Development of High Precision Level Gauge for Helium II

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1. A continuous level gauge using superconductor for helium II has been developed. Composite of tin (coated film) and hastelloy or moleculoy(substrate) were used as sensing elements. Tin was coated on substrate of 0.2[mm] by spattering. We have examined the effects of 3 factors.(1)the optimum coating depth of tin coating.(2)the optimum operating current to eliminate the error caused by film boiling or by climbing superfluid.(3)the mode of suppling the operating current to the gauge.To raise the accuracy of measurement, the electric resistance of the superconductor in normal state was made very large compared to that of hastelloy or moleculoy. By appropriate selections of coating depth of tin and operating current, it has been confirmed that the liquid level was measured with accuracy within 1[mm] by the level gauge developed. The best result was obtained when surface resistance of tin was 3.89[OMEGA/*] and the operating current was 120[mA]. The accuracy was within 0.45[mm]. The operating current was supplied only when the measurement was done. A proto-type level gauge was fabricated on the basis of these findings.2. Float-,ultrasonic- and differential pressure-type level gauges have been tested too. The float-type gauge showed poor reproducibility because of too high sensitivity. The ultrasonic-type gauge required too expensive electronics. Any appropriate material matching the condition of low temperature and very small pressure difference for a differential pressure gauge was not found.