Poseidon CCR dives: Nitrogen loading

Robert Helling helling at atdotde.de
Thu Nov 6 01:33:30 PST 2014


On 05.11.2014, at 17:37, Willem Ferguson <willemferguson at zoology.up.ac.za> wrote:

Willem,

> 1) This patch has an unintended consequence: The ending cylinder pressure for the diluent cylinder (as seen on the equipment tab) is changed to reflect that of the ending pressure of the oxygen cylinder. The diluent cylinder index for the Poseidon diluent is the dreaded and infamous 1. We hardcode the cylinder IDs here while reading in the dive log because it is very specific to the Poseidon. In addition, even though these specific two cylinders have already been assigned to enum values of oxygen and diluent respectively, we are in the process of setting up the cylinders, so we cannot already use the functions that use these enum values.
> So:  dive->cylinder[1].sample_end.mbar for this sample dive has a value of 141, that of the oxygen cylinder. The correct ending value for this variable is 137. I have carefully looked around the code around line 600 in file.c but cannot see the cause of this problem. Run the code with and without your patch and see the difference.
> 
> 2) A question just to improve my understanding of the code. If the explicit_first_cylinder() result is diluent, does this mean that the deco requirements are calculated for the gas in the diluent cylinder? In this case the diluent is air, but the displayed ceilings do not (at first sight) appear to be shallower than that for air dives to the quivalent depths. For instance, if I simulate the sample dive as an air dive using the planner, the dive extended to 70 min for sufficient deco. So if the ceilings reflect the pn2 values at the various depths as actually calculated at each point on the profile, why does it matter whether one starts with cylinder 0 or cylinder 1?


after getting a good night of sleep, I am reading this again (several times indeed) and I am not sure I understand what you are trying to tell me.

I understand that for a CCR segment there are two cylinders in use simultaneously, diluent and O2. For the deco calculation to work, it has to know that the diluent is (and figure the actual breathing gas from that and ambient pressure and set point). 

The other question is what to plot as a pressure (as in cylinder pressure, not partial pressure) graph. Ideally both, but at the moment we don’t do that. So, at least for the .txt import it seems we are plotting diluent pressure. Is that what you are complaining about?

MY question is: Where does this information (and also the information about the O2 cylinder pressure) as a function of time come from? Is that in the .txt/.csv as as far as I can see, it is not in the .xml. Or do we only have start and and pressures for the cylinders (again they seem to be missing for the xml and I don’t know about the others). In that case, for OC, cylinder pressures to be plotted get interpolated assuming a constant SAC. But for CCR that (as was discussed at length before) does not make sense. For O2 one might try a depth independent consumption (leading to a linear plot of O2 cylinder pressure) but for diluent it is ascent/descent dependent in a complicated way that we have not yet figured out how to model, so at the moment, any interpolation would be bogus.

So, what to you expect to see and what are you seeing instead?

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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                      http://www.atdotde.de

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