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< 12. Troubleshooting >
12-14
IM 11M12D01-01E
12.3 Countermeasures When Measured Value Shows
Error
The causes that the measured value shows an abnormal value is not always due to instrument
failures. There are rather many cases where the causes are those that measuring gas itself is in
abnormal state or external causes exist, which disturb the instrument operation. In this section,
causes of and measures against the cases where measured values show the following phenomena
will be described.
(1) The measured value is higher than the true value.
(2) The measured value is lower than the true value.
(3) The measured value sometimes shows abnormal values.
12.3.1 Measured Value Higher Than True Value
<Causes and Countermeasures>
(1) The measuring gas pressure becomes higher.
The measured oxygen concentration value X (vol% O
2
) is expressed as shown below, when the
measuring gas pressure is higher than that in calibration by ∆p (kPa).
X=Y [ 1+ (∆p/101.30) ]
where Y: Measured oxygen concentration value at the same pressure as in calibration (vol%
O
2
).
Where an increment of the measured value by pressure change cannot be neglected, measures
must be taken.
Investigate the following points to perform improvement available in each process.
• Is improvement in facility’s aspect available so that pressure change does not occur?
• Is performing calibration available under the average measuring gas pressure (internal
pressure of a furnace)?
(2) Moisture content in a reference gas changes (increases) greatly.
If air at the detector installation site is used for the reference gas, large change of moisture
content in the air may cause an error in measured oxygen concentration value (vol% O
2
). When
this error is not ignored, use a gas in which moisture content is constant such as instrument air in
almost dry condition as a reference gas.
In addition, change of moisture content in exhaust gas after combustion is also considered as a
cause of error. However, normally this error is negligible.
(3) Calibration gas (span gas) is mixing into the detector due to leakage.
If the span gas is mixed into the detector due to leakage as a result of failure of the valve
provided in the calibration gas tubing system, the measured value shows a value a little higher
than normal.
Check valves (needle valves, check valves, solenoid valves for automatic calibration, etc.) in the
calibration gas tubing system for leakage. For manual valves, check them after confirming that
they are in fully closed states. In addition, check the tubing joints for leakage.
(4) The reference gas is mixing into the measuring gas and vice versa.
Since the difference between oxygen partial pressures on the sensor anode and cathode sides
becomes smaller, the measured value shows a higher value.
An error which does not appear as the Error 1 may occur in the sensor. Sample gas and/or the
reference gas may be leaking. Visually inspect the sensor. If any crack is found, replace the
sensor assembly with a new one.