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. Infrared detectors are usually limited to detecting a single combustible gas.
Control Instruments infrared detector uses an integrated sampling system that extracts a sample from the process using a built-in aspirator that develops suction from compressed air. Infrared energy is passed through the sample, and the amount of infrared energy that is absorbed determines the concentration of solvent vapors. Multiple infrared detectors at select wavelengths help convert concentration data into % LFL flammability readings with greater accuracy for a wider range of solvent types.