11.3 Guard Connector
In some circumstances, the pH or mV readings may become unstable. This may
be due to static charge in the sample vessel, or electrical noise from nearby
electrical equipment. In these cases, a solution guard may eliminate the problem.
A solution earth rod is available from TPS (part no 121360). This connects
directly to the Guard socket. Alternatively, run a wire from the Guard socket to
a stainless steel fitting in contact with the sample.
11.4 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 901-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 901-PH and repeat the test.
6. If the value obtained is still outside 7.06 +/-0.05 pH, then the sensor should be
replaced.
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