ICD and switch to nx38 from normoxic trimix

[Subcooled]

THE PLAN
My target depth is 45m, with max END 30m and max ppO2 1.4 and maximum bottom time is short, half an hour or so. Such a dive is supposed to be easy. Well, no.

Due to some unfortunate circumstances, I can only get something like nitrox 38..40 as my deco gas. Using nitrox 38 does not have a major impact on anyones decompression as the deco is short. However...

ICD (IEDCS type)
My decompression planning software says that I must go as low as 21/32 with helium to use nx38 without ICD risk. On some absurdly simplified level I understand the reson for this. Ideal gasses, independent diffusion, supersaturation (total gas) before equilibrium, that sort of stuff. I knew this was coming, thanks to Powells Deco for divers. If I had only relied on the TDI trimix manual...the situation would be different.

To be on the safe side, 21/30+nx38 could be used, with a gas switch at familiar 21m for convenience.

THE PROBLEM
My next idea was to... reduce the jump in nitrogen fraction from +13% to +10% by adding some oxygen. I dialed in: 24/30+nx38, and got an ICD warning. What is happening?

The jump in nitrogen should be smaller now... hence slower inward diffusion... more time to even out... Why does the addition of oxygen complicate things? Oxygen-ieDCS?

 

[ajduplessis]

24/30 = 46% nitrogen. when you switch to 38 nitrox for deco you reduce the helium by 30% and increase the nitrogen to 70%. This will result in IBCD. You need to keep the reduction in heluim vs the increased nitrogen within 5:1 ratios.

If you use a deco mix like nitrox 80 (only 20% nitrogen) you problem will go away.

deco software are you using?

The final helium % in the bottom mix will be an input to the deco gasses. There is 5:1 ratio for calculating IBCD (decrease helium% vs increased nitrogen%). Many also believe that IBCD will not happen above a certain depth, I think number of 50-80m was used. I don't bother about the later statement, for me gasses will on/off-gas at defined rates. If there is a enough "clash" between off-gassing helium and on-gassing nitrogen, counter diffusion will result. This IMO is straight forward math, however I still believe you can take a IEDCS hit shallow using the incorrect gasses. When 10 people (helium) tries to leave the building (body) and 50 other people (nitrogen) is trying to enter the building (body) via the same door simultaneously , you will have an injury (IEDCS).

Dr. Simon Mitchell will also point that there are causes where it is believed that PFO contribute to IEDCS using non-helium dives. Divers with PFO will shunt gas left to right resulting in IEDCS. My view is if you shunt gas will while decreasingcause injuries, it’s just a matter of where in the body the injury occur.

 

[Subcooled]

Thanks, that 5:1 ratio is usefull. Gas has a physical size, so the door metaphor seems appropriate.

I am using two decompression planning software: Suunto DM3 and V-Planner. I like Suunto DM3 because it has a graphical planner and it also calculates gas consumption quickly and easily. It is more efficient in generating rough drafts and comparing them than V-Planner. The final planning I tend to do in V-Planner (Multideco) though.

 

[tomfcrist]

Legit question...

How is it that you are able to get trimix but unable to get a proper deco mix?

 

[Subcooled]

5:1
50% vs 10%
of what? All gas, or individual gas?

So if the first gas contains 50% N2 and the second contains 60%, then the increase in N2 is 10% of all gas or 20% of nitrogen.
How much is the helium fraction allowed to drop then? One half, or by 50%?

___________
I'm not 100% sure that this is caused by congestion. How could there be supersaturation in constant pressure, if gas is unable to move? However, as the pressure gradient affects the speed of diffusion, then the pressure equalization of each gas could proceed at different pace. If N2 jumps a lot and He drops a little, then He gets out slowly, while N2 gets in fast. Soon there would be a lot of extra gas inside, and that would get into the bubbles, and trouble would ensue. This model would not explain the effect of oxygen increase though, or then I have not given it enough thought. This is clearly an issue that I must study more. Any rule of thumb is dangerous when not fully understood.

 

[ajduplessis]

This calulation is in relation to backgas and the first deco gas used. If you have multiple deco gasses the same applies between the deco gasses.

Calc: If you reduce the helium content of the deco gas (compared to your backgas) by 1%, the nitrogen % can not increase by more than 5%. You have to add either 02, helium or both.

My example of 10 and 50 people entering a building is only an example and has nothing to do with calculations.

 

[rongoodman]

Is IBCD even considered to be a risk for such a relatively short, shallow dive? Deco for Divers says that it's not an issue for dives of less than 80 meters. MultiDeco(VPM +3) has no complaints with 21/35 and 40% for deco. Also, you should be aware that you're trying to plan your dive with programs using completely different deco models, so the profiles generated will probably not be similar.

 

[ajduplessis]

If I am not mistaken Don Shirley took a IBCD (inner ear) hit above 80m switching to an inappropriate mix during that fatal dive with Dave Shawn in Boesmansgat.

 

[Superlyte27]

What's 45 meters in american? About 140'? I'm guessing?

There's no chance in hell you're going to get IBCD on a 140' dive. Sure, maybe 1 time in 10,000 dives. But you're more likely to get eaten whole by a jellyfish. I joke.

This is not something I'd be concerned with even a little.

 

[Subcooled]

Out of pure curiosity I decided to read about this interesting phenomenon, eventhough it might not be relevant to my modest next dive. Nasty application wants to scare me

This link is worth a look: http://archive.rubicon-foundation.o...e/123456789/8179/SPUMS_V34N1_5.pdf?sequence=1

The abstract, introduction and case report are interesting and fairly easy to understand. Then some more technical text follows, and then the results. Correctly and fully understanding the results would obviously require an understanding of the methods and models, which is easier said than done. Fortunately there is the Discussion chapter. It describes the IEDCS as a diffusion speed issue. It also recommends deeper than usual stops following very deep dives (to allow for proper offgassing of the ear). Further, it says that the oversaturation(=problem) increases with depth or when inspired pp02 is smaller. The gas switch should be made deep or shallow, and it should be made with as high ppO2 as safely possible. (As a curiosity: The article references many other papers, this for example: [abstract] COUNERPERFUSION SUPERSATURATION. )

Now that you all have read that article,

would it not be healthy to offgass at constant depth for a while, just prior to a gas switch? Nitrogen loading would surely increase, but the likelihood of iedcs would decrease, and He is a fast gas after all. The prior one is easily solved by more deco, while the latter one is not. Unless this causes mindless amounts of extra deco, would it hurt? On the other hand, IEDCS might be a problem at such deep dives, that ongassing would be a major issue. The case study in the article described events at 9m or about 30ft though (it was a dive to 100m+/300ft+).