ELTRA CS-580A, Manual

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8.3 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.
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 infrared cell rack is temperature controlled
so that the sample gas flowing through it is kept at a constant temperature.