I had a discussion with SEEED. Unfortunately, in the case of the multichannel sensor they don’t know what they produce. Their sketch means nothing, except the voltage values. These big readings are NOT ppm or ppb. There was absolutely unadequate email conversation between me and the SEEED service engineer (for example he said, that these “ppm” of hundreds are normal LOL! It is not normal! It is deadly! I had a conversation with WINSON, the company which produces the four sensors of the multichannel sensor. Because the WIKI of the SEEED is not full (NO2 calibration curve is missing), and they said nothing about that, I asked WINSON to send me the curve. After that I wrote a sketch which reads the CO and NO2 sensors and I have made the calibration of the sensors. Actually, the sensors are enough accurate (more than I expected before the tests). If someone is interested about how to read REAL ppm from the sensors, please write me to vcoder(at)abv.bg. SEEED, take this on mind! You sell not so cheap sensor! Please inform yourself, correct your WIKI and show to your customers how to operate with that sensor!