Dive planning with subsurface

[Willem Ferguson]

I have been looking at the dive planning section of subsurface, modeling 
all the longest no-deco dive durations on the NAUI diveatble and looking 
at the shape of the Buhlmann-derived ceiling with respect to dive 
termination. I have two remarks:

1) There is still significant work required in the planning part of the 
software. It is not reliable. I attach two screenshots here:
      a) The dive planning window, the dive profile and the dive notes 
all contain different information.
https://www.dropbox.com/s/im7henod563ljku/Screenshot%20from%202013-10-12%2015%3A10%3A16.png
      b) Graph hangs up in a way that is not recoverable. Have to close 
the planning dialogue and start over.
https://www.dropbox.com/s/sczxhyvn575qfr0/Screenshot%20from%202013-10-12%2015%3A32%3A08.png
      But these are less interesting to me. The utility of the planning 
module is much more interesting as in the following point.

2) Recreational dive planning with subsurface following the NAUI no-deco 
dive duration limits. Note that NAUI defines the dive duration from 
submerging until the start of the 5m safety stop. I attach a summary of 
models of dives to 12m, 15,18,21,24,27,30,33,36 and 40m. Each diagram 
shows the NAUI dive duration limit. See the link:

https://www.dropbox.com/s/undi7pekjeaj5ta/Subsurface%20Buhlmann%20ceilings%20following%20NAUI.png

For dives from 15m to 24m, the subsurface Buhlmann model would suggest 
that the NAUI dive limit is too long if there is only a 3-min safety 
stop. The 21m dive appears to be particularly problematic. If I modify 
the dive plan to start the bottom section at 19m, descend go to 21 m and 
then gradually return to 19m, (mean bottom depth 20m) as follows:
   Transition to 19,0 m in 2:00 min - runtime 2:00 on air
   Transition to 21 m in 20:00 min - runtime 22:00 on air
   Transition to 19,0 m in 20:00 min - runtime 42:00 on air
   Transition to 5,0 m in 2:00 min - runtime 44:00 on air
   Stay at 5,0 m for 3:00 min - runtime 47:00 on air
   Transition to 0,0 m in 0:33 min - runtime 47:33 on air
then I would surface in significantly better Buhlmann shape than a dive 
with all bottom time at 21m. This probably means that (in keeping with 
previous discussion on this thread) with a realistic dive that sticks to 
the MOD, there would probably not be a ceiling-related problem.
The Buhlmann approach as implemented here would indicate that I can 
safely dive 40m for 8 min bottom time instead of 8 min total dive time 
that would be practical using the NAUI dive tables.
The GF40/80 probably do not mean much in this context because the ascent 
is brute-forced to a 5m safety stop, keeping to an maximum ascent rate 
(7-10m/min).

It is not a question of whether one approach is wrong or the right. I am 
trying to understand the interrelationships between the dive-table 
related recreational dive decisions and those that explicitly use of the 
Buhlmann algorithm within recreational limits.. Obviously the picture is 
totally different for dives requiring staged decompression where the 
gradient factors would be an important determinant.

I would appreciate comment by the experienced divers here.

Kind regards, Willem