A New Method for the Quantification and Correction of Thermal Effects on the Realization of Fixed Points

Abstract

The temperature and flatness (shape) of a fixed-point plateau depend on both the amount and nature of specific impurities and on thermal effects that are influenced by the fixed-point cell design and furnace properties. A better understanding and experimental proof of the influence of specific impurities on fixed-point realizations require the separation of impurity influences from thermal effects. In this paper the influence of heat exchange between the thermometer and furnace is quantified via a method based on changing the furnace temperature during the fixed-point measurement. It will be shown that the corresponding correction of this thermal effect has a dominant influence on the plateau shape compared to the influence of impurities. This leads to an explanation for why the maximum of an induced freeze is the most reproducible temperature. A secondary outcome is an explanation of why natural freezes have less flat plateaux compared to induced freezes, resulting in fixed-point temperatures that are too low. Furthermore, the suggested procedure is the basis of the direct and quantitative comparison of fixed-point cells and the detection of weak points within a specific design. It allows optimization of fixed-point cells and furnaces, and helps to deepen the common understanding of the phase transition in fixed-point cells.

Triple-Point Temperatures of 20Ne and 22Ne

“…The 1.5 µK uncertainty due to the hydrostatic pressure is determined by the product of half the calculated liquid height within the cell and the temperature variation with depth of the neon triple-point temperature (1.9 mK·m -1 ) [1]. The uncertainty due to stray heat flux is estimated from the sensitivity of the plateau temperature to changes in the control set-point of the cryostat shield near the fixed-point cell [13]. Ne sample.…”

Uncertainties in the SPRT Subranges of ITS-90: Topics for Further Research

“…While a poor fixed-point initiation is evident in a slow recovery from recalescence and from sensitivity to the furnace temperature, there are no guidelines or criteria for the development of initiation procedures. The furnace oscillation test [26] provides a good test of the completeness of the phase boundary, but the test has not yet been applied to the evaluation of different initiation procedures.…”

High-Purity Fixed Points of the ITS-90 with Traceable Analysis of Impurity Contents

“…This requires the lowest possible uncertainties on a level of 0.2 mK or better. It is based on slim fixed-point cells with a sample mass of about 250 g [18].…”

“…This is because of the very low dependence of this maximum temperature on the homogeneity and stability of the furnace temperature. On the other hand, the slopes of freezing and melting curves depend on both the homogeneity of the temperature profile of the furnace and the segregation of impurities [18]. In order to derive complementary information on the segregation of impurities during a fixed-point realization, better understanding and the correction of thermal effects are necessary.…”

“…This shows that the proper determination of the influence of thermal effects on melting by the measurement of immersion characteristics requires detailed investigations and the consideration of transient effects. Recent investigations at PTB have shown that for specific fixed-point cell/furnace combinations, other parts of the freezing curve, and therefore its slope, are considerably influenced by the temperature profile of the furnace [18]. This was demonstrated by using a sequence of stepwise changes of the furnace temperature during a fixedpoint realization.…”