I tried the blumat carrots in a pot of coco once. They don't work in that medium. There isn't enough of a wet/dry like there is in soil to open and close them. Mine just stayed on at a constant drip. They need to be in soil to work correctly. A coco based soil may work but I know that I had trouble in straight coco/perlite.
In soil, they work great. Definitely need a few days of tweaking to get that drip just right. FermentedFruitz has used them more often than I.