Explaining Isobaric counter diffusion

[Mark Chase]

Not even switching diluent to air? Quite a few examples of that.

Janos

Link me to them?

I have asked the question many times and no one has come up with any hard evidance. There was one incident of a inner ear bend but that dosen't necerrily mean it was ICD.


The problem with gas switching to air is narcosis not ICD. As i said i have gas switched from 60+% He to 32% nitrox many times. As has most decompresion divers doing dives in the last 20 years who wernt GUE trained.

ATB

Mark

[Scuba steve]

The oxygen window is a realitive term for increasing the rate in off gassingnor excelerated deco . The futher the window is opened the "quicker" the off gassing process . It is a great theory for explaing off gasing and excelerated deco

Im only doing He diving since 07 and not sure on the good auld days were divers were divers . What i do know is that i have only ever seen 50% ,80% and 100% mixes on the deeper diver boats . I have seen diver doing deep air dives using 32% but the are only racking up shorter decos and cutting abit off . Any of the leading deep divers iv sat with and talked to all sing it from the towers that the first switch off helium should be to Ean50 .

[dlk]

Because the Nitrogen pp gradient is huge - 1.6 bar (ambient) to zero (tissues). The Helium coming out isn't a straightforward simple diffusion across a membrane - but a complex thing involving perfusion from slow tissues to blood to lungs etc...

Here are some numbers.
Tmx 20/25 to Nx 32 @ 40m.

Saturation targets (end points):
Before switch: ppN2=2.75, ppHe=1.25, total p=4.0
After switch: ppN2=3.4, ppHe=0, total p=3.4

Gradients:
N2 2.75->3.4 = +0.65
He 1.25->0 = -1.25

Relative speed of diffusivity: He=2.65xN2

Diffusivity modified gradients:
N2: +0.65
He: -1.25x2.65 = -3.31

You seem to be off-gassing He at 5 times the rate you're on-gassing N2. If we can formulate if/where where this is reversed then we have a guideline for ICD control. I am presuming this has already been done (and is maybe the basis of the 0.5 bar rule)?

EDIT: I cannot find any switches where this is reversed, i.e., I cannot find any circumstances where you would on-gas N2 faster than off-gassing He.
Is it a transient spike in a supersaturation window??

Oh, and re perfusion limited models, that's sorta what Doolette & Mitchell propose in their IEDCS model (serial compartments), also referenced earlier.

DLK

[dlk]

The oxygen window is a realitive term for increasing the rate in off gassingnor excelerated deco . The futher the window is opened the "quicker" the off gassing process . It is a great theory for explaing off gasing and excelerated deco
...

There are at least 2, and possibly 3 (depending on your thinking) interpretations for the term Oxygen Window.
Increasing the off-gassing gradient (as you're proposing) is entirely valid and beyond contention.

Mark is referring to the O2 window partial pressure vacancy theory, or inherent unsaturation. This is an interesting model, but very contentious.

DLK

[Major Clanger]

ICD is when v-planner says you shouldn't use that mix as a deco gas. I've little interest in the science behind it other than a basic working understanding. Interestingly (to me as I use them), neither the ostc planner or ideco raise an eyebrow at it. Why you all splitting airs over what's clearly explained in deco for divers and simlar?

[dlk]

ICD is when v-planner says you shouldn't use that mix as a deco gas. I've little interest in the science behind it other than a basic working understanding.

Nothing wrong with that.

Interestingly (to me as I use them), neither the ostc planner or ideco raise an eyebrow at it. Why you all splitting airs over what's clearly explained in deco for divers and simlar?

Because it isn't.

[Major Clanger]

Explaining Isobaric counter diffusion

Explained well enough for most to know they font have to bother about it.

[JPTaylor]

Link me to them?

Not everything is on the internet!!

[dlk]

Explained well enough for most to know they font have to bother about it.

Agreed.
That's why I put this in Physics & Physiology, and not Technical Diving or Decompression.
I want answers, and I want them now, dammit!!

[matt]

Here are some numbers.
Tmx 20/25 to Nx 32 @ 40m.

Saturation targets (end points):
Before switch: ppN2=2.75, ppHe=1.25, total p=4.0
After switch: ppN2=3.4, ppHe=0, total p=3.4

Gradients:
N2 2.75->3.4 = +0.65
He 1.25->0 = -1.25

One question on this - just looking at N2 component. FN2=100-(20+25)=55 (0.55). 40m -> 5 bar * 0.55 = 2.75.

OK - I figured that bit out. Is 2.75 right or do you need to take into account supersaturation as suggested in Bruce's diagram below:



Saying:

Bowenworld Article

Depicted in Fig 1 is a comparative representation of the time courses of changes in helium, nitrogen, and sum of the two, tissue tensions for 480 min nitrogen tissue compartments and 240 min helium tissue compartments. The depth is 200 fsw with abrupt change from normoxic nitrox to normoxic heliox. Note the buildup in time of total inert gas tension, with a maxima after some 400 min. With faster tissue compartments, this maxima builds more quickly, on time scales of the slowest tissues involved. Actually the curves remain the same as shown, but axis time scales are shortened by the ratio of the fast tissue halftime, t, divided by 240 for the helium compartment, t/240, and similarly for the nitrox compartment, tissue halftime, t, divided by 480, that is, t/480. This is quite obviously not a good scenario for the mixed gas diver. If the gases were flip flopped, a minima would develop, identical in shape to inverted

Matt.

[matt]

Explained well enough for most to know they font have to bother about it.

What type of Font?

[JPTaylor]

The only *practical* guideline I'm aware of is that the percentage increase in N2 on a gas switch should be no more than 1/5th of the decrease in the He percentage.

(Covered in Mark Powell's book "Deco for Divers")

[Major Clanger]

Agreed.
That's why I put this in Physics & Physiology, and not Technical Diving or Decompression.
I want answers, and I want them now, dammit!!

Lol, I could tell you the answer but it would involve buying me lots of beer and curry. The answer may be wrong as well.

[Major Clanger]

What type of Font?

That's it, the font for fat fingers

[dlk]

One question on this - just looking at N2 component. FN2=100-(20+25)=55 (0.55). 40m -> 5 bar * 0.55 = 2.75.

OK - I figured that bit out. Is 2.75 right or do you need to take into account supersaturation as suggested in Bruce's diagram below:
...

Good link, thanks.

The supersaturation is what I'm wondering about (transient spike), but not I think as outlined here. Bruce's diagram & explanation cover slow-fast switches, which I get. I need to do some more sums to see if they also cover fast-slow. (He says not... that the maxima inverts to a minima, as per my calcs [it's an easy spreadsheet].)