When a pH measurement loop is producing unstable, erratic, or maybe no response, it's necessary to find the reason. In some cases, the application may be the problem. Process knowledge and measurement history are important factors in determining if a pH reading is correct.
If you assess that the abnormal
measurement value is not related to the process media, you need to check
the used pH loop. The main components of a typical pH loop are the
electrode, cable and transmitter. Any of these, individually or in
combination, can be the source of the problem.
Even the majority of measurement problems can be traced to the pH electrode, one of the first things you need to do is to isolate the problem. The recommended strategy to use is to interchange components in the order of electrode, cable and transmitter.
A useful tool for pH transmitter troubleshooting and fault diagnosis is
a pH Simulator. A pH Simulator is an electronically device that
simulates a pH electrode, and it can quickly identify or eliminate the
transmitter or cable as the cause of the problem.
If the pH value reading on the transmitter corresponds to the attitude of the pH simulator, the function of the transmitter and cable are okay, and you can focus on the pH sensor.
If the transmitter still displays values, although you are using a pH simulator, that are unstable and erratic, one of three things might be wrong:
To pinpoint the pH measurement loop problem, connect a new (short) cable between the transmitter and the simulator and measure again.
Have you identified the pH sensor as the issue? Then does the following:
3. If the sensor does not have any notable defects, calibrate it.
A common problem when an on-line pH system is unstable, is an electrical ground loop in the system.