Realizations of the Triple Point of Sulfur Hexafluoride in Transportable and Refillable Cells

Abstract

The Minamata Convention on Mercury has created a near-term need to develop alternative fixed points to replace the mercury triple point (Hg TP) for calibration of standard platinum resistance thermometers (SPRTs) on the International Temperature Scale of 1990 (ITS-90). The sulfur hexafluoride (SF6) TP is a good candidate to provide adequate “drop-in compatible” replacements for the lowest costs. We report our first results of SF6 TP realizations performed at the National Institute of Standards and Technology (NIST) using a new series of transportable and refillable triple-point cells. The melting curves are presented at various melted fractions F and compared to evaluate the reproducibility and overall uncertainty for the realizations. We obtained a TP temperature of 223.55587(33) K at F = 50 % and 223.55607(35) K at F = 100 % as a weighted average of realizations using two adiabatic-type cells and two immersion-type cells. (Unless otherwise stated, uncertainties are standard uncertainties corresponding to a 68 % confidence level.) Temperatures were derived using a combination of five different SPRTs as calibrated at NIST on the ITS-90. The data were evaluated over a region of the melting plateau for melted fraction F between 30 % ≤ F ≤ 80 % with a 0.2 mK wide melting range. The results from the immersion-type cells were used to derive an experimental value for the SF6 TP static head correction of −11.6(1.7) mK/m. This value implies an initial slope of the pressure-temperature (p-T) equilibrium melting line of 1.55 MPa/K, whichis in agreement with the value predicted via the Clapeyron equation. The uncertainties of these initial SF6 TP realizations are limitedby uncertainty in the realization of the ITS-90 (0.25 mK) and, to a lesser extent, static pressure head effects and chemical impurities.

On the Melting Curve of Sulfur Hexafluoride

“…Only recently has SF 6 of high purity become available; this is evident from older determinations of the triple-point temperature, where all data prior to 1993 were about 1 K too low. 3 This matches the approximately 0.9 K difference between the true triple point and that given by Vacek and Zollweg. Since the amount by which impurities depress the freezing point should not vary strongly with pressure, this suggests that all the freezing temperatures of Vacek and Zollweg are too low by roughly 0.9 K.…”

“…As a boundary for the validity of their EOS, Guder and Wagner gave an equation for the melting pressure, p m , as a function of temperature T. The melting curve p m (T) (in particular its initial slope at the triple-point temperature T t ) is important in metrology because of recent work toward developing the triple point of SF 6 (at approximately 223.555 K) as a non-toxic alternative to the triple point of mercury (234.3156 K) for realization of the temperature scale. 2,3 In the course of the work reported in Ref. 3, it was realized that the slope dp m /dT given by the Guder-Wagner correlation at the triple point was significantly larger than that given by a thermodynamic estimate based on the Clapeyron equation.…”

“…2,3 In the course of the work reported in Ref. 3, it was realized that the slope dp m /dT given by the Guder-Wagner correlation at the triple point was significantly larger than that given by a thermodynamic estimate based on the Clapeyron equation. In the following, we derive the Clapeyron slope from thermodynamic data, diagnose the probable reason for the inaccurate initial slope of the correlation of Guder and Wagner, and present a new correlation for p m (T) that is consistent with the Clapeyron slope.…”

Innovation Management Responses to Regulation—sup-Directive and Replacing Plastic

“…When products have to change, but user and consumer needs remain unchanged or are only marginally modified, new processes, products, and materials need to be actively developed. This replacement effect and market emergence occurred in the case of mercury, for example, when several new products were rapidly created to replace mercury after the Minamata convention (Born, 2001;Gnybida et al, 2014;Tew & Quelhas, 2018). Furthermore, it is possible that completely new needs, such as the need to reuse materials that have previously treated as waste only, emerge from changes in regulation (see Constantini & Crespi, 2013).…”

Comparison of xenon triple point realizations

“…Therefore, both types of non-uniqueness are expected to benefit from the use of an alternative fixed point situated about halfway between ArTP and WTP. The trend in recent years toward increasingly restrictive regulations around the transportation and use of Mercury has also spurred a search for suitable alternatives to the MTP using more benign materials [see, for example, [8][9][10]. Unfortunately, neither SF 6 nor CO 2 resolve the problem related to the closeness to the WTP.…”