A: The deterioration of a zirconium oxide probe is of interest to engineers to maintain instrumental reliability. The degradation does not follow a predictable formula.
Background
Zirconia probes are the most common method of controlling combustion and heat treatment processes. Teledyne has hundreds of systems installed in power stations, petrochemical plants, heat treatment and annealing furnaces, incinerators, cement plants and drying ovens. ?ll these process have different requirements, and pose different conditions to the components of the probe.
Failure Modes
The useful life of the probe is limited by the first component of the probe to fail. The failure may be a slow deterioration or a sudden change of the working condition of one of the vital components.
Components that may have a sudden failure rate are:
- The heater in heated probes (open or short circuit)
- The thermocouple (typically goes open circuit)
- Out Metal Sheathing (breakage through corrosion or abrasion)
- The ceramic components of the probe (physical or thermal shock)
Components that will slowly deteriorate are:
- The electrode coating of the zirconium sensor
- The thermocouple (corrosion, grain growth)
- Internal electrode and thermocouple wires (corrosion, grain growth)
Out of all these component failures, only the zirconia electrode material loss can be measured in an attempt to determine the remaining life of the probe. This failure can only be predicted with the experience of the process it is being used in.
The analyzers measure the impedance of the zirc sensors automatically. Without interfering with the process measurement, the analyzer allows an operator to read the impedance measurement and will automatically raise an alarm if the impedance is high enough.
There is no definite impedance at which a sensor can be said to have failed. ?s the electrode deteriorates, the impedance will rise. However the sensor O2 reading is not affected by the loss of electrode material over a wide temperature range over a wide range of impendence (up to tens of kilo ohms). In fact, if the temperature of the sensor is over 850C the sensor will read the correct O2 concentration without any electrode material as long as there is no other deterioration of the electrode connections. The response tie will usually increase as the electrode ages.
The sensor impudence is the sum of the electrode impedance and the electrolyte impudence. The electrode impedance is the part that will increase as the probe ages. Complicating the interpretation of the sensor impedance measurement is the electrolyte impendence will be markedly lower at higher sensor temperatures.
As an indication of probe life, the TAI analyzer will raise a sensor fail alarm when the sensor is over 9k ohms when the sensor temperature is 720C.
