This article provides short notes on Non-Dispersive Infra-Red (NDIR) Analyzer.
NDIR analyzers have been developed to monitor SO2, NOx, CO and other gases that absorb in the infra-red, including CO2 and hydrocarbons. However it is probably true to say that this is the “preferred” technique only for CO monitoring of pollutants in ambient air. The technique is of relatively low sensitivity, and is more applicable to the concentrations found in source emissions that in ambient air.
An NDIR analyzer is basically an instrument that does not disperse the light emitted from an infra-red source i.e. the light is not split up into its component wavelengths by means of a prism or grating. Instead a broad band of light is produced by means of a band-pass filter, which is chosen to coincide with an absorption peak of the pollutant molecule. The IR band centers for some common gases are shown in Table 1.
The layout of a typical NDIR analyzer is shown in Figure 7. Infra-red radiation passes through a reference cell, usually containing clean dry air, and a separate cell containing the sample. The detector is referred to as a “microphone” type. It consists of two chambers separated by a thin metal diaphragm filled with gas of the species being measured.
As the molecules in the detector absorb the IR radiation their kinetic energy increases, causing the pressure in each chamber to increase. If, however, absorbing molecules are present in the sample cell, the amount of energy reaching that side of the detector will diminish. Thus a pressure differential develops between the two chambers, resulting in displacement of the diaphragm.
This is sensed as a change in capacitance by the instrument electronics. As shown in the figure the instrument also includes a beam chopper. This serves to create an alternating signal in the detector, which makes it easier to detect and amplify.
A common problem with this type of analyzer is that other gases that absorb light in the same spectral region as the pollutant will cause a positive interference in the measurement. For CO analyzer water vapour and CO2 are potential interferon’s.
Water can be readily removed from the sample by means of an inlet filter containing a desiccant, such as silica gel. In ambient air monitoring the effect of CO2 is usually not significant.