Capacitance measurement circuits for void fraction sensors for cryogenic fluids

In the two-phase flow of fluids, the void fraction is an important parameter which determines the various aspects of the flow such as mass flow rate, heat transfer etc. Since the vapor and liquid have different dielectric constants, this results in capacitance variation of the flowing fluid. Hence, the void fraction sensors can be based on capacitance measurements. However, since variation of dielectric constants between the liquid and vapor is very small suitable electronic circuits need to be developed to measure very small changes in capacitance. Three circuits namely 555 timer circuit, Differentiator circuit using IC 741 and C to V converter using LM2917 have been developed and studied for their performances using capacitance-based level sensor. These are discussed in this paper. The studies indicate that the differentiator circuit as well as the C-V converter circuit are more suitable for the implementation in Void fraction sensor.