Reverse Profiles

[Octo-Danny]

What was the final conclusion of the Reverse Profile Dive Workshop? Are reverse profiles ok? As I understood from the sumary of the workshop in some cases they are even better.
I've read in one of the diving magazins that now it's officially ok with computers but not allowed on tables? Will someone clear this out.

Thanks
D.

 

[Dr Deco]

Dear Octo:

The story on Reverse Dive profiles is beginning to change. It is more efficient to dive in the manner of deeper to shallower when multilevel dives are being performed, since this will give the most bottom time per tank of breathing air. Based on the idea of how gas bubbles should grow and decay in the body, it seemed to some people that deeper initial dives would crush tissue micronuclei. Therefore, the reasoning went, the first dive should be deep or “crushing.” Whether this initial “crush” dive was actually deep enough to do what was claimed was open to debate.

The workshop sponsored by the Smithsonian Institution examined evidence presented by researchers with experience in this area. The result seemed to be that no one had noted any more problems when “reverse profiles” were used compared to the “traditional” or “forward” profiles.

Reverse profiles were actually dived during the initial phases of the DSAT test program for the Recreational Dive Planner. It was decided, however, not to proceed with these particular profiles since the use of such dive methods was discouraged by the training agencies.

As far as computers go, they actually calculate the gas loads and therefore will be able to handle all dive situations. Tables cannot be used for multilevel dives with the exception of “The Wheel”. Separate dives are another matter.

As far as the crushing of nuclei, these little critters do not know what you use for diving. However, I am not really sure about gas loads and tables as the format is such that they are made for “forward” dives as far as repetitive groups is concerned. Probably one could not get into trouble under recreational dive situations.

Dr Deco

 

[Rick Murchison]

Especially with tables, and to some degree with computers, reverse profiles cut down on your total available bottom time. So, as a practical matter, if you want to maximize bottom time, it's still smart to do that deep dive first.
Rick

 

[Octo-Danny]

Disregarding the bottom time factor. Looking only on bubble growth I remember that according to the RGBM reverese profiles may, in some cases, reduce bubble size or appearence.
Do you know anything about it?

 

[Dr Deco]

Dear Octo:

As far as I am aware, the Reduced Gradient Bubble Model (RGBM) predicts that the initial dives should be the deepest, as they will tend to crush the tissue micronuclei. This model derives from the work of David Yount, PhD, and his students at the University of Hawaii. Initial studies were made in aqueous solutions of gelatin, and the results were extrapolated to living humans.

The RGBM attempts to limit the growth of tissue gas bubbles since they will sequester or trap the nitrogen in the gas phase rather than allowing it to diffuse to the capillaries where it will be transported away by the circulatory system.

I do not know where the model would predict that shallow and then deep dives would be better than the reverse in terms of bottom time or safety.

Dr Deco

 

[joewr]

You guys must be reading my mind! The story:

The day before yesterday I was sitting in the passenger lounge at Truk International Airport talking to a fellow who had spent the week diving from the Thorfin. Since Truk is mostly deep dives, the discussion revolved around that subject.

In particular, we discussed multi-dive days. Since my buddies and I stayed at the Blue Lagoon Resort we did a max of 3 dives a day, but the Thorfin guys (and the Aggressor and Oddessy Guys) could do up to 5 dives a day. In my reconing that made it harder for them to avoid reverse profile diving if they changed sites. He told me that they were told that it did not matter what order one did the dives in and that the issue had been studied aboard the Thorfin with the various computer mfgs, scientists, etc.: the study indicated absolutely no difference. Interesting.

Then, I told him about my suspicion about the algorithms used in dive computers: for most human behavior there is a gausian distribution of responses, behavior, abilty, etc.; so, if you did the model so that 95% of the population fit within the curve, you would be fairly safe in assuming that most divers would be safe diving with a given computer; then, if you put in a 25-35 "safety" factor, you could be reasonably certain that, if a diver adhered to the computer's info, he would fall into the 100% "safe" category.

Finally, there is the non-technical consideration of liability. My bet is that corporate lawyers make damn certain that, if hauled into court, the company scientist/engineers would be able to state that within a 100% certainty no "healthy" diver would be at risk using the computer--i.e., the gausian distribution and safety factor would cover everyone save the diver with a medical pb or the diver who ignored the computer.

Now, that does not mean you should play fast and loose with the computer, but it would be consistent with a "relaxed" attitude toward reverse profiles.

At any rate, what I am now concluding and want to be assured about is that with a good dive computer and an adherence to its "advice" on deco, etc. reverse is okay.

It is also true that I have reversed a few times due to circustances and all seemed well. It is also true that under those circumstances I made very ample SI.

Have I drawn the correct conclusion and was the Thorfin diver correct?


Joewr

 

[Dr Deco]

Dear Joewr:

[1.] Yes, the “fears” over reverse profiles seem to be unfounded based on diving experience. Since bottom time is longer when multilevel dives are performed in an ever-shallower mode, this was the method of choice. Diving was just better that way. In addition, there were the many stories of divers who went down to free an anchor at the end of the day and acquired decompression sickness. This later occurrence seemed to indicate that deeper dives after shallower divers were a bad idea.

The common thought was that the NDL were developed for deep-to-shallow and not the other way around. If one wished to dive shallow to deep, then it would be necessary to develop new NDLs for this methodology. Since table development and testing is costly, this concept was never tested. If it were, possible someone would have found that it did not make any difference – at least within the recreation range of diving.

When a group of diving scientists with experience in this area was polled, they did not seem to find any problems in their experience. When the group as a whole found little to worry about, the caution was lifted.

Since the computers have the same NDLs and tables, and since the algorithms are the same, I would think that any computer model would be fine.

[2.] As far as the Gaussian safety factor, yes, divers do respond in a Gaussian distribution. There is actually not a bends/no bends limit for a group of divers. The NDLs are selected, usually starting with US Navy limits, and then adjusted according to the bias of the table designer.

Dr Deco