The invention relates to a membrane electrode unit for electrochemical apparatuses, in particular for direct methanol fuel cells. A valve assembly is provided to seal the interior of the conduit from its exterior when the sensing element is in its retracted position. The particles to be filtered are prevented from passing through the respective minimum widths, resulting in filtration of those particles from the fluid. The invention is particularly suited for a force-directed placer which uses a linear system solver to obtain a globally optimum solution for placement of the cells having some overlap among the cells, and thereafter spreads the cells to reduce the overlap. The invention makes it possible to simplify the design of a self-locking differential and to significantly increase the range of the blocking effect. Fitted low-frequency shape information is distinguished from a remainder of the local shape information yielding mid-frequency topographic measurements of the subapertures, which can be assembled to construct a profile measurement of the aspheric surface.