Yokogawa Single Channel Oxygen Analyzer System ZR22/ZR402 - Manual (Page 197)

IM 11M12A01-03E 12-13
12. Troubleshooting
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
⌬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 dryer)?
(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
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 or vice versa.
Since the difference between oxygen partial pressures on the sensor anode and
cathode sides becomes smaller, the measured value shows a smaller 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. Referring to Section
11.1.2, check if there is a problem with the sensor mounting. If things are abnormal
but there is no error indication, then maybe the sensor assembly isn’t properly
secured, the O-ring seal is dirty, the measurement gas and ratio gas are leaking into
each other (if measurement gas pressure is high, then it may leak into ratio gas, and
vice versa); as oxygen partial pressure difference between ratio gas and