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Skin Bends
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Rich Keller
Contributor
What can I do to help you understand? SB is a great place to learn, it works better if you listen more then you talk especially if you are talking about something you say you do not understand. In a discussion about safety you are the only one talking about money. That would suggest that buying a computer is more important for the money you make then the safety of the diver buying it. Does that clear it up for you?
I think the Navy dive tables show that the more time you spend underwater the more nitrogen your body will absorb. If the OP dove the tables he would have been out a lot sooner then he was. There are no guarantees but it seems far more likely that he would have been fine.
I hear what you're saying and at a certain level it makes sense, but I don't believe the generalizations you've been making are supported by the research.
"If appropriate decompression procedures are followed, decompression sickness is also uncommon. Rate of occurrence (per dive) in operational open water dives from minutes to several hours in duration varies according to the diving population: typically 0·015% for scientific divers, 0·01–0·019% for recreational divers, 0·030% for US Navy divers, and 0·095% for commercial divers.39,40
The number of active worldwide recreational divers is not known but is likely to be in the millions. The Divers Alert Network took a sample of 135 000 dives by 9000 recreational divers in which the rate of occurrence of decompression sickness was 0·03%."
This was taken verbatim from a paper by Vann and colleagues, linked here:
Decompression illness. - PubMed - NCBI
The 135,000 dives that DAN collected data on were from Project Dive Exploration, which uses dive profile data uploaded from computers. Navy and commercial divers use tables, though the Navy is (and has been for years) actively exploring using dive computers. The DCS rate is not higher for the PDE dives, which were conducted in a variety of diving environments and conditions.
Feel free to explore PubMed and the Rubicon Foundation website for further references.
Best regards,
DDM
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I may be insane for posting these as I'm sure people will nit pick my bouancy or ascent rate or whatever, but damn, these look pretty conservative to me - here are the profiles from the four dives....
In retrospect I was bent after the first two, and then made it worse after the 2nd batch. The 3rd dive had at least 1/3 spent at 30fsw or less - my final safety stop was a little deep I guess, but still!
Also, one hour surface intervals.
In retrospect I was bent after the first two, and then made it worse after the 2nd batch. The 3rd dive had at least 1/3 spent at 30fsw or less - my final safety stop was a little deep I guess, but still!
Also, one hour surface intervals.
I assume you were on air. I tried to run those dives through the PADI eRDPML as multi-level dives, and even if I put the deep portion of the second dive first, it runs way beyond NDLs on the second dive.
Yes air, conservative factor off, and DSAT algorithm - it would seem that the GEO 2.0 (or rather I am for trusting it) is to blame if your analysis shows me into deco. According to the data log on Mac Dive downloaded from the Oceanic computer, (including no deco or ascent rate alerts) I was within NDL limits. Maybe it's not translating to the padi program, or maybe I'm totally wrong and that would be the reason for my hit and my buddy was just super lucky.
I admit my lack of skill planning complex multilevel dives using tables and that may be the reason I had an issue. However, the point for me of using a computer is so that I can dive a more dynamic profile and still have the math on my side. This may indeed be my error as the computer's math (at least according to this discussion) is too risky to be trusted.
It is nice to know I guess, that in fact there IS a reason for the event, and that as Rich has suggested, I have been lucky trusting the machine to keep me safe all these years. In any case, I will turn conservative on and use nitrox on air tables while staying well clear of the MOD and o2 sat of 1.4.
Thanks all for the advice.
Also, I do want to say that Rich is ABSOLUTELY correct in saying that a square profile based on maximum depth would have gotten me out of the water sooner and made the dives more conservative. Would that have avoided the incident? We will never know for sure.
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I don't think you can say that your computer did you wrong. I think I can say that a less iterative method of calculating nitrogen loading found these dives to fall outside the acceptable limits. But then again, even if you do go "into deco" by the RDP, does that guarantee you'll get hit? I doubt it, especially if the decompression obligation is small.
What you may have found out is that your physiology -- or at least your physiology on that day -- is not compatible with the limits of the algorithm that computer is using. You may be better off with something more conservative, or with not approaching the limits of your device quite so closely.
What you may have found out is that your physiology -- or at least your physiology on that day -- is not compatible with the limits of the algorithm that computer is using. You may be better off with something more conservative, or with not approaching the limits of your device quite so closely.
WarrenZ
The Ultimate Bad Example
How conservative is the PADI dive calculator compared to other algorithms could the PADI limits be deliberately reduced to give a safety factor that the dsat algorithm didn't have because it calculated the dive as a multi level dive with hundreds of levels.
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There is a concept in mathematics called curve-fitting. If you look at the OP's first dive, and consider the standard tables, a table would create a plot where the dive went steeply down to 90 feet and stayed there, probably for around 50 minutes, at which time the plot would do an ascent and a 3 minute safety stop (except it wouldn't, because you would be far into deco at that point.). Such a plot fits the dive that was actually done very poorly, and would massively overestimate the nitrogen onloading during the dive.
The multi-level dive I created for the eRDPML considered about 20 minutes at 90 feet, and then 30 at 40; it's still a very crude overlay on the dive that was actually done, but a lot closer to it than the first one. This dive was permissible under the rules of the eRDPML, but probably again significantly overestimated the nitrogen uptake during the dive.
The second dive is really hard to approximate. Obviously, a square plot with a max depth close to 90 in NO way matches what was done. The eRDPML strongly dislikes the fact that the early part of the dive is shallow and the middle part is deep -- it wouldn't even permit me to input that profile without immediately saying it was outside of the parameters. Even when I put the deep part of the dive in the beginning, it did not like the resulting combination -- which again poorly matches the dive that was done. With a one hour surface interval, the program considered the second dive outside of acceptable parameters.
Now, the PADI RDP and the OP's computer algorithm may differ in what they see as acceptable levels of nitrogen loading in various compartments. They may use different numbers of compartments, with different half-lives, and make different assumptions as to the maximum tolerated overpressure gradient in each compartment, and how big a role bubbles play in the gas dynamics. There are LOT of things that are "chosen" about decompression algorithms, and the validity of those choices is argued vigorously among the experts (see the recent thread on deep stops, which has some heavy hitters participating and differing significantly). A device which takes iterative data and does continuous computation is going to estimate the nitrogen loading in the body much more accurately, IF the assumptions made in the algorithm's construction are actually valid. But if method A consistently OVERestimates the nitrogen in comparison to method B, and both draw the line at the same nitrogen loading, then method A will result in a much shorter dive, where the ACTUAL total nitrogen content in the various tissues is much lower than the program thinks it is. If you stay much further away from the acceptable limits, you should, in theory, have a lower risk of DCS.
The problem is that DCS is so rare to begin with, in properly executed recreational dives, that to see anything but an enormous difference between two approaches to computing limits would require many, many dives and a lot of careful data collection. Since it's rate that we dive profiles that are identical, under conditions that are identical, using divers with identical personal risk factors, those data really can't be collected outside of a formal study. And formal studies of this are quite difficult to get through an IRB, because if a certain percentage of the human subjects are expected to be injured by the protocol, it's unlikely to be approved. It's also extremely expensive to do such studies, which is why there are few of them, and they involve small numbers of subjects. And when you have small numbers of subjects, individual factors can become cripplingly important, and destroy the validity of study.
The multi-level dive I created for the eRDPML considered about 20 minutes at 90 feet, and then 30 at 40; it's still a very crude overlay on the dive that was actually done, but a lot closer to it than the first one. This dive was permissible under the rules of the eRDPML, but probably again significantly overestimated the nitrogen uptake during the dive.
The second dive is really hard to approximate. Obviously, a square plot with a max depth close to 90 in NO way matches what was done. The eRDPML strongly dislikes the fact that the early part of the dive is shallow and the middle part is deep -- it wouldn't even permit me to input that profile without immediately saying it was outside of the parameters. Even when I put the deep part of the dive in the beginning, it did not like the resulting combination -- which again poorly matches the dive that was done. With a one hour surface interval, the program considered the second dive outside of acceptable parameters.
Now, the PADI RDP and the OP's computer algorithm may differ in what they see as acceptable levels of nitrogen loading in various compartments. They may use different numbers of compartments, with different half-lives, and make different assumptions as to the maximum tolerated overpressure gradient in each compartment, and how big a role bubbles play in the gas dynamics. There are LOT of things that are "chosen" about decompression algorithms, and the validity of those choices is argued vigorously among the experts (see the recent thread on deep stops, which has some heavy hitters participating and differing significantly). A device which takes iterative data and does continuous computation is going to estimate the nitrogen loading in the body much more accurately, IF the assumptions made in the algorithm's construction are actually valid. But if method A consistently OVERestimates the nitrogen in comparison to method B, and both draw the line at the same nitrogen loading, then method A will result in a much shorter dive, where the ACTUAL total nitrogen content in the various tissues is much lower than the program thinks it is. If you stay much further away from the acceptable limits, you should, in theory, have a lower risk of DCS.
The problem is that DCS is so rare to begin with, in properly executed recreational dives, that to see anything but an enormous difference between two approaches to computing limits would require many, many dives and a lot of careful data collection. Since it's rate that we dive profiles that are identical, under conditions that are identical, using divers with identical personal risk factors, those data really can't be collected outside of a formal study. And formal studies of this are quite difficult to get through an IRB, because if a certain percentage of the human subjects are expected to be injured by the protocol, it's unlikely to be approved. It's also extremely expensive to do such studies, which is why there are few of them, and they involve small numbers of subjects. And when you have small numbers of subjects, individual factors can become cripplingly important, and destroy the validity of study.
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