Nondispersive infrared sensor

A non-dispersive infrared sensor or NDIR sensor is a spectroscopic device that is primarily used as a gas sensor. Particularly suitable NDIR analyzers for determining the concentration of carbon monoxide, carbon dioxide or hydrocarbons in a gas. A typical application is the analysis of the exhaust gases of internal combustion engines.

Principle

The main components are a source of infrared radiation, irradiated tube ( cuvette) with the gas to be analyzed, a wavelength filter and an infrared detector. The gas to be tested is either pumped into the sample chamber and diffuses into this chamber. The concentration of this gas is measured electro- optically by the extent of the absorption of a specific wavelength in the infrared spectrum. The light from the infrared source through the gas radiates in the sample chamber, filter, and then impinges on the IR sensor. The filter has the function of passing only a very narrow range of the spectrum of the light source, the frequency is selected so that it is effectively absorbed by the molecules of the gas under study. Ideally, light of this wavelength the other gases contained in the gas mixture of the sample should not absorb.

A second structural tube equal ( in the apparatus in the image of the other half divided by a central web tube ) contains a trapped reference gas, such as nitrogen. The sample cell and the reference cell are integrated in the same measurement system and are alternately irradiated, the switching eg by rotating perforated discs ( chopper) can take place.

Since many gases absorb light in the infrared region, a compensation for existing interfering, but not investigated gas components is often required. For example, the absorption of the gases CO2 and H2O affect the measurement result due to spectral proximity of their absorption characteristics.

The IR signals used are often modulated or pulsed in order to compensate for thermal phenomena can ..