A precision microwave level meter based on frequency moduration (FM) method has been developed for measuring the molten iron level in a converter. In order to attain as high accuracy as 20 mm in the measurement range of 20 m, new technique has been developed for improving the SN ratio of the received signal and decreasing the quantized error of the conventional FM method. The initial phase of the received signal caused by the reflection wave from the surface of molten iron always fluctuates as

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... result of fluctuation of its level. By averaging the waveform of the received signal, the noise component caused by fixed targets such as an antenna cover is extracted. Thus the SN ratio of the received signal is improved by subtracting the averaged waveform of the received signal from the instantaneous one. In order to decrease the quantized error, modulation is applied to the phase fraction of the noise-cancelled signal. During one cycle of the phase moduration, the zerocrossing counts of the signal are averaged, and so the quantized error is reduced. These level meters were installed to converters in Kakogawa Works of Kobe Steel, Ltd. and contribute effectively to improving the hitting ratio of blowing.