Product review (4 stars): Clever device.
Fill the vial with water to the line, freeze the vial with the water inside and ten place the marble on top of the ice, cover and attach somewhere I nyour freezer (which is easier said than done).

 If the power goes out or your freezer otherwise malfunctions for a while, the ice inside the vial will melt and the ball will begin falling. If power is restored soon after failing, the ball may have fallen only part way before the water freezes again. If its a while, the ball will fall all the way to the bottom. If thats the case, the manufacturer of the Freezer Meters says to discard the food. In my household, even if the ball dropped only a bit, I suspect the same thing would happen.

 The nature of our lives is such that it is highly unlikely a power failure would go undetected since we have a number of clocks and timers the would be blinking away. A refrigerator failure would likely be immediately detected as well.

 So the Freezer Meter will serve us as a secondary indicator and e especially useful if were off on a trip and come back to house full of blinking clocks. If the ball hasn't dropped, then we know it was a short term failure and the frozen food is okay.

 Worth the price of admission.

Jerry