I am often guilty of overthinking things. In this case, though, it's simply understanding some basic things. One of those is that you are undergoing decompression any time you are making an ascent.
Even if you only descend to 10 feet and stay for 1 minute, you have still on-gassed some amount of inert gas which will off-gas when you ascend back to the surface. That off-gassing is decompression.
Another of those things is that every computer, no matter the algorithm, calculates a depth where the pressure of the inert gas in your tissues will be so much higher than the ambient pressure that you will exceed the programmed risk point for incurring DCS. That point is your ceiling. If you can ascend directly to the surface with no stops and without exceeding the programmed risk point, there you are still within your NDL (aka your No-Stop Time). And, since it is the case that you are generally decompressing as you ascend, it is possible that you will have a ceiling great than 0 when you begin your ascent that has reduced to a ceiling of 0 before you get to the surface.
So, not OVERthinking. Just thinking. You can absolutely have a ceiling without exceeding your NDL - and it is pretty normal. However, a typical recreational computer will not show you your ceiling until you have exceeded your NDL. Meaning it won't show it to you until you've stayed down so long that your decompression that occurs during a no-stop ascent will not be enough to avoid breaking your ceiling. At that point, you are into mandatory deco and will have to make at least one stop.
There are two basic modes that you see in computers used for technical diving, so I'm going to tell you a little story now about how things go in practice.
As I said before, as soon as you have a ceiling the computer will display it along with a TTS (time to surface).
The two basic modes that you see can be illustrated by the differences between my computer an my buddy's computer. For our technical Nitrox dives we might be 30 minutes or so over the NDL. As I mentioned above (or was it in another thread?) I'm using an old Vytec and relying on my knowledge of deco theory to organize the ascent. My buddy uses a Perdix and follows his computer.
Both computers enter deco at roughly the same time. The older Suunto computers still used something remotely resembling Bulhmann, which is one of the reasons I still use this computer. The Vytec gives ridiculously shallow ceilings (maybe still only 4 or 5 meters even when you're 30 min over the NDL). You have to make a fairly interesting dive before it gives you a ceiling deeper than 3.8 meters and I don't think I've ever seen it give a ceiling deeper than 7 meters even when we were well over the NDL's.... The Perdix gives a ceiling for a dive where the Vytec says "ceiling" of 4m of 12 or 14 meters using 30/70. That's a ceiling, not a deep stop.
The interesting part is how it calculates TTS. The Vytec gives you a TTS based upon the gas you are on *right now*. We dive fairly regularly to 50m and at the point where we make the gas change to 50% my computer may be telling me that I have 30 or 40 min of deco and my buddy's computer is telling him 25 min. As soon as I make the change, however, the computer recalculates the TTS based on the new gas and the time gets cut in about 1/2 within a couple of minutes.
My buddy's Perdix, however, calculates the TTS based on the gasses it knows (has been told) the diver has available. it assumes the diver will make the gas changes more or less on time and displays a much more accurate TTS on the bottom than the Vytec. The risk in this, of course, is that if the diver has a lost deco gas situation -- or chooses to ride back gas longer -- the computer will not know about it and the deco time will continue to accumulate even when you are ascending.
As for ascent speeds..... Using my Vytec I've obviously played around in the course of time with various ascent speeds. We used to do 10m/min until 18-20m, extend that stop because everyone was doing gas changes and then 3m/min until just under the ceiling. VPM really liked this ascent and I used to put in longer stops at 9 and 6 meters based on theory (using a ratio) even though my computer was telling me it had a ceiling of 3 or 4m. I made hundreds of similar dives like this and never felt the least bit off when we were done so I thought we had it licked.
When we first started doing this the 3m/min ascent from 20 to 6 meters it caused the Perdix to stop counting down deco minutes. This makes sense because 3m/min is very slow..... The Vytec counted down (albeit slowly) but we were getting in a situation where I was done my deco according to the computer sometimes 8+ minutes before my buddy. We took a good look at our ascent strategy and made some changes and now ascend from 18 to 9 meters in 1 min instead of 3 -- assuming ceilings allow -- and it brings the amount of deco calculated by the Perdix to within about 3 min of what they Vytec says. We're 15 years of technological development further and it boils down to a marginal difference in deco schedule if you have some idea what you're doing during the ascent.
This is, of course, in a context of technical Nitrox diving. The Vytec can't do trimix so I'm not talking about that.
The point for Stewart, of course, is that yes, ascents make a significant difference, but how you manage that ascent depends to a large degree on understanding your deco theory. In practice there are considerations that might not seem obvious when you read about it.
R..
