reverse profiles??

[DevonDiver]

If you don't violate the deco limits with either dives, then what's the big deal?

The original concern was that reverse profile may not work with the tables/algorythms, thus the stated NDL becomes null and void... and potentially much shorter.

However, that concern was never substantiated by any evidence... which is being recognised now.

 

[miketsp]

On many tourist trips I've been on, reverse profiles are standard on the first day. Get everyone into the water for an initial shallow check-out dive and weight check then proceed to the deeper more serious dives.

 

[aquaregia]

What study showed a dramatic effect?

I can't get to the page for some reason, but Bubbletrouble linked to it once:

Relatively recent research on the topic yields the following:
McInnis et al. believe that forward dive profiles are safer than reverse profiles based on a study of guinea pigs.

 

[scubadiveilat]

Good question!!!

The theory is the deep first and shallower dives to follow. As the deep dives increase the nitrogen levels. Requiring you to SI for a longer time to dissolve from the body. Deep dives are those of deeper than 20M. If you look over dive table for the fun of it. As you read you'll notice the explanation. Deep first shallow to follow.

In your case 14 and 18m is not such a stretch. However other considerations including; Depth, Bottom Time and the Duration of dive(s) in total. Affect the NDL as well.

Have fun diving and enjoy!!!

 

[boulderjohn]

The theory is the deep first and shallower dives to follow. As the deep dives increase the nitrogen levels. Requiring you to SI for a longer time to dissolve from the body. Deep dives are those of deeper than 20M. If you look over dive table for the fun of it. As you read you'll notice the explanation. Deep first shallow to follow.

Scubadiveilat, I suggest you read through this thread to get some updated information.

Yes, the back of the PADI tables does say to do the deepest dive first, but the origin of that requirement is in doubt and research does not support it. The practical reason for it is that if you want to do the exact same two dives, you can do the second dive sooner if you do the deeper dive first.

As for the deeper dive requiring a longer surface interval theory, that does not actually work in this case, as mentioned above. (Get out your tables and try it.) More importantly, if you want a longer surface interval after the deeper dive, it would make sense to do it last, since you will have a very long surface interval until you dive again the next day (or whenever).

 

[beanojones]

The original concern was that reverse profile may not work with the tables/algorythms, thus the stated NDL becomes null and void... and potentially much shorter.

However, that concern was never substantiated by any evidence... which is being recognised now.

To be complete about that, it was assumed that the deepest dives would always be first, by the only large scale repetitive dive research done (the research done by DSAT when developing the PADI RDP). Since it was assumed, reverse profiles have not been, and never will be, large scale tested, so everyone is sort of guessing about how reverse profiles might change things. Isolated research results on non-humans can only provide guidance, but since military and commercial have no need to do repetitive dives, and dive computers have made tables (or at least human subject research) obsolete, a firm answer about reverse profiles might never really be known.

All research has to make assumptions. They made theirs (that repetitive dives would always be shallower). Unforunately the basis for that assumption was not well grounded, and thus the only large scale repetitive diving testing was done in one way.

If dive computer manufacturers were required to sell computer interfaces with dive computers, then we might get some large scale results. As it is now though, the people most likely to do repetitive reverse profiles are tropical guides, who also do a bunch of other things (rapid ascents, multiple dives over multiple days) that increase risk. So when we get bent, and we do get bent, who can say which of the risk factors contributed most?

 

[Mr Carcharodon]

I can't get to the page for some reason, but Bubbletrouble linked to it once:

Rubicon only had the abstract but it appears the paper might be summaried as "decompression dives with no decompression have a higher risk of decompression sickness than no decompression dives". Maybe I am missing it but that does not seem to be helpful in deciding to do a reverse profile within table or computer limits.

 

[divermike1011]

IIRC Mark Powell talks about this in Deco for Divers. My copy is on loan, but if someone else has one... I think he even breaks down some of the science. For the life of me I can't recall what his recommendation is.

Michael

 

[Crush]

I emailed Dr Deco some time ago and he stated that the bias against reverse profiles has no basis in science.

 

[beanojones]

It's fairly important to remember that when it comes to decompression theory, it's a guessing game. The Navy did the complete tests to develop the tables in the fifties; DSAT did complete tests to develop the RDP in the eighties; but they are based on models that are only verified by those tests.

In other words someone made a guess of how it would turn out, and then they bent a bunch of people to make corrections to their guesses. If the theory worked, they would not need the tests, and they would not have bent anyone during the tests. The theory is only verified by the tests on human subjects. Outside of the military and commercial diving, which just has no need for repetitive dives, and more important use chamber decompression as a matter of course, we are just no longer going to get anymore basic research done. We recreational divers are the only guinea pigs.

There are educated guesses about what model to use, but the fact is that randomly people get bent on dives that quite simply cannot (by theory) cause DCS (and conversely don't get bent on dives where they should). We treated a Japanese intro diver for DCS at the HTC in Hawaii who did one 20 minute dive to thirty feet, who did not go to altitude afterwards.

Yes, yes, there is absolutely no way she could have gotten bent on a twenty minute dive to thirty feet. And yet, she was. Until a model can explain the absolute the outliers in some other way than just saying "**** happens", deco theory should be thought of as very much a work in progress, and not a field in which definitive answers will be found. There are recommendations, which whether originally justified or not, become justifiable by being followed by many divers who complete dives following that recommendation without incident.

In other words, since tables are empirical, not theoretical, historically followed practices can become justified simply because people have followed them and been safe. There need not have been any original cogent rationale for the "Deep dives first" protocol for it to have become a reasonable recommendation now. Now the only rationale needed is the fact that it has been standard practice in an industry which has proven itself safe. Now, in order to reasonably rescind the "Dive Deep First" recommendation with any degree of justification would require the sort of testing which will never be done again.

So one can say it's original rationale was missing, but one cannot say continuing that recommendation now is inappropriate.