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22.5 Determining if a pH meter or an sensor is faulty
The following test can be performed to help determine if the
WP-91
or
the pH sensor is faulty.
1. Initialise the
WP-91
(see section 18).
2. Disconnect the pH sensor.
3. Connect the centre pin of the
pH/mV
connector with the outside frame
of the connector, using a short piece of wire or a paper clip etc.
4. The meter should read approximately pH7.00, depending on the
current calibration settings.
5. If the
WP-91
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. The reading should steadily drift away from 7.00 (either up or down)
at a rate of approximately 0.10 pH or less every 3 seconds. If the drift
rate is faster than this, then input circuitry of the
WP-91
is faulty and
requires servicing.