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The fridge is trickier, but for your freezer fill a small plastic cup with water and let it freeze. Then place a penny on top of it. If it gets warm enough for the water to melt the penny will drop.

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Put a bottle water in the freezer until it's frozen. Turn it on it's side. If you notice the ice-air line isn't vertical, you had a warming event.

There are heat-sensitive stickers that change color if above some threshold.

It's on packaged meat in Norway I believe.

that would be a good single use solution, we generally only a long power outage once every few years so that could work.

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