• Model: 1561-11 • Diameter: 5 1/2 inches • Length: 11 7/8 inches • Top: Open, 1 15/16” • Bottom: Open, 2 3/8” The Dimension One 1561-11 R Pool & Spa Replacement Filter Cartridge is a compatible filter replacement, made by Filbur, for the Unicel C-5402, Pleatco PDO25-4 or Aladdin 12526 filters. This pool filter cartridge is designed with open top and bottom ends that allow water to flow easily and efficiently. The diameter of the top end measures 1 15/16 inches and the bottom end measures 2 3/8 inches in diameter. For optimum performance, replace pool and spa filters as recommended.