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NDIR analyzer is shown schematically in Fig. 4.10.

•   As the absorption of IR radiations is measured only in a narrow range of wavelengths (not the entire range of wavelength of IR radiations) which has specifically a high absorbance for the particular gas, the technique is called as ‘Non-dispersive infra-red'. For example carbon monoxide has a strong absorbance in the wavelength band of 4.5-5 µm.
•   The analyzer measures differential in absorption of energy from two columns of gas; (i) the gas to be analyzed in the ‘sample cell' and (ii) a gas of fixed composition like N₂ contained in the reference cell which is free of the gas of interest and relatively non-absorbing in the infrared region.
•   The infrared beam from a single source is usually split into two beams of the same intensity, one each for the sample and reference cells.
•   The detector is divided in two compartments separated by a flexible diaphragm; one section receives transmitted IR energy from the sample cell and the other from the reference cell.
•   The detector is filled with the gas of interest, so that the energy transmitted to the detector is fully absorbed.
•   The flexible diaphragm of the detector senses the differential pressure between the two sections of the detector caused by the difference in the amount of transmitted IR energy absorbed. The deflection in the diaphragm is used to generate an electrical signal that determines the concentration of the gaseous species of interest.
•   A rotating interrupter in the path of IR beam is put to generate AC signal output that can be amplified.