CNS computing mistake

[Robert Helling]

Hi Daniel,

> On 5. Mar 2023, at 21:16, Daniel Miazek via subsurface <subsurface at subsurface-divelog.org> wrote:
> 
> 
> I've noticed this when I tried to plan a diving on 50 meters and 30minutes bottom time (including falling down), with 24litres air, 11litres nitrox 52%, and 6litres oxygen (nitrox 99%).
> I used Bulhmann algorithm with 20/80 GF, SAC 19l/17l, PO2 1,4bottom, 1,6 deco. All ascending speeds 10m/min.
> 
> Generally speaking Subsurface turned out 72 minutes diving plan indicating that CNS will be 66%, OTU 103.
> 
> I've compared this with my sharewater petrel 3 planner and manual calculations, and both turned out CNS values close to 49%.
> Diving time, gas usage and OTU was almost the same.
> 
> I can share print screens and a file with this particular plan.
> Maybe I made saome mistakes in other configuration settings, but I'v checked everything several times and everything seems to be ok, except CNS calculations.
> 
> Would be gratefull for any comments or clues.
> 

what do you use for the last stop depth? If I use 6m 69% whereas I get 47% if 3m is the last stop.

If you want more details of what Subsurface is calculating (I might have slightly different settings from you) to compare to your manual calculation, here is the 6m version


po2=740	 rate=0.000032 for t=299s
po2=1268	 rate=0.000089 for t=1501s
po2=1025	 rate=0.000056 for t=154s
po2=782	 rate=0.000035 for t=86s
po2=750	 rate=0.000033 for t=20s
po2=719	 rate=0.000031 for t=100s
po2=687	 rate=0.000029 for t=20s
po2=655	 rate=0.000027 for t=1s
po2=1568	 rate=0.000260 for t=99s
po2=1492	 rate=0.000137 for t=20s
po2=1416	 rate=0.000119 for t=100s
po2=1340	 rate=0.000102 for t=20s
po2=1264	 rate=0.000088 for t=160s
po2=1188	 rate=0.000076 for t=20s
po2=1113	 rate=0.000066 for t=220s
po2=1037	 rate=0.000057 for t=20s
po2=961	 rate=0.000049 for t=400s
po2=885	 rate=0.000042 for t=20s
po2=810	 rate=0.000037 for t=1s
po2=1619	 rate=0.000428 for t=339s
po2=1467	 rate=0.000131 for t=20s
po2=1316	 rate=0.000098 for t=640s
po2=1164	 rate=0.000073 for t=20s

and here is the 3m version

po2=740	 rate=0.000032 for t=299s
po2=1268	 rate=0.000089 for t=1501s
po2=1025	 rate=0.000056 for t=154s
po2=782	 rate=0.000035 for t=86s
po2=750	 rate=0.000033 for t=20s
po2=719	 rate=0.000031 for t=100s
po2=687	 rate=0.000029 for t=20s
po2=655	 rate=0.000027 for t=1s
po2=1568	 rate=0.000260 for t=99s
po2=1492	 rate=0.000137 for t=20s
po2=1416	 rate=0.000119 for t=100s
po2=1340	 rate=0.000102 for t=20s
po2=1264	 rate=0.000088 for t=160s
po2=1188	 rate=0.000076 for t=20s
po2=1113	 rate=0.000066 for t=220s
po2=1037	 rate=0.000057 for t=20s
po2=961	 rate=0.000049 for t=400s
po2=885	 rate=0.000042 for t=20s
po2=810	 rate=0.000037 for t=1s
po2=1619	 rate=0.000428 for t=999s
po2=1316	 rate=0.000098 for t=40s

And here is a write up of how it gets to these numbers:

https://thetheoreticaldiver.org/wordpress/index.php/2019/08/15/calculating-oxygen-cns-toxicity/

But I guess the most important part of this is that these are not in any sense high precision numbers but more like ballpark guesses (see the Hamilton quote in the article).

Best
Robert
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