lamont
Contributor
Here are some things I wonder about:
[*]Are all dives decompression dives since on all dives I on-gas Nitrogen and must off-gas it at some point?
Yes. Even on a recreational NDL dive you can have an overpressurization gradient and you will bubble before you hit the surface. Slowing down the rate of ascent will slow down bubble formation. Physiologically it will make some kind of difference after the dive if you decompress 'better' from an NDL dive (not all of them, but ones closer to the NDL limits) -- it is debatable if that leads to actual lower risk of DCS, and it is debatable as to which curve is actually 'better'.
[*]What is more important on a "recreational dive"? Ascent speed or a safety stop?
My guess is the safety stop. A rapid ascent to 15 feet will drive a gradient, but the stop will help to take that off.
[*]I read on the Internet that the safety stop was put in the tables because divers were either ignoring or lacked the skills to ascend at the table's recomended rate, so stopping them at 15' would slow them down. is this true? If so, is there any empirical evidence that safety stops matter for people ascending according to the table's ascent rate?
There is evidence that rec divers who follow the ascent rate and do the stop bubble less on surfacing than divers who only follow the ascent rate. I can't cite chapter and verse offhand though. But a 30 fpm ascent with a 3 min safety stop is a measurably 'better' decompression than just 30 fpm alone.
[*]Why 15'? Why not 10'? Or 20'? Do surface conditions such as waves or swells have anything to do with this? How about buoyancy skills? If you could hold a stop at 10' +/- 2' would it be better than 15'+/- 2'? What if you tried to hold a stop at 10' +/- 5'? Is that worse than 15' +/- 5'?
On a recreational dive it probably doesn't matter much, and typical recreational divers aren't going to be able to hold +/- 2' unless situations are very good. The more important point is to do 3 mins at 15 +/- 5 rather than at exactly 10, 15 or 20.
[*]Is a slow ascent more important near the surface, half way up the ascent, one quarter of the way up the ascent, or near the bottom. Why?
Near the surface. That is where the overpressurization gradient is highest. At the bottom of the dive there's no point in being slow.
For example, if you are diving 21% at 100 feet, then even in fully saturated compartments they only have 3.16 ata of PPN2 -- which is 72 feet in pressure depth ( ( 3.16 ata - 1.00 ata ) * 33 feet / ata ). That means that no matter how long you've been at 100 feet on that gas, there's no point in doing a slow ascent between 100 feet and 72 feet from a decompression standpoint -- you aren't offgassing into bubbles at all, so its better to get up to 72 feet to maximize the diffusive offgassing.
As a general rule, move faster closer to the bottom and slower closer to the surface -- and look at any decompression curve generated by any program or model, and it has that this general kind of shape.
[*]Some tech divers were talking about deco the other day and they said that older deco protocols had them decompressing at the surface, while newer deco protocols have them decompressing in the water. What does this mean, and does it have any relevance for recreational diving?
No useful relevance for recreational diving.
DCS seems to have a time lag before it hits, so you can use that to your advantage and climb out of the water before you are done with your decompression (blowing off decompression) and then climb into a chamber aboard a ship which will recompress you in order to finish the omitted decompression. This probably puts the diver at some additional risk, but gets them out of the water and back on a ship. It has more relevance in technical diving in discussions of the relative merits of in-water recompression 'treatment' for missed deco or treating DCS, and for lost-deco-gas situations.