Dave Talks About Rebreathers – Part 2, CO2

CO2 sensors are the next great break through in rebreathers, maybe. Scrubber media works until it doesn’t. That means the CO2 spikes rapidly when the scubber media begins to fail. CO2 sensors may be able to show the spike but can never be predictive. If the CO2 is spiking during the dive you are screwed. A much better paradigm is being conservative and not run the scubber media to it’s theoretical end point. Therefor a CO2 sensor is not as much value as it would appear on first blush. There is something called a temp stick used in a couple of rebreathers using axial canisters. The scubber media reacts on a plane within the canister generating heat. By tracking this heat you get an idea how much scubber media is left. A bit more predictive but not perfect correlation. Still left with being conservative and changing scubber media as the best method. A typical canister of scubber media will last about three hours depending on conditions.
As for a CO2 hit, the symptoms are headaches, rapid breathing then unconsciousness. When in doubt bailout.
Q: On open circuit, I breath very lightly, but it would seem to me that, that style of breathing would not lend itself to rebreather?
All rebreathers have unidirectional gas flow. This is done via flapper valves on either side of the bailout valve (BOV). Breathing type or rate is irrelevant to the movement of gas within the loop. There are some issues related to dwell time in the scubber media in extremely rapid breathing conditions. This would be a CO2 issue.
Q: Is it the outside pressure exerted on the counter lungs that forces your breath through the scrubber, and thus returning it to you?
As your lungs expand the counterlung collapses, as you exhale the counter lung expands, pressure on one cancels out the other. External pressure is not relevant, but gas density is however. Counterlungs are but one part of the loop. Work of breathing (WOB) is the resistance encountered. Commercial rebreather units have to have acceptable WOB else no one would buy them. Some units are better than others.