What's driving these changes in safety stop depths and times is research, including several of the studies pointed out by TSandM and Gene, et. al. One of the primary tools in use now is ultrasound for finding and counting subclinical vascular bubbles, the reasoning being that if you can reduce the subclinical bubbles then you can reduce the chance of getting the larger kinds which cause DCS.
My impression is that things are pointing to half-depth safety stops with moderately fast ascent rates. For example, see the new safety stop rule being proposed by NAUI (reproduced below), which I'll bet other agencies will soon adopt. Note that it emphasizes the half-stop time even more than the traditional 15' stop time.
And while I've also heard from many that they prefer just an extremely slow ascent... look at the data, because that's not necessarily the best way to avoid bubbles! One of the Bennett studies shows that the slow-and-steady ascent generated the second-highest bubble count of all the profiles under consideration.
Instead, it looks like a moderately fast (30fpm), stairstep ascent to the half-stop, and then similarly on to the 15' stop, is the safest in terms of minimizing bubbles.
My guess is the problem with the extremely slow and steady ascent is you're ultimately taking on more nitrogen at depth and overall staying down longer than someone doing a moderately fast stairstep ascent.
Remember, all these discussions about safety stops are strictly for recreational-limit, non-deco dives.
>*< Fritz
P.S. I am not a physician, but my business is digital medical imaging (PACS and related systems), so I do know a thing or two about ultrasounds and imaging technology. I am also inclined to trust hard data more than anecdotal information, especially when supported by multiple studies. I have no idea why the one poster insists on using the scare quotes around these "studies", as if there's something questionable about them. I would be inclined in that case to put all his "comments" in quotes to equally call them into question.
Here's the proposed NAUI revision (bold emphasis mine):
NAUI Rule of Halves Revised
Based upon the most current research and analysis of decompression science on the value of deep stops in "no-required-decompression" diving by Bennett et al (Undersea and Hyperbaric Medicine 2007; 34(6): 399-406, the NAUI Board of Directors approved a change to the NAUI Standards and Policies on March 7, 2008, at its annual meeting. A reprint of the study is included in this issue of Sources (pp. 48 ff).
The membership is advised to make a pen and ink change in their personal copy of the manual and mark both the title page and changed page with "rev. 1-08". Notice of these changes is included in existing inventory of the NAUI Standards and Policies Manual, and the changes will be incorporated in the next printing.
S&P Page 2.16 Current wording
It is recommended that following dives in excess of 40 feet (12m), divers make a one minute stop at a depth that is half that of the deepest depth reached during the dive and make a precautionary stop in the 10-20 feet (3-6m) zone for three to five minutes before returning to the surface. The precautionary stop time may be considered "neutral time" - not counted as either dive time or surface interval time.
S&P Page 2.16 Changed wording
It is recommended that following dives in excess of 40 feet (12m), divers make a two to three minute stop (with two and one-half minutes being optimum) at a depth that is half that of the deepest depth reached during the dive and make a precautionary stop in the 10-20 feet (3-6m) zone for one minute before returning to the surface. The precautionary stop time may be considered "neutral time" - not counted as either dive time or surface interval time.
Bennett et al have extended their study to include diving over a greater range of depths from deep (130 fsw) to shallow and will present these findings at the UHMS "Decompression and Deep Stop Workshop" on June 24-25, 2008, in Salt Lake City, Utah, sponsored by NAUI Worldwide and other dive industry members.
