Fixed-point Material Research Articles

Residual resistance ratio as indicator for the influence of impurities on fixed-point temperatures

The component caused by the influence of impurities often dominates the uncertainty budgets of realisations of temperature fixed points. The Consultative Committee for Thermometry recommended, therefore, in section 2.1 Fixed Points: Influence of Impurities of the Guide to the Realization of the ITS-90 (www.bipm.org) two methods for estimating this component. One is based on summing up the individual effects of all relevant impurities. This method causes huge efforts. The second one requires a reliable information on the overall impurity concentration, which is usually not supplied by the producers of the fixed-point materials. In the paper, it is analysed if the determination of the residual resistance ratio r 0 can aid in getting the required information [r 0 means the ratio of the electrical resistivities ρ at a sufficiently low temperature and at 293 K, usually r 0 = ρ(4.2 K)/ρ(293 K)]. The crucial criterion is the sensitivity of r 0 for the influence of an impurity on the fixed-point temperature, i.e. the magnitude of the derivative dT l/dr 0 of the liquidus-line temperature T l with respect to r 0. As examples, the developed methodology is applied to three fixed points of the ITS-90, namely the freezing points of indium, zinc, and aluminium. It can be shown that the use of r 0 can reduce the number of impurities, for which the effortful determination of the individual concentrations is necessary, by an order of magnitude, namely to about 10 or below.

Thermal Finite Element Simulation of Dry Block Calibrator Based on Multi-fixed-Point Cells

On-site thermometer calibration by the temperature scale transfer technique based on fixed points can effectively improve the calibration level of thermometers in large-diameter hot water pipes. Calibrations of contact thermometers in large-diameter hot water pipes are often carried out using dry block calibrators. Axial temperature gradients and the drift of reference thermometers are the main sources of error for the use of the traditional dry block calibrators. To overcome these disadvantages, a new dry block calibrator with small multi-fixed-point cells was designed in this work to improve the homogeneity of the inner temperature field and calibrate the reference thermometers in situ. The calibrator was based on small multi-fixed points including the melting point of Hg (234.28 K), the melting point of Ga (302.91 K), the melting point of H2O (273.15 K) and was developed for use in the temperature range from − 50 °C up to 100 °C. The calibrator was designed using thermal Finite Element simulations carried out with COMSOL Multiphysics and has an ectopic multi-fixed-point structure. The melting process of the multi-fixed points was simulated using the effective heat capacity method, and the fraction of the liquid phase for the fixed-point material was obtained at different phase-transition radius values and temperature plateaus of multiple fixed-points cells were obtained at different dT values. The simulation explored the influence of different heating temperatures on the temperature plateaus of multiple fixed-points cells, and the functional relationship between the heating temperature and the temperature plateaus of multiple fixed-points cells was obtained, calculating the error between temperature plateaus and thermometer at different heating power, providing a technical basis for the subsequent prototype experiments and the validation of the prototype design through experimental measurements in future work.

DSC temperature calibration over the range −125 °C to 50 °C

The lack of Nationally Certified Reference Materials for the calibration of DSC equipment in the sub-ambient range has been addressed by the introduction of a Certified Reference Material of purified cyclohexane by the National Metrology Institute of Japan. This sample, in conjunction with the CRM material phenyl salicylate and the ITS-90 fixed point material mercury, has enable a DSC-thermomicroscopy system to be accurately calibrated over the temperature range −90 °C to 50 °C. The DSC has then been used to measure values for the solid-solid transitions in the potential Certified Reference Materials ammonium sulphate, adamantane and ammonium dihydrogen phosphate, which have the advantage of being thermally stable and easy to handle, and for the melting point of diphenyl ether. In addition, the effect of temperature cycling on these materials has been investigated.

A systematic evaluation of contemporary impurity correction methods in ITS-90 aluminium fixed point cells

The fixed points of the International Temperature Scale of 1990 (ITS-90) are the basis of the calibration of standard platinum resistance thermometers (SPRTs). Impurities in the fixed point material at the level of parts per million can give rise to an elevation or depression of the fixed point temperature of order of millikelvins, which often represents the most significant contribution to the uncertainty of SPRT calibrations. A number of methods for correcting for the effect of impurities have been advocated, but it is becoming increasingly evident that no single method can be used in isolation. In this investigation, a suite of five aluminium fixed point cells (defined ITS-90 freezing temperature 660.323 °C) have been constructed, each cell using metal sourced from a different supplier. The five cells have very different levels and types of impurities. For each cell, chemical assays based on the glow discharge mass spectroscopy (GDMS) technique have been obtained from three separate laboratories. In addition a series of high quality, long duration freezing curves have been obtained for each cell, using three different high quality SPRTs, all measured under nominally identical conditions. The set of GDMS analyses and freezing curves were then used to compare the different proposed impurity correction methods. It was found that the most consistent corrections were obtained with a hybrid correction method based on the sum of individual estimates (SIE) and overall maximum estimate (OME), namely the SIE/Modified-OME method. Also highly consistent was the correction technique based on fitting a Scheil solidification model to the measured freezing curves, provided certain well defined constraints are applied. Importantly, the most consistent methods are those which do not depend significantly on the chemical assay.

Small Multiple Fixed-Point Cell as Calibration Reference for a Dry Block Calibrator

A small multiple fixed-point cell (SMFPC) was designed to be used as in situ calibration reference of the internal temperature sensor of a dry block calibrator, which would allow its traceable calibration to the International Temperature Scale of 1990 (ITS-90) in the operating range of the block calibrator from \(70\,^{\circ }\hbox {C}\) to \(430\,^{\circ }\hbox {C}\). The ITS-90 knows in this temperature range, three fixed-point materials (FPM) indium, tin and zinc, with their respective fixed-point temperatures (\(\vartheta _\mathrm {FP}\)), In (\(\vartheta _\mathrm {FP}\,{=}\,156.5985\,^{\circ }\hbox {C}\)), Sn (\(\vartheta _\mathrm {FP}\,{=}\,231.928\,^{\circ }\hbox {C}\)) and Zn (\(\vartheta _\mathrm {FP}\,{=}\,419.527\,^{\circ }\hbox {C}\)). All of these FPM are contained in the SMFPC in a separate chamber, respectively. This paper shows the result of temperature measurements carried out in the cell within a period of 16 months. The test setup used here has thermal properties similar to the dry block calibrator. The aim was to verify the metrological properties and functionality of the SMFPC for the proposed application.

Impurity Correction Techniques Applied to Existing Doping Measurements of Impurities in Zinc

Impurities represent the most significant source of uncertainty in most metal fixed points used for the realization of the International Temperature Scale of 1990 (ITS-90). There are a number of different methods for quantifying the effect of impurities on the freezing temperature of ITS-90 fixed points, many of which rely on an accurate knowledge of the liquidus slope in the limit of low concentration. A key method of determining the liquidus slope is to measure the freezing temperature of a fixed-point material as it is progressively doped with a known amount of impurity. Recently, a series of measurements of the freezing and melting temperature of ‘slim’ Zn fixed-point cells doped with Ag, Fe, Ni, and Pb were presented. Here, additional measurements of the Zn–X system are presented using Ga as a dopant, and the data (Zn–Ag, Zn–Fe, Zn–Ni, Zn–Pb, and Zn–Ga) have been re-analyzed to demonstrate the use of a fitting method based on Scheil solidification which is applied to both melting and freezing curves. In addition, the utility of the Sum of Individual Estimates method is explored with these systems in the context of a recently enhanced database of liquidus slopes of impurities in Zn in the limit of low concentration.

The Melting Point of Palladium Using Miniature Fixed Points of Different Ceramic Materials: Part II—Analysis of Melting Curves and Long-Term Investigation

Fifteen miniature fixed-point cells made of three different ceramic crucible materials (\(\hbox {Al}_{2}\hbox {O}_{3}\), \(\hbox {ZrO}_{2}\), and \(\hbox {Al}_{2}\hbox {O}_{3}(86\,\%)+\hbox {ZrO}_{2}\)(14 %)) were filled with pure palladium and used to calibrate type B thermocouples (Pt30 %Rh/Pt6 %Rh). A critical point by using miniature fixed points with small amounts of fixed-point material is the analysis of the melting curves, which are characterized by significant slopes during the melting process compared to flat melting plateaus obtainable using conventional fixed-point cells. The method of the extrapolated starting point temperature using straight line approximation of the melting plateau was applied to analyze the melting curves. This method allowed an unambiguous determination of an electromotive force (emf) assignable as melting temperature. The strict consideration of two constraints resulted in a unique, repeatable and objective method to determine the emf at the melting temperature within an uncertainty of about \(0.1\,\upmu \hbox {V}\). The lifetime and long-term stability of the miniature fixed points was investigated by performing more than 100 melt/freeze cycles for each crucible of the different ceramic materials. No failure of the crucibles occurred indicating an excellent mechanical stability of the investigated miniature cells. The consequent limitation of heating rates to values below \({\pm }3.5\,\hbox {K}\,\hbox {min}^{-1}\) above \(1100\,{^{\circ }}\hbox {C}\) and the carefully and completely filled crucibles (the liquid palladium occupies the whole volume of the crucible) are the reasons for successfully preventing the crucibles from breaking. The thermal stability of the melting temperature of palladium was excellent when using the crucibles made of \(\hbox {Al}_{2}\hbox {O}_{3}(86\,\%)+\hbox {ZrO}_{2}(14\,\%)\) and \(\hbox {ZrO}_{2}\). Emf drifts over the total duration of the long-term investigation were below a temperature equivalent of about 0.1 K–0.2 K.

The triple point of sulfur hexafluoride

A cryogenic fixed point cell has been filled with high purity (99.999%) sulfur hexafluoride (SF6) and measured in an adiabatic closed-cycle cryostat system. Temperature measurements of the SF6 melting curve were performed using a capsule-type standard platinum resistance thermometer (CSPRT) calibrated over the International Temperature Scale of 1990 (ITS-90) subrange from the triple point of equilibrium hydrogen to the triple point of water. The measured temperatures were corrected by 0.37 mK for the effects of thermometer self-heating, and the liquidus-point temperature estimated by extrapolation to melted fraction F = 1 of a simple linear regression versus melted fraction F in the range F = 0.53 to 0.84. Based on this measurement, the temperature of the triple point of sulfur hexafluoride is shown to be 223.555 23(49) K (k = 1) on the ITS-90. This value is in excellent agreement with the best prior measurements reported in the literature, but with considerably smaller uncertainty. An analysis of the detailed uncertainty budget of this measurement suggests that if the triple point of sulfur hexafluoride were to be included as a defining fixed point of the next revision of the International Temperature Scale, it could do so with a total realization uncertainty of approximately 0.43 mK, slightly larger than the realization uncertainties of the defining fixed points of the ITS-90. Since the combined standard uncertainty of this SF6 triple point temperature determination is dominated by chemical impurity effects, further research exploring gas purification techniques and the influence of specific impurity species on the SF6 triple point temperature may bring the realization uncertainty of SF6 as a fixed point material into the range of the defining fixed points of the ITS-90.

Investigation of self-validating thermocouples with integrated fixed-point units

Thermocouples are often exposed to harsh conditions when used for high-temperature measurements in industry. They suffer commonly from unavoidable drift effects which influence the required process efficiency and control. A self-validation concept for thermocouples to monitor their performance in the temperature range between 1000 C and about 1800 C in oxidizing atmospheres by using integrated miniature fixed-point units of different designs was tested. Two different models of fixed-point crucibles filled with high purity palladium (1553.4 °C) and platinum (1769 °C) have been constructed and assembled with type B thermocouples to be used as traceable references. Furthermore, two innovative self-validation methods by using thick wires of high purity gold (1064.18 °C), nickel (1455 °C), and palladium in multi-bore insulators as fixed-point materials were developed and investigated by assembling them with type B thermocouples. The measurement results obtained have demonstrated the suitability of the integrated fixed-point units to provide long-term confidence in industrial high-temperature measurements within about (2−3) K.

Miniature Fixed-Point Cell Approaches for $${{\varvec{In\,Situ}}}$$ I n S i t u Monitoring of Thermocouple Stability

In the framework of the European Metrology Research Project ENG08 “MetroFission,” LNE-Cnam and NPL have undertaken cooperative research into the development of temperature measurement solutions for the next generation of nuclear fission power plants. Currently, in-pile temperature monitoring is usually performed with nickel-based (Type K or N) thermocouples. When these thermocouples are exposed to a neutron flux, the thermoelements transmute, leading to large and unknown drifts in output. In addition, it is impossible to routinely recalibrate the thermocouples after irradiation for obvious reasons of safety. To alleviate this problem, both LNE-Cnam and NPL have developed, via differing approaches, in situ calibration methods for the thermocouples. The self-validating thermocouple methodologies are based on the principle of a miniature fixed-point cell to be co-located with the thermocouple measurement junction in use. The drift of the thermocouple can be monitored and corrected for by regular determination of the output at the phase transition of the fixed-point material: in effect performing regular in situ calibration checks. The two institutes have constructed miniature fixed-point cells for use at three different temperatures; the freezing point of silver $$(961.78\,^{\circ }\mathrm{C}$$ ; LNE-Cnam), the freezing point of copper $$(1084.62\,^{\circ }\mathrm{C}$$ ; LNE-Cnam and NPL), and the melting point of Fe–C ( $${\sim }1154\,^{\circ }\mathrm{C}$$ ; NPL). This paper introduces the construction and validation of the miniature fixed-point cells prior to use, to ensure traceability to the ITS-90. A comparison of the performance of the two cell designs is discussed, where typical industrial Type N thermocouples have been used for measurement of the fixed-point cells. Such initial measurements demonstrate the feasibility of each of these two approaches.

Influence of impurities on the fixed-point temperature of zinc: estimations by the SIE method and practical limitations

This publication deals with the practical challenge of describing the impurity influence on the fixed-point temperature of zinc. For this, the sum of individual estimate (SIE) approach is applied to miniaturized fixed-point cells (MFPC) filled with high-purity zinc that can be used in industrial applications. This includes comparative analyses by glow discharge mass spectroscopy as well as mass spectroscopy with inductive coupled plasma to quantify the impurity concentrations in zinc. Furthermore, the element-specific and concentration-dependent temperature deviations are presented for the fixed-point material zinc. For this, binary phase diagrams as well as thermal calculations and experimental data were analysed to extract the relevant sensitivity coefficients. Besides, results from SIE analyses of MFPCs are presented and their uncertainties are compared. On this basis, practical limits of the SIE method are identified and discussed.

Fixed-Point Comparison Uncertainties for Two Cell Geometries

To realize the ITS-90 according to its definition, among others, the melting and freezing temperatures of ideally pure metals are needed. Therefore, many national metrology institutes (NMIs) utilize a group of cells instead of one single cell as the national reference for each temperature. With direct fixed-point cell comparisons on a regular basis, it is feasible to account for the small differences between the individual fixed-point temperatures and to detect possible temperature drifts of the cells. At PTB (the German NMI), in recent years, these groups of national standard cells and the so-called transfer cells for calibrations have been complemented by newly developed slim fixed points. These cells typically contain 75% to 80% less fixed-point material compared with standard cells. Slim cells are used for homogeneity investigations of large batches of fixed-point material, for doping experiments to determine the influence of very small amounts of impurities on the fixed-point temperature with very small uncertainties, and for the investigation of contamination or purification effects after the manufacture of a fixed-point cell. These investigations have shown that the main limitation of slim cells is the quality of the phase boundary. The small dimensions of the cell do not allow the formation of a closed phase boundary (or even two of them). However, this can be compensated using a quasi-adiabatic realization procedure, and in this way, uncertainties comparable to those of standard fixed-point cells can be achieved. In this article, the design of the cells as well as typical measurement results and uncertainties for the direct comparison of fixed-point cells of both types, the standard size and slim design, are presented.

A Solidification Approach to Correcting for the Effect of Impurities in Fixed Points

Uncertainty estimation for the effect of impurities on fixed points requires an accurate assay of the fixed-point material. To apply a correction also needs a knowledge of the liquidus slope of the solute binary system. Two methods are presented here for giving a realistic uncertainty and potential for correction without detailed knowledge of the impurities present. Both methods are based on the Scheil equation. The first method uses the gradient at about 50% solid; and where some limited knowledge of impurities is available, this method can, in cases where the bulk of impurities segregate preferentially into the liquid phase, be used to apply a correction. To apply an uncertainty simply knowing typical impurities for a particular metal may be considered sufficient. The second method involves a best fit for the four variables in the Scheil equation. It is shown that this second method can work even where multiple impurities are present, but that when applied to real data, problems arise due to deviations from Scheil behavior. This deviation is thought to be due to difficulties in maintaining a uniform solid/liquid interface at the end of a freeze.

Estimation of fixed-point temperatures – A practical approach

Phase transition temperatures can be used for thermometer calibrations in accordance with the ITS-90. For this the thermometer is inserted into a fixed-point cell filled with a highly pure substance. While the substance is melting or freezing the temperature inside the cell remains almost constant and the thermometer measures a plateau in the temperature curve. From this plateau the phase transition temperature needs to be estimated for the calibration of the thermometer. This can be done using different mathematical methods, taking into account various systematic deviations as well as reproducibilities of the results depending on the chosen method, the fixed-point material’s purity and above all the amount of fixed-point material. Hence this article presents results from measurements in miniature fixed-point cells filled with zinc of various purities. The plateau curves were measured at different heating rates and comparatively analysed using five different estimation methods.

Oxides in metal fixed points of the ITS-90

In the range between 0 °C and 961 °C, the International Temperature Scale of 1990 (ITS-90) depends to a great extent on the freezing points of the pure metals gallium, indium, tin, zinc, aluminium and silver. An up-to-date realization of these fixed points is based on cells containing metals of ultra-high purity (6N or better) and should include a correction for the influence of relevant impurities. Still, chemical analyses of the fixed-point material can show large amounts of oxygen, which had to be neglected so far, because of the lack of detailed knowledge about it, presuming it could be removed from the cell by applying a vacuum (less than 1 Pa) for a few hours.In this paper we discuss an equilibrium of several forms of oxygen in a fixed-point cell, gaseous in the cell's atmosphere, dissolved in the fixed-point metal and as oxide in a separate (solid) phase. We will conclude that in many fixed points most of the oxygen is not dissolved in the metal, but bound in oxides of the fixed-point metal as well as oxides of some impurities. To demonstrate the impact that the precipitation of impurity oxides has on thermometry, two indium fixed-point cells were doped with magnesium and chromium, which leave the fixed-point temperature unchanged. Further evidence is drawn from earlier work. All these results support the presumed existence of (at least one) persistent separate oxide phase in the fixed points of indium, tin, zinc and aluminium, which renders them eutectic or peritectic points and is a more likely reason why the oxygen content of a cell does not influence the fixed-point temperature.To complement these studies, thermodynamic calculations show how to treat the equilibrium in the cell quantitatively. Using available chemical data, a list is provided that indicates for each fixed-point metal (including the other metal fixed points of the ITS-90: mercury, gold, copper) the impurities that probably build oxides. Due to the agreement of the calculated values with the presented experimental results, we suggest excluding those impurities from the correction of a fixed-point temperature (e.g. the SIE method), unless there is strong evidence of their dissolution.

Triple-Fixed-Point Blackbody for the Calibration of Radiation Thermometers

A new triple-fixed-point blackbody containing the fixed-point materials aluminum (freezing point 660.323°C), zinc (FP 419.527°C), and tin (FP 231.928°C) in one device has been developed at the Ilmenau University of Technology. It enables calibration of a radiation thermometer with direct reference to the ITS-90 at three fixed points after a single adjustment of the calibration object. The setup significantly reduces the technical effort and the time for the calibration procedure. Measurements of the phase-transition temperature and the time-dependent blackbody temperature made with a transfer radiation thermometer, the Linearpyrometer LP5 of the IKE Stuttgart, are presented in the article.

Estimating the influence of impurities on the freezing point of tin

The sum of individual estimates (SIE) and the overall maximum estimate (OME) are two methods recommended to estimate the influence of impurities on the temperatures of the liquid–solid phase transformations of high-purity substances. The methods are discussed starting with the basic crystallographic facts, and their application is demonstrated in detail by applying them to the freezing point of tin as a first example. The SIE method leads to a temperature correction with a corresponding uncertainty while the OME method yields only an uncertainty that is, perhaps not unexpectedly, larger than that of the SIE approach. The necessary sensitivity coefficients (derivatives of the liquidus lines) are tabulated, together with the equilibrium distribution coefficients. Other than the necessity of obtaining a complete elemental analysis of the fixed-point material using glow discharge mass spectrometry (or other suitable techniques), there remain no technical barriers to adopting the preferred SIE method. While the use of the method, and particularly the application of a temperature correction to account for the impurity influence, requires a paradigm shift within the thermometry community, improved interoperability and harmonization of approach are highly desirable goals. The SIE approach maximizes the application of scientific knowledge and represents the best chance of achieving these common goals.

Automated system for measuring temperature profiles inside ITS-90 fixed-point cells

The defining fixed points of the International Temperature Scale of 1990 (ITS-90) are temperature reference points for temperature calibration. The measured temperature inside the fixed-point cell depends on thermometer immersion, since measurements are made below the surface of the fixed-point material and the additional effect of the hydrostatic pressure has to be taken into account. Also, the heat flux along the thermometer stem can affect the measured temperature. The paper presents a system that enables accurate and reproducible immersion profile measurements for evaluation of measurement sensitivity and adequacy of thermometer immersion. It makes immersion profile measurements possible, where a great number of repetitions and long measurement periods are required, and reduces the workload on the user for performing such measurements. The system is flexible and portable and was developed for application to existing equipment in the laboratory. Results of immersion profile measurements in a triple point of water fixed-point cell are presented.

High-accuracy radiation thermometry at the National Metrology Institute of Germany, the PTB

Among the great variety of temperature-measurement techniques, radiation thermometry has a unique potential. As an optical contactless method, it allows remote sensing of the object temperature without influencing it, measurements of rapidly moving objects, and measurements of very high temperatures. Its high flexibility, its great accuracy potential, and the possibility of accurately tracing the measurement back to the International Temperature Scale (ITS) have made radiation thermometry a method widely used in science, industry, medicine, and everyday life. Traceability of temperature measurements is of increasing importance, as further globalisation will require comparability of temperature measurement world-wide. This can be ensured only if the measurement can be traced back to the ITS, which means that the result of a measurement can be related to stated references of the ITS through an unbroken chain of comparisons, all of them with stated uncertainties. The Physikalisch-Technische Bundesanstalt (PTB), the National Metrology Institute of Germany, maintains, provides, and further develops a wide range of instrumentation for radiation-temperature measurements, allowing temperature measurements to be carried out from -60°C to 3000 °C which are directly traceable to the ITS. After an introduction to the International Temperature Scale of 1990 (ITS-90) and a brief outline of PTB's history of thermal-radiation measurements, this article describes the present state of the art of high-accuracy radiation thermometry reached and further developed at PTB. The article deals with different temperature ranges requiring different measurement and traceability techniques. In modern radiation thermometry, heat-pipe blackbodies combined with platinum resistance thermometers and radiation thermometers equipped with photodetectors for infrared radiation are applied in the temperature range from -60°C to 961.78°C (the freezing temperature of silver). Within the scope of the European research project TRIRAT (traceability in infrared-radiation thermometry), significant progress could be achieved in traceable low-temperature radiation thermometry. This progress and recent developments aiming to extend the measurements to temperatures below -60 °C are summarised. Novel developments in high-temperature radiation thermometry from 962°C to 3000°C. which are required by the semiconductor, steel, and glass industry in particular, are dealt with in a separate section. The calibration capabilities of the PTB in this temperature range are described, and the European project HIMERT (novel high-temperature metal-carbon eutectic fixed points for radiation thermometry, radiometry, and thermocouples) is presented. In this project, considerable effort is made on the international level to further improve the high-temperature scale by developing and characterising advanced fixed-point materials. Independently of radiation-thermometric methods, which need defined temperature fixed points, the PTB performs direct measurements of the thermodynamic temperature by applying radiation detectors calibrated in absolute terms for their spectral responsivity. This radiometric method is used to further improve the ITS and might contribute to a future re-definition of the temperature unit kelvin. It is described in the last section of this review and recent results regarding the agreement between thermodynamic temperature and the ITS are given.

Radiometric observation of melting and freezing plateaus for a series of metal-carbon eutectic points in the range 1330 °C to 1950 °C

Melting and freezing plateaus were radiometrically observed for the metal-carbon eutectics Ni-C, Pd-C, Pt-C, and Ru-C, using graphite crucibles and black-body cavities. The graphite crucibles were able to withstand heat cycles without breaking. Since the crucible material is a component of the eutectic fixed-point material, the latter is inherently free of contamination from the crucible. The temperature differences between the melting and freezing plateaus were generally less than 0.1 °C. The method seems to be a promising way of realizing a series of practical reference points above the copper point.