Correlation Between Immersion Profile and Measured Value of Fixed-Point Temperature

Abstract

Assessment of thermal immersion effects in the melting and freezing points defined by the International Temperature Scale of 1990 is one of the vital issues of modern thermometry. In documents of the Consultative Committee for Thermometry, the deviation of the experimental immersion profile from the theoretical value of the hydrostatic effect at a height of about 3 cm to 5 cm from the thermometer well bottom is used for the estimation of the uncertainty due to unwanted thermal effects. This estimation assumes the occurrence of solely the hydrostatic effect all along the height of the well inner wall. Real distortions of the temperature gradient at the bottom and at the top part of the well caused by the change of heat-exchange conditions are not taken into account. To define more precisely the temperature gradient along the height of the well, a miniature PRT with a 30 mm sensitive element and a sheath length and diameter of about 60 mm and 6 mm, respectively, were used. Also, the measurements of fixed-points temperature at noticeably different slopes of immersion profiles due to variations of the thermometer heat exchange and phase transition realization conditions were produced by means of a standard platinum resistance thermometer (SPRT). The measurements were carried out at the tin and zinc freezing points. The immersion curves measured with a miniature thermometer demonstrated an increase of the temperature during its lifting in the first 1 cm to 3 cm above the bottom of the well. The measurement results at the zinc freezing point by means of the SPRT have not confirmed the correlation between the immersion curves, the received value of the Zn freezing temperature, and the estimation of its uncertainty.

A numerical and experimental investigation of the heat losses in thermometric fixed-point cells

“…In both cases the temperature rises in the first 10 mm. Such behavior was already observed by Rudtsch et al [8] and Shulgat et al [34]. With the use of a non-sandblasted well, this temperature shift in the first 10 mm is about four times larger.…”

Immersion effects in zinc ITS-90 fixed-point cells

“…This makes a thermal connection through solid metal from the thermometer to the outer surface of the crucible, and thermal exchange between the sensor and this cool surface gives rise to the breakoff from the ideal immersion profile at the deepest immersions. This thermal 'bridge' effect is also given as the reason for abnormal temperature variations near the re-entrant well bottom in the immersion profile measurements of Shulgat et al [25] and White and Mason [26]. Dendrites spanning the width of the cell near the base were also observed by McAllan in tin [27] and by Ivanova et al in indium [28].…”

Understanding immersion in zinc fixed-point cells using factorial design of experiments

“…Some authors have hypothesised that a cause may be related to the conditions within the cell, due to, for example, 'bridges' of solid in the ingot due to sub-optimal initiation of the freeze and/or thermal conditions [9][10][11][12] or the appearance of macroscopic convex solid formations [13]. Local variations in temperature distribution arising from the immersion test itself were also investigated [14].…”