Emerson ROSEMOUNT 5081-A Instruction Manual page 113

MODEL 5081-A
15.5.5 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 coat-
ing 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.
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 grad-
ual 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.
15.5.6 Sensor does not respond to changes in chlorine level.
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 trans-
mitter 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 it if necessary. Check that the holes at the base of the cath-
ode stem are open. use a straightened paper clip to clear blockages. Replace the electrolyte solution.
4. Replace the sensor.
15.5.7 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 remained constant. to correct for 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. the
time constant for return to normal is about five (5) minutes.
15.5.8 Chlorine readings are too low.
1. Was the sample tested as soon as it was taken? Chlorine solutions are unstable. test the sample immediately after
collecting it. Avoid exposing the sample to sunlight.
2. Low readings can be caused by zeroing the sensor before the residual current has reached a stable minimum value.
Residual current is the current the sensor generates even when no chlorine is in the sample. Because the residual cur-
rent is subtracted from subsequent measured currents, zeroing before the current is a minimum can lead to low results.
See Section 15.4.8 for more information.
3. Sensor response depends on flow. If the flow is too low, readings will be low and flow sensitive. Verify that the flow
past the sensor equals or exceeds the minimum value. See the sensor instruction manual for recommended flows.
15.6 TOTAL CHLORINE MEASUREMENT AND CALIBRATION PROBLEMS
Refer to the instruction manual for the SCS921 for a complete troubleshooting guide.
SECTION 15.0
TROUBLESHOOTING
107