Function description
39
5.3 Infrared cell
The measuring principle is based on the infrared radiation absorbing properties of
many gases. Each of these gases absorbs specific characteristic spectral
wavelengths of infrared radiation. The absorption spectrum is determined by the
number, configuration and type of the atoms in the gas molecules.
Fig. 15: Infrared cell
An infrared source is electrically heated emitting wide band infrared radiation. The
radiation beam is interrupted by a rotating chopper blade, resulting in alternating
light. The rotary speed of the chopper is crystal controlled, so that the chopper
frequency is very stable. The infrared radiation then passes through the measuring
IR-paths, through which a mixture of combustion gases and carrier gas flows.
Depending on the composition of the gas mixture, certain frequencies of the
infrared spectrum are absorbed. The rate of absorption depends on the
concentration of the gases.
As the infrared beam leaves the IR-path, it passes through an infrared filter, which
allows only a certain narrow band infrared radiation to pass. This narrow band
must correspond to the IR wavelength for which the gas to be detected has its
maximum absorption capability.
The intensity of the radiation after the filter thus corresponds to the concentration of
a specific gas in the path. The beam finally strikes a solid state infrared sensor,
giving an electrical signal corresponding to the intensity of the beam.
As the beam is interrupted by the rotating chopper, the detector receives an
alternating radiation creating an AC electrical signal. Temperature and aging
influences of the detector, as well as noise are thereby strongly reduced. The
signal obtained is amplified, rectified and passed thru a low pass filter so that it
leaves the infrared cell as a DC voltage.
The infrared cells utilize solid state sensors combined with infrared filters.
The lengths of all four cells can be individually optimised to obtain maximum
precision for the target analysis levels of each customer. Each of the cells can be
installed with infrared absorption lengths ranging from 1mm to 320mm. The
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