UTD Ratio deco discussion

[PfcAJ]

Ok, but where does the slide claim to reduce shallow stop time by adding deep stops when not using O2? The presumption by @Diver0001 was that RD adds deep stops to shorten the stops in the shallows which simply isn't true.


Can you please point out where I have ignored anyone? Whether you think I am on top of theory or not is a matter of opinion. I think more accurately it's when I respond with statements that seem contradictory with yours and others, and because of that you conclude that I'm not "on top of theory."


So you chose not to respond. Got it.


I want to get this straight You believe that one can draw the same conclusions regarding deep stops for an Oxygen-based decompression dive from a study which used air for decompression? A study which is vaguely related to actual deep stop diving as @rossh pointed out? You think that is a scientific method? If Peter likes Mary, and Mary likes Paul, then Peter must like Paul then too?


Where exactly?


If I ask you how you might do a dive in Truk Lagoon, does that count as PADI officially teaching me how to do a dive in Truk Lagoon? I asked my instructor personally what he might do. Don't confuse that with the agency having any official training on how to dive at altitude.

So, for the 10th time now, UTD does not officially teach altitude diving. If you ask any instructor about it, they will tell you the same thing.


Too deep for a Buhlmann model? Sure. What part of RD deep stops are "debunked?" Where's the science that disproves it? If RD used air only for decompression, based on a vaguely related NEDU deep stop study, I might agree with you more. Where's the sciences that says a dive involving oxygen-based decompression and deep stops is bad?


Please point me to the research that disproves S-Curves have no benefit and is "debunked."


You're contradicting yourself. You said it was "debunked." Now it's not?


I would agree with that study, if we were using air only for decompression. The deep stop study also used air for decompression. RD uses oxygen-based decompression like any sensible diver would.


Why couldn't it be used at altitude? I've posted a few altitude profiles as well as Kevin, but you conveniently ignored those posts. I'll ask again, do you think those profiles are acceptable to execute at the altitudes they were derived for?




Eight paragraphs to basically say you don't know exactly where you got the research from, it's just in your head. So we should trust you just because of that? That's no better than this:





I don't think he was claiming to more than the experts at NEDU, but that the deep stop study is vaguely related to actual VPM-based diving. He doesn't have to be an expert to point that out. If a smoker tells you you shouldn't smoke, does it make him wrong?

If you're going to lop off a metric **** tonne of oxygen time there's got to be some "reason" for that before you get to the oxygen stop.

Contenders are: deep stops (no evidence) and s curve (no evidence). You're all out of tricks at that point.

Dangerous nonsense.

 

[mikeny9]

If you're going to lop off a metric **** tonne of oxygen time there's got to be some "reason" for that before you get to the oxygen stop.

I was referring to Diver0001's statement in which he thought RD shortens shallow stop times because of the addition of deep stops.

I'm not quite sure what argument you're making. Are you saying the O2-time on that slide in the video linked is terribly short? If so, I don't think that's related to the argument Diver0001 was making. Could you post a sample profile or part of one and compare it to describe your argument like boulderjohn did?

 

[Storker]

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A ScubaBoard Staff Message...

Some of the posts in this thread are awfully close to another rehash of the NEDU study validity flamewars we've had enough of already. Posts that are deemed to be an attempt at that, will be removed without further notice. Repeat offenders may find that their posting privileges in this thread have been revoked.

This has been a public service announcement

---

 

[PfcAJ]

I was referring to Diver0001's statement in which he thought RD shortens shallow stop times because of the addition of deep stops.

I'm not quite sure what argument you're making. Are you saying the O2-time on that slide in the video linked is terribly short? If so, I don't think that's related to the argument Diver0001 was making. Could you post a sample profile or part of one and compare it to describe your argument like boulderjohn did?

I believe that slide shows that oxygen time = 1.5x bottom time for 300ft.

I recall a dive I did recently, 250 for 75mins. The oxygen time was like 3hrs. WAY more than 1.5x the bottom time. Same standard gasses and all that.

 

[mikeny9]

Interesting, I'll keep that in mind. I'll pose the same question to other RD divers who are able to execute a dive to that level. Those types of dives are way beyond my limits.

 

[Kevrumbo]

I believe that slide shows that oxygen time = 1.5x bottom time for 300ft.

I recall a dive I did recently, 250 for 75mins. The oxygen time was like 3hrs. WAY more than 1.5x the bottom time. Same standard gasses and all that.

Interesting, I'll keep that in mind. I'll pose the same question to other RD divers who are able to execute a dive to that level. Those types of dives are way beyond my limits.

UTD VBlog: Complex Ratio Deco Questions · UTD Scuba Diving

Check out the slide behind him. Look at the 300ft oxygen time setpoint.

Cascading Ratio Deco:
45m => 0.5 * Bottom Time;
60m => 1 * Bottom Time:
75m = >1.2 * Bottom Time;
90m => 1.5 * Bottom Time;
105m => 2.2 * Bottom Time;
120m => 3 * Bottom Time.

So for example, 40min Bottom Time at 90m depth with 12/60 Trimix, your 6m depth O2 and 21m to 9m depth Eanx50 deco total stop times are each 1.5 x 40min = 60min.

Buhlmann GF 50/80, yields 66min total deco stop time for 21m to 9m Eanx50, and nearly 90min for O2 at 6m.

(Note: both O2 profiles above assume Open Circuit along with two additional standard intermediate deco mixes of 35/25 and 21/35 used).

According to RD methodology, the criticism of Buhllmann ZHL-16 is that it does not take into account the higher diffusivity along with the lower solubility of Helium versus Nitrogen and so will "penalize" by giving you longer decompression total times with high Helium mixes.

However,

. . . it is conceivable that the long held belief that helium needs more decompression has compensated for underestimation of required decompression (to achieve acceptable levels of risk) by the decompression algorithms we all use, and that we are doing the right amount of deco but probably for the wrong reason.

Helium vs nitrogen decompression

(Good article here summarizing it too:
Eliminating The Helium Penalty - Shearwater Research)

So IMHO, in light of the NEDU Studies on Deepstops and Helium/Nitrogen Gas Kinetics, any implementation of Ratio Deco is going to require additional extension or "padding" of the O2 shallow stop times to further reduce the critical tensions of slow tissue supersaturation upon surfacing.

 

[Dr Simon Mitchell]

What part of RD deep stops are "debunked?" Where's the science that disproves it? If RD used air only for decompression, based on a vaguely related NEDU deep stop study, I might agree with you more. Where's the sciences that says a dive involving oxygen-based decompression and deep stops is bad?

Hello Mikeny9,

You cannot dismiss the relevance of the NEDU study to decompression diving by technical divers using high oxygen content gas just because the divers in the study were using air. The reasons for this were articulated at length in the original deep stops threads on rebreather world. Those threads are still available, and I would strongly suggest that if you really are interested in this subject that you read them. Put simply, if the study had incorporated oxygen decompression then the difference between deep stop and shallow stop outcomes would probably have been smaller and perhaps not detectable to a level of statistical significance in a study of pragmatic size, but there is no reason to expect that the "truth in the universe" about the disadvantage over-emphasizing deep stops would be any different. Indeed, the same disadvantageous patterns of supersaturation distribution across fast and slow tissues that form the most plausible explanation for the NEDU study results have been shown to exist in bubble model decompressions from typical technical rebreather dives.

You have asked several times about the current state of science around this subject. In this regard it is possible to make some useful general observations.

First, there is not and never has been ANY evidence supporting the emphasis placed on deep stops in decompression diving by bubble models, ratio deco, or GF approaches chosen to emulate bubble models. Whatever you think about the studies that I discuss below, you need to keep this key fact in mind at all times.

Second, every bit of related science that has emerged in recent times suggests that bubble models, ratio deco, and GF approaches with a low GF-lo place too much emphasis on deep stops. That evidence can be summarized as follows:

1. The NEDU study. Previously extensively discussed. A truly seminal piece of work which is never likely to be repeated. There has been a lot of contrived criticism of this study mainly from people with a vested interest. We could go on at length about this (and indeed we have), but the thing we should all remember is that every other bit of contemporary evidence (see below) which has emerged appears to corroborate the NEDU study findings.

2. The recent Spisni study comparing RD and GF 30/80. This study is particularly interesting because it contained a design flaw in that the RD decompression was longer than the GF one. If you want to evaluate the efficacy of the deeper stops and the RD S curve in comparison to another approach which involves shallower stops and no S curve, then you at the very least need to make the profiles the same length (because, all other factors being equal, a longer decompression should always be safer). Put simply, the experimental design imposed a significant inbuilt advantage for the RD approach. Despite this, there was a trend toward less instances of high bubble grade in the divers using the GF approach (which did not reach statistical significance) and inflammatory markers were more elevated in the divers using RD. You have asked several times what is the evidence that debunks RD. “Debunks” is probably too strong a word, but this study provides a signal that the RD approach (as it stood at the time) was inferior to another commonly used approach with shallower stops and no S curve. Moreover, there is some evidence (see the Swedish navy study below) that the comparator for RD in this study (GF-lo of 30) also places too much emphasis on deep stops. In other words RD may have looked worse if compared to an approach with even less emphasis on deep stops.

3. The Blatteau study published in Aviation Space and Environmental Medicine in 2005. They showed more venous bubbles after a deep stop decompression compared to a traditional gas content model with shallower stops.

4. A Swedish Navy study that has been presented at several conferences but which is not yet published. This compared decompression with GF lo = 30 to DCAP (which prescribed shallower stops) from decompression dives. Again, higher bubble grades in the deeper stop dives.

5. The Ljubkovic study from Journal of Applied Physiology which showed almost universally high bubble grades after decompression from trimix dives using VPM. This was not a comparative study, but it did illustrate that bubble model decompressions do not reliably control bubble formation as has been claimed in the past. This study spawned Ross’s ridiculous crusade to rewrite the DCS pathophysiology text books with his claim that venous gas emboli are irrelevant.

6. Neal Pollock’s work at inner space and on deep diving research cruises. Also not published yet, but presented at multiple conferences (including SPUMS last week) and medical courses. He has been monitoring divers for bubbles after deep decompression dives and correlating findings with the profiles. He has found apparent success in decreasing bubble grades by de-emphasizing deep stops, and (in particular), by padding shallow oxygen stops (a strategy which Ross actively disputes).

With the exception of the NEDU study all of these involved gases other than air. You are correct to point out that only one of them relates directly to RD, but RD emphasizes (or at least emphasized) deep stops as a central part of its approach. Collectively these studies offer accumulating evidence that deep stops as prescribed by bubble models and RD in decompression dives are too deep. The Spisni study (albeit a single small study) offers evidence that this concern is not adequately compensated for by imposition of the S curve and exploitation of the oxygen window as I have heard claimed. And I reiterate my first general point above: all of this needs to be considered against the fact that there is not a single study I am aware of that supports bubble model / RD type emphasis on deep stops.

Simon M

 

[dberry]

Edit: since someone misinterpretted my popcorn emoji, I'll clarify: I appreciate and support Dr. Mitchell's straightforward response, but based on past history, I'm merely bracing for the anticipated responses.

 

[RayfromTX]

Simon once again has succinctly and clearly stated the case for de-emphasizing deep stops based on available research both published and unpublished studies. He refuses to make any claims of having proof as that is a bar that is likely too high to ethically achieve due to risk to human test subjects and the size of the sampling that would be required.

In this instance, it is best to build a case based on the available data and go with the most educated guess as to the best practice while continuing to monitor outcomes. The problem with this monitoring is the unreliability of self reported cases of DCS. For that reason we are left to make our decisions based on the standard of "mounting evidence" and "building a case for".

I will always take the judgement of educated scholars that spend their careers building a body of evidence and keeping an open mind and being willing to change their views when the data leads in a different direction. I will also remember that there remains a great deal of work to be done and we may not have a proven answer in our lifetime.

Thank you for your commitment to educating divers as to where decompression science is at this time.

 

[CAPTAIN SINBAD]

Hello Mikeny9,

You cannot dismiss the relevance of the NEDU study to decompression diving by technical divers using high oxygen content gas just because the divers in the study were using air. The reasons for this were articulated at length in the original deep stops threads on rebreather world. Those threads are still available, and I would strongly suggest that if you really are interested in this subject that you read them. Put simply, if the study had incorporated oxygen decompression then the difference between deep stop and shallow stop outcomes would probably have been smaller and perhaps not detectable to a level of statistical significance in a study of pragmatic size, but there is no reason to expect that the "truth in the universe" about the disadvantage over-emphasizing deep stops would be any different. Indeed, the same disadvantageous patterns of supersaturation distribution across fast and slow tissues that form the most plausible explanation for the NEDU study results have been shown to exist in bubble model decompressions from typical technical rebreather dives.

You have asked several times about the current state of science around this subject. In this regard it is possible to make some useful general observations.

First, there is not and never has been ANY evidence supporting the emphasis placed on deep stops in decompression diving by bubble models, ratio deco, or GF approaches chosen to emulate bubble models. Whatever you think about the studies that I discuss below, you need to keep this key fact in mind at all times.

Second, every bit of related science that has emerged in recent times suggests that bubble models, ratio deco, and GF approaches with a low GF-lo place too much emphasis on deep stops. That evidence can be summarized as follows:

1. The NEDU study. Previously extensively discussed. A truly seminal piece of work which is never likely to be repeated. There has been a lot of contrived criticism of this study mainly from people with a vested interest. We could go on at length about this (and indeed we have), but the thing we should all remember is that every other bit of contemporary evidence (see below) which has emerged appears to corroborate the NEDU study findings.

2. The recent Spisni study comparing RD and GF 30/80. This study is particularly interesting because it contained a design flaw in that the RD decompression was longer than the GF one. If you want to evaluate the efficacy of the deeper stops and the RD S curve in comparison to another approach which involves shallower stops and no S curve, then you at the very least need to make the profiles the same length (because, all other factors being equal, a longer decompression should always be safer). Put simply, the experimental design imposed a significant inbuilt advantage for the RD approach. Despite this, there was a trend toward less instances of high bubble grade in the divers using the GF approach (which did not reach statistical significance) and inflammatory markers were more elevated in the divers using RD. You have asked several times what is the evidence that debunks RD. “Debunks” is probably too strong a word, but this study provides a signal that the RD approach (as it stood at the time) was inferior to another commonly used approach with shallower stops and no S curve. Moreover, there is some evidence (see the Swedish navy study below) that the comparator for RD in this study (GF-lo of 30) also places too much emphasis on deep stops. In other words RD may have looked worse if compared to an approach with even less emphasis on deep stops.

3. The Blatteau study published in Aviation Space and Environmental Medicine in 2005. They showed more venous bubbles after a deep stop decompression compared to a traditional gas content model with shallower stops.

4. A Swedish Navy study that has been presented at several conferences but which is not yet published. This compared decompression with GF lo = 30 to DCAP (which prescribed shallower stops) from decompression dives. Again, higher bubble grades in the deeper stop dives.

5. The Ljubkovic study from Journal of Applied Physiology which showed almost universally high bubble grades after decompression from trimix dives using VPM. This was not a comparative study, but it did illustrate that bubble model decompressions do not reliably control bubble formation as has been claimed in the past. This study spawned Ross’s ridiculous crusade to rewrite the DCS pathophysiology text books with his claim that venous gas emboli are irrelevant.

6. Neal Pollock’s work at inner space and on deep diving research cruises. Also not published yet, but presented at multiple conferences (including SPUMS last week) and medical courses. He has been monitoring divers for bubbles after deep decompression dives and correlating findings with the profiles. He has found apparent success in decreasing bubble grades by de-emphasizing deep stops, and (in particular), by padding shallow oxygen stops (a strategy which Ross actively disputes).

With the exception of the NEDU study all of these involved gases other than air. You are correct to point out that only one of them relates directly to RD, but RD emphasizes (or at least emphasized) deep stops as a central part of its approach. Collectively these studies offer accumulating evidence that deep stops as prescribed by bubble models and RD in decompression dives are too deep. The Spisni study (albeit a single small study) offers evidence that this concern is not adequately compensated for by imposition of the S curve and exploitation of the oxygen window as I have heard claimed. And I reiterate my first general point above: all of this needs to be considered against the fact that there is not a single study I am aware of that supports bubble model / RD type emphasis on deep stops.

Simon M

Extremely enlightening and thought provoking. Ever since Ross joined this thread I was really hoping we would be able to get your perspective on it as well.

Just wanted to clarify your position on one thing.

Am I correct in understanding that you are not opposed to deep stops at 66% depth if these are sufficiently padded at the shallowest part of the dive? In other words, UTDs present stop at 66% may not have science behind it at this point but if it is retained on the theoretical premise of bubble control or reducing the gradient, as long as it is followed by an extended shallow stop, it is not dangerous than a straight ascent to GF-lo of 50?

Thanks for showing up Doc.