Precision Ultrasonic Measurement of Fluid Level through a “Thick” Steel Wall

Issues of the fluid level measurement through a steel wall are considered. When level measuring by signal propagation time, correlation-phase reception of complex signals provides the best accuracy. However, when such signals pass through an elastic wall, distortions of their phase structure occur. These distortions are associated with vibrations of the thickness resonances of longitudinal and transverse waves and with propagation of other nonresonance vibrations in the wall, which are interferences for the useful signal passing through the wall and the fluid layer. Herewith, to ensure the small distortions of the received signal, it is necessary to choose the characteristics of the probing signal “correctly”. Therefor it is necessary to determine regularities of wave propagation of different frequencies through the wall and in the wall. The aim of this study is to determine the mechanisms of the received signal forming, to develop the methods for the optimal probing signal forming, and to determine a possibility of correlation-phase reception using and an achievable measurement accuracy. A scheme for received signal forming is proposed. It considers a signal passing through the wall and the fluid layer; the signals of the thickness resonance vibrations of the longitudinal waves propagating through the wall and the fluid layer; the signals of the thickness resonance vibrations of the transverse waves propagating in the wall, and the signals of the nonresonance vibrations propagating in the wall. The methods for the level estimation of the referred above vibrations (and their signals) based on Lamb’ wave theory are suggested. These methods permit to predict the characteristics and spectra of the received signals. A technique for estimation of the phase structure distortions and correlation coefficient of received signal in accordance with spectral component relationship is proposed. A technique for the forming of the optimal probing signal that provides small distortions of the received signal and high correlation coefficient is propound. The theoretical estimates of the received signal spectra in different frequency ranges are given. Experimental proof of these spectra found. The theoretical estimates of the probing signal parameters providing small phase structure distortions of the received signals are given. Experimental proof of the effectiveness of such signal using found. The theoretical estimates of the ratio of the useful signal to interference in different frequency ranges are given. It is shown experimentally for a plate of 14.5 mm thick that this ratio is large in high- and mid-frequency range (597–791, 395–593 kHz), in that case a qualitative operation of a meter is ensured. This ratio is fewer in low-frequency range (197–395 kHz), but the meter’s operability is hold true. The ratio is small at lower frequencies and operability ensuring is problematic. Experimental measurements show that in the case of optimal signal using the time measurement error for all frequency ranges do not exceed 1 μs, the correlation coefficient is ~0.84–0.92 at high-frequency range, ~0.78–0.90 at mid-frequency range, and ~0.45–0.65 at low-frequency one. Application of correlation-phase reception the complex signals for time (level) measurement through a “thick” steel wall is possible at optimal probing signal parameters; it provides more effective operation and greater measurement accuracy compared to other known methods and devices. One can expect that such high indicators of accuracy and operational efficiency will hold true for a wide range of wall thicknesses, level of fluid and its characteristics.