Nondispersive Infrared Gas Detectors
An NDIR gas sensor (nondispersive infrared sensors) is a simple spectroscopic device often used as gas detector. It is called nondispersive because the wavelength which passes through the sampling chamber is not pre-filtered. Instead a narrow bandpass (or NBP) filter is used before the detector.
The NDIR sensor is a broadband IR detector using one of the following technologies:
- Thermopile detector or sensor
- Pyroelectric detector or sensor
- Lead Selenide (PbSe) detector or sensor
- NDIR Gas Sensor Modules
- Mid IR LEDs and Photodiodes
The NBP filter turns the broadband detector into a very specific narrow band detector that looks at a specific IR band that the target gas absorbs.
The NDIR system needs a broadband IR source. This source can be steady state for thermopiles, but needs to be pulsed for pyroelectric and lead selenide detectors.
The target gas will absorb more IR energy as the gas concentration increases. This means that as the gas concentration increases the signal from the detector will decrease. Concentrations from % down to ppm can be measured using NDIR. The detectors can be made with multiple channels with different NBP IR filters for monitoring more than one gas.
Sensitivity of the NDIR gas sensor system is improved by:
- Increasing the gas tube length and reflectivity, optimizing the optical path
- Using a detector with more sensitivity
- Improving the IR source performance (modulation depth)