Nondispersive infrared sensor

A nondispersive infrared sensor (or NDIR) sensor is a simple spectroscopic device often used as gas detector.

NDIR-Analyzer with one double tube for CO and another double tube for hydrocarbons

Principle

The main components are an infrared source (lamp), a sample chamber or light tube, a wavelength filter, and the infrared detector. The gas is pumped (or diffuses) into the sample chamber, and gas concentration is measured electro-optically by its absorption of a specific wavelength in the infrared (IR). The IR light is directed through the sample chamber towards the detector. In parallel there is another chamber with an enclosed reference gas, typically nitrogen. The detector has an optical filter in front of it that eliminates all light except the wavelength that the selected gas molecules can absorb. Ideally other gas molecules do not absorb light at this wavelength, and do not affect the amount of light reaching the detector.

As many gases absorb well in the IR area, it is often necessary to compensate for interfering components. For instance, CO₂ and H₂O often initiate cross sensitivity in the infrared spectrum. As many measurements in the IR area are cross sensitive to H₂O it is difficult to analyse for instance SO₂ and NO₂ in low concentrations using the infrared light principle.

The IR signal from the source is usually chopped or modulated so that thermal background signals can be offset from the desired signal^[1].

Applications

• Infrared gas analyzer
• Infrared point sensor

References

1. ^ NDIR principle

External links

• NDIR Gas Sensor Lamp Selection Application Notes