pH Measurement loop

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.  

Isolate the problem

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.

pH Transmitter troubleshooting

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.

  • Disconnect the electrode from the pH transmitter.
  • Connect a pH Simulator in the position where the electrode was connected.
  • For further instruction, see the manual of the pH Simulator.

Go on troubleshooting the pH sensor

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.

Go on troubleshooting the pH transmitter and cable

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:

  • The transmitter cable might be too long.
  • The transmitter cable contains kinks, breaks, loose connectors, etc. 
  • The transmitter is broken.

To pinpoint the pH measurement loop problem, connect a new (short) cable between the transmitter and the simulator and measure again.

pH Sensor troubleshooting

Have you identified the pH sensor as the issue? Then does the following:

  1. If the sensor is dirty or fouled, clean it.

  2. Ensure there are no visible electrode defects, for example:
  • Inspect the probe tip, so it's not cracked, discolored, or contaminated.
  • Check the area around the reference junction. Clogging of the diaphragm?
  • If you have a liquid or gel-filled electrolyte electrode, check if there is any discoloration of the reference electrolyte. Contamination of the reference electrolyte will make the electrode unusable.

       3. If the sensor does not have any notable defects, calibrate it.

Is there still a problem?

A common problem when an on-line pH system is unstable, is an electrical ground loop in the system.

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