Endura AZ20 series probe
Combustion oxygen monitor Appendix A – Principle of Operation
IM/AZ20P–EN Rev. G 29
Appendix A – Principle of Operation
The Endura AZ20 probe's zirconia cell is a thimble-shaped sensing element fitted with inner and outer electrodes at its closed end.
The inner electrode is exposed to the flue gas entering the open end of the cell; the outer electrode is supplied with reference air from
a pump or regulator and is therefore exposed to a constant partial pressure of oxygen (20.95 % O
2). The cell is held at a constant
700°C (1292 °F) by a heater and control thermocouple.
Because zirconia is an electrolyte that conducts only oxygen ions at temperatures in excess of 600°C (1112 °F), the voltage
generated between the electrodes (the cell output) is a function of the ratio of the oxygen partial pressure difference between the
reference electrode and the measuring electrode and its temperature. Therefore, any change in the oxygen partial pressure of the flue
gas at the exposed electrode produces a change in the cell output voltage as dictated by the Nernst equation.
Cell output voltage increases logarithmically with decreasing oxygen, thus providing high sensitivity at low oxygen levels.
Fig. A.1 Endura AZ20 Probe Construction
High Temperature Seal
Flue
Gas
Insulator Heater Element (700°C [1292 °F])
Reference Air
Diffuser / Flame Arrester Test Gas Control Thermocouple 'K'
Cell Output
– Ve + Ve
Cell
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