Table of contents

Recently, Kose et al (2003 Metrologia40 146–53) proposed 'general principles that should guide any future discussions regarding the redefinition of the SI base units' and they illustrated their principles with an example of a revised SI. Part of their example (redefinition of the kelvin in terms of the triple point of hydrogen) is not consistent with their objective that all base units comply with '...the needs of economy, science and society'. An alternative revision of the SI might not define the kelvin as an independent unit.

Degradation corrections to solar irradiance measurements in space along with a postulated 'hyperbolic' fitting formula were recently reported (Fröhlich 2003 Metrologia40 S60–5). A simpler, alternative formula based on broadband synchrotron exposure experiments on polymeric mixtures is presented. The alternative formula is shown to be similar to the hyperbolic function in the high photon dose limit. These results complement and reinforce the physical validity of the previously reported degradation corrections.

Gas temperature gradients created during the filling stage of a pressure–volume–temperature–time (PVTt) calibration cycle, and those imposed by inhomogeneous room conditions, lead to uncertainties in the average gas temperature in the collection tank. Because these temperature uncertainties dominate the overall flow uncertainty, NIST upgraded the temperature-averaging scheme used in its 26 m³PVTt system. Instead of arithmetically averaging 10 thermistors to obtain the mean gas temperature, we now calculate this value via a volume-weighted trapezoidal integration procedure using 35 thermistors. Applying the new temperature-averaging scheme, the mean gas temperature can be determined with a standard uncertainty of 89 mK after only 2700 s of fan mixing. As a result, the flow uncertainty in the NIST 26 m³PVTt system has decreased from 0.22% to 0.13% (with a coverage factor of 2). This paper highlights the temperature improvements and presents a detailed analysis for estimating the lower temperature uncertainty.

This paper describes the main tests and precautions necessary for both reproducible and accurate results in the use of the quantum Hall effect as a means to establish a reference standard of dc resistance having a relative uncertainty of a few parts in 10⁹.

We present an evaluation of the frequency shifts in an optically pumped caesium-beam frequency standard caused by the neighbouring transitions. We analyse the shifts caused by the Rabi and Ramsey pulling effects and find a well-known result for the Rabi pulling but a new result for the Ramsey pulling. The revised Ramsey shift is found to be proportional to not the imaginary but the real part of the ground-state atomic coherence. The value of the Ramsey shift is proportional to the first power of the Rabi frequency defined by a small orthogonal component of the microwave field and the inverse value of the Zeeman splitting. We compare the shifts caused by the neighbouring transitions and show that the relative contributions of the Rabi and Ramsey pulling effects crucially depend on the quality of optical pumping. We estimate that the Rabi shift for the KRISS-1 caesium-beam frequency standard is at a relative level of 10⁻¹⁴ while the Ramsey shift is at a relative level of 10⁻¹².

Experiments, described below, revealed certain shortcomings of the current definition of the Al point: pressure in sealed type Al cells made of quartz was observed prior to their sealing and was found to steadily decrease. Consequently, the realized Al points using sealed quartz cells are in a state of drift. In addition, the initial Al point values yielded by open cells are higher than the subsequently realized ones: five different gases were used to apply the initial 1 atm pressure over the Al samples (air, N₂, Ar, He, CO₂). Each of them appeared to contaminate slightly the Al samples, lowering the initial value of the Al point by 1 or 2 mK. However, sealed quartz triple point cells remained stable at their initial value indefinitely. Thus, the triple point of Al is a more reliable fixed point than the defining Al freezing point itself.

The International Organization for Standardization (ISO) Guide to the Expression of Uncertainty in Measurement is being increasingly recognized as a de facto international standard. The ISO Guide recommends a standardized way of expressing uncertainty in all kinds of measurements and provides a comprehensive approach for combining information to evaluate that uncertainty. The ISO Guide supports uncertainties evaluated from statistical methods, Type A, and uncertainties determined by other means, Type B. The ISO Guide recommends classical (frequentist) statistics for evaluating the Type A components of uncertainty; but it interprets the combined uncertainty from a Bayesian viewpoint. This is inconsistent. In order to overcome this inconsistency, we suggest that all Type A uncertainties should be evaluated through a Bayesian approach. It turns out that the estimates from a classical statistical analysis are either equal or approximately equal to the corresponding estimates from a Bayesian analysis with non-informative prior probability distributions. So the classical (frequentist) estimates may be used provided they are interpreted from the Bayesian viewpoint. The procedure of the ISO Guide for evaluating the combined uncertainty is to propagate the uncertainties associated with the input quantities. This procedure does not yield a complete specification of the distribution represented by the result of measurement and its associated combined standard uncertainty. So the correct coverage factor for a desired coverage probability of an expanded uncertainty interval cannot always be determined. Nonetheless, the ISO Guide suggests that the coverage factor may be computed by assuming that the distribution represented by the result of measurement and its associated standard uncertainty is a normal distribution or a scaled-and-shifted t-distribution with degrees of freedom determined from the Welch–Satterthwaite formula. This assumption may be unjustified and the coverage factor so determined may be incorrect. A popular convention is to set the coverage factor as 2. When the distribution represented by the result of measurement and its associated standard uncertainty is not completely determined, the 2-standard-uncertainty interval may be interpreted in terms of its minimum coverage probability for an applicable class of probability distributions.

In the 1990s the CCQM (Comité consultatif pour la quantité de la matière) defined the term 'primary method'. It also identified some methods that are considered to be primary. One of these methods is titrimetry. In this paper, the principle of titrimetry and two practical examples performed at the highest metrological level in the frame of key comparison CCQM-K8 are discussed. It is evident that titrimetry is a very powerful method leading to highly accurate results when performed correctly, and does not depend on whether the classical metrological approach or a new experimental design using modern laboratory technology is applied.

The term 'metrology' in its modern meaning of 'science of measurement' has come into use only since the journal Metrologia was founded. Before this date, 'metrology' and synonymous terms which are likewise derived from the Greek word for measure were used in historical investigations on weights and measures only. In the present global development of science and economics, metrology plays a key role with the BIPM performing significant coordination tasks.

The inherent curvature of the wavefronts in the laser beams used in most absolute gravimeters introduces a systematic reduction in the measured gravity value. This error depends strongly on the diameter of the laser beam, but is typically of the order of 1–5 µGal. This paper describes the origin of this reduction, and provides a correction based on the laser beam diameter as well as a practical method to measure the beam's diameter.

An evaluation of the four-terminal-pair equal power 100 : 1, quadrature and 10 : 1 coaxial bridge systems for the traceable determination of NPL 10 pF primary capacitance standard from the DC quantum Hall effect is reported. This was instigated to resolve the small but significant discrepancies observed in the NPL measurements during the recent EUROMET and CCEM comparisons. A formal bilateral comparison between NPL and BIPM was conducted (involving EUROMET two-terminal-pair and BIPM three-terminal capacitance standards) to establish a new degree of equivalence between the two laboratories for the 10 pF and 100 pF capacitance measurements at 1 kHz and 1.592 kHz. The majority of results show agreement within the estimated 2σ combined relative standard uncertainty of measurement of the two laboratories. However, the best agreement was obtained for the EUROMET two-terminal-pair 10 pF and 100 pF capacitance standards with a mean difference between NPL and BIPM results of (21.9 ± 117) nF/F and (51.4 ± 117) nF/F, respectively, at 1.592 kHz.

A value for the Avogadro constant, N_A, was derived from new measurements of the lattice parameter, the density and the molar mass of a silicon single crystal. The result N_A = 6.022 135 3 × 10²³ mol⁻¹ has a relative measurement uncertainty and is in excellent agreement with other published data based on the x-ray crystal density molar mass method, indicating the high repeatability of these experiments. The value differs significantly from the Committee on Data for Science and Technology's most recent recommended value of 6.022 141 99 × 10²³ mol⁻¹ by more than 1 × 10⁻⁶ N_A.

At the Physikalisch-Technische Bundesanstalt, autocollimators are calibrated with the aid of the WMT 220 angle comparator to ensure their direct traceability to the SI unit of plane angle, the radian (rad). It is shown that high-resolution electronic autocollimators can thus be calibrated with an uncertainty of 0.007 arcsec (k = 2) in reproducible measurement steps down to 0.005 arcsec. Calibrations in very small measurement steps close to the autocollimators' resolution can inform about possible short-period deviations and resulting aliasing effects in measurements with autocollimators.