Yokogawa Integral Oxygen Analyzer ZR202 - Manual (Page 172)

IM 11M12A01-05E 12-9
12. Troubleshooting
12.3 Countermeasures When the 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.
When the process gas pressure is higher than that in calibration by np (kPa), the
measured oxygen concentration value X (vol% O
2
) may be expressed mathematically by:
X = Y[1 + (np/101.30)]
where, Y = Measured oxygen concentration value at the same pressure as in
calibration (vol% O
2
)
If an increment of the measured value by a pressure change cannot be neglected,
appropriate measures must be taken. Observe the following points for possible improve-
ment in each process.
• Can the facility’s characteristics be improved so that a pressure change does not
occur?
• Can a calibration be conducted under the average process gas pressure (internal
pressure of a dryer)?
(2) Moisture content in a reference gas changes (increases) greatly.
If air at the analyzer installation site is used for the reference gas, a large change of
moisture in the air may cause an error in the measured oxygen concentration value
(vol% O
2
). If this error cannot be ignored, use a gas in which moisture content is
constant, such as instrument air in almost dry condition as a reference gas.
(3) Calibration gas (span gas) is leaking in the sensor.
If the span gas is leaking in the sensor because of a failure of the valve provided in the
calibration gas tubing system, the measured value shows a value a little higher than
normal.
Check the valves (needle valves, check valves, solenoid valves for automatic calibration,
or the like, 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.