Product review (5 stars): Tried and tested, and they work!!
I tossed one in my safe, however left it open and 15 minutes later, the blue had turned to pink, or is it lavender? Not sure as the color is a pale color, so it could be either! There was a large silica gel packet already in the safe on arrival. So I tossed another card in and closed the safe immediately. Went back a few hours later, and now both cards have all pink spots! I'm going to change out that big bag of silica out and replace it with 3 bags of 10 Gram Dry & Dry silica gel packets. While waiting, I put the 2 cards that had turned pink into the bag of remaining 27 bags of the 10 gram D&D, and within seconds, the dots were slowly changing back to blue! Waited a few hours and the card Dot's were still blue!  So silica bag that came with it was no good.  I'm so glad I purchased this product, it's a cheap and easy way to confirm humidity levels and they work!!