Emerson Rosemount FCL Reference Manual page 100

Troubleshoot
May 2019
9.6.4 Buffer calibration is acceptable; process pH is slightly
different from expected value.
Differences between pH readings made with an on-line instrument and a laboratory or
portable instrument are normal. The on-line instrument is subject to process variables (for
example, ground potentials, stray voltages, and orientation effects) that do not affect the
laboratory or portable instrument.
To make the process readings agree with a referee instrument, see
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 transmitter by the sensor cable.
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. Check the following:
Recommended actions
1. Confirm a ground loop.
2. Ground the piping or tank to a local earth ground.
3. Simplify the sensor wiring.
100
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.
If offsets and noise appear after making the connection, a ground loop exists.
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.
If noise persists, simple grounding is not the problem. Noise is probably being
carried into the instrument through the sensor wiring. Go to
a) Disconnect all sensor wires at the transmitter except: IN REFERENCE, IN pH,
RTD IN, and RTD RETURN.
See the wiring diagrams in
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 the wiring diagrams in
Wire sensor.
Wire sensor.
Reference Manual
00809-0100-3412
Standardize pH value.
Step 3.
Emerson.com/Rosemount