Establishment of NIMT Zinc Fixed-Point Cell

One of the research programs for the Thermometry Metrology Department at the National Institute of Metrology (Thailand), NIMT, is establishment of its own fixed-point cells. Among the fixed-point cells adopted for the realization of the International Temperature Scale of 1990 (ITS-90), NIMT has chosen the zinc fixed point to start the program. The fabrication and the initial evaluation of the zinc fixed-point cell were conducted at the National Metrology Institute of Japan, NMIJ. The cell fabrication was following the design and procedures developed by the NMIJ. In the cell fabrication, a 6N nominal purity zinc metal cylinder ingot was used. The metal ingot was collected in a graphite crucible under an argon gas atmosphere. The new fixed-point cell was compared with the old fixed-point cells already owned by NIMT, namely, one open-type cell and one sealed-type cell by direct cell comparisons. Since the ingot was equipped with a detail impurity element analysis, it is possible to calculate the effect coming from the existence of the impurities based on, for example, the sum of individual estimates (SIE) method. This effect can then be used to correct for the influence impurities on the realization of the temperature fixed point.     

Comparison of Re�CC Fixed-Point Cells and Their T90 Temperatures Between NMIJ and VNIIOFI

The comparison of Re?CC fixed-point cells built at NMIJ and VNIIOFI was carried out. Both cells were built using 5N purity rhenium of the same supplier, and the crucibles had similar dimensions. The cells were heated in the same furnace and measured on consecutive days using a radiation thermometer. To eliminate the effect of possible instability of the thermometer, a third Re?CC cell was used as a reference: it was placed in another furnace and was measured each day just after the cell to be compared. The melting-temperature difference of the NMIJ cell to the VNIIOFI cell was 0.045 °C with a standard uncertainty of 0.033 °C. In the course of this comparison, a comparison of the temperature scales between the NMIJ and the VNIIOFI at the Re?CC point (2475 °C) was performed using the NMIJ Re?CC cells as transfer standards. The difference between the NMIJ and the VNIIOFI scales was found to be ?0.8 °C, which was within the combined uncertainty of 1.4 °C (k = 2).     

Procedure for the Impurity-Related Correction at the Indium Fixed-Point

Determining the influence of impurities on the fixed-point temperatures of the ITS-90 requires the completion of several tasks. In this paper, the progress made at Physikalisch-Technische Bundesanstalt (PTB) and BAM Federal Institute for Materials Research and Testing is presented and remaining questions are discussed. The projected characterization procedure at PTB, which is based on the established SIE method (sum of the individual estimates), using a new indium fixed-point cell is described as an example. This procedure includes an SI-traceable chemical analysis of the material in the fixed-point cell with sufficiently low uncertainties, the individual experimental determination of the influence of the quantified impurities on the fixed-point temperature, and the establishment of direct links to the phase-transition temperatures of the national standard and of an assumed material of ideal purity. A characteristic difference to the common practice is the chemical analysis of the fixed-point metal being done after determining the cell??s freezing temperature. This allows for the detection and consideration of contamination and purification effects due to the filling process, or due to the contact with the carbon crucible and other parts of the fixed-point cell. A chemical analysis of an indium fixed-point was carried out by BAM with relative measurement uncertainties below 30 % which have not been previously achieved. The results provide evidence for the precipitation of some impurities, which is apparently inconsistent with the corresponding binary phase diagrams, but was explained in a recent publication. Implications for the use of the SIE method shall be described briefly at the end.     

Investigation of Furnace Uniformity and its Effect on High-Temperature Fixed-Point Performance

A large-area furnace BB3500YY was designed and built at the VNIIOFI as a furnace for high-temperature metal (carbide)–carbon (M(C)–C) eutectic fixed points and was then investigated at the NMIJ. The dependence of the temperature uniformity of the furnace on various heater and cell holder arrangements was investigated. After making some improvements, the temperature of the central part of the furnace was uniform to within 2K over a length of 40 mm—the length of the fixed-point cell—at a temperature of 2,500°C. With this furnace, the melting plateaux of Re–C and TiC–C were shown to be better than those observed in other furnaces. For instance, a Re–C cell showed melting plateaux with a 0.1K melting range and a duration of about 40 min. Furthermore, to verify the capability of the furnace to fill cells, one Re–C and one TiC–C cell were made using the BB3500YY. The cells were then compared to a Re–C cell made in a Nagano furnace and a TiC–C cell filled in a BB3200pg furnace. The agreement in plateau shapes demonstrates the capability of the BB3500YY furnace to also function as a filling furnace. B. Khlevnoy and K. Anhalt are guest scientists at NMIJ at the time of investigation.     

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

The International Temperature Scale of 1990 (ITS-90) is based on thermodynamic equilibrium states of ideally pure substances. The largest contribution to the uncertainty budgets of most metallic fixed points is the influence of impurities on the fixed-point temperature. Therefore, a traceable chemical analysis of the remaining impurities with small uncertainty is the basis of further progress. Further requirements are better knowledge of the phase diagrams at very low impurity contents, impurity segregation, and the quantification and correction of thermal effects during a fixed-point realization. In this article, current and future activities at PTB and BAM in order to develop improved metallic fixed-point cells of the ITS-90 are reviewed.     

Capture of Superfluid Helium by Porous Structures

We have studied superfluid helium capture in a sample of silica aerogel of 98.2% porosity in the temperature range from 1.22 K up to 1.89 K. The high retention of He in the aerogel sample corresponds to a similar phenomenon in impurity-helium condensates, in which very high values of the ratio of helium atoms to impurity atoms (up to 60) have been seen. We have observed that removing the aerogel sample from superfluid helium in a cylindrical glass beaker caused a decrease of the helium level corresponding to the geometrical volume of the sample (≈1 cm³). This observation has allowed us to conclude that superfluid helium is completely captured by the porous sample. Superfluid helium filling aerogel and impurity-helium samples (porous structures) serves as a dispersive medium of gel-like samples which interacts strongly with impurity nanoclusters forming the dispersing system.      

Thermal Analysis of the Heater-Induced Realization of the Tin Fixed Point

In this article, work concerning the thermal analysis of the tin fixed-point is reported. First, the development of a new fixed-point furnace is described. Improvements in the design of the furnace and in the control system enable measurement of the heater power during the phase change. The furnace is sufficiently thermally insulated to produce excellent uniformity and stability, leading to high quality freeze-initiation and minimal thermal influences on the freezing point. By employing the improved furnace and newly-fabricated tin fixed-point cells, the start and end of the melting plateau and the end of the freezing plateau were accurately determined, enabling reliable evaluation of the liquid fraction during the realization of the tin fixed-point compared to conventional methods. Two open-type tin fixed-point cells were fabricated using high-purity tin that was chemically analyzed for impurity content. Thermal analysis results of freezing-point depression are compared to those based on the chemical analysis.     

New Method of Filling of High-Temperature Fixed-Point Cells Based on Metal-Carbon Eutectics/Peritectics

A new method of filling of high-temperature fixed-point cells based on metal-carbon eutectics and peritectics is suggested and tested. In this method a metal and carbon powder mixture is introduced not directly into the crucible, but into an additional container located just above the crucible. The mixture melts inside the container, and the already molten eutectic drops through a small hole in the bottom of the container and fills the crucible drop by drop. The method can be used to obtain a uniform ingot without porous or foundry cavities, to minimize the risk of contamination, and to avoid some other disadvantages. The method was applied to fabricate Re-C and WC-C cells using 5N purity materials. The cells demonstrated a good plateau shape with melting ranges of 0.2 K and 80 mK for Re-C and WC-C, respectively. The Re-C cell was compared with a cell built at NMIJ and showed good agreement with a difference of melting temperatures of only 45 mK.     

Study on the Realization of Zinc Point and the Zinc-Point Cell Comparison

Continuing our study on aluminum, tin, and silver points, a study on the realization of the zinc point was conducted. Zinc-point cells were newly fabricated using 6N-nominal grade zinc samples, impurity elements of which were analyzed extensively based on glow-discharge mass spectrometry (GDMS). The present paper reports the temperature measurements done using the newly fabricated cells during the zinc freezing process, under which the zinc fixed point is defined, and the analysis of the freezing curve obtained. Comparisons of zinc-point temperatures realized by the newly fabricated cells (cell-to-cell comparisons) were also conducted. Zinc-point depression due to impurity elements was calculated based on the sum of individual estimates and the impurity element analysis. One of the cells evaluated was drawn out from its crucible and analyzed by GDMS at four points, namely, at around the center of the top, of the middle, of the bottom, and around the outer part of the middle area. The purpose of this cell disassembly is to see whether or not there has been some difference before and after cell fabrication, as well as difference in impurity element distribution within the ingot. From the aforementioned studies, some findings were obtained. First finding is that the homogeneity of the zinc ingot was within 30%, except for Pb, which was more concentrated in the center part. Second finding is that the cell-to-cell temperature difference changes along with the progressing solidification process. As a consequence, for an accurate cell-to-cell comparison, the locus in the freezing plateau where the comparison is done should be determined. Third finding is that the slope analysis estimates accurately the cell-to-cell comparison, and is consistent with the impurity analysis. This shows that the slope analysis gives extensive information about the effect of impurity to the zinc-point realization, especially after the cell fabrication.     

Construction of Gallium Point at NMIJ

Two open-type gallium point cells were fabricated using ingots whose nominal purities are 7N. Measurement systems for the realization of the melting point of gallium using these cells were built. The melting point of gallium is repeatedly realized by means of the measurement systems for evaluating the repeatability. Measurements for evaluating the effect of hydrostatic pressure coming from the molten gallium existing during the melting process and the effect of gas pressure that fills the cell were also performed. Direct cell comparisons between those cells were conducted. This comparison was aimed to evaluate the consistency of each cell, especially related to the nominal purity. Direct cell comparison between the open-type and the sealed-type gallium point cell was also conducted. Chemical analysis was conducted using samples extracted from ingots used in both the newly built open-type gallium point cells, from which the effect of impurities in the ingot was evaluated.     

Design and Investigation of Pd–C Eutectic Fixed-Point Cells for Thermocouple Calibration at NMIA

The calibration of Pt/Rh thermocouples up to 1560 \(^{\circ }\hbox {C}\) at NMIA currently uses the conventional ‘melt-wire technique’ to realize Gold (Au) and Palladium (Pd) melting points, resulting in the loss of 20 mm of wire from the junction end for each calibration. To avoid this loss, NMIA intends to replace the melt-wire technique with the use of miniature fixed-point cells. NMIA has established Copper (Cu) and Cobalt–Carbon (Co–C) eutectic cells for calibration of thermocouples to 1324 \(^{\circ }\hbox {C}\). To extend the calibration up to 1500 \(^{\circ }\hbox {C}\), miniature Palladium–Carbon (Pd–C) eutectic cells (1492 \(^{\circ }\hbox {C}\)) have been constructed and tested in collaboration with NMIJ, AIST. Although these cells are made of high-purity reference materials, careful consideration must be given to contamination introduced during the manufacture and filling of the crucibles and by their long-term use. These issues can only be assessed by measurement of cell-to-cell temperature differences within the ensemble of cells traceable to ITS-90. In the work presented here, 3 NMIA-design mini Pd–C cells were constructed: 1 at NMIA and 2 at NMIJ. These cells were compared, together with a “large” NMIJ Pd–C cell, using type-R, type-B and Pt/Pd thermocouples and radiation thermometry. Although the cells are found to be stable and repeatable, significant problems arising from migration of Pd to the thermocouples were identified.     

Study on the Impurity Effect in the Realization of Silver Fixed Point

The application of a thermal analysis model to estimate the temperature depression from the ideal fixed-point temperature is important, especially when the chemical analysis of the sample in a cell is insufficient or the cell might have been contaminated during fabrication. This study extends previous work, on thermal analysis with the tin point, to an investigation of the impurity dependence of the silver-point temperature. Close agreement was found between the temperature depression (\(-0.36\) mK) inferred from the thermal analysis of the measured fixed-point plateau and the temperature depression (\(-0.32\) mK) inferred using the sum of individual estimates (SIE) method with an impurity analysis based on glow discharge mass spectrometry. Additionally, the results of the thermal analysis manifest no significant dependence on the rate of solidification, and the scatter of observed gradients was within 0.36 mK among five plateaux with different temperature settings of the furnace. Although the results support the application of both the SIE method and thermal analysis for the silver point, further experiments with cell-to-cell comparisons linked to thermal analysis, a study of the thermometer-furnace systematic effects, the oxygen effect, and the locus of the freezing plateau should be investigated to reach a firm conclusion.     

Effect of Ultra-Trace Impurities on the Freezing Point of Zinc

Impurities are the most significant source of uncertainty in most metal fixed points for the realization of the International Temperature Scale of 1990 (ITS-90). The methods for the estimation of uncertainties and corrections of fixed-point temperatures attributable to the influence of chemical impurities were summarized in 2005, and the sum of individual estimates (SIE) method was recommended to be used with the known concentration and liquidus slope of each impurity. This method requires the concentrations and the liquidus slopes of all impurities. For applying the SIE method, efforts still need to be made to solve a series of problems including the unsatisfactory chemical analysis, inadequate data of the liquidus slopes, and information about the dissolution and precipitation of impurities during the filling and the operation of a fixed-point cell. In the present work at the National Institute of Metrology (NIM), great attention is paid to the effect of ultra-trace impurities on the freezing point of zinc. Five slim graphite crucibles were filled with the same batch of zinc with a nominal purity of 6 N for this research. One of them was used to investigate the concentration and distribution of the impurities in the freezing point of zinc by chemical analysis. The remaining crucibles were used to carry out the ultra-trace impurity doping experiments. The liquidus slopes of Ag–Zn, Pb–Zn, Fe–Zn, and Ni–Zn were measured. All results are reported and discussed.     

The sensitivity of thermomechanical reading of data

The size of elements of modern thermomechanical data storage devices approaches the length of molecular free path for gas at room temperature and atmospheric pressure. In such devices, the heat transfer by gas molecules is the main factor defining the sensitivity of thermomechanical reading of data. This paper deals with heat transfer in devices with hardly any collisions between gas molecules. The developed mathematical model and numerical codes are used to estimate the sensitivity of thermomechanical reading of data with the device structure filled with various inert gases such as helium, neon, and argon. It is demonstrated that filling with helium makes it possible to significantly increase this sensitivity. The simulation and estimation results demonstrate that the sensitivity of the existing method of thermomechanical reading of data is not reduced catastrophically when the dimensions of reading cell are reduced at least to dimensions somewhat smaller than the molecular free path for gas.     

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.     

Simultaneous Contact and Non-contact Measurements of the Melting Temperature of a Ni–C Fixed-Point Cell

A novel fixed-point cell design that allows simultaneous measurements using contact and non-contact thermometers was developed and investigated at PTB to realize the nickel-carbon (Ni–C) fixed-point. The melting temperature indicated by the LP3 radiation thermometer amounted to (1328.86 ± 0.52)°C (k = 2). The melting temperature of the Ni–C fixed-point cell was also calculated by extrapolating the emf-temperature characteristics of two Pt/Pd thermocouples based on their calibrations at conventional fixed points of the ITS-90. The melting temperature of the Ni–C eutectic amounts to (1328.44 ± 0.45)°C using thermocouple Pt/Pd 01/04, and to (1328.53 ± 0.46)°C using thermocouple Pt/Pd 01/05, with uncertainties for k = 2. The contact and non-contact thermometers agree well within the combined uncertainties.     

气体微流量测量技术的发展

本文介绍了美国NIST、德国PTB、日本NMIJ、意大利IMGC、韩国KRISS、中国计量科学研究院NIM和国防科技工业真空一级计量站(兰州物理研究所LIP)的气体微流量测量技术的研究进展,并对其测量方法,测量装置性能指标进行了评述。     

Isotopic Correction of Water-Triple-Point Cells at NMIJ

The twenty-one participating laboratories in the international key comparison of water-triple-point cells (CCT-K7) can be classified into three groups: two laboratories that corrected the effect of the isotopic composition of water, four laboratories that had information on the isotopic composition but did not correct the effect, and the remaining laboratories that had no information. There were significant differences in the realized national standard for the triple point of water (TPW) between those laboratories that applied the isotopic correction and those that did not. The isotopic correction is now considered essential for the triple point of water. Since the National Metrology Institute of Japan (NMIJ) did not apply the isotopic correction and estimated large uncertainties at the time of the CCT-K7 comparison, we subsequently developed new cells for the TPW to improve the reliability and to reduce the uncertainty of the realization as a national reference. The isotopic compositions of seven cells were analyzed, and a chemical impurity analysis of one cell was performed. The good consistency among seven cells was shown in the results obtained when the isotopic correction was applied to the realized temperatures measured experimentally. The expanded uncertainty of the new national reference of NMIJ is estimated to be 49 μK (k = 2), and as a result of this improvement, the expanded uncertainty for calibrating a water-triple-point cell is 80 μK. The previous reference of NMIJ, reported in CCT-K7 to have an expanded uncertainty of 302 μK, is 42 μK lower than the new one. The new reference value is within the uncertainty of the previous national reference, and the new uncertainty is completely covered by the previous uncertainty. Furthermore, the new reference of NMIJ shows good agreement with the national references of the six laboratories able to apply isotopic corrects to their results for CCT-K7. These facts confirm the validity and the linkage to the CCT-K7 of both the previous and the new national references of NMIJ.     

Comparison of Co�CC and Pd�CC Eutectic-Point Cells for Thermocouple Calibration Between LNE-Cnam and NMIJ

Evaluation of the melting temperatures of two Co?CC and two Pd?CC eutectic-point cells were performed using three Pt/Pd thermocouples constructed at LNE-Cnam and NMIJ. During the comparison of cells, Pt/Pd thermocouples were evaluated at the Ag fixed point, at which their drifts and inhomogeneity represented differences within 0.03 °C and 0.14 °C, respectively. One Co?CC cell and one Pd?CC cell were designed and constructed at LNE-Cnam, while one Co?CC cell and one Pd?CC cell were designed and constructed at NMIJ. In spite of differences in cell design and in materials sources, melting points of Co?CC and Pd?CC eutectic-point cells realized using LNE-Cnam high-temperature furnace agreed within approximately 0.02 °C and 0.01 °C, respectively. Furthermore, the realizations of the Pd?CC eutectic point at LNE-Cnam and NMIJ agreed within a temperature equivalent of approximately 0.11 °C. The uncertainty of the realization was estimated to be approximately 0.14 °C (k = 2), with a major contribution from the inhomogeneity of the thermocouple.