Yokogawa Multi Channel Oxygen Analyzer System ZR22/AV550G - Manual (Page 226)

IM 11M12D01-01E 12-16
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 measur-
ing 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 Dp
(kPa).
X=Y [ 1+ (
⌬p/101.30) ]
where Y: Measured oxygen concentration value at the same pressure as in calibra-
tion (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 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 mixing 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 calibra-
tion, 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. Measured
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.
Note
Data such as cell robustness displayed in the detailed data display should also be used
for deciding sensor quality as references.