Question Isobaric counter diffusion in CC

[LFMarm]

Hi all,

As I am preparing for my MOD3 class, I have been reading on ICD starting with the great 2020 article on InDepth (Isobaric Counterdiffusion in the Real World) and also the 2003 article by David Doolette (@David Doolette) and Simon Mitchell (@Dr Simon Mitchell) that seems to be defining the latest thinking on ICD (https://journals.physiology.org/doi/full/10.1152/japplphysiol.01090.2002). Doolette and Mitchell propose a practical approach: minimizing the switch from trimix to nitrox on ascent or planning to perform these switches at depth or in shallow water to minimize supersaturation.

In CC dives, you can stay on deep mixes as long as you want and the fraction of He will decrease gradually during ascent as you replace more and more loop mix with O2 and thus following the recommendation from Doolette and Mitchell.

However, some CC divers prefer to flush the loop with deco gases poor in He to accelerate decompression. My question is: how shallow should you be before flushing He out?

As an example, for a dive to 100m/330ft, I would bring 11/74, 21/35, 40/0 and 100/0. Following the advice from the article, during the ascent, I would not plug the 21/35 and wait until getting to depths where I can switch to 40/0. From a ICD perspective, is 30m still too deep to flush the loop (pO2=1.6 for 40/0)?

Curious to hear what others do in this regard.

 

[Mobulai]

as a VERY green mod1 -- I am here to watch and see what others contribute
Curious to learn more about this

 

[tbone1004]

I personally plug in and flush at all gas switch depths. Part of it is to drop the helium down, but it also maintains proficiency with performing gas switches when on CCR since you can get rather lazy if you aren't careful which is not ideal.

@LFMarm why are you not using standard gases? Get on board, 50%=good. Also, why would you not plug the 21/35 where you are slowly reducing the helium and then slam it out with nothing in the 40%?

 

[justinthedeeps]

Some CC divers prefer to flush the loop with deco gases poor in He to accelerate decompression. My question is: how shallow should you be before flushing He out?

We have done this and yes it has a couple of advantages (in theory) which are:

1. increases the helium off-gassing gradient on longer ascents, reducing total helium deco obligations and runtime (according to the model). But that off-gassing gradient is already increasing anyway, simply because your ambient pressure decreases on ascent, as well as your increasing FO2 crowding out the loop FHe. Some CCRs with He sensors (Divesoft Liberty?) will measure the actual He (not just theoretical) in the loop, and factor it into deco?
2. if you do bail out mid-ascent, you will be bailing out from an intermediate He mix, rather than the deep mix, reducing the ICD calculations when going from CC 21/35 > OC 40/00 at 30 metres, instead of CC 11/74 > OC 40/00

But there are probably plenty of divers who think CC 11/74 to OC 40/00 is also fine, as long as the MOD is ok.

Has anyone ever experienced ICD on CC > CC diluent switches?
The most dramatic accounts I heard were about open circuit deep trimix divers going directly onto 50/00, as deeply as possible.

Another intermediate deco/bailout mix to consider is something around 32/30 (or 35/25 if you're a GUEy), but if you have good rationale and plan for 40/00 which fits the dive plan/profile better, then you are free from Institutional Protocol™© and free to 'plan your dive & dive your plan'

I have tried to point out in prior contexts why 50% alone is not always ideal or sufficient for a significant portion of Mod3 dive plans. As soon as your first real (> 5 min) deco stops are deeper than 21 metres / 70 feet, there will be a better choice than 50% for your first deco gas. Some say 21/35 can be that other gas, which is partly true, but still not actually the ideal gas for all of the stops between 60 and 20 metres. Comparing gases and gas plans in MultiDeco or Subsurface was one of the biggest components of Mod3 I think.

 

[inquis]

From a ICD perspective, is 30m still too deep to flush the loop (pO2=1.6 for 40/0)?

I am full Tmx but not MOD3, so take this as you will... If I were in the switching camp, I would actually switch to the 21/35. The guy in that study switched to air at 30 m, so the inspired N2 fraction nearly tripled (23% to 67%). Had he switched to 21/35, his inspired nitrogen fraction would have gone from 23% to 37% -- that just doesn't seem terribly risky for inducing ICD. (Don't forget that OC divers have been doing the switch to 21/35 without issue for quite a while.)

 

[Dsix36]

I am not in the camp that does dil switches on my rebreather. I simply do not do the dives if I can not do the deco. Now, with that said, I once did a dive that was planned for 5 hours which ended up being 10 hours. I had some serious deco racked up and was actually looking at about a 14 hours dive. When I got to my 32% tanks that were staged, I did a switch and dil flush to lower my deco. IBCD was a concern in my mind but I took the chance anyway. I definitely do not advocate doing what I did, but I did manage to get away with it. I got a warm and fuzzy feeling to see my deco spinning down like the wheel of fortune. It was still a long deco though. Personally we always staged 32/30 and 32. 32/30 first to mitigate IBCD then move to 32%

 

[wedivebc]

What is the advantage of changing diluent during a dive? How many dil flushes does it take to ensure a clean loop? What about helium stored in tissue that will enter the loop upon ascent? What effect will that have on the loop inert gas content?

 

[LFMarm]

What is the advantage of changing diluent during a dive? How many dil flushes does it take to ensure a clean loop? What about helium stored in tissue that will enter the loop upon ascent? What effect will that have on the loop inert gas content?

Good questions. Divers doing dil flushes (e.g., @tbone1004), do it to accelerate decompression by getting rid of He earlier, as well as for practicing gas switches. How many flushes are needed likely depends on the unit; with the ChOptima it takes 3-4 full breaths while manually continuously adding diluent and opening the OPV. Helium being released by tissues (which is one of the main reasons for doing the dil switches) will be removed at the next flush (plus it gets diluted as you add O2 to keep the pO2 during ascent).

Other divers prefer to stay on the same bottom diluent the whole time and just dilute He out by adding O2 in the shallower parts of the deco. I believe that this practice is the safest in terms of ICD but, as other have noted, OC divers have been safely switching mixes during the ascent for decades.

 

[inquis]

What is the advantage of changing diluent during a dive?

Off the top of my head,

• With a BOV+dilout configuration, it yields a consistent BO procedure, regardless of depth.
• Periodic verification of cells at a PO2 that is above the operating point
• Maximally efficient off-gassing of He
• The dil flush may reduce the presence/likelihood of condensation on one or more sensors without wrecking the PO2.

Yes, the loop is increasingly "contaminated" by off-gassing, but that happens with the non-switchers as well. I am curious as to the computer setting by those who do switch. I believe the error in computed N2 & He inspired partial pressures would be larger when switching (assuming no He sensor), but there is some evidence that there is little difference between N2 and He anyway. That is, the "helium penalty" is beneficial mainly because Buhlmann is not an iso-risk algorithm.

 

[Heat Miser]

I have spent quite a bit of time thinking about Dil switches, because when I went to Truk and Bikini (on same trip) last year we were doing Dil switches towards the end of our 3 week’s of solid deco diving. We were concerned about OTU's and therefore running 1.2 PPo2 setpoints ( i.e. more deco than 1.3 PPo2 setpoint, but hopefully less OTU's) and despite this I still ended up with a slight dose of Hyperoxic Myopia upon my return. I was there with a past instructor and when he first proposed it (a dil switch), I said I was concerned about I.C.D. and he came up with a procedure to alleviate my concern, which I became comfortable with.

I'll show you the math on say a 47 metre CCR dive (1.3PPo2 setpoint) and with TX 21/35 Dil and say 40 min bottom time, switching to NX 50 Dil. At the end of bottom time, when it’s time to ascend, switch to set point low, do a full TX 21/35 dil flush. Maybe a cell verification too. Then manually plumb NX 50 and add just enough gas to get the loop back up to 1.3 PPo2. Then ascend and continue to add NX 50 manually to maintain 1.3 PPo2 manually. Remember your probably putting in about twice as much gas in as you would normally on an ascent, because the gas you’re adding to maintain PPo2 has 50% FN2. When you have ascended to 22 Meters bump PPo2 up to 1.4 PPO2 to start deco. As you ascend past 18 meters switch dil to Nx 50 on your computer. Because you have been adding N2 on ascent the FN2% of the loop has been relatively constant and the FHe% has been gradually reducing below 15% and the Fo2% has been increasing to maintain setpoint.

I have worked out the effective DIl equivalents of the in loop gases below, if you want to want to see how much time you could save, but I was just running planning off the dive computer and dil switched at say 18 meters (rather than every meter of ascent). Where upon I used 100% O2 to manually maintain PPo2 between 18 meters and surface.

The Maths is beautiful, The time taken to work it out and write this is far greater than any time saved on deco. refer to @Dsix36 comment, I estimate it saves exactly 2 min of Deco