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.
Micro heaters can be used to raise the temperature from optical surfaces above ambient to enhance performance and to prevent condensation on the optical surfaces.
Toxic gases are measured in the low parts per million (ppm) range.
Flammable gases are measured in the 0 - 100% lower flammable limit (LFL) or lower explosive limit (LEL) range.