YOKOGAWA PH202G (S), User Manual

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IM 12B07D02-01E
8-2 Troubleshooting
8-1. Diagnostics
8-1-1. Off-line calibration checks
The EXA transmitter incorporates a diagnostic check of the asymmetry potential after a calibration has
been completed. This is a valid check for both manual and automatic calibration routines.
The actual value can be called up from the DISPLAY routine in the maintenance menu. A large value
often indicates poisoning or pollution of the reference system used. If the asymmetry potential exceeds
programmable limits, the EXA generates an error (E2).
The EXA also performs diagnostics to check for the slope of the pH electrode after automatic calibration
is completed. The actual value of the slope can be called up on the DISPLAY routine in the maintenance
menu (SL). This value is an indication of the age of the electrode. If the value stays within the limits of 70
to 110 percent of the theoretical value (59.16 mV/pH at 25°C), it is accepted. Otherwise, the unit gener-
ates an error (E3).
Activation or deactivation of the asymmetry diagnostic check and slope check is made from the Service
Codes. See Chapter 5 or Chapter 10 (Appendix).
8-1-2. On-line impedance checks
The EXA has a sophisticated impedance checking system. The sensors can be checked for their imped-
ance over a very wide range, which makes the tool equally useful for glass, enamel, reference and metal
(ORP) sensors. The measurement is temperature compensated for the characteristic of the pH glass
sensor.
In order to measure accurately over such a wide range, it is necessary to split the range into two. This is
done by a pair of jumper settings, high range and low range can be set on either input, making the sys-
tem extremely flexible.
The following error message table gives a list of problems that are indicated when the high or low imped-
ance limits are exceeded for a sensor. Such things as fouling, breakage and cable faults are readily
detected. The non-immersion of the sensors in the process fluid is also signalled.