The functional principle of an oxygen probe is based on the comparison of the two oxygen partial pressures in two separate gaseous mediums. The zirconia, representing the measuring element, has faults in its crystal structure; some of the sites which could be occupied by oxygen ions are free. At a temperature above 700°C the oxygen ions start to migrate, which is a typical feature of this ceramic material. As a result, 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 probes 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 Econox oxygen probes, 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 measuring element should be replaced when its impedance exceeds the tolerances admitted.
Cleaning air is used on our CarboProbe ZI Pro and ZS Pro to allow cleaning of the zirconia measuring element with a flow of fresh air. Cleaning air is not available for the CarboProbe CP, DS or HT.