Emerson Rosemount CCO 5500, Reference Manual

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Transmissivity of CO Figure A-4:
The characteristics of the 4.7 µm filter fix the shape of this curve, and the shape cannot
change. The curve is practically flat at CO concentrations above 10,000 ppm-meters. A
cross-duct monitor effectively measures CO molecules in its optical path so the same
concentration of CO will have a greater effect across a large measurement path than a
small measurement path. The term ppm-meters is the concentration of CO within the duct
multiplied by the gas path length (in meters) over which it has been measured.
Carbon monoxide calculation
The Rosemount CCO 5500 Analyzer takes two measurements of IR energy in the narrow
band around 4.7 µm. It takes both measurements after the beam has passed through the
gas to be measured. One beam, however, also passes through a cell containing pure CO
(the gas cell shown in
Figure A-1 ). The beam absorbs all the energy capable of being
absorbed by CO and provides a reference that is unaffected by any CO in the duct.
However, any other material (e.g., dust) which reduces the energy received from the
source unit will affect the beam.
The second beam does not pass through the cell and is very sensitive to changes in CO
within the duct.
The measurement of CO is calculated from a parameter Y, where:
Y - G-K(D2/D1)
and D2 = the live detector output
D1 = the reference detector output
K = a composite gain factor which takes account of all optical and electronic gains
G = scaling factor
Calibration
Figure A-5 shows the parameter Y plotted for the change in CO concentration.
Theory of operation
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