Fundamentals of air quality sensorsMuhammad Asim Nlazi | June 09, 2021
Air quality sensors monitor air and detect different gaseous substances, which typically contribute to air pollution or safety hazards. The chemical properties of these concentrations differ from each other, and a single sensor cannot detect all substances. Separate sensors capable of detecting individual chemical concentrations, like this CO2 Sensor, are deployed to measure individual concentrations of gas in the environment.
Air contains many substances beyond its constituent portions of nitrogen and oxygen. Commonly, air quality sensors detect substances such as carbon dioxide (CO2), nitrogen dioxide (NO2), trioxygen (O3), carbon monoxide (CO), volatile organic compounds (VOCs) and particulate matter. These concentrations are measured in ppm (parts per million) or ppb (parts per billion).
An example of air quality monitoring includes detecting particulate matter in a cleanroom. In semiconductor manufacturing, the quality department regularly inspects the cleanroom for particulates which, if allowed to contaminate the device, will present manufacturing errors. Once particulate matter passes a specified threshold, the quality department halts the production process until particles are clear from the cleanroom.
Another example could be monitoring air quality such as PM2.5, PM10, CO, CO2 in a fossil fuel power plant to monitor emissions byproducts of the electrical generation process, and employ the right reduction equipment to reduce overall pollution.
Correct selection of air quality sensors is crucial to accurate and realistic air quality readings. Inaccurate or ambiguous data is not only a key process error, it might also be illegal according to safety or pollution regulations. Regulatory bodies such as the U.S. Environmental Protection Agency (EPA) or the U.S. Occupational Safety and Health Administration (OSHA) will fine or sanction companies that fail to keep air quality measurements within the acceptable range in dangerous or dirty industries.
Correct sensor selection is also key to plant and machinery maintenance, as it can indicate when a certain component or system has failed.
There are few key criteria to consider.
Type of pollutants
Different industries and their processes emit different substances and gasses. The basic composition of these substances differs from each other; the sensing mechanism also differs for different gasses. There can be many pollutants that require monitoring. These pollutants can be categorized into chemicals, such as water or carbon monoxide, and particulates, such as dust or dirt.
The sensor’s internal structure and electronics depend on the desired pollutant testing. For example, detection of NO2, O3, CO requires metal oxide sensors to detect VOCs. Photo Ionization types are used, and for particulate matter, optical particulate counter types are used.
The pollutant source affects the concentration quantity in an area; it is essential to consider during sensor installation.
As a common rule, pollutant concentration decreases with the increase in distance from the source. Additional location-specific factors include physical environmental parameters such as the direction of airflow that can blow the pollutant. Others may consist of temperature and area humidity, which can change the pollutant's internal structure and concentration.
The sensor placement should avoid high-temperature zones, high humidity levels and stagnant air pockets. All locations having these properties can disturb the air quality value and produces faulty values.
Air quality sensors are calibrated for accurate working and correct readings. The continuous exposure to pollutants shifts the internal circuit function away from its standard and reference values. The sensor’s outer body is continuously exposed to contaminants, which changes the sensor’s material structure. These factors affect the sensor function, causing inconsistent readings.
Air quality sensors require calibration at regular intervals. The manufacturer provides recommendations for the calibration procedure and frequency. organizations typically must keep a record of the calibration procedure, as regulatory bodies require routine inspection activity.
The calibration can be done internally by the organization but more often, third-party experts perform this function. The regulatory bodies are certified and mandated to carry out calibration activities and provide certifications to the user.
Active vs. passive
There are two methods of gathering data from air quality sensors:
In active systems, the sensor actively calculates ppm or ppb measurements as air passes through the sampling unit. This may include an active readout if the sensor will be monitored, or it may store values on an internal or external memory system. The readings can then be accessed by engineers or technicians later on. Increasingly, with Industry 4.0, these devices are connected to a digital user interface, for both real-time monitoring and data storage.
Alternatively, passive systems include an air sampler that retains pollutants. The sample must be manually removed from the unit and transferred to a laboratory. A lab technician then conducts different tests, depending on the substance detection required.
As the global manufacturing communities become increasingly focused on sustainability and conservation – and governments create new legislation – air quality sensors play a pivotal role.
Not only are air quality sensors key to good, consistent manufacturing and industrial processes, they also help to ensure these processes do not come at the expense of public health. Innovation and investment in this technology will be key going forward, and air quality sensors are likely to become important IoT devices in many applications.