Product review (2 stars): Not Accurate, Cannot Adjust Humidity Reading
My Extech 445815 hygrometer is not accurate.  I ran this meter through the standard salt humidity test before I used it.  I tested it along with a few other hygrometers that I had on hand.  The salt test creates a stable known humidity in a sealed container.  An accurate hygrometer should read 75%RH.  My Extech 445815 only reached 68%.  I was surprised by this, so I repeated the test trying to find errors in my test methodology.  Each time, the 445815 read 68%. So, my unit reads 7% low.

The low reading should not be a problem because the 445815 has a multi-turn pot that allows the user to trim the RH adjustment.  The 445815 even includes clear instructions on how to do this, but the procedure proved impossible.  The trim pot was already adjusted to its upper limit.  Turning the trim pot further, did not affect the relative humidity reading.  As another reviewer wrote, there is a soft click sound when the RH trim pot reaches its upper limit. The trim pot did work in the other direction (I could decrease the RH value) but that was not helpful.

Having said all of this, I do like products from Extech.  I have an amazing Extech multimeter on the bench next to me right now.  Other than this hygrometer, Extech instruments have always proven themselves to be high quality and a good value.

Other reviewers report good results with this meter.  I must have received a dud.  You should test your meter before use.