Quote: (Originally Posted by
lamont)
But are we talking about results based on that NEDU paper? If so, I don't think you can draw conclusions about the utility of deep stops in VPN/GF/Ratio/etc from that study. It does highlight the fact that if you overstay deeper stops that you can't take time off the shallower stops and may need to do your shallower stops in thier entireity plus additional padding.
In the study they are dramtically overstaying all their stops from 80 fsw to the surface. They've gone well over the point where doing the deep stops are beneficial to allow bubbles that form to offgas while small and prevent bubble growth, so its not a surprise that they find that their deeper stops result in significantly worse decompression. I don't think you can extrapolate that backwards linearly and conclude that deep stops require even more penalty to the shallower portions of the deco -- at some point the bubble models claim that the deep stops effect on offgassing from the free phase and reduction in bubble growth is going to be significant compared to the decrease in effciency of the dissolved-phase decompression.
One of the reasons this was presented at the conference was it gave David a chance to discuss the research in some more detail. One of the points David discussed was that people on the internet have said the deep stops were too long. The way the study worked meant he felt it would not have made a material difference to the DCS incidence rate. It was his opinion that the bubbling caused by a 9m(30fsw)/min ascent rate was actually dealt with effectively by the shallow stops and did not need additional deep stops.
Some questions certainly arose in the Q&A specifically around whether the same study could be done with helium mixes and what effect it might have. Unfortunately it's unlikely they'd repeat the test with helium due to the cost of the study. David did go on to say that gas switching to higher PP02 mixes would have a substantial effect on the results. There were then some more questions around how this study should be interpretted for the sort of technical diving we do (bounce diving). The short answer was that we should ensure we don't do excessive deep stops and utilise gas switches to higher pp02 gases on the ascent.
Jarrod got and spoke afterwards and pointed out that GUE had already made a change around the practise around deep stops around a year ago. I believe he was specifically referring to the removal of the 1min at 80% ATA's in the T1 range. On that basis I'd say it was relevant to the kind of diving people are doing.
Cheers
Al