Abstract: First and second identical pseudo-random binary sequences are generated. The second sequence is delayed in a variable delay arrangement, and a reflected first sequence is compared with a delayed second sequence. The second sequence is delayed until the reflected sequence and the delayed sequence are coincident. The delay of the adjustable delay is equal to the travel time of the first sequence.

Abstract: An elongated cable has a length equal to the height of the silo, whereby, when the cable is disposed in the silo, it extends from the top end to the bottom end of the silo. An electric generator for generating an AC electric signal is automatically and continuously swept through a preselected frequency range. The generator means being connected to the top end of the cable to thereby transmit the generated signal along the cable. Thus, when the generated signal is transmitted along the cable and the frequency of the generated signal is automatically and continuously swept, peak voltages will occur at frequency intervals, the frequency intervals being a function of the level of material. The frequency intervals are detected whereby to determine the level of material. Interference signals are eliminated by an adjustable impedance device at the bottom end of the cable.