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Hi Mike, > My tentative conclusions are: 1. There are subtle symptoms associated with > hypercapnia if you are paying attention and not absorbed elsewhere (Rich > concurred, others disagree) I believe that I can detect subtle symptoms of hypercapnia, because they consistently are evident shortly before experiencing less-subtle symptoms of hypercapnia on dives where I deliberately push a canister well-beyond its rated limit. However, I'm not so sure what you were experiencing was subtle hypercapnia - at least if it was, I don't think it was a result of a failing scrubber. Have you noticed that your breathinbg pattern has changed on a rebreather? When I first started using the 'breather, I habitually breathed in scuba-diver fasion; i.e., slow deep inhale, hold it for a few seconds, slow deep exhale. It occurred to me early on that there is no reason to do this on a rebreather, because it doesn't save anything. If you still breathe like a scuba diver, then you might have actually induced your own hypercapnia headache by skip-breathing. Force of habit tends to make one exacerbate skip-breathing on deeper, low exertion dives (at least in my experience). > 2. Moving shallow removed noticeable > symptoms possibly because going on OC gave the scrubber time to absorb the > CO2 in the loop. Also there would be less CO2 moving across the scrubber > (reduced PPCO2) and thus the partially exhausted scrubber was able to keep > up. I doubt it. The sure-fire test I use to see if my scrubber is really beginning to fail is a burst of exertion. If I feel shortness of breath and a feeling of being slightly starved for air, then I know the scrubber is going. Because you felt no other symptoms in the shallow water during hard exertion, I have a hunch that if your headache was hypercapnia-induced, it wasn't from a failing scrubber. Also, as I have said before, the color indicator is not reliable. > My conclusion is: in an emergency, it may be possible to use OC to get to > shallow water and rest the scrubber, then go back on CC to take advantage > of the enriched air offered by a high set point (1.2 on my rig) for > decompression. Of course you would probably be better off skipping the > deep stop in favor of surviving the dive. > > Comments anyone? That's a tough call. By definition, in an emergency on a rebreather you will abort your dive. Unless you are in a cave or wreck that requires you increase depth before exiting, then you shouldn't need anymore diluent to operate the rebreather for the rest of the dive. Thus, your diluent is available for as much OC bailout as you can get out of it. I would be more comfortable skipping the deep stops if I knew there was boku O2 available for the shallow stops (to allow time to treat the damage done from the fast deep ascent, if indeed bubbles are at play). If you suggest that a canister will give you more hours in shallow water than in deep water, I would be skeptical. I have no doubt that absorbent efficiency is affected by pressure, but I do doubt that the difference in efficiency at depth vs. shallow is enough to significantly affect the operational time of the rebreather, if you're only considering the short period of time you're on OC. Incidentally, if indeed your PCO2 was high in the loop (as opposed to just in your body), then the amount of CO2 absorbed by the canister during your OC ascent would be tiny compared to the amount of CO2 vented from the loop due to overexpansion. If you lock in a certain PCO2 in the loop at depth, then ascend on OC while keeping the loop volume relatively constant, the PCO2 in the loop will be much lower by the time you get to shallow water not because of absorbed CO2, but because of vented CO2. Rich
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