Product review (5 stars): ... purchased 4 of these temp/ humidity clocks and I love them. I have one by my night stand
I recently purchased 4 of these temp/ humidity clocks and I love them.  I have one by my night stand, and I have 3 hanging by a paperclip to monitor the condition of the Central Air System.  Two are hanging by a paperclip on the A/C discharge vents at opposing sides of the house, and one is hanging by the A/C inlet where the filter is located.  If your A/C unit is working correctly you should see a difference of between 15 to 20 degrees, if not more between the discharge vent and the inlet.  You will also note a drop in the humidity as well at the discharge vent when the A/C is running.  They are great inexpensive diagnostic tools.  They are also a conversation piece as well.  You will get asked what is that hanging from your vent.  Then you show them, and explain. Anyway, I simply love this little inexpensive diagnostic device.  Thank You!