NDIR-procedure

Since the 1990s, simple NDIR photometers have been developed by various equipment manufacturers which operate on the basis of the interference filter correlation. In comparison to the IR photometers of the previous device technology, modulatable radiation sources are used in these system superstructures. The modulation is usually carried out by toggling the radiation source periodically at a frequency of f = 1-25 Hz. A mechanical chopper is therefore not necessary, so that very small and robust gas sensors can be implemented. However, to use this advantage, at least two radiation detectors are required. A radiation detector has as an entrance window an interference filter with a transmission curve in the maximum (λM) of gas absorption, while a second detector serves as a reference. The wavelength of the reference measurement (λR) generally is ≈4µm, since no interfering absorption bands of the accompanying gases are present there. Thermopile detectors and pyroelectric detectors are preferably used as radiation detectors. Both detector types are available in different versions (1-4 channels). The sensor manufacturers mostly use double detectors. The figure below shows an NDIR gas sensor equipped with two separate IR detectors:

IR Fotometer1

The absorption bands of the different gases are determined with FTIR spectrometers and are required for the selection of the interference filters. This results in an optimal adaptation of the measurement wavelength to the respective measurement task. The following figure shows the absorption bands of important gases. Thermography makes the local temperature distribution visible on the chip surface very precisely.

IR Gasspektren

In this case as well, the radiation absorption by the respective gas behaves according to Lambert-Beer’s law:

Lambert Beer

Usually, thermal surface emitters are used as radiation sources, which emit IR radiation over a wide spectral range according to Planck’s law. The following figure shows a radiator chip (wafer) and a thermographic image.

Axetris1     Axetris2

Reference: Wiegleb, G.: Gasmesstechnik in Theorie und Praxis (Chapter 7.4), Springer Vieweg 2016

 

  µH1                          IR6

Spotlight chip and spotlight design  Reference: Wiegleb, G.: Gasmesstechnik in Theorie und Praxis (Chapter 7.4), Springer Vieweg 2016

IR-Leuchtdioden IRLED`s

IRLEDIR light emitting diodes are also used for special applications which emit selective radiation in a narrow spectral range. However, the possible spectral range is currently limited to 5 μm, so that only a few gases can be detected with it. IRLEDs can be modulated very quickly and need extremely low electrical power.

 

Reference: Wiegleb, G.: Gasmesstechnik in Theorie und Praxis (Chapter 7.4), Springer Vieweg 2016

µH2

Different detector designs  Reference: Wiegleb, G.: Gasmesstechnik in Theorie und Praxis (Chapter. 7.6), Springer Vieweg 2016