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The TVOC (Total Volatile Organic Compounds) Index is the optimal tool to monitor TVOC conditions robustly against environments with different background levels. For this purpose, the raw signal measured by the sensor is processed by a powerful algorithm in an external microcontroller, based on a statistical normalization that allows to adapt the measurements to the specific ambient conditions. Thus, the TVOC index describes the relative TVOC status in the room, according to the recent history.
The most illuminating analogy for how the TVOC Index works is olfaction in humans. People use as a reference the composition of recently breathed air to compare it with the new air that comes into contact with our olfactory nerves. Thus, we are able to recognize more or less TVOC concentration. Similarly, the TVOC Index uses a moving average of the last 24 hours as a reference, calculated recursively and from which the updated TVOC level at each instant is detected.
TVOC sensors are MOx (metal-oxide) technology sensors based on the chemoresistive principle. The sensor consists of a hot surface with a film of metal oxide particles that changes its resistance depending on its oxygen content. Thus, upon contact with TVOCs, which are reducing gases, the oxygen content on the surface decreases and reduces the resistance, generating a change in the current that results in the measurement of the TVOC concentration. The more the resistance is reduced, the higher the TVOC concentration.
However, calibration of these sensors to measure absolute values is only possible under laboratory conditions. In field conditions, there are two complicating factors. The first is the large variability of TVOC background values, which complicates the definition of a single baseline in calibration, leading to false absolute measurements. The second is the sensitivity of the sensor to other gases, although it is possible to optimize the selectivity of the sensors for a specific gas. Considering these factors, relative measurements have great potential, as they provide accurate and robust information on the trend and intensity of changes in TVOC concentration.
In order to adapt the sensor measurement to any background value in any environment with constant possible emitting sources, the TVOC Index measures the relative intensity in reference to recent history. On the TVOC Index scale, which measures on an index from 1 to 500, this reference is mapped to the value of 100. A value above 100 means a concentration higher than the average of the last hours and therefore reflects a deterioration of air quality, possibly caused by events linked to cleaning, use of paints, solvents, glues or building maintenance, among others. On the other hand, a value below 100 reports a decrease in concentration and an improvement in air quality, as a consequence of the effective implementation of ventilation systems, the opening of windows, the use of air purifiers or any event that induces the entry of fresh air.
Furthermore, not only is the TVOC Index able to detect increasing or decreasing trends of TVOC in the air, but, like the olfactory system, it is able to sense the relative intensity of the changes that occur. Thanks to this, fixed points of the TVOC Index can be determined for which a device is desired to perform an action, such as, for example, activating an air purifier when the TVOC Index is above 150.
Although displaying data in TVOC Index is recommended, as it provides robust measures of the trend and intensity of the TVOC state in the room, it may be necessary to display data in concentration units such as ppb or µg/m3. In these cases, conversion is possible, but it must be considered that this depends on the specific gas for which the measurements are to be obtained, as different organic compounds vary greatly in composition, molecular weight and physical characteristics. For this reason, environmental certifications such as RESET or WELL choose their gases for which they determine the thresholds of accepted concentrations.
Ethanol, isobutylene, benzene, toluene, formaldehyde or ketones are some of the most common TVOCs in indoor spaces. In the case of RESET, it chooses isobutylene as the reference gas, while WELL uses a gas mixture called Mølhave that contains several gases that frequently exist in indoor spaces. For each gas, it is possible to extract the particular conversion ratio to obtain the measurements in µg/m3, and a conversion factor that allows to convert between µg/m3 and ppb as a function of molecular weight. The ratio between relative measurement and concentration must be calculated for each specific sensor from laboratory studies, and will depend on the sensitivity of the sensor to the particular gas for which it is made.
Our My inBiot platform implements the conversions to the concentration units of the gases accepted by WELL and RESET, presenting the data ready to facilitate compliance with these certifications.
