do your shallowest dives first?

[BigJetDriver]

reefraff once bubbled...
Reverse profiles, within limits, should not cause problems for normal recreational divers.

In October, 1999, a workshop was held to review whether reverse dive profiles are cause for increased risk. The workshop was organized by Michael Lang, Diving Officer and head of the Smithsonian Institution's Scientific Diving Program.

The findings of the workshop were:
[*] Historically neither the US Navy nor the commercial sector have prohibited reverse dive profiles.
[*] Reverse dive profiles are being performed in recreational, scientific, commercial and military diving.
[*] The prohibition of reverse dive profiles by recreational training organisations cannot be traced to any definite diving experience that indicates an increased risk of DCS.
[*] No convincing evidence was presented that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS.[/list]
The conclusion:

The workshop finds no reason for the diving communities to prohibit reverse dive profiles for no-decompression dives less than 40 msw (130 fsw) and depth differentials less than 12 msw (40 fsw).

Steven

Steven,

In 2000, I attended a speech given by Michael Lang on the subject in question. At that time he added one caveat. He said that the one problem profile was a long shallow exposure with a deep spike at the end of it. While I have not seen that published, I do have it in my notes.

Also, interestingly enough, he said that, if you are wearing a "wet computer", the computer will track your total exposure accurately as you move through the water column no matter which way you run your profile.

 

[Rick Murchison]

Don Burke once bubbled...
less bottom time for the reverse profile of the same two dives.

Sunnydiver, that is the bottom line for recreational no-stop diving. Recreational dive tables and computers penalize the reverse profile with either longer surface intervals required or shorter bottom times available if you do the shallower dive first.
So while there is no real proscription against doing the shallow dive first if you want to, realize you can get more diving in a day by doing the deeper one first.
Rick

 

[MikeFerrara]

A computer will track total exposure, as in depth, time and theoretical compartments but I don't know how accurate it is, at least as far as what's really going on in the body. What most computers don't take into account is gas in the free phase (bubbles).

I won't go too far because I'll botch it but a dual phase model like RGBM shows you're better off if you have a high initial P(crush) which you get if you do the deepest part of the dive first and get there pretty quick. Most computers use absorbed gas models which will not track or account for this. See BRW's "Technical Diving in Depth"

BRW has commented on this here on the board in the past and if you do a search you should be able to find it.

 

[BigJetDriver]

Michael Lang' statements were as I described, per my notes. However, as Mike Ferrara has noted, given Wienke's research and the work of Tim O'Leary's NAUI Tech Research Team on the subject, I would tend to believe that he (Wienke) has the correct take on the subject!


Further, as Mike has also noted, most computers are not set up to handle the dual-phase mechanics. The HS Explorer is the only one that I am aware of at the moment that has the full-implementation RGBM.

I would also echo Mike's recommendation of "Technical Diving in Depth" by Dr. B. R. Wienke. While I found myself having to "read around" some of the higher level math, the text is fully explanatory, and quite convincing.

 

[SDAnderson]

I would also echo Mike's recommendation of "Technical Diving in Depth" by Dr. B. R. Wienke.

For what little it may be worth, I'll add my endorsement of this book.

While I found myself having to "read around" some of the higher level math...

Yeah, what he said!

So it would seem that its more or less agreed: reverse profiles may result in some penalties (longer SI/shorter BT) but aren't inherently prohibited and the tables and computers compensate for them. For the recreational diver, at any rate.

All things in moderation, especially moderation.

 

[Dr Deco]

Hello Scuba Board Readers:

Reverse Profiles

From what I can tell, the recommendations from George Irvine stem from his style of diving at the WKPP. These are not dives typical of the recreational diver, and I believe are probably rather specific to his type of diving.

I would not generalize these methods to all recreational diving. It is obvious that they work for his situation and his cadre of divers. They are certainly interesting, to say the very least.

Dr Deco :doctor:

 

[Don Burke]

I think George's statements are not as much related to decompression as gas absorption.

If you make a big dive, say 6 hours at 300 feet, you'll go through substantial decompression and surface with quite a bit of gas in your tissues. A relatively shallow dive after that will add gas to slow tissues and could put you back into a long decompression process.

Making the shallower dive first would involve a much shorter decompression process and making the big dive after that would result in much of the gas from the first dive being rendered inconsequential since gas absorption would get to the flat part of the curve.

All of this has little to do with the dives the vast majority of people are making.

That is my take on it. Your mileage may vary.

 

[BRW]

Folks,

Contary to what you might read/hear about RPs in general,
DATA suggests that RPs with decrements larger than 40 fsw are
not a good thing. Especially as you go deeper on both dives.
And SIs are within a few hrs. And profile times are in the
same ballpark. This came out of the Smithsonian RP Wkshp.

Deco RPs are a relative bad thing -- see my earlier posts and refs therein. On both rec and tec RPs within SIs of 1 - 3 hrs.

Bub mechanics contraindicates same as decrements and
depth both increase, and SIs are shorter than a few hours.
And profile times are the same scale

Shallow (no-deco) dives followed by much deeper extended
deco dives (RPs) within SIs of 3 hrs, and vice-versa, probably
fall into the forward and reverse yo-yo category, with the deep
deco dive the main concern and the shallower ones second order
so long as they are not too deep nor long. Maybe I can
quantify when I get back.

Bottom line overall is RPs, especially for deco regimes, are
not recommended. Data says same.

Odd ball cases (disproportionate bottom times) need some
further data and analysis.

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Ldr

 

[SDAnderson]

This thread grew out of what I interpreted to be a question regarding reverse profiles as it pertains to recreational diving. For all of us, especially for those pushing past recreational limits (time/depth/mix), I hope you'll be able to weigh in with more details.

I've got no ox to gore and hope to be spared the raw data, I'm just starting to regularly push below the 200ft mark and would dearly love to not hurt myself.

Thanks,

Steven

 

[BRW]

RR,

Do you have the proceedings of The Smithsonian
RP Wkshp. If so, go to my papers (2) and check
out the stats for RPs beyond 40 fsw. DCS hit
rate for deco RPs with decrements beyond 40 fsw
is in the 15% range.

Also check out Nishi's paper on RPs
with shorter SIs. And Gerth's statistical
model implications for RPs. Plus our
admonitions overall.

Cheers, and I can certainly appreciate
your diving safety concerns.

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Ldr