Table of contents

The International System of Units (SI) was declared as a practical and evolving system in 1960 and is now 50 years old. A large amount of theoretical and experimental work has been conducted to change the standards for the base units from artefacts to physical constants, to improve their stability and reproducibility. Less attention, however, has been paid to improving the SI definitions, utility and usability, which suffer from contradictions, ambiguities and inconsistencies. While humans can often resolve these issues contextually, computers cannot. As an ever-increasing volume and proportion of data about physical quantities is collected, exchanged, processed and rendered by computers, this paper argues that the SI definitions, symbols and syntax should be made more rigorous, so they can be represented wholly and unambiguously in ontologies, programs, data and text, and so the SI notation can be rendered faithfully in print and on screen.

Circular dichroism (CD) is a spectroscopic technique that is widely used to obtain information about protein structure, and hence is an important tool with many applications, including the characterization of biopharmaceuticals. A previous inter-laboratory study, CCQM-P59, showed that there was a poor level of comparability between laboratories in CD spectroscopy. In a follow-up study reported here, we achieved our goal of demonstrating improved comparability and data quality, primarily by addressing the problems identified in the previous study, which included cell path-length measurement, instrument calibration and good practice in general. Multivariate analysis techniques (principal component analysis and soft independent modelling of class analogies) were shown to be useful in comparing large spectral data sets and in classifying spectra. However, our results also show that there is more work to be done to improve confidence in the technique as the discrepancies observed were partially due to systematic effects, which the statistical approaches do not consider. We therefore conclude that there is a need for an improved understanding of the uncertainties in CD measurement.

In this paper we review statistical models that describe measurements from a multiple-method study such as in the development of a reference material. We also review requirements for the so-called GUM compliance, as this appears to be an important criterion for choosing a model and a method for assigning a value to a measurand of interest and calculating its uncertainty. For each modelling approach, we identify groups of competing methods and evaluate their status with respect to their statistical characteristics and GUM compliance.

Surface temperatures are key parameters in many concentrated solar radiation applications. Pyrometric temperature measurement of solar irradiated material surfaces is the alternative to contact measurement techniques, which are inadequate for measuring the temperatures of such surfaces. However, reflected solar radiation is an important uncertainty variable in this non-contact methodology. A promising method for eliminating this solar perturbation is by using centred passband filters on the atmospheric solar absorption bands, creating solar-blind pyrometric systems. A commercial pyrometer has been tested in the wavelength band at around 1.4 µm in the solar furnace at Plataforma Solar de Almería, showing its advantages and limitations. An estimation of temperature measurement uncertainty for a real case is presented with theory and experiment in agreement: the higher the temperature, the lower the uncertainty. Another experiment has shown that the pyrometer measures temperature properly even through quartz windows in this spectral range.

As a single working chamber for a national air kerma standard was not considered safe and robust enough, a new set of six ionization chambers was devised to establish the French ⁶⁰Co air kerma standard. Although every new ionization chamber was treated as much as possible in the same way (manufacturing, measurements of volumes, wall effect calculations, current corrections), a maximum discrepancy of 0.2% was observed between the final measurement results from each chamber. The final value of the air kerma rate in reference conditions was determined as the mean value of the measurement results from all six chambers. Among the different factors whose determination is necessary to calculate the air kerma rate, some are considered independent of or common to all the graphite-walled ionization chambers (example: mean energy expended by an electron to produce an ion pair in dry air), while others vary for each chamber (example: air cavity ionic collection volume). Considering that the uncertainties of the individual ionization chamber measurement results seem slightly underestimated, the uncertainty on the mean of the products of the six chamber-dependent factors was taken equal to the standard deviation of the sample composed of the products of the six chamber-dependent factors (0.078%). Compared with the previous standard, the air kerma rate of the ⁶⁰Co photon beam would then increase by 0.09% and the air kerma rate uncertainty would drop from 0.38% to 0.31%.

Watt balance experiments are intended to link the mass unit to the Planck constant. Therefore, in these experiments, a reference mass standard is necessary. However, in the French apparatus, the platinum–iridium alloy that forms the international prototype of the kilogram cannot be considered because of its magnetic susceptibility. Consequently, new materials were investigated, according to stringent specifications. This led to the selection of alloys based on precious metals. The stability of the materials used in mass metrology is strongly dependent on bulk homogeneity and cleanliness: after the investigation of different production modes and heating treatments, microstructural and analytical studies were performed. The critical properties of the different materials were examined and discussed in comparison with the platinum–iridium alloy.

The SI unit of mass will probably be redefined within the next few years using an invariable natural constant. Nevertheless, dissemination of the kilogram will still be realized by weighing using physical weights prone to contamination. Published data on cleaning, humidity effects and long term stability of weights show large discrepancies, indicating that not all factors affecting adsorption characteristics of weights are known. In the work reported here, an atomic force microscope (AFM) was used to study surface effects of stainless steel weights at the nanometre scale. Effects of transfer between air and vacuum as well as effects of cleaning were studied by recording topography images of the surface before and after each procedure. An image processing method was developed for improving the sensitivity of detecting changes in images. Ultrasonic cleaning in ethanol removed contamination mainly from the grooves in the surface, while vacuum exposure caused contamination to build up in the grooves. The results show that the surface microstructure of stainless steel weights affects adsorption of contaminants in such a way that grooves seem to be preferential sites for adsorption. AFM has proven to be a valuable tool for studying surface effects of standard weights at ambient pressure with near nanometre resolution.

This paper reports results of comparison of three digital fringe signal processing methods implemented in the same free-fall absolute gravimeter. A two-sample zero-crossing method, a windowed second-difference method and a method of non-linear least-squares adjustment on the undersampled fringe signal are compared in numerical simulations, hardware tests and actual measurements with the MPG-2 absolute gravimeter, developed at the Max Planck Institute for the Science of Light, Germany. The two-sample zero-crossing method realizes data location schemes that are both equally spaced in distance and equally spaced in time (EST) along the free-fall trajectory. The windowed second-difference method and the method of non-linear least-squares adjustment with complex heterodyne demodulation operate with the EST data. Results of the comparison verify an agreement of the three methods within one part in 10⁹ of the measured gravity value, provided a common data location scheme is considered.

In this paper, the realization of an inductance scale from 1 µH to 10 H for frequencies ranging between 50 Hz and 20 kHz is presented. The scale is realized directly from a series of resistance standards using a fully automated synchronous sampling system. A careful systematic characterization of the system shows that the lowest uncertainties, around 12 µH H⁻¹, are obtained for inductances in the range from 10 mH to 100 mH at frequencies in the kilohertz range. This new measurement system, which was evaluated during an international comparison, provides a primary realization of the henry, traceable to the quantum Hall effect. An additional key feature of this system is its versatility. In addition to resistance–inductance (R–L) comparison, any kind of impedance can be compared: R–R, R–C, L–L or C–C, giving this sampling system a great potential for use in many laboratories around the world.

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