A true depth filter allows particles to penetrate the filter matrix and become captured throughout the depth of the medium. As with surface filtration, this only holds true when the particle size/pore relationship is conducive to the process for which the cartridge was designed. The depth filter matrix has a broad pore size distribution; hence, depth cartridges rely on adsorptive retention for a portion of their dirt-holding capacity. Some depth filters, such as the ARD, Nexis and DFT classic, have a gradient pore structure, with tighter pores near the center core, to maximize mechanical retention. In some depth cartridges, such as string wound, the medium is not a fixed pore matrix, as with chemically or thermally affixed pleated media. For this reason, depth cartridges should not be subjected to flows as high as those that are possible for pleated cartridges. Most depth filters are made from extruded melt blown fibers or twisted yarn fibers. Melt blown depth filters are generally made from polypropylene, polyester or nylon and can be made in both absolute and nominal retention ratings. These types of cartridges can be made to filter particles sizes from less than one micron to over 100 microns. Yarn wound cartridges, made with fibrous materials, are often brushed in order to maximize the tortuosity of flow through the filter. They are nominally rated but offer the advantage of being made from a variety of materials.
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