Oxygen Probes

MAXWELL Oxygen Probe is an entirely new method of measuring oxygen in hot gases. Its construction is in the form of a probe which has extremely fast response time and logarithmic output. It is capable of measuring very low oxygen concentrations / wide range of oxygen concentrations. The new technique has been developed by CSIRO Australia, where the Zirconia pellet is welded to super refractory tube and no chemicals are used for bonding. This technique has given excellent results in terms of reliability and long life of the sensor which enables MAXWELL to provide one year warranty for oxygen probes. It permits IN-SITU measurements to be made in a hot environment, making a sampling system, with its inherent drawbacks, completely unnecessary.

TECHNICAL DESCRIPTION

The oxygen probe in principle is a high temperature galvanic oxygen concentration cell constructed from a super refractory tube with one end welded to a stabilized zirconium oxide pellet and other end open. The zirconium oxide acts as a solid electrolyte which has the property of conducting electricity by means of oxygen ions.

OXYGEN SENSOR OPERATION

In this mainly the word probe means the complete assembly consisting of oxygen sensor, thermocouple (whenever provided), its outer protective sheath and the terminal (probe) head. Two electrodes make contact with the special catalyst coated zirconium oxide element on the tip of the sensor—one outer electrode and one inner electrode. Special alloy protection tube acts as outer electrode for the oxygen probe signal.

The probe is suitable for high temperature carburizing, heat treating, carbonitriding and endothermic gas generators as well as monitoring oxygen in combustion applications, inert gas generators, etc. This probe is not for nitriding applications.

Zirconia is a solid state electrolyte which conducts oxygen ions at temperatures above 600°C. The ion conduction results in a voltage between two electrodes. The magnitude of the voltage depends on the temperature of the Zirconia cell and the ratio of partial pressure on the reference side to the oxygen partial pressure on the process side of the electrolyte.

Nernst's Equation:

E = 0.0496 T log (P1O2 / P2O2)

Where:

T = Temperature in degrees Kelvin (°C + 273)
E = Cell output in mV
P1O2 = Reference air side oxygen partial pressure
P2O2 = Atmosphere side oxygen partial pressure