@pierrot10 Vref is an internal voltage independent from the supply voltage of the extension board. And it is, like @seb said, just used for correcting the values returned by the API. The default assumption about VRef is 1100 mV. You may also completely ignore it, and just use wm_1.value() to get the raw value; especially since your application does not seem to be a priori linked to any standard physical quantity. You have have to make your interpretation table of the returned values yourself, and it does not matter if that is mV -> humidity or number -> humidity.