ICD and switch to nx38 from normoxic trimix

[kensuf]

If I am overthinking this, please do a dive to around 270 feet on 15/55 for 30min, then start your ascent to 200feet and switch to air for 10min. This is a short shallow dive, nothing even remotely close to extreme depth. That should clearup any doubt.

I'm not sure why the "200' for ten minutes" but we used to switch from helium to air at 190 all the time back in the mid-90s when we were dumber. The typical profile was 285'-300' on gas (usually 12/55 or 12/60), then switch to air at 190'. I probably had a dozen dives with bottom times in the 25-35 minute range on that schedule before we started using 21/35 for the 190' switch. We typically ran 3 or 4 dive teams with 3 divers per team on those gas switches.

I did get bent once, but that was due to being a big fat boy and having a PFO at the time, and not ICD. We moved away from the air for two reasons: although we didn't have issues with ICD, we did find the deco on mix just made you feel better, and the switch to air from trimix at 190' was like taking a whippet while you were underwater in a cave -- no bueno.

And you really don't want to know what GF we used back then...

 

[ajduplessis]

Whats wrong with that, is it not safe?

---------- Post added January 6th, 2015 at 05:42 PM ----------

I'm not sure why the "200' for ten minutes" but we used to switch from helium to air at 190 all the time back in the mid-90s when we were dumber. The typical profile was 285'-300' on gas (usually 12/55 or 12/60), then switch to air at 190'. I probably had a dozen dives with bottom times in the 25-35 minute range on that schedule before we started using 21/35 for the 190' switch. We typically ran 3 or 4 dive teams with 3 divers per team on those gas switches.

I did get bent once, but that was due to being a big fat boy and having a PFO at the time, and not ICD. We moved away from the air for two reasons: although we didn't have issues with ICD, we did find the deco on mix just made you feel better, and the switch to air from trimix at 190' was like taking a whippet while you were underwater in a cave -- no bueno.

And you really don't want to know what GF we used back then...

Yes, dumb and lucky crossed my mind too.

 

[Superlyte27]

I love your apple vs hammer joke, because that all it is. It not about CCR vs OC, or the max depth he complete, but the depth where he made the switches and what those gas ratios was at depth.

If I am overthinking this, please do a dive to around 270 feet on 15/55 for 30min, then start your ascent to 200feet and switch to air for 10min. This is a short shallow dive, nothing even remotely close to extreme depth. That should clearup any doubt.

IBCD is a gas diffusion speed calculation. The gas diffusions speeds dont change, they are what they are.

HPNS is something I know is related to extreme depths, or the speed of descent at depth.

I really don't have the energy to argue with you much. But this plan (The OP's) and the dive you referenced are not anything near similar. You stated he took a hit above 80 meters. He didn't according to the report I read. He switched from CCR to OC when his Dive Can imploded at 250 meters. So he was probably breathing the perfect mix for 250 meters then switched to another gas, surely not the ideal gas for the depth he was at. But, he went from CCR, to OC (perfect gas to non-perfect gas) at great depth (certainly significantly deeper than 80m) and that's when he had helium issue in the ear. So, lets compare that to this dive plan.... apples to hammers. OP, if you take a IBCD hit from the dive you laid out, I'll give you both my rebreathers.

 

[kensuf]

Yes, dumb and lucky crossed my mind too.

That was standard protocol back in 1995. Many people were afraid of getting bent from helium, you can thank Hal Watts for that, and the available "deco theory" of the time suggested moving off helium as soon as possible. I'd hazard to guess that as a group we had well over a couple of hundred dives using air as a deco gas from 190' to 120'.

The truth of the matter was that ICD was not an issue, but the deco was definitely cleaner when using helium for deco gases. I don't think using 21/35 became standard protocol until around '97, and even when we started using it with great success there were people on the internet arguing that it was unsafe.

At any rate, regarding the OP's original profile, 150' on mix for 30 minutes and then switching to 40% nitrox (assuming 100' gas change) probably won't cause him to turn into a pretzel or be stuck in a wheelchair.

 

[Subcooled]

OP, if you take a IBCD hit from the dive you laid out, I'll give you both my rebreathers.

If I have a PFO then soon I own two rebreathers. Never before have I wanted to gain DCS this much, especially as I have experience in throwing up under water

Have to think about this.

________
ps. found articles like this (iedcs in air diving; enjoy): http://gtuem.praesentiert-ihnen.de/tools/literaturdb/project2/pdf/SPU03003.pdf This is of course not directly related to gas switches.

---------- Post added January 6th, 2015 at 07:00 PM ----------

I had to edit the topic of the thread, as it looked like an incident report. Now it looks like a question, as it should.

 

[Superlyte27]

Not just DCS -- but IBCD caused by Helium

 

[Subcooled]

Conclusion: Switching from 21/35 (O2/He) to EAN40 (or even EAN36) instead of EAN50 after my modest little dive is safe.

This is supported by the comments above and by two decompression planning software, and the change seems theoretically plausible too. Nice. Nitrox 32% would be more questionable, as opinions would diverge.

Further, I could use EAN50, EAN40, EAN36, or tmx 40/10 as decompression gas and differences would be minimal, and no risk of IEDCS whatsoever. I could equally well skip the deep deco gas alltogether and just use oxygen (+7min). It seems to be very difficult to screw up a dive like this. Team deco matching obviously needs to be done

The obvious benefits of EAN40 or trimix 40/10 are:
1) no time consuming partial pressure fills of pure oxygen (although this can be done simultaneously with mix)
2) no need to O2 clean tanks and tank valves
3) no need to O2 clean first stages
4) no need for an expensive O2 clean whip

And the drawbacks of EAN40
1) does not work with 18/45 anymore (ICD warning again)
2) everone else is on EAN50

And the drawbacks of 40/10:
1) helium costs more than oxygen
2) everyone else is on EAN50
3) O2 is renewable (at your local forest) whereas helium escapes to the sky and becomes hard to bring back

 

[waterpirate]

Just to noodle you up... 32/20 is in my bottle for the first gas switch after 18/45 with a bottom time of 30 minutes. I must be bent?
YMMV
Eric

 

[Dr Simon Mitchell]

Hello,

You might find this useful.
Explaining Isobaric counter diffusion - Page 7

Simon M

 

[LiteHedded]

i often do dives to 250' on 15/55 and the first switch is to 50% (this is taught in gue tech 2 and happens every day)
i must be so counterdiffused!