Air-Quality Sensors for Ventilation Systems and Outdoor Monitoring


By Kal Kaur

Introduction
Indoor Air-Quality Sensors
Outdoor Air-Quality Sensors
Reference

Introduction

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

Air-conditioning has been introduced to a range of applications including low-rise and high-rise buildings, industrial work environments, stadiums, automotive systems, public transport services, and aircrafts. Though this form of air cooling is and will continue to be a desired form of technology in its current applications, there have been concerns about the possibility of poorly maintained air-conditioning technology creating an environment idea for the growth of microorganisms such as Legionella pneumophila which if inhaled can lead to Legionnaires’ disease. The use of air-quality semiconductors could help eradicate air contaminants and make the use of a 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 will modify its electrical resistance when exposed to contaminants and generates an output signal proportional to this change in resistance. The output signal from the air-quality sensor will trigger activation of filters to increase the level of ventilation via a fan.

The semiconductor to the air quality sensor is also known as 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 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 are paramount when applied on a demand only basis, a process that can be achieved by implementing air-quality sensors. As mentioned previously, a room sensor can range from carbon dioxide (CO2), mixed gas sensor, combined occupancy CO2/mixed gases, CO2/temperature sensors, or combined CO2/carbon dioxide (CO) sensors.

Air quality sensors are clearly 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 a harmful contaminant 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 have semi-conducting material, but unlike the indoor air-quality sensor type, this material contains a heating element made out of zinc dioxide which bridges the connection between two opposing electrodes. The semiconductor to this sensor absorbs the electrons from the contaminant gases which generates electrical conductivity of the semiconductor forcing a current to flow through the sensor. This electrochemical change manifests within seconds of exposure to air pollutants. Based on the fact that this is an outdoor air-quality sensor, it is designed to be sensitive to a wider range of gases and vapours 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 application, have to be sensitive to low concentrations of gases and vapours. Furthermore, these sensors are positioned in environment that will be exposed to extreme concentrations of parameters such as gas, vapour, 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.
Date Added: Jul 25, 2012
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