Ideally, the analog input value should be converted into parts per million (ppm). This requires calibration of the sensors. Calibration involves exposing the sensor to a known gas concentration and see the voltage reading. This can then be used to calculate an unknown concentration from a reading. Since we couldn’t find a way of producing an environment with a known VOC or CO concentration, we decided to simply convert the raw voltage values into percentages via a map function.

On top of that, there are other environmental factors that affect the sensor resistance such as humidity and temperature which make a reliable data acquisition even harder.