The functional principle of an oxygen probe is based upon the comparison of the two oxygen partial pressures in two separate gaseous mediums. The zirconia which represents the measuring element has faults in its crystal structure, i.e. some of the sites which could be occupied by oxygen ions are free. At a temperature above 1100°F (600°C) the oxygen ions start to migrate, which is a typical feature of this ceramic material. Thus, the measuring element becomes conductive. The voltage generated represents a relation between the relative difference of the oxygen concentrations and the temperature of the heat treatment atmosphere.
In order to compare the partial pressures, the oxygen probe must be supplied with a gas of a known content of oxygen (ambient air : % 02 = 20.9 %) this is what we call reference air. Moreover, some of the ECONOX oxygen probe are equipped with a thermocouple allowing measurement of the actual treatment temperature.
Thus, the user who knows the CO content either by analysis or by theoretical calculation and who takes into account the values measured by the oxygen probe is able to determine the carbon potential (and the oxygen concentration) in the heat treatment atmosphere.
The measuring element of an oxygen probe has a limited lifetime. Its surface layer changes depending on the impurities which are found in the heat treatment atmosphere. Therefore the user should replace the measuring element when its impedance exceeds the tolerances admitted.