Poseidon CCR dives: Nitrogen loading

[Willem Ferguson]

On 05/11/2014 16:01, Robert Helling wrote:
>
> On 05.11.2014, at 08:02, Willem Ferguson 
> <willemferguson at zoology.up.ac.za 
> <mailto:willemferguson at zoology.up.ac.za>> wrote:
>
> Willem,
>
>> Would you be prepared to look at the sample dive log in the dives 
>> directory? There are two versions of the same dive, one in CSV, 
>> another in XML. When importing the CSV version, NO nitrogen loading 
>> is shown, however when loading the XML version, nitrogen loading is 
>> indeed shown. Are you in a position at all to say what causes this?
>
> Some debugging shows the ceiling calculation was thinking you were 
> diving pure O2 though out the dive as that is what is contained in 
> cylinder 0 which is the default starting cylinder. Here is a patch 
> that makes the diluent cylinder the default starting cylinder instead.
>
> Best
> Robert
>
Robert, thank you. This really helps. I love it that you use the code 
that makes use of enums!

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?

Thank you so much for your time.
Willem
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