In bioprocesses, oxygen is the most important gaseous substrate for microbial metabolism and carbon dioxide the most important gaseous metabolic product. One of the most critical factors in the operation of a bioreactor is the provision of an adequate gas exchange in the process. The O₂ and CO₂ concentrations are often monitored from the process exhaust air or used to ration the supply of nutrients to the bioreactor. Instruments used for monitoring purposes need to sustain harsh environments, be easy to maintain, and have minimum downtime.