Posted by MHK on December 04, 2000 at 14:30:21:
In Reply to: Safety Stops posted by JRM on December 04, 2000 at 14:09:51:
There are several good reasons behind the *safety stop*, usually completed for 3 minutes at 15'. By in large the safety stop is a way to get your ascent under control, allow a slow ascent, and provide for a shallow stop to help off-gas.
Most of the NDL models ( and I'm keeping this discussion as you have requested in the recreational environment, ie; air and within NDL limits but bear in mind my analysis will change materially in a technical environment ) use the Buhlman algorithms as a basis for the NDL limits. These studies were a byproduct of the Haldane model. The Haldane models were devopled in the very early 1900's, I believe 1907. Buhlman and also Workman in the late 50's and early 60's modified the Haldane model and produced much of what is being used today. So as you can see they are 40+ year old research. And one of the falacies that is being discovered in the Buhlman model is the notion that while you are ascending you are still *on-gasing*, which is why the tables and the agencies suggest to get to 15' and then do a safety stop...
Richard Pyle began doing some deep diving in the 80's to explore species of fish that are only found at depth. In the course of this research he noted that when he brought the fish directly to the surface ( within accepted ascent rates, ie; 60 fpm ) that they fish would die. He initially attributed this to the fact that these species simply may not be able to live above certain ATA's. He kept on researching and then began to bring the fish up real slowly ( ie; incorporating deep stops into his profile ) and he noted that the fish lived and that he felt better after his dives. The logic being that as opposed to the notion of a continued *on - gas* you are really allowing any bubble formation to slowly disapate.
These early revelations coupled with Bruce Weinke's studies repecting the RGBM ( I commented on this a few weeks ago ) are causing decompression modeler's to reconsider the value of a controlled, but yet direct, ascent to 15' and are now suggesting to build in deep stops into your profile...
As I noted above this discussion is much more detailed and complicated when intoducing He into mixes and diving deeper than recreational limits and the above analysis should not be construed as being an complete accounting of the issue.
But I hope the above answers the deep stop question as it relates to recreational diving..
Later