Table of contents

A generic method for the preparation of primary gas mixtures is described. This description forms the basis for a detailed analysis of all sources of uncertainty. Expressions are derived for the combined standard uncertainty in a single weighing cycle as well as for a transferred mass of gas. The expressions are rigorous, as they take into account all aspects, including dependencies between variables introduced by the preparation process. The strengths of these dependencies are evaluated in a generalized way, in order to find out whether the rigorous expressions can be simplified without limiting their validity under practical conditions. The model can be simplified by assessing the covariance between results of weighing cycles, as it can be demonstrated that this covariance will never become significant compared with the uncertainty in the result of a weighing.

With the aid of two diode-laser narrowband spectrometers around 633 nm, we report the measurement of the iodine hyperfine components of the R(60) 8-4, R(125) 9-4 and P(54) 8-4 transitions located −12 GHz from the classical red He-Ne laser stabilized on the R(127) 11-5 transition of ¹²⁷I₂, the P(33) 6-3 transition roughly −1 GHz away, the P(39) 6-3 transition 3.4 GHz above, and the R(59) 8-4 and P(53) 8-4 transitions roughly 19 GHz above. The Doppler-free absorption signals were resolved using a third-harmonic detection technique and hyperfine frequency splittings were measured by beat-frequency techniques. The individual hyperfine transitions were linked to a 633 nm He-Ne laser by direct frequency measurements up to 16 GHz. The experimental standard uncertainty was typically below 5 kHz (i.e. relative uncertainty <1 × 10⁻¹¹).

When measured with ac at kilohertz frequencies the quantized Hall resistance (QHR) of a quantum Hall effect (QHE) device is usually found to be current- and frequency-dependent. This is a limitation on its use as a quantum impedance standard. We develop a model for the principal ac losses arising in the QHE device and show how they are responsible for the observed QHR current and frequency coefficients. We believe that losses are mainly caused by dissipative ac charging of the device along its edges. Charging is induced by the passage of the Hall current and by capacitive coupling between an edge and any nearby conductor maintained at an ac potential different to that of the edge, as for example at shield potential. The loss power is proportional to frequency and increases more rapidly than the square of the applied voltage or current. We model losses in terms of in-phase loss currents, which are a function of the amplitude of the ac charge reaching or leaving edges. The QHR frequency coefficient is zero only when the loss current for one portion of the high-potential edge and that for a corresponding portion of the low-potential edge are equal and of opposite sign. We propose a simple method for approaching that balance condition: gates are located under the device edges and their ac potentials adjusted so that the QHR current coefficient, evaluated at a constant frequency, is zero. We report measurements of the residual QHR frequency coefficients obtained after adjustment for GaAs/GaAlAs devices of two different types. For five different devices of the most favourable type, the QHR frequency coefficients do not exceed ±2 parts in 10⁸ per kilohertz.

Measurement of amounts or concentrations of catalysts is of great usefulness in science, technology, and especially in the health services. When it is impossible or impractical to measure amount of substance or mass of catalyst, its amount may be expressed by the catalysed change in a substrate entity as evidenced by some measure appropriate to the particular measuring system. Initially, the concomitant units were specific for each catalyst and measurement procedure. Later an independent universal unit was defined as "1 µmole of substrate utilized per minute under specified conditions of pH and temperature control". Eventually, catalytic activity (as a measure of amount) became defined by catalysed rate of conversion in mole per second with the suggested name "katal" (kat). A recent petition by the International Federation of Clinical Chemistry and Laboratory Medicine to the Comité International des Poids et Mesures, asking for inclusion of "katal" among the special names for SI derived units, was discussed and recommended by the Consultative Committee for Units and the Consultative Committee for Amount of Substance, and subsequently adopted by the 21st Conférence Générale des Poids et Mesures in 1999, referring to human health and safety and promotion of SI units. This decision should improve the traceability and comparability of results.

The least-squares procedure for obtaining the parameters of a curve adjusted to a set of measured values is discussed. Two algorithms are presented in matrix notation, which can handle the general case where all input quantities are uncertain. Attention is paid to the consistency of the adjustment. Two examples show that the algorithms exhibit comparable performances.

The primary clocks CS1, CS2 and CS3 of the Physikalisch-Technische Bundesanstalt (PTB) have contributed to the realization of International Atomic Time, TAI (Temps Atomique International), throughout 1999. Their operation, their performance and the uncertainty with which they realized the International System of Units (SI) second are described in some detail, together with the results and associated uncertainties of comparisons of the realized second with the TAI scale unit for each clock.

An interlaboratory comparison of the different realizations of the triple point of water (TPW) in Europe has been organized by EUROMET (Project No. 278). This project was based on the circulation of one TPW cell and an isothermal enclosure. The national metrology institutes of thirteen countries and the Bureau International des Poids et Mesures were involved in the work. Twenty-seven TPW cells were compared. Half of the results lie between −0.05 mK and +0.05 mK of the mean value, and all but two of the results are within 0.10 mK of the mean.

An international comparison of ¹²⁷I₂-stabilized 633 nm He-Ne lasers locked either by the fifth- or the third-harmonic technique was carried out at the Finnish Centre for Metrology and Accreditation (MIKES) in September 1998. The participating laboratories were the MIKES; the Metrology Research Institute, Helsinki University of Technology (HUT); the Chinese National Institute of Metrology (NIM); the Czech Metrological Institute (CMI); the D. I. Mendeleyev Institute for Metrology (VNIIM); and the Bureau International des Poids et Mesures (BIPM). Raw measured frequency differences between lasers were in the following ranges: −5.2 kHz to +11.9 kHz and −5.8 kHz to +4.9 kHz for the third- and fifth-harmonic locking techniques, respectively. The frequency offsets between the third- and fifth-harmonic techniques for different components (d to g) were in the range 26 kHz to 35 kHz. The relative frequency stability of lasers given by the Allan standard deviations reached 2.6 × 10⁻¹³ for two lasers using the third harmonic and only 1 × 10⁻¹² for the same lasers using the fifth harmonic for a 2000 s sampling time. This comparison is part of the ongoing laser comparison carried out under the auspices of the Consultative Committee for Length (CCL, formerly CCDM) of the Comité International des Poids et Mesures (CIPM); it is also a EUROMET project (No. 462).

This second international comparison between He-Ne lasers at 543 nm exhibits similar results to the first international comparison. A standard uncertainty of 4 kHz was obtained against 2 kHz in 1993, while the average frequency difference between the lasers was −1.0 kHz against 14.2 kHz in 1993. The experimental data indicate however that a standard uncertainty below 1 kHz can be expected between two lasers. The frequency difference between the two lasers clearly lies within the limits (2.5 parts in 10¹¹) given in the revised practical realization of the definition of the metre (1997) for the 633 nm He-Ne laser.

The relativistic red shift for the new caesium fountain at the National Institute of Standards and Technology (NIST) at Boulder, USA, has been estimated using two independent methods. Method 1 uses the EGM96 geopotential model to estimate the geopotential number, C, and subsequently the frequency offset, −C/c² (c being the speed of light), obtaining the value 1797.8 × 10⁻¹⁶ with an estimated standard uncertainty of 1.1 × 10⁻¹⁶. Method 2 uses the geopotential number from the National Geodetic Survey data sheet for the NIST marker and gives a frequency offset of 1798.9 × 10⁻¹⁶, with an estimated standard uncertainty of 0.2 × 10⁻¹⁶.

Erratum to Metrologia, 2000, 37, 243-245.