NDIR is a common measurement principle for detecting gases in the environment. Many of edaphic scientific’s gas sensors, particularly our range of carbon dioxide sensors, are based on NDIR. But what is NDIR and how does it work?
In this article, a brief explanation of NDIR is given. The brief explanation provides everything you really need to know about NDIR gas measurements.
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NDIR is an acronym for Non Dispersive Infra-Red. This means that when a beam of infrared light is emitted from a light source it does not “disperse” or become scattered by substances between the light source and a detector.
A NDIR gas sensor specifically measures the abundance, or concentration, of gases in a sample chamber. If there are gases in the path from an infrared light source to a detector then, in a non-dispersive system, the light is absorbed by the gases. How much light is absorbed is a function of how much gas is between the light source and light detector.
Figure 1 is a schematic of a simple NDIR gas sensor designed to detect the gas carbon dioxide (CO2). The infrared light source is installed at one end of an enclosure and the light detector is installed at the opposite end. The light detector has a filter so it specifically only detects light in the electromagnetic spectrum that is related to CO2. An inlet and outlet between the light source and detector allows CO2 to freely move in and out of the enclosure.
Figure 1. A simple schematic of a NDIR gas sensor.
The amount of infrared light absorbed is proportional to the abundance or concentration of CO2 in the enclosure. As CO2 concentration increases, more infrared light is absorbed and less light is detected. A known mathematical equation is then applied which informs us of the CO2 concentration, in parts per million (ppm) or as a percent (%), that is present in the enclosure.
All elements and compounds absorb light spectra at numerous wavelengths along the electromagnetic spectrum. NDIR based CO2 sensors essentially employ the principles of absorbance spectroscopy. Whereas absorbance spectrometers measure across numerous wavelengths, NDIR based CO2 sensors target specific wavelengths where there is maximum absorbance of CO2 and minimum interference from other elements and compounds.
CO2 is known to have high absorbance in the infrared region of the electromagnetic spectrum at wavelengths of 2.7, 4.3 and 15 μm. The wavelength of 4.3 μm has been demonstrated to have maximum absorption and minimal interference for CO2 and, therefore, this waveband is generally used in NDIR sensors. In Figure 1, above, this is the absorption detector (note: it can also be the wavelength 4.26 μm but for simplicity we have rounded this to 4.3 μm).
A NDIR sensor also measures a reference, or incident, amount of radiation. This is measured at wavelength of 4.0 μm and is denoted as the reference detector in Figure 1.
Radiation at these wavelengths is then associated with CO2 concentration via the Beer-Lambert Law that is given with the following equation:
Id / I0 = e ^ αcl
where Id is the intensity of radiation at 4.3 μm, I0 is the intensity of the reference radiation, α is the absorption co-efficient of CO2, c is the concentration of CO2, and l is the path length between the light source and the light detectors. From Figure 1, it can be seen that Id, I0 and l can be easily measured and noted, e and α are known, therefore via algebra c, the CO2 concentration in the sample, can be calculated.
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