A Cautionary Note On Ascent Rates

For several years, DAN, the Divers Alert Network, has studied the effect rapid ascents have on decompression illness. Animal studies have shown that fewer bubbles form at a 30 foot per minute ascent rate, and in 1993 the U.S. Navy changed their rate for their air divers from 60 to 30 feet per second.

As part of DAN's Project Dive Exploration, 47 divers contributed their depth-time profiles from 357 dives made during the course of 15 dive trips. Diver's maximum depths during 20-second intervals were recorded by their computers. While none of the divers had DCI, up to one-third ascended faster than the recommended 30 feet per minute. Half took a safety stop of three or more minutes at 20 feet or less.

The results are a cautionary note to divers who believe they ascend at the proper 30 feet per minute rate: one-third don't. Researchers noted that one reason divers exceed the ascent rate is that computers with a 20-second interval recording rate can detect ascent rates greater than 60 feet per minute but are not entirely accurate at rates of less than 60 feet per minute for ascents totaling less than 20 feet.

These researchers believe that computer manufacturers should change the monitoring interval to five seconds. Meanwhile, divers should be careful to ensure a slow ascent and not depend entirely on computer monitoring.