When I looked through a stack of about 20 of these that had been sitting on a shelf for least 2 months. 
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Mine usually warms up immediately after cutting the bag, then the middle dot changes to pink within 15 minutes. It usually comes inside aluminium bag from mouser.
I like writing negative reviews on Amazon for the knock-off ones. “Thanks for including the joke humidity sensor, I gave them to friends for a laugh”.
How to spot fake ones:
- the color is way too vibrant (the real ones are muted pale blue & pink)
- The “bleed-through” to the back
- THEY NEVER CHANGE (even if you put them over a cup of tea or coffee).
Throw them out, or give them away to your enemies… they’re useless.
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I just got a handful of these with some vacuum bags and noticed the same thing. They’re all very blue and they were in a zip lock bag, so you know it wasn’t air tight.
I keep one at the bottom of my P1S for lulz (and to use as a "dustpan’ for tiny scraps that don’t make it down the chute). Works great to brush them to the front, then vacuum them out with a crevasse tool.
