Dive planning with subsurface

[Robert C. Helling]

On Oct 12, 2013, at 5:37 PM, Willem Ferguson wrote:

Willem,

sorry for replying so late, I was busy with a couple of other things over the weekend.

> 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.

The first and most important statement is that deco is more an art than a science. Anything that gets you out of the water well (for various definitions of well) is good, anything that bends you is bad obviously. Decompression depends on so many factors in your body and details of the dive and its pre-history that is completely impossible to predict all that. That means even if two algorithms give you a couple of minutes difference in non-decompression time or in deco stop time they can both still be correct (in the above sense).

Even if your algorithm had a perfect empirical basis (which if far from the fact for most real algorithms) the best kind of prediction would be something like "Following this algorithm you get bend x% of the dives". And then different people have different ideas about what x is acceptable (but most would aim for part of permille, i.e. every few thousand dives).

That said, you should compare your tables (or any others published by any agency, for a sample see http://www.seveke.de/tauchen/download/1tabellen.htm ) to Buehlmann with gradient factors set to 100/100 (your setting of GFlow=40% forces deep stops even for shallow dives). Then the "20min at 34m" dive looks perfectly fine for me in subsurface: Take 3min to get to 34m. Then leave 14min later to arrive at the 5m stop at 20min runtime (as you prescribed). Subsurface suggests then a stop of two minutes (which would be included in your safety stop).

The other thing is you should not compare ceilings during the dive as those depend strongly on settings like GFlow that you would set at your personal choice. Said differently: A ceiling with a stop time of 1 or 2 minutes would be called a slow ascent by some other diver. YMMV. So the ceiling is to some degree a matter of choice. What is more important is the total runtime, when you get out of the water. If your dive shape is somewhat reasonable this is (up to a few minutes plus or minus) what you should compare between algorithms.

Hope that helps.

Best
Robert

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Robert C. Helling     Elite Master Course Theoretical and Mathematical Physics  
                      Scientific Coordinator                                   
                      Ludwig Maximilians Universitaet Muenchen, Dept. Physik    
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