I was able to calibrate the carbon dioxide sensor earlier today. In the process, I also learned that the sensor is much simpler than I originally thought. For our purposes, the sensor will require a total of three connections:
a common ground
a source of between 6 and 12 VDC, at about 150 milliamps
an analog sink for between 0 and 4 VDC at very few milliamps of current
The 6 to 12 volts enables the onboard sensor heater and the output signal. The output signal is an analog voltage which is inversely related to the amount of carbon dioxide present at the sensor. In other words, as the amount of carbon dioxide increases, the output voltage decreases. The sensor appears fairly sensitive and responsive and seems to support a sample rate of up to twice per second. Based on these details, we should be able to both identify the changes in the peak carbon dioxide output of the subject and measure the time between the nth and (n + 1)th peaks, to infer an exhalation rate.
At 7 VDC input, the sensor draws a steady 150 milliamps. It also generates a fair amount of heat around the sensor housing. I'll have to take that into account when I mount it in the plastic snorkel tube.
I've also learned that the sensor output value responds very quickly to an increase in ambient CO2, but rather slowly when the CO2 source is removed. I'll need to see how useful it will be once the subject being measured starts breathing quickly. Instead of being able to measure the full range of CO2 output levels, we may instead only be able to determine the peaks