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Instruction Manual
IM-106-5500, Original Issue
August 2005
CCO 5500
1-10
Calibration
Figure 1-6 shows the parameter Y against the CO concentration.
Figure 1-6. Calibration Curve
This is the calibration curve for the instrument and is opposite in shape to the
transmissivity curve shown in Figure 1-5. Each is fixed by the characteristics
of the 4.7µm filter and cannot change. Rosemount Analytical 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 is 0 when the CO level is 0
If there is any drift in the measurement, it can only be due to a change in
some optical or electronic gain and can always be corrected 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 1-6 that at high CO levels, the parameter Y
becomes completely insensitive to variable CO levels in the duct, such that:
Y = a when the CO >
b
By making Y = a when CO >
b, Y = 0 when CO = 0 and all these errors are
eliminated.
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 regularly - every few seconds - to
continuously check and (if necessary) modify their zero position.
A
B
CO
In Duct
Practical
Limit
Calibration
Point
Calibration Point
Including
CO in Duct
12
4
567891011123
CO ppm.m (x1000)
0
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Parameter Y