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9060-Zirconia Analyser
measurement of impedance. If the impedance is above 10kΩ, then the probe or sensor
needs to be replaced. The probe or sensor must be over 700°C (1300°F) or above for this
measurement. The reason that impedance measurements need to be performed quickly,
is that the zirconia sensor polarises with the DC voltage from the multimeter across it.
If the probe or sensor tests reveal less than 2 mV offset and a good impedance reading,
the next step is to apply a calibration check gas. The calibration check gas should be
inserted in the calibration check port. With the calibration check gas flowing, the probe or
sensor should develop an EMF according to the tables in Appendix 2. If the EMF reading
is low then there may be insufficient calibration check gas flow. Increase the calibration
check gas until the reading is correct. An excessive calibration check gas flow will cause
cooling on one surface of the sensor, giving temperature differential errors on the sensor.
As an alternative, using the reference air port, the calibration check gas can be inserted
into the inside of a probe sensor. This requires a lower flow rate, and thus lower usage of
calibration check gas. The flow rate should be similar to that of the reference air, which
should be removed for internal calibration check. The probe or sensor EMF reading will be
identical but negative in polarity. A small flow of air should be flowing over the outside of
the sensor, when testing in this way.
Occasionally, a sensor can develop offset with a
polluted electrode caused by
contaminants in the flue gas stream. In this case the impedance may be OK but the
output incorrect. This phenomenon is rare.
6.10 PROBE OR SENSOR THERMOCOUPLE
Although some unheated probes are specified without a thermocouple, most probes, both
heated and unheated, have an integral thermocouple which is fitted in to the four bore
insulator. The transmitter has an alarm function that will advise the operator of an open
circuit thermocouple, however bench testing can be performed by simply measuring the
thermocouple continuity.
6.11 HEATER FAILURE
For heated probe or sensors, a heater failure will cause a 'SENSOR UNDER TEMP' or
'HEATER FAIL' alarm.
Heaters can be tested with a continuity test.
The heater
impedance should be approximately 100Ω. Should the heater be open or short-circuited,
replace the probe or sensor.
6.12 FILTER BLOCKAGE
For oxygen probes with filters in installations with entrained solids in the flue gas, it is
sometimes necessary to replace the filter. Filters are normally cleared with back purging.
However fine fly ash or other particles can ultimately completely block a filter necessitating
filter replacement. A new probe filter can be fitted.
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