Emerson Rosemount 6888A, Reference Manual

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4.5.1 Probe passes calibration, O
2
still reads high
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 is capped tightly between calibrations.
2. If autocal is used, make sure the check valve is sealing properly.
Probe Leakage Paths Figure 4-2:
A. Cal gas 1/4 in. tube
B. Ref gas 1/4 in. tube
C. Vent
3. If an abrasive shield is uses, a small leak at the probe flange can migrate down the
annular space between the probe OD and shield ID to the sensing cell, causing a
false high O
2
reading.
Internal reference air leak
See Figure 4-2 . There may be a leak inside the O
2
probe itself, permitting the reference air
(20.95% O
2
) to mix with the process gases at the cell. To confirm this leak condition,
pressurize the inside (reference side) of the probe by plugging the reference air exhaust
port with your finger for one minute. (The conduit ports where the signal and power wires
pass may also need to be sealed.) 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 hose that carries the cal gas to
the cell. Inspect this hose. Dislodging or improper installation of the cal gas or
reference air hose can cause a leakage path.
2. The sensing cell is fastened to the end of the probe tube and uses a corrugated
washer to separate the process gases from the ambient reference air. The
corrugated washer may be damaged by corrosion. Discard used washer.
Troubleshooting
Reference Manual 93