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10.2 Checking the reference junction of a pH sensor.
If pH readings are inaccurate or unstable, the reference junction of the
sensor may be blocked. The following test can be performed to determine
if the reference junction of a pH sensor is making adequate contact with
the sample solution.
1. Calibrate the Aqua-pH, as per section 3.
2. Dilute 1 part of pH6.88 buffer with 9 parts of distilled water.
3. Measure the pH of the diluted buffer. The result should be 7.06 +/-0.05
pH.
4. If the value obtained is outside of these limits, then clean the reference
junction as per the instructions supplied with the pH sensor.
5. Re-calibrate the Aqua-pH and repeat the test.
6. If the value obtained is still outside 7.06 +/-0.05 pH, then the sensor
should be replaced.
10.3 Determining if an instrument or sensor is faulty
The following test can be performed to help determine if the Aqua-pH or
the pH sensor is faulty.
1. Initialise the Aqua-pH (see section 8).
2. Disconnect the pH sensor.
3. Connect the centre pin of the BNC socket with the outside frame of the
socket, using a short piece of wire or a paper clip etc.
4. The meter should read approximately 7.00. If you press the key for
2 seconds, the Aqua-pH will calibrate to around 6.88 pH, depending
upon the temperature readout.
5. If the Aqua-pH 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 1 pH or less every 3 seconds. If the drift rate is
faster than this, then input circuitry of the Aqua-pH may be faulty and
could require servicing.
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