Some materials have, at high temperature, ionic conductivity; among these the zirconium oxide, yttrium stabilized, shows a high mobility of Oxygen ions and can be used as a solid electrolyte. The galvanic chain constituted by two films of porous platinum (electrodes) deposited on the ceramic material (solid electrolyte) can be configured:
where p1 and p2 represent the oxygen partial pressure in the reference and measure compartments. The e.m.f. of the cell which is generated by the different oxygen partial pressure in the reference and measure compartment is in accordance to the following equation:
With constant temperature and reference partial pressure, the e.m.f. of the concentration cell depends only on the oxygen partial pressure of the gas to be analyzed. Assuming the pressure of the reference and measure compartment to be atmospheric pressure, the oxygen partial pressure can be associated to the percentage of Oxygen to be measured. The error due to the pressure of the plant (different from the atmospheric one) is usually negligible (0,13% of theoretical value per mm of Hq) and can be adjusted using the ZERO regulation of the electronic unit.