Other Troubleshooting - Ph - Emerson Rosemount FCL Reference Manual

Reference Manual
00809-0200-3412
3. Is the pH compensation correct? If the transmitter is using manual pH correction,
4. Is the membrane clean? Clean the membrane and replace it if necessary. Check that
5. Replace the sensor.
9.5.4 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 5 minutes.
9.6

Other troubleshooting - pH

Problem
Sensor does not respond to known pH changes.
Calibration was successful, but process pH is slightly
different from expected value.
Calibration was successful, but process pH is grossly wrong
or noisy.
pH readings are moderately noisy and tend to wander.
9.6.1 Sensor does not respond to known pH changes.
Recommended actions
1. Check to see if the pH sensor is responsive to buffers. Check sensor response in two
2. Make sure the expected pH change really occurred. Use a second pH meter to verify
3. Make sure sample is flowing past the sensor. Be sure the liquid level in the constant
Rosemount FCL 56
down the tube. If necessary, disassemble and clean the overflow sampler. See
Constant head flow controller.
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.
the holes at the base of the cathode stem are open. Use a straightened paper clip to
clear the holes. Replace the electrolyte solution.
buffers at least two pH units apart.
the change.
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 Cleaning the sensor.
See Section
Sensor does not respond to known pH changes.
Buffer calibration is acceptable; process pH is slightly
different from expected value.
Calibration was successful, but process pH is grossly wrong
and/or noisy.
pH readings are moderately noisy and tend to wander.
Troubleshoot
May 2019
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