Emerson Rosemount 5081FG, Reference Manual

Page: 76 / 105

Reference Manual
00809-0100-4882, Rev AB
May 2019
Rosemount 5081FG
5-8
Figure 5-11. Fault 7, Reversed O
2
Cell
Fault 7, Reversed O
2
Cell
The O
2
cell connections are reversed. The fault
displays as shown in Figure 5-11.
1. Refer to Figure 5-4. Check the black (to
terminal 7) and white (to terminal 8) wires
for the proper placement. Rewire if
necessary.
2. Apply the low calibration check gas
(0.4% O
2
).
3. Using a multimeter measure between
terminals 7(+) and 8(-). If the cell output
reading is negative, the O
2
cell wiring is
reversed.
4. If the wiring is correct, check if the
multimeter reading is the same as the
reading shown on the O
2
CELL mV
diagnostics screen (Section 3: Startup and
Operation).
5. If the reading is different the transmitter
electronics are faulty. Replace the faulty
analog or CPU board per Electronics
Replacement in Section 6: Maintenance
and Service.
CALIBRATION PASSES,
BUT STILL READS
INCORRECTLY
Probe Passes
Calibration, O
2
Still
Reads High
There are fault conditions where no alarm indication is present and the probe
passes calibration, but the O
2
reading may still be incorrect:
External Reference Air Leak - There may be a leak that is permitting
ambient air to mix with the process gases. Since many combustion processes
are slightly negative in pressure, ambient air can be drawn into the cell area,
biasing the O
2
reading upward.
1. Make sure that the calibration gas line is capped tightly between
calibrations.
Bad Reference Side Cell Electrode - A bad reference side cell electrode can
cause an elevated O
2
reading. This fault is usually indicated by a frequent
“Calibration Recommended” alarm and increasing cell impedance readings. A
high cell impedance can be calibrated out, but if the impedance continues to
increase rapidly, the sensing cell must be replaced.