Combustible gases absorb infrared radiation at certain characteristic wavelengths. A typical non-dispersive infrared (NDIR) detector passes a source of infrared energy through the sample and measures the energy received by one of two detectors. The active detector responds to wavelengths in the same band as the sample gas, while the other detector measures a reference to compensate for changes within the instrument.
- When specific combustible gases are present, they absorb some of the infrared energy and produce a signal in the active detector relative to the reference detector. Energy absorbed by the combustible gas for a given wavelength varies exponentially with the particular gas’s absorptivity, the concentration, and the path length.
This means that infrared detectors must be specifically calibrated for a particular gas, and can have very high variations in response factors and linearity for other gases. For this reason, IR devices are often used for the detection of a single combustible gas. The technology can also be used for detecting multiple gases in limited applications if the solvent families have similar response factors, such as the use of Acetates and Alcohols in the Flexographic printing industry.