OEM quality replacement filter cartridge for Watkins Hot Spring Spas. Replacement filter cartridges from Pleatco are manufactured using the highest quality raw materials. Durable end caps and unparalleled resistance to harsh water treatment chemicals help Pleatco filter cartridges last longer than other replacements. A properly designed "Free Flow" center core provides 20% more efficient water flow than a simple drilled PVC core you might have in your filter today. While it may not seem like much, a more efficient water flow results in less wear on your pump and ultimately saves you money in energy costs from running the filter system for shorter periods of time. Outer Diameter (inches): 5 7/8 , Overall Length (inches): 10 1/2 . Top Hole Diameter (inches): 1 15/16, Bottom Hole Diameter (inches): 1 15/16. Filtration Area: 30 sq. ft. Manufacturer's Part#: 31489. Pleatco#: PWK30, Unicel#: C-6430, Filbur#: FC-3915.