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Function description

7.4 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.

: Infrared cell

An infrared source is electrically heated emitting wide band infrared radiation. The radiation is

interrupted by a rotating chopper blade, resulting in an alternating light. The chopper is crystal

controlled, so that the chopper frequency is very stable. The infrared radiation then passes thru

the measuring IR-paths, thru 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 exits the IR-path, it passes through an infrared filter which allows a narrow

band only of the infrared radiation to pass thru. This narrow band must correspond to the IR

wavelength which is absorbed by the gas which is to be measured. The loss of intensity of the

radiation on the IR sensor after this filter corresponds to the concentration of this gas in the

absorption path. The electrical signal given by the sensor corresponds to the intensity of the

radiation and therefore to the gas concentration in the absorption paths. Since the beam is

interrupted by the rotating chopper, as mentioned above, the detector has an alternating signal.

Temperature and aging influences of the detector, as well as noise, are thereby suppressed.

The signal obtained is amplified and rectified, so that the infrared cell output is DC voltage.

The detectors utilize solid state sensors combined with infrared filters. The sensors are not gas

filled, thus eliminating long term problems due to gas leakage. The CS-2000 can be equipped

with up to four independent Infrared cells.

The lengths of all four cells can be individually optimized to obtain maximum precision for the

target C and S levels of the samples of each customer. Each of the cells can be installed with

infrared absorption lengths ranging between 1 mm and 320 mm.

The infrared cell rack is temperature-controlled, so that the gas which flows through it, is kept at

a constant temperature.

ELTRA CS-2000 Manual

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