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Measurement of level of liquid by the waveguide frequency level gauge against hindrances


B.A. Atayants – Ph.D.(Eng.), General Director of Open Company enterprise «KONTAKT-1» (Ryazan)
V.M. Davydochkin – Ph.D.(Eng.), Head of Department of Microwave Oven of Devices and Aerials, Open Company enterprise «KONTAKT-1» (Ryazan)
V.V. Ezersky – Dr.Sc.(Eng.), Professor, Department of Radio Control and Communications, Ryazan State Radio Engineering University

Measurement of level of filling of tanks by liquid substances with the help wave guide a level gauge leads to occurrence of difficultly solvable contradiction. On the one hand it is necessary to increase size of a range of reorganisation of frequency for decrease in size of an error. On the other hand such increase at the big length of a wave guide causes necessity of processing of a signal with high frequency that causes sharp increase in number of processed readout. All becomes complicated in connection with dispersion presence in a wave guide which causes parasitic frequency modulation (FM) a signal difference frequencies (SDF) and, accordingly, causes the raised error of measurement of distance to liquid level in a wave guide.
Necessity to reduce influence parasitic FM has led to application of the modified discrete transformation of Fourier (MDFT) and the modified signal function (MSF). Spectrum SDF calculated by means of MDFT possesses remarkable property - all periodic continuations of a spectrum, except the first, basic period, characteristic Fourier for usual transformation, are deformed under the form, are displaced and blur on frequency. Such distortions accrue with increase in number of periodic continuation of a spectrum. The first period of a spectrum thus is not deformed also position of the main petal precisely corresponds to frequency SDF. And at increase in frequency SDF higher than the frequency of sampling the form of the basic spectrum is not deformed also it moves on an axis of frequencies strictly with change of frequency of a signal. Deformed interference periodic continuations thus are hindrance in regular intervals distributed on distance (frequency). At increase in frequency SDF the relation a signal - pomehovyj the background practically does not change. Thus, frequency SDF can be much more the frequency of sampling and at the same time measurement of distance to liquid level in a wave guide is made with the minimum error. Result of measurement of distance can influence attenuation in a wave guide and reflexion of a signal from devices of hermetic sealing of the case of the device in the top part of a wave guide and from the open end of a wave guide under a liquid layer.
For an estimation of degree of influence of these factors numerical modelling of process of measurement has been spent. A basis was model SDF offered in work and the formula for MDFT and MSF. It is received that the basic influence render attenuation of an elec-tromagnetic wave in a wave guide and reflexion from hermetic sealing devices.
Presence hindrance the background generated interference by deformed periodic continuations of a spectrum, leads to that errors of measurement of distance to a liquid about the help of a finding of position of a maximum of a spectrum of amplitudes, reaches sizes 4−6 mm at bearing frequency of the microwave oven of a signal of 8,5 GHz, a range of reorganisation of frequency of 3 GHz and a range of measured distances to 130 m. Such error can be reduced at the second stage of processing SDF by means of MSF. Thus the error of measurement is not exceeded by 1 mm. At this modelling frequency SDF increased to values in twenty times exceeding sampling frequency. The number of processed readout was equal 256.

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