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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