|The Hydro Stick provides constant freshness in your Air-O-Swiss humidifier. This water maintenance system keeps your system operating at a high level between cleanings. Begins to function as soon as it comes into contact with water. Replace annually. The Hydro Stick can be used for Air-O-Swiss models E2441, 2055, 2055A, 7131, 7133, 7135, 7142, 7144, 7145, 7147, U600, and U650.