Emerson Rosemount FCL-01 Reference Manual page 91

Reference Manual
00809-0100-3412
8.5.6 Sensor does not respond to changes in chlorine levels
Recommended actions
8.5.7 Chlorine readings spike following sudden changes in pH
(automatic pH correction)
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 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 minutes.
8.5.8 Chlorine readings are too low
Recommended actions
Rosemount FCL 1056
b) Confirm that the nozzle is properly positioned in the flow cell. Line up
your eye with the bottom of the membrane retainer.
No gap should be visible between the end of the nozzle and membrane
retainer.
6. If the sensor is new or has been recently serviced, wait a few hours.
New or rebuilt sensors may require several hours to stabilize.
1. Make sure the grab sample test is accurate and that the grab sample is
representative of the sample flowing to the sensor.
2. Make sure that sample is flowing past the sensor, that the liquid level in the
constant head sampler is level with the central overflow tube, and that excess
sample is flowing down the tube. If necessary, disassemble and clean the
overflow sampler.
See Constant head flow
3. Make sure the pH compensation is correct. If using manual pH correction, verify
that the pH value in the transmitter equals the actual pH within ±0.1 pH. If using
automatic pH correction, check the calibration of the pH sensor.
4. Make sure the membrane is clean. Clean the membrane with a stream of water
and replace it if necessary.
a) Check that the holes at the base of the cathode stem are open. Use a
straightened paper clip to clear blockages.
See Step 3 in Zero current is unstable
b) Replace the electrolyte solution.
5. Replace the sensor.
1. Test the sample immediately after collecting it. Avoid exposing the sample to
sunlight.
Chlorine solutions are unstable.
controller.
Troubleshooting
October 2023
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