Poseidon CCR dives: Nitrogen loading

[Robert Helling]

On 07.11.2014, at 10:29, Willem Ferguson <willemferguson at zoology.up.ac.za> wrote:

Willem,

> 1) Please could you send the mail with the two patches to me privately? I appeared in my email system but was gone two minutes later. It's not in the junk folder. I suspect my finger trouble.
> 

done. You could find those in the mailing list archive as well

http://lists.subsurface-divelog.org/pipermail/subsurface/2014-November/thread.html

> 2) I have the cylinder pressure calculations working correctly as far as I can see. Will do a few cross checks over the week end. Important: I want to confirm with you that changing the return value of explicit_first_cylinder() is the way to go for correct deco calculations. Does the second patch in your last mail affect this at all because I saw some code affecting deco? The over-printed cylinder contents on the left of the profile widget is because of the return value of explicit_first_cylinder(): Subsurface sees that as a cylinder change to cylinder 1  right at the start of the profile, printing the labels for both cylinders 0 and 

Not quite I would say. For the deco calculation you have to know what gas (in particular: which cylinder) you are breathing from at the moment. In the data from dive computers (as well as in the xml), this is, however, not how that data is represented. There you find only the gas change events that tell you that the gas changes. The latest gas change of course determines the gas used at later times but that possibly does not tell you which is the current gas before the first gas change event. For that reason, some dive computers spit out a gas change event at the beginning of the dive. Others silently assume, that dives always start with gas zero. This is the information that explicit_first_cylinder() returns: Which cylinder number is used before the first gas change event. It has no effect after the first gas change.

Now, in the CCR case, things are a bit more complicated as there is not _the_ current cylinder, there are two, diluent and oxygen. For the purpose of deco calculation (and anything that is related to what gas is the diver currently breathing), I would still argue that the “current” cylinder should be the diluent as we have to know the diluent for fill_pressures to determine the breathing gas. 

To compute gas use (and on a related matter: tank pressures) of course both are in use but behave very differently: O2 is (to zeroth approximation) used at a constant rate irrespective of depth. So you could, if you wanted, interpolate values (in effect linearly in time). Diluent is used on descents and use is thus complicated so, in a first go, I would not attempt to interpolate it in any way.

So, what shall we do? I would keep the concept of one current cylinder (and explicit_first_cylinder determines that lacking previous gas change events as above) and in the CCR gas let that be the diluent and not O2. This lets deco/ceiling calculations work as for OC (assuming fill_pressures is used) as well as the partial pressure graphs. I would only change the gas use calculations to use the current cylinder as is only for OC and do that differently (now with two gases and adopted interpolation rules) for CCR.

What do you think?

Best
Robert



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Robert C. Helling     Elite Master Course Theoretical and Mathematical Physics
                      Scientific Coordinator
                      Ludwig Maximilians Universitaet Muenchen, Dept. Physik
                      Phone: +49 89 2180-4523  Theresienstr. 39, rm. B339
                      http://www.atdotde.de

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