Trimix ICD gas switches

[FIXXERVI6]

DISCLAIMER: I am not an expert, I do not do this for a living, I'm just some guy on the internet, I could be way wrong here.

I don't know how many of you here follow deco stop but I got going on a converstation and thought I would post here with the recent threads here regarding Trimix.

The thread is regarding gas switches from 15/55 to 50%

And can be found here

The Deco Stop

Quote:
Originally Posted by DeepSea
Not spiking your PN2 after coming off a deep mix.

15/55 = 30% N2
50% = 50% N2

Dont' forget its not just the hike in N2, its the drop in He as well.

Sure this is only 20% hike in N2,

HOWEVER

At the same time, your taking a massive drop in He pressure difference between tissue pressure and inspired mix.

You drop your He by 55% AND increase your N2 by 20%

If your making your switch at 70 feet, look at the difference.

Your going from 1.71 pp he to 0, and your nitrogen is increasing from .9 to 1.6, the difference is pretty big, VERY quickly, 2.41 atmosphers of pressure difference as soon as you start breathing (not sure how fast maybe someone can chime in, but it has to be pretty fast).

Now keep in mind, the pressure of He INSIDE yoru tissues will be HIGHER than 1.71 based on bottom time and depth, so your looking at a pressure delta HIGHER than 2.41 between the He pressure in the tissue and inspired mix.

What does that mean? well, look at those numbers and keep in mind, helium is faster and easier to come out of solution.

People can do whatever they want, I have no interest in playing with an IBCD hit, I don't care if people come up to 32 or 50 all the time, I won't do it on deep high helium mix dives.

I suspect as more and more people start pushing trimix and start pushing deeper we'll see more and more ibcd hits, I mean look, 2.0 ppo2 used to be acceptable until enough people rolled the dice and lost.

If you look at GUE standard mixes, when your coming up from deep dives you'll be going onto trimix, NOT 32% or air.

So to you fellows that keep saying I do it its fine, I'm happy for you, keep rolling your dice, I just don't like those odds.

Now my disclaimer, I am by no means an expert, I just read a lot, and research ALL I can to increase my chances as much as possible that I'll be one of the ones that make it back.

If I am wrong in what I have typed above, please don't flame, correct me so that we can all learn.

next post

This solubility difference is a big one:

Don't forget as well solubility issues, N2 is 4.5X more solubale than Helium


Solubility of He .015

Solubility of N2 .067


15/55 to 50%

Partial pressures

N2.93 He1.71 N21.56 He0

.93x.067 + 1.71 x .015 = .087

1.56 x .067 + 0 = .104

from .087 to .104, rapid N2 loading, where will the Helium go - the only place it can, out of solution

 

[The Chad]

From what I know there is not alot of info/testing on the subject. I agree it is quite a slam. That being said I think if you were to ween yourself by carrying a travel mix in the end you would be better off. But then again it's another bottle. I've done the 13/55 to NX 36 or 50 like everyone else without trouble. guess we'll just have to wait and see

 

[Dark Eternal]

Disclaimer: I do NOT have any training in this, so these are merely questions to help me understand better before I take the classes.

what is an acceptable pphe level?

are people going straight to ean32 or 50 due to cost or due to deco times?

what are the exact causes of an ibcd hit?

 

[battles2a5]

I probably don't have enough background on the issue but why don't we see similar issue with N2when going from a lean nitrox mix or air (really any nitrox mix for that matter) to 100%. Especially diving on earlier Haldanean tables without deeper stops?

 

[ianr33]

Interesting topic. I am no expert on deco either but I do have a chemistry/physics background.

from .087 to .104, rapid N2 loading, where will the Helium go - the only place it can, out of solution

I *think* there is a flaw in this argument. You are looking at the partial pressures in the gas phase. i.e. in the lungs. If you switch from ,say,15/55 to 32% then yes,the pHe drops drastically and the He will come out of solution but it will go INTO THE GAS IN THE LUNGS which is a good thing as it will leave the body. This is one of the reasons that deco times are shorter using say 21/35 and 50% rather than air and 50%.

Obviously the concern is He forming bubbles in,say,the blood and the nitrogen pressures in blood or any compartment will not change instantly on a gas switch but will change according to the half life of that compartment.

What form do IBCD hits take? I seem to recall that it can be a problem if using He mixes to inflate a drysuit and some inner ear hits. i dont *think* it shows up as regular DCS,but I could well be wrong.

The highest He I have ever used is 35% and I figure that IBCD is a non issue at that concentration,although V planner certainly throws out warnings when switching from 21/35 to 32%.

 

[FIXXERVI6]

I probably don't have enough background on the issue but why don't we see similar issue with N2when going from a lean nitrox mix or air (really any nitrox mix for that matter) to 100%. Especially diving on earlier Haldanean tables without deeper stops?

The issue is when your dealing with two inert gases, oxygen is not inert and our body motabolizes it, and increase in oxygen actually increases your boddies ability to carry out nitrogen so it is not an issue.

 

[FIXXERVI6]

From what I know there is not alot of info/testing on the subject. I agree it is quite a slam. That being said I think if you were to ween yourself by carrying a travel mix in the end you would be better off. But then again it's another bottle. I've done the 13/55 to NX 36 or 50 like everyone else without trouble. guess we'll just have to wait and see

Bottom time will play a major factor in it, coming up from say 15/15 to 21/35 then 50 would help buffer the slam.

 

[FIXXERVI6]

Disclaimer: I do NOT have any training in this, so these are merely questions to help me understand better before I take the classes.

what is an acceptable pphe level?

are people going straight to ean32 or 50 due to cost or due to deco times?

what are the exact causes of an ibcd hit?

accptable pphe level isn't really an issue, you adjust He based on what you want your END to be.

Lots of people go straight off nitrox, the old days people would get off of helium as soon as they could, to air at 216 feet and yes people got hit, read shecks caverns measureless to man.

I don't know enough about it to know the exact causes of an ibcd hit, I've given some of the math in the first post to try to wrap my mind around why it happens, but what it comes down to is the pressure difference change can be so great at a gas switch you can cause enough bubbles to bend in the water.

 

[ianr33]

I probably don't have enough background on the issue but why don't we see similar issue with N2when going from a lean nitrox mix or air (really any nitrox mix for that matter) to 100%. Especially diving on earlier Haldanean tables without deeper stops?

My limited understanding is that IBCD may happen when you have 2 different inert gases that diffuse at different rates.Lets imagine 3 compartments,slow,medium and fast.
After switching from a high He mix to ,say 32% what can happen is that He is going from the slow compartment to the medium one as it offgases. At the same time N2 is going from the fast compartment to the medium one. The net result is that the poor middle compartment has too much gas going into it,causing bubbling and DCS.

Switching to 100%O2 is not a problem as no inert gas is diffusing in,just the N2 coming out which is O.K as long as it does not come out too fast (i.e. appropriate depth dedco stops)

 

[FIXXERVI6]

Interesting topic. I am no expert on deco either but I do have a chemistry/physics background.



I *think* there is a flaw in this argument. You are looking at the partial pressures in the gas phase. i.e. in the lungs. If you switch from ,say,15/55 to 32% then yes,the pHe drops drastically and the He will come out of solution but it will go INTO THE GAS IN THE LUNGS which is a good thing as it will leave the body. This is one of the reasons that deco times are shorter using say 21/35 and 50% rather than air and 50%.

Obviously the concern is He forming bubbles in,say,the blood and the nitrogen pressures in blood or any compartment will not change instantly on a gas switch but will change according to the half life of that compartment.

What form do IBCD hits take? I seem to recall that it can be a problem if using He mixes to inflate a drysuit and some inner ear hits. i dont *think* it shows up as regular DCS,but I could well be wrong.

The highest He I have ever used is 35% and I figure that IBCD is a non issue at that concentration,although V planner certainly throws out warnings when switching from 21/35 to 32%.

I looked at partial pressures in gas phase but also am thinking about them in a disolved phase, maybe I'm not using the right terms or something but if you look at pressure of a gas inside a tissue I believe you can address it as pp, actually I'll pull my doc hold on.. yes you can so I'm along the right path (if I'm reading right), I have a document by a guy, Johnny E Brian Jr MD at University of Iowa College of Medicine about PP gradients and the o2 window that discusses this in detail, I'd like to post it but I'm not sure about the rules, can I attach documents in PM?

There have been several DCS type IBCD hits, vomiting paralysis etc, I wish I had the links handy but David Shaw's support diver got a bad IBCD hit, and there are others I just don't know there name, there are people posting on deco stop about these hits as well.

My understanding is IBCD is a non issue for most divers as most of us don't dive deep enough long enough on high enough helium mixes for it to be an issue, otherwise we'd all be dropping like flys.

The second block of numbers I threw up there was addressing the helium and nitrogen disolved in the tissue, the part I left out was tissue half times and time, which for the purpose of the numbers that doesn't change anything, I put them up there to show that there is a sudden increase when you do the switch, add the other factors and it would just make the gap larger.