mccabejc:
I've always figured when it comes to ascents, slower is better. Well, except for deep dives, in which you may still be ongassing even while ascending, and your remaining NDL time may keep ticking down.
What I'm trying to figure is this: is there a goal, an ideal ascent rate? Of course, too fast (over 30ft/min) is bad, and my computer will throw in a mandatory stop if I exceed 33 fpm continuously or 39 fpm momentarily. But too slow is also bad, cause you may suddenly run out of NDL time.
So if you are deep, and need to start offgassing, is the goal to ascend at around 30fpm, or is slower really better?
There are some important points to make about this:
1) If you ascend slowly you will continue to ongass from a deep ascent until you get to a certain depth. Your ascent to the shallow zone should be done at 10 metres per min to avoid additional ongassing. If you ascend any slower than 10 metres per min from a deep ascent then you run the chance of "running of of NDL". If you are already over the NDL then you will accumulate extra deco.
2) Ascending over the bottom (what may of us do during shore dives) will often give you slower than 10 metres per minute. In the case of where I live, a swimming ascent over the bottom can bring you up as slowly as 2 metres per min. The point here is to watch this and get a feeling for how fast an ascent over the bottom goes at various sites in your local area.
3) Slower is better once you get into the shallow zone. You should start your ascent from a deep dive at 10 metres per min, slow it to 3 metres per min when you get in the shallow zone (maybe 12 metres) and *arrest* your ascent at about 6 metres and ascend slowly from that point. From 6 metres to the surface you do not ongass in any significant (theoretical) compartment anymore and this part of the ascent should be done as slowly as it's practical to do. The reason we do safety stops in this zone is to arrest and slow the ascent towards the surface to allow for extra offgassing.
So that's just a long winded way of saying, start your ascent with a good tempo and slow down more and more as you get shallower and shallower.
4) It's proabably (theoretically) better for you to leave the bottom a little early (not right at the NDL) and ascend slowly than to run it up to the line and ascend with 10 metres per min.
5) Your tables, if you use them, are designed with a particular rate of ascent in mind. If you don't follow that ascent rate then your N2 load will be higher than you think. Over multiple dives this could become significant. This is in relation to Doc Intrepid's comment that the bottom time ends at the beginning of the ascent. That's partly true. The entire truth is that your bottom time ends at
the beginning of an uninterrupted ascent done at the speed with with your tables were designed. It's not just a difference of nuance. It makes a difference in your N2 load.
6) Watching the ascent indicator on your computer can be misleading but it's better than nothing. What's much better is to watch your clock as you ascend and make a few little stops along the way.
Here's an example.
Say you're on a 30 metre dive and you start your ascent at 20 minutes (that's the NDL for air IIRC). That means that you should be at 20 metres at 21 minutes and 10 metres at 22 minutes. Follow?
What I would do is start my ascent and just swim at a good tempo from 30 metres to 20 metres. Once at 20 metres I would look at my bottom time. If it doesn't say 21 yet then I would stop and wait for it to flip to 21 and then start ascending again. If it already says 21 then I would keep going. Once at 12 meters (12 is a factor of 3, and it's the deep end of the shallow zone so you won't hit your ceiling here unless you botched the ascent beyond belief) I would stop again and look at my bottom time. If it didn't say 22 minutes then I would wait at 12 metres until my clock goes to 22. Once at 22 I would ascend to 9 metres and wait until it flips to 23, then ascend to 6 metres and wait until it says 24 and then start with my safety stop at either 6, 5 or 4,5 metres depending on which tables you're using. After the safety stop I would ascend slowly to the surface.
The reason I think that this is better than using your ascent indicator is that watching your clock makes you 100% sure that you didn't ascend too fast. Ascent indicators only give you a picture of a particular moment in time and are unable of warning you if your *total* ascent is too fast or too slow. In my mind they're completely usless but YYMV.
R..