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MODEL FCL-1056 SECTION 9.0
TROUBLESHOOTING
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 analyzer limits
the offset to ±60 mV. If the standardization causes an offset greater than ±60 mV, the analyzer 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 pH 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, disassemble and clean the over
flow sampler. See Section 8.4.
4. Is the sensor properly wired to the analyzer? See Section 3.2.
5. Is the glass bulb cracked or broken? Check the glass electrode impedance. See Section 9.6.1.
6. Is the analyzer working properly? Check the analyzer by simulating the pH input. See Section 9.9.
9.6.4 Buffer Calibration Is Acceptable, Process pH Is Slightly Different from Expected Value.
Differences between pH readings made with an on-line instrument and a laboratory or portable instrument are
normal. The on-line instrument is subject to process variables, for example ground potentials, stray voltages, and
orientation effects that may not affect the laboratory or portable instrument. To make the process reading agree
with a referee instrument, see Section 6.4.5.