Structure of NDIR Gas Sensors
When the infrared radiation interacts with the measured gas molecules, the infrared ray is absorbed by the measured gas molecules at a specific wavelength, causing the gas molecules to vibrate. NDIR gas sensors can detect a decrease in transmitted infrared light proportional to the gas concentration. The transmittance, the ratio of transmitted radiation energy to incident energy, depends on the target gas concentration.
NDIR gas sensors consist of an infrared source, detector, optical filter, gas cell, and electronics for signal processing. A single light source, dual wavelength type gas sensor has two detectors and two optical filters of different wavelengths which are placed in front of each detector. Infrared light that is absorbed by a target gas passes through the active filter with a particular bandwidth for the detection of the target gas. Infrared light that does not interact with the target gas passes through the reference filter. The difference between transmitted light intensities in these two bandwidths is converted into gas concentration. The dual wavelength sensor ensures stable measurements for a long period of operation as the aging effects of the light source or the gas cell are automatically compensated by output signals at the reference wavelength.
Mid-infrared radiation through sample gas causes resonance of gas molecules at their natural frequency with the infrared light in the spectrum region where the energy level of infrared is equivalent to the natural frequency of gas molecules, resulting in absorption of infrared by gas molecules in the form of molecular vibration.
Relationship between infrared transmittance and gas concentration is expressed by the Lambert-Beer law:
Where T is transmittance, I is the intensity of light passed through sample gas and optical filter, I0 is the initial light intensity emitted from the source, εis the molar attenuation coefficient, c is gas concentration, and d is light path length.
Because εof target gas and the light pass length d are fixed with an NDIR gas sensor, gas concentration can be measured by measuring the transmittance within the spectrum region of the absorbed energy (wavelength) by target gas.
The initial light intensity emitted from the light source I0 is preset by calibration using zero gas which does not absorb infrared light. Initial value of the molar extinction coefficient ε is set by calibration using calibration gas of known concentration.
Absorption of infrared light occurs at the IR energy (i.e. wavelength) specific to a target gas, meaning high selectivity to a specific gas. This is the most distinctive feature of NDIR gas sensors. Another feature is that exposure to high concentration of target gas for a prolonged time would not result in irreversible sensitivity drift in principle.