Miniature gallium phase-change cells for in situ thermometry calibrations in space

Abstract

The melting point of gallium, 29.7646 °C, makes it a very convenient fixed point for in situ calibration of thermometry in space applications such as Earth observation. NPL and RAL Space have collaborated to develop a miniature ‘phase-change cell’ using gallium. This report describes the modelling, construction and thermal metrological performance of miniature phase change gallium cells. These cells are intended to be embedded in the calibration blackbody structure, near the temperature sensors, for the purpose of in situ recalibration of those sensors. In this study, the blackbody structure was simulated by a monolithic aluminium block, and the phase-change cell and thermometers were embedded within the block. A thermal model of the system was also developed using Comsol Multiphysics® and validated against the experimental results, which provides useful information on further optimisation of the system. Under the (realistic) test conditions in this study, the phase-change cell, containing 2.1 g of gallium, was able to produce clearly defined melting curves with a duration of several hours and melting range of less than 5 mK, which represents sufficient performance for the envisaged application. Overall it has been shown that the current design methodology is fit for purpose and represents a solid foundation for raising the technology readiness level sufficiently for in-flight use.