Dive planner CNS calculations

[Willem Ferguson]

Hi Robert, Anton,

Robert, apologies for ascribing the CNS calculations to you. Anton, any 
comments on the text below would be extremely valuable.

As you might have deduced, the CNS and OTU calculations can rapidly 
become pretty complex for dives with long-ish segments. To be absolutely 
accurate, one needs to divide each dive segment up into parts that fit 
into each 0.1 bar PPO2 interval. E.g., a segment that runs from 10m to 
20m using EAN50 runs through five intervals of 0.1 bar PPO2 (PPO2=1.0 at 
10m to PPO2=1.5 at 20m). Then the CNS calculations for each of these 
five parts (1.0-1.1; 1.1-1.2;  1.2-1.3;  1.3-1.4;  1.4-1.5)of the single 
segments needs to be added to get a CNS value that pertains only to the 
segment from 10m to 20m. Before you know what happens, Subsurface is 
likely to spend 80% of the computing needs just to calculate CNS. One 
cannot simplify CNS calculations by using mean PO2 during a segment: it 
is a sort-of exponential relationship, not linear.

This brings me to the topic of segment lengths in the planner. I know 
that planning segments are actually implemented as much shorter profile 
segments. The big question is what happens to an initial descending dive 
planner segment that goes from surface to 40m in two minutes. Using 
EAN32, PPO2 would run from 0.30 at surface to 1.5 at 40m, spanning some 
12 PO2-classes of 0.1 in the lookup table, each with its own maximal O2 
exposure duration and its own set of calculations.

What is the profile segment duration in the planner, as opposed to 
planner segments defined in the dive planner points table? I am trying 
to make an assessment of what level of additional complexity to expect 
in the planner. This is complicated by the fact that one needs to do 
different calculations in horizontal segments, as opposed to segments 
that ascend or descend. OTU calculations are affected in an exactly 
similar way. Any comments would be highly appreciated.

Kind regards,

willem





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