Help me understand the rule of fifths for IBCD

[rjack321]

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?

You are assuming this 1/5ths rule is actually based on good science instead of conjecture and arm waving. That article is: 1) seventeen years old and 2) has no accounting for depth of the switches or the actual gas loadings and 3) is not supported by empirical data or even common modern practices.

If you stay out of trimix filled bells, keep your ENDs <100ft, and don't do any deep air switches, it's basically a hypothetical problem that just doesnt manifest in the world of recreational diving.

 

[boulderjohn]

You are assuming this 1/5ths rule is actually based on good science instead of conjecture and arm waving. That article is: 1) seventeen years old and 2) has no accounting for depth of the switches or the actual gas loadings and 3) is not supported by empirical data or even common modern practices.

If you stay out of trimix filled bells, keep your ENDs <100ft, and don't do any deep air switches, it's basically a hypothetical problem that just doesn't manifest in the world of recreational diving.

A few years ago, we had a case in which a new ScubaBoard member had a username that suggested he was an official representative of Suunto. It turned out he was a Suunto employee, but fairly far down the chain of command and not an authorized representative. He was, however, an obsessive believer in RGBM and Bruce Wienke, and in that context he described the switch from 21/35 to 50% as "suicidal" because of IBCD, despite the fact that it may be the most common gas switch in technical diving. He was a bit flummoxed when someone posted a video in which Wienke said there was no problem switching from 21/35 to 50%.

I have yet to see anything that convinces me that anything typically done in most technical diving has any real danger for IBCD.

 

[rsingler]

So now I'm creeping towards getting an answer to my theoretical question. Thank you @rjack321 and @boulderjohn . Yes, I've heard the "don't worry about it" part, and yet I still see the "rule of fifths" bandied about when I read about deep trimix (or bailout from great depth). Perhaps everything I've chosen to read has been dated.
I didn't notice the date of my article above - thank you for pointing that out. I chose poorly in attaching that - perhaps akin to advocating for deep stops in 2021.

My question remains. Assuming that I'd like to read about deco gas choices in deep trimix diving, where can I read newer data? Is it now accepted that deco with hypoxic mixes can just dump all Helium as soon as you've reached a suitable END? I'm guessing not, and I was asking where to read how intermediate mixes are chosen, besides walking up the list of "standard deco gases."

Can you point me in the right direction?

 

[MichaelMc]

A 2020 data point for your conversation:
How Two Tech Agencies Address Isobaric Counterdiffusion
(Actually three agencies as they mention BSAC's guidance.)

 

[tbone1004]

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

I'll have to agree to disagree on this one. I 100% agree in being a competent OC technical diver in terms of gas switches, deco theory, etc. but the practicality of OC trimix diving, especially hypoxic depths is going away very rapidly due to the cost and availability of helium. The discussion about the rule of fifths is still fully relevant to rebreather diving due to bailout, and arguably more so to CCR diving since the unit may encourage people to "skip" a deco bottle to minimize the bottles they are carrying so emphasizing the importance of gradual gas switches *which the rebreather obviously does for you* is important. Some of us also do dil flushes with the offboard bottles when we get to deco for cell validation. I will not run a 1.6 ppO2 for deco until I can verify that the cells are behaving because I run a lean ppO2 on the bottom. The first time you can validate that on ascent is at the first gas switch. I will go through the gas switch procedure which instead of being a second stage in my mouth, is the whip plugged into my unit and then do a dil flush to check the cells. Just as important as an OC gas switch.

 

[rsingler]

A 2020 data point for your conversation:
How Two Tech Agencies Address Isobaric Counterdiffusion
(Actually three agencies as they mention BSAC's guidance.)

Both agencies make a big point of saying "IBCD is real!" That's not to say it's a huge problem, just that it's a real phenomenon with transition from hypoxic mixes.
I don't know if it's still true, but per the article, NAUI doesn't ever go to a more nitrogen rich mix, but supposedly ascends on backgas until 100%O2 at 20ft. I can see that posing some challenges. And they acknowledge "First-" and "Second-order Rules", implying that lots of folks DO deviate from "back gas to 20'."
And the article quotes GUE's stance as "too large a bump in inspired N2 can be a problem, but going from 15/55 to EAN50 seems to be okay."
Again, even this article from 2020 cited no scientific work later than 2004. Finally, BSAC's approach to limiting PN2 increases to 0.5 bar is mentioned without explanation.
At least the "rule of fifths" justified its math based upon differing gas solubility.
What I see is mainly acknowledgement that there is a potential issue, followed by "the way we do it now works just fine."
That's not unlike the resistance to change that followed deep stops confronting NEDU.
My take thus far is that the NAUI "zero-order" approach is most conservative, followed by the rule of fifths, followed by a 0.5 bar PN2 change limit. The "standard deco gases" exceed both the rule of fifths and 0.5 bar in at least one switch, even if you used all of them ascending from 300+ feet.
But as with deep stops, the denominator of documented hypoxic mix deco dives is perhaps too small to give us meaningful data for a number of years to come. I'd love to hear how the deep trimix (or deep CCR) folks choose their sequence of deco/bailout gases.
This was a nice summary from Dr. Mitchell, however: BSAC published new gas density recommendations

 

[boulderjohn]

I don't know if it's still true, but per the article, NAUI doesn't ever go to a more nitrogen rich mix, but supposedly ascends on backgas until 100%O2 at 20ft.

The only NAUI tech instructor I know uses standard gases.

 

[Scuba Client]

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?

In the mix: Closed circuit rebreather gas planning

 

[RainPilot]

RAID uses a 3:1 rule. Attached a pic from the normoxic course materials (Deco60). There's a typo where they talk about nitrogen being "increased 37%" where they mean it is 37% in an 18/45, so going from 18/45 to EAN50 is a 13% increase in N2.

Using standard gases, 18/45 to 50% switch is fine, as is 21/35 to 50. Basically, their recommendation is that the decrease in He should not be more than 3 times the N2 increase.
At some point, usually the 50% switch, your preceding gas should have a low enough He that that switch is ok, which then avoids the whole "arrow chasing a tortoise" problem.

 

[lermontov]

[QUOTE="
Using standard gases, 18/45 to 50% switch is fine, as is 21/35 to 50. Basically, their recommendation is that the decrease in He should not be more than 3 times the N2 increase.

View attachment 663362[/QUOTE]

just for clarity I think youve worded this incorrectly - the N2 increase is one third of the He decrease (not 3 times the N2 increase )- if you use your wording then 18/45 to 50 is a N2 increase of 13% - 3 times the increase is 39% so your 18/45 (ie 45% He ) decrease is more than the 39%