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jpStandard multi-gas detection
NDIR gas sensors are increasingly being adapted for multi-gas detection, allowing for the simultaneous measurement of multiple gases in a single setup. This capability is particularly valuable in industrial and medical applications. Common approaches are:
- Multiple IR sources: NDIR systems utilize multiple infrared light sources, each tuned to specific wavelengths associated with different target gases, e.g. CO2, CO and CH4.
- Multichannel IR detectors: Modern systems often employ multichannel detectors with up to eight channels (see Insights 24/04) by using different bandpass filters each adapted to a specific target gas for a simultaneously signal capturing from multiple wavelengths.
These common NDIR multi-gas sensors face several limitations in performance mainly due to the fixed and equal absorption path length for all channels (Fig. 1). This applies in particular to the dynamic and measurement ranges as well as the detection limits of the multi-gas sensor.
Adjustable absorption path length
However, each gas has a specific absorption coefficient, is present in different concentrations in a gas mixture, and depending on the application, different emission and exposure limits apply which must be complied with. Therefore, an adjustable absorption path length for each spectral channel increases performance of NDIR multi-gas sensors.
A novel NDIR multi-gas sensor approach enables different and adjustable absorption path lengths in a single optical path (Fig. 2). Single-element detectors are used, each of which is positioned at a reflection point. At each reflection point, a spectral filter transmits a narrow wavelength band (according to a specific gas) to the detector positioned behind it, while reflecting the remaining broadband radiation back into the optical path toward the next reflection point (Fig. 3). The system is modular making it very cost-efficient with a simple adjustment of the path lengths and target gases. In addition, it enables the simultaneous use of different IR detector types, e.g. pyroelectric, thermopile and even photoacoustic detectors, depending on the needs (measurement range, detection limit, costs, etc.).
