Isobaric Countertransport And Mix Switch Strategies

[BRW]

All,

Got some emails about optimal switch strategies in modern
deco frameworks, and impacts of isobaric countertransport.

So for those who asked me, here's my read using dual
phase dynamics (RGBM)), what tec sectors dive and report,
some earlier experiments and observation by Chris Lamberstsen,
field tests, plus building on earlier posts.

1) -- light-to-heavy switches are the ONLY safe switches,
and once started, should never be reversed back
the other way;

2) -- good way to deco overall is riding He to 20 fsw zone
and switching pure O2 or 80/20 if you can (you will
most likely feel better on He deco, be in better position
to repet, and have roughly equal deco time as with some
switch to nitrox earlier);

3) -- the lighter the He mix, the less benefits of switching
to nitrox;

4) -- heavy nitrox switches (depths beyond 70 fsw) are not
an optimal (nor good) strategy for short or long
exposures;

5) -- EAN50 at 70 fsw is usually N2 light enough for longer
exposures

What this all adds up to is the following isobaric switch
prescription, which is, of course, outside of most dive
logistics. But useful on first principles -- and some places
DO have the support in place for just such logistics.

So, it goes like this.

Best deco strategy is to increase O2 on way up in same proportion
as He is reduced, while keeping N2 relatively constant. Switch to
a nitrox mix with less N2, and thus outgas both He and N2. The
He to O2 changes look a little like an RB, and the lower N2
nitrox switches avoid gradient slams and further ingassing of
N2.

Practically, this means all the above plus EAN50 at the 70 fsw
level for long exposures, and/or He to the surface for shallow
exposures, plus O2 in the 20 fsw zone either way.

Cheers, and safe diving always,

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

 

[Genesis]

down to something a bit more practical.

Is the bottom line that from a bubble mechanics point of view, one should never increase the percentage of an inert gas as one "rides up" the decompression chain?

So, for instance, if you are using Trimix 18/45, the FN2 in the bottom mix is 37%. Thus, switching to a gas with more than 37% FN2 as a deco mix is inadvisable?! That precludes a 70' 50/50 bottle, but might allow a 60/40 bottle, which is pretty close to 37%.

Or, for example, if you're using 21/35, the FN2 is 44% in that mix; as a consequence the "maximum FN2 permitted" is 44%? That might permit 50/50, as that's pretty close, but even better might be that 60/40 mix again....

In both of the above cases you COULD ride the Trimix up to the 30' depth, in that neither would become hypoxic and, in fact, you could breathe the 18/45 on the surface (its a bit hypoxic but not significantly so.)

I think I understand what the prescription is here, in that increasing the Fx of an inert gas can cause counterdiffusion (actually increasing your inert gas load rather than decreasing it in that gas, even as the other gas vacates your body) but this is majorly counter to what I've understood about general deco strategy.

(The odd thing is that for a deco dive on Nitrox, for example, taking a single bottle of 50/50 might remain an optimum strategy, in that there is no counterdiffusion problem since the FN2 is always decreasing, and if something goes wrong on the shallow stops you've done some of the deco with an advantageous oxygen window .vs. no benefit from that, but that same deco gas option on a 21/35 dive would be inadvisable, with either a single bottle of 80/20 or 100% being preferred.)

Did my distillation end up correct?

 

[Kendall Raine]

BRW;

Sounds like you would advocate 50/25/25 at the 70 over 50/50 nitrox coming off, say, a 15/50 at 250 kind of dive. Is that correct?

 

[barb]

if my understanding is correct (usually, it's only partly so) then the best situation is to stay on the helium mix as long as possible before switching to the nitrox mix to avoid reversing the off-gassing quality of the helium in place of the on-gassing quality of nitrogen.
In your example of 21/35, you can't switch to nitrox 60/40 at 70' (not safe) so I'd guess that you would have to stay on the 21/35 until you reached 50' then switch there. I think that you have to look at the dive profile to determine the ideal depth for a gas switch. If 70' is the depth, then 50/50 is the mix so you'd "ideally" be using a richer blend of helium or you would have already had a switch to a higher 02 pp. maybe 32/25?
I'm struggling to understand all this myself and really appreciate your post as it helps me to think about a "practical" application.

Thank you BRW for taking the time to teach us this.
I'm looking forward to Spring - so I can get back in the water to practice this !
Barb

 

[vlada]

And would appreciate any furhter comments BRW may have.

The way i interpreted the lead post is that for exposures using 21/35 and only one deco gas, the most optimal way is to ride that mix to 20 ft and switch to O2 - which is not what i've been taught in some of my tech classes, so i have a lot more research to do to figure out which way i want to dive

It also looks to me that for 18/45 mix, switching first to EAN50% and then to O2 is an OK way to do the deco while sticking to standard gasses

By the same token, it looks like, for doing deco from 15/55 mix, one will be better off adding some He to 70 ft gas - i've read many reports about people using some He in the mixes and "feeling better afte the dives" - that puts some theory behind that

I do have a quesiton though - what about the breaks to back gas for long O2 exposures - in most cases, that will be a break from 100% O2 to some back gas with some % of N2 and back - these would appear to break the rule of "light to heavy only"? Along the same lines, what about WKPP practice of going to backgas at 30 ft stop before switching to O2?

Thanks a lot,

Vlada

 

[Genesis]

I think that's the point, yes.

You always want the Fx of a given inert gas to drop as you do gas switches on deco, if I'm reading BRW correctly. Since the only non-inert gas is Oxygen, this ends up being the control you can didle, but of course that's subject to your PO2 and thus depth at which the switch can be made.

This is contrary to the accepted practice of essentially all of the existing agencies, and makes the dogma of "standard mixes" that some agencies preach real difficult or even impossible to deal with, as your deco mix, other than 100%, suddenly becomes something that has to be locked to the FN2 of the bottom mix, lest you get that counterdiffusion.

As you noted, this view would make 21/35 undiveable with a 50/50 deco mix, as the FN2 of the backgas is 44%. Its even worse if you're diving 18/45 as a bottom mix, as the FN2 of that mix is 37%, meaning that even 60/40 is ill-advised.

The paradox is that as you go deeper, you want more helium and less nitrogen to get an END that is acceptable. However, as you go deeper (with time at depth being constant) your first stop is at a greater depth, and you have more inert gas to elmiinate.

I suppose the obvious "fix" is to "cut" the deco mix with Helium as well, so that the FN2 never exceeds that of the bottom mix, but then you lose the window effect of removing all Helium from the decompression gas. I'm not sure how that trade-off would work out in practice, and it definitely requires a complete re-write of the concept of "standardized gasses."

As noted, it also raises the question of "cleanup breaks" and backgas breaks to control Pulmonary O2 effects.

Comments BRW?

 

[padiscubapro]

vlada once bubbled...
And would appreciate any furhter comments BRW may have.

The way i interpreted the lead post is that for exposures using 21/35 and only one deco gas, the most optimal way is to ride that mix to 20 ft and switch to O2 - which is not what i've been taught in some of my tech classes, so i have a lot more research to do to figure out which way i want to dive

It also looks to me that for 18/45 mix, switching first to EAN50% and then to O2 is an OK way to do the deco while sticking to standard gasses

By the same token, it looks like, for doing deco from 15/55 mix, one will be better off adding some He to 70 ft gas - i've read many reports about people using some He in the mixes and "feeling better afte the dives" - that puts some theory behind that

I do have a quesiton though - what about the breaks to back gas for long O2 exposures - in most cases, that will be a break from 100% O2 to some back gas with some % of N2 and back - these would appear to break the rule of "light to heavy only"? Along the same lines, what about WKPP practice of going to backgas at 30 ft stop before switching to O2?

Thanks a lot,

Vlada

a nitrogen only mix is probably better (and used in hyperbaric therapy), the mass of nitrogen and oxygen is comparable only about 4 amu different,plus nitrogen diffuses very slowly compaed to He, while going to a helium mix, there is a tremenous difference something like 28AMU (less the mass of the nitrogen and oxygen percentages) and the Helium diffuses rapidly.
If it comes down to risking DCS or risking Oxygen CNS issues, I'll choose the DCS chances.. You can always hang a few minutes longer, risking CNS is too great a risk..

 

[IndigoBlue]

Bruce,

I am looking forward to poring over the new NAUI deco tables, to see what deco regimens you built into those. I ordered them today.

Were you able to publish into them your ideal switches? Or did the NAUI tech execs slant them another way, presumably on the side of being more conservative?

I am also curious to know what "reduced" gradients you built into them. With diving software, users are able to set their own gradients. What gradients did you assign?

When I am diving with "your" tables, some one on the boat is going to ask me these questions.

 

[vlada]

to clean up break questions above

clean up breaks answers

 

[BRW]

Folks,

I am on the fly and hope I can patch some replies mucho vite.

This is a copy of a related post on same subject at another site.

Just a few comments after perusing thru this thread:

1) -- since 911, I have been doing more diving than I
care to ever do again (Diesel, DRE, GI3), and I can
assure you that my head is only part of our dive
operation;

2) -- fact is iso-saturation risks induced by heavy-to-light
switches on the way up increase with time on
light gas after the switch because relative iso-sat
increases (sum of both gas tensions) with time;

3) -- this ain't just theory -- it's been shown in
experiments dating back to Lambertsen, and
following after. References abound -- check
some of my monographs, or the UHMS ICD Wkshp
Proceedings on same subject (some 15 -20 yrs ago)

4) -- it ain't rocket science either;

5) -- for folks who like to make those deep nitrox switches,
keep in mind that relative DCS risk increases directly with
ppN2 (reported RP Wkshp Smithsonian, Deep Diving
Procedures UHMS) from data;

6) -- same for any deep air divers floating around these
days;

7) -- risks for detox switches from heavy-to-light are indeed
small for the kinds of exposures OC bottle folks in the
300 fsw range log, but as a figure of merit, time
scales for isobaric switch effects to "kick in" are in the
15 - 25 minute range (see TDID, Chapter 7, Isobaric
Countertransport, UHMS ICD Wkshp Proceedings)

8) -- avoid OC probs with isobaric countertransport totally
and switch to RBs with heliox diluent;

9) -- and solve the bottle problem while optimizing
deco;

10) -- and, for some, maybe culling time to check the real
skinny on ICD.

Seems there is confusion. I probably didn't spell out
situation on backgas detox far enough. Nor did I emphasize
enough that short breaks are not really problematic.

And we all know it's done. And hits have not been a
real problem.

But they can be when ingradients for the lighter gas are
large and outgradients for the heavy gas are small. And
times get longer on the lighter mix.

Here at C & C, we do not do OC heavy-to-light switches.
And on RBs, it's a non-issue. We are not a bottles-and-jump
operation on OC. Luxury, yes. And OC is not our preferred MO,

If backgas breaks are short (N2 switches back to He),
there isn't much risk (though the risks ARE higher).
"Short" relates to He and N2 partial pressures, and
especially the gradients between inspired air and tissues.
And also the tissue compartment and bubble sizes excited
into growth. It's complicated.

So to quantify "short" go to TDID and plug some numbers
into the iso-saturation expression for your switch regime.
Time scales for iso-sat fall out. And iso-sat is to be avoided.

Hopefully, the following is illustrative

For nominal trimix dialed to MOD of 1.4 atm, and N2 narcotic
depth of 60 fsw or less, isobaric effects start to kick in at
15 - 25 min off a switch from 50/50 to backgas. As the
EAN staging goes deeper (O2 drops, N2 increases) the
effects kick in sooner. Obviously, as the EAN deco switches
start deeper, N2 levels stay higher, and countergradients for
He ingassing off backgas increase (in the presence of higher
N2). And that marches to iso-sat, which is not a good thing.

That's really the long and short of it -- on isobaric switches
maximize ALL outgassing gradients on the switch. Or,
minimize all ingassing gradients. One way is to avoid
heavy-to-light gas switches. And on light to heavy switches,
also minimize ppN2 levels because DCS risks track with ppN2.

And, yes heliox vs nitrox switches are less risky coming off
helium bottom gas (while overall deco times off heliox switches
are a bit LONGER than nitrox). But, for the record again,
if switches to nitrox are/must be used, do so above 70 fsw
and maximize O2 (so that means 50/50 and higher) and increase
to pure O2 in the shallow zone. In such regime, ICD risks
are also decreased on backgas switches (detox or otherwise).

Why?

Same as before.

N2 levels get minimized, and outgassing is maximized
for both He and N2.

I feel like I am beating a dead horse here a bit but thanks
for the questions anyway. And I am "out of here"

Oops, forgot Indigo.

Indigo, RGBM does NOT use GFs. Bubble dynamics dictate
permissible gradients on acsent. GFs as brandished in the tec
community are knob twiddling without substantiation. Twiddler
beware. RGBM tables are top to bottom consistent -- no fudge
GFs. And validated and tested -- only phase model sitting so.

Want some fun?

Concoct any arbitrary mixed gas deco dive, and ask twiddlers
for the GFs for the dive on first principles. Make it fun, like
550 fsw for 15 minutes on heliox RB with ppO2 set to 1.2,
and toggle switch to ppO2 = 1.0 at 300 fsw, and then toggle
switch back to 1.2 atm in the 100 fsw range. Tell them they can't
use RGBM first -- generate GFs without any reference points to
full phase models.

Cheers,

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