TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The plate stack comprises planar stacked cells for electrolysis. At least one pressure cushion is arranged between the plate stack and at least one base plate, and/or between the plate stack and at least one cover plate. The clamping elements are provided, which are trained to tension at least a base and cover plate together as support elements for the plate stack or to clamp the plate stack together. At least one pressure cushion is a pneumatic and/or hydraulic pressure cushion, which has a terminal for a pneumatic and/or hydraulic system or control of the pressure in the pressure cushion. At least one side of at least one pressure cushion is mechanically deformable, preferably elastic depending on the deformation of base or cover plate and/or the plate stack, where at least one side of the side of at least one pressure cushion, with which one side of at least one base or cover plate and/or one side of the plate stack are in mechanical contact, and/or at least one side of the side of pressure cushion, which is at least formed as an edge, and arranged as a circumferential edge between the sides is in mechanical contact with at least one base or cover plate and with the plate stack. The pressure cushion in at least one base and/or cover plate is spatially integrated, and/or the pressure cushion spatially projects from at least one base and/or cover plate partially or completely. The power conversion unit is a hydrogen and oxygen implementing unit, preferably a proton exchange membrane (PEM) high pressure electrolysis unit. The unit is adapted for an operating pressure of greater than or equal to 20 bar, and/or the unit comprises electrode faces with a surface area of greater than 500 cm. At least one pressure cushion causes an isostatic pressing of the plate stack. A contact pressure is affected on the plate stack, which is adjusted independently controlled of the energy conversion unit by an operating pressure, or is controlled by a pressure in the pressure cushion. The pressure in the pressure cushion is controlled or adjusted by the pneumatic and/or hydraulic pressure control unit, which is connected to at least one controlled pressure cushion. An edge of at least one pressure cushion mechanically deformed in the manner of a bellow, and contact pressure is adjusted between stacked cells of the plate stack to a constant value.