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Oxymitter 4000
8-22
Instruction Manual
IM-106-340, Rev. 4.0
May 2006
CALIBRATION PASSES,
BUT STILL READS
INCORRECTLY
There are a few fault conditions where no alarm indication is present and the
probe passes calibration, but the O
2
reading may still be incorrect:
Probe passes calibration, but still appears to read high
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 sucked into the cell area, biasing the O
2
reading upward.
1. Make sure that the calibration gas line is capped tightly between
calibrations. If autocal is used, make sure the check valve is 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.
There may be a leak inside the probe itself, permitting the reference air
(20.95% O
2
) to mix with the process gases at the cell. To confirm this leak
condition, instrument air will need to be connected for reference. Pressurize
the inside (reference side) of the probe by plugging the reference air exhaust
port with your finger for 1 minute. The O
2
reading should decrease slightly. If
the O
2
reading increases during this test, there is a leak inside the probe.
1. Acid condensation inside the probe can degrade the red silicon tube
(28, Figure 9-3) that carries the cal gas to the cell. Remove housing (21)
to inspect this hose. (See Section 9: Maintenance and Service). Black
vitan material is optionally available with greater chemical resistance.
Figure 8-18. Probe Leakage
Paths
2. The sensing cell is bolted to the end of the probe, and uses a corrugated
metallic seal (5, Figure 9-3) to separate the process gases from the
ambient reference air. This seal can be used only one time, so always
replace this seal when a cell is removed or replaced. Always apply
anti-seize compound on both sides of the corrugations.
BOTTOM VIEW
PROBE HEAD
Corrugated
Seal
Reference Air
Exhaust Port
Probe
Flange
Gasket