CO₂ absorbs Infra red light at 4.3 µm

Infra-red spectrographs are more compact and less expensive than the other methods of measurement. This has been the most popular technique for monitoring C02. The wavelength of IR rays exceeds 1.0 milli micron while the visible spectrum is between 0.4 and 0.8 milli microns. The IR rays are absorbed by polyatomic gases (non-elementary gases such as nitrous oxide (N20), C02, and water vapour. Carbon dioxide selectively absorbs specific wavelengths (4.3 milli microns) of IR light. Since the amount of light absorbed is proportional to the concentration of the absorbing molecules, the concentration of a gas can be determined by comparing the measured absorbance with the absorbance of a known standard. The CO₂ concentration measured by the monitor is usually expressed as partial pressure in mmHg, although some units display percentage CO₂ (FCO₂), obtained by dividing CO₂ partial pressure by the atmospheric pressure.1,2