A fuel cell bipolar plate including a plurality of reactant channels defining respective inlets and outlets and at least two flow restrictors respectively associated with at least two adjacent reactant channels. The structure includes a housing, a hub rotatably coupled to the housing, a plurality of blades, each with an inner edge attached to the hub, and a plurality of end-pieces. As an aid to memorization of a keyboard arrangement, symbols are assigned to keys such that a substantial number of keys in a column of keys belong to the same attribute class. The locking pin assembly is movable between a first position, wherein the bore is unobstructed and the contact pin can be freely inserted and removed, and a second position, wherein the cylindrical section of the locking pin extends into the bore. The sensors evaluate the item in 3 dimensional space and the results are compared against expected results.