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Instruction Manual
IM-106-5500, Original Issue
August 2005
CCO 5500
1-8
Carbon Monoxide IR Absorption Spectrum
Carbon monoxide absorbs IR energy in a band between wavelengths of
approximately 4.5 and 4.9 µm. The absorption spectrum is complex and is
illustrated in Figure 1-3 below.
Figure 1-3. CO IR Absorption
Spectrum
However, two other common flue gas constituents - carbon dioxide and water
vapor - also absorb energy within this wave band. Fortunately, at 4.7µm, IR
absorption by each of these gases is at a minimum. Figure 1-4 demonstrates
how the absorption spectra of CO, CO
2
and water vapor affect wavelengths of
between 4.5 and 4.9 µm.
Absorption Spectra of CO, CO
2
and Water Vapor
By using a narrow band pass filter which only passes IR energy at
wavelengths of around 4.7µm, correctly designed CO analyzers are able to
ignore the effects of water vapor and CO
2
. (The filter characteristics are
shown in Figure 1-4). No other flue gases absorb IR energy in this band.
Figure 1-4. Comparison of
Spectra
4.5 4.6 4.7 4.8 4.9
Wavelength (microns)
Absorption (%)
0
4.5 4.6 4.7 4.8 4.9
Wavelength (microns)
Absorption (%)
0
CO
2
HO
2
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