The Aprilaire 4640 is the correct replacement for the following humidifiers:
The Aprilaire 4640 Humidifier Scale Control Insert is for use in your 500 series model humidifier. This Aprilaire humidifier scale control is a U-shaped piece that encases your humidifiers water panel evaporator.
The Aprilaire 4640 Specifications:
The Aprilaire humidifier scale control will direct mineral build up back into your humidifiers evaporative pad, keeping it out of your clean, humidified air. This humidifier scale control is a key feature to keeping your Aprilaire 500 series model humidifier working at its best.