cornfed
Mindless lemming
Underlying assumption is that you're using the same models, table, etc.
Reverse dive profiles
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Underlying assumption is that you're using the same models, table, etc.
Don Burke
Contributor
That isn't what the thread is about.
In the context of this thread, I can't, since this thread has been about repetitive diving from the start. Your question doesn't apply to this scenario since the diver has known two dives were planned since well before the boat left the pier.
I'm sure we agree on many subjects. We aren't doing so well on the subject at hand.
If you want to start over with another scenario, we can do that, but I'm not going to pretend that these two dives are unrelated.
cornfed
Mindless lemming
It's relevant to my argument. I was hoping to use it to help clarify my position.
Don, you don't have a clue as to what I'm arguing, do you? The "simple linear algebra" which you blew off laid it out very clearly. Since you didn't see this I thought the non-repetative dives I asked about would help set things up.
I'm not going to start over, but I will try to spell it out.
My proposition is that, within the no-stop limits, the only way to increase safety is to leave a margin between your bottom time and the NDL time. The greater the difference between NDL and BT, the greater the margin of safety. Under this assumption, if two dives have the same NDL-BT margin you can't say one is safer than the other. The entire point of the non-repetitive dive example was to get you to address this point. My argument centers around it and it doesn't matter whether we're talking about repetitive dives or not. If you want to attack my argument do so based on this and not based on some perceived lack of relevance.
If you understand my non-repetitive dive example then you can easily extrapolate it to repetitive dives. This is what I tried to show with the "linear algebra" example. If you can work out a forward and reverse profile such that the difference in bottom time and NDL for the second dive is the same in both cases, then you can't say one is safer than the other. I admit this might be tricky while trying to conserve total bottom time like you did in your example. However, conserving total bottom time when reversing the profile has never been of concern to me.
My concern has always been maximizing total bottom time. But this included the implicit assumption of preserving the NDL-BT safety margin. This is why I said we agree yet you don't realize it. When reversing the profiles you found that the NDL-BT difference went from 11 minutes down to 1 minute and concluded that this wasn't enough of a safety margin. I agree. Now you can gain that safety margin back, but it will cost you something. You'll have to trade bottom time or surface interval, but the point is you can get it back. So you can either conserve safety and sacrifice bottom time or you can conserve BT and sacrifice safety. Most people opt for safety and subsequently conclude that reversed profiles are a bad idea. But once you've imposed the criteria of maintaining the safety buffer, the recommendation against reversed profiles is not longer based on safety. You can do reversed profiles under the same safety requirement, they will just be shorter.
Don Burke
Contributor
On that we agree. The reverse profile with the same bottom times is more dangerous, so in order to get the same degree of safety for both profiles, the reversed profile needs to be shorter. Those are no longer the same dives.
Now I understand why you didn't want to work out an example on the tables. Instead of talking about two dives that might be done in two different orders, you are talking about four different dives, which isn't the given problem.
Why not just skip the dives? Since reducing the length of the dives is fair game, what about deleting them? That would seem to fit under the same rules.
pdoege
Contributor
Reverse profiles are discouraged because they tend to screw with the assumptions made by the creators of the various dive tables.
They do not appear to actually impact the resolution of DCS symptoms and as of 1999 there was no statistical evidence that reverse profiles have contributed to a greater risk of DCS.
See http://www.umich.edu/~oseh/reverse.pdf for the actual numbers, statistics, and discussion presented at the Reverse Dive Profiles Workshop.
Another good discussion can be found at http://www.harcourt-international.com/e-books/pdf/317.pdf
One thing to note is that bubble formation and DCS is an inexact science. Nobody knows exactly how and why it occurs, although we seem to be getting closer with the bubble models and etc.
It does not hurt to avoid reverse profiles and dive conservatively in general. Remember that you can get bent inside the NDL. The table cannot guarantee safety. Neither can the computer.
Be careful out there. Don't let blind dogmatism lead you into a dangerous position, one way or another.
Peter
They do not appear to actually impact the resolution of DCS symptoms and as of 1999 there was no statistical evidence that reverse profiles have contributed to a greater risk of DCS.
See http://www.umich.edu/~oseh/reverse.pdf for the actual numbers, statistics, and discussion presented at the Reverse Dive Profiles Workshop.
Another good discussion can be found at http://www.harcourt-international.com/e-books/pdf/317.pdf
One thing to note is that bubble formation and DCS is an inexact science. Nobody knows exactly how and why it occurs, although we seem to be getting closer with the bubble models and etc.
It does not hurt to avoid reverse profiles and dive conservatively in general. Remember that you can get bent inside the NDL. The table cannot guarantee safety. Neither can the computer.
Be careful out there. Don't let blind dogmatism lead you into a dangerous position, one way or another.
Peter
cornfed
Mindless lemming
But a reversed profile is simply a shallow dive followed by a deeper dive, so what does it matter if they're different?
At the time I didn't have tables and felt that the "linear algebra" example would work.
I think I mentioned this at one point. You're free to make whatever choices you want, including skipping the second dive.
Don Burke
Contributor
There's the problem.
Tissue gas loading and offgassing happens on a half-life, or logrithmic basis. There are at least several compartments involved, and there is quite a bit of debate on the subjects of how many compartments to use, how long the longest one should be, and the acceptable differentials or gradients that are safe. All of the algorithms say that putting the deeper dive first results in longer bottom times, a greater safety margin, or some combination of the two.
Somewhere around here I have a list of how many compartments various tables and computer programs use.
There are some anomalies in the tables. The one in the Navy tables I point out most often is a 30 minute dive to 60 feet, followed by a 45 minute surface interval and a second dive to 60 feet. The no-stop limit on the second dive is 24 minutes.
Contrast that with the same first dive with a surface interval of 9 minutes. The second dive is actually a continuation of the first dive and has a no-stop limit of 30 minutes.
Extend the surface interval to 46 minutes and the second dive again goes to a no-stop limit of 30 minutes.
I would expect to find similar things in any table. DecoPlanner does some interesting things if you fiddle with the shallow stops a bit, so there are anomalies in the computer programs too.
cornfed
Mindless lemming
From the article,
"Many other variables exist that were not collected or considered in this analysis. Of
interest would be maximum depth of series and if the profiles were within the limits of the decompression tool (table or computer) utilized, if any."
Don Burke
Contributor
The paragraph immediately below the one quoted:
"This review hints at the potential for more severe DCS following reverse dive profiles versus forward profiles and that this potential may increase as the MRDD increases. If there is an increase in this risk, more data needs to be included in the analysis to obtain a significant degree of certainty. Assistance in obtaining this data could come from DAN and/or other chamber facilities."
cornfed
Mindless lemming
That doesn't change the fact that they still haven't detemined whether people were getting bent from diving reverse profiles within the limits of their table, computer, etc.
I need to reread the article but it seems devoid of information useful to the average recreational diver.
I need to reread the article but it seems devoid of information useful to the average recreational diver.
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