10
5.2
Comparison with Another O
2
Probe
The probe may be checked by comparison with another of
known reliability, either by replacing the suspect probe with the
known probe or by mounting the known probe in close
proximity and monitoring both probes continuously for a short
period.
5.3
Returning the Probe
to the Manufacturer for Checking
If it is not possible to carry out the above tests on site and
failure or malfunction is suspected, the probe may be returned
to the Company for checking.
If returning the probe, carefully dismantle the probe
components by reversing the procedure detailed in Fig. 2.2.
Repack in the original packing to ensure safe carriage.
5.4
Continuity Check
Connect a 100k
Ω
resistor across the probe output. If the
output drops to near zero millivolts and then drifts when the
resistance is removed, this indicates that the probe may be
open circuit or have a high impedance.
Warning. The probe operates at high
temperatures. Take all necessary precautions to avoid
injury through burns.
During its working life, under normal recommended
conditions, the probe output remains accurate and without
drift. However, probe malfunctions can result from a fault in the
probe or from incorrect operating conditions.
The testing of a suspect probe can only be carried out
satisfactorily in its working position or in a furnace controlled
within the normal probe operating temperature range.
If a probe failure is suspected, first perform a test gas check as
described in the following sections.
5.1
In Situ Checking Using a Test Gas
This procedure introduces a test gas of known O
2
concentration around the outer electrode of the probe in the
space between the filter washer, at the bottom of the sheath,
and the wadding around the lower end of the probe. Four vent
holes (see Fig. 2.2) prevent the gas from being trapped in the
upper part of the sheath.
a) Gain access to the interior of the probe head by
unscrewing the lid fixing screws – see Fig. 4.1.
b) Remove the screwed plug from the test gas connector –
see Fig. 4.1.
c) Fit
1
/
4
in i.d. x
3
/
8
in o.d. plastic or similar tubing and supply a
test gas of known oxygen between 0.25 and 25% O
2
concentration to the probe at a steady flow rate of 800 to
1000ml/min. Allow at least 5 minutes for the system to
stabilize before making a measurement.
d) Check that the oxygen concentration measured by the
4680 or 4685 transmitter (mounted in a safe area),
compares with the specification for the test gas used.
✶
Note. The O
2
concentration measured by the
probe can be checked locally by taking readings from
the probe thermocouple (for temperature) and the
cell O
2
mV input and entering the results in to the
Nernst Equation.
e) Disconnect the test gas and ensure that the screwed plug
is replaced in the test gas connector. Failure to do this may
result in serious measurement errors due to the entry of air
into the probe.
f)
Proceed to Table 5.1.
5
FAULT FINDING
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Table 5.1 Fault Finding
