Development of a high-accuracy capacitance-type densimeter for slush hydrogen

Slush hydrogen is a mixture of solid hydrogen particles and liquid hydrogen, and is being considered as a spaceplane fuel or as a means of transport with hydrogen used as a source of clean energy. In this paper we describe the fabrication of three varieties of capacitance-type densimeters using combinations of a flat plate and two cylinders for the electrodes, and present the subsequent density measurement of slush hydrogen obtained using the freeze-thaw method. The density values measured for slush hydrogen with a solid weight % of up to 30% were generally found to correspond to within ±0.5% of the density values estimated from the latent heat of evaporation of liquid hydrogen. Since the flat-plate/cylinder electrode-type densimeter allowed easy penetration of solid hydrogen particles within the gap between the electrodes, highly accurate density measurements were obtained compared to those simultaneously measured using densimeters with parallel flat plate electrodes. The practical applicability of the new type of slush hydrogen densimeter was thus confirmed.