Instruction Manual
IM-106-910Xi, Original Issue November 2010
Xi Advanced Electronics
Probe Passes
Calibration, O2 Still
Reads High
Figure 5-2. Probe Leakage Paths
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 O2 reading upward.
1.Make sure that the calibration gas line is capped tightly between calibrations.
2.If autocal is used, make sure the check valve is seating properly.
Internal Reference Air Leak - See Figure 5-2. There may be a leak inside the O2 Probe itself, permitting the reference air (20.95% O2) 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 1 minute. (The conduit ports where the signal and power wires pass may also need to be sealed.) 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 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.
NOTE
You should aways install a new corrugated washer whenever you remove the sensing cell from the probe.
Bad Reference Side Cell Electrode - A bad reference side cell electrode can cause an elevated O2 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.
