Product review (5 stars): Perfect little device
I bought this along with a humidifier so that we could determine what settings to use for the humidifier when our furnace is running.  It requires two AAA batteries which do not come with it, so if you buy one be sure to get batteries, also.  It does everything it is stated to do and works perfectly.  It has an opening in the back to allow you to hang it, so we mounted ours next to our electronic thermostat and have noticed that the two agree on the temperature of the room.  I would recommend this unit for anyone buying a humidifier, so that you understand the humidity levels of your home.