Carbon dioxide sensor

{{main|Nondispersive infrared sensor}}

{{see also|Capnography}}

File:CO2Mini monitor TFA Dostmann.jpg]]

NDIR sensors are spectroscopic sensors to detect {{CO2}} in a gaseous environment by its characteristic absorption. The key components are an infrared source, a light tube, an interference (wavelength) filter, and an infrared detector. The gas is pumped or diffuses into the light tube, and the electronics measure the absorption of the characteristic wavelength of light. NDIR sensors are most often used for measuring carbon dioxide.{{cite journal |last1=Lang |first1=T. |last2=Wiemhöfer |first2=H.D. |last3=Göpel |first3=W. |title=Carbonate based CO₂ sensors with high performance |journal=Sensors and Actuators B: Chemical |volume=34 |issue=1–3 |pages=383–7 |date=1996 |doi=10.1016/S0925-4005(96)01846-1 }} The best of these have sensitivities of 20–50 PPM. Typical NDIR sensors cost in the (US) $100 to $1000 range.

NDIR {{CO2}} sensors are also used for dissolved {{CO2}} for applications such as beverage carbonation, pharmaceutical fermentation and {{CO2}} sequestration applications. In this case they are mated to an ATR (attenuated total reflection) optic and measure the gas in situ. New developments include using microelectromechanical systems (MEMS) IR sources to bring down the costs of this sensor and to create smaller devices (for example for use in air conditioning applications).{{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 |bibcode=2016SeAcB.236..954V |url=https://www.researchgate.net/publication/301241843}}

Another method (Henry's Law) also can be used to measure the amount of dissolved {{CO2}} in a liquid, if the amount of foreign gases is insignificant.{{explain|date=August 2014}}

{{CO2}} can be measured using photoacoustic spectroscopy. Concentration of {{CO2}} can be measured by subjecting a sample to pulses of electromagnetic energy (such as from a distributed feedback laser{{Cite thesis|last=Zakaria|first=Ryadh|title=NDIR Instrumentation Design for CO₂ Gas Sensing|type=PhD|date=March 2010|pages=35–36 |chapter=3.5 Photoacoustic Spectroscopy (PAS) |chapter-url=https://dspace.lib.cranfield.ac.uk/bitstream/handle/1826/6784/Ryad_Adbdullah_Zakaria_Thesis_2010.pdf |publisher=Cranfield University |hdl=1826/6784}}) that is tuned specifically to the absorption wavelength of {{CO2}}. With each pulse of energy, the {{CO2}} molecules within the sample will absorb and generate pressure waves via the photoacoustic effect. These pressure waves are then detected with an acoustic detector and converted to a usable {{CO2}} reading through a computer or microprocessor.{{Cite web|last=AG|first=Infineon Technologies|title={{CO2}} Sensors - Infineon Technologies|url=https://www.infineon.com/cms/en/product/sensor/co2-sensors/|access-date=2020-11-10|website=www.infineon.com}}

Chemical {{CO2}} gas sensors with sensitive layers based on polymer- or heteropolysiloxane have the principal advantage of very low energy consumption, and that they can be reduced in size to fit into microelectronic-based systems. On the downside, short and long term drift effects, as well as a rather low overall lifetime, are major obstacles when compared with the NDIR measurement principle.{{cite journal |last1=Zhou |first1=R. |last2=Vaihinger |first2=S. |last3=Geckeler |first3=K.E. |last4=Göpel |first4=W. |title=Reliable CO₂ sensors with silicon-based polymers on quartz microbalance transducers |journal=Sensors and Actuators B: Chemical |volume=19 |issue=1–3 |pages=415–420 |date=1994 |doi=10.1016/0925-4005(93)01018-Y }} Most {{CO2}} sensors are fully calibrated prior to shipping from the factory. Over time, the zero point of the sensor needs to be calibrated to maintain the long term stability of the sensor.{{cite web |title=CO₂ Auto-Calibration Guide|url=http://sstsensing.com/sites/default/files/AN0117_4_CO2SensorAutoCalibrationNote.pdf |access-date=2014-08-19 |url-status=dead |archive-url=https://web.archive.org/web/20140819214423/http://sstsensing.com/sites/default/files/AN0117_4_CO2SensorAutoCalibrationNote.pdf |archive-date=2014-08-19 }}

For indoor environments such as offices or gyms where the principal source of {{CO2}} is human respiration, rescaling some easier-to-measure quantities such as volatile organic compound (VOC) and hydrogen gas ({{chem2|H2}}) concentrations provides a good-enough estimator of the real {{CO2}} concentration for ventilation and occupancy purposes.{{Citation needed|date=January 2022}} Furthermore, inasmuch as ventilation is a factor in the spread of respiratory viruses,{{cite journal | doi=10.1146/annurev-virology-012420-022445 | title=Seasonality of Respiratory Viral Infections | journal=Annual Review of Virology | date=29 September 2020 | volume=7 | issue=1 | pages=83–101 | last1=Moriyama | first1=Miyu | last2=Hugentobler | first2=Walter J. | last3=Iwasaki | first3=Akiko | pmid=32196426 | s2cid=214601321 | doi-access=free }} {{CO2}} levels are a rough metric for COVID-19 risk; the worse the ventilation, the better for viruses and vice versa.{{cite journal | doi=10.1021/acs.estlett.1c00183 | title=Exhaled CO 2 as a COVID-19 Infection Risk Proxy for Different Indoor Environments and Activities | journal=Environmental Science & Technology Letters | date=11 May 2021 | volume=8 | issue=5 | pages=392–397 | last1=Peng | first1=Zhe | last2=Jimenez | first2=Jose L. | pmid=37566374 | pmc=8043197 | bibcode=2021EnSTL...8..392P }} Sensors for these substances can be made using cheap (~$20) Microelectromechanical systems (MEMS) metal oxide semiconductor (MOS) technology. The reading they generate is called estimated {{CO2}} (e{{CO2}}){{cite journal|last1=Rüffer|first1=D|last2=Hoehne|first2=F|last3=Bühler|first3=J|date=31 March 2018|title=New Digital Metal-Oxide (MOx) Sensor Platform.|journal=Sensors (Basel, Switzerland)|volume=18|issue=4|page=1052|doi=10.3390/s18041052|pmc=5948493|pmid=29614746|bibcode=2018Senso..18.1052.|doi-access=free}} or {{CO2}} equivalent ({{CO2}}eq). Although the readings tend to be good enough in the long run, introducing non-respiration sources of VOC or {{CO2}}, such as peeling fruits or using perfume, will undermine their reliability. H₂-based sensors are less susceptible as they are more specific to human breathing, although the very health conditions the hydrogen breath test is set to diagnose will also disrupt them.{{cite journal |title=MOS gas sensor technology for demand controlled ventilation |journal=Proceedings of the 4th International Symposium on Building and Ductwork Air Tightness and 30th AIVC Conference on Trends in High Performance Buildings and the Role of Ventilation |date=2009 |url=https://www.aivc.org/sites/default/files/members_area/medias/pdf/Conf/2009/AIVC_Herberger_fullpaper_engl.pdf |location=Berlin |vauthors=Herberger S, Herold M, Ulmer H}}

• Examples:
• Modified atmospheres
• Indoor air quality
• Stowaway detection
• Cellar and gas stores
• Marine vessels
• Greenhouses
• Landfill gas
• Confined spaces
• Aerospace
• Healthcare
• Horticulture
• Transportation
• Cryogenics
• Ventilation management
• Mining
• Rebreathers (SCUBA)
• Decaffeination
• For indoor human occupancy counting{{Cite journal|last1=Arief-Ang|first1=I.B.|last2=Hamilton|first2=M.|last3=Salim|first3=F.|date=2018-06-01|title=RUP: Large Room Utilisation Prediction with carbon dioxide sensor|journal=Pervasive and Mobile Computing|volume=46|pages=49–72|doi=10.1016/j.pmcj.2018.03.001|s2cid=13670861 |issn=1873-1589}}{{Cite book|last1=Arief-Ang|first1=I.B.|last2=Salim|first2=F.D.|last3=Hamilton|first3=M. |chapter=SD-HOC: Seasonal Decomposition Algorithm for Mining Lagged Time Series |series=Communications in Computer and Information Science |date=2018-04-14|volume=845 |title=Data Mining|trans-title=SD-HOC: Seasonal Decomposition Algorithm for Mining Lagged Time Series|publisher=Springer|pages=125–143|doi=10.1007/978-981-13-0292-3_8|isbn=978-981-13-0291-6}}
• For HVAC applications, {{CO2}} sensors can be used to monitor the quality of air and the tailored need for fresh air, respectively. Measuring {{CO2}} levels indirectly determines how many people are in a room, and ventilation can be adjusted accordingly. See demand controlled ventilation (DCV).{{cite web |title=Demand Control Ventilation Benefits for Your Building |date=2013 |publisher=KMC Controls |url=http://www.kmccontrols.com/docs/DCV_Benefits_White_Paper_KMC_RevB.pdf|archive-url=https://web.archive.org/web/20140627030355/http://www.kmccontrols.com/docs/DCV_Benefits_White_Paper_KMC_RevB.pdf |archive-date=2014-06-27 }}

• Exhaust gas analyzer
• Oxygen sensor
• Gas detector
• Colorimetric capnography
• Carbon dioxide recorder

{{Reflist|30em}}

{{HVAC}}

{{DEFAULTSORT:Carbon Dioxide Sensor}}

Category:Gas sensors

Category:Carbon dioxide

Category:Indoor air pollution