Emerson Rosemount5081, Instruction Manual, Page: 115 / 152

Chlorine readings drift
1. 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.
2. 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 tissue to wipe the membrane.
3. Is the sample flow within the recommended range? Gradual loss of sample flow will
cause a downward drift.
4. Is the sensor new, or has it been recently serviced? New or rebuilt sensors may
require several hours to stabilize.
5. 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) remains 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.
Sensor not responding to chlorine changes
1. Is the grab sample test accurate? Is the grab sample representative of the sample
flowing to the sensor?
2. Is the pH compensation correct? If the transmitter is using manual pH correction,
verify that the pH value in the transmitter equals the actual pH to within ±0.1 ph. If
the transmitter is using automatic pH correction, check the calibration of the pH
sensor.
3. Is the membrane clean? Clean the membrane and replace if necessary. Check that
the holes at the base of the cathode stem are open. Use a straightened paper clip to
clear blockages. Replace the electrolyte solution.
4. Replace the sensor.
Chlorine readings spike
Chlorine readings spike following sudden changes in pH.
Changes in pH alter the relative amounts of hypochlorous acid (HOCl) and hypochlorite ion
(OCl
-
) in the sample. Because the sensor responds only to HOCl, an increase in pH causes
the sensor current (and the apparent chlorine level) to drop even though the actual free
chlorine concentration remains constant. To correct the pH effect, the transmitter
automatically applies a correction. Generally, the pH sensor responds faster than the
chlorine sensor. After a sudden pH change, the transmitter will temporarily over-
compensate and gradually return to the correct value. A time constant for return to
normal is about five minutes.
Diagnostics and troubleshooting
Instruction Manual 105