Carbon dioxide sensor | Wikipedia audio article

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 monitoring
indoor air quality, the function of the lungs in the form of a capnograph device, and many
industrial processes.==Nondispersive Infrared (NDIR) CO2 Sensors
==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. 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).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.==Chemical CO2 sensors==
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. 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.==Estimated CO2 sensor==
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 (H2) concentrations provides a good-enough estimator of the current
CO2 concentration for ventilation and occupancy purposes. Sensors for these substances can be made using
cheap (~$20) MEMS metal oxide semiconductor (MOS) technology. The reading they generate is called estimated
CO2 (eCO2) or CO2 equivalent (CO2eq). 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 conditions the hydrogen breath test
is set to diagnose will also disrupt them.==Applications==
Examples: Modified atmospheres
Indoor air quality Stowaway detection
Cellar and gas stores Marine vessels
Greenhouses Landfill gas
Confined spaces Cryogenics
Ventilation management Mining
Rebreathers (SCUBA) Decaffeination
For indoor human occupancy counting 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).==See
also==Exhaust gas analyzer
Oxygen sensor

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