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> At greater depths, any CO2 buildup in the loop would, by necessity, rise to > the concentration allowed by depth - i.e. ppCO2. I'm not sure what you mean by "allowed by depth". At a given PCO2 the FCO2 will be lower at greater depth. This doesn't mean there are fewer molecules of CO2 in the loop - it just means there are more other molecules. In other words, the numerator stays the same for a given PCO2 at any depth; what changes is the denominator. Our bodies are concerned primarily with the numerator, not the denominator. > 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. With a given PCO2, the severity of symptoms should be essentially identical at any/all depths. PCO2 is not affected by depth, except when you ascend it drops; but it only drops because CO2 molecules are being vented (or, if the counterlung was infinitely expandable, it would drop because fewer & fewer CO2 molecules would be inhaled with each breath, because the total gs density is less). > 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. It's not just perceived - it's real. > 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. The body reacts to the "concentration" of CO2 in the blood. Another way to express concentration is partial pressure. The partial pressure in the blood is a function of the PCO2 in the inspired gas mixture, and the rate that metabolic processes are pumping more CO2 into the bloodstream. The increased PCO2 in the inspired breathing gas simply deminishes the rate at which CO2 leaves the body via the lungs. > 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. Yes, because the inspired PCO2 drops, so the rate that CO2 comes out of the body via the lungs increases, which means the concentration in the blood drops. > 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. Not sure I follow you here. Although PCO2 decreases on ascent, it does NOT increase on descent (unless there is CO2 in the diluent or O2 supply). Whether or not the PCO2 in the breathing loop increases depends only on the difference between the rate your body produces the stuff, and the rate your canister binds it. If the rate of production is greater than the rate at which the canister pulls it out, then you will have a net PCO2 in the loop. If the canister is not keeping up with your rate of production at a high workload, it might still keep up just find at a low workload. If a canister is keeping up with your workload on the surface, it should also keep up with you at depth (except for the fact that the canister is probably not working as efficiently at depth, but I don't think this is the variable you are talking about). > 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. Oh, I think it matters - I'm just not sure how the magnitude of how much it matters compares with the magnitude of how much workload matters. You could have a 6-8 fold change in the rate of CO2 production at the different exertion level extremes. I don't think the impact of canister efficiency at the depths we're looking at amounts to a 6-8 fold change. > 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. It essentially is. The caveat is that work of breathing at depth is greater due to increased gas density, so the amount of CO2 you produce just from breathing (part of the CO2 "overhead") is proportionally greater (leaving proportionally less left over for other effort). > 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? Probably partly because of increased workload due to increased gas density (& work of breathing), but mostly because of a decrease in the chemical binding efficiency of the absorbent at depth. Here's one way to think about it: Any CO2 that gets through the canister is getting through by not impacting with any particles of absorbent. At greater gas densities, more CO2 has the opportunity to slip passed, because more molecules are moving past at any given rate of breathing. This is a bit essoteric, but if you think about it, the number of molecules that pass through the channels between particles is proportional to volume, but the number of molecules impacting on particles is proportional to surface area. This means that as you increase ambient pressure and increase the density of the gas molecules, then the number of molecules impacting on the absorbent goes up with the square of the the pressure, while the number of molecules moving through the canister goes up with the cube of the pressure. It's after midnight, so I might be way off on this one - any physics/engineer types care to proof this? > 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. Don't ignore the USN guys - they know a HELLUVA lot more about this stuff than we do. I trust them. However, I think they are concerned with a bunch of black boxes, whereas we have the luxury of experimenting with our own bodies and figuring out how to optimize for our own physiologies. > 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. Yup, I know. It's hard for me to believe too - but I still believe it. Rich
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