Emerson ROSEMOUNT 5081-A Instruction Manual page 117

MODEL 5081-A
15.8.7 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 prob-
lem arises from the process or installation. It is not a fault of the transmitter. the problem should disappear once the sen-
sor 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 elec-
trical 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, 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.
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. First, verify that pH sensor wiring is correct.
b. disconnect all sensor wires at the transmitter except pH/mV IN, REFERENCE IN, Rtd IN and Rtd REtuRN. See
the wiring diagrams in Section 3.0. If the sensor is wired to the transmitter through a remote junction box contain-
ing a preamplifier, disconnect the wires at the sensor side of the junction box.
c. tape back the ends of the disconnected wires to keep them from making accidental connections with other wires
or terminals.
d. Connect a jumper wire between the Rtd REtuRN and Rtd SENSE terminals (see wiring diagrams in Section 3.0).
e. If noise and/or offsets disappear, the interference was coming into the transmitter 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. If the sensor cable is run inside conduit, there may be a short between the cable and the conduit. Re-run the cable
outside the conduit. If symptoms disappear, there is a short between the cable and the conduit. Likely a shield is
exposed and touching the conduit. Repair the cable and reinstall it in the conduit.
b. 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.
c. If ground loops persist, consult the factory. A visit from a service technician may be required to solve the problem.
15.8.8 Process readings are noisy.
1. What is the conductivity of the sample? Measuring pH is samples having conductivity less than about 50uS/cm can be
very difficult. Special sensors (for example, the Model 320HP) are often needed and special attention must be paid to
grounding and sample flow rate.
Measuring free chlorine in samples having low conductivity can also be a problem. Generally,
for a successful chlorine measurement, the conductivity should be greater than 50 µS/cm.
2. Is the sensor dirty or fouled? Suspended solids in the sample can coat the reference junction and interfere with the
electrical connection between the sensor and the process liquid. the result is often a noisy reading.
3. Is the sensor properly wired to the transmitter? See Section 3.0.
4. Is a ground loop present? Refer to Section 15.8.7.
NOTE:
SECTION 15.0
TROUBLESHOOTING
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