Trimix gas switching

[saturated]

I was just curious. How many of you switch to air and how many of you remain on trimix through out deco and why. Also if you could I know many planners give reduced deco time if you switch to air please list your planning software. I currently use pro-planner and do not switch to air. As the amount of helium in the loop is reduced on ascent;however, my deco(according to pro-planner would be reduced if I perform a gas switch to air). While I do carry staged air, it does'nt make sense to switch. I have always believed that any deco mix should not contain more nitrogen than the bottom mix because of solubility/permeability problems between helium and nitrogen.

Sorry I just did a dive with 4 other turtle divers and we were split on "gas switches".

This has also been posted on the inspiration owners board;but, nobody seems to be able to substantiate the reasoning with physiological fact. Please no I thinks, I would ect. We are only interested in physiological facts.

[rbdave]

Hi Saturated,

I'm unsure if I should post or not. Initially you ask for opinions but conclude with " We are only interested in physiological facts." ???

Anyway here is what I do. Instead of flushing with Air I flush with my standard onboard dil 25/25 which gets you out of the water faster than staying on bottom mix. I do the flush at 42 metres which keeps my END at about the same as my bottom mix.

I use V-planner. Of course you would be able to get out of the water faster if you did an air flush.

My apologies if this is irrelevent to the thread.

Cheers

Dave

[fins wake]

... cross-post you question on the 'Dr Deco' section of this board, Saturated ... :doctor:

The likelihood of you getting 'physiological facts' there may be stronger than on the Inspiration owner's list (although there actually are some pretty decent 'scientific types' there too). ;)
I have always believed that any deco mix should not contain more nitrogen than the bottom mix because of solubility/permeability problems between helium and nitrogen. Not quite sure what you mean? Is this another classic 'Trey musing'? As has been mentioned on the Dr Deco subsection of this forum a few times, commercial outfits use this type of gas-switching a lot. There would certainly appear to be solid theoretical reasons for doing so, certainly from a neo-Haldanean perspective.

The disadvantage, as I see it, is that if you do the switch too deep, you will end up heavily narked, by anecdotal evidence more narked than you would have been if you used the high-nitrogen mix as bottom gas in the first place. (I haven't tried this myself at depth, so I speak from hearsay. I think Exley's early experiments pointed to this, as well.)
I flush with my standard onboard dil 25/25 which gets you out of the water faster than staying on bottom mix. :confused: :confused: How is the ppO2 of your 'bottom mix' markedly lower than in your diluent? Are you running very low set points at depth?

BTW, I use V-planner (VPM-B) myself, mainly.

[saturated]

sorry guys, what I meant was I dont want to here some guy thats never used a ccr telling us how it should be done. I just want to here the reasoning behind both ways. I am currently researching this, in fact I got so interested I'm taking a couple days off. I know I really should get a life.

Finswake
No I dont lower my po2 at depth, what I mean is the nitrogen content of the dilutent. N2 is heavier than He and thus saturates/desaturates slower. When replacing he with n2 a period of supersaturation occurs due to the slower desaturation of the N2.

Most of my experience with mixed gas has been military and commercial and primarily heliox. And it has been awhile so please bear with me.

I have indeed done gas switching to air, but only when my bottom mix was not breathable on the surface. For example a 45 minute dive to 85msw at a USN approved po2 of 1.4 would give me a He mix of 14/86, so I would dive air to 40'. However, even with a dilutent mix of 14/86 my ccr would automatically add the o2 necessary to keep my po2 at optimum levels on the surface.
but also allow my dilutent to be safe at my intended target depth.Should I need to go into scc mode for an emergency, ect.

As I stated above most of my experience is with heliox. We dove heliox with o2 decompression with great success. And a ccr in effect does the same thing;however pro-planner gives a sig break in deco times if a gas switch to air is performed. I have been going through all my formulas and cannot figure out the reasoning behind the difference.

Thanks in advance for any help, I've been working abit too much and am all but braindead. I hope this makes sense.

[saturated]

hey guys, going to post tidbits of info as I get it. Most info is obtained from the Navy dive manual(by the way if anyone would like I have copies of all Navy dive, Salvage ect. manuals in pdf converted) Pm me and will send you your request.

Ok starting with straight Heliox21/79 Vs. Air profiles.

sample dive to 130' for 20min bottom time.

Heliox

130' 20 min descent rate 60 fpm-Ascent rate 30 Fpm ppo2 .7ata

time to first stop. 4:00min 9min deco at 10' total ascent=13:20min

Air

130' 20 min descent rate 60 fpm-Ascent rate 30 Fpm ppo2 .7ata

time to first stop. 4:00min 6min deco at 10' total ascent=10:20min

Of course this does'nt say much in itself;however, if we look at the times we see that for a very modest dive we are penalized 3 minutes for diving heliox. I would conclude from this that on a modest dive the solubility is a factor. As HE is more soluable than N2, it would make sense that more HE would enter the tissues in this time frame, even though HE also desaturates quicker than N2. Since the absorbtion rate is quicker more HE would be absorbed, Thus more He would have to be removed. Thus the additional alotted time for deco on He. This does make sense to me.

What I am trying to figure out is the effects of breathing air on the desaturation of the He. Would the intruduction of N2, a much heavier gas, reduce the tissues abbility to rid itself of the He?


As I recall, the addition of nitrogen results in a time of supersaturation. I believe that the addition of a "heavier" gas N2 would also inhibit the desaturation of the Lighter gas(He).

If not then maybe a gas switch does make sense. Since Nitrogen is absorbed slower, and helium desaturates quicker, then maybe the air switch allows the desaturation of the helium without a quick saturation of N2. But I dont think this is the case. When I get closer to some books will do some research and post more info.

Sorry for rambling, I'm feeling dumber by the moment.:confused:

[fins wake]

No I dont lower my po2 at depth, what I mean is the nitrogen content of the dilutent I know. This wasn't directed at you, but a question aimed at rbdave: I found - and still find - the remark on not "staying on the bottom mix" somewhat confusing.
When replacing he with n2 a period of supersaturation occurs due to the slower desaturation of the N2. This is an interesting point and makes sense if we are talking about only one 'generic inert gas'. We are however talking about two different gases, each with its own saturation and de-saturation half-times.


however pro-planner gives a sig break in deco times if a gas switch to air is performed. I have been going through all my formulas and cannot figure out the reasoning behind the difference. I think this is for the reason you yourself outline below.
As HE is more soluable than N2, it would make sense that more HE would enter the tissues in this time frame, even though HE also desaturates quicker than N2. Since the absorbtion rate is quicker more HE would be absorbed, Thus more He would have to be removed. Thus the additional alotted time for deco on He. This does make sense to me. Me, too.
Would the intruduction of N2, a much heavier atom, reduce the tissues abbility to rid itself of the He? Why should it? It's not the same gas.
Since Nitrogen is absorbed slower, and helium desaturates quicker, then maybe the air switch allows the desaturation of the helium without a quick saturation of N2. Precisely how I'm thinking. And because you specifically dive heliox (not trimix), the pressure differentials would really work in your favour with air or nitrox decompression, although it should work reasonably well with trimix/heliair as well. But of course, I'm merely a well-read diver, not a hyperbaric specialist.

This thread would ideally be answered by Dr Deco himself ... :doctor: :D

[saturated]

Thanks fins, like I said I am braindead. I do remember something on the gas models involving N2 and He;however, that may have been 20 years ago......This has been a good post for me though. I have been diving the same way for a longgggg time. Heliox and O2. And to be honest its gotten to the point where it has'nt required much thought. Just trying to make sure I'm not caveman diving.....:wacko:

thanks for your input

[fins wake]

I do remember something on the gas models involving N2 and He;however, that may have been 20 years ago You're absolutely right! Some 27 years ago Bill Hamilton published a paper on the effects, specifically of switching from heliox at depth to air during decompression and found it seemed to allow accelerated decompression with no superficial counterdiffusion problems. It is a well-established practice for deep heliox bounce diving (Hamilton 1976).
Just trying to make sure I'm not caveman diving I don't think you are. On the contrary, relying on the established practices of the US Navy Diving Manual and diving the rebreathers you do, I'd argue the reverse, you're at the forefront. :)

[saturated]

Ok guys heres the answer(kind of). I finally got access to all my materials, and doc deco confirms. Uptake of N2 and He have no bearing whatsoever on each other. He saturates and desaturates at a rate 2.645 times the rate of N2. So by doing a gas switch to air you eliminate uptake of He and allow the desaturation of He to begin. Since N2 saturates/desaturates at approx 1/3 the rate as He your uptake of N2 during deco is minimal. So by switching to air the rapid loss of He begins while the slower uptake on N2 also begins......This explains the big break you get on planners for doing the switch...

I still have alot of math to do to show specific comparisons, but will post when completed....

Hey fins thanks for unconfusing me

[madmole]

Interestingly 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

Nitrogen and He do interact, its a thing called isobaric counterdiffusion and causes such nasties as Vestibular bends (a bend almost exclusive to folks who swap from He to Nitrogen mixes)

I think the current models are wrong on deep dives and the difference between He and nitrogen. The newer ideas that these guys are proposing mean that in the future we will have models that get you out faster on He than on Nitrox or air. 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. A sort of RGBM plus

Time for another batch of goats to go diving methinks

[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 ... :D )

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....:D

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?:D :D

[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

[BigJetDriver]

SORRY FOR THE DOUBLE POST! PLEASE SEE NEXT MESSAGE!

[BigJetDriver]

[QUOTE]madmole once bubbled...
[B]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

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

As much as I like Diver Mole, and usually use him as an expert resource about rebreathers, especially the Inspiration, in the case above his remarks are well off target and completely WRONG!

"Isobaric countertransport simply denotes isobaric diffusion of two gases in opposite directions. Perhaps a better descriptor is countercurrrent diffusion. Historically, both terms have been used, with the former mostly employed in the decompression arena. Countertransport processes are a concern in mixed gas diving, when differing gas solubilities and diffusion coefficients provide a means for multiple inert gases to move in opposite directions under facilitating gradients. While ambient pressure remains constant, such counterdiffusion currents can temporarily induce high tissure gas supersaturation levels, and greater susceptibility to bubble formation and DCI. In general, problems can be avoided when diving by employing LIGHT TO HEAVY (BREATHING) GAS MIXTURE SWITCHES (emphasis mine), and by using more slowly diffusing gases than the breathing mixture inside enclosure suits (drysuits). Such procedure promotes isobaric desaturation, as termed in the lore."

The above paragraph is from "Technical Diving in Depth" by B.R. Wienke, Ph.D., Los Alamos National Laboratory. The cite is on page 78.

Again, on page 81: "For HELIUM-TO-NITROGEN SWITCHING (the usual case for technical and commercial divers), a state of gas desaturation would ensue due to isobaric counterdiffusion." (Again emphasis is mine.)

I do not know what Mr. Ellyatt means (in his rambling statement posted by Diver Mole) when he says: "Having conversed with some doctors who seem pretty clued up on deco theory (not the IT professional type (oxymoron)", but I would have to make the assumption that he cannot be referring to Dr. Wienke.

Dr. Wienke is a Program Manager in the Nuclear Weapons Technology / Simulation and Computing Office at Los Alamos National Laboratory. His Ph.D. is in Particle Physics. He is a member of the LANL Nuclear Emergency Strategy Team (NEST) and functions as an adviser to Special Warfare Units in their diving activities. He is also an Instructor Trainer / Technical Instructor for NAUI. He is the developer of the RGBM and an adviser to DAN.

I also do not know what Mr. Ellyatt is referring to when he says: "what can best be described as "the usual bollox deco software", except to say that he cannot be referring to RGBM, since it was designed by physicists using supercomputers, and tested for some thousands of dive hours by NAUI's technical people.

Whatever else may be so, in reference to Mr. Ellyatt's remarks, his conclusion about Isobaric Counterdiffusion is in error, and it was an unusual error on the part of Diver Mole to post such misinformation.

I would like to request that Dr. Michael Powell, (who writes as Dr. Deco on this forum and is employed by NASA in the Bioastronautics Division), review the above and comment when he has a chance.

Doc, over to you!=-)

P.S.---I STILL want to know what this kinky thing is between the deco researchers and goats!:wacko: =-)

[madmole]

a) they are not my remarks, I was very open about the source

but more importantly they seem to match a lot of anecdotal evidence coming from the RB foruns where we are seeing a lot of vestibular bends when swapping off of helium mixes to denser N2 containing mixes. But we also see a few of them on air dill all the way and on He based dil all the way

We just dont understand what is going on here.

In the meantime I'm staying on the helium and taking the extra 5 mins stops. Having suffered minor vertigo on several dives due to ear temperature differentials (I hate wearing thick hoods, so I wear thin neoprene which sometimes lets water to the ears if you move wrong), I'd hate to have a major attack. Its quite fun surfacing when the surface is at 45 degs:upset:

[BigJetDriver]

madmole once bubbled...
...anecdotal evidence coming from the RB forums where we are seeing a lot of vestibular bends when swapping off of helium mixes to denser N2 containing mixes. But we also see a few of them on air dill all the way and on He based dil all the way...

In the meantime I'm staying on the helium and taking the extra 5 min stops. Having suffered minor vertigo on several dives due to ear temperature differentials (I hate wearing thick hoods, so I wear thin neoprene which sometimes lets water to the ears if you move wrong), I'd hate to have a major attack. Its quite fun surfacing when the surface is at 45 degs:upset: [SIZE=3][FONT=times new roman][COLOR=darkblue]

This is an area of particular concern to me, since I am working my way toward the 500+ fsw mark with my Extended Range Inspiration.

It is unclear what is occurring here, but I would be the last to ignore reports of trouble from the field. It is perplexing that these events are happening on straight air diluent, and helium dil surface to surface as well. Fortunately, we have some extremely good sources to contact, such as Doc Deco on this board, O'Leary of NAUI, Wienke, Savatsky, et al. I will post this report to them via private e-mail and see what information they can offer.

As for the rest, let's see now...ice cold water pouring into my ears...air temp 45 degrees Farenheit on the surface...God, how I envy you! :confused:

Cheers, and happy no-bubbles diving!:D :D

Eh, dove il Dottore Deco? Over to you, Doc!;)

[madmole]

I meant an angle of 45 degs, Youd be in trouble here if the water was 45 degs temp wise (we use C!!)

is 45 F hot or cold?

Heres a fun suggestion

a) hang on deco line at 3m
b) Watch surface
c) lift hood off one ear
d) Watch surface move
:D

[BigJetDriver]

madmole once bubbled...
I meant an angle of 45 degs, Youd be in trouble here if the water was 45 degs temp wise (we use C!!)

is 45 F hot or cold?

Heres a fun suggestion

a) hang on deco line at 3m
b) Watch surface
c) lift hood off one ear
d) Watch surface move
:D

Hey, that sounds like stacks of fun! A really exciting alternative to playing "Octo" on your VR3!

Ta muchly! :D

[Dr Deco]

Dear Readers:

I did not see this thread and it was pointed out to me that there were some questions with which I might be of assistance. The replies are not in any particular order.

Switches

Switching from a less soluble to a more soluble gas is a decades-old method. It was instigated by the Swiss mathematician and diver Hans Keller in the late 1950s. It has been found that the gas switches (e.g., helium to nitrogen mixes) must be performed slowly to prevent vestibular problems. This is more easily done breathing the gases in a decompression chamber than with a mouthpiece.

Helium and Nitrogen Solubility

Nitrogen is more soluble than helium in both aqueous and lipid tissues (and any combination of the two components).

Goats ?

The reason goats were used in the early research by Haldane was that their body mass was large and they had decompression characteristics somewhat similar to human divers. The goats would also lift their sore leg if they had a joint pain (the bends).

Goats were easy to work with, and their only problem was eating anything they could reach. [Once a goat remarked to Haldane, “I liked your report, but I thought your book was better.”=-) ]

Two Gases and Diffusion

When a material diffuses, the motive power is heat. Molecules jostle around at any temperature above absolute zero and will randomly change their position (e.g., Brownian motion). They do this independently of the other molecules around them.

One molecule (e.g., nitrogen) is not hindered (or blocked) in its motion when another molecule (e.g., helium) is present in a solution (or in a mixture of gases). Thus, the out flux of helium is not changed by the in flux of nitrogen in a solution. {The distance that the molecules travel, however, is modified by the fact that they hit water molecules, but that is a different question (the “mean free path”) .} There is no coupling this diffusion process, that is, the molecules move independently. This might not be true if the molecules possessed an electrical charge, for example.

“Steady State” Counter Diffusion

This is one of two general types. It was the first found in barophysiology during deep dive experiments with neon as a breathing gas. It involved the diffusion of a lighter gas through the skin and into tissue perfused with a less diffusible gas. This is termed “steady state” or “superficial” countertransport. { Lambertsen CJ, Idicula J. A new gas lesion syndrome in man, induced by "isobaric gas counter diffusion". J Appl Physiol. 1975; 39(3):434-43.} This case produced lesions (sores) on the skin of divers in hyperbaric chambers. This occurred without decompression, thus the term isobaric (= “same pressure”). Tissue nuclei present can grow in this situation even in the absence of decompression.

“Transient” Counter Diffusion

The other type appears with the switching of breathing gases where a less soluble gas (helium) is transported to a tissue containing a more soluble one (e.g. nitrogen). Apparently, one gas will dissolve faster than the other can exsolve and the sum of the partial pressures exceed the ambient pressure. Nuclei present can grow in this situation even if there is not a decompression.

Vestibular DCS

This malady is hypothesized to result fro the inward flux of helium from the space in the middle ear into the vestibular apparatus. Slow gas switches seem to alleviate this problem.

Attachment?

“Its has to do with the larger nitrogen molecules 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. A sort of RGBM plus.”

Tissue helium bubbles are not mobile and neither are nitrogen ones. They will move with muscle motion, but this displacement is fluid driven. Extravascular bubbles basically stay in one place until they dissolve. Nitrogen replacing helium will not alter the chemical characteristics of the bubble to any significant degree with respect to DCS.

Surface Tension

“Interaction of a He and N bubble 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.”

There are indeed questions concerning surface tension and the changes thereof on the surface of gas bubbles. The adsorption of biomacromolecules (surfactants) to the gas/liquid interface would not be appreciable changed by the content of these gases in a bubble. There might be time-dependend changes in surfactants dependent on what is dissolved in the tissue fluid; this is the well-known Vroman effect.

I doubt I hit every topic under consideration. :eek:

Dr Deco :doctor:

Readers, please note the next class in [I] Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm