Emerson Process Management IM-106-340 Calibration PASSES, but Still Reads Incorrectly

Oxymitter 4000

Instruction Manual

IM-106-340, Rev. 4.0 May 2006

CALIBRATION PASSES, BUT STILL READS INCORRECTLY

Figure 8-18. Probe Leakage Paths

There are a few fault conditions where no alarm indication is present and the probe passes calibration, but the O2 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 O2 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% O2) 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 O2 reading should decrease slightly. If the O2 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.

Corrugated	Reference Air	PROBE HEAD
Seal	Exhaust Port

Probe

Flange BOTTOM VIEW

Gasket

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.

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