Liquid Level Indicator for Condensed Gases at Low Temperatures

Abstract

The instrument described here is designed to measure, indicate, record, and control the level of liquefied gases inside a closed vessel. It is especially designed for use at low temperatures with liquefied gases such as nitrogen, oxygen, hydrogen, and helium. It operates on a capacitance principle, and makes use of the difference in dielectric constants of the liquid and vapor. The sensing element is a cylindrical capacitor whose capacitance is a function of the height of the liquid column. The sensing capacitor is one element of an automatically balanced bridge which both indicates and records the liquid level. An over-all accuracy of ±1.0 percent has been obtained. The sensitivity is adequate for use where the difference in dielectric constants of the liquid and vapor is 0.05 or greater. The instrument also incorporates a control function and supplies a controlled air pressure to a pneumatically operated valve for maintaining the level at a predetermined point.