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MODEL 54eA SECTION 16.0
TROUBLESHOOTING
16.9.2 Warning or error message during two-point calibration.
During standardization, the millivolt signal from the pH cell is increased or decreased until it agrees with the pH
reading from a reference instrument. A unit change in pH requires an offset of about 59 mV. The controller limits
the offset to ±1400 mV. If the standardization causes an offset greater than ±1400 mV, the analyzer will display the
Calibration Error screen. The standardization will not be updated. Check the following:
A. Is the referee pH meter working and properly calibrated? Check the response of the referee sensor in buffers.
B. Is the process sensor working properly? Check the process sensor in buffers.
C. Is the sensor fully immersed in the process liquid? If the sensor is not completely submerged, it may be meas-
uring the pH of the liquid film covering the glass bulb and reference element. The pH of this film may be dif-
ferent from the pH of the bulk liquid.
D. 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.
E. 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.
To check the reference voltage, see Section 16.15.
16.9.3 Controller will not accept manual slope.
If the sensor slope is known from other sources, it can be entered directly into the controller. The controller will not
accept a slope (at 25°) outside the range 45 to 60 mV/pH. If the user attempts to enter a slope less than 45 mV/pH,
the controller will automatically change the entry to 45. If the user attempts to enter a slope greater than 60 mV/pH,
the controller will change the entry to 60 mV/pH. See Section 16.9.1 for troubleshooting sensor slope problems.
16.9.4 Sensor does not respond to known pH changes.
A. Did the expected pH change really occur? If the process pH reading was not what was expected, check the
performance of the sensor in buffers. Also, use a second pH meter to verify the change.
B. Is the sensor properly wired to the analyzer?
C. Is the glass bulb cracked or broken? Check the glass electrode impedance. See Section 16.1
D. Is the analyzer working properly. Check the analyzer by simulating the pH input.
16.9.5 Calibration was successful, but 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 nor-
mal. The on-line instrument is subject to process variables, for example ground potentials, stray voltages, and ori-
entation effects that may not affect the laboratory or portable instrument. To make the process reading agree with
a reference instrument, see Section 12.4.