CNS calculation headaches

Willem Ferguson willemferguson at zoology.up.ac.za
Sat Aug 10 05:16:51 PDT 2019


Robert, Linus,

The bug that that was encountered during dive planning by one of the 
users leads to an interesting point about the handling of dives that are 
not well-behaved, i.e. with pO2 above 1.6. The NOAA tables for 
calculating CNS toxicity only go up to a pO2 of 1.6. Above that our 
knowledge is so sparse that we do not really have a basis for 
calculating CNS toxicity. Using the standard algorithm, a negative CNS 
value is created when attempting to calculate CNS% at pO2 higher than 
1.65. Following the Shearwater web site, they implement a rule of "1% 
CNS for every 4 minutes above pO2=1.6". This means that, after 400 
seconds above pO2 of 1.6, an additional 100% of CNS is accrued. There 
are two problems.

1) The basis for using the Shearwater rule of 1% per 4s is unclear. I 
have read most of the literature around this (including Clark & 
Lambersen) and I admit I did not come across the 4s-1% generalisation. 
But the literature is really lengthy and I could easily have missed it. 
So I am note sure there is enough information available about CNS 
toxicity above 1.6 to really make any firm statement.

2) In diving practice there is a real problem. If I have 9 min of deco 
to do at 6m with oxygen I am right at the 1.6 pO2 limit. I might be in 
sea water at 6m, in which case I will be some 3% over the 1.6 limit. In 
this case I will accrue ~ 125% of CNS toxicity during my 9 min deco 
stop. This is blatantly not helpful and probably not realistic: using 
the 1%- 4s yard stick does not contribute to a sane CNS% value during 
that particular deco stop.

Assuming that Subsurface should attempt to take real dive conditions 
into account in which the pO2 may well exceed 1.6, the question is how 
to handle such cases since this is an unknown domain. One approach could 
be to treat all pO2 values > 1.65 as if it was at 1.65. In cases with 
pO2 above 1.65 this results in an underestimate of CNS%, but in the case 
of my deco stop at 6m it would be immensely more helpful than using a 
1%-4-minute thumb-suck.

Do you have any opinion?

Kind regards,

willem




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