Graph documentation [was: Re: Meaning of GF settings]
Robert C. Helling
helling at atdotde.de
Thu Jan 10 14:53:43 PST 2013
On Jan 10, 2013, at 7:29 PM, Jan Schubert wrote:
>> Let me add a final remark: Deco is an art, not a science.
> If we speak about implementing Bühlmann I tend more to science than art.
The Buehlmann model is sufficiently well defined (once you fix the numerical constants) that you either implement it or not. That is true. But I think, everybody agrees these days that you don't want to use plain vanilla Buehlmann for dives that are deeper and have significant deco obligations. And thus everybody adds their own fudge/safty factors etc and you don't know what exactly they are calculating unless you see the source code. As I said, not even Buehlmann himself in the book that lays out his algorithm used it himself for the tables in the book (and he does not say what he used).
> The reason I stick to this topic is that I'd really like to be sure
> about the outcome of the planning part of subsurface (which right now
> shows extreme differences to other dive planning software).
>> Thus I would not worry at all if two different models (or
>> implementations) vary in their prediction for your stops by one or
>> even two meters.
> I'm not worrying about the differences in depth (as it is quite obvious
> where this comes from) but the much longer deco in the shallow part.
My point is, this is effectively the same. If you for example change the rule for "end of deco" from "the ceiling is at the surface" to "the ceiling is at 0.5m" for an arbitrary number where the change is likely not very significant as the pressure difference it implies is quite small. And using a different rule how you round your depth to meters has exactly this effect. But the resulting change in stop time at the last stop seems quite significant. This is because the ceiling graph at the end is almost flat, the slope is so small. What I am saying is that even though several minutes difference seem quite important at the end of deco they are equivalent to very small variations in the (allowed) ambient pressure and thus are not relevant for the quality of your decompression.
> Please note that the profile is cut, I expect the calculated deco
> ceiling to be much longer.
Yes, but the ceiling is only a few centimeters from the surface.
> As we use real profile data the real depth should be taken into account.
And it is taken into account. But you don't want to specify the profile to a higher precision than you can actually realize it. That would be virtual precision.
> And as my dives are done at much less than 1013mbar ambient pressure
> this might result in even more difference...
Robert C. Helling Elite Master Course Theoretical and Mathematical Physics
Ludwig Maximilians Universitaet Muenchen, Dept. Physik
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