NDIR gas analysis

Many gases absorb infrared radiation in a characteristic absorption spectrum, which is determined by the specific molecular structure of the respective gas. Non-dispersive infrared gas analysis (NDIR gas analysis) is a comparatively inexpensive method for an exact determination of the concentration of individual gases, which utilizes individual substance-typical absorption bands of the gases to be detected.

Principle of gas concentration measurement using NDIR gas analysis

NDIR gas analysis
NDIR gas analysis

The gas mixture to be analyzed is located in a defined volume between a broadband IR emitter and an infrared sensor. The broadband absorbing IR sensor is equipped with a narrowband filter which has its peak transmission exactly at a typical absorption band of the gas to be analyzed. If a sensor with multiple spectral channels is used instead of a single-element sensor, several discrete gases can be detected simultaneously. The emission spectrum of the emitter must cover at least the spectral range of the absorption bands used.
If there is no gas to be detected between the emitter and the sensor, the emitted radiation hits the sensor almost unhindered and generates the maximum possible signal there. With increasing concentration of a gas to be measured, the absorption on its typical absorption spectrum increases and thus the sensor with the narrowband filter tuned to it emits a lower signal. The other channels, if any, react accordingly only to other gases

Gas Absorption band
CH4 3.33 µm
HC 3.40 µm
Reference 3.91 µm
CO2 4.26 µm & 4.43 µm
CO 4.64 µm
NO 5.30 µm
NO2 6.22 µm
SO2 7.30 µm
Freon R134a 10.27 µm
Freon R12 11.30 µm

 

For standard measurement tasks, the sensors of the type series PYROSENS LTA and LTM are perfectly suitable. With the type series PYROSENS LTMI and LTSI DIAS Infrared GmbH offers highly developed pyroelectric IR sensors which allow NDIR gas analysis even for lowest gas concentrations in the single digit ppm range. The respective detection limit depends on the construction of the measuring cell as well as on the gas to be detected. The typical spectral range is between approx. 1 µm and 20 µm, but our sensors can even be used at wavelengths far above 100 µm.

Für Standardmessaufgaben sind die Sensoren der Typenreihen PYROSENS LTA und LTM hervorragend geeignet. Mit den Typenreihen PYROSENS LTMI und LTSI bietet die DIAS Infrared GmbH hochentwickelte pyroelektrische IR-Sensoren an, mit denen NDIR-Gasanalyse auch für geringste Gaskonzentrationen im einstelligen ppm-Bereich möglich wird. Die jeweilige Nachweisgrenze ist dabei sowohl von der Konstruktion der Messzelle als auch vom zu detektierenden Gas abhängig. Der typische Spektralbereich liegt dabei zwischen ca. 1 µm und 20 µm, unsere Sensoren sind aber selbst bei Wellenlängen weit über 100 µm einsetzbar.

Die Sensoren der Typenreihe PYROSENS gibt es in zahlreichen Ausführungen.
Die Sensoren der Typenreihe PYROSENS gibt es in zahlreichen Ausführungen.

 

PYROSENS sensors are used for leak detection or leak testing of refrigerators and air conditioning systems as well as for highly accurate and temperature stable concentration measurements of gases such as CO, CO2 and CH4 with very low cross sensitivity between CO and CO2, which are used especially in human and veterinary medicine.

In the semiconductor industry, they are used, among other things, for gas monitoring in the end-point monitoring of vacuum chamber cleaning processes.

 

NDIR-Gasmesszelle mit PYROSENS (Foto: mit freundlicher Genehmigung des Herstellers)
NDIR gas sensing module with PYROSENS (Photo: courtesy of the manufacturer)

More information

Please read our applicaton note “NDIR gas analysis with pyroelectrical detectors PYROSENS“.