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> Richard you state
>
> "Looking only at decompression math, that diver
> should have a higher probability of DCI due to the additional ongasing
> that occurs during this 100fsw stop (assuming he/she did not add that time
> to their total bottom time)."
> 
> But if there is a pressure gradient at that point (and not equilibrium), then
> there will not be ongasing as the gradient will be in the opposite direction.
>
>Ian

Whether or not there is a net movement of nitrogen molecules from the
blood to the alveoli, or from the alveoli to the blood (i.e., whether
there would be a net off-gasing or a net on-gasing) during a "deep stop"
depends on the degree to which the diver's tissues are saturated with
nitrogen.  If the average tissue saturation is relatively high (i.e., if
it had been a very long dive), then yes, there would be a net offgasing
even at a deep stop (assuming that the deep stop is still onsiderably
shallower than the depth at which most of the dive was spent).  However,
if on a "bounce"-type dive (say, 15-20 minutes at 250fsw), then the
average tissue saturation may be somewhat lower than the PN2 in the lungs
at a given "deep stop", in which case the net movement of N2 is from the
blood to the tissues.  Blood returning from the tissues to the lungs in
this case would have less dissolved PN2 in it than the PN2 in the lungs
(because it would be at equilibrium with average tissue saturation), so
there would be a net movement of N2 from the lungs to the blood (i.e.,
ongasing).

But even if there *is* a net offgasing during a deep stop, this can still
adversely affect decompression (looking at it only from a Haldanian-based
math perspective) because the gradient during a deep stop is not as great
as the average gradient would have been on a direct ascent to the first
*required* decompression stop.  Unless the deep stop time is added to the
total bottom time (a conservative approach), then the total
ongasing/offgasing ratio is biased towards the ongasing side (looking only
at the decompression algorithm) if deep stops are included on the dive, but
not included in the decompression calculations.

Did I sufficiently confuse the issue?

Aloha,
Rich

deepreef@bi*.bi*.ha*.or*

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