Dive planner CNS calculations
Willem Ferguson
willemferguson at zoology.up.ac.za
Thu Nov 8 22:08:53 PST 2018
There are some difficulties with calculating CNS% values de novo.
Attached is a document by Eric Baker: I have no idea when or where this
was published, probably early 1990s. It gives information for
calculating CNS% as well as OTUs for dives. In both cases the ultimate
sources appear to be NOAA documents in the late 1980s.
As far as OTUs are concerned the case is a bit simpler. I implemented
this for a few simple dive profiles and it appears that Subsurface
overestimates the pulmonary toxicity (OTU) count for a specific dive.
As far as CNS% calculations are concerned, I am not sure that the Baker
publication is correct. The CNS calculations include a lookup table of
slopes and intercepts to be used at different PPO2 levels. The
intercepts represent the max dive duration (minutes) at fixed PPO2
values, the slope parameter allowing for interpolation between fixed
PPO2 values. However, the lookup table in the Baker publication applies
to OTU calculations, *not* CNS calculations. The last line in the lookup
table is a total outlier with no apparent relevance. This appears to be
a gross error in the Baker publication. The example FORTRAN code (the
DATA block) that follows this appears to use this wrong (OTU)
information for calculating CNS% values. I digitised the graph from the
NOAA diving manual, presented as Figure 2 in the Baker document and
created a lookup table of intercepts and slopes for use in CNS%
calculations (this graph was presented in one version of the NOAA diving
manual but has been dropped since). Using these digitised values I
obtain realistic values of CNS% for some simple dives that broadly agree
with both the dive planner in Subsurface as well as with Multideco.
Obviously the calculation of CNS% is problematic and I would not like to
use these calculations in attempting to benchmark the CNS% calculations
in Subsurface.
Does anyone have any better information?
Kind regards,
willem
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