A nondispersive infrared sensor (or NDIR sensor) is a simple spectroscopic

sensor A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...

often used as a

gas detector A gas detector is a device that detects the presence of gases in an area, often as part of a safety system. A gas detector can sound an alarm to operators in the area where the leak is occurring, giving them the opportunity to leave. This type of ...

. It is non-dispersive in the fact that no dispersive element (e.g a prism or

diffraction grating In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structur ...

as is often present in other

spectrometer A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...

s) is used to separate out (like a

monochromator A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is from the Greek roots ''mono-'', ...

) the broadband light into a narrow spectrum suitable for gas sensing. The majority of NDIR sensors use a broadband lamp source and an optical filter to select a narrow band spectral region that overlaps with the absorption region of the gas of interest. In this context narrow may be 50-300nm bandwidth. Modern NDIR sensors may use

Microelectromechanical systems Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...

(MEMs) or mid IR LED sources, with or without an optical filter. NDIR

Principle

The main components of an NDIR sensor are an

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

(IR) source (lamp), a sample chamber or

light tube Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 terah ...

, a

light filter An optical filter is a device that selectively transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference coatings. The optic ...

and an

infrared detector An infrared detector is a detector that reacts to infrared (IR) radiation. The two main types of detectors are thermal and photonic (photodetectors). The thermal effects of the incident IR radiation can be followed through many temperature depen ...

. 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 Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

. The gas in the sample chamber causes

absorption Absorption may refer to: Chemistry and biology *Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which s ...

of specific wavelengths according to the

Beer–Lambert law The Beer–Lambert law, also known as Beer's law, the Lambert–Beer law, or the Beer–Lambert–Bouguer law relates the attenuation of light to the properties of the material through which the light is travelling. The law is commonly applied t ...

, and the attenuation of these wavelengths is measured by the detector to determine the gas concentration. 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 however some cross-sensitivity is inevitable. For instance, many measurements in the IR area are cross sensitive to H₂O so gases like CO₂, SO₂ and NO₂ often initiate cross sensitivity in low concentrations. The IR signal from the source is usually chopped or modulated so that thermal background signals can be offset from the desired signal. NDIR sensors for carbon dioxide are often encountered in

heating, ventilation, and air conditioning Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality ...

(HVAC) units. Configurations with multiple filters, either on individual sensors or on a rotating wheel, allow simultaneous measurement at several chosen wavelengths.

Fourier transform infrared spectroscopy Fourier-transform infrared spectroscopy (FTIR) is a technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid, or gas. An FTIR spectrometer simultaneously collects high-resolution spectral data over a wide spectra ...

(FTIR), a more complex technology, scans a wide part of the spectrum, measuring many absorbing species simultaneously.

Research

Miniature IR sources based on

microelectromechanical systems Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...

(MEMS) have been experimentally applied to NDIR systems since 2006 and is useful since 2016. The low energy of MEMS emission means a sensitive detector circuit based on lock-in amplification is needed. Other useful detectors include the photoacoustic gas sensor which use a MEMS microphone to detect IR-gas interactions.

Gases and their sensing wavelengths

Mid-infrared absorption spectra of Gases

Gases do not have a specific sensing wavelength, rather there are regions of the IR spectrum where there are typically many thousands of closely spaced absorption lines. See the Hitran database for more information. * O₂ - 0.763 μm * CO₂ - 4.26 μm, 2.7 μm, about 13 μm * CO - 4.67 μm, 1.55 μm, 2.33 μm, 4.6 μm, 4.8 μm, 5.9 μm * NO - 5.3 μm, NO₂ has to be reduced to NO and then they are measured together as NOx; NO also absorbs in ultraviolet at 195-230 nm, NO₂ is measured at 350-450 nm; in situations where NO₂ content is known to be low, it is often ignored and only NO is measured; also, 1.8 μm * NO₂ - 6.17-6.43 μm, 15.4-16.3 μm, 496 nm * N₂O - 7.73 μm (NO₂ and SO₂ interfere), 1.52 μm, 4.3 μm, 4.4 μm, about 8 μm * HNO₃ - 5.81 μm * NH₃ - 2.25 μm, 3.03 μm, 5.7 μm * H₂S - 1.57 μm, 3.72 μm, 3.83 μm * SO₂ - 7.35 μm, 19.25 μm * HF - 1.27 μm, 1.33 μm *

HCl HCL may refer to: Science and medicine * Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia * Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development * Hollow-cathode lamp, a s ...

- 3.4 μm * HBr - 1.34 μm, 3.77 μm * HI - 4.39 μm * hydrocarbons - 3.3-3.5 μm, the C-H bond vibration * CH₄ - 3.33 μm, can also be used, 1.3 μm, 1.65 μm, 2.3 μm, 3.2-3.5 μm, about 7.7 μm * C₂H₂ - 3.07 μm * C₃H₈ - 1.68 μm, 3.3 μm * CH₃Cl - 3.29 μm * H₂O - 1.94 μm, 2.9 μm (CO₂ interferes), can also be used to avoid interference from CO₂, 1.3 μm, 1.4 μm, 1.8 μm * O₃ - 9.0 μm, also 254 nm (UV) * H₂O₂ - 7.79 μm * alcohol mixtures - * HCHO - 3.6 μm * HCOOH - 8.98 μm *

COS Cos, COS, CoS, coS or Cos. may refer to: Mathematics, science and technology * Carbonyl sulfide * Class of service (CoS or COS), a network header field defined by the IEEE 802.1p task group * Class of service (COS), a parameter in telephone syst ...

- 4.87 μm

Applications

Infrared gas analyzer ] An infrared gas analyzer measures trace gases by determining the Absorption (electromagnetic radiation), absorption of an emitted infrared light source through a certain air sample. Trace gases found in the Earth's atmosphere become excited un ...

* Infrared point sensor *

Carbon dioxide sensor A carbon dioxide sensor or CO2 sensor is an instrument for the measurement of carbon dioxide gas. The most common principles for CO2 sensors are infrared gas sensors ( NDIR) and chemical gas sensors. Measuring carbon dioxide is important in monito ...

References

External links

NDIR & CO2 Sensors Explained
The Gas Detector Encyclopedia, Edaphic Scientific Knowledge Base
NDIR Technology for gasoline exhaust
{{Webarchive, url=https://web.archive.org/web/20141017061042/http://www.lumasenseinc.com/EN/solutions/techoverview/ndir/ , date=2014-10-17
NDIR detectors for CO&CO₂ in internal combustion engine exhaust
Gas sensors Spectrometers Infrared spectroscopy Absorption spectroscopy