IM 12B6B3-E-H
8-2 Troubleshooting
8-1. Diagnostics
8-1-1. Off-line calibration checks
The EXA PH402 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 a poisoning or pollution of the reference system used. If the asymmetry potential exceeds pro-
grammable limits, the EXA PH402 generates an error (E2).
The EXA PH402 also performs diagnostics to check for the slope of the pH electrode after automatic cali-
bration is completed. The actual value of the slope can be called up on the DISPLAY routine in the mainte-
nance 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 PH402 has a sophisticated impedance checking system. The sensors can be checked for their
impedance 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 system
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 detect-
ed. The non-immersion of the sensors in the process fluid is also signalled.
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