MODEL | SECTION 9.0 |
| TROUBLESHOOTING |
9.6.5 Calibration Was Successful, but Process pH Is Grossly Wrong and/or Noisy.
Grossly wrong or noisy readings suggest a ground loop (measurement system connected to earth ground at more than one point), a floating system (no earth ground), or noise being brought into the analyzer by the sensor cable. The problem arises from the process or installation. It is not a fault of the analyzer. The problem should disappear once the sensor is taken out of the system. Check the following:
1.Is a ground loop present?
a.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.
b.Strip back the ends of a heavy gauge wire. Connect one end of the wire to the process piping or place it in the process liquid. 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.
c.If offsets and noise appear after making the connection, a ground loop exists.
2.Is the process grounded?
a.The measurement system needs one path to ground: through the process liquid and piping. Plastic piping, fiberglass tanks, and ungrounded or poorly grounded vessels do not provide a path. A floating system can pick up stray voltages from other electrical equipment.
b.Ground the piping or tank to a local earth ground.
c.If noise still persists, simple grounding is not the problem. Noise is probably being carried into the instrument through the sensor wiring.
3.Simplify the sensor wiring.
a.Disconnect all sensor wires at the analyzer except, IN REFERENCE, IN pH, RTD IN and RTD RETURN. See the wiring diagrams in Section 3.2.
b.Tape back the ends of the disconnected wires to keep them from making accidental connections with other wires or terminals.
c.Connect a jumper wire between the RTD RETURN and RTD SENSE terminals (see wiring diagrams in Section 3.2).
d.If noise and/or offsets disappear, the interference was coming into the analyzer through one of the sensor wires. The system can be operated permanently with the simplified wiring.
4.Check for extra ground connections or induced noise.
a.To avoid induced noise in the sensor cable, keep the unit as far away as possible from power cables, relays, and electric motors.
b.If ground loops persist, consult the factory. A visit from an experienced technician may be required to solve the problem.
9.6.6 pH Readings Are Moderately Noisy and Tend to Wander.
pH readings that are moderately noisy (±0.1 pH) and tend to wander are probably caused by bubbles getting trapped against the pH sensor. Although the overflow sampler is designed to allow bubbles to escape before they reach the pH sensor and the sensor itself is designed so trapped air bubbles don’t interfere with the measurement, bubbles may occasionally be a problem. Shaking the sensor will dislodge the bubbles. If bubbles remain a problem, contact the factory.
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