Emerson Process Management OXYMITTER 4000 IM-106-340 - Manual (Page 115)

Instruction Manual
IM-106-340, Rev. 4.0
May 2006
8-23
Oxymitter 4000
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?
The O
2
signal's speed of response will degrade. The O
2
trend in the control
room will 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.
Install all protective equipment covers and safety ground leads after troubleshooting. Failure
to install covers and ground leads could result in serious injury or death.