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CO2 Sensors vs. VOC Sensors for IAQ - What's the Difference?

People sometimes ask about the differences between VOC (Volatile Organic Compounds) sensors and CO2 sensors. While both can be used to measure indoor air quality (IAQ), these sensors are not interchangeable. They measure very different things.

CO2 sensing technology by IR (infrared) is stable and is not subject to the short-term, random drift found in air quality sensors. Most IR carbon dioxide sensors only measure CO2.

A CO2 sensor is designed to control the ventilation rate in occupied spaces. People are the principal source of CO2 in indoor air. Whereas outside air carbon dioxide levels tend to be relatively low and constant, an indoor CO2 measurement can be used to determine whether a room or building is occupied. In other words, higher levels of CO2 correspond to larger numbers of people inside, and therefore, the rate of air exchange required.

Unlike a CO2 sensor, a VOC sensor cannot indicate the rate of ventilation needed, only the level of VOCs in the air. It also cannot necessarily indicate whether safe or harmful concentrations of VOC contaminants are present. It can indicate a general change in the concentration of contaminants. This makes air quality sensors a better solution in applications where unusual, non-occupant-related sources periodically may be present.

As a control, a CO2 sensor can activate an alarm or mitigation strategy (activate filters or ventilation). Because carbon dioxide is an inert gas, it is one of the few elements that will not cause an air quality sensor to react. Both approaches can be applied to a demand-controlled ventilation strategy, but the results may be very different.

When you use CO2 sensing, energy savings can result because ventilation is based on actual occupancy of the space rather than the design occupancy of the space. Energy is saved when pollutant loads are low and ventilation can be reduced, which may occur during or after occupied hours. Where a CO2 sensor would specifically reduce ventilation during unoccupied periods, a VOC sensor may actually maintain ventilation rates during unoccupied periods if there is a significant pollutant level in the building.

In the case of VOC sensors, ventilation is regulated based on the actual presence of some pollutants sensed by the air quality sensor. This may or may not conflict with established ventilation codes. These sensors can also be used to sense periodic episodes of high pollution that might occur when special equipment is being used, or when potent chemicals from cleaners are released into the air.

All VOC air quality sensors are basically the same. Some manufacturers of air quality sensors are now providing an output in “CO2 equivalent units.” This measure is considered misleading and may confuse many new to the indoor air quality industry.

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