Product review (4 stars): Works and is smaller than ice cube method
The classic way to determine if a freezer has undergone a thaw is to put some ice cubes in a container and see if they have melted and refrozen. This "meter" basically does the same thing, though with a visual indicator that is a bit easier to see, and is smaller than the typical ice cube solution. My major gripe with it is that the Velcro loop used to secure the meter to a basket in the freezer tends to let the tube droop or tilt away from vertical. You'll need to place it with care.