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:Nondispersive infrared sensor

{{Short description|Spectroscopic sensor}}

A nondispersive infrared sensor (or NDIR sensor) is a simple spectroscopic sensor often used as a gas detector. It is non-dispersive in the sense that no dispersive element (e.g., a prism, or diffraction grating, as is often present in other spectrometers) is used to separate (like a monochromator) 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 correspond to the absorption region of the gas of interest. In this context, the narrow band can be 50-300 nm bandwidth. Modern NDIR sensors may use microelectromechanical systems (MEMs) or mid IR LED sources, with or without an optical filter.

File:NDIR.JPG

Principle

The main components of an NDIR sensor are an infrared (IR) source (lamp), a sample chamber or light tube, a light filter and an infrared detector. 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 gas in the sample chamber causes absorption of specific wavelengths according to the Beer–Lambert law, 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.{{cite web|title=NDIR Gas Sensor Light Sources|url=http://www.intl-lighttech.com/applications/light-source-apps/ndir-gas-sensor/ndir-gas-sensor-index|website=International Light Technologies|accessdate=9 May 2016|archive-url=https://web.archive.org/web/20121205033418/http://www.intl-lighttech.com/applications/light-source-apps/ndir-gas-sensor/ndir-gas-sensor-index|archive-date=5 December 2012|url-status=dead}} 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.{{citation needed|date=October 2014}}[http://www.ecfr.gov/cgi-bin/text-idx?sid=b09e0b356089f50988feae6565ed426a&c=ecfr&tpl=/ecfrbrowse/Title40/40tab_02.tpl Title 40: Protection of Environment, Part 1065—Engine-Testing Procedures, Subpart D—Calibrations and Verifications, §1065.350 H2O interference verification for CO2 NDIR analyzers]

The IR signal from the source is usually chopped or modulated so that thermal background signals can be offset from the desired signal.{{cite book|last1=Seitz|first1=Jason|last2=Tong|first2=Chenan|title=SNAA207 – LMP91051 NDIR CO2 Gas Detection System|date=May 2013|publisher=Texas Instruments|url=http://www.ti.com/lit/an/snaa207/snaa207.pdf}}

NDIR sensors for carbon dioxide are often installed in heating, ventilation, and air conditioning (HVAC) units.

Configurations with multiple filters, either on individual sensors or on a rotating wheel, allow simultaneous measurement at several selected wavelengths.

Fourier transform infrared spectroscopy (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 (MEMS) have been experimentally applied to NDIR systems since 2006 and are useful since 2016. The low energy of MEMS emission means a sensitive detector circuit based on lock-in amplification is needed.{{cite journal |last1=Vincent |first1=T.A. |last2=Gardner |first2=J.W. |title=A low cost MEMS based NDIR system for the monitoring of carbon dioxide in breath analysis at ppm levels |journal=Sensors and Actuators B: Chemical |date=November 2016 |volume=236 |pages=954–964 |doi=10.1016/j.snb.2016.04.016 |url=https://www.researchgate.net/publication/301241843}} Other useful detectors include the photoacoustic gas sensor which use a MEMS microphone to detect IR-gas interactions.{{cite journal |last1=Popa |first1=Daniel |last2=Udrea |first2=Florin |title=Towards Integrated Mid-Infrared Gas Sensors |journal=Sensors |date=4 May 2019 |volume=19 |issue=9 |pages=2076 |doi=10.3390/s19092076 |pmid=31060244 |pmc=6539445 |bibcode=2019Senso..19.2076P |doi-access=free}}

Gases and their sensing wavelengths

File:Mid-infrared absorption spectra of Gases.pngGases 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.

  • {{chem2|link=oxygen|O2}} — 0.763 μm
  • {{chem2|link=carbon dioxide|CO2}} — 4.26 μm,{{cite web|url=https://www.photonics.com/a56392|title=Optical Filters Open Up New Uses for MWIR, LWIR Systems|first=Jason |last=Palidwar |publisher=Iridian Spectral Technologies|date=|website=photonics.com|accessdate=16 April 2018}} 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
  • {{chem2|link=nitrogen monoxide|NO}} — 5.3 μm, {{chem2|NO2}} has to be reduced to NO and then they are measured together as {{chem|NO|x}}; NO also absorbs in ultraviolet at 195-230 nm, {{chem2|NO2}} is measured at 350-450 nm;{{Cite web |url=http://nett21.gec.jp/CTT_DATA/AMON/CHAP_4/html/Amon-084.html |title=2.2 Nitrogen oxide (NOx) |work=2. Measuring instruments for stationary source, Air Pollution Continuous Monitoring Technology in Japan |publisher=Global Environment Centre Foundation |access-date=2020-01-16 |archive-url=https://web.archive.org/web/20170916032727/http://nett21.gec.jp/CTT_DATA/AMON/CHAP_4/html/Amon-084.html |archive-date=2017-09-16 |url-status=dead }} in situations where {{chem2|NO2}} content is known to be low, it is often ignored and only NO is measured; also, 1.8 μm
  • {{chem2|link=nitrogen dioxide|NO2}} — 6.17-6.43 μm, 15.4-16.3 μm, 496 nm
  • {{chem2|link=nitrous oxide|N2O}} — 7.73 μm ({{chem2|NO2}} and {{chem2|SO2}} interfere),{{cite journal|doi=10.1080/08940630.1989.10466559 | volume=39 | issue=5 | title=Continuous Infrared Analysis of N2O in Combustion Products | journal= Journal of the Air & Waste Management Association | pages=721–726| year=1989 | last1=Montgomery | first1=Tami A. | last2=Samuelsen | first2=Gary S. | last3=Muzio | first3=Lawrence J. | s2cid=56277453 | url=http://www.escholarship.org/uc/item/959894fk | url-access=subscription }} 1.52 μm, 4.3 μm, 4.4 μm, about 8 μm
  • {{chem2|link=nitric acid|HNO3}} — 5.81 μm
  • {{chem2|link=ammonia|NH3}} — 2.25 μm, 3.03 μm, 5.7 μm
  • {{chem2|link=hydrogen sulfide|H2S}} — 1.57 μm, 3.72 μm, 3.83 μm
  • {{chem2|link=sulfur dioxide|SO2}} — 7.35 μm, 19.25 μm
  • HF — 1.27 μm, 1.33 μm
  • HCl — 3.4 μm{{cite book |chapter=Table 14.4 Absorption lines of several gases and vapors used for gas analysis |chapter-url={{GBurl|iSzhAAAAQBAJ|p=331}} |series=Handbook of Gas Sensor Materials: Properties, Advantages and Shortcomings for Applications |volume=1 |title=Conventional Approaches|first=Ghenadii|last=Korotcenkov |date=2013 |publisher=Springer |isbn=978-1-4614-7165-3}}
  • HBr — 1.34 μm, 3.77 μm
  • HI — 4.39 μm
  • hydrocarbons — 3.3-3.5 μm, the C-H bond vibration
  • {{chem2|link=methane|CH4}} — 3.33 μm, {{val|7.91|0.16|u=μm}} can also be used,{{Cite press release |url=http://www.lasercomponents.com/fileadmin/user_upload/home/Images/_Presse/Englisch/2016-Q2/New_Filters_for_Pyroelectric_Detectors.pdf |title=Moisture and Gas Detection with NDIR Measurement Devies |publisher=Laser Components |date=10 May 2016 |access-date=2020-01-16 |archive-url=https://web.archive.org/web/20180224052731/https://www.lasercomponents.com/fileadmin/user_upload/home/Images/_Presse/Englisch/2016-Q2/New_Filters_for_Pyroelectric_Detectors.pdf |archive-date=2018-02-24 |url-status=dead }} 1.3 μm, 1.65 μm, 2.3 μm, 3.2-3.5 μm, about 7.7 μm
  • {{chem2|link=acetylene|C2H2}} — 3.07 μm
  • {{chem2|link=propane|C3H8}} — 1.68 μm, 3.3 μm
  • {{chem2|link=chloromethane|CH3Cl}} — 3.29 μm
  • {{chem2|link=water|H2O}} — 1.94 μm, 2.9 μm ({{CO2}} interferes), {{val|5.78|0.18|u=μm}} can also be used to avoid interference from {{CO2}}, 1.3 μm, 1.4 μm, 1.8 μm
  • {{chem2|link=ozone|O3}} — 9.0 μm, also 254 nm (UV)
  • {{chem2|link=hydrogen peroxide|H2O2}} — 7.79 μm
  • alcohol mixtures — {{val|9.5|0.45|u=μm}}
  • HCHO — 3.6 μm
  • HCOOH — 8.98 μm
  • COS — 4.87 μm

Applications

References

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External links

  • [https://web.archive.org/web/20190307133039/https://www.edaphic.com.au/knowledge-base/articles/gas-articles/ndir-explained/ NDIR and CO2 sensors explained], The Gas Detector Encyclopedia, Edaphic Scientific Knowledge Base
  • [https://web.archive.org/web/20071012104947/http://intl-lighttech.com/applications/ndir-gas-sensors.html NDIR gas sensor lamp selection application notes]
  • [https://www.lumasenseinc.com/EN/solutions/techoverview/ndir/ 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 }}
  • [https://www.cambustion.com/products/ndir500/operating-principle NDIR detectors for CO and CO2 in internal combustion engine exhaust]

Category:Gas sensors

Category:Spectrometers

Category:Infrared spectroscopy

Category:Absorption spectroscopy