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I've been doing consistent repetitive o/w diving on nitrox at recreational
depths (90-110') for the past several years, using an Oceanic nitrox
computer. Basic stuff--no technical questions here, right?
However, during a two-day series of four one-hour 100FSW dives that were
completed within 24 hrs. (dove late Sat. and early Sun.), the computer
displayed an O2 violation mid-way through the fourth dive. So I started
researching. After exhausting known resources (Oceanic has so far not
responded to my polite queries), I return here, hoping for a correct, or at
least an enlightened answer.
The most fundamental guidelines published for CNS-tox are the NOAA per-dive
and 24 hr. exposure limits, too crude to even be considered a "model". I've
come across a table published by IANTD that I think is based on Dr.
Hamilton's research and uses 90-min. half-times to predict the decay of
previous CNS O2 exposure effects over interval time. It indicates that
after just a 6 hr. interval, the residual effect of previous CNS exposures
has diminished to just 7% of the starting value (i.e., the CNS clock value
at the *beginning* of the surface interval).
I'm seeking a definitive explanation of the decay or diminishing effect of
the CNS exposure over varying length surface intervals and multi-day
repetitive diving.
My dive computer typically ends dive #2 Saturday afternoon with 50 to 60% of
the O2 clock "consumed". The following day, 18 or more hours later, dive #3
begins at the same 50 to 60%. Bullshit! I don't believe that's reflective
of physiology!
I've purchased Abyss 2.0 and used it to model a series of four dives over
24+ hrs. and, surprisingly, I get the same result: "CNS clock exceeds 100%"
on what I think is an O2-conservative mix and profile (PO2max for all four
dives is just 1.27ATM). For the entire 24 hr. surface interval between
dives #2 and #3, the CNS clock remains the same!
Repetitive profile used for Abyss 2.0:
-------------------------------------
(hypothetical, for purposes of analyzing O2 exposure only!)
O2/N2 = 34%/66%
SI 1:15 SI 24:00 SI 1:15
___ _____ ___
| #1 ____/ | #2 ____/ | #3 ____/ | #4 ___/
| / 10' | / 10' | / 10' | / 10'
| / :12 | / :34 | / :18 | / :35
|____/ | |____/ | |____/ | |____/ |
90' | 90' | 90' | 90' |
:50 | :50 | | :50 | :50 |
| | | | |
CNS O2 --> 27% 55% 55% 83% 111%
Clock !!!
(from Abyss output)
Any guesses as to why the O2 clock remains at 55%, after a 24-hr. surface
interval?
I'll be willing to accept that my "recreational" dive computer has a flawed
(simplistic and/or ultra-conservative) O2 algorithm, but I'm not going to
violate it or replace it until I understand the theory and/or physiology
much better than I do now. We've purchased the books "Diving Physiology in
Plain English" and Tom Mount's "Technical Diver Encyclopedia" which are
interesting and helpful, but they've not completely illuminated the nature
of, nor the reason for the differences between the NOAA CNS guidelines, the
behavior of both Abyss and my dive computer, and the "half-time" table
included in Mount's book. They all seem to contradict significantly.
Who's right? Where else to look?
Thanks in advance,
tj
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