Most commercially available monitor systems for bed levels in chemical reactors or in storage bins have the drawback that their calibration is based on specific properties, which are often very difficult if not impossible to determine, of the bed and bed material. This report describes a method for a continuous monitoring of the levels of any kind of bed (liquid or granular, moving, fluid or fixed bed) utilizing the technique of time domain reflectometry. The transit time of an electromagnetic pulse, which travels downward along an electrode inside the vessel and is partially reflected by the surface of the bed, is measured. The principal novel feature of this method is the use of the propagation velocity of an electrical signal in a gas as the basis of measurement, making the calibration very reliable and stable. An additional advantage is the simplicity of the necessary probe system in the interior of the reactor vessel or container.