Emerson Rosemount FCL, Reference Manual

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Another way of checking for a faulty sensor is to replace it with a new one. If a new
sensor can be calibrated, the old sensor has failed.
7. Is the transmitter faulty? The best way to check for a faulty transmitter is to
simulate pH inputs. See Simulating inputs - pH .
9.6.2 Calibration error during standardization.
During standardization, the millivolt signal from the pH cell is increased or decreased until
the pH agrees with the pH reading from a referee instrument. A unit change in pH requires
an offset of about 59 mV. The transmitter limits the offset to ±60 mV. If the
standardization causes an offset greater than ±60 mV, the transmitter will display the
Offset Error screen. The standardization will not be updated. Check the following:
1. Is the referee pH meter working and properly calibrated? Check the response of the
referee sensor in buffers.
2. Is the sensor fully immersed in the process liquid? If the sensor is not completely
submerged, it may be measuring the pH of the liquid film covering the glass bulb
and reference element. The pH of this film may be different from the pH of the bulk
liquid.
3. Is the sensor fouled? The sensor measures the pH of the liquid adjacent to the glass
bulb. If the sensor is heavily fouled, the pH of liquid trapped against the bulb may be
different from the bulk liquid.
4. Has the sensor been exposed to poisoning agents (sulfides or cyanides) or has it
been exposed to extreme temperature? Poisoning agents and high temperature
can shift the reference voltage many hundred millivolts.
9.6.3 Sensor does not respond to known pH changes.
1. Is the pH sensor responsive to buffers? Check sensor response in two buffers at least
two pH units apart.
2. Did the expected pH change really occur? Use a second meter to verify the change.
3. Is sample flowing past the sensor? Be sure 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, dissassemble and clean the overflow sampler. See
Constant head flow controller .
4. Is the sensor properly wired to the transmitter? See Wire sensor .
5. Is the glass bulb cracked or broken? Check the glass electrode impedance. See
Calibration error during two-point calibration. .
6. Is the transmitter working properly? Check the transmitter by simulating the pH
input. See Simulate pH input .
Reference Manual Troubleshoot
00809-0100-3412 May 2019
Rosemount FCL 1056 99