Alan, Some issues to contemplate with using NITROX with an ambient PSUB: 1. Without constant exhaust to the water, the issues become much more complicated. 2. If you use a scrubber you now need to have an oxygen supply and a NITROX supply, the O2 to make up consumed oxygen and the NITROX to maintain your bubble volume in the PSUB with increases in depth. a. If you use a constant flow of oxygen, optimally this should be equivalent to your rate of consumption of O2 and is usually accomplished with an orifice. The problem is that the size of the orifice is very small and can be clogged easily. Rebreathers that in the past have used this method have been plagued with clogged orifices and the Navy had numerous warnings concerning this issue in their course “Principles of Navy Diving”. b. If you were to just add NITROX as the ambient atmosphere is scrubbed the partial pressure of nitrogen would continue to creep up. 3. If you just consume the O2 and let its partial pressure drop you are using the equivalent of a constantly changing O2 concentration in the NITROX instead of having a fixed mix and there are no decompression tables for this case. 4. If you are to depend on just a flow of NITROX without the addition of make-up oxygen, your optimized to the equivalent of discharging every fifth breath to the sea. The deeper you go, the more breaths you can go before venting to the sea. 5. Careful appraisal of the above issues would suggest that oxygen levels should be constantly monitored if you are planning long dives in your ambient NOTROX sub. Again falling back on rebreathers, 3 independent O2 monitors are employed and constantly compared. A further concern with oxygen monitoring cells is that extreme humidity can be an issue if not carefully placed. A very experienced technical diver and the lead engineer for a Navy rebreather project was pulled from the bottom of a pool during a test dead (no heart beat) but was fortunately revived…the oxygen sensors provided faulty readings due to high humidity. Needless to say there was a revision in the design of the rebreather. As Jon noted, many items that are not normally thought of as combustible, will ignite and burn vigorously under increased partial pressures of oxygen. If you deal with all of these issues, it would have probably been easier to build a dry sub. All of these issues have previously been explored in depth previously in the archives. R/Jay Resepectfully, Jay K. Jeffries Andros Is., Bahamas Save the whales, collect the whole set. -----Original Message----- Thanks for your responses. I've seen Phils white paper, but that system has a bellows at 1atm that responds to pressure changes in the cabin to release additional O2. It won't work in an ambient. I'll look into the fire risk a bit more. O2 doesn't combust but I'm not sure wether the flash point of a fuel will be lower with the higher % O2 or how much more ferocious a fire would be. I have a friend who is a dangerous goods expert that accompanies fire fighters to industrial fires as an adviser. I'll pick his brains & update you on that. Re the physiology; you just stick to the Nitrox no decompression dive tables. In general you get double the bottom time & your limits round 100ft with no decompression limit above 40ft. This idea was originally suggested to me by a pretty clued up guy who runs a dive shop & instructs in rebreather & mixed gasses. Alan |