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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).

The BIPM Watt Balance: Improvements and Developments

This paper summarizes the various improvements to and new developments for the Bureau International des Poids et Mesures (BIPM) watt balance, which have been implemented over the past two years. The main distinctive feature of the BIPM apparatus is its capability for simultaneous force and velocity measurements. Our first campaign of measurements in air at atmospheric pressure has been carried out with simultaneous force-velocity measurements. The results obtained among 11 series showed a relative reproducibility of the Planck constant h of 5.0 × 10-6. The relative difference between our Planck constant result and the CODATA 2006 recommended value is -4.8 × 10-6, with a relative uncertainty of 4.8 × 10-5.

Progress on the BIPM watt balance

Since the beginning of the development in spring 2005, considerable progress has been made on the BIPM watt balance. We have continued the development of a room temperature experiment to test the feasibility of simultaneous force and velocity measurements. We are now able to simultaneously acquire the velocity and voltage of the moving coil and to deduce their ratio. This article briefly recalls the main ideas of the BIPM watt balance and reports the progress to date and the preliminary results.

A review of Josephson comparison results

In March 2009, the Consultative Committee for Electricity and Magnetism (CCEM) reviewed and re-endorsed its recommendations originally proposed during its 25th meeting (April 2007) for changes in the SI electrical units (Recommendation E-1). Among its considerations, the document stated that the representation of the volt using the Josephson effect and the conventional value of the Josephson constant, KJ−90, has provided practical, accessible, reproducible, low noise and highly linear references worldwide since 1990. We summarize the measurement results of comparisons between Josephson voltage standards that help support this statement, especially concerning the accuracy and the simplicity of the Josephson relationship between voltage and frequency. We also detail a list of influence parameters and the ranges over which the Josephson relationship has been tested.

Direct comparison between a programmable and a conventional Josephson voltage standard at the level of 10?V

A Josephson voltage standard (JVS) from the BIPM, incorporating a PTB 10 V programmable array, has been compared with a JVS from the LNE, operating with a conventional hysteretic array. The metrological characteristics of both JVSs were carefully studied. We report on the experiments performed for this comparison and on the uncertainty components. The final comparison result is (UPJVS − UJVS)/UJVS = −3.2 × 10−11 with a relative total combined standard uncertainty of 6.1 × 10−11 (k = 1).

Allan Variance Analysis of Josephson Voltage Standard Comparison for Data Taken at Unequal Time Intervals

Stochastic serial correlations are often ignored in the statistical uncertainty (Type A) analysis of measurement values. For repeated measurements, the standard deviation of the mean is often used to specify Type-A uncertainty, but it is frequently underestimated by assuming that it is given by the standard deviation divided by the square root of the number of measurements. Methods of time-series analysis such as the Allan variance (Avar) formalism give realistic estimates of Type-A uncertainty, but they require equal time intervals between successive measurements. This requirement is difficult to satisfy in comparisons of Josephson voltage standards (JVSs) because of the instability of the voltage steps and the small number of repeated measurements. A JVS comparison was made using a completely automatic compact JVS. The time intervals were uneven; thus, we studied the effect of their irregularity on the Avar by simulating data having the same noise model and uneven time intervals as the measured data. We found that, for this JVS comparison, the Avar is only slightly affected by uneven intervals. The noise was found to be a mixture of white and 1/f noise. The latter limits the Allan deviation to 0.64 nV after 14.3 h of measurement. The method of simulating data this way should be applicable to other complex measurement situations.

Limits to the Accuracy of 10-V Josephson Standards Revealed by BIPM On-Site Comparisons

In September 2004, the Bureau International des Poids et Mesures (BIPM) began a new round of its large-scale international comparison of Josephson array voltage standards with national metrology institutes. As an option, it offers a new simplified measurement procedure, whereby the other participants simply measure the BIPM array as if it were an electronic voltage standard. This presentation discusses the accuracy of JAVS, the problems that were encountered in the new round, and how most of them were resolved

Alignment Procedure Used in the BIPM Watt Balance

The Bureau International des Poids et Mesures (BIPM) is developing a watt balance to measure accurately the Planck constant h by linking it to the kilogram. The kilogram is currently the only base unit of the International System of Units still defined by a material artifact. A fixed numerical value of h could be the basis for the expected future redefinition of the kilogram in terms of fundamental constants. One main concern of the watt balance experiment is the alignment of the involved vectors. In particular, the electromagnetic force exerted on the watt balance coil needs to be carefully aligned parallel to the gravitational force exerted on the test mass. To address this requirement, we have developed an original experimental procedure that has been validated with a test coil which is a prototype for the ultimate watt balance coil. The result is accurate enough to enable a determination of the Planck constant with a relative uncertainty of about 1·10-8 , which is the ultimate goal of the BIPM watt balance.

The BIPM watt balance: Improvements and developments

This paper summarizes the various improvements to and new developments for the BIPM watt balance which have been implemented over the past two years. The main distinctive feature of the BIPM apparatus is its capability for simultaneous force and velocity measurements. The fact that all the major components of the apparatus are now available (although not yet necessarily in final form) has enabled crucial tests of this. In addition we expect to soon make a first determination of the Planck constant h, albeit with a relatively large uncertainty.

The BIPM compact Josephson voltage standard

The BIPM has developed a new Josephson Voltage Standard (JVS) that will replace the present transportable standard in the near future. The old system has been involved in 17 international comparisons at the level of 10 V in the framework of the BIPM direct on-site JVS key comparison since 1994. The heart of the new primary standard will still consist of arrays of SIS junctions. It will be able to use future 10 V programmable arrays with minor modifications. We present here the technical details of the key parts of the compact primary JVS. The preliminary results of a direct comparison against the current BIPM 10 V primary reference have shown an agreement within a total combined relative uncertainty of 1×10−10 (k = 1).