NOVELTY - A fuel supply for a fuel cell (10) comprises an outer casing (21) containing fuel, and an information storage device (23) supported by the casing. The information storage device stores encrypted data that is readable by a controller (18) capable of decoding the encrypted data.    USE - The fuel supply e.g. fuel cartridge (12) is used for a fuel cell used in fuel cell system (claimed). This fuel cell, e.g. direct methanol fuel cell, enzyme fuel cell and reformat fuel cell, is used for powering electronic device, e.g. computer or personal digital assistant.    ADVANTAGE - The fuel supply has the ability to store information or memory relevant to the fuel contained in the supply, and the ability to communicate relevant information to the fuel cell that is supplies and/or the electronic device that the fuel cell powers.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) an information storage device associated with a fuel cell, where the storage device comprises information for hot swap procedure;    (2) fuel cell system comprising a controller operatively connected to a first information storage device disposed on fuel supply; a fuel regulator comprising a regulating valve; and a fuel cell comprising a membrane exchange member, where e fuel supply is in fluid communication with the fuel regulator and the fuel regulator is in fluid communication with the fuel cell;    (3) filter for use with fuel cell comprising a medium made from ion-conducting polymer where fuel exiting the filter contains less metal ions than fuel entering the filter, and where the medium comprises a polymer that has been irradiated and sulfonated;    (4) ion gauge comprising two spaced apart nodes, and electrical source applying a voltage across the two nodes, where fuel is present between the two nodes, and where a measured electrical property between the two nodes is related to the amount of ions in the fuel; and    (5) method for measuring an ion level in fuel comprising applying a voltage between the two nodes; measuring an electrical property between the two nodes; and comparing the measured electrical property to known values.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view of a fuel cell system.    Fuel cell (10)    Cartridge (12)    Pump (14)    Controller (18)    Outer casing (21)    Information storage device (23)