NOVELTY - The assembly (2) has a water vapor transmission device (4) provided between a pair of end plates (6), where the end plates include a set of outwardly extending ribs (8). A housing (10) e.g. single or multi-piece housing, includes a set of channels (20) formed adjacent to dry stream apertures (16, 18) of the housing, and is linked with wet stream apertures (12) of the housing. The ribs cooperate with the channels to define a tortuous bypass flow path between the wet stream apertures of the housing. Elastomeric seals (22) are provided for each inlet and outlet of wet and dry streams (3, 5).    USE - Water vapor transmission assembly for a fuel cell i.e. polymer electrolyte membrane fuel cell, system that is utilized as a power source in a vehicle as a replacement for an internal combustion engine. Can also be used as a stationary electric power plant in a building and residence and as a portable power in a video camera and a computer.    ADVANTAGE - The outwardly extending ribs of the end plates cooperate with the channels to define the tortuous bypass flow path between the wet stream apertures of the housing, thus simplifying installation of the assembly into a fuel cell module, eliminating usage of compression sealing in desired sealing planes of the assembly and minimizing pressure differential across the end plates. The elastomeric seals are provided for each inlet and outlet for the wet and dry streams, thus producing the assembly with compact structure. The elastomeric seals are not provided at the wet stream apertures, thus allowing larger tolerances for an assembly interface with mating components of the fuel cell module.    DETAILED DESCRIPTION - The wet stream comprises water vapor, and the dry stream comprises atmospheric air.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of a water vapor transmission assembly with a water vapor transmission device arranged within a housing, and a direction of flow of wet stream and dry stream through the water vapor transmission device.    Water vapor transmission assembly (2)    Wet stream (3)    Water vapor transmission device (4)    Dry stream (5)    End plates (6)    Outwardly extending ribs (8)    Housing (10)    Wet stream apertures (12)    Dry stream apertures (16, 18)    Channels (20)    Elastomeric seals (22)