Product review (4 stars): You only need two of these, you should be using the refill granules.
Do not pay more for to replace the full cartridge every time.  One comes with your Air-O-Swiss (AOS), buy 1 more CARTRIDGE here and you are set.  AOS sells demineralization GRANULES without the cartridge.  Once you have two CARTRIDGES, just buy the GRANULES.  It cuts down on plastic waste and will save you a ton on the refills.  This is something AOS condones and it does not void your machine's warranty.

Every 2-3 months (depending on use), you remove the cartridge, take the top off (it should unscrew), and pour used granules in the trash.  Follow the instructions that AOS provides to refill the new granules (its very straightforward).

I always have 1 spare cartridge on hand because you must soak the newly refilled cartridge for 24 hours.  If you are fine going a day without your humidifier every 2-3 months then you just need the one cartridge that comes with your AOS.

Thanks for taking the time to read my review, and I hope you found it helpful in making a purchase decision.