Emerson Rosemount CCO 5500, Reference Manual

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Calibration Curve Figure A-5:
This is the calibration curve for the instrument, opposite in shape to the transmissivity
curve shown in
Figure A-4 . Characteristics of the 4.7 µm filter fix each curve, and the curves
cannot change. Rosemount analyzers make full use of this scale shape to provide an easily
attainable calibration point.
It is not necessary to calculate K because we know that when the constant K is correct (Y =
0 when the CO level = 0) any drift in the measurement can only be due to a change in some
optical or electronic gain. You can always correctthe drift by setting Y to zero when the CO
level is zero.
In practice, however, it is not always possible to produce a zero CO level. But if we consider
the calibration curve, we can see that:
if Y = 0 when CO = 0
then Y = a when CO = b
We can also see from Figure A-5 that at high CO levels the parameter Y becomes
completely insensitive to changing CO levels in the duct, such that:
Y = a when CO > b
By making Y = a when CO > b and Y = 0 when CO - 0, you can eliminate all errors.
You can introduce a gas cell containing pure carbon monoxide can be introduced into the
IR beam at the source. This cell represents a value of 10,000 ppm-meters and provides a
reference point for the calibration of the instrument. Any further CO in the duct will have
negligible effect on the reference point because the calibration curve is flat at these high
concentrations of CO. Well designed cross-duct analyzers introduce this gas cell every few
seconds to continuously check and (if necessary) modify the zero position.
Theory of operation
112 Rosemount CCO 5500