Are you running very low set points at depth?

I'm not sure I get what you mean. I always run a setpoint of 1.3

To expand I run 25/25 Dil in my onboard 3lt tank. I then carry 2 sling tanks either matching 7lt or matching 12lt.

In one of the 7lt tanks I currently have 13/54 and in one of the 12lt tanks I have 9/71.

As an example, 40 minutes at 60 metres.

I'll just use the 7lt 13/54 plugged in as my bottom diluent. The other 7 lt tank will have nitrox 32.

This gives me an END of about 18 metres on the bottom. On my ascent I stop below the 40 metre stop and disconnect the offboard 13/54 diluent and plug in my onboard 25/25. I do a flush and allow the PPO2 to settle and move up to the 40 stop.

This drops the HE content but not as much as doing an air flush. It has the advantage of getting you out of the water quicker than riding bottom mix diluent to the surface. You always keep the END low. And often we do multiple dives in a day and I think it preferrable to keep your nitrogen loading down.

Cheers

Dave

 

[madmole]

Interaction of a He and N buibble isn't as simple as one inert gas meets another, there are surface tension issues and they are both in an active and dynamic medium which they do interact with.

This is why its a grey area, nobody fully understands the interactions, let alone is able to make an accurate model. We still have a lot to learn

And yes, the results I quoted where on swapping after VERY deep diveas and hence a whole different ball game to a 30m bimble. (mad buggers, who in their right mind goes to 300m??)

Other suggestions mentioned in other forums is that the larger N molecule physically blocks the outgassing of the He, this has been quoted as a cause for the Vestiular bends

I'm not an expert on this and am just quoting other postings. I suspect at prsent we just dont know enough about the interactions going on. More Goats needed :wacko:

 

[BigJetDriver]

saturated once bubbled...
There we go, so the answer may have been incorrect after all. Well I've survived on Heliox for a long time, so I guess I'll stick to what works.

PS: There are plenty of DIR goats out there

My all-time favorite diving medical text is "Diving and Subaquatic Medicine" by Edmonds, Lowry, and Pennefather. Comparing that with the information in"Technical Diving in Depth" by Wienke, some comments from Savatsky of DCIEM (now DRDC), and some historical stuff from COMEX, the consensus seems to be that light to heavy is OK, but not the reverse. A helium to a nitrox switch on ASCENT will not do harm. The helium will continue to off-gas rapidly, while any on-gassing of nitrogen will be slow.

The major problem in a lot of these arguments about run-times and gas switching (or not) is that we are comparing apples to tangerines. Due to the design of the older deco models, helium is not handled as well as modern research shows it could be.

I believe, please note that this is indeed personal opinion, that the RGBM is the tool we have needed to properly schedule helium in the decompression environment.

BTW, I've wondered for a long time what it is with these deco researchers and goats. It's a kinky kind of thing, eh?

 

[madmole]

From Mark Ellyatt from the CCR forums, on this very subject. Showing that light to Heavy is VERY bad. He is a DIR person so was using this as a dig at CCR but its relevent never the less and sums up the discussions on vestibular bends taking place lately

------------------------------------------

A couple of months back;I was after some answers re counter diffusion; here is the gist of it

As I was just on the receiving end of counter diffusion related antics. I went looking for answers

Having conversed with some doctors who seem pretty clued up on deco theory (not the IT professional type (oxymoron) It would seem that counter diffusion is a fairly predictable outcome when using decompression profiles giving incomplete decompression. When ascending from a dive using what can best be described as "the usual bollox deco software" and certain values are reached in the ear compartments, the last thing you would need is a gas change to accelerate deco.

When gas changes take place generally a heavier gas (o2 or n2) has its relative Pg increased ;when this occurs a compartment laden with He will be counter infused with heavier gas as the heavier gases is invariably more soluble. The helium super saturates as its pathways are blocked by the denser inward gases. The super saturation normally takes place in the Inner ear area (because of its unusual dual action on/off gassing mechanisms)

Counter diffusion will super saturate any type of compartment depending on that compartments current loadings, When this occurs in the Vestibular canals your attention will be directed solely to the symptoms that will follow.

Deco software out there at the moment are very poor interpretations of others work ( er..I could not do better though) Most use tissue overpressure values derived from archaic research (although from a time when test dives existed). These OP values are then fondled by IT professionals adhering to the next latest fad..ie faster deco.

On the upside, current software seems adequate; when used upto 80m (max 30mins?) beyond this level of saturation then counter diffusion is likely unless ascent protocols akin to commercial schedules are adhered to.

Shallower and shorter helium based dives with Rocket ascents switching to air or nitrox would provide similar outcomes as the deeper longer dives following insufficient decos…quite literally when the gas switches take place the Inner ear values are chronically exceeded causing inner ear deco sickness / Vestibular DCI.

Until research is done (the non petri dish type) then maybe to extend the last pre gas switch stop by 5-10 mins(?) to further lower tissue tensions (lengthening deco) would be advised. Decompression software should give warnings when counter diffusion thresholds are reached or EVEN simpler just give proper deco schedules based AROUND subsequent gas changes and adequate deco.

Counter diffusion is as likely on CCR, as constant Po2 ascents place the diver much closer to an EAD (symptoms edge) than an OC diver would be exposed to. The trick is to lower setpoints below current recommendations when doing big dives. A longer deco will be incurred but necessarily so. Diving to an EAD is as unsafe as diving to an air limit. The high Po2 will get you quite shallow when you switch to high Po2 oxygen shallow, the lack of helium will cause a problem.

Combine all this with Co2 and OXTOX I can see why constant po2 boxes are even more popular ;-D

The lower the Po2 the longer the stops , rocket science I know. Deeper stops have a place im sure ;but should be implemented as a matter of course by the deco program ; not added by the user. More recent Free gas models would seem ideal, but have been created without thought for extreme dives (below 80) and I can give examples of dives where they fail.

I could womble on more but the simpsons has begun!

This aint going to get everyone or anyone if they stay relatively shallow, but for the lemmings going deep then…read some stuff being published soon. The improved software will follow after the lawsuits J Currently ; the truth is not out there !


M