Apparatus for determining and/or monitoring the filling level of a product in a container

Apparatus for determining and/or monitoring the fill level of a fill substance or the position of an interface between two media or phases in a container, comprising:

• a signal-generating unit, which generates high-frequency measuring signals;
• a coupling-in unit;
• a surface waveguide in the form of a flexible wire cable;
• and
• a receiving/evaluating unit, which determines, directly, or indirectly, via the travel time of the measuring signals reflected on the surface or the interface of the fill substance, the fill level of the fill substance or the position of the interface in the container, wherein:
• the measuring signals are coupled via said coupling-in unit onto said surface waveguide and guided via said surface waveguide in the direction of the fill substance;
• and
• said flexible wire cable has the following structure:
• a plurality of coaxial plies of solid, twisted, single wires of a predetermined diameter arranged about a solid, single wire of said predetermined diameter;
• and the twisting of the solid, single wires in the individual plies is of opposite sense.