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.
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
- 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.