Historically, thermally bonded or melt-blown depth cartridges were defined a nominal filters, and the term absolute filter＇ was reserved for surface cartridge types. With the arrival of improved manufacturing techniques, however absolute ratings can now be achieved with depth filters. In this article Chris Faulder, European product manager for Parker Hannifin＇s Fulflo Division, explains the changes in definition, and outlines the benefits of both types of filter for the chemical process industry.