Dive plan: Taking into account isobaric counterdiffusion

[Willem Ferguson]

I have been looking at ways of providing information in the dive plan 
that aids in managing isobaric counterdiffusion (ICD) during a dive, a 
problem that almost exclusively concerns trimix dives.

I use the IANTD approach (when changing cylinder during deco, increase 
in fraction of N2 in gas should not exceed  a fifth of the corresponding 
decrease in fraction of He). I know this has been appropriated by at 
least some other agencies. I am not aware of other formal approaches to 
this, except for V-planner/Multideco that implement an increase in 
N2-partial pressure beyond a declarable critical value as being 
dangerous. I would like to have some opinion/advice/comments from 
individuals who have dealt with this issue for possible implementation 
in the Subsurface dive planner.

Attached are two files:

1) A screenshot of the resulting dive plan.

Counterdiffusion data are only shown for cylinder changes involving 
trimix during the ascent. It shows the increase in % of N2 during the 
gas change, compared to the recommended maximum using rule-of-fifths. If 
the first number is significantly higher than the second, a different 
selection of trimix gases should be considered. Is this an efficient way 
of presenting the information or is there a better way?

1) A patch file allowing others to see how I have implemented some of 
this in a rather ham-fisted way.

I am not sure my code lies in the best possible place. Firstly, there 
are at least two alternative approaches here. Since the dive structure 
and the linked list of dive points are readily available during 
planning, one could step through the linked list, look up the cylinder 
used at each point and collect ICD information whenever the cylinder 
changes during ascent. I followed the second approach by using the code 
for printing the dive plan because the appropriate where the exact 
events that should trigger an ICD-related calculation already exist. 
Ideally, one would not like to do calculations in plannernotes.c because 
that is not the function of that specific code. But putting the 
calculation in that C function is extremely convenient. Comments, 
please, especially from Robert?

Secondly, assuming that my approach is seen as the preferred one, I am 
not convinced that my code lies in the best place within plannernotes.c. 
There may be a much better approach.

Comments, please? This communication is not meant at all to propose a 
formal patch. It is a proof-of-concept that would require formal 
implementation. There are three issues:

1) approach to managing ICD for trimix dives within the planner;

2) How the ICD data should be presented (see attached image);

3) An efficient way to implement this (see attached patch file).

Kind regards,

willem




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