Help me understand the rule of fifths for IBCD

[rsingler]

This great article gives you a quick and dirty understanding of the rule of fifths as a way to avoid Isobaric Counterdiffusion when diving Helium.
Isobaric Counter Diffusion - Scuba Engineer
Basically, "limit your rise in N2 to one fifth of the drop in Helium as you ascend."
So in the article, going from 20/25 to EAN32 was a no-no with a long deco run, because when He dropped from 25% to zero, the safe max jump in N2 was 5% rather than 55% to 68% (13%).

But when does it become unnecessary? I gather that for short deco, going from (for example) 15/55 to EAN50 is both common and generally accepted as safe. Theoretically, it exceeds the rule: He - 50% to 0%, while N2 - 30% to 50%. But your intermediate stops are very short, and no deco gases are hypoxic.

But using the first example discussed in the article, going from 20/25 would then progress to 32/8, and was noted to be safe. But this violates the rule too! The smaller the drop in Helium, the smaller the allowed rise in N2. Here, one fifth of a 17% drop in He is 3.7%, while the rise in N2 was 5%.

So when does it become silly that a small drop in Helium "allows" an even smaller rise in N2?
How does one properly use the rule to determine intermediate deco mixes?

 

[fsardone]

This great article gives you a quick and dirty understanding of the rule of fifths as a way to avoid Isobaric Counterdiffusion when diving Helium.
Isobaric Counter Diffusion - Scuba Engineer
Basically, "limit your rise in N2 to one fifth of the drop in Helium as you ascend."
So in the article, going from 20/25 to EAN32 was a no-no with a long deco run, because when He dropped from 25% to zero, the safe max jump in N2 was 5% rather than 55% to 68% (13%).

But when does it become unnecessary? I gather that for short deco, going from (for example) 15/55 to EAN50 is both common and generally accepted as safe. Theoretically, it exceeds the rule: He - 50% to 0%, while N2 - 30% to 50%. But your intermediate stops are very short.

But using the first example discussed in the article, going from 20/25 would then progress to 32/8, and was noted to be safe. But this violates the rule too! The smaller the drop in Helium, the smaller the allowed rise in N2. Here, one fifth of a 17% drop in He is 3.7%, while the rise in N2 was 5%.

So when does it become silly that a small drop in Helium "allows" an even smaller rise in N2?
How does one properly use the rule to determine intermediate deco mixes?

Disclaimer: I am not a deco guru so my opinion is worth what you paid for it - Nothing and do not blame me if you get bent.

Once that out of the way ...
The issue with IBCD is that the different solubility and speed allow the sum of the gas partial pressure to exceed the tissue maximum allowable supersaturation pressure. The problem happens at gas switch. Why?

Because you switch gas immediately after raising to the next stop: i.e. when the limiting compartment is at maximum allowable supersaturation. Therefore if you switch gas to a “wrong” mix that compartment will be exceeding the max supersaturation. Because the amount of He exiting the tissue (fast but less soluble) is less than the amount of Nitrogen entering (slower but more soluble). If you stay long enough at that stop, you will decompress enough that the IBCD becomes a non issue. This is the case with short enough dives (the limiting compartments are the fast ones that desaturate fast enough and are less prone to IBCD) while the slow compartment are far from critical supersaturation.

In deep dives are the intermediate and slow compartment limiting and therefore there is enough time at those stops for the counter diffusion to become a factor.

The 1 to 5 rule stops being necessary when you are far enough from critical supersaturation in the limiting compartment. Especially if it is a slow compartment and in tissues with limited perfusion.

Again my 2 worthless cents ...

Cheers.

 

[RIHappyDiver]

I misread your title. I, for the life of me, couldn’t figure out what the rule of fifths for Irritated Bowel Disease would be.

Erik

 

[kensuf]

I thought the rule of fifths was if you drank a fifth of Jack Daniels you'd likely feel a bit of vertigo.

The switch to 50% at 70' doesn't exceed a PN2 of 1.6. Interestingly enough, that kind of lines up nicely with the maximal theoretical m-value tissue tension for nitrogen.

 

[Grantt]

I am only normoxic trimix certified, so I don't know if what is described is standard practice beyond my range and experience.
The way the article describes Sheck Exley's phased gas switch procedure, I think, would be effective in that it gives your fast tissue time at that depth to start off gassing while slowly switching you to the appropriate mix for that depth. If a procedure like this is implemented, I think the more drastic the change would require a more gradual transition. Maybe something like 1/3, 2/2, 3/1... or potentially even more gradual. No idea how to calculate that though.
Again this is just my theorizing on my limited experience.

 

[fsardone]

I am only normoxic trimix certified, so I don't know if what is described is standard practice beyond my range and experience.
The way the article describes Sheck Exley's phased gas switch procedure, I think, would be effective in that it gives your fast tissue time at that depth to start off gassing while slowly switching you to the appropriate mix for that depth. If a procedure like this is implemented, I think the more drastic the change would require a more gradual transition. Maybe something like 1/3, 2/2, 3/1... or potentially even more gradual. No idea how to calculate that though.
Again this is just my theorizing on my limited experience.

Gas switching is the most stressful moment of the dive. The more complicated you make it the most dangerous and prone to error becomes. Additionally for the slow tissues which are under perfused I don’t think that the progressive switching illustrated above would allow enough time to offgas sufficiently to avoid IBCD.
The avoidance of gas switching it is one additional advantage of the mixed gas rebreather for deep dives.

 

[Grantt]

The avoidance of gas switching it is one additional advantage of the mixed gas rebreather for deep dives.

This is one of the many reasons why I stopped where I was on my OC progression and switched over to CCR. I am comfortable with doing gas switches, but if I don't have to, all the better.

 

[PfcAJ]

This is one of the many reasons why I stopped where I was on my OC progression and switched over to CCR. I am comfortable with doing gas switches, but if I don't have to, all the better.

I’m sure it’ll go smoothly when you have to bail out.

 

[rsingler]

I’m sure it’ll go smoothly when you have to bail out.

This is what prompted my thread. I'd like to be able to better prepare for bailout. I'm ahead of myself as only a Helitrox CCR diver doing modest deco, and I'm okay with planning best mixes or standard mixes except for that intermediate Helium mix at a gas switch. The article gave a rule of thumb, and there's a computer program for really adventuresome dives, but when the example in the article doesn't seem to follow the rule, I don't know where to turn.
Any IBCD planning suggestions for that intermediate mix on a dive I won't get to for awhile? I'm interested in a bit more than "your instruction will cover that. "

 

[PfcAJ]

This is what prompted my thread. I'd like to be able to better prepare for bailout. I'm ahead of myself as only a Helitrox CCR diver doing modest deco, and I'm okay with planning best mixes or standard mixes except for that intermediate Helium mix at a gas switch. The article gave a rule of thumb, and there's a computer program for really adventuresome dives, but when the example in the article doesn't seem to follow the rule, I don't know where to turn.
Any IBCD planning suggestions for that intermediate mix on a dive I won't get to for awhile? I'm interested in a bit more than "your instruction will cover that. "

This is part of why I think one should be a competent OC trimix diver before ccr.