Probe Passes Calibration, But Still Appears To Read Low; How Do I Detect A Plugged Diffuser - Emerson Rosemount Oxymitter 4000 Reference Manual

July 2018
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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. In applications that have a heavy particulate
loading (coal or wood fired boilers, cement and lime kilns, catalyst regeneration,
recovery boilers, etc.), this diffusion element will eventually plug.
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
diffuser is installed. As the diffuser plugs do not adjust the flow rates upward.

How do I detect a plugged diffuser?

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become smoother. When calibrating, the calibration gas flow rate will be noted to be
lower. Never readjust this flow upwards. Adjust this flow only when a new diffuser is
installed.
Always note the time it takes for the cell to recover to the normal process value after
the cal gas is removed. As the diffuser plugs this recovery time will get longer and longer.
Use the Calibration Record form provided in this manual.
Can I calibrate a badly plugged diffuser?
It may not be possible to immediately replace a plugged diffuser while the process is on
line. One can calibrate the probe without pressurizing the cell by adjusting the
calibration gas flow rate downward before calibration. For instance, say the process is at
3% and the first calibration gas is 8%. Adjust the flow of cal gas downward until the
reading begins to migrate from 8% to lower values indicating that process gases are now
mixing in with the calibration gases.
Adjust the flow rate back up until this mixing is just eliminated. Calibrate at this flow rate.
Replace the diffuser at the first opportunity.
Troubleshooting
The sensing cell is bolted to the end of the probe and uses a corrugated
metallicseal (item 25, 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.
signal's speed of response will degrade. The O
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WARNING
Install all protective equipment covers and safety ground leads after installation.
Failure to install covers and ground leads could result in serious injury or death.
Rosemount Oxymitter 4000
trend in the control room will
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