Aprilaire 4231 yellow orifices are the tiny nylon components which, accurately meter the flow of water into your humidifier. They allow a specific amount of water to go into the humidifier, unique to your model humidifier. Used in Models 400, 400A, 400M, 500, 500A, 500M, 550, 550A, 558, 560, 560A, 568, 600, 600A, and 600M.
