Thermo Scientific AquaSensors
™
AnalogPlus
™
Differential pH Sensor User Guide
26
4.3.
pH Sensor Troubleshooting
General Inspection
If the sensor is not providing reasonable signals to the analyzer, check the following:
1.
Inspect the integrity of the glass electrodes. If the electrode is broken, replace the sensor.
2.
Inspect the integrity of the salt bridge junctions. Be sure that they are clean and moist. If the salt bridge has
been allowed to dry out it may be necessary to replace the salt bridge and filling solution.
3.
Inspect the sensor cable for damage to the outer jacket. Any cuts or kinks may damage signal
connections.
4.
Inspect terminal block connections to be sure wires are not corroded or loose.
5.
Make sure all sensor wires are connected to the correct places on the analyzer terminal block. Depending
on the analyzer, some terminal block jumpers may be required and it is common to put those jumpers in the
wrong place.
6.
The sensor electrodes should be immersed in a solution of known pH.
7.
Establish that the analyzer electronics are working correctly by verifying operation with another sensor.
Note:
Any new sensor that has been in storage for more than a few days should be soaked
in tap water or a solution of known pH for at least 30 minutes before reviewing
measurement performance.
Note:
The salt bridge of any pH sensor in storage must be kept moist at all times.
Protective caps should be filled with Thermo Scientific AquaSensors storage solution for
best performance. The protective cap should be placed securely over the sensor head.
If the sensor passes the general inspection, there are four basic tests that can verify the temperature
measurement, the pH offset, the pH span and the pH span on sensor power. If the sensor passes these tests
it should be ready for service.
Temperature Span (For Sensors that Connect to the P99 or GLI/Hach Analyzers)
Disconnect the BLACK and YELLOW sensor wires from the analyzer. Use an ohmmeter to measure the
resistance between the BLACK and YELLOW wires. The meter should measure a resistance corresponding
to the sensor temperature as outlined in the following table.
Sensor
Temperature
5°C 15°C 25°C 35°C 45°C
Thermistor Resistance
630
Ω
430
Ω
300
Ω
215
Ω
155
Ω
If the measured resistance is within 10 to 20 ohms of the expected value, the element should be satisfactory.
If not, please consult the factory.
