Table of contents

Since its first edition, published in 1984, the International Vocabulary of Metrology (VIM) has become a landmark for the language of measurement, and in its three editions it has evolved together with the evolution of measurement science and its applications. This paper discusses the fundamental features of the VIM as a concept system and proposes some highlights about the way in the VIM some basic and general concepts of measurement have changed their definitions in the last thirty years.

The explicit use of the number '1' in quotient-based expressions of units for intensive quantities involving counting, together with a full description of the quantity being measured, is recommended as an improvement on product-based expressions that omit the number 1 and do not fully explain the quantity presented. The benefits of these recommendations over proposals for new 'units' as special names for 'one' is also highlighted. In particular the nomenclature emphasizes the aspect of pure number counting associated with these quantities and helps to provide a distinction between intensive and extensive quantities which would otherwise be lost, especially if the quantity being measured is not fully described.

While a new type of oscillating-cup viscometer which uses a rotating magnetic field has been developed and tested, the uncertainty of such measurements has been considered to only a cursory level. This paper reports work in which the sensitivities of this new method's output are studied for the measurement of mercury with a particular device. It is concluded from the analysis that while the new method requires dramatically shorter measurement times than previous methods, the resultant uncertainty seems to be nearly the same as the classical oscillating cup method. As such, since the results are just as precise as other oscillating-cup methods, the new method should be employed for its speed advantage, allowing the viscosity of materials as functions of temperature or pressure to be economically determined.

A certified reference material (CRM) for poly(ethylene glycol) with no distribution in the degree of polymerization was developed. The degree of polymerization of the CRM was accurately determined to be 23. Supercritical fluid chromatography (SFC) was used to separate the molecularly uniform polymer from a standard commercial sample with wide polydispersity in its degree of polymerization. Through the use of a specific fractionation system coupled with SFC, we are able to obtain samples of poly(ethylene glycol) oligomer with exact degrees of polymerization, as required for a CRM produced by the National Metrology Institute of Japan.

We report on the relative length fluctuation of two fixed-spacer Fabry–Pérot cavities with mirrors fabricated from silica/tantala dielectric coatings on fused silica substrates. By locking a laser to each cavity and reading out the beat note $\hat{\nu}={{\nu}_{1}}-{{\nu}_{2}}$ of the transmitted beams, we find that, for frequencies from 10 Hz to 1 kHz, the power spectral density of beat note fluctuation is ${{S}_{{\hat{\nu}}}}(f)={{\left(0.5\text{Hz}\right)}^{2}}/f$. By careful budgeting of noise sources contributing to the beat note, we find that our measurement is consistent with the fluctuation in this band being dominated by the Brownian noise of the mirror coatings. Fitting for the coating loss angle ϕ_c, we find it equal to 4 × 10⁻⁴. We then use a Bayesian analysis to combine our measurement with previous observations, and thereby extract estimates for the individual loss angles of the silica and tantala constituents of these coatings. With minor upgrades, the testbed described in this article can be used in the future to measure the length noise of cavities formed with novel mirror coating materials and geometries.

Quantum Hall effect (QHE) is the basis of modern resistance metrology. In quantum Hall array resistance standards (QHARS), several individual QHE elements, each one with the same QHE resistance (typically half of the von Klitzing constant), are arranged in networks that realize resistance values close to decadic values (such as 1 kΩ or 100 kΩ), of direct interest for dissemination. The same decadic value can be approximated with different grades of precision, and even for the same approximation several networks of QHE elements can be conceived. This paper investigates the design of QHARS networks by giving methods to find a proper approximation of the resistance of interest, and to design the corresponding network with a small number of elements; results for several decadic case examples are given. The realization of these networks with multiterminal QHE elements requires a new multiple bridge connection, here described.

The International System of Units (SI) is supposed to be coherent. That is, when a combination of units is replaced by an equivalent unit, there is no additional numerical factor. Here we consider dimensionless units as defined in the SI, e.g. angular units like radians or steradians and counting units like radioactive decays or molecules. We show that an incoherence may arise when different units of this type are replaced by a single dimensionless unit, the unit 'one', and suggest how to properly include such units into the SI in order to remove the incoherence. In particular, we argue that the radian is the appropriate coherent unit for angles and that hertz is not a coherent unit in the SI. We also discuss how including angular and counting units affects the fundamental constants.

Measurements of mechanical quantities such as pressure often take place under dynamic conditions, yet no traceable standards for the primary dynamic calibration of pressure sensors currently exist. In theory, shock tubes can provide a close to perfect step-function ideal for the calibration of pressure transducers. In this paper we investigate a system consisting of a shock tube and an ultra-fast fiber-optical sensor that is designed to be a future primary system for dynamic pressure calibrations. For reference, the fiber-optical sensor is compared to a piezoelectric sensor, and their corresponding frequency spectra are calculated. Furthermore, an investigation of the repeatability of the fiber-optical sensor, as well as a comparison with a second shock tube, is performed.

Four platinum resistance thermometer data sets of resistance ratio as a function of temperature from 13.8033 K to 273.16 K are fitted to two different functions. One has the form of the ITS-90 reference function (the 'Kemp model') while the second is the mathematically more complex model of Nicholas. Both are shown to fit the resistivity data well, though the residuals for the Kemp model are lower. Both models also cope well with adjustments to the input temperatures consistent with the most recent consensus estimates of T–T₉₀. In particular, the refitting of the data on which the low-temperature reference function of the ITS-90 is based to the Kemp functional form provides updated coefficients that could be part of a revised International Temperature Scale, or incorporated into the Mise en pratique for the kelvin. The residuals of both the Kemp and Nicholas models have similar magnitudes whether fitting either the original (W, T₉₀) data pairs or the (W, T) data pairs obtained by adjusting the temperatures to incorporate the recent estimates of (T–T₉₀). On that basis, there appears to be no evidence to suggest that either of the models is useful as an arbiter of thermodynamic temperature data—a reduction in the magnitude of the residuals might have been expected after revising the temperatures of the data pairs for better accord with thermodynamic temperature if a discriminating model function had such predictive ability.

Results are presented from a series of comparisons between two independent femtosecond frequency comb systems at NPL, which were carried out in order to assess their systematic uncertainty. Simultaneous measurements with the two systems demonstrate agreement at the level of 5 × 10⁻¹⁸ when measuring an optical frequency against a common microwave reference. When simultaneously measuring the ratio of two optical frequencies, agreement at the 3 × 10⁻²¹ level is observed. The results represent the highest reported level of agreement to date between Ti:Sapphire and Er-doped femtosecond combs. The limitations of the combs when operating in these two different manners are discussed, including traceability to the SI second, which can be achieved with an uncertainty below 1 × 10⁻¹⁶. The technical details presented underpin recent absolute frequency measurements of the ⁸⁸Sr⁺ and ¹⁷¹Yb⁺ optical clock transitions at NPL, as well as a frequency ratio measurement between the two optical clock transitions in ¹⁷¹Yb⁺.

Measuring amplifiers are used for transducer output signal conditioning in many dynamic measurement applications. For a traceable measurement, a calibration of all components of the measuring chain—and therefore of the conditioning amplifiers, too—is mandatory. In this paper methods for a dynamic calibration of different types of conditioning amplifiers are presented. Measurement uncertainties and calibration results for typical amplifiers are discussed.

In this paper we describe a new solution of active delay stabilization for fibre-optic distribution of time and RF-frequency signals, which allows one to obtain both high precision and a potentially unlimited range of compensation of the fibre delay fluctuations. The solution is based on a hybrid system exploiting a pair of continuously tuned electronic variable delay lines, and a set of switched optical delays. We present a fully operational prototype of the time and frequency distribution setup based on this idea, which is capable of compensating more than 1 µs of the fiber delay fluctuations, and thus may be used in very long-haul links up to about 1000 km, without the need for any seasonal maintenance. We also report measurements of the time and frequency distribution stability, and the verification of the time transfer calibration.

The surface contamination of various metallic samples upon cyclic transfer from vacuum to ambient pressure air or dry nitrogen was studied using x-ray photoelectron spectroscopy (XPS) for surface chemical analysis. The venting methods were vacuum–air–vacuum, vacuum–N₂–air–vacuum and vacuum–N₂–vacuum. In addition, the dependence on the initial surface chemical state was investigated by using different cleaning methods such as the traditional washing and cleaning (nettoyage-lavage), low-pressure hydrogen plasma and UV/ozone as starting conditions. We found that cyclic exposure to air after UV/ozone and nettoyage-lavage has very little effect on the surface contamination. In contrast, hydrogen plasma removes the initial contamination almost completely. Until a first layer of contaminants is built up, the growth is very rapid but slows down exponentially with the number of cycles. Exposure directly to ambient pressure air causes the fastest and largest recontamination, followed by the cyclic transfer vacuum–N₂–air–vacuum. Venting with dry nitrogen exclusively exhibits the slowest growth of contaminants but continues with further cycling until the first layer is completed. A list of precedence for existing and potential new materials and for the processes applied is presented in order to test and evaluate the best materials for future primary mass standards and to find the best working practice.

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) μK when using neon of 'natural' isotopic composition or to 30 μK 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.

The National Institute of Metrology has developed a new oil manometer that covers the absolute pressure range from 100 Pa up to 10 kPa. The manometer is based on the ultrasonic measurement of transit time in oil columns, and a novel dual U-tube system has been designed to measure the speed of sound in real time as the pressure changes. The working fluid, di-2-ethylhexyl sebacate, was chosen for its sufficiently low vapor pressure and low sound attenuation. Each tube has a coating of Teflon to resist wetting by the oil. To obtain a uniform and stable temperature environment, the dual U-tube system is located inside a guard vacuum chamber that is wrapped with foam and aluminium foil. A vertical temperature difference of less than 20 mK, a horizontal temperature difference of less than 5 mK and a temperature stability better than 10 mK were achieved. The overall standard (k = 1) uncertainty of the oil manometer is estimated to be approximately (0.015 + 1.63  ×  10⁻⁵ p Pa⁻¹) Pa for absolute pressure measurements.

Reflection and transmission of microwaves in coaxial devices are usually described by S-parameters. The current definition of S-parameters requires that the reference plane is in a section of ideal wave guide. Due to this, tremendous effort is necessary to facilitate the dissemination of standards, for the comparison of measurement values and for cascading devices. These processes can be simplified by extending the definition of S-parameters to reference planes in sections of non-ideal wave guide, e.g. in connectors. Extended S-parameters can be approximated with conventional simulation programs. Practical experiments show that extended S-parameters can be compared and cascaded without effort.

The inhomogeneities within a thermocouple influence the measured temperature and contribute the largest component to uncertainty. Currently there is no accepted best practice for measuring the inhomogeneities or for forecasting their effects on real-world measurements. The aim of this paper is to provide guidance on the design and performance assessment of thermocouple inhomogeneity scanners by characterizing the qualitative performance of the various designs reported in the literature, and developing a quantitative measure of scanner resolution. Numerical simulations incorporating Fourier transforms and convolutions are used to gauge the levels of attenuation and distortion present in single- and double-gradient scanners. Single-gradient scanners are found to be far superior to double-gradient scanners, which are unsuitable for quantitative measurements due to their blindness to inhomogeneities at many spatial frequencies and severe attenuation of signals at other frequencies. It is recommended that the standard deviation of the temperature gradient within the scanner is used as a measure of the scanner resolution and spatial bandwidth. Recommendations for the design of scanners are presented, and include advice on the basic design of scanners, the media employed, operating temperature, scan rates, construction of survey probes, data processing, gradient symmetry, and the spatial resolution required for research and calibration applications.

Methods of absolute radiometric calibration of backscatter ultraviolet (BUV) satellite instruments are compared as part of an effort to minimize pre-launch calibration uncertainties. An internally illuminated integrating sphere source has been used for the Shuttle Solar BUV, Total Ozone Mapping Spectrometer, Ozone Mapping Instrument, and Global Ozone Monitoring Experiment 2 using standardized procedures traceable to national standards. These sphere-based spectral responsivities agree to within the derived combined standard uncertainty of 1.87% relative to calibrations performed using an external diffuser illuminated by standard irradiance sources, the customary spectral radiance responsivity calibration method for BUV instruments. The combined standard uncertainty for these calibration techniques as implemented at the NASA Goddard Space Flight Center's Radiometric Calibration and Development Laboratory is shown to less than 2% at 250 nm when using a single traceable calibration standard.

We discuss the treatment of the systematic frequency shifts due to microwave lensing and distributed cavity phase in Heavner et al 2014 Metrologia51 174–82. We explain that the microwave lensing frequency shift is generally non-zero and finite in the limit of no applied microwave field. This systematic error was incorrectly treated and we find that it contributes a significant frequency offset. Accounting for this shift implies that the measured microwave amplitude dependence (e.g. due to microwave leakage) is comparable to the total reported inaccuracy. We also discuss the importance of vertically aligning the fountain perpendicular to the axis of the cavity feeds, when the cavity has only two independent feeds. Finally, we note that background gas collisions have a different behavior for cold clock atoms than for clock atoms at room-temperature, and therefore room temperature measurements do not directly apply to laser-cooled clocks.