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On Thu, 9 Mar 1995, Scott Cherf wrote:
> At 1:12 AM 9/27/69, Richard Pyle wrote:
>
> >Yes, fully-closed systems have an
> >increased potential for problems of hypoxia and O2 toxicity (I'm not sure
> >I buy that they have an increased potential for hypercapnia).
>
> Why? I would have thought the reverse was true (in fact, I do think the
> reverse is true :).
>
> With a constant volume semi closed system, the FO2 is variable, which would
> make hypoxia or hyperoxia an inescapable fact if the unit is breathed outside
> a fixed depth envelope defined by the mix. Conversely, the fully closed
> systems you've mentioned (Phibian, Cis-Lunar, Bio-Marine) are all
> continuous blend devices with a operator configured PO2 setpoint. Unless
> the unit (or your brain) fails you can't go hypo/hyperoxic, no?
>
> Are you suggesting the semi closed units are safer *assuming the diver
> does not venture outside the mix envelope?* If not (if you're trying
> to argue they're safer across the board), I would think you'd need to
> consider the relative probabilities of a catastrophic failure of
> the gas blending system and an error in depth on the part of the operator.
> My gut tells me operator depth error is more likely to occur in practice.
First of all, I *don't* think semi-closed units are safer (that was the
point of my message). Yes, I was assuming the diver would not deliberately
venture outside the range allowable with gases used. I *COMPLETELY* agree
with you that operator error is FAR more likely a problem than rig
malfunction, both for fully closed and for semi-closed (I think this is the
*main* problem rebreathers have had all along). However, I was only
comparing rig designs, not considering operator variables. I think both
semi-closed and fully-closed are about equally complicated operationally
(I actually think semi-closed are a little more complicated to
operate...but since I have very little experience with semi-closed, I
can't say for sure).
The reason for my stating that fully-closed had a higher *potential* for
hypoxia/hyperoxia than semi-closed is because the replacement of
metabolized O2 on a fully closed rig is usually almost entirely dependant
on electronics (electronic sensors, solenoid, etc.), whereas the addition
of oxygen in semi-closed systems is mechanical. While such mechanical
systems are not immune to malfunction, the probability of mechanical
malfunction is usually much lower than the probability of electronic
malfunction (especially for constant mass-flow systems). Also, just
*having* pure oxygen in the system on a fully closed rig introduces the
possibility of hyperoxia (solenoid sticks open, sensors fail, etc.) On a
semi-closed rig, really the only way to run into hyperoxia problems is by
going too deep. The thing is, on fully-closed systems, you generally have
to rely on electical sensors to tell you the O2 content of the mix
(although smart designs have clever manual ways to control it); whereas in
semi-closed, fluctuations are pretty-much only a function of your
metabolism and depth.
I did *not* mean to imply that fully-closed rigs had a higher PROBABILITY
of hypoxia/hyperoxia ("probability" is different from "potential"),
although I guess I wasn't clear on that. With multiple sensors and
highly reliable (or redundant) electronics, I think the *probability* of
hypoxia/hyperoxia is no greater on fully-closed rigs than semi-closed.
Thanks for your comments,
Aloha,
Rich
deepreef@bi*.bi*.ha*.or*
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