Diving inverted profiles?

[Teamcasa]

I’ve done a few reverse profiles. Not huge differences in depth but reverse no the same. I always double or triple my 20’ safety stop times and add a 3 min stop at 10’, just to minimize my risk. I’m not a fan of reversing but sometimes opportunities present themselves and we want to take advantage of them.

Note: Just because something worked for me, don’t trust you will have the same results.

 

[Gombessa]

You just used GUE and computers in the same sentence. That's probably not ideal

Seriously, I think there is a difference in tech and recreational that doesn't transfer well. IN tech you assume that you do not have access to the surface for emergencies and a "proper" ascent with stops is involved in every dive. Unfortunately, in recreational you can't count on that.

I had a feeling I'd get called on the "dive computer" bit so for the incredulous I dug up the quote straight from the horse's mouth (which seems to opine on both the recreational and the computer points):

Current research seems to make it fairly clear that there are no particularly obvious problems associated with reverse dive profiles in recreational, commercial, military or scientific diving. It is also clear that, on a regular basis, divers seem to be doing reverse profiles without incident. When occasional instances of DCS did occur with reverse profiles, it was at a statistically lower rate than for forward profiles. What this means is, in the field, decompression algorithms and dive computers are adequately handling the question of reverse dive profiles.

-Jarrod Jablonski, Getting Clear on the Basics--The Fundamentals of Technical Diving

 

[jeckyll]

Gombessa: Good quote

Of course, he was talking about what was being observed in the field by "general" divers, but wasn't specifically referring to DIR divers using computers

As Teamcasa, I've done slight reverse profiles with differences less that 20 feet in total depth. And plenty of time spent above 20 feet Typically for good reason (once to recover a camera & strobe lost on dive one). I think my total reverse profiles may account for ... maybe 2 in 150 dives or so ...

FWIW

 

[amascuba]

I had a feeling I'd get called on the "dive computer" bit so for the incredulous I dug up the quote straight from the horse's mouth (which seems to opine on both the recreational and the computer points):

-Jarrod Jablonski, Getting Clear on the Basics--The Fundamentals of Technical Diving

To use a computer, or not to use a computer... It's not really the question of this thread, but regardless here you go:

A Baker's Dozen: Problems With Computer Diving

1. Dive computers tend to induce significant levels of diver dependence, and undermine the awareness essential to all diving, but particularly essential to divers just beginning decompression diving.
2. Dive computers prohibit proper planning; they discourage divers from “studying” the impact of various mixtures and decompression choices.
3. Dive computers are of little educational benefit because they promote neither questioning nor proper planning discussions.
4. Dive computers often use algorithms that heavily pad decompression time; this sometimes results in odd and ridiculous levels of conservatism.
5. Dive computers are expensive, and prevent divers with limited resources from purchasing truly useful equipment.
6. Dive computers significantly limit the likelihood that divers will track their residual nitrogen groups, leaving them less informed in the event of computer failure.
7. Dive computers do not allow for diving helium in any format but the bulkiest and most questionable. It is very likely that new helium-based decompression computers will be inordinately conservative and suffer from all the limitations of air and Nitrox dive computers.
8. Dive computers often generate longer decompressions than an astute, well-educated, experienced diver generates.
9. Dive computers often confuse matters by providing the diver with too much useless information, sometimes even obscuring depth and time in favor of blinking CNS and/or decompression limitations.
10. Some dive computers become very difficult to use if a decompression stop has been violated. Some computers will lock up completely, while others will just beep or generate erroneous and distracting information.
11. Dive computers do not allow the educated diver to properly modify his/her decompression profile to account for advances in knowledge, e.g., the use of deeper stops in a decompression profile.
12. Dive computers do not offer divers much flexibility to generate profiles with varying conservatism. For example, the right mix would allow 100 minutes at 60 ft rather than 60 minutes at 60 ft, but a diver might prefer to do one or the other or a hybrid of the two. Computers confuse this issue by not providing divers with the proper information.
13. Dive computer users often ignore table proficiency and therefore do not learn to read tables properly. When faced with a situation where they can't dive a computer (e.g., failure or loss) these divers are seriously handicapped.

-- gue.com/Equipment/Config/index.html

 

[tc]

Did anyone figure out what "tc246" suggested? I'm too lazy to do it. And "jeckyll", thanks for the always friendly comments.

Assuming square wave dives (Yours were most likely multi level) and fudging a minute off of the 105 fsw dive to stay within the NDL limits on the second example-

46 fsw for 29 min. comes out in group E
45 min. SI stays in group E- you would need 54 min. to move up a group
16 min RNT for an E diver going to 110 fsw, 21 min. actual= 37 min., NDL is 20 so you would have been 17minutes into deco.

Now reverse them-

105 fsw for 20 min. comes out a G diver
45 min. SI moves you to F
47 min. RNT for an F diver going to 50 fsw, 100 min. NDL - 47 RNT = 53 min. available, plenty of time for the 29 min. actual dive time.

Going shallower to deeper in the first example puts you into some major deco, reversing that in the second example gets rid of the problem.

 

[tc]

Evidently, the tables are screwed up.

Not necessarily, just because you exceed the table does not mean you are instantly bent, just that the chances of getting bent start to go up.

 

[TheHobster]

Well, the paper I quoted from and referenced above was presented at the "Reverse dive profile workshop" in 1999 at the Smithsonian Institute... maybe that was the one?

i THINK THAT'D BE CORRECT

 

[Buoyant1]

I looked over my profiles from the past week, and a few were reversed...some were shallow dives and probably not really a concern, however, on my dives on the Thunderbolt, I logged 112 on the first and 113 on the second...

 

[tc]

Can't recall where I heard/read this but I seem to recall someone stating that the reverse profiles were not significant enough to worry about until a later dive exceeded the earlier dive by 150% or so.

Anyone else recall this or am I hallucinating again?

 

[TeddyDiver]

Can't recall where I heard/read this but I seem to recall someone stating that the reverse profiles were not significant enough to worry about until a later dive exceeded the earlier dive by 150% or so.
Anyone else recall this or am I hallucinating again?

Not hallucinating, allthough there have been a bit different percentages too.
What's significant here to remember. When pushing NDL on a shallow dive will saturate more the slow tissues, and generate more comlex on and of gassing situation during the second dive.
Instead doing first (shallower) dive using less of the available bottom time. Anyway not exceeding significantly the time which could have been used to dive to the depth of the second dive (any sense?) makes no increased risk bcs orf the reversed profile.
Just my 5c.