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.     

Isotopic Composition of Water Used in Triple-Point Cells

Isotopic analysis of the water used in KRISS triple point of water (TPW) cells was performed by three separate laboratories. The δD and δ ¹⁸O isotopic composition of six ampoules, made from two TPW cells, were analyzed by isotope ratio mass spectrometers. The analysis data showed that δD and δ ¹⁸O were − 62.17‰ and − 9.41‰ for the KRISS-2002-Jan cell, and − 36.42‰ and − 4.08‰ for the KRISS-2005-Jun cell. The temperature deviation of the triple point of water for these cells calculated from Kiyosawa’s data and the definition of the TPW were + 45.07μK for the KRISS-2002-Jan cell, and + 25.49μK for the KRISS-2005-Jun cell. The KRISS TPW temperature was + 92μK higher than the CCT-K7 KCRV after correcting for the deviation of the isotopic composition from Vienna Standard Mean Ocean Water.     

The Impact of Isotopic Concentration,Impurities, and Cell Aging on the Water Triple-Point Temperature

In 2005, the National Institutes of Standards and Technology (NIST) and Fluke’s Hart Scientific Division initiated a study to validate the isotopic correction algorithm applied to the realization temperature of triple point of water (TPW) cells. Additionally, the study quantified the impact of water sample impurities on the TPW cell realization temperature. For this study, eight TPW cells containing water of the same nominal isotopic concentration as Vienna Standard Mean Ocean Water (VSMOW) were used. Five of the cells were manufactured with fused-quartz envelopes and the remaining three with borosilicate envelopes. One TPW cell of each type was uniquely designed so that water samples could be periodically removed to analyze the isotopic composition and to monitor any changes in water purity with time and thereby correlate changes in composition with changes in realization temperature. The borosilicate TPW cells gave an average drift of −13 μK · yr⁻¹ and the more stable fused-quartz TPW cells gave an average drift of −2 μK · yr⁻¹.     

VSMOW Triple Point of Water Cells: Borosilicate versus Fused-Quartz

To investigate an ideal container material for the triple point of water (TPW) cell and to reduce the influence to the triple-point temperature, due to the deviation of the isotopic composition of the water, both borosilicate and fused-quartz glass shelled TPW cells with isotopic composition substantially matching that of Vienna Standard Mean Ocean Water (VSMOW) were developed and tested. Through a specially designed manufacturing system, the isotopic composition, δD and δ¹⁸ O, of the water in the TPW cell could be controlled within ±10‰ (per mil) and ±1.5‰, respectively, resulting in control of the isotopic temperature correction to better than ± 8 μK. Through an ampoule attached to the cell, the isotopic composition of the water in the cell could be individually analyzed . After manufacture, the initial triple-point temperatures of the two types of cell were measured and compared to assess the quality of the cells and manufacturing process. Cells fabricated with the new system agree within 50 μK. Two innovatively designed borosilicate and fused-quartz TPW cells were made, each with six attached ampoules. One ampoule was removed every 6 months to track any changes in purity of the water over time.     

Comparison of Five Isotope-Corrected Water-Triple-Point Cells with the NMIA-2002 WTP Ensemble

A comparison of NMIA’s new water-triple-point (WTP) ensemble with a previously established ensemble is reported. Until 2007, the kelvin in Australia was defined as the average of an ensemble of WTP cells that were selected for stability and purity and collected over a period of several years from a variety of sources. As a result of the recent CCT-K7 comparison, a clarification of the SI definition for the kelvin was adopted, explicitly specifying the isotopic composition of the water in WTP cells. Although NMIA’s results were within the estimated uncertainties, NMIA initiated a project to acquire cells with isotope information from several manufacturers and batches to establish a new ensemble. We find that the standard deviation of the isotope-shift-corrected temperatures of five cells from three manufacturers to be 6 μK, which is significantly lower than that of the cells in the previous ensemble, which was 24 μK. The average temperature of the new ensemble is found to be approximately 107 μK higher than that of the previous ensemble. This difference is consistent with the findings of CCT-K7, which identified a group of laboratories controlling isotope effects, and is displaced 73 μK from the mean of the other laboratories.     

Realization of the Triple Point of Water in Metallic Sealed Cells at the LNE-INM/CNAM: A Progress Report

A miniature metallic cell for the water triple point (TPW, temperature 273.16 K) was developed for capsule-type thermometer calibrations for realizations with adiabatic calorimetry techniques. The LNE-INM/Cnam previously developed a copper cell for the water triple point and the techniques for cleaning, filling, and sealing. On the basis of previous work, a new copper cell prototype for the TPW was developed and filled at the LNE-INM/Cnam. Measurements were performed using an appropriate calorimeter and a comparison block containing several thermometers. Preliminary results show a scatter of the temperatures measured at the phase transition of the order of 0.2 mK when measurements are repeated over a short-term period (1 month). A positive drift in the phase transition temperature of about 30μK·month⁻¹ was observed over several months. Studies are in progress to improve the cell, to reduce the reproducibility uncertainty to less than 0.1 mK and to have a phase transition with better temporal stability.     

Isotopic Effects on the Temperature of the Triple Point of Water

An investigation into the effects of isotopic composition on the triple point temperature of water has been carried out at the National Institute of Metrology (NIM), China, since redefinition of the kelvin with respect to Vienna Standard Mean Ocean Water (V-SMOW) was officially proposed by the Consultative Committee for Thermometry (CCT) in 2005. In this paper, a comparison of four cells with isotopic analyses and relevant results corrected for isotopic composition, employing the isotope correction algorithm recommended by the CCT, is described. The results indicate that, after application of the corrections, the maximum temperature difference between the cells drops from 0.10 mK to 0.02 mK and that these cells are in good agreement within 0.02 mK. Also, temperature deviations arising from isotopic variations fall in the range from −55.9 μK to + 40.7 μK. We consider that the distillation temperature and degassing time of the production procedure lead to isotopic variations.     

CCT-K7水三相点容器国际关键比对

在国际互认框架内,国家计量标准的国际等效性是通过国际计量委员会（CIPM）的咨询委员会组织的一系列国际关键比对来确定的。温度咨询委员会（CCT）委托国际计量局（BIPM）作为主导实验室组织了由20个国家实验室参加的CCT-K7水三相点容器国际关键比对。比对结果表明：这些国家基准水三相点值在0．171mK范围内一致。此外,为了减小国家实验室复现水三相点的系统差,需要进一步研究同位素组成对水三相点温度的影响以及同位素修正。     

Analysis of the Comparison of TPW Realizations in Europe in Light of CCT Recommendation 2 (CI-2005)

Three comparisons of different triple-point-of-water (TPW) realizations in Europe have been organized under the auspices of EUROMET (EUROMET Projects 278, 549, and 714). Thirty European national metrology institutes were involved in these three comparisons that took place from 1994 to 2005. The aim of these successive projects was to assess the uncertainties associated with the practical realization of the triple point of water in Europe. Fifty-four TPW local cells were compared to a traveling standard cell (ref 679) circulated with an isothermal enclosure. The same equipment was used for the three projects, and LNE-INM regularly checked the stability of the TPW standard cell. Recently, LNE-INM has devoted efforts to bring the French standard at the triple point of water into close agreement with CIPM Recommendation 2 (CI-2005). The isotopic fractionation between water and ice when the cell is in use was experimentally studied. Several new TPW cells delivered by the manufacturer with water samples were added to our batch of reference cells. A French laboratory analyzed the isotopic compositions of these samples. These actions allow the French national definition of temperature at the triple point of water to be changed. A new temperature was associated with TPW cell 679 in agreement with the CIPM recommendation. In this presentation, the latest TPW cell measurements carried out by LNE-INM are presented. The results from EUROMET Projects 278, 549, and 714 are investigated in light of these changes.     

高准确度水三相点容器

水三相点的高精度复现及准确测量是保证国际温标ITS-90实施的关键。水三相点容器内高纯水的同位素组成会影响复现的水三相点温度值。为了提高水三相点复现水平,减小氢氧同位素的影响,研制了带有氢氧同位素分析的石英及硼硅玻璃高准确度水三相点容器。为了评价容器的性能,开展了硼硅玻璃和石英水三相点容器的比对。实验结果表明:同位素修正前,石英玻璃和硼硅玻璃水三相点容器复现的水三相点在0.058mK范围内一致;同位素修正之后,容器之间的差异在0.017mK范围内一致。采用高准确度水三相点容器复现水三相点的扩展不确定度为0.066mK(k=2)。     

金属外壳水三相点容器

水三相点是ITS-90国际温标中最重要的定义固定点,其复现不确定度是传递到整个温标的.目前,通常采用不同的冻制方法在硼硅玻璃或石英水三相点容器内冻制均匀的冰套来复现水三相点.冻制过程中,由于在水三相点容器内生成冰桥,会造成容器的破裂.为了解决此难题,研制了金属外壳水三相点容器,利用高纯水自发相变原理,在液体槽内自动冻制...     

Method for Compensating Heat Flux Errors in Mini-TPW Cell Measurements

Small triple-point-of-water cells (mini-TPW) are used in laboratories to monitor the stability of PRTs. Compared with a standard TPW cell, heat flow in the thermometer well usually disturbs the apparent equilibrium temperature to a larger extent in a mini-TPW cell due to its smaller dimensions. In this paper, the heat flow effect is studied on the basis of experimental data. Special attention is paid to the thermal conduction along a thin thermometer probe and to the self-heating of the probe. A new method for compensating the error due to the heat flow is presented. It is shown that the compensated results are in good agreement with results obtained with standard TPW cells. The determined differences were well within the estimated expanded uncertainty of 2 mK (k = 2). The heat flow effect was studied experimentally by controlling the temperature of the upper part of a PRT inserted in a mini-TPW cell. Also, the effect of different fillings of the measurement well of the cell was studied. Without the compensation, thin metal-sheathed PRTs (1.6 and 2.2 mm) indicated 3 to 9 mK differences between mini-TPW and standard TPW cells.     

The effect of natural variations in the isotopic composition on the reproducibility of the temperature of the triple point of water

The effect of long-term natural variations in the isotopic composition on the temperature of the reproduction of the triple point of water – the main reference point of the ITS-90 International Temperature Scale – is investigated. Translated from Izmeritel’naya Tekhnika, No. 12, pp. 31–33, December, 2008.     

System for Realizing the Triple Point of Mercury to Calibrate Industrial-Type Platinum Resistance Thermometers

Most platinum resistance thermometers (PRTs) sent to CENAM for calibration are of the industrial type (IPRT). The cells used to calibrate both standard PRTs (SPRTs) and IPRTs form part of the national standard of temperature. In order to reduce their use, we built a set of fixed-point cells, furnaces, and baths to calibrate IPRTs using fixed points, including an apparatus and cells to reproduce the Hg triple point. To realize the triple point of mercury (TP-Hg), an apparatus that operates from room temperature to  − 45 ^◦C using a CENAM-constructed heat pipe was designed and built. With the heat pipe, it is unnecessary to use a recirculation bath to provide a temperature-controlled environment, the temperature gradient is reduced, and the system is more efficient because its thermal mass is reduced. In this way, it is possible to reproduce the TP-Hg for long periods of time to facilitate IPRT calibration, using cells of smaller size than is conventional and without using expensive liquid baths. The system was tested with cells having 400 g and 800 g of mercury, and a reproducibility of about 0.1 mK was obtained.     

Effect of Impurities on the Triple Point of Water: Experiments with Doped Cells at Different Liquid Fractions

Recent international comparisons showed that there is still room for improvement in triple point of water (TPW) realization uncertainty. Large groups of cells manufactured, maintained and measured in similar conditions still show a spread in the realized TPW temperature that is larger than the best measurement uncertainties (25 µK). One cause is the time-dependent concentration of dissolved impurities in water. The origin of such impurities is the glass/quartz envelope dissolution during a cell lifetime. The effect is a difference in the triple point temperature proportional to the impurities concentration. In order to measure this temperature difference and to investigate the effect of different types of impurities, we manufactured doped cells with different concentrations of silicon (Si), boron (B), sodium (Na) and potassium (K), the glass main chemical components. To identify any influence of the filling process, two completely independent manufacturing procedures were followed in two different laboratories, both national metrology institutes (VSL, Netherlands and UME, Turkey). Cells glass and filling water were also different while the doping materials were identical. Measuring the temperature difference as a function of the liquid fraction is a method to obtain information about impurities concentrations in TPW. Only cells doped with 1 µmol·mol^?1 B, Na and K proved to be suitable for measurements at different liquid fractions. We present here the results with related uncertainties and discuss the critical points in this experimental approach.     

AC-Mode Short-Wavelength IR Radiation Thermometers for Measurement of Ambient Temperatures

Recent improvements in the fabrication of short-wave infrared (SW-IR) quantum detectors have opened a new era in radiation thermometry. Ambient and higher temperatures can be measured with low uncertainties using thermoelectrically (TE) cooled extended-InGaAs (E-IGA) and short-wave photovoltaic-HgCdTe (SW-MCT) detectors. Since these detectors have low cut-off wavelengths (2.5 μm and 2.8 μm, respectively), they do not respond past cut-off and are less sensitive to the background infrared radiation, resulting in orders of magnitude lower background noise than traditional broad-band infrared detectors such as cryogenically cooled quantum detectors or thermal detectors. At the same time, the cut-off is far enough in the infrared to obtain a large enough signal from the source of interest. Because of the low detector cut-off wavelength, traditional glass-based optics can be used in the radiation thermometers. A chopper-produced alternating-current (AC) signal was used to measure low temperatures by separating the AC signal from the background-radiation-produced direct-current (DC) signal and its fluctuations. Design considerations and characteristics of a newly developed SW-IR radiation thermometer are discussed. A noise-equivalent temperature difference (NETD) of < 3mK for a 50°C blackbody was measured. At the human body temperature of 36°C, the obtained NETD of ~10mK indicates that these detectors can be used in non-contact temperature measurements to replace thermopile- or pyroelectric-based radiation thermometers.     

Realization of Low-Temperature Fixed Points of the ITS-90 at NMIJ/AIST

A new model of sealed cells with three thermometer wells for calibration of capsule-type thermometers at low-temperature fixed points of the International Temperature Scale of 1990 has been developed at the National Metrology Institute of Japan (NMIJ). The melting curves of Ar and O₂ obtained using the new cells show very flat plateaux and a linear temperature dependence as a function of the inverse liquid fraction (1/F) over the range 1/F = 1 to 1/F = 20 with a narrow melting curve width of 0.1 mK. The melting curves of Ne obtained with the new cell also show very flat plateaux and approximately linear temperature dependence versus 1/F and a narrow melting curve width of 0.1 mK, though with a slight concave structure at high 1/F. The melting temperatures with the new cells agree with previous NMIJ sealed cells within 10 μK, which is similar to the reproducibility of the realization of the triple points at NMIJ. The source dependence of the triple-point temperature of Ne was investigated by filling two of the new cells from different sources of Ne. The difference in the realized triple point temperatures between the two sources is 0.031 mK, consistent with that estimated from isotope analysis. The uncertainties in the calibration of standard platinum resistance thermometers at the low-temperature fixed points are summarized. The uncertainty of the calibration at the triple point of e-H₂ has been reduced to about one-third of its value without the correction by making the isotopic correction on the basis of the technical annex for the ITS-90 in the mise en pratique for the definition of the kelvin.     

Long-Term Stability of 0.65-μm Radiation Thermometers at NMIJ

Narrow-band radiation thermometers with center wavelengths near 0.65 μm are frequently used as standard thermometers at high temperatures. The long-term stability of ten Topcon 0.65-μm radiation thermometers was assessed at NMIJ by using fixed-point blackbodies and spectral responsivity measurements. Most of the changes are due to shifts in the center wavelength of the interference filters to longer wavelengths. Even when the center wavelengths shifted, the filter widths and transmittances remained quite stable for some radiation thermometers, but one was found for which the bandwidth increased from 15.7 to 17.2 nm and the transmittance decreased by 6%. Three Barr filters were found to be very stable in wavelength. The output signals of 0.65-μm Topcon radiation thermometers were within 2% · year⁻¹ without correcting for the wavelength change and within 0.2% · year⁻¹ after the correction. Keeping the objective lens clean is very important for radiation thermometers. Large output decreases were observed in early 2000 for many radiation thermometers at NMIJ. The output changes were as large as 1% and were recovered by cleaning the objective lens.     

Assembly and Study of Different Mercury Cells with Known Impurity Content and Isotopic Composition

The “Centro Español de Metrología” is carrying out a project to improve the knowledge of the influence of impurities and isotopic composition on the temperature of the mercury triple point. High-purity mercury from the Almaden mine (stated purity of 99.9998%) was further purified by vacuum distillation. Three mercury fractions, the original mercury from Almaden and two distilled fractions, were characterized in terms of both impurities and isotopic composition and used to measure the mercury triple point. The original mercury sample contained silver at 560 ng · g^?1 as the main impurity while the impurity levels were much lower (silver < 1 ng · g^?1) in the two distilled fractions. The isotopic composition of the distilled fractions showed delta values, expressed as $1,000\times(^{198/202}{\rm Hg}_{\rm sample}-^{198/202}\,{\rm Hg}_{\rm reference})/^{198/202}{\rm Hg}_{\rm reference}The “Centro Espa?ol de Metrología” is carrying out a project to improve the knowledge of the influence of impurities and isotopic composition on the temperature of the mercury triple point. High-purity mercury from the Almaden mine (stated purity of 99.9998%) was further purified by vacuum distillation. Three mercury fractions, the original mercury from Almaden and two distilled fractions, were characterized in terms of both impurities and isotopic composition and used to measure the mercury triple point. The original mercury sample contained silver at 560 ng · g⁻¹ as the main impurity while the impurity levels were much lower (silver < 1 ng · g⁻¹) in the two distilled fractions. The isotopic composition of the distilled fractions showed delta values, expressed as , of 1.37±0.07 (1σ) for the first distilled sample and −1.55±0.03 (1σ) for the second distilled sample with reference to the original Almaden mercury. For the measurement of the mercury triple point, an alcohol stirred bath was used that allowed two cells to be compared nearly simultaneously. It was observed that the presence of the silver impurities in the high-purity mercury modified slightly the mercury triple point while the effect of variations in the isotopic composition can be considered negligible.     

The New KRISS Low Frost-Point Humidity Generator

A new low frost-point humidity generator (LFPG) has been designed, and its performance has been tested, in order to extend the calibration capabilities to the low frost-point range at KRISS. The water vapor–gas mixture is generated by saturating air with water vapor over a surface of an ice-coated saturator under the conditions of constant temperature and pressure. This LFPG covers a range of frost point from  − 99 °C to  − 40 °C. The temperature of the saturator, which is controlled by thermoelectric devices and a two-stage mechanical refrigeration system, is stable within 5 mK, and the difference between the saturator temperature and the frost point generated at the saturator outlet is less than 20 mK. This stability is achieved by using oxygen-free high-conductivity copper materials as the saturator body, and applying a precision PID temperature control system. The performance of this new LFPG system is compared with the KRISS standard two-temperature generator in the frost-point range ( − 80 to  − 40) °C, and its performance is tested with a quartz crystal microbalance (QCM), which was built at KRISS, to  − 91 °C.