K. D. Hill

Final Report on CCT-K7: Key comparison of water triple point cells

The triple point of water serves to define the kelvin, the unit of thermodynamic temperature, in the International System of Units (SI). Furthermore, it is the most important fixed point of the International Temperature Scale of 1990 (ITS-90). Any uncertainty in the realization of the triple point of water contributes directly to the measurement uncertainty over the wide temperature range from 13.8033 K to 1234.93 K. The Consultative Committee for Thermometry (CCT) decided at its 21st meeting in 2001 to carry out a comparison of water triple point cells and charged the BIPM with its organization. Water triple point cells from 20 national metrology institutes were carried to the BIPM and were compared with highest accuracy with two reference cells. The small day-to-day changes of the reference cells were determined by a least-squares technique. Prior to the measurements at the BIPM, the transfer cells were compared with the corresponding national references and therefore also allow comparison of the national references of the water triple point. This report presents the results of this comparison and gives detailed information about the measurements made at the BIPM and in the participating laboratories. It was found that the transfer cells show a standard deviation of 50 ?K; the difference between the extremes is 160 ?K. The same spread is observed between the national references. The most important result of this work is that a correlation between the isotopic composition of the cell water and the triple point temperature was observed. To reduce the spread between different realizations, it is therefore proposed that the definition of the kelvin should refer to water of a specified isotopic composition. The CCT recommended to the International Committee of Weights and Measures (CIPM) to clarify the definition of the kelvin in the SI brochure by explicitly referring to water with the isotopic composition of Vienna Standard Mean Ocean Water (VSMOW). The CIPM accepted this recommendation and the next edition of the SI brochure will include this specification. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Isotopic and other influences on the realization of the triple point of hydrogen

Within an international collaboration of the eight metrological institutes represented by the authors, the dependence of the triple-point temperature of equilibrium hydrogen on the deuterium content at low concentrations has been precisely determined so that the uncertainty in realizing the triple point as a temperature fixed point might be reduced by nearly one order of magnitude. To investigate the thermodynamic properties of the hydrogen–deuterium mixtures and to elucidate the factors that influence the melting temperature, 28 sealed fixed-point cells have been filled and measured, and some of these have been compared with an open-cell system. Hydrogen gas with a deuterium content ranging from 27.2 µmol D/mol H to 154.9 µmol D/mol H was studied using cells containing five different types of spin-conversion catalyst, with different catalyst-to-liquid volume ratios (a few per cent to more than 100%) and of different designs. The latter consideration is especially influential in determining the thermal behaviour of the cells and, thus, the temperature-measurement errors. The cells were measured at the eight participating institutes in accordance with a detailed protocol that facilitates a direct comparison of the results. Through analysis of the measurements, significant inter-institute deviations due to different measurement facilities and methods have been ruled out with respect to the determination of both the melting temperatures and the thermal parameters of the cells. The uncertainty estimates for the determination of the deuterium content have been verified by including isotopic analysis results from four different sources. The slope of the dependence of the triple-point temperature of equilibrium hydrogen isotopic mixtures on the deuterium content has been deduced from the melting temperatures of those sample portions not in direct contact with the catalysts. Evaluation of the data using different mathematical methods has yielded an average value of 5.42 µK per µmol D/mol H, with an upper bound of the standard uncertainty of 0.31 µK per µmol D/mol H. This is close to the literature value of 5.6 µK per µmol D/mol H that was obtained at higher deuterium concentrations. (Some figures in this article are in colour only in the electronic version)

A useful reflection

We discuss a simple, practical transformation between the probability density function (PDF) of the measurand and the associated PDF of the process for producing measurement results, both of which are used in metrology. This transformation is a reflection through the reported value and captures the essential reversal of skewness of these two perspectives for one-dimensional cases (i.e. any scalar quantity with no free parameters). It is useful for reconciling the skewness of the measurand and measurement-results perspectives, identifying cases where the choice of perspective should not matter and helping to treat the exceptional cases, involving PDF asymmetry, where it can matter.

The triple point of xenon

The triple point of xenon is shown to be a suitable fixed point for incorporation into the next revision of the International Temperature Scale (ITS) as a means of reducing the non-uniqueness in the important 84 K to 273 K range. Isotopic effects, once thought to be a limiting factor, are shown to be negligible. Purity remains the overriding concern, but high-purity xenon gas is obtainable, giving a very flat plateau with a melting range within ±10 µK from 50% to 90% of the melted sample fraction. The triple point temperature of xenon is shown to be 161.405 96 K ± 0.32 mK (k = 1) on the ITS-90. The propagated calibration uncertainty of the platinum resistance thermometers and the component attributed to the non-uniqueness of the ITS-90, as evidenced by the differences among the seven calibrated thermometers, dominate the overall uncertainty of the estimated triple point temperature. The xenon triple point itself is highly reproducible, with a standard deviation of 48 µK for the eight melts of this study and a total realization uncertainty of just 76 µK.

Investigation of low-temperature fixed points by an international star intercomparison of sealed triple-point cells

An overview of the results of an international star intercomparison of low-temperature fixed points is given. Between 1997 and 2005, 68 sealed triple-point cells (STPCs) of the twelve laboratories represented by the authors were investigated at PTB. The STPCs are used to realize the triple points of hydrogen, neon, oxygen and argon as defining fixed points of the International Temperature Scale of 1990, ITS-90. The melting curves (MCs) of all STPCs have been measured on the same experimental equipment, adhering strictly to a single measurement program. This protocol enables separation of the effects influencing the MCs and direct comparison of the thermal behaviour of the STPCs, which are quite different with respect to design, age, gas source and filling technology. In the paper, special emphasis is given to the spread of the liquidus-point temperatures and to the uncertainty of their determination. Connections between the star intercomparison and completed and ongoing international activities are also discussed.

An Apparatus for Realizing the Triple Point of Mercury

A mercury triple point apparatus has been developed which permits the observation of long freezes and melts with little intervention. We have developed a sealed glass cell containing 2,5 kg of mercury which was filled simply by pouring rather than by vaccum distillation. The cell is installed in a temperature-controlled bath of our own design. Melting plateaux of more than 24 hours duration are recorded with the bath setpoint approximately 3 °C higher than the mercury triple point temperature. Much longer plateaux are possible by setting the bath closer to the triple point temperature. The temperature of the triple point of mercury has been determined as 234,308 5 K on the IPTS-68 with a standard deviation of 0,1 mK based on measurements with five platinum resistance thermometers. The result is in excellent agreement with the values 234,308 3 K and 234,308 6 K as determined at the NIST (formerly the NBS, USA) by Furukawa and Bigge and at the NPL (UK) by Chattle and Butler, respectively. A platinum resistance thermometer tracks the hydrostatic gradient within the cell over a distance of 10 cm and is in agreement with the value of 7,1 mK per metre of liquid as given in the texts of the IPTS-68 and the ITS-90. The measurements yield a value of 234,315 9 K on the ITS-90 for the triple point of mercury cell investigated.

CCT-K2.4: NRC/INTiBS/LNE-Cnam trilateral comparison of capsule-type standard platinum resistance thermometers from 13.8 K to 273.16 K

A trilateral comparison of capsule-type standard platinum resistance thermometers (CSPRT) was carried out between NRC (Canada), INTiBS (Poland) and LNE (France) in 2006 over the temperature range 13.8 K to 273.16 K. The NRC/INTiBS comparison made it possible to link the INTiBS realization of the International Temperature Scale of 1990 to the results of the Consultative Committee for Thermometry Key Comparison CCT-K2 over this temperature range. It revealed that calibrations at INTiBS were in agreement with the key comparison reference values of CCT-K2 within the expanded uncertainties for all temperatures of the comparison based on the data obtained with one of CSPRTs, the other CSPRT of the comparison being considered as anomalous. The linkage to the CCT-K2 data supports the inclusion of the INTiBS CMCs in the BIPM key comparison database (KCDB) and also supplies evidence to support the revision of the uncertainties of the LNE CMCs in the KCDB at the triple points of neon and hydrogen. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

SIM.T-K9.1: Canada/Peru bilateral comparison of standard platinum resistance thermometers from 0.01 °C to 419.527 °C

The Consultative Committee for Thermometry has organized Key Comparison 9 (CCT-K9) as an updated replacement for CCT-K3. A comparison of the fixed-point temperature standards of Peru with those of Canada was carried out following a protocol that closely resembles CCT-K9. The comparison was limited to the fixed points of tin and zinc, with NRC the pilot laboratory and the eventual link to CCT-K9 (currently, CCT-K9 remains a work in progress). The comparison protocol required INDECOPI to select, characterize, and transport (i.e. hand carry) the SPRTs to NRC. Following the measurements at NRC, the SPRTs were returned and re-measured at INDECOPI to close the loop and assess any changes that occurred in the SPRTs over the course of the measurements. The following degrees of equivalence, D, and expanded uncertainties, U (k = 2), were obtained via this comparison. T / ?C DINDECOPI / mK U / mK 419.527 1.58 4.6 231.928 -2.12 3.9 SIM.T-K9.1 has demonstrated the compatibility of the fixed points of INDECOPI with those maintained by NRC within the combined uncertainties, and therefore serves to verify the intended calibration and measurement capabilities envisioned for INDECOPI, Peru. The results can be linked to NMIs globally following CCT-K9, with NRC as the linking participant. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

CCT-K2.3: NRC/NMi-VSL bilateral comparison of capsule-type standard platinum resistance thermometers from 13.8 K to 273.16 K

A bilateral comparison of capsule-type standard platinum resistance thermometers (CSPRT) was carried out between NRC (Canada) and NMi-VSL (The Netherlands) in 2006 over the temperature range 13.8 K to 273.16 K. It made it possible to link the NMi-VSL realization of the International Temperature Scale of 1990 to the results of the Consultative Committee for Thermometry Key Comparison CCT-K2 over this temperature range. It revealed that calibrations at NMi-VSL were in agreement with the key comparison reference values of CCT-K2 within the expanded uncertainties for all temperatures of the comparison with the possible exception of the triple point of neon at 24.5561 K, where the difference from the key comparison reference value for one of the CSPRTs exceeds the expanded uncertainty. The linkage to the CCT-K2 data supports the evaluation of the NMi-VSL CMCs in the BIPM key comparison database (KCDB). Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).