Monitor For the Quality Factor in Two-Phase Helium Flow Using a Low Temperature Oscillator

This report describes a system to measure the quality factor X in a two-phase helium flow. The two-phase helium flows through a concentric cylindrical capacitor governing the frequency of an oscillator placed at low temperature. The difference between the dielectric constants of liquid and gaseous helium results in a capacitance change, when the ratio of liquid to gas varies in the stream. The capacitance probe is optimized for minimum pressure drop and flow perturbation. Two different modes (analog and digital) are shown to obtain a reading of the quality factor with a precision of about 1% of full scale. The oscillator has excellent stability and reproducibility with respect to temperature variations. The measured quality factors are compared with the results obtained by separate gas flow and liquid helium measurements after phase separation.