36 Configuration, Startup and Operation
Section 3: Configuration, Startup and Operation Reference Manual
January 2019 00809-0100-4892
purchasing the acid resistant stoichiometer cell and the stoichiometer feature inside the
6888Xi.
3.3.3 Programmable Reference
The zirconium oxide sensing technology has historically measured process oxygen by using
ambient or instrument air as a reference (20.95% oxygen). The sensor develops most of its sig-
nal at the low oxygen levels typically found in combustion flue gasses (2-4% oxygen), and is
most accurate at these levels. When measuring near 20.95% O2, the sensor develops only a few
millivolts of signal, and accuracy degrades.
The programmable reference feature permits the user to use a bottled reference gas of low oxy-
gen value (0.4% oxygen recommended). When measuring at or near 21% oxygen, a strong neg-
ative oxygen signal results, with much improved accuracy. A bottle of reference gas typically
lasts about a month at the low flows required. Typical applications would be:
Flue gas recirculation - controlling the mixing of flue gasses into the burner windbox prior ahead
of the burner to reduce NOx emissions.
Moisture monitoring - measuring the amount of moisture coming off of industrial dryers by not-
ing the dilution effect water vapor has on the normal 20.95% ambient drying air. (Non-combus-
tion drying processes only.)
Enriched oxygen concentration - pure oxygen is sometimes mixed in with the combustion air to
increase heat at the flame. This is used in steel and other metals reduction processes and in
some catalyst regenerators.
3.3.4 Plugged Diffusion Element Diagnostic
In situ analyzers do very well in high particulate flue gases resulting from processes as such coal
or biofuel boilers, or lime and cement kilns because the passive filter, or "diffusion element" does
not foul or plug off easily. Since the probe sensing cell is inserted entirely into the flue gas stream,
the process gasses can diffuse (migrate) into the cell area with minimal fouling of the filter
media. After many months or years of operation, however, the diffusion element may plug off.
The operator at the DCS console may notice that the O
2
measurement is not as active as previ-
ously or that the speed of response back to the process (purge time) after calibration gases are
removed has increased considerably. A new diffusion element will start to come back to the
process value in 3-5 seconds (T
initial
), and will be all the way back to the process reading in 30-40
seconds (T
final
). As the diffuser plugs off over many months, these times will get longer and
longer.
Another indication of a plugged diffuser is a large increase of the "cell constant" after a
calibration. A slower speed of response not only delays O
2
information for the operator or the
automatic O
2
trim control loop, it can also cause technicians to induce a calibration error while
doing calibrations. Published specifications call for a 5 SCFH flow of calibration gases with a new
diffuser. This slightly pressurizes the cell area with cal gas, ensuring that no flue gases mix in
with the calibration test gasses. As the diffuser plugs off over time, the calibration flow rate will
drop and the cell area becomes increasingly more pressurized. Pressurizing the sensing cell
NOTE
Make sure the DCS is configured for the same range as the 6888Xi. For instance: -1 O2 to 10 O2.
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