NDIR (non-dispersive infrared)

NDIR stands for Non-Dispersive Infrared spectroscopy, a gas-analysis method that measures a gas by how strongly it absorbs infrared light at a wavelength characteristic of that gas. "Non-dispersive" means it does not split the light into a spectrum; instead it uses an optical filter to isolate the target wavelength. It is the standard, robust method for measuring carbon monoxide (CO) and carbon dioxide (CO₂), and is also used for some hydrocarbons.

NDIR is valued for being continuous, stable, low-maintenance and selective, which makes it the workhorse for CO/CO₂ channels in both ambient analysers and stack continuous-emission monitors. It is the reference approach for CO in the NAAQS suite (CO has 1-hour and 8-hour ambient limits). Because CO₂ is also measured, NDIR analysers double as the tool for the oxygen/CO₂ correction applied to many stack emission standards.

For recyclers, NDIR-derived numbers matter most for combustion completeness. Carbon monoxide is the key indicator of incomplete combustion — high CO means the burner or reactor is not getting enough oxygen or residence time, which simultaneously signals poor energy efficiency and elevated PAH/dioxin formation risk. Monitoring CO is therefore both a compliance and an operational-optimisation tool, especially for pyrolysis, incineration and any combustion process.

The practical relevance is that a pyrolysis or thermal recycler should treat the CO reading as a live combustion-quality gauge: keeping CO low confirms the non-condensable gas and burners are combusting completely, which keeps PAH, benzo(a)pyrene and dioxin formation down. The CO₂ channel, meanwhile, provides the reference correction needed to report stack emissions on the proper basis. NDIR is the method behind both.