Pentair 25021-0224S Small Inner Cartridge Replacement Sta-Rite System 3 SM-Series S8M500 Pool and Spa Cartridge Filter (Discontinued by Manufacturer) Small inner cartridge replacement Fits Pentair Sta-Rite System 3 SM-Series pool and spa cartridge filter model S8M500 209-square feet filtration area Measures 14-1/2-inch length by 31-inch height by 14-1/2-inch width