High-frequency microwave pulses transmitted by the guided wave radar propagates along the detector component (wire cable or steel bar), and are reflected on the surface of the medium. After reaching the dielectrics to be measured, part of the pulse energy is reflected. The time interval between the emission of the pulses and their arrival is proportional to the distance between the surface of the medium and the reference plan of the instrument. The instruments have a low consumption; it can be installed on metallic or non metallic tanks. Their use is not harmful to humans or environment.