Carbon dioxide sensor

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

Main article: Nondispersive infrared sensor

See also: Capnography

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 measures 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.

New developments include using microelectromechanical systems to bring down the costs of this sensor and to create smaller devices (for example for use in air conditioning applications). 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.

Another method (Henry's Law) can be also 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 a very low energy consumption, and 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.

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)

• 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).
• In applications where direct temperature measurement is not applicable, NDIR sensors can be used. The sensors absorb ambient infrared radiation (IR) given off by a heated surface.