Trimix ICD gas switches

[FIXXERVI6]

Ok here we go I found where I took some of my notes from, I have a HD full of info so I can't link people to my HD but I did find this.

Again I"m not trying to cause a trimix scare as most of us do not have to worry about this, you'll never see (well I hate to say never but I'd lay big money) that you will never see an IBCD hit come out of travis diving no matter what gasses you use.

Isobaric Counter Diffusion

Background
Type III DCS caused ICD is predominantly a problem when carrying out Trimix dives that are deep enough and long enough to generate formal decompression stops that require the use of a hypoxic Trimix deeper than 40meters/130ft. An analysis program embodying these concepts to identify known unsafe ICD gas switching practices and unsafe decompression is made available here

Assumption
Formal decompression can only be carried out safely when the TOTAL quantity of dissolved gas from all partial pressures of inert gases in a tissue is less than the tissues overall M-value. It is very important to view decompression of the Trimix diver in this manner to get an understanding of what’s going on with ICD.

Since, the quantity of a dissolved gas for a saturated medium is equal to:-

Current saturation pressure x solubility factor in the medium

For lipid tissues, the solubility factors of Nitrogen and Helium are as follows

Solubility of Helium=0.015

Solubility of Nitrogen=0.067 (approx 4.5 times more soluble)

(Ref:- Solubility factors from ‘Scuba Diving in Safety & Health by Chris Duer, MD) ISBN 0-9614638-0-5)



Note that the solubility of Nitrogen in lipid tissues is more than 4 times that of Helium

But, the diffusability (the speed the gas goes into and out of solution) will also come into the dynamics of the overall model. The speed of diffusivity of Helium is 2.65 times faster than Nitrogen.

So lets look at the aiming conditions by examining a gas switch ‘at the ascent ceiling’ from Trimix 20/25 to Nitrox32 at 40 meters deep

These are the partial pressure ‘aiming points’ for the tissues in (bars)

Depth Ambient pressure Trimix 20/25 Nitrox32
pp-n2 pp-He pp-N2 pp-He
40m/130ft 5 bars 2.75 1.25 3.4 0

Although the sudden jump in ppN2 appears small, one must remember that the ‘aiming point’ for the TOTAL amount of dissolved gas in the tissue is actual as follows

Before switch

(2.75x0.067) + (1.25x0.015) =0.18425+0.01875 =0.203 eq(1)

Immediately after the switch

(3.4x0.067) + ( 0) =0.2278+0 =0.2278 higher than eq(1) !

Hence the effect of switching to Nitrox from Trimix has the effect of INCREASING the overall gas loading within the tissue since the loss of 25%helium is more than taken up by the increase in Nitrogen from 55 to 68%(13%). This is not a good thing if you happen to be sitting on the ascent ceiling at this time. This will cause an immediate fast tissue DCS Hit.

 

[FIXXERVI6]

Here is one way to look at it

You have a can of coke, I don't know how much stuff they put in it but lets say its under 3 ata of Co2 pressure, let it sit long enough the disolved gas inside the coke itself could be cald 3 ata of pressure inside the fluid, but this is not the same as pressure of pushing against something its gas disloved in a fluid.

Open the coke, the ambient pressure drops to 1 ata of different gases, co2 comes out fast coke fizzes, if you opened the coke up to 1 ata of Co2 it would not come out as fast because the difference in pressure between the disolved gas (3ata) and ambient (1ata) is only 2 ata difference, with air its 3 ata difference.

So take that sealed coke can, keep it sealed up but put some nitrogen in it and take some co2 out, now we have a pressure differnece between the disolved gas, 3ata of co2, and ambient, say, 2.5 ata of co2 and .5 ata of nitrogen, there will be a gas exchange, nitrogen will go into the coke, co2 will come out into the gas, it will try to achieve equallibrium, bigger the pressure difference the faster the change.

Now throw into that solubility of different gases into a tissue and different diffusion speeds and your looking at the causes of IBCD hits, theory I guess.

You have one gas that isnt' very soluable, so its not going to take as much gas into the disolved gas state to reach an equallibrium presure state, and you have another gas thast a lot more soluable, N2, and they diffuse at different speeds, throw the pressure differences in on top of that and you have Nitrogen slamming in pushing He out, thats not really a bad thing it speeds up deco, but when you exceed the M value of the tissue being subjected to this slam, its bender time.

I'm a visual guy I can draw this on paper better than I can explain it.

 

[Mack Diver]

Like many who have posted, I know very little about the effects of trimix and deco. My simple mind says the HE would stay in solution due to the pressure. Is it a matter of the heavier N2 pushing the HE out of the tissues? And, would that only happen if you exceeded a 100% load on a given tissue?

 

[FIXXERVI6]

Like many who have posted, I know very little about the effects of trimix and deco. My simple mind says the HE would stay in solution due to the pressure. Is it a matter of the heavier N2 pushing the HE out of the tissues? And, would that only happen if you exceeded a 100% load on a given tissue?

Its counter diffusion issue, the tissue is going to attempt to equallize with surrounding pressures but its not just about pressure, its about the gas too, they will try to achive a homogenious mixture in the tissue and ambient gasl so gases will trade places, thats why its called isobaric counter diffusion, the pressure does not change but gases diffuse anyway.

The N2 is highly soluable, you can disolve a lot of nitrogen into fluids, helium not as much, when you have such a large difference between the ambient gas and gas inside the fluid the diffusion is going to be rapid, helium fast out nitrogen fast hard in, couple that with the fact nitrogen likes to go into fluids and it goes in big time, you end up with a pressure jump inside the tissue, if that jump is enough to exceed the M value then you bend. If what you mean is exceed 100% load on a given tissue is its tollerance to hold gas before it bubbles then you have IBCD hit potential, gas will come out of solution in that tissue into gas form because the tissue can't hold any more.

Imagine a bath tub, thats your tissue, that bath tub can only hold so much before it over flows, fill it with some water, then top off with ping pong balls, water is nitrogen ping pong balls are helium, fill it to the top, there is your tissue, loaded to max for its ambient pressure, for example purposes we could say this path tub is at its max pressure difference and up against its m vlaue, you are at your ascent celing. now dump a bunch of water in all at once, this is like switching to a low helium to high nitrogen mix, the bath tub can only hold so much, the nitrogn is going to blow the ping pong balls out of the bath tub.

Not sure if that makes sense, like I said I'm a picture guy, and I'm not the expert on the matter either.

 

[Scubakevdm]

I think this is the best thread I've ever read on ScubaBoard.

 

[The Chad]

Wow this thread is giving me a headache with all the number crunching!!!! lol no seriously its one of the better ones I've come across. One of the problems I see is how it pertains to real world diving. It would be nice if we always got the exact mix and pressure fill we asked for. Since this happens rarely we adjust our dive accordingly. Unless you bring your own tanks, HE, O2 and a booster everywhere you go.

Another thing to consider is just how long you can stay at a deep depth on OC. Being deep on OC your time is very limited if you are following the rules (1/3rd's). I don't know if you guys looked at the TDL book, but it discusses this very subject and recommends carrying a 3rd deco/travel gas with HE in it. End result is your deco time shrinks a little, Your HE dosen't go from 55 (or whatever) to 0, you use less back gas, have another contingency gas....but ya gotta lug around more junk and deal with getting it filled.

Hey how deep is Travis anyways? My wife keeps telling me we gotta move to Austin

 

[k-valve]

Like many who have posted, I know very little about the effects of trimix and deco. My simple mind says the HE would stay in solution due to the pressure. Is it a matter of the heavier N2 pushing the HE out of the tissues? And, would that only happen if you exceeded a 100% load on a given tissue?

After reading the page from Scubaengineer.com that FIXXERVI6 posted what I understand is that switching to Nitrox, or air, from Trimix has the effect of increasing the overall gas loading within the tissue. The loss of helium in the tissues is more than replaced 1:1 by the increase in nitrogen from breathing a nitrox mix or air. It sounds like it happens very quickly too.

The symptoms sound more like an inner-ear hit, but they seem to think it's really a hit to certain parts of the brain. Yikes!!

Like I said, that's the way I interpret it, but I'm no doctor or physicist!

 

[FIXXERVI6]

The deepest I have been in travis was 170 feet.

For cave diving carrying a 3rd deco gas is no biggie, I drop it at the start of the dive.

I like to talk about these types of subjects because I want to know more, and its hard to find people that have nitty gritty detail level knowledge of this stuff.

Of course remember too these are all models and theories, and a good example of how these therioes have flaws is saw tooth profiles will bend you but your computer won't tell you that, it's model is not accurate enought to deal with that, it proves our lack of knowledge of what is really going on, all these numbers, m vlaues, its all smoke and mirrors in an attempt to try to understand what is happening.

 

[FIXXERVI6]

After reading the page from Scubaengineer.com that FIXXERVI6 posted what I understand is that switching to Nitrox, or air, from Trimix has the effect of increasing the overall gas loading within the tissue. The loss of helium in the tissues is more than replaced 1:1 by the increase in nitrogen from breathing a nitrox mix or air. It sounds like it happens very quickly too.

The symptoms sound more like an inner-ear hit, but they seem to think it's really a hit to certain parts of the brain. Yikes!!

Like I said, that's the way I interpret it, but I'm no doctor or physicist!

Look, see, thats why I gotta post and talk about this stuff, this is it exactly I just don't do a good job of explaining and I tend to get to rambling.

I have read to that inner ear hits may actually be spinal tissue hits in the areas that affect inner ear function and such, this would make sense to me as the spinal tissue I believe is the fastest tissue group.

 

[k-valve]

Hey how deep is Travis anyways? My wife keeps telling me we gotta move to Austin

Ha! There's plenty of reasons to move to Austin, but Lake Travis wasn't one of my considerations when I moved back home. Seriously, it's usually pretty good for lake diving and 165-170' is easy to find. I've heard of deeper spots, but haven't found them yet.