Yokogawa EXA PH202 2-Wire pH/ORP Analyzer - Manual (Page 83)

IM 12B07D02-01E
Troubleshooting 8-1
8. TROUBLESHOOTING
The EXA 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 also checks the electrodes to establish whether they are still functioning within
specified limits. The transmitter checks the glass-electrode impedance for a low value to determine if it is
broken or cracked, and for a high impedance to check for internal breakage or disconnection.
The reference system is prone to more faults than the glass electrode in general. The unit measures the
impedance value and compares it to the programmed value in memory to determine acceptance during
testing. A high impedance signals pollution or poisoning of the reference electrode diaphragm.
Also, the EXA checks the electrodes during calibration to determine if the reaction time is suitable for pH
measurement. A specially timed check can be activated following each cleaning cycle. After calibration,
the unit checks the calculated asymmetry potential and the slope to determine if they are still within limits
specified by the software.
The slow shift of asymmetry potential could signal a poisoning of the reference electrode system by the
process. The decrease of slope equals a decrease of sensitivity of the glass electrode or can show a
coating buildup at the electrode.
The EXA makes a distinction among diagnostic findings. All errors are signaled by the FAIL flag in the
display. Only faults in the measuring circuit can be set as HARD FAIL, with “Burn-up or Burn-down” sig-
nals on the mA output.
What follows is a brief outline of some of the EXA troubleshooting procedures, followed by a detailed
table of error codes with possible causes and remedies.
NOTE :
The diagnostic function of the EXA gives a variable time interval between impedance checks, up
to 5 minutes. When trouble shooting, a manual impedance check can be initiated by following the
procedure in section 5-1-4.