Patent ID: 6979361

Claim:
An end of service life indicator comprising: a housing member ( 10 ), said housing member having a front side and a back side, said housing member being constructed of a solid material, and having a plurality of housing openings ( 10 a ) extending from said front side to said back side of said housing member; each of said housing openings being of sufficient cross-sectional area to allow a fluid medium (F) and particulates (P) to pass; a filter member ( 20 ) disposed adjacent to said housing member, said filter member comprising a porous solid through which said fluid medium can pass, said filter member being adapted to adhere to said particulates; a template member ( 30 ), said template member being disposed adjacent to said filter member, and said template member comprising a solid material, said solid material being substantially impervious to said fluid medium; said template member ( 30 ) having a front side and a back side and a template opening pattern ( 50 ), said template opening pattern comprising an opening in said template member extending from said front side to said back side of said template member.