Product review (3 stars): Necessary!
Need these for my William Stadler Form humidifier. Hard to find but found them on Amazon. Ordered 6, and they came right away, thank you Amazon, but the yellow sand inside was moving around and yellowed the outside of the cartridge. I emailed them in Switzerland, and they said  that was okay as long as none of them were lighter than the others. (What were we suppose to do, weigh them?) A little disconcerting. The fit was  a little funky and we thought it wasn't tight enough of a fit as it was hard to screw in and didn't appear to line up with the grooves; and it appeared to be leaking after we did. But we messed with it a little more, and gave it a minute. It did appear to work out okay. And we love the humidifier so we were quite relieved as it won't work correctly without changing out the cartridge every month.