Nondispersive infrared sensor
A nondispersive infrared sensor (or NDIR sensor) is a simple spectroscopic sensor often used as a gas detector. It is nondispersive in the sense of optical dispersion since the infrared energy is allowed to pass through the atmospheric sampling chamber without deformation.
The main components of an NDIR sensor are an infrared 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. 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. Part 1065.350(b) states that H2O can interfere with an NDIR analyzer's response to CO2. (http://www.ecfr.gov/cgi-bin/text-idx?sid=b09e0b356089f50988feae6565ed426a&c=ecfr&tpl=/ecfrbrowse/Title40/40tab_02.tpl)
NDIR sensors for carbon dioxide are often encountered in HVAC units.
Configurations with multiple filters, either on individual sensors or on a rotating wheel, allow simultaneous measurement at several chosen wavelengths.
FTIR, a more complex technology, scans a wide part of the spectrum, measuring many absorbing species simultaneously.
Gases and their sensing wavelengths
- O2 - 0.763 µm
- CO2 - 4.26 µm, 2.7 µm, about 13 µm
- carbon monoxide - 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 - 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, ±0.16 μm can also be used, 7.91 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), ±0.18 μm can also be used to eliminate CO2 interference, 5.78 1.3 µm, 1.4 µm, 1.8 µm
- O3 - 9.0 µm, also 254 nm (UV)
- H2O2 - 7.79 µm
- alcohol mixtures - ±0.45 μm9.5
- HCHO - 3.6 µm
- HCOOH - 8.98 µm
- COS - 4.87 µm
- "Dispersive Waves". University of Saskatchewan. Akira Hirose. Retrieved 9 May 2016.
- "NDIR Gas Sensor Light Sources". International Light Technologies. Retrieved 9 May 2016.
- Seitz, Jason; Tong, Chenan (May 2013). SNAA207 - LMP91051 NDIR CO2 Gas Detection System (PDF). Texas Instruments.
- Korotcenkov, Ghenadii (18 September 2013). "Handbook of Gas Sensor Materials: Properties, Advantages and Shortcomings for Applications Volume 1: Conventional Approaches". Springer Science & Business Media. Retrieved 16 April 2018 – via Google Books.
- Technologies, Jason Palidwar, Iridian Spectral. "Optical Filters Open Up New Uses for MWIR, LWIR Systems". photonics.com. Retrieved 16 April 2018.
- "Continuous Infrared Analysis of N2O in Combustion Products". JAPCA. 39: 721–726. doi:10.1080/08940630.1989.10466559.
- NDIR Gas Sensors Explained, The Gas Detector Encyclopedia, Edaphic Scientific Knowledge Base
- NDIR Gas Sensor Lamp Selection Application Notes
- NDIR Technology for gasoline exhaust
- NDIR detectors for CO&CO2 in internal combustion engine exhaust