It is imperative to use it as the support bag for the correct fit and strength when the Tru-Seal Filtration system is being used. This support bag stays in the unit and the Tru-Seal Microfilter disposable bag fits properly inside it. This support bag fits into all MD, Modern Day, Silent Master, and Flo-Master power units no matter the height or previous filtration type. Sold individually. Diameter is 13-7/8 inches. Length is 13.5 inches.