Instruction Manual

IB-106-340 Rev. 3.0 December 2003

Oxymitter 4000

8-6 OXYMITTER 4000 PASSES CALIBRATION,BUT STILL READS INCORRECTLY

There are a few fault conditions where no alarm indication is present and the probe passes cali- bration, but the O2 reading may still be incorrect:

a.Probe passes calibration, but still appears to read high.

1.There may be a leak that is permitting ambient air to mix with the process gases. Since many combustion proc- esses are slightly negative in pressure, ambient air can be sucked into the cell area, biasing the O2 reading upward.

(a)Make sure that the calibration gas line is capped tightly between cali- brations. If autocal is used, make sure the check valve is seating properly.

(b)If an abrasive shield is installed to protect the entire probe from par- ticulate 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 re- installing a probe.

2.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 con- dition, instrument air will need to be connected for reference. Pressurize the inside (reference side) of the probe by plugging off the reference air ex- haust 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.

(a)Acid condensation inside the probe can degrade the red silicon tube (item 28, Figure 9-3)that carries the cal gas to the cell. The blue electronics housing or probe head needs to be removed to inspect this hose (See Section 9, MAIN-

TENANCE AND SERVICE).

CORRUGATED

REFERENCE AIR

PROBE HEAD

SEAL

EXHAUST PORT

 

PROBE

 

FLANGE

BOTTOM VIEW

GASKET

 

37260045

Figure 8-18. Probe Leakage Paths

(b)The sensing cell is bolted to the end of the probe, and uses a cor- rugated metallic seal (item 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 replaced. Always apply anti-seize compound on both sides of the corrugations.

b.Probe passes calibration, but still appears to read low.

The diffusion element at the end of the probe is a passive filter. It plugs very slowly, since there is no active flow being drawn across it. This diffusion element will eventu- ally plug in applications that have a heavy particulate loading (coal or wood fired boil- ers, cement and lime kilns, catalyst regen- eration, recovery boilers, etc.) It is important not to pressurize the sensing cell during calibrations by flowing excessive cal gas against a plugged diffuser. Calibration flow rates should be set only when a new dif- fuser is installed. As the diffuser plugs, do not adjust the flow rates upward.

How do I detect a plugged diffuser?

1.The O2 signal's speed of response will degrade. The O2 trend in the control room will become smoother.

2.When calibrating, the calibration gas flow rate will be noted to be lower. Never readjust this flow upwards. Ad- just this flow only when a new diffuser is installed.

8-20 Troubleshooting

Rosemount Analytical Inc. A Division of Emerson Process Management