Another 10m question

Dutchy

After reading the thread that Paul started on a ascent from 10m to the surface I remembered having another 10m question. I hope the expertise is here to a) answer it. b) provide sufficicient background so as to be able to predict how the "equation" changes when it's variables change.
Now the question is this:

If I were to dive to 10 m for 60 minutes with 35% Helium in my diluent (only reason being that it's always in there) then I were to bail out having a 40% Nitrox only (because it was the only thing I had lying around)

Now will I develop a type III dcs (or put more precisely have a high probability) because of the sudden drop in ppHe? (> 0.5Bar)

I'm interested in where the < .5 Bar planning rule originates from.

What if the 60 minutes were 2 minutes or 120 minutes? (I'd guess the longer the exposure the higher the probability because of the greater gas load) Would this practise be safe under certain circumstances? I'm looking for the bigger picture here. Is there any reading on this?

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Underwaterbear

I'm sure you have your reasons for asking... personally the bigger picture for me is that I would do that dive OC but if I had to dive CC I would use the Nx40 as dil and bail out (ie leave the onboard dil turned off) rather than worry about complicating a muppet dive and wasting He.

tibby

I'm no expert in inner ear DCS hits but from what I read and been trained to is that Isobaric Counter Diffusion is first not completely understood, second it has historically been recorded on dives deeper than 80m, third it happens when switching from high He mixes to high N2 mixes causing spikes in N2 while switching greater than 0.5 such as switching to air from 10/50 at 50m when ascending from 100m.
What I'm trying to say is that there may not be enough data out there to answer your question.
There's an interesting article I can't find right now (read try to find it on the web) that explains the IBCD at the cellular level in the cerebellum. I still don't understand the concept completely and I'm sure I'm not the only one.
Regards,
Tibby

Dutchy

This type of diving isn't uncommon for me on CC. Doing the same on OC would scare away bream, salmon-trout and pike-perch. But yes if (once) I add a connection for off-board dil I should be fine.

I'm still curious though about the physics and fysiology and I was wondering wether the phenomena that I've read about in relation to extreme dives can occur in a different setting.

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divetheworld

ICD will only be a factor when you are close to the compartment M value and the jump in END is sufficiently large.

Deep gas switches;
generally impact fast tissues, particularly the vestibular apparatus. IEDCS (Inner Ear Decompression Sickness) examples have been recorded numerous times at deep stop gas switches where the new mix contained insufficient Helium and a resulting in a jump in END.
IEDCS;
is the most well known symptom with its debilitating extreme vertigo and vomiting.
Gas switching;
Jumps in END can occur during Air Breaks...Back Switching ...Gas switches. If the END changes dramatically at or near a decompression ceiling...BEWARE. Shallow gas switches (shallower than 21 metres) can cause similar consequences to slower controlling tissues. Slow tissues are less sensitive to jump in END - they bubble - but you don't often notice it.

Several divers have made panicked gas switches after a rapid ascent. Rapid Ascents will bring decompression ceilings much deeper and even send them below the current stop depth.

Summary.
Gas switches;
Nitrogen --> Helium : This one is dangerous for the diver. It can induce an oversaturated status even in isobaric conditions.
Helium --> Nitrogen : This one should be good for the diver (accelerate the offgasing), but it's not and we don't really know why.
So, the best is to avoid the situation where one inert gas is ongasing while another one is offgasing.
With CCR, it's easy : don't switch !
With OC, only use trimix with the same nitrogen percentage (more or less) in every mix. The best would be to keep the same PN2, but not easy to do.
And do not forget that Surface is another breathing gas switch.

This is my tuppence, hope it helps. Its what i teach.

Brent


P.S.
Go to http://www.inspired-training.com/Isobaric%20Counter%20Diffusion.htm
Some nice stuff to glean. Marks software models ICD, its free for <40m stuff, cheap for deeper. Might be worth considering.

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JS1scuba

It's moot. 10 m for 60 minutes is a no-stop dive -- you can go directly to the surface. In fact if you were using 35% helium and flying a 1.3 PO2 set point you can go directly to the surface and then climb a mountain and go to 33,000 foot elevation. At a PO2 of 1.3 you are diving an equivalent FO2 of .68 or 68% EAN. You virtually have unlimited no-stop time and are only limited by your CNS exposure time.

Don't sweat it. Just make a slow ascent then equalize your ears when you get on the surface.

Cheers

JDS

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Dutchy

The PS linked to interesting reading. The link itself containes a reference to:


A biophysical basis for inner ear decompression sickness -- Doolette and Mitchell, 10.1152/japplphysiol.01090.2002 -- Journal of Applied Physiology

Oh and uh Joel great perspctive on this type of diving i loved the mountain climbing idea

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JS1scuba

The whole purpose of using constant PO2 is to minimize decompression. NAUTILUS Dive Planner shows your post dive elevation availability. We put that in there so folks can get a true idea of where they are at.

Cheers
JDS

dantheman

That's a cool feature.

--dan

decoweenie

"Some" other deco program also had that feature since 1999...

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