You shouldn't say that being warm and off-gassing fast topside causes less deco stress. First dive, second dive, overall, whatever: just don't.
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You shouldn't say that being warm and off-gassing fast topside causes less deco stress. First dive, second dive, overall, whatever: just don't.
What he means is that the the pressure gradient on the surface is much greater than it is when still under pressure in the water. After a recreational dive, for a diver on the surface, all tissues are off-gassing as quickly as they possibly can. For a diver in the water, the pressure gradient is not as great, so the tissues that are off-gassing will not do it as quickly, and some tissues will still be on-gassing.
For technical divers using oxygen for decompression, they will off-gas faster in the water than on the shore breathing air. In fact, after a long final decompression stop, their fastest tissues will on-gas during the surface interval. This is possibly true to an extent for recreational divers breathing a high percentage of nitrox during the dive. They can, however, get that same advantage by breathing off their nitrox tank for a while on the surface.
I know what he meant, and it doesn't follow that that diver on the surface has lower decompression stress that the diver at the safety stop. Maybe, maybe not: I read it somewhere on the Internet that not is the assumption that makes RGBM favour longer safety stops. Allegedly.
johndiver999
Contributor
The deco stress is going to be manifested upon a second dive if the surface interval is much shorter. The diver doing the extra slow ascent is probably safer, if that is his last dive.
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However, according to one of the leading hyperbaric physicians in my country, the functional difference between many of them is pretty much negligible as long as you're staying within NDLs. Maybe even for short deco times and moderately deep deco dives.
I use a Suunto, which is infamously known ("known"?) to be excessively conservative, but still, if I compare the predictions of the Suunto RGBM dive planning tool with the predictions of PADI's RDP, the difference in minutes of no-stop bottom time can easily be counted on one hand. With a pretty decent margin.
johndiver999
Contributor
When you say below do you mean deeper or numerically smaller?
Regardless, I still don’t understand your point. Not saying you are incorrect, just that I don’t understand????
Perhaps breathing nitrox for a long time on a safety stop
might provide a benefit, relative to the buddy who is breathing air on the surface? I hadn’t considered that in my conceptual model.
The question is, who enters the water with more residual nitrogen, the guy who stayed down shallow or the guy on the boat???? If we were only considering air, then I think the answer is clear.
johndiver999
Contributor
Yes and the difference widens over the day of repetitive dives.
Yes.
Let's compare 3 dives to 100 feet, 70 feet, and 60 feet, with a 1 hour surface interval after each. In each case, we will dive to the maximum allowed NDL. Here are the compared allowed bottom times
US Navy (NAUI, SSI)
1. 100 feet for 25 minutes
2. 70 feet for 19 minutes
3. 60 feet for 8 minutes.
PADI RDP
1. 100 feet for 20 minutes
2. 70 feet for 25 minutes
3. 60 feet for 34 minutes
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