Slow ascent rates

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Posted by Kendall Raine on February 20, 2001 at 08:50:19:

Peter Bennett's editorial in the latest issue of Alert Diver speaks to the efficacy of slowing ascent rates beyond the 30 fpm typically used in Buehlemann tables and meters. Specifically, he notes growing evidence that hits following bounce dives tend to be neurological in nature and thereby symptomatic of fast tissue bends. He suggests that an ascent rate of 10 fpm is more effective at eliminating microbubbles from fast tisues. As a proxy for 10 fpm, he suggests using much deeper stops than the gospel 15 for 3-5 minutes. For a 100 fsw bounce dive, for example, Bennett recommends a 5 minute stop at 50 feet (half the depth) prior to the 5 at 15 assuming an ascent rate of 30 fpm. For shallower dives, he suggests taking a five minute stop at half the max depth in addition to the standard 15 fsw stop.

While DAN can hardly be viewed as either the first or the last word in decompression research, this supports post-Haldane modeling (VPM and RGBM) and empirical evidence from the technical community which has for years suggested that pulling short deep stops-much deeper than recommended by neo-Haldanean schedules-results in substantially reduced silent bubble grades post dive. Perhaps the most interesting facet of this is that the concept of deep stops isn't new. Brian Hills observed the same thing in a 1965 report on pearl divers in the Torres Straight.

While Dr. Bennett's editorial raises more questions than answers, it does suggest that blind faith in any model, whether in table or computer form, is just that-blind faith. As Karl Huggins said to me recently "The more I learn about decompression the less I know."

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