Isobaric Counter Diffusion....Type III DCI.....Vestibular Hit.

[Peter McCamley]

A number of years ago, I learned about Isobaric Counter Difussion, when doing my OC Trimix Course. . Its to do with “Narcotic Shock”
Divers breathing Helium Trimix, as a bottom mix, can suffer Type III DCI. (Vestibular Decompression Sickness) following a Trimix to Nitrox or Air. The resulting inner ear barotraumas of vestibular hit results in physical injury to the inner ear.
As a result if this, the diver looses their balance, they can’t do their stops, as they would,nt have a clue where they are.

They would’nt even know which way’s up or down. The dizziness causes vomiting,   and thereafter,  the time left doing deco can at the best be uncomfortable and that’s if they are lucky and have been able to hold onto the shot line.

I was told that when doing a gas switch from a helium mix to another breathing gas,  (eg doing a bale out from my unit onto Open Circut) the partial pressure of N2, should not be more than 0.5 bar.
Eg
Say 10-70 @ 70 mtrs = 8bar 
(N2 content is 20% x 8 bar =  1.6 bar)
Baling out onto say a 15-60 is 25% x 8 bar = 2 bar

Baling out onto say. 18-30 is 3.4 bar, would even have a difference of 1.8 bar

My question?
I know some divers who dive to  70 mtrs on units, using air as a bale out.  What’s the reason?
Yes, I am aware that helium is expensive for putting in lift bags, and not everyone wants to put helium in their suit, (although some do) however if carrying 10-70, and they were forced to bale out, this would give a N2 difference of 4.72 bar.  Well above the advisable limit of half a bar.

I am interested in your comments.

[Ger Dooly]

Hey Peter,

I'm fairly sure that they do not consider the air cylinder as a bailout, instead they are dependent on drop cylinders (as we all are once we go past a certain depth/dive time) ie they only have one bailout cylinder on them.

However that being said I'm sure that if they do not get the drop cylinders in time, then they may take their chances with counter-diffusion and breath from the air cylinder (as a last resort)

cheers,

[Fintan Lowney]

Hi Peter,

The air bail out would not be used at depth, more as the rich mix. Most guys carrying air as bail out are also carrying a trimix bailout. So if the dil is 10/70tx, they would have a side mount with 10/70tx (ish) to get out of depth and I assume using the air at shallower depths.

Regards,
Fintan.

[Stephen McMullan]

That'll still do it Fintan - going from 10/70 bailout to air bailout say @45m will run a risk of ICD because of the sudden spike in the nitrogen and ongassing into tissues that are already slightly overpressurised with helium. As Ger said the air in the right hand stage is used for spidge gas and suit gas. If its primarily bailout then it should be something like 20/30 (followed by richer nitroxes up the line) and smooth out the introduction of nitrogen.

By the way I dived all last year and half of this year with 10/70 and 50% as bailout and that will give you a nice red warning in V-planner if you run the bailout schedule.

[Peter McCamley]

Well Ger
Many thanks for your reply.
Nope, on the 70+ meter dives, there was no drop cylinder at depth, apart from the 80%, which was at 6 meters.

When I did my MOD 2, I was told “ask plenty of questions, listen to all, question everything,  and then when you are happy with the answers take it away and try it. What suits you might not suit others. However if it does not work, you will be lucky to end up in the pot…….Your call”

I believe that a lot of problems are caused by the training, or rather the lack of it and the subsequent lack of / wrong information. People follow like sheep, fully dependant on what is being taught, is Gospel.  I know, as I have had a bad experience with an Instructor. It’s a case of not knowing what you had, till you get it. Once you have got it, then you can fairly notice and see the difference! …Correct information is paramount.
Regards
Peter

[Alan Price.]

That'll still do it Fintan - going from 10/70 bailout to air bailout say @45m will run a risk of ICD because of the sudden spike in the nitrogen and ongassing into tissues that are already slightly overpressurised with helium. As Ger said the air in the right hand stage is used for spidge gas and suit gas. If its primarily bailout then it should be something like 20/30 (followed by richer nitroxes up the line) and smooth out the introduction of nitrogen.

By the way I dived all last year and half of this year with 10/70 and 50% as bailout and that will give you a nice red warning in V-planner if you run the bailout schedule.

Stephen what are you using now as your bail-out???

[Stephen McMullan]

Well on the week up in Downings I've been using 10/70 and 20/30.

I think the equation changes if you're wrecking, without a buddy for team bailout, go past a certain depth or don't have a dropset sorted

Don't take the above to be right. I'd be lying if I said I had it sorted - still thinking about what the right thing to do is. Its a good one for a chat over a pint.

[scuba steve]

i had a good long chat with my OC tri mix instructor about this subject on my course .

how regular an occurance is it ? from my chat with my instructor its seems to be very rare . but is talked about ( and blamed for bends) in another forum (UK based). it can also be mistakeidly blamed on burst ear drum and vertigo .

in my deco plans i use 50% and 80% and it doesnt raise any red flags on my dive plans

i understand the theory behind it but as i asked how regular an occurance would it be .

[emmbee]

Hi,


Say 10-70 @ 70 mtrs = 8bar
(N2 content is 20% x 8 bar =  1.6 bar)


On CCR, wouldn't this be ppN2=(8-1.3)*.2/.9=~1.49 bar?

Cheers,

Matthieu

[Stephen McMullan]

Its 10/70 on open circuit Mathieu - this whole question is about CCR divers going OC for the purpose of bailout and possibly getting clobbered by the gas switch on ascent. On CCR of course the norm these days is not to gas switch diluent and introduce higher levels of an new inert into the loop but that can tend to lead us into a wee bit of false security with the bailout scenario.

in my deco plans i use 50% and 80% and it doesnt raise any red flags on my dive plans

@ScubaSteve - you're not using enough helium in your mix mate!  

[emmbee]

Well, yes.
So if you have 10/70 as a diluent, and you're set to 1.3, your ppN2 before the switch is 1.49 bar?
Then you want to bail out to OC, with a ppN2 difference less than .5 bar, so a maximum ppN2 of 2 bar, hence the 15/60?

Cheers,

Matthieu

[Stephen McMullan]

There's too many snorkellers on this forum  Generally CCR divers use in an around the same mix for lean bailout and dil. The problem is the next gas switch.

[arturconrad]

It seems like in a majority of wreck dives in a range of 60-70m (exept when you do some serious wreck penetration or when you experience some serious CO2 hit at depth and your breathing rate's hitting the roof)) you should be able to reach your EAN50 at -21m using your dil/bailout Alu80 with Tx10/70 (and provided it's more or less full, not just 60B for the dive ) without problems.
Still, as Stephen said, there's quite a bit a shock to go from 10/70 to EAN50 and some transition gas seem to be desired. And why not? It's cheap ( it is!  - 20/30, come on...), relativly light due to He so not much hassle to bring it as a third stage ( I guess alu 40 would do the job on most of the dives in that range) and you're not wasting it every dive.

Nevertheless I think that you're still in a comfort position, the deep wreck divers,  coz you can relativly quickly get out of depth should any problem arise.

What about this:

In my current project 450m from the cave entrance the depth is -72m. A hundred metres further the depth is already -80m and continues like that for another 250m (800m from the entrance total) . A planned exploration dive to 1km assumes that if my rebreather fails at the furthest point of exploration I'm 35 minutes of rather brisky finning to get out of -80m and another 10min to reach my first,out of three, deco tankat -50m. For that 45minutes I need a cover of 5 full Alu80. Would you take a chance here and put an air in it?

And someone said recently to me that I'm lucky coz cave diving is so much cheaper than deep sea diving...)

artur

[Peter McCamley]

Well Artur
Fair play to you. Thats proper divin. Makes me feel like a snorkeler. Would really like to hear how you get on with your project.

[emmbee]

Hey, that's my title

Out of curiosity, Artur, what do you call cheap?

Steven, I have to agree, this place is going to bits! Dreadful!

I blame you

Still the question was about not doing what you said, and switching to something else. Might as well get the arithmetic right - if it wasn't. Considering that then 15/60 has a certain optimum quality.

Anyhoo, I'm off to do AN&DP, 20-24 (the AN+dives option didn't pan out). Hopefully I'll eventually be able to talk about this stuff less non-intelligently.

Cheers,

Matthieu