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A response to Peter St. Onge's note on DCS and computers, which points out that experienced divers seem to have trouble with bends when using computers: Consider the way many divers use computers and tables. Standard table protocol requires that you use the table based on the nearest depth & time increments greater than your planned maximum depth and bottom time. A computer, on the other hand, constantly monitors time and depth to within a few seconds and feet, respectively. The rounding to the next greater increment done by tables users automatically introduces some safety which computer users do not have, since computers measure actual depth and time continuously. For ascending multilevel dives, such as many people do on a generic recreational dives, using tables based on max depth adds a lot of safety. Square wave dives on tables, in which a diver stays at the max depth for his entire bottom time (this is common among wreck divers) are well known to produce more bends hits than ascending multilevel dives. The reason for this should be obvious - a square wave diver spends all his time at the max depth absorbing inert gas, whereas a multilevel diver spends relatively little at the max depth. A square wave diver is pushing the algorithm on which the table is based much closer to its limits. I haven't seen the figures, but I would bet that bends incidence for computer users is similar for square wave and multilevel dives, all other factors being equal. This is because in each case the computer is computing tissue compartment N2 tension based on actual time at depth, as opposed to assuming max depth as in tables. Where computer users may get into real trouble is in deco stop diving. A typical recreational diver on a shallow dive will come up when he runs out of air in his (usually single) tank. Typically, the computer will indicate that he has plenty of no-decompression time left, which is to say that his computed N2 tensions are well below critical bubbling thresholds for the model used. Now suppose the diver straps doubles on his back, or dives deep, and nears or exceeds his computer's NDL. Many divers do not realize that doing stops exactly as indicated by the computer means diving the computer's algorithm to its very limits. I.e., when a computer says that you need to hang at 10 fsw for 6 min., this means that if you do that then your computed tissue tensions will be just barely below the critical threshold established in the computer's model. A diver making a planned decompression dive using tables which specify the same hang will probably have lower tissue gas tension than a computer user, since tables assume he has been at the maximum depth for his entire bottom time. Sinced most people doing decompression diving have some experience, my argument could explain why experienced computer users are disproportionately represented in DAN stats. Even when avoiding decompression, sport divers who are more experienced are likely to use less air and thus go much closer to the No-D limits of their computers than their inexperienced counterparts. Jheimann@sc*.gt*.co*
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