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On Thu, 3 Nov 94 16:37:41 GMT Tom Wylie wrote:
> Does the Prism not contain a bottle of pure oxygen and if so,
> is it not the orrifice that the o2 flows through that dictates the
> depth you can go to and not the bottles...
No. Both bottles in the Prism contain Nitrox (or heliox...or trimix?).
The O2 breathed is metabolised & the diluent goes around again. Depleted gas
is replenished with mix. If this was fully-closed circuit you would end up
with
a hypoxic mix in the counterlung. This is avoided by the 'constant mass'
trickle feed that causes the counterlung to purge every minute or so...less
frequently at depth because of 'Boyle' and his Law.
The pre-mixed fill in the cylinders dictates the max dive depth, so you
must plan the dive BEFORE filling (or plan the dive depth around the mix you
already have). The Prism has an O2 sensor so you can see what's going on in
the
counterlung. It also has a CO2 indicator. I imagine the Cis Lunar has this
too...it seems to have everything else..!
If you manually turn down the Prism flow knob and carefully monitor the
O2 meter, you could adjust the ppO2 to less than the cylinder mix and in theory
go deeper than the ppO2 limit of the pre-mix. The dangers are going hypo-oxic
and getting an O2 hit (ouch!) if the flow rate goes too high. Alternatively
you
could go hyper-oxic and pass out if you have wound down the flow too far (not
too good either...!).
Also, if you need to bail out using the Prism, the set is simply turned
over to open circuit and the pre-mix breathed directly. I imagine that the
Cis-Lunar has problems here as you can't breath 100% O2 at depth nor 100% He at
ANY depth. You would need to rely on the triple redundancy of the Cis-Lunar
system not going wrong (sounds OK to me) or carry an extra bailout bottle of
'bottom mix' as well as the rebreather. Using the Prism with the ppO2 manually
adjusted below the pre-mix level, an open circuit bailout at depth could risk
an
O2 hit too.
The Prism is 'modular' so you can change the cylinder size, CO2 scrubber
size etc. It can also be converted to a fully closed circuit system, but I
have
no details of what is involved, what the safety systems are or what the costs
are.
>
> Or do you actually put nitrox bottles in it if so how do you get 6 hours..
You can get 6 hours out of the Prism by using large enough cylinders. I
think twin 7 litres would be enough (?). The size of cylinders selected
depends
not only on the planned dive time using the rebreather, but also on the amount
of gas you need to have on board to 'bail-out' in open circuit mode. Sufficient
to get to the surface, including all deco stops.
Another problem we were made aware of on the 1-day Prism course is that
of inadvertant exhalation through the nose. Some divers do this all the time
without realising it (as well as doing it to clear water from the mask). I know
I often do this to remove water continually leaking pass my moustache into the
mask. Your endurance time on any rebreather will go down RADIDLY if you do
this using a rebreather.
The time limitation seems to be the scrubber capacity rather than the
gas volume capacity. Using the Prism, you can select a suitably sized scrubber
for your set. The scrubber limits are
#1 - The active filler gets used up. This should be easy to calculate.
#2 - You let water into the scrubber by removing the mouthpiece
underwater without first turning the mouthpiece valve off. Using modern coated
scrubber granules, water ingress will not cause a 'caustic cocktail' to come
foaming up the hoses. Sodium or Lithium Hydroxide granules can be made very
resistant to water as long as no agitation occurs (unlikely in a well packed
scrubber container). However, water ingress will cause the inhalation or
exhalation resistance to increase to the point where breathing becomes
difficult
or impossible.
#3 - 'Tracking' of the gas mix through the scrubber occurs. This is a
settlement of the granules so that pathways through the scrubber form and
incomplete CO2 removal results. Your CO2 meter/detector should show this up as
well as your increased breathing rate.
There are MANY more techdivers out there on this BBS who know A LOT more
about all this than I do. Please correct any misconceptions I have and add
more
info. We are ALL interested in rebreathers....! Some more technical details
of
the Cis-Lunar here would be very welcome.
Regards, Steve M.
*******************************************************************************
* * *
* Dr. S. G. Millard * E-Mail : ec96@li*.ac*.uk*
*
* Department of Civil Engineering, * *
* University of Liverpool, * Tel : 051 794 5224 (UK) *
* PO Box 147, * 44 51 794 5224 *
* Liverpool L69 3BX, * (International) *
* UK. * *
* * Fax : 051 794 5218 (UK) *
* * 44 51 795 5218 *
* * (International) *
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