Question Isobaric counter diffusion in CC

[inquis]

This all seems to be very biased toward thinking the gas programed into your computer is what is actually happening. If I'm on the loop with a 10/80 dil, it is not 10/80 as I ascend until I do an O2 flush at 20'. Even with a perfect ascent, and needing to add no dil, your inspired gas will increase in PO2 and decrease in PHe as you add O2 on ascent.

It won't be 10/80 when you do an O2 flush either; but you're right, the loop contents will be nowhere near 10/80. That is actually part of the calculations in the first example because it uses a PO2 of 1.3 throughout. At 20 ft, for example, the planner is using PO2=1.3 / PHe=0.3 (an effective mix of 81/17) compared to the bottom PO2=1.3 / PHe=7.8 (an effective mix of 13/77). I *think* this was the point you were trying to make, but again, this aspect was actually present in the above calculations.

 

[tbone1004]

If you are doing "really long runtimes" are you really flushing your po2 up to 1.4 and above on deco? I sort of doubt it.

What is a "really long runtime" that people are actually doing in the ocean that necessitates dil flushing to validate cells on deco?

yes, I typically bump up to 1.6 then let it naturally decay back down, vent the gas then bump back up to 1.6 and let it fall back. I do not ride constant ppO2's on deco as it is incredibly inefficient in terms of gas use. In the ocean very few people are doing those run times, but I tend to follow the same protocol I do on the really long stuff for most of the short stuff.

i don't think i would do that unless i suspected something was off, but then again i usually check cells for current limiting at the start of the descent and i don't really run my po2s above 1.3 anyways

linearity checks are done on land since i can barely do the math above water

Your current limit and linearity checks on descent are useful but they are not valid for the end of a 3+hr dive where the cells are not under the same test conditions as they are not in a 100% RH environment and they behave differently, they have also been subjected to high ambient pressure and they do react differently at the end of the dive than they do at the beginning. Does it matter for 2-3 hour run times, probably not, but they can become current limited and exhibit wildly different linearity behavior at the end of a longer dive, particularly down at 100m

 

[LFMarm]

yes, I typically bump up to 1.6 then let it naturally decay back down, vent the gas then bump back up to 1.6 and let it fall back. I do not ride constant ppO2's on deco as it is incredibly inefficient in terms of gas use.

I do exactly the same. The reason for me is that keeping a high SP when very shallow makes it very hard to control minimum loop volume. Is that what you mean by inefficient or there are other aspects I am missing? Thanks

 

[tbone1004]

I do exactly the same. The reason for me is that keeping a high SP when very shallow makes it very hard to control minimum loop volume. Is that what you mean by inefficient or there are other aspects I am missing? Thanks

correct, maintaining minimum loop volume while maintaining a constant ppO2 requires constant dumping and adding of O2 to the loop which is very gas inefficient. I prefer to bump up to my high setpoint, for me that is 1.5/1.6, and then I let it fall back down until the loop volume becomes a nuisance and then dump and add O2 to get to min loop. At the beginning of each stop it does not stay there for very long

 

[LFMarm]

@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%?

On big dives, if possible, I optimize my mixes based on the profile. In this case difference between 40/0 and 50/0 is small but it allows to switch 10m deeper (only relevant for bailout calculations).

@LFMarm 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%?

This is probably the main doubt that had me start this thread. The recommendation from Doolette/Mitchell to minimize ICD is to limit trimix/nitrox switches only when shallow and hence I was considering only switching when above 20-30m.

 

[inquis]

The recommendation from Doolette/Mitchell to minimize ICD is to limit trimix/nitrox switches only when shallow

A subtle point is that they recommended switching from *helium-rich to nitrogen-rich* mixes when shallow. Switching from He-rich to "middle-class" N2 mixes broadens the acceptable switch zone.

 

[GF99/99]

One thing that has not been mentioned yet and should be.

When you get into these really long run times and the cells are hot and humid, you need to be able to perform cell validation. Cell validation is done with dil flushes to ensure that the cells are responding predictably and to check for current limitation. You can certainly do this by dil flushing with whatever you have plugged in but I really couldn't be bothered if my cells are accurate below a ppO2 of 1.0 when I'm on deco, I really want to know what they're doing in the 1.3-1.6 range and by doing the gas switch and doing a dil flush you are validating that the cells can still read up to 1.5-1.6 and that they are doing it correctly. This is risk mitigation for oxtox on the ascent. To each their own, but I won't run a ppO2 up above about 1.3 unless I can get a dil flush in and make sure the cells still get up there, if they can't get it up, then at least you aren't trying to get it up and do the funky chicken.

This is spot on!! And very overlooked. For big long dives I almost always dil switch a to a gas that has a O2 percentage that matches the SP im going to dive at that depth. So if I dil switch at 100m Ill switch to a 10/60 and dive at 1.1 then at 60m Ill switch to a 17/40 and dive a 1.2 SP, then my go to next dil switch is usually 18m to a 50% and then ill bump up to 1.4 SP and last at 6m ill o2 flush validate that im hitting 1.6 and then metabolize it down to 1.4 a finish my deco off at 1.4/1.5'ish. Catching my trend.. the O2 % at that depth is the SP im going to dive so I am always checking my cells hit the SP I want to dive at befor I actually dive that SP. This something that not alot of people do.

Also another thing to note about dil switching that not alot of people think about and this is actually one of the main reason I was taught to dil switch is for Bailing Out, yes shaving deco time off is a great byproduct. The main topic of this post was ICD and it actually falls right in line with why I was taught to dil switch on ascent. If your doing a 180m dive on a 5/80 (or something) and you do not dil switch on ascent let say you have an issue at 21m with your unit and you need to bail out what gas do you bail out to?? Do you know your 50% is safe to go to with out getting an ICD hit?? (chance are probably not) if not then what trimix do you go to and then how long do you have to stay on that until you can switch to your 50%. Or what if you at 45m and you need to bailout what gas can you safely switch to directly from your unit with out getting at ICD hit? And How long do you have to stay on that gas to then switch to you next one? The bail out to O/C what if scenarios to check for ICD hits become very dive planning intense. I am not saying it can not be done but it make the B/O planning fairly complex. And I would venture to guess that most people do not even check these kind of things and if they are very minimally. There are ALOT of different what if scenarios to run through for B/O on ascent and checking for ICD.

Now if your dil switching the O/C B/O planning becomes alot less complex to plan as your not going to run into any where near as many ICD what if scenarios. Hence for the dil switching.

If you are doing "really long runtimes" are you really flushing your po2 up to 1.4 and above on deco? I sort of doubt it.

What is a "really long runtime" that people are actually doing in the ocean that necessitates dil flushing to validate cells on deco?

Absolutely if your not running a high set point on deco your dive become stupid long.

A long runtime is not really the factor that is driving dil flushing its the depth and time at depth. Honestly a for a 100m dive I probably am not going to dil flush just because im lazy and it really does not shave that much time off and I dont want to have to top up Trimix B/O but for example last month my dive buddy and I were smashing out quit a few big dives in the 150m-170m range with some down to 200m and on a dive like a 200m dive you end up with close to a 6hr run time with dil flushing (and high SP). If you do not dil flush or bump up to a higher SP your looking at 9hr run time. Now yes technically your correct it is not necessary to dil flush and many will argue that dil flushing in dangerous and you should never do it. But getting out of the water 3hr sooner sounds safer to me than sitting on deco racking up more CNS risking o2 tox convulsions and don't forget that 3hr more ontop of your 6hr already used scrubber. I am lucky that my unit has a 4kg radial scrubber so 9hr would be no issue for me but there are alot of unit out that that 9hr would be very questionable.

Dil switch is a very very very controversial subject and some people have very strong opinions about it. Personally I have never had any issue with it and I think it is a great tool for deep diving. But it is just that, its a tool and there is no exact science to it and you have to know your limitations and what tools work for you and what do not. Its kind of like gradient factors. Everyone has there own opinions about the and everyone does that own thing for them. For big dives sub 200m some believe is not dil switching but running super aggressive G/F highs like upwards of 90 so the deco is not so crazy. Personally I dive the opposite I actually go more conservative and will dive a around a 70-75 high but I dil switch.

Honestly I don't think this topic has a correct answer. There is just not enough hard data to back anyone of it and there is just not alot of people doing dive to depth where it actually plays a big role.

 

[tbone1004]

On big dives, if possible, I optimize my mixes based on the profile. In this case difference between 40/0 and 50/0 is small but it allows to switch 10m deeper (only relevant for bailout calculations).


This is probably the main doubt that had me start this thread. The recommendation from Doolette/Mitchell to minimize ICD is to limit trimix/nitrox switches only when shallow and hence I was considering only switching when above 20-30m.

Run the calculations, use standard gases, 50% is better.
Regarding the recommendation to limit mixes with helium to those without, you may be trying to read too far between the lines. If you look at the gases in the case study it was a switch from 8/60 to 21/0 at 30m. Going effectively from a pHe of 2.4 to 0 and from a pN2 from 1.3 to 3.1. If you look at what a better choice in gases would be, you'd be going from 10/70 at 100m then switch to 21/35 at 60m ish *some may choose 18/45 to have a "deeper" get out of jail free card with the switch but you're not spending a ton of time down here and if you're going to 100m you should be on a CCR but that's another topic for another day*, so cutting helium in half while doubling the nitrogen to 21/35 or slightly less with 18/45, not something that anyone is going to balk at. Then you get to 20m and kick over to 50% because you're going to be in that range for a hot minute but you're now what I would consider "shallow". Now we are making a gas switch at a similar depth to the case study but we are going from 21/35 to 50% so we already have offgased a huge amount of helium since the ppN2 is pushing it all out on the 21/35 or 18/45. Those are standard normoxic dives that no one ever questions ICD. The real issue is slamming the helium out of the tissues by seeing huge swings in inspired pHe, and going from 1.0 to 0 is a lot less scary than 2.4 to 0.

 

[inquis]

LFMarm, I really think you're applying those guidelines needlessly for where you are right now. We've had some VERY experienced contributors indicate that inner ear ICD is simply not a problem for 2-3 hr runtimes at 300 ft. Here is a bit of empirical justification for that:

Recall that inner-ear ICD is due to inspired nitrogen being the "straw that breaks the camel's back" -- i.e., that tissue was already near its limit before the switch. Obviously, the time at depth plays a huge role. Doolette & Gerth did a study in 2013(*) in which they made a switch from 16/84 to Air with ZERO incidence of DCS (4 atm ambient pressure). The part I'd like to point out was that the inner-ear tissue tension due to helium was about 6.5 bar, and the lack of DCS implies there was sufficient room above that for the rapid influx of N2 following the switch.

You didn't give a time or gradient factors for your example, but the tissue tension after spending 18 minutes at 300 ft on your 11/74 is about 5.5 bar -- well under the level in that study. It actually takes nearly 30 minutes to reach that 6.5 bar level of tension (ignoring loading during descent for simplicity), so we're looking at ballpark 4 hour runtimes to even match that level, let alone exceed it. Further, your first stop will be significantly deeper than 100 ft, so the degree of supersaturation -- the real issue, after all -- will be even lower than that in that study.

ETA: my original takeaway when I did full trimix was that ICD is a theoretical concern until much deeper than I'd probably be going. (Lots to see above 100m.) Thanks for this thread, as it has only reinforced that position in my mind.

(*) Doolette and Gerth, NEDU TR 12-04, "SAFE INNER EAR GAS TENSIONS FOR SWITCH FROM HELIUM TO AIR BREATHING DURING DECOMPRESSION", 4/2013.

 

[LFMarm]

Very helpful comments and thanks for sharing your procedures for on a controversial topic.