Ratio deco #1

[Slamfire]

If you actually took a deco course from AndrewG you'd discover that this not how RD is presented today.

What do you mean? I was on a live AG webinar and one of the first things he said was to acknowledge it has no scientific proof. Throughout his presentation he takes breaks to give a chance to the audience to ask questions. On the first break I asked him directly so this is based on what you've seen that works for you throughout your many years of experience and thousands of dives. There are no statistical trials, no sampling size, etc. He answered, "yes".

You are "supposed to" conservatively try it and gradually build an understanding of what works for you.

You are supposed to do this with everything in scuba.

Most of the elements of RD (deep stops, "O2 window although that's a bit misnamed, and shallow stops for dissolved gas are founded on a mix of what's understood about deco theory). Pure Buhlmann is for instance only a dissolved gas theory which does not address inevitable bubbling. There's not alot of trust, more "here are the concepts we agree upon how do we put them together into a plan?" If you don't agree with the 3 basic pillars of RD then use one of the other products which incorporates the concepts you do agree with. In simple terms:
Buhlmann = dissolved only (GFs being a bubble kludge)
VPM = dissolved + bubble
RD = dissolved + bubble + deep stops

So the three pillars are dissolved, bubble and deep stops...

Dissolved: How many tissue compartments does RD take into account? What are the off-gassing half-lives of each compartment? Can you give me the M-values for each compartment? Did the creators looked at the ionic strength of the solvent in relation to the solute for each compartment? Did they study what a slight change in pH would do to the solubility properties in each compartment? Uhmm, no? Such things do not exist for RD... So then how in the world do you claim that it is a pillar? A pillar how? Where is the connection? There is no connection... So then is it just an exercise of throwing in buzzwords at it to make it sound cool?

Bubble: What is the range of bubble diameters that RD works with? How does it track bubbles in a body? Doppler? What are the bubble surface tensions in different compartments? Did you do any kind of molecular modeling to assess microbubble interactions within tissues? ...No. No connection again... ok let's move to the 3rd pillar.

Deep stops:What do you mean? Pyle? I would have thought deep stops are simply a strategy to manage risk from the first two pillars: dissolved, and bubble. But since it's been promoted to "pillar" status there must be something more to it. Even Pyle himself said that the thing for what he is most known in scuba is wrong. I guess a third pillar had to be created because a two legged stool will fall down under its own weight.

If you are demanding scientific "proof" of "safe" deco before proceeding then you should probably quit diving.

Richard, I have no interest in discrediting RD. I know it works. You ask a question about why people want it to "match". I simply answered why people want it to "match." I already have have a clear concept of what it is and, more importantly, what it is not. I have at least a clear enough concept to know where I want it to fit within my own personal diving.

 

[Slamfire]

VPM-B from v-planner with +2 conservatism doesn't reproduce anything like the 1200 minute deco obligation, closer to 500-600. If set it to nominal and eliminate the helium from the deco gases (eliminating the helium weenie factor from the algorithm) and include 18%, 21%, 30%, 50%, 100% deco gases then I can get it down to 480 minutes of deco.

AG said it, not me. To be fair, he said "a Bulhmann CCR" model. I placed the question in here without first playing with deco software. If, like you did, I had run a couple of scenarios through deco software first I probably would have answered my own question before getting an answer from James.

 

[Rainer]

What setpoint are you using? I'd assume 0.7 for such a dive. The deco I'm seeing for this dive (VPM+2) is a lot higher than your 500-600.

VPM-B from v-planner with +2 conservatism doesn't reproduce anything like the 1200 minute deco obligation, closer to 500-600. If set it to nominal and eliminate the helium from the deco gases (eliminating the helium weenie factor from the algorithm) and include 18%, 21%, 30%, 50%, 100% deco gases then I can get it down to 480 minutes of deco.

 

[rjack321]

Dissolved: How many tissue compartments does RD take into account? What are the off-gassing half-lives of each compartment? Can you give me the M-values for each compartment? Did the creators looked at the ionic strength of the solvent in relation to the solute for each compartment? Did they study what a slight change in pH would do to the solubility properties in each compartment? Uhmm, no? Such things do not exist for RD... So then how in the world do you claim that it is a pillar? A pillar how? Where is the connection? There is no connection... So then is it just an exercise of throwing in buzzwords at it to make it sound cool?

Bubble: What is the range of bubble diameters that RD works with? How does it track bubbles in a body? Doppler? What are the bubble surface tensions in different compartments? Did you do any kind of molecular modeling to assess microbubble interactions within tissues? ...No. No connection again... ok let's move to the 3rd pillar.

Deep stops:What do you mean? Pyle? I would have thought deep stops are simply a strategy to manage risk from the first two pillars: dissolved, and bubble. But since it's been promoted to "pillar" status there must be something more to it. Even Pyle himself said that the thing for what he is most known in scuba is wrong. I guess a third pillar had to be created because a two legged stool will fall down under its own weight.Richard, I have no interest in discrediting RD. I know it works. You ask a question about why people want it to "match". I simply answered why people want it to "match." I already have have a clear concept of what it is and, more importantly, what it is not. I have at least a clear enough concept to know where I want it to fit within my own personal diving.

I'm not going to go into every one of this issues because most of them are demanding precision and accuracy where there is none in even the most "proven" mathematical models.

But a few points:
Buhlmann is marginally accurate, we know gas dissolves (duh) but even within M-values everyone bubbles. Shoot even with GFs like 30/85 everyone bubbles. So why calculate 16 diffferent purely theoretical compartments to "track" something which is known to be inaccurate? Its rediculously precise but remains inaccurate. Look up the definitions of precision and accuracy.

Funny thing about VPM is that during development it was fitted to Buhlmann GFs because the answers didn't make sense. And the first iteration didn't account for bubbles growing during ascent (oops). And different implementations give wildly different answers (GUE Forums). So what's accurate? Not much more than we know already. Bubble elimination is greater at depth due to surface tension (which probably varies between tissues and hydration states among other unknown variables). Bubbles grow during ascent. Everything else you demand is a WAG. Shoot many bubble modeling concepts have only been "proven" in jello.

Pyle stops (and various other implementations like in RD) are a great example. According to many models that you like so much they are theoretically completely unnecessary. Yet they seem to work, isn't that "proof" enough of their validity?

You seem to only like mathematical proof. Any epidemiological or empirical evidence from actual dives is considered invalid. If the dives in question were tracked in a Sequel database with doppler profiles and peer-review & published statistics of hits would that consititute "proof"? There are thousands of natural phenomenon which defy complete human understanding, human decompression is one of them despite 16 Buhlmann compartments and the purported surface tension of a nitrogen bubble in blood.

 

[TSandM]

I do think it's really important, when we start talking about this stuff, to acknowledge that the "standard" decompression models are no more an accurate representation of what is actually happening inside the body than RD is. They may have a more involved mathematical basis, as they attempt to model gas transport using data from simpler systems, but aside from some very old data that I understand the Navy has, nobody has put probes into divers to measure gas tensions in various tissues as people are compressed and decompressed.

The recreational tables, or at least some of them, are based on mathematical modeling but have also been subjected to a great deal of empiric testing. Tables for staged decompression diving, for the most part, have not; where they have been tested, the numbers of divers and dives are comparatively quite small.

Richard, that "deep stops" pillar may be somewhat dubious . . . I found the proceedings of the DAN Symposium on Technical Diving to be very interesting in that regard. Of course, the studies cited use a proxy for decompression stress, which is Doppler-detectable bubbling, and this has a far from perfect correlation with symptoms, at least in the short term. But it's sort of the best we have at present.

 

[rjack321]

The recreational tables, or at least some of them, are based on mathematical modeling but have also been subjected to a great deal of empiric testing. Tables for staged decompression diving, for the most part, have not; where they have been tested, the numbers of divers and dives are comparatively quite small.

Richard, that "deep stops" pillar may be somewhat dubious . . . I found the proceedings of the DAN Symposium on Technical Diving to be very interesting in that regard. Of course, the studies cited use a proxy for decompression stress, which is Doppler-detectable bubbling, and this has a far from perfect correlation with symptoms, at least in the short term. But it's sort of the best we have at present.

They bent alot of people (in the chamber) developing the DSAT recreational tables. My wife was a chamber tech at the time and had to ride with a few of them to "fix" them. I don't think they could do such a study with substantive decompression dives because the risks would be too high.

Well the way RD is currently taught deep stops are integral to the whole package. I think they start a little too deep personally and the "rules" scale poorly for the shallow stuff. But most of the models don't have any deep stops at all. I was just trying to say that something "deeper than we can mathematically predict" remains integral because it works and has a proven track record. Even if that proof is merely anecdotal, it extends across a couple of decades and to many different divers who follow a wide variety of deco profiles.

 

[Slamfire]

According to many models that you like so much they are theoretically completely unnecessary.

How do yo know which models I like?

You seem to only like mathematical proof. Any epidemiological or empirical evidence from actual dives is considered invalid. If the dives in question were tracked in a Sequel database with doppler profiles and peer-review & published statistics of hits would that constitute "proof"?

I like math, but it is not the only way to prove things. An SQL database with properly documented stats would rank higher in my book than just throwing unfounded buzz words at a methodology. Does such a database exist for RD? If it does, can I have a look at it?

I do think it's really important, when we start talking about this stuff, to acknowledge that the "standard" decompression models are no more an accurate representation of what is actually happening inside the body than RD is.

100% acknowledged.

I'll say it again, RD works. My pet peeve here is characterizing it as something that it is not. The models that have the math and the science behind them may not be more accurate than RD. This does not mean that all of a sudden you can then say RD is based on (i.e. RD pillars are) dissolved gases and bubble mechanics, because that simply is not a true statement. And when you acknowledge that, then the questions still remain: What is it based on? Where does it come from? I asked the questions directly to AG and I got the answers I wrote above.

Please note that the question is not: Does it work? Personally, I think it does.