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In an effort to further reduce the uncertainties in the triple point temperatures of 20Ne and 22Ne, already established in previous publications, at INRIM a cell for each of the two high-purity isotopes was mounted on the same copper block and their triple points were realized subsequently one after the other, allowing a reduction of the total experimental uncertainty on their temperature difference due to a full correlation of some of the uncertainty components arising from this type of realization. The two meltings remain perfectly distinct and do not interfere with each other, being the temperature difference of 0.147 K while the melting ranges are below 0.2 mK. The main difference with respect to single measurements (mounting a single cell at a time), lies in the adiabatic conditions during measurement, which are hardly reproducible when mounting a single cell at a time, leading to one of the dominant uncertainty contributions, 15 μK (single). In the present work, the uncertainty component due to the adiabatic conditions is reduced by using a measurement in sequence (mounting the two cells at a time), 1 μK. Thus, an improved reliability in the experimental realization is obtained as reflected in the experimental contributions to the uncertainty budget.

The neon triple point is one of the defining fixed points of the International Temperature Scale of 1990 (ITS-90). Although recognizing that natural neon is a mixture of isotopes, the ITS-90 definition only states that the neon should be of ?natural isotopic composition?, without any further requirements. A preliminary study in 2005 indicated that most of the observed variability in the realized neon triple point temperatures within a range of about 0.5?mK can be attributed to the variability in isotopic composition among different samples of ?natural? neon. Based on the results of an International Project (EUROMET Project No. 770), the Consultative Committee for Thermometry decided to improve the realization of the neon fixed point by assigning the ITS-90 temperature value 24.5561?K to neon with the isotopic composition recommended by IUPAC, accompanied by a quadratic equation?to take the deviations from the reference composition into account. In this paper, the uncertainties of the equation?are discussed and an uncertainty budget is presented. The resulting standard uncertainty due to the isotopic effect (k = 1) after correction of the calibration data is reduced to (4 to 40)??K when using neon of ?natural? isotopic composition or to 30??K when using 20Ne. For comparison, an uncertainty component of 0.15?mK should be included in the uncertainty budget for the neon triple point if the isotopic composition is unknown, i.e. whenever the correction cannot be applied.

This note discusses some problems that should be taken into account in the implementation of the proposed use of fundamental constants in the definition of measurement units: use of CODATA adjusted values of the constants for this specific purpose; how the CODATA Task Group could continue performing meaningful least squares adjustments of the fundamental constants taking into account future data.

The expression of uncertainty in the field of metrology is based, since 1993, on the Guide to the Expression of Uncertainty in Measurement. According to this, 'it is assumed that the results of a measurement have been corrected for all recognized significant systematic effects'. Since the International Temperature Scale of 1990 considers the substances used for the realization of the 'fixed points' to be ideally pure, to fully implement the intent of the GUM corrections should be applied for any chemical impurities that affect the value of the measurand. The present paper aims at reviewing an aspect that must be tackled to arrive to reliable and scientifically sound corrections: the use of an appropriate statistical method. In addition to the SIE, OME and hybrid methods recommended by the CCT, two new approaches are proposed in this paper, called one-sided OME and Average Overall Estimate (AOE). They are illustrated and their merits compared with the previous one, by applying them for the correction of the measured values of the triple-point temperature of the four gaseous substances (hydrogen, neon, oxygen and argon) used for the realization of the ITS-90 reference points in the range 13.8 K to 273.16 K. Some suggestions are drawn from the resulting evidence.

New comparisons are reported of the triple-point temperatures, Ttp, of seven neon samples with different 22Ne amount concentrations, 22x, in cryogenic cells sealed by INRIM, NMIJ, NPL and PTB. This work stems from a collaboration between those laboratories aiming at establishing the relationship between Ttp and 22x. Several problems remained unsolved in the published results. The new measurements supply more thermal data of lower uncertainty, to help solve some of these problems. The detailed results from 35 meltings are reported, characterized by improved uncertainty, and include an analysis of calorimetric, thermometric and sample-related effects. A broad range of collected and processed information is analysed in detail in order to confirm expanded uncertainty levels of the order of 30??K for a single cell and 50??K for a comparison of pairs of cells. The quality of the INRIM results is then discussed. In most cases the uncertainty budget for the INRIM measurements, including analysis of chemical impurities and the hydrostatic head correction, reached the targeted improved uncertainty. Finally, the results are discussed and compared with previous studies in support of the future assignment by the CCT of an accurate and reliable relationship Ttp versus 22x. The paper discusses various improvements regarding the influence of the thermal measurements on both the uncertainty of the relationship and the ability to recognize either possible isotopic fractionation during sample preparation or incorrect assignment of 22x values to some measured samples.

The ISO TS21748:2003 statement that the laboratory component of 'bias' included in the model and indicated by B in ISO 5725-2:1994 has zero expectation is evaluated critically. It is found that this condition should apply not to the Bk of each laboratory modelled in ISO 5725-2:1994, but to the expectation of the distribution of the Bk in a comparison when the method of choosing the participant laboratories supports the hypothesis that it becomes a 'random effect'. A few related inconsistencies are also discussed.

According to the Guide to the Expression of Uncertainty in Measurement (GUM) on which the expression of uncertainty in the field of metrology is based, since 1995, ‘it is assumed that the results of a measurement have been corrected for all recognised significant systematic effects’. Since the International Temperature Scale of 1990 specifies that the substances used for the realization of the ‘fixed points’ be ‘ideally pure’, to fully implement the intent of the GUM corrections should be applied for any chemical impurities that affect the value of the measurand. In general, thermometrists' opinion is that significant laboratory research and more literature search are still needed for further progress towards reliable corrections. This paper, reviewing the available literature data, shows that the situation is more favourable in the case of the substances used for the realization of the Scale reference points in the range 13.8 K to 273.16 K based on the use of hydrogen, neon, oxygen and argon. The appendix reports a similar review also for nitrogen. Then the paper discusses the other conditions, physical–chemical and thermal, of the substances inside the thermometric cells, concurring with the chemical impurity effects to the overall state of knowledge of the realized triple point temperature relevant to the solution of the problem of performing the corrections.

The identification of the measurand in a comparison of standards may seem a simple matter. This choice can, however, be complicated by (extremely) varied interpretations, as this paper seeks to show. Cases affected by possible ambiguities are presented, involving the intrinsic nature of the standard, the way the standard is implemented or the way the comparison is conducted. Ambiguities can lead to controversial scientific and formal decisions. The paper reviews and discusses a broad variety of aspects of measurand identification that have arisen in CIPM key comparisons, mainly in thermal metrology.

We present accurate measurements of the triple point of nitrogen. The quality of this triple point is once more confirmed, and its temperature value is found to be T90 = 63.152 6 K (expanded uncertainty U = 0.30 mK, reproducibility better than 0.1 mK). This is the first direct determination of this point on ITS-90 since the introduction of the scale. This point is useful as a non-uniqueness checkpoint.

This paper discusses the issue of repeated versus non-repeated measurements on standards. Starting from the general issue of replication of measurement operations, we consider the consequences of the differences that these two types of measurements entail in the comparison of standards. We look at MRA key comparisons (KCs) as a specific case. Recent developments seem to have somewhat obscured the fact that the replication and combination of observations is the everyday task of NMIs. Therefore, this paper first recalls the many facets of intra-laboratory measurements on standards, some not very different from the inter-laboratory exercises. The meaning of the replicated observations is analysed starting from this context. Then the issue is examined in the context of inter-laboratory comparisons, namely, in the case of KCs. Some of the consequences for the statistical treatment of repeated and non-repeated measurement data are illustrated and solutions proposed.

The definition of the International Temperature Scale, ITS-90, requires the substances used for the realization of the fixed-points, except 3He, to be of 'natural' isotopic composition. The uncertainty of the best fixed-point realizations for some of these substances is today significantly affected by the natural variability of the isotopic composition of the available samples. The paper discusses the main implications of this problem, which is becoming an obstacle to further improvements for some of the fixed-point realizations. In addition, the two main solutions are compared: (i) to define only a reference composition and (ii) to define a reference composition and allow the use of a correction for the deviations from a reference composition.

Several studies highlight the need for appropriate statistical and probabilistic tools to analyse the data provided by the participants in an interlaboratory comparison. In some temperature comparisons, where the measurand is a physical state, independent realizations of the same physical state are acquired in each participating institute, which should be considered as belonging to a single super-population. This paper introduces the use of a probabilistic tool, a mixture of probability distributions, to represent the overall population in such a temperature comparison. This super-population is defined by combining the local populations in given proportions. The mixture density function identifies the total data variability, and the key comparison reference value has a natural definition as the expectation value of this probability density.

An investigation is presented on the influence of impurity concentration and magnetic field on the superconducting transition of high-purity commercial titanium samples. Run-to-run and sample-to-sample reproducibility is evaluated for the purest titanium specimen. Past work on the superconducting transition of titanium is reviewed, and the suitability of titanium as a thermometric reference point is discussed.

The paper by Kacker et al (2004 Metrologia 41 340?52) takes a systematic approach to the key comparison reference value and the degree of equivalence. Their approach seems of particular interest on three issues that are discussed here.

Recently the values of the various instrument constants and correction terms for the BIPM/JAEGER manobarometer of the Istituto di Metrologia "G. Colonnetti" (IMGC) have been re-assessed, resulting in smaller uncertainties for the pressures obtained. The international intercomparison of 1991/1992 provided an opportunity to test the error budget.

A modelling procedure is introduced for improvement of the reduction of data with the least-squares method, when the data are subdivided in series and cannot be considered as replicated. It combines both the advantage of preserving the individuality of each series and that of improving the variance estimate which arises from fitting the overall data. The procedure assumes - and includes in the model - a translational bias for every series. The method has been applied to a thermal expansion database of copper below 300 K and to high-accuracy low-temperature gas thermometry data, using several least-squares solvers.

The triple point temperatures (Ttp) of argon and oxygen were realized by a calorimetric technique with an accuracy of ±015 mK. Their values lie within 0.5 mK of the most recent determinations carried out in other laboratories. Errors in Ttp due to the cryostat and to impurities were extensively investigated. After initial conditioning, the gases were sealed into triple point cells, in which Ttp was stable at the quoted accuracy over a two-year period. The triple point pressure of oxygen was found to be 146.25 ± 0.4 Pa. An extensive bibliography of solid-solid equilibria of mixtures with Ar and O2 is also given.

The use of the triple point of gases as a method for obtaining highly reproducible pressure fixed points is described. In order to obtain pressure transfer standards, the method is implemented with transportable, self-contained sealed cells, containing the sample of gas, which can be easily shipped around for intercomparisons in the pressure range 0.1 to 100 kPa. Experimental results on these devices are reported for three gases, including also their absolute pressure value. Oxygen: p = 146.25 ± 0.3 Pa, T = 54.3610 K, reproducibility of the transfer standard: ±0.1 Pa; methane: p = 11 696 ± 2 Pa, T = 90.6856 K, reproducibility: ±0.7 Pa; argon: p = 68 890 ± 1.5 Pa, T = 83.7980 K, reproducibility: ±1 Pa. The temperature of the normal boiling point of argon was also measured and found to be 87.2952 ± 0.0005 K. All temperatures are given in IPTS-68, realized with the triple point of argon.

An International Intercomparison of Fixed Points by Means of Sealed Cells has been conducted under the auspices of the Comite Consultatif de Thermometrie (CCT) between 1978 and 1984. Forty-one sealed cells, realizing the triple point of seven different substances, defining both primary fixed points of the IPTS-68 and secondary fixed points in the temperature range from 14 K to 90 K, were supplied by nine laboratories. They were measured in eleven national laboratories around the world, against the fixed points realized in these laboratories (both in open cryostats or in other sealed cells). Some 150 independent series of data were produced, from almost 300 melting experiments, representing some 2,300 equilibrium temperature values. The basic sets of results are presented, concerning the agreement between different cell realizations and the comparison of national IPTS-68 realizations. Data connecting the results of this intercomparison with that performed at NPL in 1975 using calibrated thermometers are also given.

A study specifically intended to measure the triple-point temperature of both n-D2 and e-D2 containing a volume fraction of HD not higher than 0.0001 has been completed. It was performed by measuring samples of gas in metal sealed cells. The cells, after being checked for a non-increasing HD contamination within the stated limits, were filled by directly connecting them to the outlet of a thermal-diffusion column for isotopic separation, known to produce D2 with a HD volume fraction not larger than 0.00003. In the case of e-D2, additional studies were performed on the effect of deuterating the catalyst or on high-temperature activation of the catalyst. Results proved that it is possible to obtain good results for e-D2 with partially deuterated catalyst, but this does not allow a reliable control of the contamination. An alternative procedure to full deuteration has been found, which uses commercial hydrous ferric oxide heated to about 400 °C, and commercially available deuterium, produced by isotopic separation and showing a purity better than 99.99%. Temperature values (18.7327 ± 0.0003) K for n-D2 and (18.6983 ± 0.0003) K for e-D2 on NPL-IPTS-68 were obtained at the liquidus point; the best thermodynamic values for these triple points are (18.7232 ± 0.001) K and (18.6888 ± 0.001) K respectively; e-D2 can be used as a first-quality temperature reference point.

From accurate measurements on a set of sealed cells, each reproducible to within ±0.2 mK, the temperature value for the triple point of methane was found to be 90.6856 K, with a range of 1 mK depending on which one of ten thermometers was used. All thermometers were calibrated on IPTS-68 using T(Ar) = 83.798 K. On the contrary, when IPTS-68 calibration of the same thermometers was considered, based on the condensation point of oxygen as realized in different laboratories, a spread of 3.6 mK was found for the values of both the argon and the methane triple points.