Volatile organic compounds (VOCs) are significant indoor air contaminants with adverse effects on our health. Following our previous discussion on VOCs, the next crucial inquiry arises: How are VOCs quantified, or what is the operational principle behind a TVOC sensor?

Various methods exist for measuring VOCs, each with its advantages and drawbacks depending on factors like the situation and available budget. Historically, laboratory techniques such as flame ionization detectors or gas chromatography–mass spectrometry were employed, offering precise identification of specific gases within samples.

Although lab-based measurements offer high accuracy, they lack the capability to provide continuous TVOC measurements, which are crucial and arguably more significant than obtaining a perfectly accurate value for a particular gas.

What Is an TVOC Sensor?

For the continuous monitoring of MOS, TVOC sensors are commonly utilized. While MOS sensors vary in quality across this broad category, they share a similar underlying technology, as outlined below:

TVOC sensors function by heating a thin film or surface of metal oxide particles. In Kaiterra products, this entails heating a thin film of metal oxide nanoparticles to approximately 300°C, hence the warm-up period after powering on a Kaiterra device.

During operation, oxygen particles are adsorbed on the surface, subsequently reacting with the target gas. This reaction results in the release of electrons from the oxygen present on the surface, leading to a change in the electrical resistance of the metal oxide layer.

What the sensor detects is the electrical resistance of the metal oxide layer. This real-time measurement and output of resistance constitute the initial step in obtaining our TVOC reading.

The alteration in resistance across the sensor's surface is directly correlated to fluctuations in the concentration of the target gas(es). The precise extent of resistance alteration (i.e., the ratio or equation governing this relationship) may vary depending on the specific gas present in the environment. For instance, formaldehyde and ethanol may exhibit a similar relationship, although not necessarily identical.

The sensor's sensitivity extends to a broad spectrum of VOCs rather than being limited to a single individual VOC.

How does the alteration in resistance translate into a TVOC reading in parts per billion (ppb)?

While similar, different VOCs may elicit slightly varied reactions. For instance, an equivalent increase in formaldehyde versus ethanol might not yield the exact same change in resistance on the sensor. Consequently, to correlate a change in resistance with a change in VOC levels in ppb, either the precise gas composition in the air must be identified, or an assumption about the air's composition must be made.

Mølhave et al. delineate a "Typical IAQ Mix" comprising 22 VOCs at concentrations akin to those typically observed in residential indoor environments. This Typical IAQ Mix serves as a reference to interpret alterations in resistance on the sensor's film and convert them into a TVOC reading in ppb.

This approach enables uninterrupted monitoring, an extended sensor lifespan, and ppb-level readings that closely align with measurements obtained using laboratory techniques when applied in a standard indoor environment.