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I have a few in various sizes and I find them useful and beneficial, they do have limitations however:
1/ In my opinion the heat just doesn't last long enough. They are great while pumping it out, but the amount of heat given out declines fairly quickly after the first 20-30 minutes. You almost need to own several to get you through sometimes (I'm working on it, still building my collection)
(I have however used a small (pocket) one that was past it's main heat output and put it in a sealed box with my ep's to keep them warm, after my hands have sucked the big heat out of them - worked a treat!)
2/ They take a LOT more energy (ie boiling) to drive the chemical reaction back the other way and "Set" them again, and as a result I did have one melt and lose it's essence in the pot once while boiling it up yet again - my fault though, a small one got trapped under 2 large ones, it was held against the hot metal on the bottom of the saucepan and melted like cheese on an over-nuked cheeseburger.
And yet I continue to use them and love them - a larger one inside my observing gear, held against my kidneys on really cold nights, hand warmer size in the pocket, etc. Great stuff!
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