Zirconium Oxide Oxygen Sensors

This type of sensor is occasionally referred to as the “high temperature” electrochemical sensor and is based on the Nernst principle [W. H. Nernst (1864-1941)]. Zirconium oxide sensors use a solid state electrolyte typically fabricated from zirconium oxide stabilized with yttrium oxide. The zirconium oxide probe is plated on opposing sides with platinum which serves as the sensor electrodes. For a zirconium oxide sensor to operate properly, it must be heated to approximately 650 degrees Centigrade. At this temperature, on a molecular basis, the zirconium lattice becomes porous, allowing the movement of oxygen ions from a higher concentration of oxygen to a lower one, based on the partial pressure of oxygen. To create this partial pressure differential, one electrode is usually exposed to air (20.9% oxygen) while the other electrode is exposed to the sample gas. The movement of oxygen ions across the zirconium oxide produces a voltage between the two electrodes, the magnitude of which is based on the oxygen partial pressure differential created by the reference gas and sample gas. The zirconium oxide oxygen sensor exhibits excellent response time characteristics. Another virtue is that the same sensor can be used to measure 100% oxygen, as well as parts per billion concentrations. Due to the high temperatures of operation, the life of the sensor can be shortened by on/off operation. The coefficients of expansions associated with the materials of construction are such that the constant heating and cooling often causes “sensor fatigue”. A major limitation of zirconium oxide oxygen sensors is their unsuitability for trace oxygen measurements when reducing gases (hydrocarbons of any species, hydrogen, and carbon monoxide) are present in the sample gas. At operating temperatures of 650 degrees Centigrade, the reducing gases will react with the oxygen, consuming it prior to measurement thus producing a lower than actual oxygen reading. The magnitude of the error is proportional to the concentration of reducing gas. Zirconium oxide oxygen sensors are the “defacto standard” for in-situ combustion control applications.

Other types of oxygen measuring techniques are under development and in some cases being used for specific applications. They include, but are not limited to, luminescence polarization, opto-chemical sensors, laser gas sensors, et al. As these techniques are further developed and improved, they may represent viable alternatives to the major oxygen sensor types currently in use.