I have never found accurites to be accurate at all. I have owned 3 two returned as they were outside manufacturers tolerances (more than 2ºF high or low and more than 20% RH (high or low) the third one I was inside barley and I gave it anyway. Digital hygrometers take time to read properly hours, I believe, according to the specs. Definitely more than 30 min in any case. They are slow altering, it maybe nothing more than time and the expected deviation of this style of unit.