Re: Dive planner CNS calculations

```On 09 November, 2018 - Willem Ferguson wrote:

> 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.```
```
The cns calculator we currently got was never intended to be used in the
planner. Back when it was written, subsurface didn't have a planner.

The model it uses works just fine with a 1-3-10-30-60 second sample
interval from a divecomputer.

Btw. It works just fine for the diving I tend to do, IE, drop to rock
bottom, swim around and deco.

I'd suggest we "expand" the planner dive points into some sane sample
size, like 10s intervals before doing this sort of calculations.

//Anton

--
Anton Lundin    +46702-161604
_______________________________________________
subsurface mailing list
subsurface@subsurface-divelog.org
http://lists.subsurface-divelog.org/cgi-bin/mailman/listinfo/subsurface
```