I am looking for instrumentation to measure oxygen consumed by primates during exercise. We are unable to attach the device to the monkey so we will construct a containment vessel.
Two O2 sensors. One on the air inlet and one on the air outlet. Solve for the delta. Set it up so you can, with the flip of a lever, swap which is sensing which outlet. Then drift and calibration can be offset easily.
The inlet / outlet measurement also requires measuring the air flow rate. I suspect that the total error in the three signals and the associated calculations may be imprecise.