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>From: story@be*.en*.sg*.co* (David Story) >> >Dan Nafe writes: >> >: >> Dave! "the table models" do not assume a specified descent rate, at least >> not the ones used by tech divers. Our implementation (MiGPlan, >> MiGPlan.Mac) of the Buhlmann model assumes an instant descent. > >Actually, all *tables* do assume a particular descent and ascent rate. >The current USN Tables, for instance, assume 75fpm descent and 60fpm >ascent. Specifying an ascent and descent rate is necessary in order >to account for the gas loading during those phases of the dive. > >However, I assume that by saying "table model" you are aware tables >and models are two different things. Models generally allow any >ascent and descent rate, and if they are calculated in real time (as >opposed to tables) then the user can use any ascent or descent rate. > >Assuming an instant descent penalizes the diver for gas loading which >is not taking place. However, it is a conservative method. OK, I think I need to be a little more precise! The original poster was discussing table based diving. I contend that tables do assume a specified descent rate (as David Story writes above). Whether a computer program model such as MigPlan or DECOM assumes an instant descent or not in calculating its gas loadings is immaterial to the use of tables. My understanding of the development methodology of tables is as follows: - a model is designed - tables are cut from the model - said tables are tested with both open water and chamber dives - tables are "adjusted" to achieve an acceptable bends rate for as many profiles as possible. I assume that in the testing phase, volunteers are NOT compressed intantaneously to the working depth!!! I assume that they are compressed at a SPECIFIED descent rate (e.g 75 ft/min for USN tables). Hence, if you descend faster than the specified descent rate, you WILL ongas more inert gas than the tables are designed for and hence increase your risk of DCI. Whether this is a significant increase is an open question, but the effect will become more significant the deeper you go. These comments ONLY apply to published tables. Cheers, David. ______________________________________________________________________________ David Giddy, | Voice: +61 3 253 6388 Telstra Corporation, | Fax: +61 3 253 6144 P.O. Box 249, Rosebank MDC, 3169, AUSTRALIA | Internet: d.giddy@tr*.oz*.au* ------------------------------------------------------------------------------
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