An electrical capacitance method is described for the measurement of multi-interface levels of gas/liquid/solid, including the foam layer. It uses a novel multi-electrode capacitance sensing element. A capacitance transducer based on the charge/discharge principle is used, which is stray immune and capable of operating at high frequencies to reduce the effects of the liquid conductivity on the measurement. The presence of foam is detected by processing the fluctuating component of the capacitance signal, which reflects the inherently unstable feature of the foam, whereas the multi-interface levels are reconstructed by processing the steady-state component of the measured capacitances.
Shi, T. M., Xie, C. G., Huang, S. M., Williams, R. A., & Beck, M. S. (1991). Capacitance-based instrumentation for multi-interface level measurement. Measurement Science and Technology, 2(10), 923-933. https://doi.org/10.1088/0957-0233/2/10/006