Single-beam

A simple single-beam analyzer

Like dispersive analysis, non-dispersive analysis begins with a light beam passing through a sample substance, often enclosed in a windowed sample chamber (typically called a cell ). Certain “species” (compounds) of gas introduced into this cell absorb part of the incident light, leaving the light exiting the cell partially depleted of specific wavelengths. We may easily imagine the concentration of light-absorbing gas increasing within the cell, resulting in more of the light being absorbed by the gas and converted into heat, the detector at the other end of the cell receiving less and less light as a result. The simplest style of non-dispersive analyzer uses a single light source, shining continuously through a single gas cell, and eventually falling on a small thermopile (converting the received infrared light into heat, and then into a voltage signal):

This crude analyzer suffers from multiple problems. First, it is non-selective: any light-absorbing gas entering the sample cell will cause a change in the detector’s signal, regardless of the species. It might work well enough in an application where only the gas of interest absorbs light in the wavelength range of the source, but most industrial analyzer applications are not like this. In most cases, our process sample contains multiple species of gases capable of absorbing light within a similar range of wavelengths, but we are only interested in measuring one of them. An example would be the measurement of carbon dioxide (CO2) concentration in the exhaust gas of a combustion furnace: most of the gases exiting the furnace do not absorb infrared light (nitrogen, oxygen) and CO2 does, but carbon monoxide (CO), water vapor (H2O), and sulfur dioxide (SO2) also absorb infrared light and are all normally present in the exhaust gas of a furnace to varying degrees. Our crude infrared analyzer cannot tell the difference between carbon dioxide and any of the other infrared-absorbing gases present in the exhaust gas.

Another significant problem with this analyzer design is that any variations in the light source’s output cause both a zero shift and a span shift in the instrument’s calibration. Since light sources tend to change output with age, this flaw necessitates frequent re-calibration of the analyzer. Finally, since the detector is a thermopile, its output will be affected not just by the light falling on it, but also by ambient temperature, causing the analyzer’s output to vary in ways completely unrelated to the sample composition.