MQ135 Gas Sensor Calibration
By my opinion, it is impossible to calibrate these solid electrolyte based sensors. I achieved best results when I measured the CO2 level in 5 minute intervals, by heating the sensor for 100 seconds. I cant remember these times precisely, but by heating and allowing it to cool down I was able to get quite reliable results. But then I realized that the sensor is getting some offset. If you want to do a two-point calibration, you can put it to fresh air, it should read about 0.04%, or you can breathe directly to the sensor for a short while and it shoud read 4%. But I highly recommend using NDIR CO2 sensor, those are used in home air monitoring devices.