The wet expansion according to Fenchel is a measuring instruction which is oriented to a device design. A load of 1 g per 1 mm sample width is defined. A paper strip of 100 mm length and 15 mm width is inserted between two clamps. The lower clamp is rigid and the movable weight of the upper clamp is balanced by a counterweight and connected to a display. The strip expands when immersed in water. This extension is displayed as a percentage.