scaleD inhibitors

[John Stumbles]

Sic: scaled up! I've come across a couple of the permutit plug-in cartridge types where the waterways feeding the cartridge have become blocked up (whether through limescale or the phosphor salt getting back into the feed and precipitating out again, I'm not sure) and one 'Combimate' type which seems to have the same problem as it still has phosphor balls in the chamber despite not having been touched for years. I suspect it's an affliction of all such designs, but I wonder if there are any that don't suffer from this problem (and if so how they achieve this).

[John Stumbles]

Sorry, meant to add: I opened up an ancient Permutit type cartridge (I know it has been there for at least 3 years) and it's still full of a white salt. My understanding is that these phosphor-dosing inhibitors simply allow the phosphor salt to dissolve slowly into the system being dosed so the salt should eventually be used up with nothing left. Is this correct, or is there something else in beside the active salt, or is it some sort of ion-exchange system?