17.3.5 Determining if an instrument or electrode is faulty
The following test can be performed to help determine if the 90-FL or the pH electrode is faulty.
1. Initialise the 90-FL (see section 12).
2. Disconnect the pH electrode from the pH connector.
3. Connect the centre pin of the pH connector with the outside frame of the connector, using a
short piece of wire or a paper clip etc.
4. The meter should display approximately pH7.00, depending on the current calibration settings.
5. If the 90-FL is operating correctly, the reading should be totally stable with the wire firmly in
place. If not, the meter requires servicing.
6. Now carefully disconnect the wire from the centre pin only (make sure the other end of the
wire remains connected to the outside frame of the connector).
7. After an initial jump, the reading should steadily drift away from 7.00 (either up or down) at a
rate of approximately 1 pH or less every 3 seconds. If the drift rate is faster than this, the input
circuitry of the 90-FL is faulty and requires servicing.
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