I have seen some pretty beat up Ultratherms work just fine. I would imagine that the spot where the ridges of the vision cages pinch the heat pad cause a hotter hot spot right there than throughout the heated area. I didn't like Ultratherms with visions, half of the hot spot would be 82 and the other half would be 105 because of how the wavy visions inconsistantly insulated the heat

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