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(In rec.scuba or bit.listsev.scuba-l?) Jay Hack wrote an interesting posting on computer versus dive table safety. His interpretation of DAN=B4s data seems sound and interesting. His conclusion: it takes more negative factors to produce DCS for a diver relying on his computer than for someone relying on written tables. Several replies to this posting argue that tables can be as safe and list all kinds of healthy precautions. As somebody else has pointed out already, that is besides the point. The main message is, provided that a diver (for whatever reason) accepts negative attributes, a computer diver is more likely to get away with it. I have got one objection which goes past the statistical interpretation: Could it be that the negative attributes for table divers are underestimated? When somebody gets bent, there will always be a wish to avoid looking foolish. People lie (or "forget") about maximum depth and bottom time. The patient will be treated for symptoms anyway. The recompression facility does not really have to know your dive profile. However, the "adjusted records" mess up our much wanted feedback on the validity of tables. We are not going to flog anybody for having carried out a stupid dive - our only concern is: are the tables we use safe or will some entries have to be changed? As an example: Joe Diver goes to 200 feet for 15 min and skips most of his decompression. He gets bent. Upon recompression he is asked what kind of dive this was. He feels like a jerk and presents a nicer version: I dove to 130 feet for 15 min. Moreover, I decompressed according to my tables. I assume I am just one of the unlucky ones. He gets treated, no problem. But, we are left with a case that makes us doubt the safety of the 130 feet/ 15 min entry - here is somebody who did everything right and still he got hit. If Joe Diver had told us the true story, we would not have to doubt our tables. He would not be condemned for admitting his "sin" - in fact we would feel good about it since the truth would confirm the validity of our tables. So, the moral is: Do not lie about your dive. Physicians want to learn which table entries work and which do not. The doctor is there to try and fix you, not to fine you according to your violations. Computer divers would like to tidy up their stories too. But, it is a lot harder. When that little gadget recalls the latest dive, maximum depth will be displayed as well as dive time. Some will recall the profile also. Negative attributes are much harder to conceal. In effect, computer divers will have to admit the true colors of their dive, while table divers can go on lying about it. In Norway bends victims are treated for free. No special insurance is needed. If the relevant U.S. insurance can be waived on the basis of gross negligence, that could be a reason for presenting a modified record. I do not know whether special terms apply and would appreciate some feedback. Will an off-color dive waive the insurance? If so, we all loose. The voluntary field-testing of tables, our best means of learning what they are worth, will be fraught with errors. On the other hand, suppose the data is correct. What then, could be the explanation to an inherently higher risk for table dives? In general, a table dive is a square profile bounce dive. The computer diver will be more prone to choose a multi-level approach. Could it be that computer divers tend to have less provokative ascents? That is, in spite of other adverse factors the multi-level ascent will provide for a better decompression. In any event, provided there is a different risk factor, the reason should be sought in the profile. Physiology stays the same whether one is wearing a computer or not. I would suggest that multi-level ascents could be the answer. It would also be interesting to look into what kinds of negative attributes we are dealing with. As mentioned by others, there is no need to discuss why some divers accept stupid risks. But, it could be productive to look at the various factors and see whether there is a pattern to it. That is, do computer divers and table divers accept different risks? Could it be that computer divers pick up four subtle factors to get bent, while table divers go for two serious ones? If "excessive ascent rate" is listed for a computer dive, that dive will not provide the typical computer profile protection. On the other hand, if the unlucky computer divers have taken advantage of a gradual ascent, picking up their high INA score from other factors, my above argument is valid. That is: It takes several negative factors to contract DCS despite a healthy, shallow "tail" to ones dive profile. Also, I would like to see a distinction between presumably correct dives and violations. If the computer beeps on the surface, that is no computer dive - it is a case of missed decompression. The same goes for table dives. I am interested in dives that should have been OK, but turned out to be bad. Hans-Petter Roverud currently using Geir Johhannesen mail address
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