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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.