Aprilaire 4640 Scale Control Insert for Humidifier Model 500
The Aprilaire 4640 Scale Control Insert is the component which encases the Water Panel. Use with Water Panel Number 10 and Water Distribution Tray number 4331 — both sold separately. This part is for use in Humidifier Models 500, 500A, and 500M. All parts mentioned above are included in the number 4792 Maintenance Kit.