MikeFerrara:
as I see it there are a few important points for divers to consider.
The so called NDL is a wide line rather than the thin one.
Being aware of and in control of your profile is important.
Understanding some decompression theory is never a bad thing. Why?...
A diver strictly following a table may be lead to believe that as they approach an "NDL" the thing to do is to make a bee-line for the magic 15 ft safety stop depth. The magic of the number 15 of course is that time spent there needn't be added to bottom time. What about 18 ft or 20 ft or maybe even 30ft?
A diver with a computor hitting his NDL may see his computor suddenly screaming for them to get to a 10 ft stop. If I was trying to get bent that's exactly how I'd go about it.
The thing to realize is that some models/devices try to get you as close to the surface as fast as the math says is possible. That's all well and good but MY goal is to get to the surface with as little potentially harmful bubbling as possible. A conflict? I think so.
Time spent shallow after a dive is a good thing as is slow ascents at shallow depths. How shallow and how slow? That's where the study comes in and knowledge is almost always a good thing.
Profile depth (average or otherwise)? The least important. If you don't have any idea what your profile looked like beyond knowing max depth then you have no choice. By all means, use max depth if that's what your comfortable with and/or that's what makes sense for the dive. It's a valid choice.
The other thing that takes away our choices is not having anything left to breath. We've all see divers not only race to that magic 15 ft because they think they need to but we have all seen them cut that pause short for lack of breathing gas. They were certainly in no position to pause any deeper.
All good stuff ... and things that a good instructor will cover either in OW or AOW.
To the point, most instructors I know teach that ...
- Decompression theory is not an exact science, and there is no magic formula. Your NDL is simply a guideline.
- Dive computers are only as good as your ability to interpret the data is provides. It's nothing more than an electronic table. YOU need to control your dive, not rely on your computer to do it for you.
- When ascending, slower is better ... 30 FPM is the maximum, not the rule. The importance of going slow increases as you ascend.
Personally, I correlate this to my discussion of Boyle's Law when I show how the fraction of pressure change increases as you get closer to the surface. Instead of using the "balloons" they show in the textbook, I discuss what decreasing pressure does to the gas bubbles in your body, and explain that what can really hurt you is for those bubbles to get big enough to touch and combine with neighboring bubbles. The trick is to let them come out of your system before that happens. That's why slow ascents and deep stops are important, even (or perhaps especially) for the recreational diver. That's why NAUI teaches the 1-minute stop at half your max depth. From that point you should slow down. I give my students a rule of thumb ... after your deep stop, cut your ascent rate in half each time you halve your depth. In other words, make your ascent rate correspond to the rate at which the water pressure is changing as you ascend. After your safety stop you should take at least a full minute to the surface ... two minutes is better.
Most students can grasp this concept and the reasoning behind it without going into anything that wasn't covered in their OW textbook.
As to the limitations of breathing gas, that's why teaching gas management skills is so important. I consider it one of the biggest failures of all the major agencies that they've neglected to cover these skills in their standard texts. Personally, no student of mine gets past AOW without knowing how to plan their dive around the amount of gas they're carrying ... and why all the stuff you've discussed above makes that skill so important.
... Bob (Grateful Diver)
