100% agree... unless you have to bail out, effectively turning you into an open circuit diver for the purposes of your deco.Originally Posted by floridabob
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100% agree... unless you have to bail out, effectively turning you into an open circuit diver for the purposes of your deco.
Who is talking about o2 gas breaks? Aren't we discussing gas switches between deco mixes when multiple bottles are involved?
The reason I brought up deep dives with a heavy helium mix as back gas is because this may now be a 3 mix decompression plan, and because gas SWITCHING, not gas breaks, is exactly the time that ICD becomes a problem... particularly for the inner ear.
In real life, switches between sensible gases don't cause ICD issues.
Look at the bajillions of successful dives out there making switches from gasses like 15/55 and 18/45 to 50%. No issues.
10/70 or 12/65 or whatever to first deco mix of likely 35/25 or 30/30 or something around there is not going to cause ICD. Put helium in your ~100ft deco gas and then you don't have an issue. I wouldn't go from 10/70 to EAN32 though, that's a bad idea.
^^This
Not entirely true... in this article discussing ICD, there's a case included of an individual experiencing inner ear DCS related to a seemingly benign gas switch. Mind you, it was a rebreather diver, but the mechanics are no different...
https://journals.physiology.org/doi/...iol.01090.2002
i haven't read the whole thing, but it doesn't seem to be the case.
it reads like it was a 1.3 setpoint the whole way and he got bent in the ear at 9 meters. not after any gas switch
I'm with Litehedded, the gas switch probably didn't cause any issues, his tissues were probably quite unhappy after being at 1.3 for 2 hours *rough time when ambient pressure was 2ata's since graph resolution isn't great*. It's one thing on OC where your ppO2 falls drastically until gas switches. Says he switched from 8/60 to air at 30msw, so at 4ata he would be been at 4*8=.32ppO2, 4*60=2.4ppHe, 4*.32=1.3ppN2 switching to 4*.21=.84ppO2, 4*.79=3.1ppN2. Instead, he was running constant ppO2 of 1.3 through that and his tissues were likely quite aggravated and not offgasing efficiently.
Also, 8/60 is a stupid gas, and switching to air that deep is probably not the most brilliant of gas switches, but since his body would have been dumping He into the loop I'd be shocked if the gas switch was determined as a contributing factor since he was on a CCR and there is no mention of doing a dil flush when he made that gas switch.
Rule of fifths says that you get to increase ppN2 by 1 for every drop of ppHe by 5. 18/45 has 37% Nitrogen so switching to 50/50 has ppHe going down by 45 which means ppN2 can go up by 9 so technically that would be a max of 46 and 54/46 is a weird mix so it is deemed close enough and is deemed a "sensible gas switch.
So... he didn't switch to air, he switched to air as his diluent. I absolutely know people who have been taught to make that switch around 100 feet to help with helium wash out. I also know people that lie to their computer to tell it they made that kind of switch to decrease their helium penalty, even if no actual switch was made.
That said, your calculations don't really hold up for his nitrogen exposure, since air was dil, not directly inspired gas. Since the diver was holding 1.3, he would have needed to add o2 at 30m, thus decreasing the relative concentration of nitrogen in the loop.
As far as if this relates to an ICD issue or not, all I know is that Simon Mitchell felt it was, and that's why he included it as a case in his article about ICD. I don't pretend to know more about this issue than he does.
I do propose, however, that their must be SOME known problems related to ICD, otherwise why would they bother including it in curriculum for trimix diving?
As a final note... all of this is a big drift from the initial topic. The reason I asked my initial question was to see if people would treat the gas switching problem differently if high % helium trimix was involved. If a change of procedure would be needed based on the composition of the back gas, then I personally wouldn't really entertain a change in procedures involving other mixes, since consistency is paramount to folks like me that don't dive nearly as often as I would like. Admittedly, I don't really give much thought to this type of diving, anyway, as I suspect open circuit trimix diving is likely becoming less common with the high cost of helium and ready access to rebreathers. I personally won't be loading a set of doubles with high % helium any time soon...
his article was not about ICD, it was about IEDCS, two very different things, position of the C and D are important. One often causes the other, but you can get inner ear hits without helium.
Also note that I said I'd be shocked if the gas switch was determined to be a contributing factor, ergo the gas switch was not relevant unless he dil flushed whcih there was no mention of.
Your last comment however is actually very relevant as the rebreather running too high of a constant ppO2 is likely to be a contributing factor to his IEDCS and that would not have been an issue on OC or SCR. Be careful about what ppO2 you are running on those dives and don't be afraid to let it decay on the way up
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