Solving ground loop problems
in pH process installations

If you are struggling with unstable pH values for your process measurement, there may be ground loop problems. A ground loop exists when an electric circuit is connected to earth ground at two or more points with different potentials. Different earth grounds are supposed to be at the same potential, but the potential of the earth varies from point to point.

In a typical process pH measurement, the electrode is connected through the process liquid and piping to earth ground. The pH transmitter is in most cases grounded with a grounding wire to the power outlet or safety ground and via the electrode to the process liquid. Those are two grounding points, and what will probably happen is that current will flow through the electrode wiring. The currents created by ground loops are often fluctuating a lot, and will therefore, produce an erratic and unpredictable pH measurement.

Ground loop symptoms

The pH transmitter reading is:

  • offset from the actual process pH value by a consistent amount.
  • frozen on one value. It can be any value, maybe even off the scale. 
  • drifting, either up or down, and can start and stop at any time.

How to verify ground loop problems

  • Remove the pH electrode from the process. 
  • Calibrate the electrode according to your normal procedure.
  • Place the electrode and a sample of the process liquid in a plastic or glass beaker.
  • Read the displayed pH value and make a note of it. 
  • Place the pH electrode back into the process and take another reading

If the two readings differ more than ±0.1 pH, a ground loop exists.

Conclusion

A clear evidence of a ground loop is a pH sensor that reads correctly in buffers, but gives an error reading when placed in the process liquid.

Ground loops will ruin your pH electrode

The unwanted ground current will electrolytically destroy the reference element of your electrode. The result is short electrode life.

Possible ways to avoid ground loop problems

  • In most applications there are external mixers, pumps, motors and switches in contact with the process liquid. If possible, unplug each of these devices one at a time and watch for changes in the pH readings. Try plugging the offending equipment into a different outlet or circuit.
  • Ensure the pH measurement system is connected to a single ground point. Ground the piping or tank to a local earth. Is there still a problem? Force the grounds to the same potential by connect a wire from ground of the piping or the tank to the pH transmitter's ground.
  • Is your process installation containing of plastic piping, fiberglass tanks or ungrounded vessels? That mean your pH electrode system is floating and is highly susceptible to any noise from an electrically powered device in the media. If applicable, immerse a ground element into the process liquid constructed of a metal which will not react with the solution and hook the element to the correct terminal on your pH transmitter.
  • A common cause of ground loops is improper wiring and moisture or corrosion in a junction box. The pH sensor wiring should not run parallel to power lines as they may pick up noise. Try simplifying the wiring, to reduce the possible noise paths.
  • A signal isolator is an ideal device to use for solving earth loop problems in process control systems. The signal isolator provide galvanic isolation from one signal (input) to another (output). The input and output are electrically isolated from each other while the correct signal pass with little or no loss of accuracy. The ground loop is broken.
  • A differential pH sensor will minimize your ground loop problems.



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