Posted by Ken Kurtis on December 04, 2000 at 20:39:34:
In Reply to: Safety Stops posted by JRM on December 04, 2000 at 14:09:51:
The short answer is that safety stops are a good thing. The secondary benefit, as Mike pointed out, is that doing the stop forces you to bring your ascent under control which in and of itself has benefit.
Perhaps the best reason I ever saw for safety stops came through a demonstrtaion that Andy Pilmanis did a number of years ago (probably early- to mid-80s) at a decompression workshop in Long Beach. At the time Andy was doing research and working with high-alittude piltos (who face the same sort of deco problems that divers do) at Brooks AFB. Andy had previously been director of the Catalina Chamber.
What Andy discussed was the idea of asymptomatic bubbles (aka "silent" bubbles) that formed during a no-d dive. These bubbles by themselves didn't cause any problems. But since the tables models are based on the assumption that NO bubbles are present in the system, and since the science of bubble mechanics postulates that it's easier to form bubbles around already-existing bubbles, the presence of "silent" bubbles was thought to be something that might encourage a bends hit (bubbles to form) on a subsequent dive that was otherwise within tables limits.
The amazing thing Andy showed us was a video Doppler of a test subject's heart. At the conclusion of a dive with NO safety stop, you could literally see what seemed like a big cluster of bubbles shooting through the chambers of the heart. Pretty dramatic stuff and you could literally hear the audience (myself included) audibly gasp. Again, these were asymptomatic bubbles. (And Kendall has mentioned previously about Doppler bubble size being a factor but I don't have that info as it relates to this test.)
Andy then showed a video Doppler of the same guy doing the same profile (different day I believe) but with a 3-minute stop at 15'. This time, there were just about zero bubbles.
Andy said that what their research showed was that doing a safety stop at 15' for 3 minutes reduced the number of "silent" bubbles by about 90%, which should also result in a reduction of the risk of an unexpected bends hit on what would otherwise have been a no-d profile.
As to why 15' is the magic number, it really isn't. (This is from my research and understanding, not Andy's.) The important thing is to make a stop at some shallow depth, and shallow can be a relative term, especially if you're incorporating deep stops into the plan.
If you look at the original Navy tables, the mandatory deco stops are in incremements of 10'. When safety stops were first being proposed, 10' was the number used. However, from a practical stanbdpoint, divers have a tough time maintaining neutral buoyancy at 10' because if you go up a little or take a bigger breath, etc., etc., you wind up at the surface and you've blown the safety stop.
When PADI first beagn pushing for safety stops as a regular part of diving (my recollection is that they were first on the bandwagon), they suggested 15', mainly because it was an easier depth to maintain and you had a bit bigger margin of error if you should start to drift up. NAUI and other agencies followed suit, and 15' became the standard. (And even now, it's taught by some as 15-20'.)
But the bottom line is that the safety stop should encourage outgassing in critical tissues, drop your overall N2 level down a bit before you break the surface, reduce dramatically the numnber of "silent" bubbles, and force you to get your buoyancy under control, all of which should contribute to a lesser risk of an unexpected hit.
Ken Kurtis
NAUI Instr. #5936
Co-owner, Reef Seekers Dive Co.
Beverly Hills, Ca.