Emerson Rosemount Oxymitter 4000, Reference Manual, Page: 146 / 198

8.7 Calibration passes, but still reads incorrectly
There are a few conditions where no alarm indication is present and the probe passes
calibration, but the O
2
reading is still incorrect.
8.7.1 Probe passes calibration, but still appears to read too
high
Potential cause
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 caqn be sucked
into the cell area, biasing the O
2
reading upward.
Recommended actions
1. Ensure that the calibration gas line is capped tightly between calibrations. If using
Autocal, make sure to check the valve seating properly.
2. If an abrasive shield is installed to protect the entire probe from particulate erosion,
a leak in the probe flange gasket can allow ambient air to migrate down the annular
space between the probe and shield and then into the cell. Always install a new
probe flange gasket when reinstalling a probe.
3. Check calibration gas hoses or tubing for leaks. Repair leaks in hose or tubing.
4. Check cell flange corrugated seal for leaks. Replace seal.
Potential cause
There may be a leak inside the probe itself, permitting the reference air (20.95 percent O
2
)
to mix with the process gases at the cell.
Recommended actions
1. To confirm this leak condition, connect instrument air for reference. Pressurize the
inside (reference side) of the probe by plugging the reference air exhaust port with
your finger for one minute.
The O
2
reading should decrease slightly. If the O
2
reading increases during this test,
there is a leak inside the probe.
Acid condensation inside the probe can degrade the red silcone tube that carries
the cal gas to the cell.
2. Remove the housing (11, Figure 2-18 ) to inspect this hose.
See Maintenance and service .
Troubleshoot Reference Manual
October 2019 00809-0100-4340
146 Emerson.com/Rosemount