ra 1 6 the surface on both faces has to be scratch free to aid in sealing in the central part of the foil 78 there is provided a recess 80 penetrating the foil the recess defines the entire geometry of the fff channel 13 i e thickness length width diverging and converging sections recess 80 is disposed completely inside foil 78 i e the material of foil 78 completely surrounds it the holes 82 are not connected to the recess 80 defining the channel 13 the shape of the recess 80 can be described as a longitudinally stretched hexagon with slots 86 fig5 b longitudinally extending away from the farthest corners of the hexagon the remaining four corners are disposed on the corner of what can be described as a rectangle alternatively the shape could be described as a longitudinal rectangle with a triangle each joined to the short sides of the rectangle and the apexes of the triangles terminating in the slots 86 at each end of the slots 86 there is an opening 87 in the form of a segment of a circle with the diameter of the circle defining the opening 87 being at least equal to the width of the slots 86 and preferably being larger than the width of the slots 86 in an assembled apparatus these openings 87 are aligned with holes 71 of inner spacer 70 with holes 71 being larger than openings 87 fig6 a to c show the holder 12 in detail fig6 a shows a side or axial view of the holder 12 the holder 12 is formed of an aluminium zinc manganese copper alloy which has been hard coated its outside surface is smooth with the surface roughness on the outside being 0 35 lt ra lt.