Uncertainties in the Realization of the SPRT Sub-ranges of the ITS-90

Working Group 3 of the Consultative Committee for Thermometry is responsible for recommending methods to assess uncertainties in contact thermometry. Accordingly, it has now completed a guide summarizing the uncertainties in the realization of the standard platinum resistance thermometer subranges of ITS-90 between the triple point of neon (24.5561K) and the freezing point of silver (961.78 °C). The document provides guidance to assess the uncertainties of both SPRT calibrations and temperature measurements. The document describes all known sources of uncertainty and influence variables, identifies key references in the literature that discuss, model or evaluate each effect, gives an indication of the typical magnitudes of the uncertainties, and provides propagation laws. This article is an overview of the guide emphasizing aspects that may be different from common practice, which includes: associating all uncertainty terms with a physical cause to ensure they can be propagated and to prevent double counting; uncertainty due to the oxidation state of the SPRT; uncertainty due to the isotopic composition of fixed-point substances; uncertainty due to impurities in fixed-point substances; and uncertainty due to non-uniqueness of the SPRT interpolations. The article gives a graphical summary of the total uncertainties in ITS-90 over the SPRT temperature range.     

Thermal metrology after the introduction of the ITS-90

The adoption of the international temperature scale of 1990 (ITS-90) is producing significant innovation in thermal metrology. The temperature range of the ITS is wider than that of the previous editions. Also the scale precision and the accuracy of the thermodynamic temperature values forming its base have been improved. A short presentation of the ITS-90 will introduce its most important features, describing briefly the types of interpolating instruments and fixed points. To realize the ITS-90, many national laboratories will have to develop within their premises relatively new techniques of measurement such as gas thermometry to interpolate between fixed points from 3 K and 24.6 K and high-temperature platinum resistance thermometry. The highest precision is acheived in the temperature range –40°C to 30° C, where the ITS-90 combines the most reproducible fixed points with the interpolating instrument exhibiting the highest accuracy and the lowest nonuniqueness. Since a reproducibility better than 0.2 mK can be achieved everywhere in that range, except in the proximity of the triple point of water where it is lower than 0.1 mK (all estimates being at the 1 level), the size of the kelvin is reporoduced to within 7·10^–6 in the worst case, and to within 2·10^–7 at the triple point of water. Some practical problems may result from the dissemination of the iTS-90 to users of temperature measurements in the temperature range 420–1085°C. They are related to the construction and use of high-temperature platinum resistance thermometers (HTPRTs), to the practical exploitation of radiation thermometry, and to the availabiality of suitable transfer standards in this temperature range. Several practical realizations of the ITS-90 that are underway in various national laboratories will enable one to better estimate its precision. Such an operation will essentially be carried out through intercomparisons, which will be successful if highly-reproducible transfer standards like the sealed cells for fixed points will be made available. The basic level of the scale precision, i.e., its irreducible component, is given by the scale nonuniqueness. Some results are already available and some experiments have recently been proposed in order to provide further data in critical areas. Another important aspect is the scale smoothness and its effect on the measured thermal properties. With the introduction of the ITS-90 the projects for the determination of thermodynamic properties have not completely stopped. Radiometric determinations and noise thermometry are mainly carried out at temperatures between 660°C and 1085°C. Other experiments involving gas thermomery may lead to better determinations below 3 K. Thermal noise measurements are presently considered for the redetermination of the Boltzmann constant, k_B. Their uncertainty should be within ± 10ppm, still too high to propose such a method for the definition of the kelvin in terms of k_B and of the SI unit of energy. As regards the determination of thermophysical properties of matter, special importance is now attributed to the temperature dependence of the vapor pressure of sodium. Such an interest in connection with the use of pressure-controlled sodium heat pipes for temperature reference. In addition, some information is provided on methods for the determination of pulsed-energy excitation. There, advantage is taken from improvements in radiation thermometry.Translated from Izmeritel'naya Tekhnika, No. 12, pp. 50–58, December, 1993.     

Applying Isotopic Effect in ITS-90 SPRT Calibrations

The International Temperature Scale of 1990 (ITS-90) defines exact values for all fixed-point temperatures. For example, for the standard platinum resistance thermometers (SPRT), at each fixed point, the measured SPRT resistance and the temperature defined in the ITS-90 are used as input data into the correction equations of the ITS-90. Starting from 2006, formal equations were added to the Technical Annex for the ITS-90 for computing the fixed-point temperatures of the substances of different isotopic compositions, presently the \(e\hbox {-H}_{2}\) triple and vapor-pressure points, Ne triple point, and \(\hbox {H}_{2}\hbox {O}\) triple point. This paper addresses the issue of the method required to apply the procedure defined in the ITS-90 for the calibration of a SPRT, according to the new requirements. The required procedure does not involve a “correction” of the fixed-point temperatures, since they are defined exactly by the ITS-90, but requires instead, the re-computing of the measured resistances at the relevant fixed points. In those cases where resistance ratios with respect to the triple point of water are required, the re-computation must be first applied separately to the specific fixed points and to the triple point of water. In case the re-computation is not possible because of insufficient information on the isotopic composition of the sample used, an additional component must be added to the total uncertainty budget.     

Present Estimates of the Differences Between Thermodynamic Temperatures and the ITS-90

At the request of the Consultative Committee for Thermometry (CCT), Working Group 4 (WG4) has critically reviewed all available measurements of the differences between thermodynamic and ITS-90 temperatures, (T ? T ₉₀), and documented the conversion of older data to the ITS-90. Particular attention has been given to the uncertainties. Based on this review, we provide consensus estimates of T ? T ₉₀ for selected measurements from 0.65 K to 1358 K. We provide two analytic functions for T ? T ₉₀, one for use from 8 K to the triple point of water (T _TPW) and one for use above T _TPW. The small discontinuity of the derivative dT ₉₀/d T at T _TPW is discussed. We also identify temperature ranges where researchers are encouraged to undertake high-accuracy measurements of T ? T ₉₀.     

关于1990年国际温标(ITS—90)

自1990年元旦开始,1990年国际温标(TTS-90)已取代1968年国际实用温标(IPTS-68、1975年修订版)。本文评述了制订 ITS-90的背景,以及与 IPTS-68相比它在准确性、一致性和光滑性等方面的重大改进。这里还对 ITS-90的构成进行了分析,并对它可能达到的准确度作出讨论。文章也指出了为贯彻 ITS-90所涉及到的一些问题。     

国际90温标关键比对3的评述

1998年BIPM温度咨询委员会 (CCT)组织了从氩三相点到铝凝固点 90温标定义固定点的复现值的国际比对 ,称为CCT关键比对 3。中国计量科学研究院作为中国国家实验室参加了这一比对 ,给出了全部 8个定义固定点的复现值。本文通过对已公布的结果进行分析和评述 ,使国人了解国际ITS 90温标复现情况的现状 ,以及我国在此次比对结果中的位置 ,进而指导我们今后进一步提高ITS 90复现和维护水平。     

Realization of ITS-90 Above the Silver Point at the NIM

The high-temperature primary standard system was gradually improved at the National Institute of Metrology (NIM) in China after 2004. A new primary standard pyrometer (PSP) was developed, one with a size-of-source effect of 1 × 10^?4, and regional thermostats for interference filters, photoelectric detectors, and I/V converters. The relative spectral responsivity of the entire PSP was calibrated by means of a new facility. A new LED-based measurement facility and novel systematic error correction model were utilized to characterize the PSP nonlinearity and extend the photocurrent to PSP temperature readings of about 2680 °C. As an improved scheme, the fixed-point blackbody pyrometer assembly was utilized to realize and disseminate the International Temperature Scale of 1990 above the silver point. This scheme can avoid the influences of instability and inhomogeneity of tungsten strip lamps and corrects pyrometer drifts, thereby improving the realization uncertainty and simplifying the transfer chain. The expanded uncertainties of the scale realization ranged from 0.04 °C at the silver point to 0.48 °C at 2474 °C.     

NMIA Realization of the ITS-90 Based on Fixed-Point Cell Ensembles

NMIA has recently moved from an ITS-90 realization based on single cells of each fixed point to one based on ensembles of up to five cells of each fixed point from argon to silver. It has been suggested that relying on Raoult’s law to estimate the concentration of impurities in fixed-point cells, and to thereby estimate the likely shift introduced to the temperature of the melting or freezing phase transition, is inadequate. Measurements of NMIA’s present set of 36 cells confirm that using Raoult’s law alone is inadequate, and underestimates the temperature depression of some cells. Material purity assays are usually available for the metals (or gases); however, this will not include contamination introduced during the cell construction process or in-use impurity migration into the sample. At NMIA, we have (1) established ensembles of up to five cells of each fixed point and (2) established techniques and uncertainties for comparisons of cells at the 0.2 mK level. It is concluded that, at the sub-mK accuracy level, fixed-point cells should be considered as artifacts requiring calibration or validation to confirm their suitability as intrinsic reference standards.     

Realisation, Dissemination, and Comparison of the ITS-90 and the PLTS-2000 Below 1 K at PTB

The International Temperature Scale of 1990 (ITS-901 and the Provisional Low Temperature Scale (PLTS-2000)² are the internationally agreed temperature scales in the low temperature range. At temperatures between 0.65 K and 5 K, the ITS-90 is defined by polynomials relating the vapour pressure p _v of ⁴He and ³He to the temperature T ₉₀, whereas the PLTS-2000 is defined by a polynomial relating the ³He megting pressure p _m to the temperature T ₂₀₀₀ in the range from 0.9 mK to 1 K. Here, we describe the methods and the cryogenic facilities used for the realisation and dissemination of the ITS-90 and the PLTS-2000 as well as the level of accuracy, which is achieved at PTB. We focus on how the pressures are measured traceably to the national pressure standard. Using this experimental basis, we plan to compare directly the vapour and megting pressure of ³He in the overlapping temperature range from 0.65 K to 1 K.     

ITS-90 Density of Water Formulation for Volumetric Standards Calibration

A new formulation of the density of air-saturated water as a function of temperature on the 1990 International Temperature Scale (ITS-90) is presented. Also, a new equation for calculating isothermal compressibility as a function of temperature on ITS-90 was developed. The equations are to be used to calculate the density of water, in the temperature range 5 to 40 °C on ITS-90, used in the gravimetric determination of the volume of volumetric standards.     

Uncertainties in the SPRT Subranges of ITS-90: Topics for Further Research

The CCT has completed the guide summarizing the uncertainties in the realization of the SPRT subranges of ITS-90 between the triple point of neon (24.5561 K) and the freezing point of silver (961.78 °C). This article identifies aspects of standard platinum resistance thermometry where either data or models are lacking and further research is required. In the calibration of SPRTs, the two main concerns are the need for data on liquidus slopes for the different impurities in the fixed points and improved understanding of the impact of the thermal environment of the fixed point on the realized temperature. In the use of SPRTs, the two largest sources of uncertainty are Types 1 and 3 non-uniqueness and oxidation. The causes of Type 3 non-uniqueness are not yet understood, especially at low temperatures, and there is a paucity of data for the high-temperature subranges. In respect of oxidation, there is a need for validation of the models developed in the 1980s, especially in light of the reduced partial pressure of oxygen used in modern SPRTs. A range of other effects including vacancy effects in SPRTs, isotopic effects in fixed points, and improved statistical methods are discussed.     

A system approach to the description of the properties of the ITS-90

The use of a system approach to describe the properties of the International Temperature Scale (ITS-90) is proposed. A model is presented which enables the error of the difference between the ITS-90 and the thermodynamic scale to be estimated. The possibility of estimating the static chaos of the results of temperature measurements is considered.     

Realization of the ITS-90 at IMGC in the range of long stem PRTS

IMGC has established the fixed points required by the International Temperature Scale of 1990 (ITS-90) for long-stem platinum resistance thermometers (PRTs) at the highest accuracy level. The present paper describes the fixed point apparatus and the measuring procedures used at IMGC for the accurate realization of each fixed point, and gives some data on the typical phase transitions thereby obtained. Moreover, since IMGC has developed both traditional and sealed cells for the realization of the freesing or melting points of metals, the different types of cells and the procedure used for their construction and preparation are also supplied. Finally, the reproducibilities of the freezing points of Sn, Zn, Al, and Ag were evaluated by means of several determinatiaons with one standard thermometer.Translated from Izmeritel'naya Tekhnika, No. 12, 58–62, December, 1993.     

Evaluation of Small-Sized Platinum Resistance Thermometers with ITS-90 Characteristics

Many platinum resistance thermometers (PRTs) are applied for high precision temperature measurements in industry. Most of the applications use PRTs that follow the industrial standard of PRTs, IEC 60751. However, recently, some applications, such as measurements of the temperature distribution within equipments, require a more precise temperature scale at the 0.01 °C level. In this article the evaluation of remarkably small-sized PRTs that have temperature?Cresistance characteristics very close to that of standard PRTs of the International Temperature Scale of 1990 (ITS-90) is reported. Two types of the sensing element were tested, one is 1.2 mm in diameter and 10 mm long, the other is 0.8 mm and 8 mm. The resistance of the sensor is 100 ?? at the triple-point-of-water temperature. The resistance ratio at the Ga melting-point temperature of the sensing elements exceeds 1.11807. To verify the closeness of the temperature?Cresistance characteristics, comparison measurements up to 157 °C were employed. A pressure-controlled water heat-pipe furnace was used for the comparison measurement. Characteristics of 19 thermometers with these small-sized sensing elements were evaluated. The deviation from the temperature measured using a standard PRT used as a reference thermometer in the comparison was remarkably small, when we apply the same interpolating function for the ITS-90 sub-range to these small thermometers. Results including the stability of the PRTs and the uncertainty evaluation of the comparison measurements, and the comparison results showing the small deviation from the ITS-90 temperature?Cresistance characteristics are reported. The development of such a PRT might be a good solution for applications such as temperature measurements of small objects or temperature distribution measurements that need the ITS-90 temperature scale.     

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.     

NMIJ Constant-Volume Gas Thermometer for Realization of the ITS-90 and Thermodynamic Temperature Measurement

The constant-volume gas thermometer (CVGT) of the National Metrology Institute of Japan (NMIJ), AIST with ³He as the working gas is used as an interpolating gas thermometer to realize the International Temperature Scale of 1990 (ITS-90) from 3 K to 24.5561 K and as a relative gas thermometer for thermodynamic temperature measurement calibrated at the triple point (TP) of Ne. The standard uncertainties of the realization and measurement are estimated to be 0.58 mK and 0.86 mK at a maximum in the mentioned temperature range, respectively. The maximum difference between both temperatures is about 1 mK. In the calibration of the CVGT, the TP of equilibrium hydrogen (e-H₂) is corrected for isotopic composition as specified in the Technical Annex for the ITS-90. The ambiguity of the TP of Ne due to the variability in isotopic composition is included in the uncertainty. Although the CVGT was also used in 2004 to realize the ITS-90, it was modified for the present experiment to reduce some measurement uncertainty components and the working gas was replaced with a higher-isotopic-purity gas. The results from 2004 were recalculated by correcting for the isotopic composition of e-H₂ and differ insignificantly from the present results, except for a wider scatter.     

Disseminating the ITS-90 Traceability in Industry��An Intercomparison of Temperature Block Calibrators

An international intercomparison has been carried out with a commercial dry block calibrator as a transfer standard by the Danish Technological Institute (DTI). The intercomparison involved 16 participating laboratories from five European countries. The intercomparison comprised five measurement points in the range from ?20 °C to +150 °C. The purposes of the intercomparison were twofold: to compare the results of the participating laboratories during calibration of a dry block calibrator and to establish the dry block calibrators?? reproducibility and suitability both as a transfer standard and as a working measurement standard for disseminating the ITS-90 traceability in industry. The characterization and performance of state- of-the-art multi-zone dry block calibrators and the results of the intercomparison are presented.     

A Comparison of the ITS-90 Among NPL, NIM, and CEM, Above the Silver Point Using High-Temperature Fixed Points

A prototype comparison of the ITS-90, as realized by NPL, NIM, and CEM, using high-temperature fixed points (HTFPs) of Co-C (1324 °C), Pt-C (1738 °C), and Re-C (2474 °C), is reported. The local realizations of ITS-90 temperatures were assigned by NPL, NIM, and CEM to their own set of HTFPs. NIM and CEM then transported their cells to NPL, and the ITS-90 temperatures of all three sets of cells were measured using a linear pyrometer. From these measurements, a comparison reference value (CRV) was derived. At the Co-C and Pt-C points, the deviation from the CRV was <0.1 °C for all three institutes; at the Re-C point, the deviation was <0.4 °C. These deviations are significantly less than the scale realization uncertainties ascribed by the individual institutes indicating that these uncertainty estimates are conservative and could be revised to smaller values. In addition, thermodynamic temperatures were determined for these HTFPs using the current value of the thermodynamic temperature for the copper point, namely, 1357.82 K. Given the consistent performance of the HTFPs, they should be seriously considered as scale comparison artifacts of choice when comparing primary realizations of the ITS-90 and of the thermodynamic temperature.     

The effect of covariance on uncertainty when constructing the ITS-90 temperature scale

The effect of covariance between the resistances of a calibrated standard platinum resistance thermometer at the fixed points on the uncertainty when constructing the ITS-90 temperature scale in the 0–660°C range is considered. The proposed method of calculating the uncertainty complies with the international recommendations. This paper was presented at the 3rd All-Russia Conference “Temperature-2007” (see the selection in Measurement Techniques, Nos. 8, 9, 11, 2007). __________ Translated from Izmeritel’naya Tekhnika, No. 4, pp. 44–49, April, 2008.