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Infrared light - IR/NDIR

Many gasses absorb infrared light extremely well, which makes the use of infrared light (IR) very useful for the analysis of many gasses such as CO, CO2, NO, SO2 and CH4.

One of the IR measuring principles is the NDIR (Non Dispersive Infrared) principle. The measurement is made by a gas flow which is led through a cuvette where the IR light source and an optical filter has been placed by one end of the cuvette, and a detector has been placed by the other end. The IR light source sends out a scattered IR light, and the wave length of the light that is sent through the gas in the cuvette is determined by the optical filter that has been placed between the light source and the cuvette. Different kinds of wave lengths from IR light are used for analysis of different gasses. The absorption of the light that is sent into the cuvette is an expression of the concentration of the gas to be analysed. The amount of light passing through the gas is measured by the detector at the other end of the cuvette.

As many gasses absorb well in the IR area, it is often necessary to compensate for interfering components. For instance CO2 and H2O often initiates cross sensitivity in the infrared spectrum. As many measurements in the IR area are cross sensitive to H2O, it is some times not possible to analyse for instance SO2 and NO2 in low concentrations (concentrations less than 500 mg/m3) using the infrared light principle. In such cases, it is necessary to use the UV light principle instead.
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