cornfed
Mindless lemming
I have a sneaking suspicion this has a lot to do with it.
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I have a sneaking suspicion this has a lot to do with it.
Scuba_Vixen
Guest
Thanks Drew
Darlene
Darlene
cornfed
Mindless lemming
LOL, yeah you can probably take him off your ignore list!
Dr Deco
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Dear Scuba Board Readers:
I have been gone a couple of days and have returned to find considerable exchange has occurred on this topic.
Research Methods
Might I start by saying that, as a research scientist, I would like to point out differences in research since we see the term research and research results quite frequently. There are prospective trials and there are retrospective trials.
Theprospective study where all variables [that are known] are controlled and a direct comparison is made of the results under one condition and compared with the results under another condition. These are very often laboratory trials. An example might be comparing two groups of divers (if it is the same subjects, then it is termed a cross over study) in a warm-water and a cold-water environment. The population would be observed for signs and symptoms of DCS and probably monitored for bubbles with a Doppler flow meter. The DAN Flaying After Diving study is an example of a laboratory test with controlled conditions that will vary from one test series to another and comparisons will be made at the end.
The retrospective study statistically analyses past events and attempts to find meaningful correlations. It cannot control for many variables since the activity is already completed. Some control is possible by admitting data only when certain conditions have been fulfilled. Such studies are often employed where direct testing would be dangerous. An example might be the incidence of auto vehicle deaths depending on the blood alcohol levels of the drivers. Another example might be accidental poisoning of children in relation to the educational level of the parents.
This retrospective technique can also collect data from a large number of subjects since it does not involve direct laboratory testing but is instead field data, usually a much larger population. Most data concerning recreational scuba diving and DCS is from this type of field research. There is a definite problem with data of this type. In the vast majority of the cases, the dose of nitrogen is much smaller than the limits imposed by the model and the results are often without much scientific meaning.
The data presented by Dr Wienke are of this latter type. In fairness to Dr Wienke, it must be remember that he does not have research money or a charter (from Los Alamos National Laboratory) to perform manned research in recreational scuba diving (other than modeling). I am in the same position at NASA. Our NASA charter is to study decompression in space and not in recreational scuba diving.
The information I present in this FORUM concerns either my NASA research on micronuclei formation (derived from simulated null gravity research) or information that I gathered in my laboratory experience since entering barophysiology in 1969.
Reverse Profiles
The conference at the Smithsonian Institution was a gathering of scientists, each of whom analyzed retrospective dive data and presented it to the group. The group reviewed what was before them and decided to issue an announcement. Within the depth differential of 30 fsw, repetitive dives could be of the reverse profile type. This was not a new finding, and God and Nature did not change on that day. You always could do this and many were aware of it. This simply was a large-scale consensus and published announcement.
Tables
Tables will have different systems for managing the situation of repetitive dives. Early Royal Navy tables added bottom times together (I believe) to determine decompression from the second dive. Provision was made in the decompression as the second dive increased in duration from the first. The US Navy had its Residual Nitrogen System for repet diving tables. The current RGBM NAUI tables do not use repetitive groups and thus state that the subsequent dives must by of a certain shallower depth. This is for ease in the reading of table. The original algorithm could compute any dive but requires the computer, naturally. Dive computers will actually perform the calculations, but these might be different from what is on the printed table especially if it is a simplified table.
RGBM Data
The problem -- I find -- with the RGBM data is that it is all field data. I do not know how it would perform if the same in-water decompression was performed in a dry chamber. If the test subjects walked around, I would bet that the results would be different. If, however, the subjects remained seated, then the results would be very good (similar to the good results in the water). I say this by comparison to the results from the Germany DFVLR test program. There, the divers remained seated and decompressed from hundreds of meters (30 minute bottom time) in a few hours with little problem. Subjects had few Doppler bubbles at depth and many bubbles when they surfaced and walked around. The system employed an ad hoc deep stop, and gas switches, but the algorithm was calculated as a straight Haldane type otherwise.
Based on my NASA studies, depending on whether individuals are walking or suspended in water (or in null gravity), there is a big difference in DCS incidence. Thus, there is not any given DCS incidence for a certain decompression. The ground-based incidence is much larger than the base line incidence ( ground state incidence ).
Dr Deco :doctor:
I have been gone a couple of days and have returned to find considerable exchange has occurred on this topic.
Research Methods
Might I start by saying that, as a research scientist, I would like to point out differences in research since we see the term research and research results quite frequently. There are prospective trials and there are retrospective trials.
Theprospective study where all variables [that are known] are controlled and a direct comparison is made of the results under one condition and compared with the results under another condition. These are very often laboratory trials. An example might be comparing two groups of divers (if it is the same subjects, then it is termed a cross over study) in a warm-water and a cold-water environment. The population would be observed for signs and symptoms of DCS and probably monitored for bubbles with a Doppler flow meter. The DAN Flaying After Diving study is an example of a laboratory test with controlled conditions that will vary from one test series to another and comparisons will be made at the end.
The retrospective study statistically analyses past events and attempts to find meaningful correlations. It cannot control for many variables since the activity is already completed. Some control is possible by admitting data only when certain conditions have been fulfilled. Such studies are often employed where direct testing would be dangerous. An example might be the incidence of auto vehicle deaths depending on the blood alcohol levels of the drivers. Another example might be accidental poisoning of children in relation to the educational level of the parents.
This retrospective technique can also collect data from a large number of subjects since it does not involve direct laboratory testing but is instead field data, usually a much larger population. Most data concerning recreational scuba diving and DCS is from this type of field research. There is a definite problem with data of this type. In the vast majority of the cases, the dose of nitrogen is much smaller than the limits imposed by the model and the results are often without much scientific meaning.
The data presented by Dr Wienke are of this latter type. In fairness to Dr Wienke, it must be remember that he does not have research money or a charter (from Los Alamos National Laboratory) to perform manned research in recreational scuba diving (other than modeling). I am in the same position at NASA. Our NASA charter is to study decompression in space and not in recreational scuba diving.
The information I present in this FORUM concerns either my NASA research on micronuclei formation (derived from simulated null gravity research) or information that I gathered in my laboratory experience since entering barophysiology in 1969.
Reverse Profiles
The conference at the Smithsonian Institution was a gathering of scientists, each of whom analyzed retrospective dive data and presented it to the group. The group reviewed what was before them and decided to issue an announcement. Within the depth differential of 30 fsw, repetitive dives could be of the reverse profile type. This was not a new finding, and God and Nature did not change on that day. You always could do this and many were aware of it. This simply was a large-scale consensus and published announcement.
Tables
Tables will have different systems for managing the situation of repetitive dives. Early Royal Navy tables added bottom times together (I believe) to determine decompression from the second dive. Provision was made in the decompression as the second dive increased in duration from the first. The US Navy had its Residual Nitrogen System for repet diving tables. The current RGBM NAUI tables do not use repetitive groups and thus state that the subsequent dives must by of a certain shallower depth. This is for ease in the reading of table. The original algorithm could compute any dive but requires the computer, naturally. Dive computers will actually perform the calculations, but these might be different from what is on the printed table especially if it is a simplified table.
RGBM Data
The problem -- I find -- with the RGBM data is that it is all field data. I do not know how it would perform if the same in-water decompression was performed in a dry chamber. If the test subjects walked around, I would bet that the results would be different. If, however, the subjects remained seated, then the results would be very good (similar to the good results in the water). I say this by comparison to the results from the Germany DFVLR test program. There, the divers remained seated and decompressed from hundreds of meters (30 minute bottom time) in a few hours with little problem. Subjects had few Doppler bubbles at depth and many bubbles when they surfaced and walked around. The system employed an ad hoc deep stop, and gas switches, but the algorithm was calculated as a straight Haldane type otherwise.
Based on my NASA studies, depending on whether individuals are walking or suspended in water (or in null gravity), there is a big difference in DCS incidence. Thus, there is not any given DCS incidence for a certain decompression. The ground-based incidence is much larger than the base line incidence ( ground state incidence ).
Dr Deco :doctor:
IndigoBlue
Contributor
Dr. Powell aka Deco definitely is an excellent authority by any definition.
And now we seem to have conflicting authorities.
That is what makes life so very interesting!
I will confess to having dived reverse profiles, however both dives being shallower than 75 ft. But I would not teach anyone else to do so. That is the difference between me and George Irvine. We both do irresponsible things. But I do not teach people to do irresponsible things as well.
As Dr. Powell pointed out, NAUI's new RGBM dive tables require you to dive shallower repetitive dives. They are also easier to use than all prior Navy-like tables. I like the looks of them, so far. They are perfect for basic open water students. And they are easier to teach.
I still agree with the Mares legal dept and their caveat. Dive computers let you do very stange and irresponsible things, and then continue to crank out data for you. Their use should include caveats. For example, never dive twice to a depth over 100 ft in one day, even though your dive computer lets you do it. Unless you are decompression trained and configured.
Dr Deco
Contributor
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Hello Indigo:
This is not meant to indicate that we have conflicting authorities. It is rather to indicate that there can be varying interpretations depending on what data and what study is being used. There will naturally be some degree of bias from the scientists; each will believe his interpretation to be the better one. A study might be developed around that interpretation. One would certainly imagine that no one would be his/her idea to be wrong but move to prove it anyway knowing full well that it will fall on its face.
If you wish to see some real fireworks, watch one of our NASA meetings involving decompression when there are four scientists in the room, and each has a different opinion of what the same data means.
It is almost impossible to believe!
Dr Deco :doctor:
This is not meant to indicate that we have conflicting authorities. It is rather to indicate that there can be varying interpretations depending on what data and what study is being used. There will naturally be some degree of bias from the scientists; each will believe his interpretation to be the better one. A study might be developed around that interpretation. One would certainly imagine that no one would be his/her idea to be wrong but move to prove it anyway knowing full well that it will fall on its face.
If you wish to see some real fireworks, watch one of our NASA meetings involving decompression when there are four scientists in the room, and each has a different opinion of what the same data means.
It is almost impossible to believe!
Dr Deco :doctor:
chrpai
Contributor
Unfortuantly its not that simple. I wish it was. If it was, I'd agree with you, I don't do or teach people to do irresponsible things.
The problem comes from the fact that irresponsible can not be simply defined.
One person can decide that its responsible, and one can decide that its not. Then you get the one person looking at the other person like they are an idiot when in fact both people are on solid ground in their decision making process. One way of doing something is not always so cleanly right as to make all other ways wrong.
cornfed
Mindless lemming
LOL, That sounds an awful lot like you're trying to defend GI!
chrpai
Contributor
I can't help it that you never take anything serious.
cornfed
Mindless lemming
I can't help it that you make me laugh.
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