The AprilAire 4231 yellow orifice is a small but important nylon component which accurately meters the flow of water into your humidifier. If you determine that the rate of water flow to your AprilAire humidifier is too high or too low, check the orifice to determine if it has been damaged or dislodged. Replace the orifice if any defect is noted. Attempting to clean the hole in the orifice using pins and needles can enlarge and damage the hole. To ensure that the correct amount of water is flowing to your humidifier for maintaining recommended humidity levels in your home, always use the correct orifice replacement part unique to your model humidifier. Yellow Orifice 4231 is intended for a flow rate of 3GPH (gallons per hour) and is used in Models 400, 400A, 400M, 500, 500A, 500M, 550, 550A, 558, 560, 560A, 568, 600, 600A, and 600M. Also used for Model 700 manufactured after June, 2019 (check your Model 700 for the AprilAire label to locate the date code).