Dual source and dual receivers are used for self compensation of changes in [[alignment]], light source intensity and component efficiency. The transmitted beams from two [[infrared]] sources are superimposed onto an internal [[beam splitter]]. 50% of the overlapping sample and reference signal is passed through the gas measuring path and reflected back onto the measuring detector. The presence of combustible [[gas]] will reduce the intensity of the sample beam and not the [[reference beam]], with the difference between these two signals being proportional to the concentration of gas present in the measuring path. The other 50% of the overlapped signal passes through the beam splitter and onto the [[compensation detector]]. The compensation detector monitors the intensity of the two infrared sources and automatically compensates for any long term drift.
The main components are an [[infrared]] source (lamp), a sample chamber or light tube, a [[wavelength]] filter, and the infrared detector. The gas is pumped (or [[diffusion|diffuses]]) into the sample chamber, and gas concentration is measured electro-optically by its [[Absorption (electromagnetic radiation)|absorption]] of a specific wavelength in the infrared (IR). The IR light is directed through the sample chamber towards the detector. The detector has an [[optical filter]] in front of it that eliminates all light except the wavelength that the selected gas molecules can absorb. Ideally other gas molecules do not absorb light at this wavelength, and do not affect the amount of light reaching the detector.
As many gases absorb well in the IR area, it is often necessary to compensate for interfering components. For instance, [[carbon dioxide|CO<sub>2</sub>]] and [[water (molecule)|H<sub>2</sub>O]] often initiate cross sensitivity in the infrared spectrum. As many measurements in the IR area are cross sensitive to H<sub>2</sub>O it is some times not possible to analyse for instance [[sulfur dioxide|SO<sub>2</sub>]] and [[nitrogen dioxide|NO<sub>2</sub>]] in low concentrations using the infrared light principle.
The IR [[signal processing|signal]] from the source is usually chopped or modulated so that thermal background signals can be offset from the desired signal.
[[Mean time between failures]] may go up to 15 years.
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==Micro heaters==
Micro [[Heating element|heaters]] can be used to raise the temperature from optical surfaces above ambient to enhance performance and to prevent condensation on the optical surfaces.
