Phase-shift detection for capacitance sensor measuring void fraction in two-phase flow

Capacitance sensors are frequently applied for void fraction measurements in two-phase flow. A capacitance sensor is formed by two electrodes of various shapes, usually a fragment of a cylinder, mounted at the outside of the wall of a dielectric pipeline through which a two-phase medium flows. The capacitance of the sensor changes when the mean dielectric constant between the electrodes varies due to variations in the percentage of gas and liquid flowing in the pipe. For the detection of the measuring signal, an impedance bridge or a frequency-deviation counter are usually used. In this paper, a new measuring procedure was tested utilizing the phase shift between the sinusoidal signal passing the capacitance sensor and a reference signal. The void fraction is determined directly from this phase shift instead of measuring the capacitance as void fraction indicator. It was noticed that the phase shift is almost proportional to the void fraction.