Emerson Rosemount5081, Instruction Manual, Page: 121 / 152

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. If the sensor is wired to the transmitter through a
remote junction box containing a preamplifer, 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.
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 simplified wiring.
4. Check for 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 the
power cables, relays, and electric motors. Keep sensor wiring out of crowded
panels and cable trays.
c. If ground loops persist, contact the factory. A visit from a service technician may
be required to solve the problem.
Process readings noisy
1. What is the conductivity of the sample? Measuring pH in samples having
conductivity less than about 50 µS/cm can be very difficult. Special sensors (for
example, the 320HP) are often needed and special attention must be paid to
grounding and sample flow rate.
NOTICE
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.
Diagnostics and troubleshooting
Instruction Manual 111