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Not to take everyone's attention away from scooters and sharks, but... The other day I breathed my sofnolime scrubber blue. (It was really quit attractive when I poured it out of the canister.) It occurred four hours short (8hrs.) of the life of the last canister fill (10hrs.). With the last canister fill I was relatively inactive and shallow, 80ft., preferring to pay more attention to the rig than to what was going on around me. Immediately after refilling the scrubber I began to pick up the pace by diving deeper, 140ft., and staying through longer deco's. I also began to follow sharks around and generally get involved with the marine life, something I find unique to rebreather diving. On the last dive, 4 hrs earlier than my previous scrubber change and 2 hours earlier than I had planned to change it, I developed a slight headache at 130ft. while hovering in blue water watching for a whaleshark (sans scooter). The headache was similar to those persistent headaches you get from skip breathing and I took this as a hint to get someplace safer just in case. I went OC to about 30ft., then decided to play it out on CC to see if any more symptoms would manifest themselves. None did. I then swam hard for a minute in a straight line paying very close attention to any change in me or the air I was breathing, OC reg in hand. Nothing else. I stayed CC and surfaced to get picked up. Later when checking the canister I discovered the color change. 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) 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. 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? Mike
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