A carbon dioxide sensor or sensor is an instrument for the measurement of

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

gas. The most common principles for sensors are infrared gas sensors ( NDIR) and chemical gas sensors. Measuring carbon dioxide is important in monitoring

indoor air quality Indoor air quality (IAQ) is the air quality within buildings and Nonbuilding structure, structures. Poor indoor air quality due to indoor air pollution is known to affect the health, comfort, and well-being of building occupants. It has also be ...

, the function of the lungs in the form of a capnograph device, and many industrial processes.

Nondispersive infrared (NDIR) sensors

NDIR sensors are spectroscopic sensors to detect in a gaseous environment by its characteristic absorption. The key components are an

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

source, a

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

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 In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

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 sensors are also used for dissolved for applications such as beverage carbonation, pharmaceutical fermentation and 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 (micro-electromechanical systems) is the technology of microscopic devices incorporating both electronic and moving parts. MEMS are made up of components between 1 and 100 micrometres in size (i.e., 0.001 to 0.1 mm), and MEMS devices ...

(MEMS) IR sources to bring down the costs of this sensor and to create smaller devices (for example for use in

air conditioning Air conditioning, often abbreviated as A/C (US) or air con (UK), is the process of removing heat from an enclosed space to achieve a more comfortable interior temperature, and in some cases, also controlling the humidity of internal air. Air c ...

applications). Another method (

Henry's Law In physical chemistry, Henry's law is a gas law that states that the amount of dissolved gas in a liquid is directly proportional at equilibrium to its partial pressure above the liquid. The proportionality factor is called Henry's law constant ...

) also can be used to measure the amount of dissolved in a liquid, if the amount of foreign gases is insignificant.

Photoacoustic sensors

can be measured using photoacoustic spectroscopy. Concentration of can be measured by subjecting a sample to pulses of electromagnetic energy (such as from a distributed feedback laser) that is tuned specifically to the absorption wavelength of . With each pulse of energy, the molecules within the sample will absorb and generate pressure waves via the

photoacoustic effect The photoacoustic effect or optoacoustic effect is the formation of sound waves following light absorption in a material sample. In order to obtain this effect the light intensity must vary, either periodically (''modulated light'') or as a singl ...

. These pressure waves are then detected with an acoustic detector and converted to a usable reading through a computer or microprocessor.

Chemical sensors

Chemical gas sensors with sensitive layers based on polymer- or hetero polysiloxane 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 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 sensor

For indoor environments such as offices or gyms where the principal source of is human

respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellul ...

, rescaling some easier-to-measure quantities such as

volatile organic compound Volatile organic compounds (VOCs) are organic compounds that have a high vapor pressure at room temperature. They are common and exist in a variety of settings and products, not limited to Indoor mold, house mold, Upholstery, upholstered furnitur ...

(VOC) and

hydrogen gas Hydrogen is a chemical element; it has symbol H and atomic number 1. It is the lightest and most abundant chemical element in the universe, constituting about 75% of all normal matter. Under standard conditions, hydrogen is a gas of diatomi ...

() concentrations provides a good-enough estimator of the real concentration for ventilation and occupancy purposes. Furthermore, inasmuch as ventilation is a factor in the spread of respiratory

virus A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...

es, levels are a rough metric for

COVID-19 Coronavirus disease 2019 (COVID-19) is a contagious disease caused by the coronavirus SARS-CoV-2. In January 2020, the disease spread worldwide, resulting in the COVID-19 pandemic. The symptoms of COVID‑19 can vary but often include fever ...

risk; the worse the ventilation, the better for viruses and ''vice versa''. Sensors for these substances can be made using cheap (~$20)

Microelectromechanical systems MEMS (micro-electromechanical systems) is the technology of microscopic devices incorporating both electronic and moving parts. MEMS are made up of components between 1 and 100 micrometres in size (i.e., 0.001 to 0.1 mm), and MEMS devices ...

(MEMS)

metal oxide semiconductor upright=1.3, Two power MOSFETs in amperes">A in the ''on'' state, dissipating up to about 100 watt">W and controlling a load of over 2000 W. A matchstick is pictured for scale. In electronics, the metal–oxide–semiconductor field- ...

(MOS) technology. The reading they generate is called estimated (e) or equivalent (eq). Although the readings tend to be good enough in the long run, introducing non-respiration sources of VOC or , such as peeling fruits or using

perfume Perfume (, ) is a mixture of fragrance, fragrant essential oils or aroma compounds (fragrances), Fixative (perfumery), fixatives and solvents, usually in liquid form, used to give the human body, animals, food, objects, and living-spaces an agre ...

, 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 A hydrogen breath test (HBT) or hydrogen-methane breath test is a breath test used as a diagnostic tool for small intestine bacterial overgrowth (SIBO), and carbohydrate malabsorption, such as lactose, fructose, and sorbitol malabsorption. The ...

is set to diagnose will also disrupt them.

Applications

* Examples: ** Modified atmospheres **

Indoor air quality Indoor air quality (IAQ) is the air quality within buildings and Nonbuilding structure, structures. Poor indoor air quality due to indoor air pollution is known to affect the health, comfort, and well-being of building occupants. It has also be ...

Stowaway A stowaway or clandestine traveller is a person who secretly boards a vehicle, such as a ship, an aircraft, a train, cargo truck or bus. Sometimes, the purpose is to get from one place to another without paying for transportation. In other c ...

detection ** Cellar and gas stores **

Marine vessel A watercraft or waterborne vessel is any vehicle designed for travel across or through water bodies, such as a boat, ship, hovercraft, submersible or submarine. Types Historically, watercraft have been divided into two main categories. *Raf ...

s **

Greenhouse A greenhouse is a structure that is designed to regulate the temperature and humidity of the environment inside. There are different types of greenhouses, but they all have large areas covered with transparent materials that let sunlight pass an ...

s **

Landfill A landfill is a site for the disposal of waste materials. It is the oldest and most common form of waste disposal, although the systematic burial of waste with daily, intermediate and final covers only began in the 1940s. In the past, waste was ...

gas ** Confined spaces **

Aerospace Aerospace is a term used to collectively refer to the atmosphere and outer space. Aerospace activity is very diverse, with a multitude of commercial, industrial, and military applications. Aerospace engineering consists of aeronautics and astron ...

Healthcare Health care, or healthcare, is the improvement or maintenance of health via the preventive healthcare, prevention, diagnosis, therapy, treatment, wikt:amelioration, amelioration or cure of disease, illness, injury, and other disability, physic ...

Horticulture Horticulture (from ) is the art and science of growing fruits, vegetables, flowers, trees, shrubs and ornamental plants. Horticulture is commonly associated with the more professional and technical aspects of plant cultivation on a smaller and mo ...

Transportation Transport (in British English) or transportation (in American English) is the intentional Motion, movement of humans, animals, and cargo, goods from one location to another. Mode of transport, Modes of transport include aviation, air, land tr ...

Cryogenics In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th International Institute of Refrigeration's (IIR) International Congress of Refrigeration (held in Washington, DC in 1971) endorsed a universa ...

Ventilation Ventilation may refer to: * Ventilation (physiology), the movement of air between the environment and the lungs via inhalation and exhalation ** Mechanical ventilation, in medicine, using artificial methods to assist breathing *** Respirator, a ma ...

management **

Mining Mining is the Resource extraction, extraction of valuable geological materials and minerals from the surface of the Earth. Mining is required to obtain most materials that cannot be grown through agriculture, agricultural processes, or feasib ...

Rebreather A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantial unused oxygen content, and unused inert content when present, of each breath. Oxygen is a ...

s (SCUBA) **

Decaffeination Decaffeination is the removal of caffeine from coffee beans, cocoa, tea leaves, and other caffeine-containing materials. Decaffeinated products are commonly termed by the abbreviation decaf. To ensure product quality, manufacturers are required ...

* For indoor human occupancy counting * For

HVAC Heating, ventilation, and air conditioning (HVAC ) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. ...

applications, sensors can be used to monitor the quality of air and the tailored need for fresh air, respectively. Measuring levels indirectly determines how many people are in a room, and ventilation can be adjusted accordingly. See demand controlled ventilation (DCV).

References