Mark also goes into the rationale; the conclusion isn't his, he just relays the concensus reached by the symposium.
Reverse Dive Profiles
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Mark also goes into the rationale; the conclusion isn't his, he just relays the concensus reached by the symposium.
OP
FPDocMatt
Contributor
Thanks for the clarification.
But this is even stronger. It means that some of the best minds in scuba diving have come to this conclusion.
And then you do a little more research and find out your college professor was off his rocker. It is certainly true that Shakespeare's original version was corrupted in the nearly 20 years before it was published, but I don't know any credible source who thinks he was not the original author.
Here is a little more information that may be useful.
In the famed discussion on this about a decade ago, PADI revealed that the first indication of the ban on reverse profiles was found in a RECOMMENDATION in a 1972 PADI manual. There was no clear explanation of why that recommendation was made, and PADI did not know who put that recommendation into the manual. As the years went by, the recommendation essentially turned into an edict, again with no explanation of why that was true. The PADI RDP includes this reverse profile statement, again with no explanation. It is therefore not surprising that an instructor will be in a position of telling you what to do without knowing why.
The explanation given here--you get more bottom time using the tables that way--is probably the reason, but it is not stated anywhere that I know of.
If your tables, computer, or whatever you are using to plan your dives tell you that you have enough SI to do the next RECREATIONAL dive, you should be OK, reverse profile or not.
On the other hand, in that famed workshop, one participant, Bruce Weinke, creator of the RGBM decompression theory, was able to convince the group that reverse profiles were still bad for technical divers doing decompression dives. As I recall, his reasoning had to do with bubble formation on one dive followed by bubble crushing (and potential migration into the arterial side) on the second.
I think I did found out why .....
I put into divePAL a series of 2 dives (I called them: Deep first - Urban legen #1 and #2)
Dive #1
Max depth: 120ft
Bottom time: 19 minutes.
Descent rate: 60ft/min
Ascent rate: 30ft/min
Safety Stop: 3min at 15ft
Surface Interval: 60min
Dive #2
Max depth: 80ft
Bottom time: 25 minutes.
Descent rate: 60ft/min
Ascent rate: 30ft/min
Safety Stop: 3min at 15ft
Analyzing the series with ZH-L16C Moderate, there was no deco situation and the the Nitrogen loading at the end of dive 2 was 75%
Using the new feature Estimated Pressure Group, the EPG at the end of dive 2 is R
Here image of dive 2 of this series:
I created the series "Reverse Profile - Urban Legen?" where dive 1 has the profile of dive 2 of the series above and dive 2 has the profile of dive 1. Surface interval is still 60 minutes.
Surprise surprise ....
Analyzing this series with the same decompression algorithm, the Nitrogen loading at the end of the second dive is 78% (very similar to the previous series) but .... the EPG is OFF THE CHART !
Here image of dive 2 of the "Reverse Profile" series:
Basically it looks like that the Tables are indeed heavily penalizing reverse profiles.
My team (that is much smarter than me .... AND has just completed the integration of the tables into divePAL) is telling me that the way the tables are structured make them "asymmetric" :shocked2:
Were the tables designed this way? Or it just happened?
Anyway, we are not advocating that one way is better than the other .... or viceversa.
I just wanted to present some data.
Alberto (aka eDiver)
I asked the same question many years ago to a very well known diving medic in the UK, who was at the time one of the old and bold hands at the DDRC.
The answer that I got was quite different.
It goes back to the time of J.S. Haldane, who did all his initial research assuming that his goats reached saturation. He decompressed them and sent them back down. His *original* theory suggested that the second dive should be shallower and shorter, a bit like "topping-up" the N2 level.
Now, he did all his experiments in this way, and since then nearly all the experiments have been done in this way. Everything has been optimised to work in this way. All the tables and models currently used are based on theories and subsequent testing in chambers and real life dives.
As I understood it at the time, the lack of any experimental data, and a bit of CYA is the most likely cause of the "rule" and the best reply to the original poster really should be that deco theory was developed in this way, and nobody has ever gone out and tested reverse profiles.
It would be interesting to do the maths and see what in the Buehlmann example above causes the large penalty for the reverse profile. It's late and my maths doesn't work this late, but I can't immediately see which part of the mode l would cause the penalty.
Thanks for the calculations Alberto, this and many of your other posts have been highly informative.
Jon
The answer that I got was quite different.
It goes back to the time of J.S. Haldane, who did all his initial research assuming that his goats reached saturation. He decompressed them and sent them back down. His *original* theory suggested that the second dive should be shallower and shorter, a bit like "topping-up" the N2 level.
Now, he did all his experiments in this way, and since then nearly all the experiments have been done in this way. Everything has been optimised to work in this way. All the tables and models currently used are based on theories and subsequent testing in chambers and real life dives.
As I understood it at the time, the lack of any experimental data, and a bit of CYA is the most likely cause of the "rule" and the best reply to the original poster really should be that deco theory was developed in this way, and nobody has ever gone out and tested reverse profiles.
It would be interesting to do the maths and see what in the Buehlmann example above causes the large penalty for the reverse profile. It's late and my maths doesn't work this late, but I can't immediately see which part of the mode l would cause the penalty.
Thanks for the calculations Alberto, this and many of your other posts have been highly informative.
Jon
actually .... that is not correct.
In fact, for the Buhlmann algorithm the 2 series are almost similar; in one case NL at the end of the series is 75% while in the other is 78% and the distributions by compartment are quite similar too.
The huge difference is just for the tables as in the reverse series the EPG is DECO.
See image:
Thank you your kind words. Tell your friends about our products.
Actually the issues lays beyond the experimental evidence supporting the tables.
Despite decades of research the physiological processes underlying decompression are poorly understood.
So we have models of body compartments that are not really coupled with real body organs or tissues and we rely on a set of equations to model dives and decompression limits.
We are using those models just because they works fine after a set of testing and some improvements.
The driving force for improvement is usually a sad increase in accident statistics, so if in a given situation a model thought to be good shows a dramatic increase in decompression sickness under certain circumstances, some new parameter is added to the model to penalize this class of profiles and reduce the risk of DCS.
The original Haldane model hypotheses a symmetric intake and wash out of nitrogen for all compartments (same half-time both in and degassing), more extensive testing of the model revealed the poor performance of the algorithm, so the model was slightly changed to account for slower nitrogen washout.
Further testing pointed out that a set of profiles, including reverse profiles, are associated with an increased risk of DCS, so more recent models added an "epsylon-factor" penalizing inverse profiles (inverse profiles or short SI are penalized by reduced NDL bottom time).
The increased surface interval for reverse profiles allows for increased N2 washout and lower risk of DCS.
It is not granted to be the best solution, but it is applied "because it works", in the sense that a set of experimental evidence shows that using this factor significantly reduces the risk of DCS diving inverse profiles. My personal feeling is that this factor over-penalizes reverse profiles, but till more accurate models will be available I like to stay on the safe side.
Probably future models will figure out under what conditions reverse profiles are dangerous and under other conditions they are not o risky, and newer models will be more precise.
With reference to model model comparison, any model is as good as it is able to represent the biology of the individual diving a given profile, if the model is a bad approximation of the subject physiology a particular dive profile can be associated with DCS despite the model allows it.
It is not rare that the same individual diving the same profile on different days will be fine one day and have problems on the other day. One explanation for those effects can be searched in the number of parameters neglected by the model, like hydration, general fitness, health, body aggressant e.g. cold water, stress, excertion and so on.
Despite decades of research the physiological processes underlying decompression are poorly understood.
So we have models of body compartments that are not really coupled with real body organs or tissues and we rely on a set of equations to model dives and decompression limits.
We are using those models just because they works fine after a set of testing and some improvements.
The driving force for improvement is usually a sad increase in accident statistics, so if in a given situation a model thought to be good shows a dramatic increase in decompression sickness under certain circumstances, some new parameter is added to the model to penalize this class of profiles and reduce the risk of DCS.
The original Haldane model hypotheses a symmetric intake and wash out of nitrogen for all compartments (same half-time both in and degassing), more extensive testing of the model revealed the poor performance of the algorithm, so the model was slightly changed to account for slower nitrogen washout.
Further testing pointed out that a set of profiles, including reverse profiles, are associated with an increased risk of DCS, so more recent models added an "epsylon-factor" penalizing inverse profiles (inverse profiles or short SI are penalized by reduced NDL bottom time).
The increased surface interval for reverse profiles allows for increased N2 washout and lower risk of DCS.
It is not granted to be the best solution, but it is applied "because it works", in the sense that a set of experimental evidence shows that using this factor significantly reduces the risk of DCS diving inverse profiles. My personal feeling is that this factor over-penalizes reverse profiles, but till more accurate models will be available I like to stay on the safe side.
Probably future models will figure out under what conditions reverse profiles are dangerous and under other conditions they are not o risky, and newer models will be more precise.
With reference to model model comparison, any model is as good as it is able to represent the biology of the individual diving a given profile, if the model is a bad approximation of the subject physiology a particular dive profile can be associated with DCS despite the model allows it.
It is not rare that the same individual diving the same profile on different days will be fine one day and have problems on the other day. One explanation for those effects can be searched in the number of parameters neglected by the model, like hydration, general fitness, health, body aggressant e.g. cold water, stress, excertion and so on.
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