CODEL Energy Tech 401 Installation, Operation And Maintenance Manual page 22

EnergyTech 401
Operating and Maintenance Manual
2.2.5 Principles of Cross-Duct
Gas Analysers
2.2.5.1 Carbon Monoxide IR
Absorption Spectrum -
Figure 3 : CO IR Absorption Spectrum
2.2.5.2 Absorption Spectra
CO, CO & Water Vapour
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Cross-duct analysers work on the basic principle that infrared (IR) energy is
absorbed by particular gases in a manner very specific to the gas.
Although cross-duct analysis will differ from gas to gas, the basic principles are
similar for all measured gases. This section examines the analysis of carbon
monoxide in detail, but briefly considers the analysis of NO.
Carbon monoxide absorbs IR energy in a band between wavelengths of
approximately 4.5 and 4.9µm.
illustrated in Figure 3 below.
However, two other common flue gas constituents - carbon dioxide and water
vapour - also absorb energy within this wave band. Fortunately, at 4.7µm, IR
absorption by each of these gases is at a minimum. Figure 4 demonstrates how
the absorption spectra of CO, CO
between 4.5 and 4.9 µm.
By using a narrow band pass filter which only passes IR energy at wavelengths of
around 4.7µm, correctly designed CO analysers are able to ignore the effects of
water vapour and CO . (The filter characteristics are shown in Figure 4). No
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other flue gases absorb IR energy in this band.
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The absorption spectrum is complex and is
and water vapour affect wavelengths of
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