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Rich, Here's what I believe is going on as regards CO2 at depth: First, lets assume you've got a diver with old, inefficient scrubber: At greater depths, any CO2 buildup in the loop would, by necessity, rise to the concentration allowed by depth - i.e. ppCO2. Since our bodies react to increased levels of CO2 in a variety of ways, including the "need to breathe" response, it seems logical to me to make the assumption that a higher ppCO2 would accentuate this response, making the perception of hypercapnia much greater at depth. Once an ascent is made, all the gasses present in the loop reduce their partial pressure, including CO2. By virtue of this, the perceived hypercapnia (notice the word "perceived") should diminish. This would be true, unless the body reacts to CO2 on a Ratio basis (i.e. the ratio of CO2 to other gasses present). This, I highly doubt, since it (the body) reacts with Oxygen based solely on partial pressure, not ratio. Therefore, someone like Mike, with a failing scrubber at depth, SHOULD feel some "relief" upon ascending, due to the decrease in ppCO2 in his loop. This would also explain how someone could breathe the rig on the boat - feel fine, decide to dive with the used scrubber, then run into problems at depth. You are absolutely right when you say that you don't think it has anything to do with an increase in the scrubber medium's ability under lower pressures to absorb CO2. I doubt that it matters much at all. This is an interesting question that I discussed with Rod a while back. The metabolism of O2 and the subsequent release of CO2 as a product of that metabolism should actuallly be INDEPENDENT of depth. In other words, you don't make MORE CO2 at 200 feet than you do at the surface. That is why I don't quite understand why the Navy rates the scrubber life by depth. Why would it matter? This is a good question for Dr. Wolov - I notice he's back on the list. Maybe he'd have an answer. As for the Navy dude stating that some of their tests indicate that you can pass out from CO2 buildup before noticing any symptoms of hypercapnia: I don't buy it. Maybe, like you said, some shalllow breathers out there could have it happen, but I did several tests, including the famous "Pyle Sit-on-your-couch-and-breathe-the-rig-with-no-scrubber-until-you-can't-stand -it" test, and I tell you, I KNOW what the symptoms are. It sucks really bad. Anyway. My $.02 Kevin.
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