MEMS and Sensors

What are eCO2 sensors?

12 April 2022
Indoor air quality sensors measure the amount of CO2 to avoid any problems with the air in closed spaces or offices. Source: Morn/ CC BY-SA 4.0

Indoor air quality is being talked about more and more these days, especially as the world reopens after shutting down for COVID-19. While temperature, humidity and the level of particulates are all important aspects of indoor air quality, CO2 is now being seen as equally important.

Carbon dioxide (CO2) is emitted by all animals through respiration. While the baseline CO2 in the atmosphere is around 410 ppm, human breath often has a carbon dioxide concentration above 40,000 ppm. Indoor spaces tend to have a carbon dioxide concentration between 400 and 2,000 ppm depending on how the space is being used and the quality of the ventilation system. While the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends a concentration below 1,000 ppm for a closed room, many HVAC installers and local jurisdictions set the target maximum concentration at 800 ppm. What do those target maximums mean and are they even really that important?

Why does CO2 matter?

Carbon dioxide tends to affect air quality in two ways. As the CO2 in a space goes up, people tend to feel drowsier and duller. As the CO2 in the air increases, CO2 in the bloodstream increases as well, limiting the amount of oxygen available to the brain. Ever been in a stuffy conference room for a few hours? That drowsy, tired feeling is often in part due to higher CO2 concentrations.

Additionally, if the CO2 starts going up in an occupied space, there is typically a higher chance for virus transmission as well. Many newer HVAC systems are using CO2 measurements as inputs to control the amount of outdoor air that is used as ventilation air. Because it is typically more expensive to condition outdoor air, using CO2 as an indicator allows the system to trim the amount of outdoor air being introduced, and thus energy used, while still protecting occupants and optimizing indoor air quality. In addition to HVAC systems using CO2, more air quality monitors on the market are choosing to include CO2 as one of the measured values.

How do CO2 sensors work?

The most common type of carbon dioxide sensor is the nondispersive infrared (NDIR) type sensor. Air is drawn into the CO2 sensor where an infrared lamp directs waves of light through the air sample. On the other side of the sensor is a detector that measures how much light can pass through the sample.

Carbon dioxide absorbs light with a frequency near 4.26 microns, the frequency output by the infrared lamp. Because of how precisely tuned the infrared lamp and the detector are to the carbon dioxide sample, NDIR sensors can output a highly accurate carbon dioxide concentration measurement. The downside to such a powerful sensor? The cost.

What is eCO2?

As CO2 has become a more prevalent metric when considering indoor air quality, the market has searched for an alternative to the accurate but expensive NDIR sensors. To bring the price down, the industry has searched for different ways of measuring carbon dioxide. This brings us to a parameter called eCO2, or equivalent carbon dioxide.

Many devices that claim to measure CO2 actually only report eCO2. An eCO2 measurement is derived from a total volatile organic components (TVOC) measurement. The EPA defines volatile organic compounds as “compounds that have a high vapor pressure and low water solubility.” Think of the smell in the air after you finish cleaning a room with chemicals. Those are VOCs creating those scents. Methane, alcohols, ketones, amines and aromatic hydrocarbons are all examples of VOCs that can contribute to a TVOC reading. High concentrations of VOCs are damaging to human health and should be avoided.

The main assumption for eCO2 is that people are the main driver of VOCs, the pool of pollutants in the air. As TVOC increases, CO2 should increase. Sounds good in theory, but does it work?

How do eCO2 sensors differ?

Because VOC sensors typically only provide the total VOC concentration, they are unable to measure the concentration of particular compounds. An eCO2 estimate usually comes from a metal oxide TVOC sensor. These sensors work by measuring the resistance of a special reactive layer that is exposed to the air being measured. The special layer is heated up to several hundred degrees Celsius to allow oxidation to occur, which changes the resistance of the sensor. These readings are used as inputs, along with temperature and humidity, and a great deal of processing to determine an equivalent or estimated value for the CO2 concentration in the air. The accuracy of these readings is highly dependent on the algorithm used and the initial tuning process for the sensor.

When should eCO2 be used over CO2?

When deciding whether you need a dedicated CO2 sensor or if an eCO2 measurement is good enough, consider whether you truly need to know the carbon dioxide concentration in the air or if you just need to know directionally if the air quality is degrading or improving. If directionality is all that’s needed, eCO2 may work just fine. For applications where the actual concentration of carbon dioxide in the air is an important parameter, such as grow room applications that are piping in CO2, a true CO2 sensor is absolutely key. For indoor air quality monitoring applications, having a dedicated CO2 sensor and a TVOC sensor lets you look at both parameters separately to have more informed control of a system.

As the importance of indoor air quality and carbon dioxide concentration increases, the need for these measurements will only increase as well. The greater demand for these sensors, as well as new ways of measuring carbon dioxide, will hopefully lead to cheaper and more robust solutions. Until then, knowing when eCO2 can be used helps drive down product costs while providing more value.

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