Air-Quality Sensors for Ventilation Systems & Outdoor Monitoring

This article was updated on the 4th October 2019.

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Applications of air conditioning systems date back to 1902 when the first electrically powered air-cooling system was engineered by American scientist Willis Havailand Carrie. Air conditioning involves changing the physical properties of the air passing through the cooling system, such as temperature, humidity, and quality, with an aim to generate an outflow of air that achieves the desired properties for a particular closed environment.

Air conditioning systems are now commonplace in many low-rise and high-rise buildings, industrial work environments, stadiums, automotive systems, public transport services, and aircraft.

Although this form of air cooling is and will continue to be, an integral part of building infrastructure, there have been concerns about the possibility of poorly maintained air conditioning units creating an environment ideal for the growth of microorganisms such as Legionella pneumophila that can lead to Legionnaires’ disease.

Modifications to air conditioning systems, such as the use of air quality semiconductors, could help eradicate air contaminants and make the ventilation system more energy efficient.

Indoor Air Quality Sensors

Air quality sensors can be used to detect contaminants, including smoke particles, perspiration, oil remnants, and poisonous gases. A basic air quality sensor is designed with an element that changes electrical resistance when exposed to contaminants. This then generates an output signal proportional to this change in resistance that can be used to trigger activation of filters to increase the level of ventilation via a fan.

The semiconductor in the air quality sensor is a metal-oxide sensor that monitors electrical conductivity in the presence of gas particles. The process of gas detection works by sorption of gas particles into the semiconductor surface. During this process, gas particles cluster on the hot surface of the semiconductor, where oxygen atoms become negatively charged. For example, the presence of carbon monoxide results in the attraction of negatively charged oxygen atoms, a reaction which forms carbon dioxide molecules.

As this process involves the exchange of electrons, any free electrons change the electrical resistance of the semiconductor as a proportional amount to the concentration of gas particles in the sensing chamber.  

Constant use of ventilation is not energy efficient, which is why varying airflow systems that supply airflow on a demand-only basis are so important. This process can be achieved by implementing air quality sensors. As mentioned previously, a room sensor can range from carbon dioxide (CO₂), mixed gas, combined occupancy CO₂/mixed gases, CO₂/temperature, or combined CO₂/carbon dioxide (CO) sensors.

Air quality sensors are useful for making indoor ventilation systems energy efficient based on demand, though it is questionable as to how sensitive these sensors are at distinguishing between harmful and harmless gas contaminants.

There is also no known measure of contaminant concentration in an ambient environment which means calibrating these sensors will also be difficult as there may not be a standard measure of contaminant concentration to calibrate against. An additional problem with air quality sensors is that they are not built to detect a wide range of gas contaminants.

Outdoor Air-Quality Sensors

Air quality sensors are not restricted to their use in ventilation systems; they are also used to measure the air quality in outdoor space surrounding industrial buildings that use chemicals in a production process giving off harmful fumes that may pose a health risk (figure 1).

Figure 1. Typical air quality sensor. Source: Awbi, H.B. (2003). Ventilation of Buildings. New York: Taylor and Francis Group.

Outdoor air quality sensors still contain semi-conductors, but unlike the indoor air quality sensor type, this material contains a heating element made from zinc dioxide, which bridges the connection between two opposing electrodes. The semiconductor in this sensor absorbs the electrons from the contaminant gases, which increases the electrical conductivity of the semiconductor, forcing a current to flow through the sensor.

This electrochemical change manifests within seconds of exposure to air pollutants. As this is an outdoor air quality sensor, it is designed to be sensitive to a wider range of gases and vapors, including hydrogen, CO, a range of hydrocarbons, esters, and petrochemical molecules, including benzene.

Considering how the human nose is sensitive to extremely low concentrations of gases, air quality sensors, whether for indoor or outdoor applications, have to be sensitive to low concentrations of gases and vapors. Furthermore, these sensors are positioned in an environment that will be exposed to extreme concentrations of parameters such as gas, vapor, and temperature, and so they have to be designed to function effectively under such conditions.

Reference

• Sinclair, I.R. (2001). Sensors and Transducers. UK, Oxford: Elsevier.
• BAS, E. (2004). Indoor Air Quality: A Guide for Facility Managers. Georgia, Lilburn: The Fairmont Press, Inc.
• Bullinger, H. (2009). Technology Guide: Principles, Applications, Trends. New York: Springer.
• Awbi, H.B. (2003). Ventilation of Buildings. New York: Taylor and Francis Group.
• Siciliano, P. (2003). Sensors for Environmental Control: Proceedings of the International Workshop on New Development on Sensors for Environmental Control. UK, London: World Scientific Publishing Co. Pte. Ltd.