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The FTIR measuring principle is a measurement with IR light. Contrary to NDIR with a narrow wave length area by means of an optical filter, the scan area of the IR wave length by use of the FTIR measuring principle is large. The principle of FTIR is that the gas to be analysed is led through a cuvette with an IR light source at one end sending out scattered IR light, and a modulator that "cuts" the infra red light into different wave lengths. At the other end of the cuvette, a detector is measuring the amount of IR light to pass through the cuvette.

Like the NDIR measuring principle, it is the absorption of light at different wave lengths that expresses the concentration of gasses to be analysed. By data processing, Fourier Transformation mathematics is used to turn the measured absorption values into gas concentrations for the analysed gasses. As the light, when using the FTIR measuring principle, is modulated into many different wave lengths, it is possible to analyse many different gasses in the same instrument; such as CO, H2O, SO2, NO, NO2, HCl, HF, NH3.

Using the above measuring principle also produces a much larger data material (as compared to the conventional NDIR principle), from where the concentrations of the different gasses can be measured. The large data material supply excellent calibration curves and correlation values, thus providing very reliable analysis results.
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