Product review (1 stars): You get what you pay for !
I bought 3 of these to put into some of my guitar cases... but before I ever had a chance to put them in the cases, I noticed that all three were sitting on the same 'reading' and never changing.  I have other electronic ones I use in this room as it is full of guitars hanging on the wall also... but during the time I switched from using the humidifier to needing to use the de-humidifier in the room ... they NEVER CHANGED their reading.  So I'd say they don't work.  You get what you pay for.