FAS622en_pH measurement

3 Quality assurance
Drifting measured values
The phenomenon referred to as „drift“ is a normal characteristic of an electrode. The measurement
setup (or, in the case of a control system, the hand-held meter) indicates permanently increasing or
decreasing pH values.
Measured variable
Ref. value
Meas. value
Observation period
Fig. 15:
Drifting measured values
The cause of the drift is normally the reference electrode. Loss of electrolyte, increasing sensitivity
to flow, and also contaminations, change the potential of the electrode. This change in potential
shows itself in the drift of the measurement setup. The drift caused by the electrolyte loss accelerates
at the end of the service life to such an extent that it can only be remedied by changing the
electrolyte solution or the electrode.
Poisoning may occur if, for instance, sulfide ions penetrate the reference electrode from the water,
thereby converting the silver/silver chloride system into a silver/silver sulfide system, or block the
diaphragm. Cyanide ions can damage the silver/silver chloride system, as they dissolve the silver
chloride.
Electrolyte consumption and contamination cause a pronounced shift in the system zero during
calibration. A sensitivity to flow is often not noticeable during calibration. The effect can be seen
more clearly by moving the electrode gently in the water. The value indicated by the setup when
the electrode is being moved is different from the value when the electrode is kept stationary.
28 3 Quality assurance JUMO, FAS 622, Edition 04.07