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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