A cross-section drawing of an assembly of these fixtures is shown in Fig. . Atop the mounting platform, which is mechanically coupled to the lab bench via rigid framing, the GH fixture is affixed and is intended to remain stationary. The CE fixture on top can be retracted via a spring-loaded translation stage and motor. When in the "engaged" position, the spring-loaded stage and motor enable controlled application of ca. 200 N of downward force to maintain the o-ring seals between the CE fixture and membrane, the membrane and WE fixture, and the WE and GH fixtures. With the CE fixture in the "disengaged" positions, the WE fixture is removable along with the GDE sample, which is held in place with a friction fit of the GDL paper with a regular octagon profile whose ca. 8 mm outer diameter is ca. 5% larger than the bored hole in the WE fixture.