Miniature gallium phase-change cells forin situthermometry 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.

Digital twins for metrology; metrology for digital twins

“…The requirements for high precision measurement on the factory floor and for reliable measurement in challenging environments has led metrologists to develop approaches to in situ realisation of SI units [29][30][31] and self-validation of sensors [32,33]. This approach shrinks the traceability chain and removes the need for calibration (or allows for remote or self-calibration), but it is likely that some monitoring of these in situ realisations will be necessary to maintain end user confidence in their stability and accuracy.…”

Progress towards in-situ traceability and digitalization of temperature measurements

“…So far, no evidence of measurable drift of the miniature fixed points has been found, even in quite harsh environments such as aerospace heat treatment processes. Calculations indicate that contamination by transmutation in ionising radiation environments is even less important in most situations, although the extreme case of operation in the core of a nuclear reactor may cause significant drift [5].…”

“…This device has been extensively characterised [6] A similar concept has been employed for an application in space-borne instrumentation, where the phase-change cell is part of the system whose temperature is to be measured. Such an embedded fixed-point has been demonstrated by NPL in collaboration with RAL Space on a prototype blackbody calibrator designed for operation as part of a spacecraft-borne earth observation instrument suite [5]. The phase-change cell, containing approximately 2 g of gallium (melting point 29.7646 °C), is embedded in the aluminium blackbody calibrator base, close to an embedded platinum resistance thermometer (PRT).…”

Towards Digitalization of Temperature Measurements

“…A similar concept has been employed for an application in space-borne instrumentation, where the phase-change cell is part of the system whose temperature is to be measured. Such an embedded fixed-point has been demonstrated by NPL in collaboration with RAL Space on a prototype blackbody calibrator designed for operation as part of a spacecraft-borne earth observation instrument suite [7]. The phase-change cell, containing approximately 2 g of gallium (melting point 29.7646 °C), is embedded in the aluminium blackbody calibrator base, close to an embedded platinum resistance thermometer (PRT).…”