Peter,


> O2 analyzer? Atlantis/Dolphin ? These systems don't have a oxy gauge.

You are at least partly wrong here.  The oxygauge is available as an
accessory on the Dolphin! Take a look at the following 2 web pages.

http://www.draeger.com/english/st/DraegerDive/products/rebreather/dolphin/index.htm

http://www.draeger.com/english/st/DraegerDive/products/rebreather/dolphin/index.htm

The following is taken from the "Data and Facts" about the *DOLPHIN*...

Accessories:
                                   Compressed air bailout system,
sodalime DiveSorb,
                                   dosage device for 32/68, 4 L and 5 L
mixed gas cylinders,
                                   buoyancy bladder (for Dolphin T 52
200), Oxygauge

Notice that last little word there - Oxygauge...

The point is that with the Dolphin, when you are working hard your PPO2
drops.  You can NOT count on the added benefits of diving any nitrox
mixture.  When you take the course they give you a couple charts for
the various nozzles and show the inspired PPO2 for projected workloads.

I don't remember off the top of my head, but even with the 60% nozzle,
under a heavy workload you can REALLY drop the PPO2 - I think down to
somewhere around 32%.  So, these systems suck - you HAVE to plan the
dive with an MOD for the nozzle PPO2 and a nitrogen uptake according to
the *expected* workload using the EXPECTED inspired PPO2.

That's a GREAT unit (NOT).

Not to mention the FACT that if you DO run out of gas there is NO WAY
to tell. Sure, you MIGHT notice that your bag isn't burping, but then
again, you might not and then it's lights out ZZZzzzzzzzzzz.......
Good night Irene!!!

On a GOOD semi closed system like the Halcyon, which exhausts a fixed
fraction of breathe each time you will notice your breaths getting
shorter and therefore have time to react and switch gas or take
whatever emergency procedures you deem necessary.

Of course, the Halcyon is also depth compensated.  To put it
basically, the deeper you go the less gas gets added each time.
Think about doing a 100' dive (4 ATA) - that means that you only
have to add 1/4 the gas you would add at the surface, so you just
extended your gas time by a factor of 4.  Then add to the fact
that it's RMV driven (i.e. it only breathes when YOU do) and
the gas gets extended even more.

Carl