What is a CO2 sensor?

A carbon dioxide or CO2 sensor is an instrument for measuring carbon dioxide gas. The most common principles for CO 2 sensors are infrared gas sensors (NDIR) and chemical gas sensors. Measuring carbon dioxide is important for monitoring indoor air quality.

What are the types of CO2 sensors?

NDIR CO2 sensor

NDIR sensors are spectroscopic sensors for detecting CO2 in a gaseous environment by its characteristic absorption. The key components are an infrared source, a light tube, an interference filter (wavelength) and an infrared detector. The gas is pumped or diffused in the light tube and the electronics measure the absorption of the characteristic wavelength of light. NDIR sensors are most often used to measure carbon dioxide. The best of them have a sensitivity of 20 to 50 PPM. Typical NDIR sensors cost between € 80 and € 1000.

NDIR CO2 sensors are also used for dissolved CO2 for applications such as carbonation of beverages, pharmaceutical fermentation and CO2 storage applications. In this case, they are coupled to an ATR optic (attenuated total reflection) and measure the gas in situ. New developments include the use of IR sources from microelectromechanical systems (MEMS) to reduce the costs of this sensor and to create smaller devices (for example for use in air conditioning applications).

Another method (Henry's Law) can also be used to measure the amount of CO2 dissolved in a liquid, if the amount of foreign gas is insignificant.

Chemical CO2 sensors

The chemical CO2 sensors with sensitive layers based on polymer or heteropolysiloxane have the main advantage of very low energy consumption, and of being able to be reduced in size to adapt to systems based on microelectronics. In return, the short and long term drift effects, as well as a rather short overall lifespan, are major obstacles compared to the NDIR measurement principle. The GUILCOR CO2 sensors are fully calibrated before being shipped from the factory. Over time, the zero point of the sensor must be calibrated to maintain the long-term stability of the sensor.

Estimated CO2 sensor

For indoor environments such as offices or gymnasiums where the main source of CO2 is human respiration, the resizing of certain quantities that are easier to measure such as the concentrations of volatile organic compounds (VOCs) and hydrogen gas (H2) provides a good sufficient estimator of the current CO 2 concentration for ventilation and occupancy. Sensors for these substances can be manufactured using inexpensive (~ $ 20) technology of metal oxide semiconductor MEMS (MOS). The reading they generate is called estimated CO2 (eCO2) or CO2 equivalent (CO2 eq). Although the readings tend to be good enough in the long run, the introduction of non-respiratory sources of VOCs or CO2, such as chipping fruit or the use of perfume, will compromise their reliability. H2-based sensors are less sensitive because they are more specific to human respiration, although the very conditions that the hydrogen breath test is set up to diagnose will also disturb them.

CO2 sensor applications

- Modified atmospheres
- Indoor air quality
- Cellar and gas stores
- Marine ships
- Greenhouses
- Landfill gas
- Confined spaces
- Aerospace
- Health care
- Horticulture
- Transportation
- Cryogenics
- Ventilation management
- Mining
- Recyclers
- Decaffeination
- For counting indoor human occupation

For HVAC applications, CO2 sensors can be used to monitor air quality and the personalized need for fresh air. Measuring CO2 levels indirectly determines the number of people in a room and the ventilation can be adjusted accordingly. 

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