Deco dynamics on a CCR..

[Padowan]

Whilst enjoying (!) a 50 minute hangtime yesterday my mind was wandering thinking about deco-ing on the CCR. Here's my question:

Whilst deco-ing, your body if off-gassing inert gasses from your tissues into your bloodstream to be transferred into the lungs (if there is a sufficient partial pressure gradient) then breathed out. Now on OC, this "inert rich" (or just richer than the inspired gas) is breathed out into the water, never to be seen again, so this gas is most definitely removed. On a CCR, this inert gas is breathed out into the breathing loop, which you breathe in again. Surely over time the inert loading of the breathing loop increases, thus decreasing the efficiency of the off-gass process (because of a decreasing PP gradient across the lungs).

I guess that as the inert loading increases (and therefore volume in the loop) you vent this gas, thus dropping the loading again. However when you "burp" gas around the mouthpiece (or however you do it) you can't JUST be getting rid of inert gasses, there's some O2 in there too (which the controller will put back), but the aggregate inert loading of the loop gas must surely be slightly higher than if you were on OC.

Is this why some advanced technical diving teams switch to OC for the shallow stops, because it offers a possible more efficient, or certainly different deco dymanic?

Anyone got any comments on this?

[nigelH]

Originally Posted by Padowan

Whilst deco-ing, your body if off-gassing inert gasses from your tissues into your bloodstream to be transferred into the lungs (if there is a sufficient partial pressure gradient) then breathed out. Now on OC, this "inert rich" (or just richer than the inspired gas) is breathed out into the water, never to be seen again, so this gas is most definitely removed. On a CCR, this inert gas is breathed out into the breathing loop, which you breathe in again. Surely over time the inert loading of the breathing loop increases,

Stop right there.
The loop contains oxygen at the set point.
Provided you haven't changed the diluent then the gas you are off gassing will match the loop gas (helium/nitrogen ratio). The computer will be working on the right numbers. Even if you suddenly burped a great blob of inerts into the loop the controller would promptly get the O2 back up to spec and you'd blow off the residual.

I can see no reason to switch to O/C except for an air break or because the scrubber is getting past it's sell by date.

nigelH

[Juz]

I don't think that's what Simon meant.


Let's take an OC dive, at 6m, decoing on 80% (PPO2 1.28 & PPN2 0.48) and perhaps your body's N2 loading is (example only) 1.2.

Here you have an N2 pressure gradient from Body @ 1.2 to OC gas @ 0.48.


Ok, for the CCR example, Body is still at 1.2 PPN2, but because you are off gassing N2 INTO the loop, perhaps the loop has an increasing PPN2, so that the loop has a PPN2 of 0.7 for example (compared to the 0.48 OC example), thus reducing the pressure gradient and the speed of off gassing when compared to the OC equivalent.


Thought? Comments?

Juz

[Janos]

And the amount of Nitrogen dissolved in your blood is Tiny when compared to the amount you inhale / exhale.

Laters,
Janos

[Pierre Farrugia]

Hello,

What we know for sure is the PpO2, so lets say it's 1.3 at 6m, then using Dalton's Law of partials pressures, the remeinder should be other gasses, nitrogen/helium, I think ! If you are exhaling more inertgas in the loop then the O2 solenoid will inject more O2 to compensate and you will end up with your lungs full and positive boyancy! Forcing you to remove gass from loop!

[Janos]

Originally Posted by Juz

I
Ok, for the CCR example, Body is still at 1.2 PPN2, but because you are off gassing N2 INTO the loop, perhaps the loop has an increasing PPN2, so that the loop has a PPN2 of 0.7 for example

Sorry. Just reread the question.

I would guess that on a 1.3 setpoint you're going to have a PPN2 of 0.3 no matter what you do. Irrespective of how much N2 is off-gassed, your PPO2 is fixed (by the controller), your loop volume is fixed (by the user venting as necessary) and so I don't see how the PPN2 can change.

Put it another way, if you're right, and N2 is off-gassed in significant quanitites into the loop, I would expect you to start your deco on minimum loop volume and after 50 minutes your loop volume would have increased significantly. If it doesn't increase (which I guess it doesn't) then it's not a problem.

Laters,
Janos

PS - I'm a bit worried by the fact I can talk the talk without even owning a unit! Scary stuff. Probably means I'm talking crap too.

[Juz]

Yeah, all seems to make sense, as long as you keep the loop voulme constant, you must be venting the excess N2.

Juz

[Brass Monkey]

Originally Posted by Pierre Farrugia

What we know for sure is the PpO2

Enough said.

Padowan it seems to me like a case of overcomplicating things. Put this down to boredom on your long stop and being too smart looking for the probem that doesn't exist

Dave.

[nigelH]

There is a slight question about the loop inert gas ratio on mix. However I would guess that they will off-gas BY VOLUME at the same ratio as they on gas so the difference will be minor if any.

Actually the volumes of helium, the 'fast' gas, you on and off gas during a dive are measured in litres. This is why blowing a stop on helium puts you on a helicoptor while blowing the equivalent time on nitrox leaves you wondering if it's denial or you really are OK.

nigelH

[Janos]

Originally Posted by nigelH

Actually the volumes of helium, the 'fast' gas, you on and off gas during a dive are measured in litres.

Blimey. That surprises me. I'm afraid I didn't believe you so I did some looking on the internet.

I've just looked up the solubility of Helium to be 0.008, so at say 40m (5 bar) then it's going to be 0.04. I have around about 6 litres of blood, which I know is going to be saturated, but I would have thought that the remaining 94 litres (I'm a big lad) wouldn't have such a high solubility co-efficient, but yes, I think you're right and I can see how you might get a litre or two of Helium out of me.

Well, you learn something new everyday.

Laters.
Janos