It’s a lot of text with a number of side conversations, but there’s a lot of information in there too.
https://forum.bambulab.com/t/filament-drying-preliminary-results
Short answers are that regular filament dryers are flawed because of how negatively ambient relative humidity affects drying. Think the difference between the dry heat desert and a steamy jungle. Sweat evaporates so fast in the desert you can get chilled. Manufacturer’s tell you how to dry filament using industrial drying ovens. Not the same thing.
What I’ve been working on is turning ambient air into dry air and pumping that into a filament dryer. Filament dries consistently faster and it’s easy to dry to low humidity in a filament dryer.
Filaments like PETG HF need to be dried and this does it. I’ve just dried one spool of PETG HF but it printed great. It actually printed pretty good out of the shipping box, but printed better after drying.