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> Richard, I missed something somewhere in there. > > It is my understanding that a fully-closed-circuit rebreather has to > compensate for respiration of oxygen into carbon dioxide and for ascents > and descents. I sort of had the idea somewhere that the former required an > oxygen sensor, because you (a) don't want to wind up with a counterlung > full of diluent and you (b) don't want to push your ppO2 over 1.4 ATA or > so. > > How does the MkV get around this? Or what is it doing that it can be said > that it is not dependent on electronics? It's not just the Mk-5 - this is true of most fully closed rebreathers. The advantages of the Mk5 are the things that reduce the probability of ever experiencing a total electronics failure. I could spend megabytes explaining the series of recovery options that take place between a totally functioning rig and a total loss of all electronics. But for the interest of brevity, let's cut straight to the big one - total electronics failure (including all three sensors). The naive person will say you have no idea what's in the breathing loop, so you'll probably soon die of O2 toxicity or hypoxia. But you CAN know, at least within some margin of error, what the mixture is. Here's what you've got to work with: - At least two known gas mixtures - oxygen and a diluent (the latter has a known fraction of oxygen in it and is breathable at maximum depth of the dive). - Two known loop volumes - The volume when the counterlungs are bottomed out on a full inhalation (we'll call this "low", and the volume when the counterlungs are fully inflated on full exhalation (we'll call this "high"). For most rebreathers, the "low" volume is more than half of the "high" volume. - A rough idea of how much O2 you need to manually inject to keep the PO2 within breathable ranges during a direct ascent (you've worked this out during your many, many practice dives). Therefore, you have access to at least four breathing gases: 1) 100% diluent 2) A mix of 100% diluent when the volume is "low", plus the amount of oxygen it takes to make the volume "high". 3) A mix of 100% oxygen when the volume is "low", plus the amount of diluent it takes to make the volume "high". 4) 100% oxygen. Given these, plus the fact that you know the only loss to loop volume not associated with depth change is due to O2 metabolism, you can farily easily (with a lot of practice - i.e., a well-programmed brain) get yourself out of any situation. Here's an example: You're on a decompression dive and you loose all of your O2 sensors. You know the mix was breathable when you lost the electronics, so you start your abort. If you're in a cave and have to swim horizontally for a long distance, you simply maintain the loop volume at whatever it presently is by adding O2 manually. If you're in open water and begin your ascent, then you periodically add O2 manually to keep the O2 with a safe window (it's a pretty damn big window, when you think about it). When you arrive at your first decompression stop, you have four mixtures to choose from (described above). If it's Mix 1, you just flush the loop with diluent and maintain te loop volume with O2 injections. If it's mix 2, then you flush the loop with diluent, then drop the loop volume to "low" (counterlungs bottoming out on full inhalation), then fill the counterlungs with oxygen until they are totaly full on full exhalation, then allow the gases to mix for a few breaths, then vent off loop gas until the total volume is comfortable to breathe. The you replace any drop in volume over time with manual O2 injections. You follow your decompression stops (Open circuit decompression schedules, based on your four known mixes that you calculated in advance with some extra conservatism for the innaccuracies of your on-the-fly blending) until the depth is shallow enough to switch to mix 3. Then keep following 'till you can switch to 100% oxygen. Please spare me the arguments that this is too much task loading in a stressful situation, blah, blah, blah....it's not as difficult as you might think (if you've been a good boy or girl and have practiced practiced practiced), and if it's still too much task loading, then your cranial processor is probably not powerful enough (or there's a bug in the programming) to be rebreather diving in the first place. As intellectually stimulating as all this is, it has virtually no practical value for a well-designed rebreather - because it is INCREDIBLY unlikely that it will ever come to this. In 160+ hours of rebreather diving, I have had a total of 2 sensor failures - both were manufacturing defects, so both were evident the moment I got below 10 feet - a very simple dive abort. But let's say I've been lucky. Let's say the probability of an O2 sensor failure during the dive (I've never had one like this) is one in 50 hours. With three sensors, that means the probability of losing all three is is one in 125,000 hours of diving. But it will never come to this because the minute the first one fails, you abort the dive. O.K., I've wasted enough bandwidth for now....
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