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Hi Matthias,

last weekend I made the following observations:
On two consecutive days I have been diving a Trimix (23/38) on a CCR. The dives went all great. Towards the end of the dives I switched diluent to a nitrox.
I changed also the Diluent on the OSTC. I did my decompression with oxygen at 6 m as ususal and left the TC at the 1.2 Setpoint with the Nitrox, so the TC surely had still to calculate with a Nitrogen fraction in the mix..
At the end of the dive according to the graphs all my tissues were cleared from Nitrogen! Whereas the still was residual Helium in the middle compartments.
I tried to figure out how this should be possible, especially with similar Helium fractions in my mixes but not switching the Diluent towards the end of the dive. The Helium load had been significantly lower in the single compartments than the Nitrogen load. I would expect this effect to be accelerated switching to a Nitrox Diluent but experienced somehow the oposite.
Is there a potential flaw??
Or am I missing something in my thoughts?

Ah and I am running 1.7 on OSTC Mk2

Bye,
Frank

Hello,

sounds like a counter-diffusion issue to me.

From my point of view i don't see any benefit changing form a 23/xx Triox to whatever Nitrox ( < NX60 ) for decompression as you most propably increase the N2 partial pressure with that and hinder the offgasing of Helium.

Just my two cents.

Reiner

Hello Frank,

the graphical output of the nitrogen relates to 78% N2 for zero / equilibrium while the Helium output relates to 0% He. For that reason the Helium bars might be higher than the N2 bars.


regards,
Christian Weikamp

Hi Christian,

Good to know.
This makes sense. At least it is a plausible explanation.

@Reiner:
I am not following you. Remember we are talking closed circuit here.
Counterdiffusion might at least theoretically an issue here in OC, but with such 'light' Helium, I would change any time to any Nitrox /reater 21percent even OC and 'take the chance' with a 21/35 you don't get that deep to encounter counter diffusion issues. Just IMhO though.
At the part of hindering Helium from offgasing I am loosing you completely. If there was no Helium offgasing you would never get into a counter diffusion issue (here since ascent only inner ear DCS possible anyway since changing from 'faster to slower inert gases).
Offgasing in my world is depending on the gradoients of each individual inert gas. Thus no Helium in the mix the better offgasing after Helium in the mix..

Cheers from the Red Sea,
Frank

Hi Frank,

counter-diffusion happens also on CC when switching to a diluent with a higher PN2 and flushing the loop.
PN2 is the key factor, not the missing Helium.

Reiner

Hi Reiner,

I am aware that counter diffusion can also happen on CC, it is just less of an issue, since you switch during ascent, during ascent you increase your O2 fraction and thus decrease any remaining other gas fraction. Thus even when you have a little "jump" at the moment of switching this will shortly be "eliminated".
In any case counter diffusion is much more an issue when increasing the PHe after a high PN2..as far as I followed the matter..

Cheers Frank