A ground loop exists when a circuit is connected to earth ground at two or more points. Because the potential of the earth varies from point to point, two or more connections to ground cause currents to flow. If the current flows through a signal carrying wire, the result is a noisy, offset signal.
The classic symptom of a ground loop is a sensor that reads correctly in buffers, but gives a reading grossly in error when placed in the process liquid. In a typical process measurement, the pH sensor is connected through the process liquid and piping to earth ground. If the circuitry in the pH analyzer becomes connected to a second earth ground, current will flow through the reference electrode. A voltage proportional to the current and the electrode resistance develops across the reference electrode. Because the voltage is in series with the other cell voltages, the ground loop current causes the pH reading to be substantially different from the expected value. The currents created by ground loops are often unstable, so pH readings affected by ground loops are often noisy.
Use the following procedure to check for ground loops:
- Remove the pH sensor from the process liquid.
- Calibrate the sensor in buffers. Be sure there is no direct electrical connection between the container holding the buffer and the process liquid or piping.
- Strip back the ends of a heavy gauge wire. Connect one end of the wire to the process piping or, better, place it in the process liquid. Place the other end of the wire in the container with the buffer and sensor. The wire makes an electrical connection between the process and sensor.
- If the pH reading changes or becomes noisy after making the connection, a ground loop exists. If no symptoms develop a ground loop probably does not exist.