Table of contents

We report a comparison between two absolute gravimeters: the LNE-SYRTE cold atom gravimeter and FG5#220 of Leibniz Universität of Hannover. They rely on different principles of operation: atomic and optical interferometry. Both are movable which enabled them to participate in the last International Comparison of Absolute Gravimeters (ICAG'09) at BIPM. Immediately after, their bilateral comparison took place in the LNE watt balance laboratory and showed an agreement of (4.3 ± 6.4) µGal.

Heat flow transducers have been implemented within a thermometric fixed-point cell operated with indium. Then a local heat flow rate was monitored simultaneously with the classical temperature measurement. A specific configuration, so-called cell-within-cell, allowed us to control a melting transition plateau and to observe the behaviour of the heat exchanged during the whole process of the melting transition. Owing to their dynamic character, heat flow measurements prove to be the most appropriate for defining specific boundaries on a given transition plateau, and relevant sensors are found to be suitable tools to be coupled with enhanced temperature measurements. This piece of work is presented in two parts, with the description of the set-up and measurement results in this first paper and their subsequent thermodynamic implications in the following one.

This second part of the work is devoted to the thermodynamic implications of the results presented in the preceding paper, as heat flow meters (HFMs) were installed within a thermometric fixed-point cell operated with indium. Then, simultaneous measurements of both temperature and heat flow allow us to pinpoint the specific boundaries (solidus and melt-off points) of a given transition plateau. To the best of our knowledge, this work is the first experimental illustration being reported on how to consider a plateau as a specific thermodynamic regime marked with a minimum amount of generated entropy in a metrological fixed-point cell. As a result, a significant relationship linking the boundaries of the plateau and the melting temperature of the material studied is presented. In order to obtain a quantitative understanding of the quality of a given transition process, straightforward thermodynamic considerations allow us to consider both theoretically and experimentally an adimensional parameter defined as the ratio of the entropy generation over the entropy change.

An accuracy evaluation of the caesium fountain NPL-CsF2 as a primary frequency standard is reported. The device operates with a simple one-stage magneto-optical trap as the source of cold atoms. Both the uncertainty in and magnitude of the cold collision frequency shift are reduced by taking advantage of the dependence of the cross section on the effective collision energy in an expanding atomic cloud. The combined type B uncertainty (typically 4 × 10⁻¹⁶) is dominated by an estimate of the frequency shift due to the distributed cavity phase. When operated at single density, the short-term fractional frequency instability of NPL-CsF2 is 1.7 × 10⁻¹³ at 1 s and limited by the noise of the room temperature quartz-based local oscillator. During a typical frequency measurement campaign, the fountain is operated in an alternating mode at high and low density in order to measure and correct for a residual collision shift. This increases the effective fractional frequency instability to 5.4 × 10⁻¹³ at 1 s; consequently the averaging time required for the type A uncertainty level to match that of the type B is 20 days.

The variation of the response of instruments as a function of neutron energy has to be determined in well-characterized mono-energetic neutron fields. These fields are measured in terms of neutron fluence and mean energy of the mono-energetic neutron peak, providing values that are required to determine the related dosimetric quantities. At the IRSN AMANDE facility, the reference measurement standard for neutron energy is based on the time of flight (ToF) method, i.e. on a determination of the neutron velocity, requiring a pulsed beam and fast detectors. In this study, we describe the experimental and theoretical aspects of the development of this method for neutron energies above 1 MeV using liquid scintillators. We present the adopted experimental protocol and give full details of the estimation of uncertainties associated with the neutron energy measurement. To extract as accurately as possible the mean energy from the experimental neutron ToF distribution, a simulated energy distribution is fitted to the measurements. This simulation includes a detailed analysis of the energy resolution, taking into account all the contributions influencing the ToF measurements. Comparison with neutron energy calculated from kinematics leads to the conclusion that the ToF method at the AMANDE facility can be considered as a reference measurement standard for neutron energies between 1 MeV and 20 MeV, with a relative uncertainty lower than 1.5%.

We report on acoustic and microwave measurements made with a purified helium sample maintained close to a single thermodynamic state (T_exp ∼ 273.16 K, p_exp ∼ 410 kPa) within a 2.1 L volume stainless steel spherical cavity. From these measurements and ab initio calculations of the non-ideality and the refractive index of helium, we determine a value for the Boltzmann constant k_B which is consistent with the recommended 2006 CODATA value: (k_B − k₂₀₀₆)/k₂₀₀₆ = (−7.5 ± 7.5) × 10⁻⁶. We discuss the current limits of the experiment and the prospects of a further reduction in the uncertainty associated with the determination of k_B.

Platinum–iridium (Pt/Ir) kilogram mass prototypes are known to gain contamination from the environment in which they are stored. The current method of cleaning these mass prototypes is called nettoyage–lavage and involves the physical rubbing of a kilogram with a chamois leather cloth soaked in a solvent followed by removal of any solvent residue using a jet of steam water. The manual nature of the technique means the effectiveness of the cleaning process is reliant on the human operative. An alternative cleaning method involving exposure to ultraviolet light and ozone (UV/O₃) has been tested on Pt/Ir foils and kilogram mass prototypes. The changes to the surface of the Pt/Ir foils as a result of this process have been quantified using x-ray photoelectron spectroscopy and have shown a clear reduction in the quantity of carbonaceous contamination. Variation of the UV intensity, ozone concentration and exposure duration enabled the optimum cleaning conditions to be established. The UV/O₃ cleaning method was then used to clean two Pt/Ir kilogram mass prototypes and gravimetric weighing of the kilograms before and after cleaning gave the amount of contamination removed. These gravimetric weighing results demonstrated that UV/O₃ cleaning was as effective as the nettoyage–lavage process.

In view of the proposed redefinition of the kilogram, in 2005, 2007 and 2010 the CCM adopted recommendations and conditions to be met before the redefinition is implemented. Some of these conditions concern the uncertainty by which the kilogram will be realized after the redefinition. This uncertainty has become a point of discussion in other committees, because the needs for this value were not sufficiently transparent. Here, examples of the realization of the new kilogram with different uncertainties, along with the experimental results available from the Avogadro and watt balance experiments, the uncertainties of mass determinations and their propagation with reasonable assumptions are presented, in order to compare possible realization uncertainties with the requirements in practice. It turned out that the CCM conditions are reasonable, if major changes in the dissemination chain of the kilogram are to be avoided.

In 2004, the National Institute of Standards and Technology established the ultraviolet spectral irradiance scale from 200 nm to 400 nm using the calculable irradiance of the Synchrotron Ultraviolet Radiation Facility (SURF). Since the establishment of the scale, spectral irradiance calibrations of many customer lamps have been performed in direct comparison with synchrotron radiation. However, to ensure long-term stability of the scale, three check-standard deuterium lamps were calibrated using SURF III at the same time as customer lamps were being calibrated. Here, we present the results of the long-term monitoring of the scale using these check-standard lamps to ensure customer lamps are calibrated within the expanded calibration uncertainty (k = 2) of 1.2%.

This paper proposes a measure for assessing the equivalence between the results of two laboratories. The measure is called asymmetric interchangeability. It is asymmetric since, based on this measure, one laboratory may be considered interchangeable with another laboratory but the converse may not be true. Such a situation can arise when the accuracy and precision of one laboratory are noticeably greater than that of the other. The proposed measure of interchangeability depends on the parameters of the measurement models, which include means, variances and the correlation of the two laboratories being compared. Since the level of correlation is often difficult to assess, it is assumed to be zero in this paper. Fiducial procedures are presented for testing the hypothesis that a laboratory is directionally interchangeable with another. The procedure is based on comparing a probability measure, which is shown to be closely related to asymmetric interchangeability, with an agreed threshold. Computer programs for calculating the p-value of the tests, written by use of open-source software, are listed.

Three methods for the determination of the key comparison reference value (KCRV) and degree of equivalence of inconsistent comparison data are proposed in this study. These methods are, respectively, based on the premises of (1) unknown biases of individual measurement values, (2) underestimated uncertainties of individual participants or (3) additional and common uncertainty. Bayesian statistics were employed for the analysis using locally uniform priors. In the case of the first premise, Procedure B in the CIPM guidelines (2002 Metrologia39 589–95) can be derived in the Bayesian context. In the case of the second and the third premises, the weighted mean is a possible candidate for the KCRV. These methods are exemplified using the key comparison data of CIPM CCM.FF-K3 and APMP.L-K1. Markov chain Monte Carlo simulations were conducted for calculations based on the latter two premises. From the results obtained, it is considered that, in addition to Procedure B in the CIPM guidelines, the method based on the second premise is also a robust method for the estimation of the KCRV.

We present the connection of two programmable Josephson arrays generating synchronous waveforms to measure impedance ratios—the Josephson two-terminal-pair impedance bridge. This approach is more flexible than conventional bridges at the same level of uncertainty. The Josephson bridge can measure over a wider frequency range, over a wider range of impedance ratios than conventional two-terminal-pair bridges. Furthermore, the phase angle between the two impedances can take any value. As a first application, we present measurements of a 1 : 1 resistance ratio at the 10 kΩ level in the frequency range between 25 Hz and 10 kHz. The uncertainties are better than a few parts in 10⁸ and hence comparable to those of conventional impedance bridges. Quantization at up to 10 kHz was confirmed by varying the bias current of the Josephson arrays, resulting in constant resistance ratios within the measurement resolution.

A new method applying isotope dilution mass spectrometry to determine the molar mass of a silicon sample, highly enriched with respect to ²⁸Si, has been proposed recently. This paper describes a different way of calculating the molar mass by solving an equation linking the measurement results to the ratio between the amount-of-substance fractions of ²⁸Si and ³⁰Si in the sample. The mathematical model and the final measurement equation are much more straightforward than those previously reported, though fully equivalent.

A traceability chain for the derivation of the farad from dc quantum Hall effect has been recently implemented at INRIM. The main components of the chain are two coaxial transformer bridges: a resistance ratio bridge and a quadrature bridge, both operating at 1541 Hz.

Both bridges are energized and controlled by a polyphase sinewave direct-digital-synthesizer; the automated control of relative amplitudes and phases of the generator voltage outputs permits principal and auxiliary balances of each bridge to be achieved. Such a technique, which may be called digital assistance, is implemented for the first time in an ohm–farad traceability chain.

The relative standard uncertainty in the realization of the farad, at the level of 1000 pF, is estimated to be 64 × 10⁻⁹. A first verification of the realization is given by a comparison with the maintained INRIM capacitance standard, where an agreement within a relative combined standard uncertainty of 420 × 10⁻⁹ is obtained.

We report on a successful 20-year study, involving a national metrology institute and a commercial instrument manufacturer, in the field of gauge block measurement by optical interferometry. The National Physical Laboratory (NPL) has made measurements of length artefacts on its own equipment and on 30 copies of this instrument that are now in use worldwide. Several NPL length artefacts have been measured on all 31 instruments including repeated re-measurement of the artefacts on the same equipment at NPL, over a period of 20 years. This pool of data therefore allows for quantitative examination of the temporal stability of these standards as well as the long-term performance of the instruments and of the evolving instrument design. Many of the laboratories which now own these instruments have taken part in international comparisons under the Comité International des Poids et Mesures' Mutual Recognition Arrangement. Combining the data from these key comparisons with the data obtained by NPL allows for a novel analysis—these data can be regarded as coming from an international key comparison, where both the artefact and the measuring instrument are circulated amongst the participants—the remaining variability being due to operator skill at the different laboratories, and the quality of their independent traceability chains.

This paper reports work undertaken to assess the change in the mass values of stainless steel and platinum–iridium weights transferred between air and vacuum and to determine the repeatability of this change. Sets of kilogram transfer standards, manufactured from stainless steel and platinum–iridium and with different surface areas, were used to determine the effect of transfer between air and vacuum on the values of the mass standards. The SI unit of mass is the only unit of the seven base SI quantities which is still defined in terms of an artefact rather than by relation to a fundamental physical constant. Work is underway to identify a means of deriving the SI unit of mass from fundamental constants and at present the two principal approaches are the International Avogadro Coordination and the watt balance projects. Both of these approaches involve realizing a kilogram in vacuum and therefore the traceability from a kilogram realized in vacuum to mass standards in air is crucial to the effective dissemination of the mass scale. The work reported here characterizes the changes in mass values of standards on transfer between air and vacuum and thus will enable traceability to be established for an in-air mass scale based on a definition of the unit in vacuum.