I.E. Kuznetsova, B.D.Zaitsev, A.S.Kuznetsova, I.A.Borodina
At present for measuring gas consumption in trunk pipeline one uses the methods based on acoustic waves. In order to achieve the high accuracy of measurement and to use of acoustic wave of low intensity it is necessary to have a perfect acoustical match of standard piezoceramics with gas. Bad matching leads to the necessity to use the acoustic wave of high intensity and to increase the sensitivity of recording equipment and also arouses long “ringing” the launching transducer. These facts decrease the measurement accuracy and complicate the recording equipment. As long as the values of acoustic impedance of piezoelectric ceramics and gas significantly differs this cause the need of using the matching layers with intermediate value of acoustic impedance. The matching devices of modern acoustic transmitters and receivers of gas flowmeters are based on such low impedance materials as epoxy with various compounds. However the variation of composition and quality of such compounds does not lead to significant decrease of impedance value and quality of matching of acoustic transmitters and receivers with gas is unsatisfactory. In this paper we carried out the search of new materials use of which as matching layer can lead to significant increase of intensity of acoustic wave in gas. For this purpose we analyzed a few situations differing by the number of matching layers and determined the ranges of acceptable values of elastic constants and density. The analysis was based on the solving the problem of transmitting longitudinal acoustic wave through multilayer structure. We used the mechanical boundary conditions namely the continuity of mechanical displacement and stresses on all interfaces. The second side of the transducer was loaded by the medium with the total attenuation of acoustic wave. The conducted analysis has shown that the amplitude of output signal may be significantly increase if matching device contains at least three layers. The values of elastic constant, density and layer thickness corresponding to the maximum amplitude of acoustic wave in gas were determined. Then we carried out the search of commercially accessible materials with acceptable parameters. Then the optimal values of their thicknesses providing the maximum amplitude of output signal were calculated. The conducted calculations have shown that use of such matching device structure allows to increase the amplitude of acoustic wave in gas about 2 orders respectively to widely used acoustic gas flowmeters.