Peter, you've just verfified what we have been saying all along. I own a
BioMarine CCR155 (the $15K unit) and dive it. The CCR500 is a downsized
version of the 155. $5000 for a fully closed circuit rebreather like the
CCR500 is a deal. For the divers who have been clamoring for a rebreather
that is relatively affordable, it is time to put up or shut up. These
units are not going to get much cheaper. I should warn you that you and 
the list will likely bombarded with posts about how having a more complex 
unit like the CCR500 means that there are more components to worry about 
failing, like the O2 sensors and the electronics. Go to the tech diver 
archives and drag out all the documents relating to this- try spring 
1996. Thanks for the post. I'll copy it to Dick King. Rod 

On Mon, 24 Jun 1996, Peter N.R. Heseltine wrote:

> 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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