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> 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 To summarize problems with a "semi" CCR - they don't provide enough PPO2 when you need it most (i.e. when you're working hard). Steve went on to talk about adjusting the flow rate, but then you have a feedback loop to *closely* monitor. People on this list talk about having too high of a task load when you're switching regulators on a dive with twin independants - performing the tasks that Steve is describing is a *quantum* leap above that. I seriously doubt that they will teach this or than any diver with such a unit should attempt such a maneuver. > 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. First, if you've been down quite a while, *NO* bailout bottle or dilent tank that you're carrying will be enough to see you through a long deco schedule if the rebreather fails for some reason. Second, if you go into "bailout" mode with a prism there is a chance of getting a hyperoxic mixture at depth depending on the gas your carrying. Third, since you're only carrying one type of gas, the gas must be breathable at the surface (i.e. must have sufficient PPO2 to maintain life, and the ability to maintain consciousness would be good too). Regarding the CIS-Lunar unit. This is not entirely true. The "diluent" used in the CIS-Lunar unit does *not* have to be a pure helium mixture. I think that normally, for deep dives, using some type of heliox or even tri-mix would be best. I believe standard tank sizes on the CIS-Lunar unit are 13 cu ft for the O2 and 30 cu ft for the diluent, however, it's very easy on this unit to add offboard gas If I had to carry some type of "bailout" bottle with the CIS Lunar unit it would depend greatly on the type of dive and amount of surface support that I have. I just don't forsee people going out and doing 600' dives off of their Zodiac! This is not what they're meant for! Depending on the amount of surface support available (are there emergency hang bottles) I would pick an appropriate mixture that is hyperoxic at the bottom but hypoxic at the surface. With a mix like this you get off the bottom and head up as soon and fast as reasonably possible. I would most likely err on the side of avoiding hyperoxic on the bottom because if the rebreather gives out and I'm on a deco schedule, there better be backup bottles and plenty of them (or a backup rebreather somewhere) for the deco!!! If the dive isn't too deep of a dive, the mix can avoid being hyperoxic at the bottom *or* hypoxic at the surface and, for shallow dives, the diluent would most likely be some type of nitrox mixture!!! > 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.
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