Emerson Process Management 53eA Sensor can be calibrated, but the current is too low

MODEL 54eA	SECTION 16.0
	TROUBLESHOOTING

16.5.3 Sensor can be calibrated, but the current is too low

A.Is the temperature low? For both the 499ACL-01 and 498CL-01 sensors, sensor current is a strong function of temperature. The current decreases about 3% for every ºC drop in temperature.

B.Is the pH high? The 499ACL-01 sensor current is a strong function of pH. Sensor current decreases as pH increases. Above pH 7, a 0.1 unit increase in pH lowers the current by about 5%. The 498CL-01 sensor cur- rent is practically independent of pH.

C.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.

D.Low current can be caused by lack of electrolyte flow to the cathode and membrane. See step D in Section 16.5.2.

E.Is the membrane fouled or coated? A dirty 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.

F.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.

16.5.4 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 16.5.2.

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 (499ACL-01) or electrolyte slurry (498CL-01). Refer to the sensor instruction manual for details.

16.5.5 Readings drift

A.Is the sample temperature changing? Membrane permeability is a function of temperature. The time constant for the 499ACL-01 and 498CL-01 sensors 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? (Applies only to the 499ACL-01 sensor.) 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.

92