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Hi All,

According to the PR, the odessy rebreather works by dumping a
proportion of gas overboard, equivalent to 1/5 of the surface
respiration, regardless of depth.  This lost gas is made up
by the action of an addition valve, which allows very hyperoxic
mix into the loop.  The idea is that the PPO2 remains almost
constant at some value which is set by the FO2 of the addition
gas, and it's rate of addition, against the rate of O2 metabolism,
and the rate of O2 dump overboard.

Addition of the hyperoxic mix depends on the collapse of the
counterlung, to trigger the addition valve.

I don't have figures for the size of the breathing loop (lung
volume + rebreather internal volume) but after some "back of
the envelope" calculations I came to an interesting conclusion.

If you have a breathing rate of 30 litres/minute (high) which
expels 6 surface litres/minute of loop gas from the system
(1/5P of 30, where P = The absolute pressure in bar) then
the loop volume is being maintained by the addition of 6 litres
of hyperoxic mix, plus a volume equal to the amount of oxygen
that has been metabolised.  (In most semiclosed systems this
is taken as 0.5-3 litres per minute, and 1.5 as the maximum
sustainable O2 consumption)

Hence you are adding about 7.5 litres/minute of hyperoxic mix
to the system.

However if you were to ascend, such that the loop gas expands
by 7.5 *surface* litres/minute or more, then *NO MIX WILL BE
ADDED TO THE SYSTEM*

Given that the loop volume is 10 litres, and the loop has the
highest possible non toxic mix of PPO2 = 1.4, then the loop can
only contain 14 litres of O2.  Given also that a certain percentage
of O2 is expeled from the system with each breath, and that
the O2 is being consumed at about 1.5 litres/minute, my figures
suggest that the O2 volume in the loop will fall to zero at
about 10 metres, during a slow (about 10m/minute) ascent from
50 metres.

This was with a breathing rate of 30 litres/minute (1.06 cuft/min).

I know lots of people with a consumption of half this.  A lower
breathing rate, and a lower ascent rate means that you could
run out of loop O2 even deeper.

1.4 is also *very* high PPO2 in a system that works like this.
Clearing your mask, or sneezing could lose enough gas from the
loop to spike you up to very dangerous levels.  I would expect
that the loop would be set to provide around the 0.7 mark.

Of course, 0.7 would provide only very little scope for swiming
around without any mix being added to the loop.  (About 5 minutes,
even if the system *weren't* dumping some gas overboard)

Can anyone poke holes in my logic?

Cheers Jason =:)

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