Emerson Rosemount5081 - Buffer Calibration Is Acceptable; Process Ph Is Slightly Different from Expected Value; Buffer Calibration Is Acceptable; Process Ph Is Grossly Different from Expected Value

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.
Buffer calibration is acceptable; process pH is grossly
different from expected value
The systems suggest a ground loop (measurement system connected to earth ground at
more than one point), a floating system (no earth ground), or noise being induced into the
transmitter by a 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.
1. To confirm a ground loop...
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 similar symptoms develop after making the connection, a ground loop exists. If
no symptoms develop, a ground loop may or may not exist.
2. Check the grounding of the process.
a. 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.
b. Ground the piping or tank to a local earth ground. Metal tees, grounding rings, or
grounding rods may be required.
c. If problems persist, connect a wire from 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.
d. 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 3.
3. Simplify the sensor wiring.
a. Disconnect all sensor wires at the transmitter except: TB-4 (RTD SNS), TB-5 (RTD
IN), TB-7 (REF IN), and TB-10 (pH/ORP IN). If a remote preamplifier is being used,
disconnect the wires at the input side of the junction box.
b. 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.
Diagnostics and troubleshooting
Instruction Manual 71