BONECO Demineralization Cartridge 7531 Provides water mist without minerals or limescale Easy cartridge replacement or easy replacement of the granules every 2  3 months Can be refilled with top-up granules A7533. Compatible products : U650, 7135, 7142, 7144, 7145, 7147, U200, U700 Use with all BONECO & AIR-O-SWISS ultrasonic humidifiers except the U7146 Travel & U300 series