Question Isobaric counter diffusion in CC

[wedivebc]

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.

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.

 

[tbone1004]

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.

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.

 

[GF99/99]

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.

Once again spot on!

This is exactly how I dive "big/long dives" and im talking 6hr+ runtimes down to 200m. Very similar to what was just described, my go to is usually 17/40 dil switch at 60m (I dive a 1.2 SP here hence the 17% as a cell check plus I like having the 84m get out of jail free b/o card) and then dil switch to 50% at 18m (Ill bump up to 1.4 SP here so 50% is a good cell check) and then 6m o2 flush (once again cell check) and metabolize down and finish the deco off around 1.4/1.5'ish

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.

On top of what @tbone1004 just mentioned check what I just described on the previous page what benefits changing dil provides.

 

[Heat Miser]

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.

Not to try and steal @tbone1004 street cred (which granted is better than mine) I suggested a dil flush too, for a cell verification at the end of the 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.

... its because after you have been diving for a couple of hours some cells can have moisture on them, some cells can just gradually fade with time and depth, they all respond slightly differently. But if you can work out which ones may be cactus when they are all supposed to read ppo2 1.1 the margin for error is less likely and gives you a good feel for which cells may be cactus when they read ppo2 1.4 (or higher) in deco.

sorry its happy hour in Perth, Western Australia

this leads to another discussion about rich dils ... how can you work out whether your cells are screwed or not if your dil is 1.3 ppo2 at depth @beldridg and Ben Lair, Paragon Dive (in Depth) mag on Choptima - Brittanic

 

[Heat Miser]

and Ben Lair, Paragon Dive (in Dpeth) mag on Brittanic

p.p.s. just joking ... all good from me don;t bounce me from the the Solomons trip 2026 .... vv excite

 

[beldridg]

this leads to another discussion about rich dils ... how can you work out whether your cells are screwed or not if your dil is 1.3 ppo2 at depth @beldridg and Ben Lair, Paragon Dive (in Dpeth) mag on Brittanic

I personally ran a Dil of 7/70 on the Britannic which equates to a PPO2 of about 0.9 at max depth so I'm not sure what you are referring to? My deep bailout was 10/72 which is about 1.3 at max depth.

Regards,

- brett

 

[inquis]

Not to try and steal tbone1004 street cred (which granted is better than mine) I suggested a dil flush too, for a cell verification at the end of the bottom time.

Haha, join the club, amigo:

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.

I'm just happy to have confirmation from the pros that I wasn't totally in the weeds.

 

[Heat Miser]

I personally ran a Dil of 7/70 on the Britannic which equates to a PPO2 of about 0.9 at max depth so I'm not sure what you are referring to? My deep bailout was 10/72 which is about 1.3 at max depth.

Regards,

- brett

I dont want to deflect from a good line of discussion about the original topic .. so Brett or Brett's friends if your a moderator and want to set up a new thread ... I wont be hurt .... But isnt the point of the indepth article (not yours) the one about choptima gas planning --- to say that having a rich dil (dilout) allowed them to take one less stage, and possibly bail out to a slightly richer ppo2 .... isnt the point of being able to do a dil flush at depth the ability to do a cell verification ... shouldn't you neing on the revo with 5 cells have a stattistically higher proibablity of picking up cell problems then someone with only 4 cell .... I'm certed on both units ... I'm just putting it out there.

 

[DiveTucson]

I dont want to deflect from a good line of discussion about the original topic .. so Brett or Brett's friends if your a moderator and want to set up a new thread ... I wont be hurt .... But isnt the point of the indepth article (not yours) the one about choptima gas planning --- to say that having a rich dil (dilout) allowed them to take one less stage, and possibly bail out to a slightly richer ppo2 .... isnt the point of being able to do a dil flush at depth the ability to do a cell verification ... shouldn't you neing on the revo with 5 cells have a stattistically higher proibablity of picking up cell problems then someone with only 4 cell .... I'm certed on both units ... I'm just putting it out there.

That’s the point. It better to verify the cells with a dil flush that brings the PO2 up as you are also checking for current limiting. It’s a lot more useful for me to know my cells are working at 1.4-1.6 vs a .9-1.0.

That’s exactly the point @tbone1004 was trying to make during the ascent. You want to know if your cells are current limited and dil flushing down to a 1.0 or so won’t help with that at all.

And the article was not about carrying less gas at all…

 

[inquis]

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.

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.