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could you comment how the long-term stability is handled? My experience with a simillar CO2 sensor (the TGS4161 ) showed that even if it is calibrated, it's sensivity tends to decrease with the number of times it is heated. And the initial calibration also get's off once it is stopped for a day or so. And in addition it may take up to 2-3 days heating until the sensor can report correct data. Having in mind that the TGS4161 costs 20+ eur, and the MQ-series sensors are 1-2 Eur, i wouldn't expect much difference.Also i wonder - keeping those 3 heaters so close to each other (emiting ~2.5w of energy) doesn't it also affect the readings, as those electrochemical sensors tend to be sensitive to ambient temperature https://www.hackster.io/vlast3k/tgs4161-co2-research-f2e834
in fact now i noticed that the Temperature sensor is put 2-3 cm from the heaters. my experience with emitted heat is that a USB2Serial cheap that radiates 60mW (12mA @ 5V) increases the ambient temperature in radius 2cm by 2 degrees (Celsius). Eventually i had to put it 10 cm away to avoid the interference of those 60mW. so to make this design work, i believe the Temperature sensor should be connected with at least 20 cm jumper wires