boulderjohn:
[Note: for clarity purposes, I edited what I thought was an obvious typo above; please pardon me if I was wrong.]
My guess is that changes in his are in the offing based upon recent research that was not available when those tables were made. The recent DAN study, for example, clearly shows that a deep stop is very beneficial in all dives, not just for a deco dive. I do a deep level stop for any dive other than a relatively shallow dive in which I did not even come remotely near NDL's.
Yes. Even the DAN study came a little late and many of us had a "well duuuuuh" reaction to it. Other models, buhlmann (old and pretty well tested) with gradient factors applied using decompression software, VPM, RGBM ect were all pretty widely used long before that DAN study. All of those will result in stops starting deeper and in some cases more total decompression. Even straight buhlmann isn't going to let you do a 10 minute 130 ft dive as a no stop dive...that "NDL" is a moving target and from table to table (even for recreational depths and times) could vary by 40% or so. Would the real NDL please step forward?
Additional factors that make models/tables other than the navy tables attractive is the common use of helium mixes and/or multiple gasses for accelerating decompression.
Personally, in almost all cases, including little dives that only require a little bit of decompression time, I prefer to have at least one decompression gas and redundant breathing equipment. Anytime you are in a situation where you would consider staged decompression manditory you can't afford to run out of gas. Generally speaking, I would consider a "recreational dive" (using common terminology) one where you are shallow enough and far enough from an "NDL" that an immediate ascent is a viable way out. Get too deep or have an overhead (real or virtual) and it becomes a "technical" and you should treat it as such.
All that was a long way of saying that I don't plan 20 minute 100 ft dives or 10 minute 130 ft dives (a couple PADI NDLs) as no stop recreational "dives". If there's anything worth seeing or doing that would put me anywhere near there, it's much more enjoyable to have the right equipment and gas so that I can relax and enjoy the dive without comming close to any of the limits of my plan (time or gas). On the most basic level, any sensible gas plan for any dive is going to have you ending your dive with at least twice as much gas as you plan on needing for your ascent (enough for you and a buddy).
The fact that we see questions like this on the board at all, IMO, is an indication of a very real problem and that is that gas management is rarely taught at any level of recreational dive training. They tell you to watch your SPG but don't tell you how to figure what you need to actually complete the dive or have enough for unplanned events. So, great you're looking at the SPG, now what? Get out with a few hundred psi left? Why? When do you start back/up? What if you have to donate gas to your buddy during the trip back or the ascent, do you still have enough? The answers to those questions belong in every entry level diving text. then we wouldn't have to worry about having enough gas for a few minutes of additional decompression even if we have to share our gas with a buddy.
Of course, even though the math needed for such gas management is pretty simple, some of the ten year olds currently enrolled in scuba classes might have some trouble with it. I guess the current industry answer is to watch your SPG and stick close to your DM guide and hope that he has a plan.
