Question Isobaric counter diffusion in CC

[LFMarm]

I did want to point out an oversight that I made in the above post. Unlike the 2013 study, @LFMarm, you have inspired nitrogen that I neglected to include. This raises your total tissue tension of the inner ear to about 6.8 bar (before any off-gassing during ascent), which is comparable to the study's loading that incurred zero DCS when the flood gate of N2 was opened (again, after minor off-gassing). Contrasting to the 2003 case study, that case had a total tissue tension of about 9 bar prior to ascent and subsequent firehose of N2. Somewhere between the two seems to be a gray line where ICD ceases to be just a theoretical concern.

A critical factor, however, is how much He off-gassing takes place before the Dil switch and how large the N2 gradient is at the switch. Using the 8/60 bottom Dil in the case study all the way to 100 ft kept the He content higher than a switch to something like 21/35 earlier on. The cup "runneth over" soon after the large N2 spout was opened (i.e., the switch to Air). Switching to 21/35 accelerates the removal of He creating headroom (compared to the case study), and a later switch to 50% instead of Air would have filled the cup more slowly with N2 while maintaining maximal He elimination. More room + slower fill = less risk.

Thanks to this thread, I am fully convinced of the benefits of doing dil switches during ascent and that ICD is not a big concern for the types of dives I am doing (TTS<3h, depth<100m). I fully agree with the importance of cell validation when planning to go to high pO2 during deco and the only way to do it is by dil flushing. Faster deco is a very minor benefit for my type of diving.

Out of curiosity, how do you calculate total tissue tension?

 

[inquis]

Out of curiosity, how do you calculate total tissue tension?

P = Pi - (Pi-P0)*0.5^(t/h), where Pi is the inspired partial pressure, P0 is the initial tissue tension, t is the elapsed time, h is the tissue half-time. Calc for He and N2 separately & add for the total. Inner ear halftime is about 9 minutes.

 

[LFMarm]

P = Pi - (Pi-P0)*0.5^(t/h), where Pi is the inspired partial pressure, P0 is the initial tissue tension, t is the elapsed time, h is the tissue half-time. Calc for He and N2 separately & add for the total. Inner ear halftime is about 9 minutes.

Thank you! How is P0 defined? At the beginning of the dive?

 

[inquis]

Thank you! How is P0 defined? At the beginning of the dive?

Yes. I did ignore the uptake during descent, but I also ignored the outgassing during ascent to switch depth. There is an equation that works when changing depth, but you can just ballpark it with the average depth during the transition, then use that as P0 for the bottom phase, etc.

 

[lermontov]

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

Since the O2 cells measure only O2, what benefit does changing dil provide? I have never felt the need to validate cells with a dil flush except when my cells are acting strangely. Seems like a lot of effort for little result.

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.

^ these are my picks-

Personally I like to keep it as simple as possible if you have a complicated switching scenario (or trying to make it so ) and you have some problem on top of that then you will have a, lot of stress- I think stress is where you start making mistakes and if you have 3-4 hours of deco in front of you then you cant run home when it starts falling apart

If your doing a 100m + dive your committed to one dive for the day - and i'Il prefer to do a longer bottom time and just accept the deco - fretting about saving 20 min is fruitless in my view

I dont tworry about a flush unless theres an obvious reading problem as @wedivebc mentioned -youve already planned what po2 you are expecting at the bottom and at 21m no? so maybe at 6m you could flush but i run it manually at 6m anyway similar to what @tbone1004 said mostly so I can control the loop volume

on the ascent you burping the loop so surely thats enough of a flush - not sure what changes in the helium volumes are - maybe someone has done the math on this

 

[inquis]

I fully agree with the importance of cell validation when planning to go to high pO2 during deco and the only way to do it is by dil flushing.

Not necessarily the only way. The response to a depth increase is fairly deterministic, even with Dil addition back to min loop volume. If sensors don't increase to the expected value, that's an indication there may be a problem. This can be used in some cases to validate cell operation above the Dil PO2. (Just another tool in the box.)

For instance, I'm sitting at a PO2 of 1.2 at whatever depth. Moving 20 ft deeper & addition of 17/x Dil should take me to 1.3. If a cell maxes out at 1.25, I get suspicious as that's a bit too far off. The initial descent is another good opportunity to verify things work as expected under pressure, with zero gas or time wasted.

Details: PO2 increases by the Dil fraction added for every 33 fsw or 10 msw, excluding metabolism. I have an mCCR, so that takes care of metabolism during that brief time. An eCCR will increase a bit less, so adjust based on experience.

 

[wedivebc]

It allows you to validate the cells at a useful ppO2 if you need to. If your cells start acting weird but your dil ppO2 is 0.6, what does that tell you? It tells you what the cells are doing at 0.6, but you don't care what they're doing at 0.6 because you're running at 1.2 and it doesn't do anything to confirm what they're doing at 1.2. Dil flushes for cell validation only work if the validation is done at a ppO2 close to the ppO2 you are trying to run. If you aren't doing 3+hr run times at those depths then you will likely never experience this kind of erratic cell behavior, and exceedingly few of us are actually doing those kind of run times and/or depths, but it is very real. I hope it goes away with solid state cells, but we aren't there yet.

I see so only you and a select few of your buddies are doing dives that warrant dil switches? That is very presumptive of you.

 

[Alekseolsen]

I see so only you and a select few of your buddies are doing dives that warrant dil switches? That is very presumptive of you.

Given that I - a newbie diver - easily can find a bunch of +4 hour, plus 100 meters dives in my "local" 6°C mine, I doubt someone with serious experience would make such a claim. I rather understood it as: "It may never get prooved by agencies or research as the safest approach due to the low sample size", and to communicate that it may not be needed for a "MOD 3 certification dive" due to the runtimes involved

 

[tbone1004]

I see so only you and a select few of your buddies are doing dives that warrant dil switches? That is very presumptive of you.

how did you get that from what I posted? I was referring to experiencing the highly erratic behavior on ascent from the cells, nothing in that post was lobbying against dil switches. Go reread that post as well as my previous ones in this thread that are all lobbying for dil switches

 

[wedivebc]

how did you get that from what I posted? I was referring to experiencing the highly erratic behavior on ascent from the cells, nothing in that post was lobbying against dil switches. Go reread that post as well as my previous ones in this thread that are all lobbying for dil switches

t allows you to validate the cells at a useful ppO2 if you need to. If your cells start acting weird but your dil ppO2 is 0.6, what does that tell you? It tells you what the cells are doing at 0.6, but you don't care what they're doing at 0.6 because you're running at 1.2 and it doesn't do anything to confirm what they're doing at 1.2. Dil flushes for cell validation only work if the validation is done at a ppO2 close to the ppO2 you are trying to run. If you aren't doing 3+hr run times at those depths then you will likely never experience this kind of erratic cell behavior, and exceedingly few of us are actually doing those kind of run times and/or depths, but it is very real. I hope it goes away with solid state cells, but we aren't there yet.

This post is focused on the use of multiple diluents for the purpose of validating cells is it not? It seems you are claiming to be carrying multiple diluents for the purpose of validating cells during your dive, If that is the case it seems like a lot of effort , expense for a very limited result. Then you imply that only you and a select few of diving elite are doing the type of diving that warrants such extravagance .
I would accept the argument that multiple diluents can be used to mitigate the effects of IBCD in certain dive profiles but to carry multiple dil cylinders for the purpose of validaitng cells at depth seems highly impractical in real life.