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>Maybe 20 years from now, for tri-mix dives we'll have computer controlled
>servo-valves that dynamically mix gases from 3 or 4 bottles on your back,
>feed them into a semi-closed-circuit rebreather, servo-controlled valve from
>an argon bottle into the dry-suit, and we'll have 12 hour bottom times
>at 600fsw.  ???

Why 20 years.  The CIS Lunar unit is currently available.  It should
be able to be used down to somewhere in the 300+ ft range (I can understand
why a mfg wouldn't want to really quote a "max" depth for a unit such
as this) for 6 hrs or so.  If you carry more offboard gas and perhaps
reconfigure
the unit with a larger scrubber pack, your desired dive would
theoretically be possible with this unit.  Of course, how much diluent you
use is dependant on how often you clear your mask, how much of a "sawtooth"
profile you dive, etc etc etc.  The aquaCorps C2 issue had an article that
stated that a "working" diver uses roughly 3 cu ft of O2/hour - independant
of depth (makes sense) so O2 usage is easy to figure.

I believe it is relatively easy to hook up an offboard gas supply(ies) to 
this unit, btw.  Current standard diving practice is that the diluent gas
*should* be able to sustain life at the surface (or some very shallow
depth) although with this type of extreme diving, you'll have suface
support (no swimming offshore with your rebreather and diving down into
the abyss!!! - sounds obvious but you know people these days!!!), hang bottles, 
etc so I don't think that's quite as important an issue - for this type of 
diving you could have a diluent that is pure helium, He/H2 mix, (or whatever 
other inert gas you choose).  There will be many other logictical problems
on a 6 hour dive to 600' - food, calories, hydration, not to mention exposure,
other normal biological functions - both #1 and #2 :^), and probably some
other physiological problems we haven't even considered yet.

I think one of the big uses for rebreathers like this (i.e. full CC) is the
underwater videography aspect - *long* times at shallow (i.e. <100')
depths.

-Carl-

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