Emerson Rosemount 6888A, Reference Manual

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NOTICE
Always 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 O
2
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.
4.5.2 Probe passes calibration, O
2
still reads low
The diffusion element at the end of the probe is a passive filter. It plugs very slowly,
because 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.
NOTICE
It is important not to pressurize the sensing cell during calibrations by flowing excessive cal
gas against a plugged diffuser. Always use a two-stage regulator for setting calibration gas
pressure. Calibration flow rates should be set only when a new diffuser is installed. As the
diffuser plugs, do not adjust the flow rates upward.
4.5.3 How do I detect a plugged diffuser?
The O
2
cell'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 to correct for a plugged diffuser. 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
a calibration record to record and track calibration response times.
The Rosemount 6888Xi Advanced Electronics is available with an enhanced software
feature to automatically characterize the rate of diffuser pluggage during a calibration
cycle.
4.5.4 Can I calibrate a badly plugged diffuser?
It may not be possible to immediately replace a plugged diffuser while the process is on-
line.
Troubleshooting
94 Rosemount 6888A