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Miniaturized multi channel infrared optical gas sensor system

 
: Wöllenstein, J.; Eberhard, A; Rademacher, S.; Schmitt, K.

:

Schmid, U. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Smart sensors, actuators, and MEMS V : 18-20 April 2011, Prague, Czech Republic
Bellingham, WA: SPIE, 2011 (Proceedings of SPIE 8066)
ISBN: 978-0-8194-8655-4
Paper 80660Q
Conference "Smart Sensors, Actuators, and MEMS" <5, 2011, Prague>
English
Conference Paper
Fraunhofer IPM ()
infrared spectroscopy; gas sensor; filter photometer

Abstract
Infrared spectroscopy uses the characteristic absorption of the molecules in the mid infrared and allows the determination of the gases and their concentration. Especially by the absorption at longer wavelengths between 8 μm and 12 μm, the so called "fingerprint" region, the molecules can be measured with highest selectivity. We present an infrared optical filter photometer for the analytical determination of trace gases in the air. The challenge in developing the filter photometer was the construction of a multi-channel system using a novel filter wheel concept - which acts as a chopper too- in order to measure simultaneously four gases: carbon monoxide, carbon dioxide, methane and ammonia. The system consists of a broadband infrared emitter, a long path cell with 1.7m optical p ath length, a filter wheel and analogue and digital signal processing. Multi channel filter photometers normally need one filter and one detector per target gas. There are small detection units with one, two or more detectors with integrated filters available on the market. One filter is normally used as reference at a wavelength without any cross-sensitivities to possible interfering gases (e.g. at 3.95 μm is an "atmospheric window" - a small spectral band without absorbing gases in the atmosphere). The advantage of a filter-wheel set-up is that a single IR-detector can be used, which reduces the signal drift enormously. Pyroelectric and thermopile detectors are often integrated in these kinds of spectrometers. For both detector types a modulation of the light is required and can be done - without an additional chopper - with the filter wheel.

: http://publica.fraunhofer.de/documents/N-186245.html