Using pH CO 2 in equilibrium with a solution. Difficult to get adequate speed for breath by breath analysis. Oxygen Polarographic Noble metal electrode, usually platinum, made cathode in a solution in equilibrium with the gas. Higher voltages produce H 2 . Voltage current curve shows a plateau region where current is determined by oxygen availability.
Main problem is variability of oxygen availability with stirring or movement of the electrode.
If the electrode is made very small, then the surface layer does not move. If the electrode is coated with a permeant layer, this defines the diffusion distance. Either a membrane over an electrode, or a recessed electrode. Thicker undisturbed layers lead to slower response times. Oxygen reading instruments simply monitor the current output of the sensor.