How does the Chemical (VOCs) sensor work and calibrate over time? (UK)

Awair uses VOC(Volatile Organic Compounds) sensors to calculate the chemical readings.

What it measures and how does it react to different types of pollutants

Awair uses a tin dioxide semiconductor gas type VOC sensor which has a high sensitivity to various air pollution sources (e.g. VOCs) with a fast response speed. The sensor contains an element which changes its electrical resistance in the presence of VOCs. While the sensitivity differs for volatile gases, our sensors can measure volatile compounds in the range of tens ppb to tens of thousands of ppm. This resistance of the sensor value is what is measured, and what “Rs” represents in the image below. The sensor responds toMethane, Ammonia, Ethanol, CO, Hydrogen gas, Hydrogen sulfide, Ammonia, Ethanol, Toluene (smell associated with paint thinner) and more. The image below is reproduced from the sensor datasheet and shows that sensitivity to various pollutant gases are different.

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<Sensor response (electrical resistance) relative to a clean air condition>

The VOC sensor will respond to a variety of organic compounds, but the individual components cannot be distinguished from each other, and sensor will respond with varying sensitivity to the same concentration of each compounds. The reported “total VOC” (tVOC) value is in “isobutylene equivalent units”, the standard reporting figure for the gas measurement industry, meaning that if the only pollutant present was isobutylene, the sensor would accurately reflect the concentration in the atmosphere.

Some VOC’s are relatively benign, e.g. diatomic hydrogen, while some are extremely toxic, e.g. methyl ethyl ketone. The output from the VOC sensor should be taken as a guideline of the potential presence of a pollutant presence, rather than a definite trace of concentration of any specific compound

How it calibrates over time

The VOC sensor continuously calibrates itself over the lifetime of the device, starting from when the Awair unit first comes online. Each VOC sensor reading is compared to the observed minimum for the lifetime of the device to establish the baseline for the sensor  This minimum is expected to correspond to a background VOC level for a clean, outside environment. If the analysis concludes there is drift, a small correction factor is made to the sensor calibration to adjust for this change.

The useful lifetime of the sensor is semi-permanent if used under usual ambient atmospheric conditions.

 

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