Semi-closed rebreathers are likely to be used by the advanced recreational
diving community in some numbers. The tech community may be a better
source of info on SCR's than the manufacturers, who have traditionally
made rebreathers for the military. The safety performance equation needs
to be pushed hard in direction of safety rather than performance, if the
whole concept is not to get bad press and poor acceptance from
recreational diving enthusiasts, to say nothing of potential injuries.

So here is my list of physiologic requirements for semi-closed, fixed
flow, rebreathers - the type most likely to be used in the next year or
so. e.g., Atlantis, Phibian, Prism. These are my minimum safety
parameters for the advanced recreational diving range (0 - 150 fsw). The
list should not be regarded as either complete or a recipe for diving
SCRs. Please add your own and/or debate these suggested parameters.

1.  The SCR must be capable of supplying 2 litres O2/min, whatever the gas
(nitrox mix) used, at all operational depths, particularly swimming at or
near the surface.

Yes, that's a high oxygen consumption, but that's close to what George III
can probably do and will, fighting a 3 knot current.

2.  The pPO2 at the surface under maximal use (2L O2/min) has to be at
least 0.20 ATA

The margin between 0.20 and 0.16 (poor brain performance) is narrow enough
for most of us as it is! Let's not prove the point that divers do it
dumber.

3.  The MOD of the unit is the MOD of the gas (nitrox) supply, using a
maximal pPO2 of 1.4 ATA.

No self respecting techhead will accept a limitation of a pPO2 of 1.4 ATA.
But PADI and NAUI do and these are safety parameters for recreational
users. Maybe we should go with 1.3 ATA? My rationale for not doing so is
that the actual in-use ATA will be less than the MOD for the nitrox gas,
because it's diluted.

4.  The EAD is calculated by assuming the FiO2 the at the MOD is
calculated by subtracting *maximal* O2 consumption (2L O2/min)
from the operational FiO2 (i.e., not including the contribution of any
override) at the MOD, with all remaining gas considered nitrogen.

So, now the real truth is out. Your EAD will actually be a lot deeper than
you would have predicted from the equation for your nitrox used on open
scuba. You're not just breathing from that nice little yellow bottle,
your breathing your own nitrogen too from the bag. So your NDLs will be
*shorter*.

5.  The SCR must deliver a volume of gas at the MOD that exceeds the
minute volume of the user by at least 10%.

Others may disagree, but I think this is *minimal* to blow off CO2.

6.  The SCR must be capable of maintaining a volume of gas (may use an
override) that permits a tidal volume of 2L during a descent at a rate of
50 ft/min.

Only (small) elephants have a tidal volume of 2L, but I think 100%
overbuilt is not to much to expect to prevent people panicking. Anyway
this is supposed to be a *re*breather - it shouldn't be too hard.

7.  During ascent at 30ft/min, the pPO2 must not fall below 0.20 ATA nor
available O2 fall below 2L O2/min (may use an override).

This is probably where people will die - bolting for the surface: If you
go up much faster than this from 150 fsw with a pPO2 < 0.20, you'll arrive
at the surface unconscious but alive.

8. If the SCR's MOD is deeper than 66 fsw, a bail-out system capable of
sustaining the diver at 15 ft for 20 mins must be available.

This is nice - but I could be persuaded that the system just has to
operate as open scuba. The problem is that people are going to SCRs to
extend their bottom time and will find themselves (I believe) more
frequently in a deco dive than they plan - even if only because the nitrox
will be more nitrogen and less oxygen than they thought.

9. The manufacturer must specify the maximal descent and ascent rates
under which the SCR will perform as noted above.

What are we? Crash test dummies - or just dummies? I already know your
answer George III.

10. Given that pPO2 meters may not be accurate under in-water use, (an
understatement if ever there was one!) depth, ascent and descent alarms
should be used with the SCR.

11. The effort of work breathing should not result in a positive end
expiratory pressure (PEEP) of greater than 3 cms of water.

You might consider specifying the cracking effort too - as with
regulators. However PEEP will tire the heck out of you and does weird
things to gas exchange and may (consequently) significantly increase O2
lung toxicity.

How about the instructors have to have done more with SCRs than just
have seen one demo'd at DEMA! (i.e., see one, dive one, teach one)?

"Make it idiot proof and someone will make a better idiot"

Peter Heseltine

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