Understanding the TVOC Sensor: Functionality, Limitations, and Calibration

Q. Can Atmotube PRO’s TVOC sensor detect specific VOCs?

The Atmotube PRO uses a type of sensor called a MOx (metal oxide) sensor to measure total volatile organic compounds (TVOCs), which is the overall mix of VOCs present in the air. The sensor's primary function is to provide a general indication of air quality trends, rather than offering precise readings for individual VOCs or differentiating between specific types of VOCs.

Q. Why do you need calibration? What gas do you use for Atmotube PRO’s TVOC sensor calibration?

The TVOC sensor detects a wide range of organic chemicals. However, it’s necessary to calibrate a TVOC sensor with a reference gas. Typically, it’s a gas with a well-defined concentration of a specific VOC or a gas mixture representative of TVOCs. This process allows the sensor's response to be adjusted and aligned with the known concentration, establishing a baseline for accurate measurements. 

Atmotube PRO sensors use ethanol for TVOC sensor calibration. By comparing the sensor's response to ethanol and an ISO-normed VOC gas mix representing TVOC, the sensor exhibits similar gains for both compounds across a wide concentration range. 

Q. Do TVOC sensors react to different VOCs differently?

The nature of MOx sensors is that they react to different VOCs (volatile organic compounds) differently. 

One of the factors is variations in their chemical properties. Different VOCs exhibit different reactivity with the sensing material because of their chemical composition and volatility. Another factor is what calibration gas has been used. 

It results in some of the VOCs having a stronger signal compared to others. 

For example, ethanol will have a stronger signal compared to formaldehyde. See the illustration below of how a MOX sensor reacts to different VOCs:

(Figure taken from Sensirion's Compliance of Sensirion’s VOC Sensors with BuildingStandards)

In the picture, we have two situations with the same TVOC concentration (partial pressure). However, the composition of TVOCs differs between the two situations. 

Situation 1: The TVOC value is primarily influenced by less-toxic ethanol.

Situation 2: The main contributor to the same TVOC value is the harmful toluene.

(Figure taken from Sensirion’s Compliance of Sensirion’s VOC Sensors with BuildingStandards)

Q. How can you interpret the data from Atmotube PRO?

TVOCs encompass hundreds of organic compounds emitted from various sources, including building materials, furnishings, cleaning products, and human activities. These compounds have different chemical structures, volatility, and toxicity levels. Some TVOCs, such as formaldehyde or benzene, are known to be harmful at certain concentrations, while others may be relatively less toxic or even non-toxic.

If you observe elevated levels of TVOCs measured by Atmotube PRO, we recommend further investigation with professional-grade equipment to identify the specific VOC and its health impact. The presence of harmful TVOCs, even if not precisely quantified, can be a concern for indoor air quality and occupant health.

Q. How do you ensure the reliability and accuracy of the Atmotube PRO TVOC sensor?

Inside the Atmotube PRO, there is a well-known sensor called the Sensirion SGPC3 TVOC sensor, which samples the air every 2 seconds. The TVOC sensor establishes a baseline reading based on the surrounding environment. This baseline may change over time, depending on where the sensor has been. The live readings provided by the sensor describe the current TVOC condition relative to its baseline. The sensor calculates the concentration of VOCs compared to the cleanest air it has previously encountered.

Most MOx-based TVOC sensors tend to adjust to the environment they are in, which may lead to gradually decreasing readings unless recalibration is performed. However, the Sensirion TVOC sensor in the Atmotube PRO incorporates a dynamic baseline adjustment algorithm that compensates for these changes and continuously corrects the baseline. As a result, you can trust the accuracy and consistency of TVOC readings over time.

In addition to in-sensor algorithms, there are some additional measurement normalizations applied, like temperature and humidity normalizations.

The sensor is also protected by a water and dust membrane, reducing variations caused by changes in airflow. If precise TVOC readings are critical for your use case, we recommend considering the average TVOC data (per hour or per day) as a helpful guideline.

What’s the accuracy of Atmotube PRO’s TVOC sensor?

The accuracy of MOx-based TVOC sensing technology is limited to around 15% of the measured value. You will find a comprehensive description of Atmotube’s TVOC sensing principle, including sensor data sheets and accuracy specs for the TVOC sensor, in this article.

How does Atmotube PRO’s TVOC sensor differ from other sensors?

It's important to note that most TVOC sensors, including professional reference monitors, operate in a similar relative manner due to the sensing principle they use. 

However, the Atmotube PRO's TVOC sensor offers real-time readings and allows manual baseline recalibration, which is not commonly found in other similar devices. Manual recalibration can be useful if you believe that the initial environment Atmotube PRO was exposed to isn’t an adequate reference.  It means that there may be more variation in the readings compared to devices that use low-powered VOC sensors.

Does an activated PM sensor affect TVOC readings?

When the Atmotube PRO is connected to a smartphone, the particle matter (PM) sensor mode is automatically set to "Always ON" to provide real-time readings in the Atmotube app. The activation of the PM sensor fan during this mode introduces changes in the airflow around the device, which can result in lower TVOC readings. This variation is within the typical range observed with TVOC sensors and does not significantly affect the average air quality score (AQS) reading in the Atmotube application. The TVOC sensor can detect "high-VOC events" when the VOC level exceeds its usual detection range.