Density determination of slush nitrogen by the improved capacitance-type densimeter

Slush nitrogen is considered to be a better coolant than subcooled liquid nitrogen for the high-temperature superconducting cable cooling because of its greater density and cooling capacity. The capacitance-type densimeter is utilized to determine the density of slush nitrogen in this paper. In order to improve the densimeter performance, the bulk shielding method, based on the application of the double-shielded cables, is introduced, and the influence of the frequency of the applied voltage is investigated. The parasitic capacitance of the densimeter system is significantly reduced, and the fluctuation of the capacitance is depressed within ±2.0 × 10−4 pF at a frequency of 1.0 MHz, and the typical sensitivity of the differential type densimeter is 3.718 pF and the high accuracy of within ±0.25% for density measurement is obtained. In process of producing slush nitrogen by the freeze-thaw method, the discharge rate is 4 l/s and the time of the freeze and thaw cycles is 10 s and 5 s respectively to obtain slush nitrogen with fine solid particles, and slush density is measured by the densimeter system. According to the experimental results, the rotating speed, higher than 50 rpm in this study, is necessary to homogenize slush nitrogen for the high accuracy of the density measurement.