Product review (4 stars): A Simple Way To See If The Temperature Rises In A Freezer
This is a simple to use item that will indicate whether the temperature in a freezer had risen above freezing.  One big plus would seem to be if a freezer has stopped working for a period of time and then started again this would indicate that something had happened. One negative is it does not show the exact temperature in a freezer.

The idea is simple. You put water in the container and let it freeze.  Once frozen you put a ball on top of the frozen water.  If the water melts the ball starts dropping and you know there is a problem with your freezer.  How low the ball drops would seemingly indicate how high the temperature is getting and how long  the temperature has been above freezing.  I assume if your freezer stops working and starts working again the ball would freeze in the container at the point the temperature was the highest indicating there was a rise in temperature. The problem with the freezer meter is that it does not show the exact temperature in the freezer which can indicate if there is a problem as well.

I  have always used thermometers in my freezer. I use the nonelectric ones that tell you the exact temperature.  The problem with these is that if the electricity goes out, or a freezer stops working for an extended period of time when you are not home and then starts up again, you can't tell something has happened.  While a regular thermometer shows the current temperature it does not show what happened in the past.

I am using this with a regular thermometer in my freezer.  This will tell me if the temperature in my freezer has risen above freezing at a certain point when I am not home or not opening my freezer, while a regular thermometer will tell me the exact temperature.