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> ... Now a question for the decompression modellers based > loosely on the recent thread of repetitive diving. The old RNPLs > put a no decompression limit at 9m, other tables put it at lower > limits (5 m on the BSAC '88s/Buhlmann ? - I haven't got any tables > handy). The RNPL tables I have stipulate decompression stops at 5m and 10m. > Based on current knowledge, what is the safe depth to carry out > numerous ascents (eg lots of training drills) ? Please note, I'm IMHO the depth is not the issue. It is the number of dives, the surface intervals, the ascent rates and the profiles. The main problem with the RNPL tables is that they are only valid for two dives, and even then it was only pressure from the sport diving community which persuaded them to provide the repetitive information. The problem with training is that most of the ascents tend to be either rescues or air sharing. In these situations your are unlikely to make a safety stop. You are also dealing with, potentially, poorly controlled ascents. So they are intrinsically more dangerous than your normal recreational dive. There is also pressure to do lots of them in a short period. For example, do both the donor and recipient shared ascents in the same session for a number of students. After the first two ascents you are probably outside the RNPL tables. Even repetitive tables (USN, Buhlmann, etc) are not intended for these extremely aggressive profiles. The ascent rate on my RNPL tables is 15m/min. Most computers allow 10-12m/min. I have seen a great many training ascents carried out at much greater speeds but this is not usually taken into account when the trainee/instructor goes down for their next attempt. The instructor may say "that was a little fast, let's try it again with a bit more control". This is not conducive to safe diving. I have seen instructors do 5 or 6 ascents from 20m in less than an hour. Each dive may last about 5 minutes while the student descends gets the buddy breathing going and then ascends again. > Is it safe to use the RNPLs under restricted > conditions ? (Eg, depth limit of 30m, no repetitive dives, 10 m/s > ascent rate etc). (some scuba-l subscribers may remember my > original post on a similar subject some time ago - what I need now > is a more technical comment on the subject). I think many problems arise because people do not use them properly. I know many people who claim they dive the tables but in reality dive according to their computers. In the lectures they treat them as the bible but they seem to get ignored on the dive site. Of course this does not always get reported in the accident report. There may also be an insurance problem for dives which do not conform to your organisation's tables. So people may be reluctant to admit after an incident that they broke the table rules. Alan
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