ABSTRACT

Nondispersive infrared (NDIR) photometry provides a method of utilizing the integrated absorption of infrared energy over most of the spectrum for a given compound, to provide a quantitative determination of the concentration of that compound in a gas mixture. Specifically, the technique involves determining the difference in infrared energy absorption over all wavelengths passed by the optical system between a gas sample containing the compound of interest and a reference path. The assumption is made that the difference in infrared energy absorbed is proportional to the concentration of the subject compound in the sample gas. There are several different arrangements to detect this difference in commercially available instruments. The sample and reference paths can be separated in space or time. In the first case, the reference path is physically separate from the sample path. It may be a sealed cell containing a non-absorbing gas or a flowing cell from which the compound of interest has been selectively removed. In the time separation case, there is only one optical path: infrared energy passes sequentially through a gas filter cell containing the compound of interest and then an infrared-transparent cell prior to passing through the sample cell.