Phonak recommends these, their original replacement parts from Hearing Aid Battery Club. These tubes fit Certena, Excelia, Savia, Versata, and Naida Open Fit Hearing Aids. Audiologists recommend changing these hearing aid tubes every 6 months. The size is printed on the tiny end; we recommend using a loupe and bright light to determine the correct type and size. Please review the Product Description before ordering, and contact us if you need assistance with ordering the proper size Slim Tube.