Isobaric Countertransport And Mix Switch Strategies

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Genesis,

The real issue here is the breaks to "back gas" that many do to control O2 exposure.

It may be ideal to reduce the fraction of inert gas as O2 is added to the mix but it isn't the issue. The problem comes in with a heavy to light switch (like switching from EAN50 to trimix).

It doesn't take much of a dive to totally blow away the NOAA max recommended O2 exposure. For many years "AIR" breaks have been recommended when spending prolonged periods on O2 (which we do on most dives). It's certainly worked as far as avoiding whole body/pulmanary symptoms and has more recently been used on exposures that are many many times that of what has conventionaly thought to be possible (or at least wise). So, it seems that it may älso fend off CNS symptoms. Isobaric countertransport becomes a topi when helium is used in the bottom gas and nitrox of any flavor is used for decompression and one wishes to break from a high PPO2 of the decompression gas to the low PPO2 of the bottom gas. The exact flavors of gas don't have anything to do with it.

BRW, dives a rebreather so the diluent stays the same and O2 is added on ascent.

The "dogma" as you put it about standard gasses probably shouldn't get confused with this. One concern is efficient safe decompression. Another is having every one on the same page with their gasses and standardizing therefore becoming very familiar with schedules because the same gasses are always used for the same depths. Some standardization in gasses also simplifies mixing. For instance, if you bank 32% and base all your mixes on topping the the helium with 32%. Now, just as with heliair, if you analyze the O2, you know what your mix is without buying a helium analyzer. That still doesn't give standard mixes but if you take that a step further and decide to use 21/35 to 150, 18/45 to 200, 15/55 to 250 and 10/70 to 400 you have the standard mixes of one group. Mixing is easy because you add the desired amount of helium and top from your 32% bank. Carry one "standard" set of tables and every one is always on the same page as far as decompression and those tables are always in your pocket (though after a while you might not need to pull them out and look at them). It's about simplification. Oh, and since you're banking 32% you just use that for shallow dives.

You could gain many of the same advantages by banking some other flavor of nitrox and stepping up the He in whatever incriments you like.

Also, adding helium to decompression gasses is getting to be SOP even for those who have fallen victim to the "dogma".
 
it seems that what BRW is saying is the "better" pattern is to stay on backgas (provided the PO2 allows, and it generally does) all the way to the oxygen stop!

That would, presumably, solve the problem of breaks back to backgas being "bad" (in that the gradient would never increase) but it breaks virtually every other rule in the "dogma" book :wink:
 
Genesis once bubbled...
it seems that what BRW is saying is the "better" pattern is to stay on backgas (provided the PO2 allows, and it generally does) all the way to the oxygen stop!

That would, presumably, solve the problem of breaks back to backgas being "bad" (in that the gradient would never increase) but it breaks virtually every other rule in the "dogma" book :wink:

That can result in a very long decompression and a monster dose at 20 ft.

I think BRW said the best is to add O2 on the way up without adding N2. BRW dives a rebreather so the diluent stays the same and the FO2 is the only thing that changes. In that case adding O2 removes the same percentage of inert gas. So, I think he's saying to stay on helium which may still not be the same as back gas or bottom mix.

But you're right, some of what BRW recommends is different than what some others recommend. For instance switching from EAN50 to bottom mix for a clean up break at 30 ft before moving up to 20 ft and switching to O2.

If I have 45 minutes at 20 ft I'm going to break to something. If I've been on EAN50 from 70 ft to 20, I guess there's some potential for problems by breaking back to trimix. But, how long do I have to be on a mix with 36% N2 at 20 ft to ongas enough N2 for it to be a problem? Is breaking back to EAN50 a good enough break? Should we have a dedicated gas for breaks? It might be ideal to use 50/50 (or some flavor of heliox) as apposed to EAN but...

Lots of unknowns here but one thing I do know is that I can get a little bent and try to do better next time but with an O2 hit there's probably only going to be one chance. On long dives the O2 exposure is something we have to deal with one way or the other.

20 years from now they'll wonder how we survived any of this knowing as little as we do.
 
Breaking to 50/50 doesn't help you though, because above 0.5 PPO2 you get no 'credit' (at least according to current theory) on the pulmonary (and CNS) side; so breaking to a 50/50 mix at 30 or 20' doesn't do any good.

The cleanup/break gas becomes the real problem from what I can see in following this prescription.
 
Hi Mike and Genesis,

Yes, we do RBs for longer and deeper stuff, but we also
do OC in the mid ranges above 250 fsw. And ICD is not
a problem then.

Yes, we ride He mixes to either 50/50 nitrox at 70 fsw then
pure O2, or up to pure O2 at 20 fsw. And we do backgas breaks
on He when necessary for oxtox. And ICD is a concern for
staging and we worry about backgas break times along the
lines posted. And He mixes are changed at switch points
ala the prescription I mentioned before 50/50 at 70 fsw and/or
pure O2 at 20 fsw.

We have tried many, many things, and overall this works
best for safe and efficient deco plus the most important
thing -- we feel better :) and ready to jump back into the
water in 2 - 3 hrs on He rich mixes.

Earlier dogma was not quite correct as far as deep N2,
so this corrects it. Also, as you both know, earlier staging
ala Haldane was also incorrect. And phase staging correctly
helps to also minimize deco and oxtox concerns.

Cheers, and thanks :)

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Ldr
 
Thanks Bruce.
 
Is the 50/50 you're talking about 50% He/50% O2 or the common 50/50 N2/O2?

I thought the argument here was about it potentially being wiser to ride "reasonable" exposure He mixtures (say, above 250' depth) all the way up to the 20' O2 stop?
 
Genesis,

Thanks for that query.

I was referencing 50/50 nitrox at 70 fsw with regards to
ICD and backgas switches in the earlier posts. And that
was only directed at the nitrox switch folks brought up on
old dogma.

On OC, we only dive He rich bottom mixes and do switch
to 50/50 heliox at 70 fsw, and then pure O2. Deco is
slightly longer than for nitrox switch, but we feel better
always on 50/50 heliox switch. And yes, we usually detox
on backgas. And ICD is no prob.

Sorry if I confused the flow.

And thanks again, :)

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team
 
So basically you're doing something like this...

Say you're using 15/55 for a dive to 250' for 20 minutes.

Instead of using 50/50 (Nitrox) for deco, you'd choose instead to use 50/50 Heliox - no nitrogen at all - and 100% at 20'.

This would stay within the view that the Fxs of the various inert gasses should always fall.

However, the backgas breaks still happen with an increase of FN2.

At first blush it would seem that the selection of a 50% Helium first deco mix might actually increase the risk of counterdiffusion nailing you on the gas breaks, since the washout of Nitrogen from your bottom mix would be much higher, yet the washout (comparatively) of Helium would be lower. Is this mitigated to some extent by the fact that Helium offgasses faster than Nitrogen can ongas, and that you may have very effectively washed out the Nitrogen since your deep deco gas had zero nitrogen in it - so the "window effect" helps immensely?

I am assuming that the controlling factor here in terms of bubble formation is that the TOTAL inert gas load cannot be execeded (that is, its not just the partial pressure of ONE gas that's important in terms of whethe you bubble, but the partial pressure of ALL inert gasses.

Interestingly enough, the total runtime for this dive on VPM-B is 106 minutes, and the CNS is 85% - a bit higher than I'd like. This is with a 12/4 O2 break schedule to backgas. (Both of these with conservatism set to +2) I don't have an RGBM package to run the profiles on for comparison.

Using 50% Nitrox the total runtime is 94 minutes and the CNS exposure is lower as a consequence, staying under the 80% limit.

Here's the VPM-B output for this dive with Heliox:

V-Planner 3.30 by R. Hemingway, VPM code by Erik C. Baker.

Decompression model: VPM-B

DIVE PLAN
Surface interval = 2 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 200ft (4) on Trimix 15.0/55.0, 50ft/min descent.
Dec to 250ft (4) on Trimix 15.0/55.0, 60ft/min descent.
Level 250ft 15:10 (20) on Trimix 15.0/55.0, 1.28 ppO2, 74ft ead, 94ft end
Asc to 170ft (22) on Trimix 15.0/55.0, -30ft/min ascent.
Stop at 170ft 0:20 (23) on Trimix 15.0/55.0, 0.92 ppO2, 44ft ead, 58ft end
Stop at 160ft 1:00 (24) on Trimix 15.0/55.0, 0.88 ppO2, 40ft ead, 54ft end
Stop at 150ft 1:00 (25) on Trimix 15.0/55.0, 0.83 ppO2, 36ft ead, 49ft end
Stop at 140ft 1:00 (26) on Trimix 15.0/55.0, 0.79 ppO2, 33ft ead, 45ft end
Stop at 130ft 1:00 (27) on Trimix 15.0/55.0, 0.74 ppO2, 29ft ead, 40ft end
Stop at 120ft 2:00 (29) on Trimix 15.0/55.0, 0.69 ppO2, 25ft ead, 36ft end
Stop at 110ft 2:00 (31) on Trimix 15.0/55.0, 0.65 ppO2, 21ft ead, 31ft end
Stop at 100ft 2:00 (33) on Trimix 15.0/55.0, 0.60 ppO2, 17ft ead, 27ft end
Stop at 90ft 4:00 (37) on Trimix 15.0/55.0, 0.56 ppO2, 14ft ead, 22ft end
Stop at 80ft 3:00 (40) on Trimix 15.0/55.0, 0.51 ppO2, 10ft ead, 18ft end
Stop at 70ft 4:00 (44) on HeliOx 50.0, 1.56 ppO2, 0ft ead, 18ft end
Stop at 60ft 4:00 (48) on HeliOx 50.0, 1.41 ppO2, 0ft ead, 13ft end
Stop at 50ft 6:00 (54) on HeliOx 50.0, 1.26 ppO2, 0ft ead, 8ft end
Stop at 40ft 8:00 (62) on HeliOx 50.0, 1.10 ppO2, 0ft ead, 3ft end
Stop at 30ft 11:00 (73) on HeliOx 50.0, 0.95 ppO2, 0ft ead, 0ft end
Stop at 20ft 1:00 (74) on Oxygen, 1.60 ppO2, 0ft ead
Stop at 20ft 4:00 (78) on Trimix 15.0/55.0, 0.24 ppO2, 0ft ead, 0ft end
Stop at 20ft 12:00 (90) on Oxygen, 1.60 ppO2, 0ft ead
Stop at 20ft 4:00 (94) on Trimix 15.0/55.0, 0.24 ppO2, 0ft ead, 0ft end
Stop at 20ft 12:00 (106) on Oxygen, 1.60 ppO2, 0ft ead
Asc to sfc. (106) on Oxygen, -30ft/min ascent.

Off gassing starts at 196.2ft

OTU's this dive: 125
CNS Total: 85.3%

182.8 cu ft Trimix 15.0/55.0
38.6 cu ft HeliOx 50.0
20.2 cu ft Oxygen
241.6 cu ft TOTAL


DIVE PLAN COMPLETE

I can certainly see the value of a rebreather in that kind of environment, in that a CC RB effectively holds the relationship between N2 and He constant, as the diluent is a constant, and the FO2 is varied to give the desired PPO2 from depth all the way up to the 20' stop (where it begins to fall off towards 1.0), where essentially the entire loop is full of O2. It would seem that you'd get MUCH more efficient decompression this way. The same dive computed on VPM-B with a maximum 1.3 PO2 and diluent of 15/55 gives me a total runtime of 84 minutes and only 42% CNS O2! That's a MAJOR difference! My concern on a RB dive like that would be OC bailout if the loop was to fail; carrying enough bailout around to cover a potential failure has to become a problem, does it not?
 
Genesis,

Yep, you got most of it. :) :)

One thing to add on backgas breaks and ICD on He rich
bottom mixes is that N2 is low in such. So both N2 levels are
low and counterdiffusion gradients for ingassing are also
small.

Cheers -- and that's how we "ride He to the surface"
at C & C and at NAUI Tec (most important there, might
you agree). :mean:

And that's all folks. :eek:ut: (love these little peduncles)

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Ldr
 

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