Reverse Profile.

[Liquid]

I've seen "reverse profile" being mentioned in several threads around here.

First I'd like to give you my oinion (according to my knowledge) on the subject. than I'd like you to give me your opinion, aspecialy Dr Deco, whom according to a message he wrote, I think may disagree with me.

My opinion-

1) reverse profile using a standard diving table (not one of the new RGBM tables, which I havent had the chance to look at yet):

While using a standard diving table, reverse profile is a big NO NO. the reason is that the tables work in the assumption that you go always from deep to shallow. Affectively, what it means, that if you go from shallow to deep, the critical compartments will be different ones (when diving shortly deep it's the faster compartments, when diving shalow for long it's the slower compartments). It can be seen on deco-planners, very nicely, on the screen of you own PC. same time, reverse setting, and whoops! you'v crossed an m-value.

2) Reverse profile using a decometer/dive computer:

While diving with a computer, the risk of reverse profile is aliminated. The reason is very simple- the computer calculates the compartments on REAL-TIME. It does not assume anything about order of depths, it simply adhust to the condition, and will worn you anyway, adjusting your NDL acording to what your dive really is like.

That's about it.

 

[Diversauras]

Originally posted by Liquid


While using a standard diving table, reverse profile is a big NO NO. the reason is that the tables work in the assumption that you go always from deep to shallow. Affectively, what it means, that if you go from shallow to deep, the critical compartments will be different ones (when diving shortly deep it's the faster compartments, when diving shalow for long it's the slower compartments). It can be seen on deco-planners, very nicely, on the screen of you own PC. same time, reverse setting, and whoops! you'v crossed an m-value.

I don't think it is so much of a no-no as it is a move that costs you dearly in bottom time or adds dramatically to the deco time. I am at work and don't have a Navy manual in front of me but this is what I seem to remember.

 

[Rick Murchison]

Let's plug some numbers...
Let's use two dives - 90 feet for 20 minutes and 60 feet for 30 minutes.
Using the Navy Doppler limits table, if you do the deep dive first, you must have a surface interval of an hour and a half to remain within the no-stop limits. If, on the other hand, you do the shallow dive first, your required surface interval is a whopping seven hours and six minutes.
Same dives, using a computer - I had an Oceanic handy... If I do the deep dive first, I need a surface interval of 20 minutes. If I do the shallow dive first, the required surface interval goes up to 65 minutes, or over three times as long.
Now those are both Haldanean models, and the RGBM may give a little different results.. I look forward to someone running the numbers through that model to see if there's any penalty for doing the shallow dive first.
Rick

 

[Dr Deco]

Dear Readers:

[A.] The comments on the deep-to-shallow diving at one time revolved around the [assumed] fear that the tables would not really work successfully since the M-values had been determined from deep-to-shallow diving. This reservation would hold even if one were using a dive computer. The limiting supersaturations simply were not tested. It is very costly to test all of the multilevel combinations that are possible. Some can be checked as examples of profiles, but you can never check as many as you like.

The second reason involved the already-stated comment that bottom time is reduced (or the necessary surface interval is too long) in the case of shallow-to-deep dives.

[B.] For the RGBM, I do not believe that this will show much of a difference over a deco computer. The reason for saying this is that this is what Dr Wienke told me. But, realistically, the model is more devoted to deep diving and will not show big differences when recreational scuba profiles are used. (So I am told.) The RGBM is designed to insert deep stops for technical diving in order to maximize the elimination of dissolved inert gas.

When one ascends too quickly to the surface, the small nuclei (with their considerable inward "Laplace pressure" from surface tension) will expand in accordance with Boyle’s law. The inward, surface tension force is now smaller and the nuclei can expand to be decompression bubbles; the dissolved inert gas now diffuses into them and is sequestered or “trapped.”

This is somewhat complicated, I know, but I hope it helps to some degree. If there is still confusion, ask again, and I will endeavor to straighten it out.

Dr Deco :doctor:

 

[Spectre]

Originally posted by Rick Murchison
Now those are both Haldanean models, and the RGBM may give a little different results.. I look forward to someone running the numbers through that model to see if there's any penalty for doing the shallow dive first.

FWIW:

I ran the numbers with my Suunto Dive Manager, as my Stinger uses an RGBM. Running the dive profiles Rick provided (30@60 and 20@90) and including 30 feet/min ascending to 30 ft, then 15 fpm to 15 ft and 3 minutes @ 15:

To stay within NDL limits, you need 59 minutes between the 90 and the 60. To stay within NDL limits doing the 60 first, you need 1 hour, 47 minutes.

 

[Dr Deco]

Funky monks:

It sounds to me that the surface interval times reflect a difference in the no-decompression limits between the Suunto and the Oceanic decompression computers.

Dr Deco
:doctor:

 

[Spectre]

Originally posted by Dr Deco
Funky monks:

It sounds to me that the surface interval times reflect a difference in the no-decompression limits between the Suunto and the Oceanic decompression computers.

Dr Deco
:doctor:

Yep... which fits with your previous comment....

[B.] For the RGBM, I do not believe that this will show much of a difference over a deco computer.

 

[Rick Murchison]

So, the bottom line we're seeing is that despite the "acceptance" of reverse profiles - or should we say despite the removal of the virtual taboo on reverse profiles, existing decompression models do favor the deepest dive first when viewed from the "more bottom time/shorter surface interval" perspective. And for me, that's a pretty practical reason to do the deepest dive first unless there's some other overriding consideration.
----------------------
That said, I'll give a specific example of when I plan for and do a reverse profile - Saturday morning I have an OW class at Vortex, where our max depth will be 40' - we'll do two dives of about a half hour each. Saturday afternoon I'll be doing two dives in the 100' range in the Gulf with Nitrox and night divers. I'll have a surface interval of about five hours between the shallow ones and the deep ones, and an interval of about two and a half hours between each of the deep ones, all well within the NDL's for the gasses I'm using... an example of a reverse profile that's just fine, thank you.
Rick

 

[madmole]

Or you can do what WKPP are doing and make sure that the last dive saturates you to a higher level than the first. They then totally ignore the earlier dives and just decompress for the last one (At least thats what I understand from Irvine's rantings)

they are making several assumptions here.
1) That the latest dive does indeed saturate you to a higher level than the earlier dives
2) Due to the earlier N2 uptake the N2 uptake on the later dive is slowed as there is a lower pressure gradient
3) Lady luck is on their side

But they seem to getting away with this (I'm not trying it nor would I advise anyone else to).

This suggests to me that we need to do more research on the subject and that the current models are not very accurate in this respect. but I'll let the WKPP lot be the guinea pigs for that. Me I'll carry on doing the deep dive first if possible and if not I'll trust my computers

 

[The Iceni]

Without going into scientific detail, which is well beyond my limited understanding, this is the way I see it.

If the deeper dive is first the decond dive is equivalent to a deep stop on the first so the slower tissue compartments will effectively be offgassing during the whole of the second dive.

If the deeper dive is second, these compartments will become even more saturated than they were at the end of the first dive.

(I do realise it is far more complex than this.)

As stated above, this must add considerably, to decompression penalties. More importantly you are entering the unknown, as there is little eveidence on the safety of reverse pofile dives from field evidence. So few divers do them!

To my mind there is one thing for sure, you are likely to be increasing the statistical risks of an "unexpected" DCI even if you dive within the calculated limits with a reveresed profile.

by madmole
3) Lady luck is on their side

I would prefer to err on the side of caution!