Yokogawa EXAxt SC450 4-Wire Analyzer for Conductivity/Resistivity - Manual (Page 48)

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IM 12D08N05-01E
8-1. General
The EXAxt is a microprocessor-based analyzer
that performs continuous self-diagnostics to
verify that it is working correctly. Error mes-
sages resulting from faults in the micro-proc-
essor systems itself are monitored. Incorrect
programming by the user will also result in an
error, explained in a message, so that the fault
can be corrected according to the limits set in
the operating structure. The EXAxt also checks
the sensor system to establish whether it is still
functioning properly. In the main display screen
is a “Status Information” button that will show
For information
For warning - a potential problem is diag-
nosed, and the system should be checked.
For FAIL, when the diagnostics have confirmed
a problem, and the system must be checked.
This button gives access to a status report
page, where “ The most applicable error” will
be displayed. (“ No errors” is displayed during
proper operation)
Explanation >> Description or error message
and possible remedies
Advanced troubleshooting >> Error code
screen that is used in conjunction with the
service manual. This data will also be needed
in the event that you request assistance from a
Yokogawa service department.
What follows is a brief outline of the EXAxt
troubleshooting procedures including possible
causes and remedies.
8-2. Calibration check
The EXAxt SC450G converter incorporates a
diagnostic check of the adjusted cell constant
value during calibration. If the adjusted value
stays within 80-120 % of the factory value, it is
accepted, otherwise, the unit generates an er-
ror message, and the calibration is rejected.
8-3. Polarization check
The EXAxt SC450G performs on-line moni-
toring to detect polarization. This is an early
indicator for sensor fouling. The detection
of polarization in the measurement gives a
warning of the onset of sensor coating, before
significant measuring errors build up.
8-4. Predictive maintenance
EXAxt has a unique prediction feature. Calibra-
tion, and polarization check data are stored in
software data logbooks. This data is then used to
calculate a prediction for maintenance purposes.
8-5. Prediction of cleaning needed
The date when the next maintenance is needed
is calculated from on-line polarization checks.
The trend of polarization measurements on the
sensor is used to calculate when to tell the user
when to clean the sensor.
8-6. Poor calibration technique
When the calibration data is not consistent
this fact is used as a diagnostic tool. The
significance of this error message is to require
the user to improve his calibration technique.
Typical causes for this error are attempting
to calibrate dirty sensors, calibration solution
contamination and poor operator technique.
8-7. Error displays and actions
All errors are shown in the “Main Display”
screen, however, the EXAxt makes a distinc-
tion between diagnostic findings. The error
messages may be set to OFF, WARN or FAIL.
For process conditions where a particular diag-
nostic may not be appropriate, the setting OFF
is used. FAIL gives a display indication only of
that the system has a problem and inhibits the
relay control action, and can be set to trigger
the “Burn” function. “Burn-up” or “Burn-down”
drives the mA output signal to 21 mA or 3.6 mA
respectively.
8. TROUBLESHOOTING