Figure 1: Schematic structure of a potentiometric planar oxygen sensor. (A) Measuring atmosphere. (B) Porous protective layer. This layer is optional as it prevents from electrode degradation and sometimes it improves the sensor response due to gas species diffusion. (C) Measuring electrode, catalytically active, generally made of platinum. (D) O2- ion-conductive ceramic electrolyte. The most used electrolyte is Y2O3-doped ZrO2 (YSZ). (E) Reference electrode, in contact with (F) Air reference atmosphere. Some sensors have been occasionally described without a reference atmosphere. Also, a heating element (a dissipative low resistance) is optionally put in front of this sensor structure to achieve stable, high enough temperatures.