< 12. Troubleshooting >
12-12
IM 11M12D01-01E
(1) Cause
In general, sensor aging causes a change in the slope of the compensation line. If calibration is
performed when the detector tip is clogged with dust from the sample gas, however, this can cause
a shift in both zero and span calibration results. Such an offset can be compensated for by zero shift
alone. This alarm occurs if, when the last two calibration results are compared, the shift indicates such
a tip-clogging pressure-related effect.
cell e.m.f. mV
1st calibration
2nd calibration
3rd calibration
O
2
value
0.51%
21%
F12-11E.ai
Example: When the second calibration result is compared with the first, the change in slope indicates
sensor degradation.
However, when the third calibration result is compared with the second, the slope has not changed,
only the zero offset, suggesting clogging and pressure effects.
Figure 12.11 Cause of Alarm
(2) Conditions for generating an alarm
When the current result and the two previous results -- for the oxygen concentration resulting in a
given cell e.m.f. -- are compared, if both of the following conditions are satisfied then an alarm is
issued:
A
B
C
21%
0.51%
O
2
value
Current calibration
Previous calibration
Calibration
before previous one
1 A-B≥0.125%O
2
2 B-C≥0.125%O
2
cell e.m.f. mV
_
_
F12-12E.ai
Figure 12.12
Conditions for Alarm
A repeatability of 0.125% O
2
on the 0 to 25% range is determined from the specified precision.
(25% O
2
x 0.5% = 0.125% O
2
)
However, the following restrictions apply:
(a) When either span or zero calibration is performed, not both, the results are ignored in
applying the above criteria.
(b) If calibrations are separated by an interval of less than 5 days, then the results are ignored in
applying the above criteria.
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