Non- Dispersive InfraRed sensor (NDIR)
A Non-Dispersive Infrared Sensor is used in gas analysis to measure gas concentrations. Thereby the missing infrared radiation, absorbed from the respective gas through physical interaction of the radiation and the molecules, is determined. Subsequently the concentration can be calculated with the law of Lambert-Beer. Non-dispersive means in this context, that radiation energy is absorbed.
Many gases have a specific wavelength range in the infrared range, in which they absorb the infrared radiation. The gas is filled in a cuvette of a defined length and is irradiated by infrared light. The length of the cuvette is chosen in such a way that the measurement range requirements can be fulfilled. There is an infrared sensor on the other end of the cuvette. A bandpass filter which suits for the respective gas is used that the sensor doesn’t response to every wavelength.
In the ideal case only this gas should absorb the light of this wavelength and no other gas of this gas mixture. Absorption ranges can also overlap which results a cross sensitivity. In order not to distort the measurement results this has to be compensated or avoided by a skillful choice of the frequency bands.
Over 100 different gases can be detected by NDIR sensors from the ppm- up to the percent range. In many fields of application they are classified as standard method because the measurement method is non-contact and wear-free.