Yokogawa Integral Oxygen Analyzer ZR202 - Manual (Page 164)

IM 11M13A01-04E 12-10
12.3.2 Measured Value Lower Than True Value
<Causes and Measures>
(1) The measuring gas pressure becomes lower.
Where an increment of the measured value due to pressure change cannot be
neglected, take measures referring to subsection 12.3.1 (1).
(2) Moisture content in a reference gas changes (decreases) greatly.
If air at the analyzer 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 (zero gas) is mixed into the sensor due to leakage.
If the zero 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 lower 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.
(4) Combustible components exist in the measured gas. If combustible components exist
in the measured gas, they burn in the sensor and thus oxygen concentration
decreases. Check that there are no combustible components.
(5) Temperature of the sensor cell reaches 750˚C or more.
If the sensor temperature is 750˚C or higher, this may indicate that measurement
gas has leaks into the reference gas side, corrosion. Also check that the
thermocouple resistance is no greater than 15
⍀.
12.3.3 Measurements Sometimes Show Abnormal Values
<Cause and Measure>
(1) Noise may be mixing in with the converter from the detector output wiring.
Check whether the equipment is securely grounded.
Check whether or not the signal wiring is laid along other power cords.
(2) The converter may be affected by noise from the power supply.
Check whether or not the converter power is supplied from the same outlet, switch,
or breaker as other power machines and equipment.
(3) Combustible components in the measured gas may be getting into the sensor.
If the combustible components show signs of dust, the abnormality may be im-
proved by mounting a dust-removing filter K9471UA.
(4) There may be a crack in the sensor or leakage at the sensor-mounting portion.
If the indication of concentration varies in synchronization with the pressure change
in the furnace, check whether or not there is a crack in the sensor or whether the
sensor flange is sticking tightly to the probe-attaching face with the metal O-ring
squeezed.
(5) There may be leakage in the calibration gas tubing
In the case of a negative furnace inner pressure, if the indication of concentration
varies with the pressure change in the furnace, check whether or not there is leakage
in the calibration gas tubing.