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Air Quality Sensors

Air quality sensors are devices used to detect contaminants in the air. This includes particulates, pollutants and noxious gases that may be harmful to human health. They are used in applications like air quality monitoring, gas detection in industry, combustion controllers and oxygen generators in aircraft. Volatile organic compound (VOC) sensors are available that are capable of detecting volatile chemicals and odorous pollutants.

A common device for detecting particulate matter in the air is the household fire alarm. Smoke is detected using two types of sensor: ionization detectors and photoelectric detectors. Ionization sensors work by using a small radioactive source (Americium-241) produces alpha particles at a constant rate. Smoke particles enter the ionization chamber and interrupt the small leakage current between electrodes. This is detected, setting off the alarm.
Dust sensors are available for air quality monitoring. These work by using an infrared diode and phototransistor pair to detect reflected light from dust particles. They are highly effective cigarette smoke detectors, and can use the patterns of reflected energy to distinguish between smoke and house dust. They are found in air purifiers, air conditioners and monitors.
CO2 gas sensors are available in a number of ‘ppm detection ranges’. They utilize non-dispersive infrared (NDIR) to detect the presence of the gas. CO2 molecules absorb specific wavelengths of light and this is detected in the sensor. Heating and ventilation systems (HVAC) use these to provide demand controlled ventilation (DCV).
Methane gas sensors are another example of air quality sensors used in industry. These are also used to detect LPG, alcohol fumes and hydrogen. A sensor is made from a special circuit comprising an alumina tube and a tin dioxide sensor with electrodes and a heating element to maintain the sensor at the calibrated temperature. Similar sensors are used for alcohol breath testing and testing for the presence of carbon monoxide.
Oxygen sensors heat zirconia oxide disks on the ends of a hermetically sealed tube. By heating to 700C one of the disks acts as a ‘chemical pump’ and the time taken for pressure to change in the tube indicates the partial oxygen pressure of the environment. These sensors have a warm-up time and response time that allow for the sensor heat to cycle between measurements.
Many sensors are not selective over a range of different gases. For this reason, there are often erroneous readings or false alarms that are caused by an unknown combination of compounds.
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