CO2 is a calculated/estimated value based on an algorithm designed by Bosch Sensortec. Whilst there is no training required, the sensors need to be burned in and you will need to understand saving and loading state in order to shorten the time required for the sensor to achive stability (quality 3). The BME688 is a MOX sensor, with the ability to change the heater profile in order to better characterise VOC (volatile organic compounds) being sampled. The IAQ (Indoor Air Quality) is derived from resistance readings using an algorithm designed by Bosch Sensortec. If you wish to identify a specific compound - aka sniff - then you have to train on what it is you are trying to detect. You really need a Bosch Sensortec Dev Kit (8 sensors) to sample enough data for sucessful learning, and data on up to 4 seperate compounds can be used in a single model . A single sensor can be used to deploy the generated module, in scanning mode sniffing for a match. When you download the BESC2.X library from Bosch Sensortec there is an Integration Guide which I suggest you read carefully. Bosch Sensortec say that the BME 688 compensates for the heater impact, but there are often other sources of heat, so in my experience this is an issue and you may need to calibrate, depending on your requirement/design. Regards Keith
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