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There are several reasons why you may want limit the descent rate: 1) HPNS: For the normal mortal this is a no issue. As far as I recall the first symptoms of HPNS can occour at ~280ft while diving Helium- Oyxgen and exceeding 100fpm. Small amounts of nitrogen largely delay this effect to even deeper dives (>400ft). For hyperoxic trimix this is certainly not the limit. 2) Tables: If you _plan_ your dive with a descent rate of 30fpm and you descent faster you will indeed exceed the table. Example: for a 300ft dive with 15min real bottom time your table would tell you a decompression schedule that starts 25 min after begin. If you descent with 60fpm you hit the gound 5 mins earlier, hence when you stick to your table and leave the bottom at 25mins you already made 20 min. real bottom time (=you are more saturated than planned). The solution to this problem is either to stick to the table (ie. don't be faster than recommended) or get tables for the descent rate you use. If you use a computer program selecting faster the descent rates will make the schedule more conservative. For my private tables I calculate the schedules based on a 60fpm descent rate. For the depth where hyperoxic trimix makes sense (<200ft) the differences in the decompression schedule between a 30fpm and a 100fpm should be very small. 3) Stress: This is the real reason to reduce your descent rate. If you descent slower: * bouyancy control is easier and more precise * equalization is easier * you have time to look for your buddies and your equipment * you reach the bottom much more relaxed For my taste a descent rate of 45-35fpm is just fine. Greetings, Frank Riffel Jbdvr@ao*.co* wrote: > Hi, the current issue of Advanced Diver Magazine has a article called Oxygen > Enriched Trimix, which deals with mixes with a 02 of 21 to 28%. The article > states that the descent rate should be 30fpm, and that ''The danger in > descending too fast is that the inert gas uptake may exceed what is planned > for the schedule, which can increase your risk of DCI''. Question: What is > the current conventional wisdom on proper descent rates for trimixes? > thanks.......Joe Boone -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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