You have 2 choices: The simple way : Indeed, send a constant value of 4.6 celsius to BSEC with each datapoint, knowing that the output will wrong until your board has actually heated up. The accurate way : You can model the thermal response of your module using a reference ambient sensor. Use this model to get a good BSEC_INPUT_HEATSOURCE estimation. If your device is always-on and boot times of a few minutes do not scare you, then the simple way may provide an acceptable output for your use-case. If you have multiple heat sources (CPU, wireless chip, Battery charger, display, etc) which are not constant, then modelling the thermal behavior of your design is essential, if the BME680 cannot provide an accurate measurement using placement alone o_o
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