Ground Testing of a Compression Mass Gauge

The development and design of a cryogenic liquid quantity gauge for low-g applications is described. The gauge, named the Compression Mass Gauge (CMG), operates on the principle of slightly changing the volume of the tank by an oscillating bellows. The resulting pressure change is measured and related by thermodynamics to the volume of vapor in the tank, from which the volume of liquid is computed. Data at several bellows frequencies are obtained to enable minor real-gas effects to be quantified and thereby to obtain a target gauging accuracy of ±2% of tank volume, depending on the tank fill level. A development unit was fabricated using flight-approvable mechanical components for the in-tank hardware in conjunction with commercially available controllers. Test results at room temperature and in a liquid nitrogen environment confirm the predicted measurement uncertainty for the laboratory test conditions. The measurement uncertainty anticipated for flight conditions in spacecraft tank sizes is projected from the mathematical model of the ground test article.