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How do combination CO₂ + VOC sensors work together to produce better Indoor Air Quality?

By: Bob Byrom, Sensors Product Manager, Siemens Smart Infrastructure USA

Today more than ever indoor air quality plays an important role in the overall health and well-being of building occupants. What people don’t always think about are the Volatile Organic Compounds, or VOCs, which are chemicals found in many products we use in our daily lives. These chemicals are then released into the indoor air we breathe and can irritate the eyes, nose and throat, cause difficulty breathing and may result in other serious health problems.


VOCs, are not detected by Carbon Dioxide (CO₂) sensors, yet are major contributors to “stale air”. Common examples of VOCs include acetone, benzene and xylene. These may seem extreme, but VOCs are also emitted by many common building materials, including carpeting, hardwood flooring, upholstery, and even marble surfaces. People can also be VOC emitters. In a typical office or school environment, it is possible to have CO₂ levels within the acceptable range yet still have poor air quality due to VOCs. This problem can be worse after lunch – depending on what the cafeteria was serving that day.


Demand Control Ventilation (DCV) strategies typically measure Carbon Dioxide levels to determine space ventilation requirements. At a very basic level, the calculation is simple:


More People = More CO₂ = More Fresh Air Required

Most CO₂ sensors used in HVAC applications have a measuring range of 0-2000 parts per million (ppm). Generally speaking, the acceptable indoor air quality is considered to have a maximum CO₂ concentration between 800 and 1000 ppm. However, there is much more to indoor air quality than CO₂ concentration.


In a condition of acceptable CO₂ levels and high VOC levels, a DCV system using traditional CO₂ sensors will be of little value. Siemens combination CO₂/VOC sensors are the perfect way to help ensure optimum air quality at all times. These sensors have two separate sensing elements, one measures CO₂ and the other measures VOCs. The sensor algorithms then adjust the output signal to reflect the higher of the two values.


In the scenario shown in the graph below, the CO₂ level is approximately 900 ppm, which is acceptable in most situations. If the sensor output signal is reflecting only the CO₂ level, the DCV controller would see approximately 4.5V (900 ppm) and take no action. However, since the VOC concentration is relatively high, the Siemens combination CO₂ / VOC sensor will output a 6V signal, which would indicate to the controller that air quality is outside the desired range and the controller then initiates the appropriate DCV strategy.

It is important to note that the CO₂ portion of the signal is the typical 0-2000 ppm, while the VOC portion is simply a general indicator of potentially unpleasant odors. From a comfort perspective, a lower VOC level is roughly equal to a lower CO₂ level.


Siemens combination CO₂ + VOC sensors seamlessly integrate into any DCV strategy without any reprogramming of existing applications. These sensors are the perfect way to help ensure optimum air quality at all times, providing peace of mind for occupants. Accurately measure the required environmental variables with sensors for a healthy and productive indoor climate.


Learn more about IAQ and CO₂ + VOC sensors

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