Product review (2 stars): Humidity reading drifted, calibration doesn't work
I've had this for 2-1/2 years. At first it seemed to compare well to two other humidity sensors I had. Over time the humidity reading has drifted downward, and it now reads about 15% low compared to the other sensors. As other reviewers have reported, the humidity calibration screw does not work -- unlike the temperature calibration screw, which works fine.

So I'd echo the suggestion made by another reviewer that if you buy this, you should check the operation of the calibration screws right away instead of waiting until you need to use them. (On mine these screws were covered by small foam pads that I had to peel off. Don't make the mistake of trying to "adjust" the nearby Phillips-head screws that hold the case together.)

I would have expected better from Extech.