Testing A Probe Or Sensor; Probe Or Sensor Thermocouple; Heater Failure; Filter Blockage - Novatech 1635 User Manual

5.9

TESTING A PROBE OR SENSOR

With the probe or sensor heated to approximately 720 °C, either from a small test furnace or its own internal heater,
connect a digital multimeter to the probe or sensor electrode conductors. Connect the multimeter positive to the internal
electrode conductor. Connect reference air to and apply a gentle purge of air to the probe calibration check port.
Reference air flow should be the smallest flow available (less than 50 cc per minute). The multimeter should read zero
millivolts + two millivolts. If not, then there is a problem with the probe electrodes and the sensor needs refurbishing.
Normally a faulty probe electrode is indicated with a high source impedance. 1234 sensors do not require reference air
but a gentle flow of air should be admitted into the sample connection.
To test the source impedance, set the multimeter to read ohms and take a measurement, within a couple of seconds, of
the sensor impendence. Reverse the multimeter and repeat the reading. Take the average of the two readings for an
approximate measurement of impedance. If the impedance is above 10kΩ, then the probe or sensor needs to be
replaced. The probe or sensor must be at 720 °C 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 gas stream. In this
case the impedance may be OK but the output incorrect. This phenomenon is rare.

5.10 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 analyser has an alarm function which will advise
the operator of an open circuit thermocouple, however bench testing can be performed by simply measuring the
thermocouple continuity.

5.11 HEATER FAILURE

For heated probe or sensors, a heater failure will cause a 'Probe Temp' or 'HEATER FAILURE' alarm. Heaters can be
tested with a continuity test. The heater impedance should be approximately 110Ω. Should the heater be open or short
circuited, replace the probe or sensor.

5.12 FILTER BLOCKAGE

For oxygen probes with filters in installations with entrained solids in the gas, it is sometimes necessary to replace the
filter. Filters are normally cleared with back purging. However particles can ultimately completely block a filter
necessitating filter replacement. A new probe filter can be fitted.
August 2009
1635 Water Vapour Analyser 49