Precision Ultrasonic Meters of Fluid Level in Sealed Containers

Abstract

The problems of measuring the fluid level in steel containers through a wall by acoustic methods are considered. When measuring the level by sound propagation time in a fluid in case of an inelastic wall the best accuracy is provided by the correlation-phase reception of compound signals. But for measurements in steel containers, such a technique is usually not used due to phase structure distortions of a signal with a broad frequency bandwidth when transmitting through an elastic wall (having multiple resonances in this band). The use of correlation-phase reception in such cases may be possible by choice a probing signal, a phase structure distortion of which will be small when transmitting through a wall. The aim of the work is to determine the possibilities of using the correlation-phase reception and the achievable accuracy of level measuring in the conditions of different wall thickness of steel containers. The significance of research is determined by the possibilities of a multiple increase in the measuring accuracy. Acoustic methods and level measuring devices are considered; the advantage of time-pulse (time-of-flight, TOF) methods over interferometric (including swept-frequency acoustic interferometry, SFAI) and other measuring methods in large containers is shown. A method of level measuring using the correlation-phase reception of compound signals and a scheme for calculating the broadband impulse signal transmission through an elastic wall (and building the amplitude-frequency and phase-frequency characteristics) taking into account the longitudinal and shear waves of thickness wall vibrations and standing waves of its resonant vibrations along the length (diameter) are proposed. A method for selecting the probing signal frequency range according to an amplitude-frequency characteristic of elastic wall which provide the low phase structure distortions when transmitting through a wall is suggested. The achievable accuracy estimates of measuring the sound velocity and the fluid level in rail tank wagons are obtained. Experimental works carried out on a thin-walled (0.8 mm) barrel with signals of the frequency range of 250-750 kHz (i.e. frequencies below the first thickness resonance frequency of barrel bottom) are confirmed the small phase structure distortion of signals when transmitting through the bottom and the high efficiency of the correlation-phase reception. With a signal-to-noise ratio of 0.4, a high time measuring accuracy of ~0.15 μs corresponding to a level measuring accuracy of ~0.1 mm is obtained. For signals of frequency range 250-750 kHz the interval of wall thickness is defined as 0.3-3.6 mm at which the efficiency of the correlation-phase reception and the level measuring accuracy should be high. The results showed that the use of correlation-phase reception of compound signals when measuring a fluid level through the bottom of a thin-walled steel barrel is possible and provides both high operating efficiency in the noise conditions and high measuring accuracy. It can be expected that the same accuracy factors and operating efficiency will be kept in a bottom thicknesses range at which the probing signal frequencies will be less than the first bottom thickness resonance frequency. In cases of large bottom thicknesses, if the first bottom thickness resonance frequency is low, the probing signal spectrum may include frequency regions between the thickness resonance frequencies. In this case, it becomes possible to use the correlation-phase reception for the level measuring in containers with a wide range of bottom thicknesses. Ref. 20, tab. 1, fig. 7.