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MODEL 54eA SECTION 15.0
TROUBLESHOOTING
15.5.3 Sensor can be calibrated, but the current is too low
A. Is the temperature low or is the pH high? Sensor current is a strong function of pH and temperature. The sen-
sor current decreases about 3% for every °C drop in temperature. Sensor current also decreases as pH
increases. Above pH 7, a 0.1 unit increase in pH lowers the current about 5%.
B. Sensor current depends on the rate of sample flow past the sensor tip. If the flow is too low, chlorine readings
will be low. Refer to the sensor instruction sheet for recommended sample flows.
C. Low current can be caused by lack of electrolyte flow to the cathode and membrane. See step D in Section
15.5.2.
D. Is the membrane fouled or coated? Adirty membrane inhibits diffusion of free chlorine through the membrane,
reducing the sensor current and increasing the response time. Clean the membrane by rinsing it with a stream
of water from a wash bottle. DO NOT use a membrane or tissue to wipe the membrane.
E. If cleaning the membrane does not improve the sensor response, replace the membrane and electrolyte solu-
tion. If necessary, polish the cathode. See the sensor instruction sheet for details.
15.5.4 Process readings are erratic
A. Readings are often erratic when a new sensor or a rebuilt sensor is first placed in service. The current usual-
ly stabilizes after a few hours.
B. Is the sample flow within the recommended range? High sample flow may cause erratic readings. Refer to the
sensor instruction sheet for recommended flow rates.
C. Are the holes between the membrane and the electrolyte reservoir open. Refer to Section 15.5.2.
D. Verify that wiring is correct. Pay particular attention to shield and ground connections.
E. If automatic pH correction is being used, check the pH reading. If the pH reading is noisy, the chlorine read-
ing will also be noisy. If the pH sensor is the cause of the noise, use manual pH correction until the problem
with the pH sensor can be corrected.
F. Is the membrane in good condition and is the sensor filled with electrolyte solution? Replace the fill solution
and electrolyte. Refer to the sensor instruction manual for details.
15.5.5 Readings drift
A. Is the sample temperature changing? Membrane permeability is a function of temperature. The time constant
for the 499ACL-01 sensor is about five minutes. Therefore, the reading may drift for a while after a sudden
temperature change.
B. Is the membrane clean? For the sensor to work properly, chlorine must diffuse freely through the membrane.
A coating on the membrane will interfere with the passage of chlorine, resulting in slow response. Clean the
membrane by rinsing it with a stream of water from a wash bottle. DO NOT use a membrane or tissue to wipe
the membrane.
C. Is the sample flow within the recommended range? Gradual loss of sample flow will cause a downward drift.
D. Is the sensor new or has it been recently serviced? New or rebuilt sensors may require several hours to sta-
bilize.
E. Is the pH of the process changing? If manual pH correction is being used, a gradual change in pH will cause
a gradual change in the chlorine reading. As pH increases, chlorine readings will decrease, even though the
free chlorine level (as determined by a grab sample test) remained constant. If the pH change is no more than
about 0.2, the change in the chlorine reading will be no more than about 10% of reading. If the pH changes
are more than 0.2, use automatic pH correction.