Yokogawa EXA SC202 2-Wire Conductivity Transmitter/Analyzer - Manual (Page 70)

IM 12D08B02-01E
Troubleshooting 8-1
8. TROUBLESHOOTING
The EXA SC202 is a microprocessor-based analyzer that performs continuous self-diagnostics to verify
that it is working correctly. Error messages resulting from faults in the microprocessor systems itself are
few. Incorrect programming by the user can be corrected according to the limits set in the following text.
In addition, the EXA SC202 also checks the sensor to establish whether it is still functioning within speci-
fied limits.
What follows is a brief outline of some of the EXA SC202 troubleshooting procedures, followed by a
detailed table of error codes with possible causes and remedies.
8-1. Diagnostics
8-1-1. Off-line checks
The EXA SC202 transmitter incorporates a diagnostic check of the adjusted cell constant value at cali-
bration. If the adjusted value stays within 80 - 120 % of the nominal value set in service code 03, it is
accepted. Otherwise, the unit generates an error (E3). With a HART® communication package it is pos-
sible to scroll the calibration data in a logbook function.
The EXA also checks the temperature compensation factor while performing manual temperature com-
pensation as described in section 5.2.5. If the TC factor stays within 0.00% to 3.50% per °C, it is accept-
ed. Otherwise, E2 will be displayed.
8-1-2. On-line checks
The EXA performs several on-line checks to optimize the measurement and to indicate a fault due to the
fouling or polarization of the connected sensor. The fault will be indicated by the activation of the FAIL
flag in the display.
During measurement the EXA adjusts the measuring frequency to give the best conditions for the actual
value being measured. At low conductivity there is a risk of error due to the capacitive effects of the
cable and the cell. These are reduced by using a low measuring frequency. At high conductivity the
capacitive effects become negligible and errors are more likely to be caused by polarization or fouling of
the cell. These errors are decreased by increasing the measuring frequency.
At all values the EXA checks the signal from the cell to search for distortion which is typical of capacitive
or polarization errors. If the difference between pulse front and pulse rear is > 20% an error E1 will be
displayed and the FAIL flag in the display is activated. In service code 05 it is possible to turn this check
on and off.