Hi Keno1,
Whether calibration is required depends on the backend BSEC algorithm. The output of BSEC includes calibration accuracy. If the accuracy is 3, calibration is not required, and usually calibration is required if the accuracy is lower than 3. Calibration does not affect physical sensors, which is relevant to the BSEC algorithm. 28 days means it takes longer than 4 days to complete the calibration.
You can refer to integration guide in BSEC package after you download BSEC2.4.0.0.
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