Atmotube constantly monitors air pollution in real time. We utilize a MOx-based (metal-oxide) VOC sensors in our products, which change conductivity at gas exposure, and these changes can be externally measured and analyzed.
The sensing principle is based on a heated film of metal-oxide (MOx) nanoparticles. Adsorbed oxygen on the metal-oxide particles reacts with the target gas and thereby releases electrons. This results in a change of the electrical resistance of the metal-oxide layer that is measured by the sensor.
Atmotube VOC gas sensor is responsive to a broad range of volatile organic compounds (VOC) and other gases relevant for indoor air quality, the present gas sensing technology is well suited for monitoring TVOC concentrations and for translating those into IAQ levels. Each Atmotube VOC sensor is production calibrated.
The term total VOC (TVOC) refers to the total concentration of VOCs present simultaneously in the air. The TVOC concept is used as a practical time and cost-effective method of surveying indoor environments for contamination. Global consensus has resulted in the emergence of guidelines for TVOC standards of indoor air quality (IAQ) issued by governmental organizations in different countries (e.g. Australia, Finland, Germany, Hong Kong, Japan).
Recommended TVOC levels of IAQ that are considered acceptable range from 0.6 to 1 mg/m³.
The ISO norm for indoor air quality IS016000-29 provides standardized test methods for metal oxide based VOC detectors. The norm compares a simulated VOC mixed gas, with more than 40 individual constituents, to three different test gases. Listed test gases for TVOC are toluene, a two kinds of VOC mixed gas (listed in Table below) and six kinds of VOC mixed gas.
Two kinds of VOC mixed gas is composed of n-octane and m-xylene, representing the two VOC classes of saturated and unsaturated hydrocarbons, respectively.
According to the ISO norm, the difference of indications between the simulated VOC mixed gas (more than 40 individual constituents) and the three different test gases toluene, the two kinds of VOC mixed gas, and the six kinds of VOC mixed gas are -18.1%, -1.6% and -0.46%, respectively. Considering realibility and lower costs of mixed gases with less components, the ISO norm quotes that “the most suitable test gas for metal oxide semiconductor detectors is the two kinds of VOC mix”.
According to ISO16000-29, the six kinds of VOC mix is based on the idea, that VOC can be classified into seven kinds of groups, i.e. aromatic hydrocarbon, aliphatic hydrocarbon, terpene, halide, ester, ketone and aldehyde. Note that aldehyde is omitted due to a poor stability. For each of the other six groups, one representative was chosen, such as toluene, n-decane, αpinene, p-dichlorobenzene, butyl acetate and methyl i-butyl ketone.
Ref.: Indoor air – test methods for VOC detectors, ISO16000-29:2104(E).
The Sensirion SGP is a broadband VOC detector, which is capable of detecting various VOCs and therefore TVOC. Sensirion’s SGP gas sensors are calibrated using ethanol since ethanol serves as a stable, reliable and economical proxy for TVOC. This is verified by linking the SGP sensor response to ethanol with the response to the ISO normed two kinds of VOC mix (see Table above). According to the ISO norm for indoor air quality, this two kinds of VOC mixed gas is a suitable test gas for simulating ambient TVOC concentrations.
Image below illustrates production/calibration, verification and application of SGP TVOC sensors.
SGP sensors are calibrated with ethanol, which serves as a reliable and economical proxy for TVOC. This can be verified by comparison between the reponse of the SGP to ethanol and to an ISO normed two kinds of VOC gas mix that represents TVOC. Within a large target gas concentration range, the SGP reveals a similar gain for both of those target gas compounds (compare similar slopes in Figure below). That allows for calibration and testing with ethanol in production regarding real life TVOC applications.
Figure above shows the SGP response for different concentrations of ethanol and of the two kinds VOC gas mix, respectively.
Over a concentration range relevant for indoor air quality the TVOC concentration output of SGP sensors when exposed to different ethanol concentration is directly proportional to the observed response for the two kinds of VOC gas mix of the ISO norm.
Furthermore, this allows for directly converting between concentrations of the different test gases and ambient TVOC concentrations by employing the relative response factors a listed in Table below.