MODEL 3081 pH/ORP | SECTION 12.0 |
| TROUBLESHOOTING |
12.5.8 Buffer Calibration Is Acceptable; Process pH is Grossly Different from Expected Value.
The symptoms suggest a ground loop (measurement system connected to earth ground at more than one point), a float- ing system (no earth ground), or noise being induced into the transmitter by sensor cabling. The problem arises from the process or installation. It is not a fault of the transmitter. The problem should disappear once the sensor is taken out of the system.
A.To confirm a ground loop...
1.Verify that the system works properly in buffers. Be sure there is no direct electrical connection between the buffer containers and the process liquid or piping.
2.Strip back the ends of a heavy gauge wire. Connect one end of the wire to the process piping or place it in the processliquid. Place the other end of the wire in the container of buffer with the sensor. The wire makes an electrical connection between the process and sensor.
3.If similar symptoms develop after making the connection, a ground loop exists. If no symptoms develop, a ground loop may or may not exist.
B.Check the grounding of the process.
1.The measurement system needs one path to ground: through the process liquid and piping. Plastic piping, fiber glass tanks, and ungrounded or poorly grounded vessels do not provide a path. A floating system can pick up stray voltages from other electrical equipment.
2.Ground the piping or tank to a local earth ground. Metal tees, grounding rings, or grounding rods may be required.
3.If problems persist, connect a wire from the the ground connection at the dc power supply to the transmitter case. Connect a second wire from the transmitter case to the process. These connections force the grounds to the same potential.
4.If the problem persists, simple grounding is not the problem. Noise is probably being carried into the instrument through the sensor wiring. Go to step C.
C.Simplify the sensor wiring.
1.Disconnect all sensor wires at the transmitter except:
2.Tape back the ends of the disconnected wires, including all shield and drain wires, to keep them from making accidental connections with other wires, terminals, or the transmitter case.
3.Connect a jumper wire between
4.Place the sensor back in the process liquid. If diagnostic messages such as GLASSFAIL or REF WArn appear, turn off the sensor diagnostics. See Section 8.4.
If the symptoms disappear, interference was coming into the transmitter along one of the sensor wires. The measurement system can be operated permanently with the simplified wiring.
If symptoms still persist, go to step D.
D.Check for extra ground connections or induced noise.
1.The electrode system is connected to earth ground through the process. If other ground connections exist, there are multiple paths and ground loops are present. Noise enters the measurement either by a direct connection, usually between the cable and grounded metal, or by an indirect connection, usually EMI/RFI picked up by the cable.
2.If the sensor cable is run inside conduit, there may be a short between the cable and the conduit.
3.To avoid induced noise in the sensor cable, run it as far away as possible from power cables, relays, and electric motors. Keep sensor wiring out of crowded panels and cable trays.
