Product review (5 stars): I LIKE THIS LITTLE HYDROMETER SEEMS TO BE WORKING WELL
I LIKE THIS LITTLE HYDROMETER SEEMS TO BE WORKING WELL.  iNSTALLED ON THE WALL JUST ABOVE MY THERMOSTAT BY THE COLD AIR RETURN. THE MAGNITE IS PRETTY STRONG, . TO STICK IT TO THE WALL I USED A FENDER WASHER WITH DOUBLE SIDED 3M TAPE TO HOLD THE WASHER ON THE WALL now you have a metal surface for the magnet.