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