I had a look at the Biomarine CCR500 this weekend at the SCUBA96 show in
Long Beach CA. It seems to me, after spending more than three hours over
two days grilling various and sundry on all aspects, as well as pulling it
apart and putting it together, that this machine and the concept have a
great deal more to offer, not only in performance, but also in safety than
the Drager/Uwatec Atlantis I and the Fieno, both of which were present at
the show (the Fieno was there incognito).

In all our exchanges about CNS O2 toxicity, we come back to the concept
that, while there is likely a proportional relationship realtionship
between exposure duration and pPO2 at 1.6 ATA and higher, there is no
sound way to predict who will take a CNS "hit" at pPO2s between 1.3 ATA
and 1.6, and while these are rare, they can be fatal. (I call this Brett's
Dilemma) See the DAN 94 death stats for a diver who switched to 100%
between 15 - 20 fsw and almost immediately had a witnessed convulsion and
was lost.) This reminded me that it works both ways - the "off O2 effect".
Also CO2 retention, as measured by arterial CO2, rises progressively and
almost in a straight line when breathing O2 at 2.0 ATA.

Clark's chapter in The Physiology and Medicine of Diving (3rd ed) by
Bennett & Elliot, also relates a variety of other organ toxicities,
including myopia (!), but concludes that at low pPO2s <1.6 ATA, CNS
sensitivity to O2 toxicity varies as much for the same individual, from
day-to-day as it does for a large group studied. I read this as - you
can't predict an CNS O2 hit for an individual diver and just because the
diver was at 1.6 ATA yesterday without incident doesn't mean he won't take
a hit today at 1.4 ATA. The prudent choice is to keep the working pPO2
below 1.4 and/or do as the commercial divers and wear a full face-mask.

Another choice is to dive a system that maintains your pPO2 where you want
it. This gives you the benefit of nitrox without the disadvantages of a
significantly limiting MOD (say 95 fsw for EAN36). To dive the fixed mass
flow SCR units like the Atlantis, you have to limit your depth to the MOD
of the nitrox mix (60 fsw for EAN50 in the current Atlatic config, with
*maybe* a few fsw leeway because your FiO2 will be lower) and your NDL are
based on the worse case scenario that if you actually consume O2 at any
significant rate, you are diving close to air. So right there you have all
the disadvantages of nitrox and few of the advantages.

However, with a variable addition of O2 and diluent, based on realtime
measurement of pPO2, you don't have a (theoretic) MOD and your NDL is
dictated by the EAD of the system. Usually the FiG is the fixed part of
the equation on a dive; not true if your pPO2 is now the constant in the
equation. But even if you set your pPO2 at a conservative 1.0 ATA, not
until you go below 130 fsw (US sport diving recommended limits), will the
FiN2 rise above 80%, increasing your EAD.

The CCR500 seems to do all this: Two separate systems measure pPO2 from
three different sensors average the readings or throw out a reading that's
very anomalous. Two readouts present the pPO2 in different ways, analog
and digital. The sensors are mounted *before* the O2 and diluents are
added, so that it can correct the pPO2 *before* you breathe it. You might
not be able to read the displays that indicate you have just *taken* two
or three breaths of pPO2 0.16 mixture. The solenoids that add gas can
*only* fail in the closed position. You are then alerted to the falling
pPO2 by the displays (I would like a sonic alert on mine, but the light
array display is OK) and you can then add the O2 by pushing a manual
bypass button or bail out. Key to not being able to outbreathe the unit is
that the O2 addition is at 5 L/min. The only difference between the
military unit ($15,000) and CCR500 (~$4000) is the size of the scrubber
and the two internally coated luxifer 15 cf gas cylinders. These replace
the exotic non-magnetic metal gas spheres in the military unit. The
electronics, counterlung, plumbing, solenoids etc. are all mil spec.
Because of the extensive military and commercial experience, they have
time-to-failure data on all critical components and most others.

When I get a chance to dive one at the end of July, I'll report more. But
this one is the first I've seen that makes physiologic sense for us
non-marine mammals.

Safe diving through happier physiology,

Peter Heseltine

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