Table of contents

Deviations from flatness of the gauging surfaces of material artefacts do not practically affect the results of new parallax-free methods of length measurements based on optical differential measurements (DM). The surface texture deformations, resulting from the wringing procedure of a gauge block to a reference plate and defined for the wringing contact without an excessive wringing film thickness, are highly reproducible and can be included in the definition of the mechanical length of a block without any tangible increase in the total uncertainty.

This communication invites attention to a simple differential gas thermometer at liquid helium temperatures for detecting the heat power released at either end of some heat resistor. In conjunction with the most sensitive models of commercially available differential pressure sensors, this type of cryogenic calorimeter promises sub-nanowatt sensitivity with a time constant of heat relaxation of the order of 60 s.

This paper describes improvements to a Hilger gauge block interferometer, including a fibre optic feed for a traceable laser radiation, use of a CCD camera for computer-controlled acquisition of fringe images and automatic analysis of the fringe image to obtain the fringe fraction. The gauge measuring process is now automated, computer-controlled, more accurate and significantly faster and easier to operate than the original design.

The objective of this paper is to provide general principles that should guide any future discussions regarding the redefinition of the SI base units. The suggestion is to use the envisaged redefinition of the unit of mass, the kilogram, as an opportunity to generally check the compliance of all base units with modern demands and the needs of economy, science and society. In order to meet these requirements it is proposed principally to separate the 'carrier' of the definition and the 'carriers' of the realization of the SI units. By providing an example of a revised SI it is shown that these considerations could add value to the International System of Units.

We compared the IMGC and NIST standards for small gas flows at the IMGC. The IMGC standard is a recently developed primary flow meter that extracts a large piston out of a temperature-controlled chamber. Controlling the piston's speed holds the pressure in the chamber constant. The NIST standard is a recently developed transfer standard that was calibrated against two primary standards at NIST. The first NIST primary extracts a large piston out of an oil-filled chamber in which a metal bellows is suspended. Controlling the piston's speed holds the pressure in the bellows constant. The second NIST primary is a static gravimetry method.

The results include 49 nitrogen flow rates from 0.22 µmol s⁻¹ to 770 µmol s⁻¹ (1 µmol s⁻¹ = 1.3448 cm³ min⁻¹ of an ideal gas at the 'standard' conditions of 0 °C and 101 325 Pa). For all but one of the comparisons, the agreement between IMGC and NIST is better than 0.06%.

The von Klitzing constant R_K has been previously determined at the Bureau National de Métrologie (BNM) in 1986 and 1993. Considering the recommendation of the Comité Consultatif d'Electricité working group on the quantum Hall effect in 1988, as well as the differences between the values of the fine structure constant obtained from different methods, it appears to be particularly important that we continue to develop and improve the direct determinations of R_K.

Since 1993, the BNM calculable capacitor, comparison bridges and transfer standards have been considerably modified and the value was determined anew in 1999–2000. The result is R_K = R_K−90×(1 + 4.1 × 10⁻⁸) (1σ = 5.3×10⁻⁸)

This paper describes the different investigations, modifications and characterizations of the measurement chain elements that were carried out during the last fifteen years in order to obtain this result.

In 2000, the Bureau National de Métrologie (BNM, France) decided to develop a new watt balance experiment. Among numerous design studies, the choice of the transfer mass is particularly important. Because of the proximity to a source of high magnetic intensity, this mass must have a magnetic susceptibility as weak as possible. Gold–platinum alloy seems to meet this requirement, as well as additional requirements for density and hardness values, making it a possible candidate for mass standard realization. Five different gold–platinum alloys were studied, their volume magnetic susceptibility ranging from −2.8×10⁻⁵ to −2.1×10⁻⁵ for two of them and from +1.1×10⁻⁵ to +8.8×10⁻⁵ for the other three.

A measurand θ of interest is the ratio of two other quantities, μ_p and μ_q. A measurement experiment is conducted and results P and Q are obtained as estimates of μ_p and μ_q. The ratio Y = P/Q is generally reported as the result for the measurand θ. In this paper we consider the problem of computing an uncertainty interval for θ having a prescribed confidence level of 1−α. Although an exact procedure, based on an approach due to Fieller, is available for this problem, it is well known that this procedure can lead to unbounded confidence regions in certain situations. As a result, practitioners often use various non-exact methods. One such non-exact method is based on the propagation-of-errors approach described in the ISO Guide to the Expression of Uncertainty in Measurement to calculate a standard uncertainty u_y for Y. A confidence interval for θ with a presumed confidence level of 95% is obtained as [Y−2u_y, Y+2u_y]. In this paper we develop a highly accurate approximation for the coverage probability associated with the interval [Y−ku_y, Y+ku_y]. In particular, we demonstrate that, using n−1 degrees of freedom for u_y, and the corresponding Student's t coverage factor k = t_{1−α/2 : n−1} rather than k = 2, leads to uncertainty intervals [Y−t_{1−α/2 : n−1}u_y, Y+t_{1−α/2 : n−1}u_y], that are nearly identical to Fieller's exact intervals whenever the measurement relative uncertainties are small, as is the case in most metrological applications. In addition, they are easy to compute and may be recommended for routine use in metrological applications. Improved coverage factors k can be derived based on the results of this paper for those exceptional situations where the t-interval may not have coverage probability sufficiently close to the desired value.

The classical time transfer method used to realize International Atomic Time (TAI) is based on the common view technique, with GPS observations collected by C/A code receivers. The resulting clock offsets between the laboratory clock and GPS time are obtained from a fixed procedure defined by the Consultative Committee for Time and Frequency (CCTF). A similar procedure can be applied to the Receiver INdependent EXchange (RINEX) observation files produced by geodetic receivers driven by a stable external frequency. If the link between the receiver clock and the external clock is stable and precisely determined, the geodetic receivers can then be used for time transfer to TAI. In that case, we propose some modifications to the CCTF procedure to adapt it for the links between geodetic receivers, in order to take advantage of the P codes available on L1 and L2. This new procedure forms the ionosphere-free combination of the P1 and P2 codes as given by the 30 s RINEX observation files, the standard of the International GPS Service. The procedure is tested using the Ashtech Z-XII3T geodetic receivers and the results are compared with those obtained with the classical CCTF procedure based on the C/A code by computing the fractional frequency stability (Allan deviation) of the time links. Over short baselines, the two techniques are equivalent, while the new technique provides a factor 2 improvement for a transatlantic time link. For time links between a time receiver and a geodetic receiver, the differential satellite delays (P1-C/A or P2-C/A) must additionally be introduced. We show here that these biases do not, however, alter the long-term (>3 days) stability of the time transfer results. The corrections associated with tidal station displacement are also investigated, and the results indicate that they do not significantly improve the results at the present level of precision.

A procedure for linking the results of a Regional Metrology Organisation (RMO) key comparison to those of a related Comité International des Poids et Mesures (CIPM) key comparison is proposed. The RMO results are linked to the CIPM results by a factor which is determined as the ratio of the CIPM key comparison reference value and the weighted mean of the RMO results of the linking laboratories. Correlations of the results of the linking laboratories in the two comparisons are taken into account. The uncertainties associated with the linked RMO results and the degrees of equivalence (DOEs) are explicitly given.

The influence of correlations of the results of the linking laboratories in both comparisons is examined. It is shown that these correlations can decrease the linking uncertainty, whereas DOEs are expected to be influenced less.

The proposed linking procedure is illustrated by its application to linking the results of a recent CIPM key comparison on accelerometer calibrations to that of a corresponding RMO key comparison.

A method is described to relate the parameters appearing in interpolation equations for radiation thermometry directly to the radiometric characteristics of the thermometer itself. It is shown that for sufficiently narrow bandwidths these parameters are independent of the shape of the spectral responsivity and can be expressed solely in terms of its mean wavelength and bandwidth as determined by the variance. This allows the parameters to be determined either by direct measurement of the spectral responsivity, by measurements at fixed points, or by a combination of the two, in effect unifying the ITS-90, interpolation, and absolute thermometry methodologies for determining temperature. The development of high-temperature metal–carbon and metal-carbide–carbon eutectic fixed points means that temperatures up to and exceeding 3500 K may be determined by implementing a single simple interpolation equation in a variety of ways.

Figure 7 in this paper is inaccurate. The correct figure 7 is shown below.

Figure 7. Allan deviation plot of each Block IIR satellite clock as well as a plot of the average Allan deviation of the Blocks II/IIA cesium and II/IIA rubidium satellites during December 2001. All clocks are referenced to IGST and no data editing has been applied.

The beginning of table 1 on page 580 should read:

and not:

Evaluating the Measurement Uncertainty is a book written for anyone who makes and reports measurements. It attempts to fill the gaps in the ISO Guide to the Expression of Uncertainty in Measurement, or the GUM, and does a pretty thorough job. The GUM was written with the intent of being applicable by all metrologists, from the shop floor to the National Metrology Institute laboratory; however, the GUM has often been criticized for its lack of user-friendliness because it is primarily filled with statements, but with little explanation. Evaluating the Measurement Uncertainty gives lots of explanations. It is well written and makes use of many good figures and numerical examples. Also important, this book is written by a metrologist from a National Metrology Institute, and therefore up-to-date ISO rules, style conventions and definitions are correctly used and supported throughout. The author sticks very closely to the GUM in topical theme and with frequent reference, so readers who have not read GUM cover-to-cover may feel as if they are missing something.

The first chapter consists of a reprinted lecture by T J Quinn, Director of the Bureau International des Poids et Mesures (BIPM), on the role of metrology in today's world. It is an interesting and informative essay that clearly outlines the importance of metrology in our modern society, and why accurate measurement capability, and by definition uncertainty evaluation, should be so important. Particularly interesting is the section on the need for accuracy rather than simply reproducibility.

Evaluating the Measurement Uncertainty then begins at the beginning, with basic concepts and definitions. The third chapter carefully introduces the concept of standard uncertainty and includes many derivations and discussion of probability density functions. The author also touches on Monte Carlo methods, calibration correction quantities, acceptance intervals or guardbanding, and many other interesting cases. The book goes on to treat evaluation of expanded uncertainty, joint treatment of several measurands, least-squares adjustment, curve fitting and more. Chapter 6 is devoted to Bayesian inference.

Perhaps one can say that Evaluating the Measurement Uncertainty caters to a wider reader-base than the GUM; however, a mathematical or statistical background is still advantageous. Also, this is not a book with a library of worked overall uncertainty evaluations for various measurements; the feel of the book is rather theoretical. The novice will still have some work to do—but this is a good place to start. I think this book is a fitting companion to the GUM because the text complements the GUM, from fundamental principles to more sophisticated measurement situations, and moreover includes intelligent discussion regarding intent and interpretation. Evaluating the Measurement Uncertainty is detailed, and I think most metrologists will really enjoy the detail and care put into this book.

Jennifer Decker