Oops, you're right, I meant dehydration (not hydration!).
I'm still not clear on the reasoning behind diminishing NDL during an ascent, regardless of the rate. My computer is the Suunto Vyper, and I have contacted them regarding the calculations behind it, but I haven't received a response.
The scenario was this: wreck dive with most of the dive spent doing penetrations at 110-120ft, followed by a bit of time on the deck at 85ft. We began up the ascent line with a few minutes of NDL left (admittedly too little), at which point I concentrated on watching my depth rather than my NDL since I have become used to NDL increasing as you head up the line, so I assumed that there was no point in monitoring it once we were headed for the surface. (As you can guess, I have learned my lesson here!) Upon reviewing my profile, NDL continued to be consumed as we went up since it was an excessively slow ascent rate (something like 10ft/min) which was intentional since I have been bent before and I thought I was being conservative by going so slowly. We stopped for a one minute safety stop at half our maximum depth (60ft), and that is the point that I ran out of NDL and went into deco. Somehow, I didn't notice that my computer was telling me to ascend immediately to a deco stop at 10ft (I think, may have been a slightly different depth) and so we continued our slow ascent up the line. We stopped at 15ft for 3 minutes, and though the deco obligation was violated, it actually cleared by the time I reached the surface.
The skin bends I received several hours after that dive was extremely minor, but it was a warning and it caused me to go back and check my profile, at which point I realized that I had blown a deco obligation. There may have been other factors that contributed, namely the fact that I got quite cold on the boat before that dive.
I have discussed this with many people, and most of them agree that as long as you are ascending, the pressure gradient should be such that you will be offgassing as you ascend regardless of the rate, until you reach an equilibrium and begin to build up nitrogen again. According to this theory, you should never lose NDL during an ascent. Perhaps I don't understand decompression theory well enough to see the full picture here, so if anybody out there has any insight, please share the wealth!
