Table of contents

We report on characterizations of single-electron pumps at the highest accuracy level, enabled by improvements of the small-current measurement technique. With these improvements a new accuracy record in measurements on single-electron pumps is demonstrated: 0.16 µA · A⁻¹ of relative combined uncertainty was reached within less than 1 d of measurement time. Additionally, robustness tests of pump operation on a sub-ppm level revealed a good stability of tunable-barrier single-electron pumps against variations in the operating parameters.

Previously (Underwood et al 2015 Meteorol. Appl. 22830) we reported first tests of a device capable of simultaneous, non-contact, temperature and humidity (NCTAH) measurements in air. The device used an acoustic thermometer and a tuneable diode laser absorption spectrometer (TDLAS), a combination which should be capable of an extremely rapid response to changes in humidity as it does not require moisture in a solid-state matrix to equilibrate with the surrounding air. In this paper we report recent developments of the instrument focussed on reducing its response time so that it can be used as a reference instrument for assessing the response time of conventional humidity sensors. In addition, the interdependence of the temperature and humidity estimates is now accounted for in real-time using an iterative procedure, which eliminates the need for data post-processing.

The TDLAS measures water molecule number density based on the transmission of an infrared beam (approximate wavelength 1360 nm) through a 0.6 m path length. The acoustic thermometer is based around a fixed-path acoustic interferometer. The improved NCTAH device now produces estimates of the water molecule number density every 20 ms and the temperature output displays an RC filter-like response, with a time constant of approximately 30 ms.

The instrument has been tested in a climatic chamber through a temperature range of  −40 °C to  +40 °C and a dew point range of  −43 °C to  +38 °C, at atmospheric pressure, comparing the instrument readings with those from a calibrated hygrometer and four platinum resistance thermometers. In steady-state conditions, the instrument readings are in good agreement with the conventional sensors, with temperature differences less than 1 °C (repeatability 0.1 °C), and humidity differences mostly within 5% of mixing ratio. Under transient conditions, we demonstrate how the instrument can be used to evaluate the response times of conventional sensors.

Claims that proximity to the Sun causes variation of decay constants at permille level have been investigated for alpha decaying nuclides. Repeated decay rate measurements of ²⁰⁹Po, ²²⁶Ra, ²²⁸Th, ²³⁰U, and ²⁴¹Am sources were performed over periods of 200 d up to two decades at various nuclear metrology institutes around the globe. Residuals from the exponential decay curves were inspected for annual oscillations. Systematic deviations from a purely exponential decay curve differ in amplitude and phase from one data set to another and appear attributable to instabilities in the instrumentation and measurement conditions. The most stable activity measurements of α decaying sources set an upper limit between 0.0006% and 0.006% to the amplitude of annual oscillations in the decay rate. There are no apparent indications for systematic oscillations at a level of weeks or months. Oscillations in phase with Earth's orbital distance to the sun could not be observed within 10⁻⁵–10⁻⁶ range precision.

Claims that proximity to the Sun causes variations of decay constants at the permille level have been investigated for beta-minus decaying nuclides. Repeated activity measurements of ³H, ¹⁴C, ⁶⁰Co, ⁸⁵Kr, ⁹⁰Sr, ¹²⁴Sb, ¹³⁴Cs, ¹³⁷Cs, and ¹⁵⁴Eu sources were performed over periods of 259 d up to 5 decades at various nuclear metrology institutes. Residuals from the exponential decay curves were inspected for annual oscillations. Systematic deviations from a purely exponential decay curve differ in amplitude and phase from one data set to another and appear attributable to instabilities in the instrumentation and measurement conditions. Oscillations in phase with Earth's orbital distance to the Sun could not be observed within 10⁻⁴–10⁻⁵ range precision. The most stable activity measurements of β⁻ decaying sources set an upper limit of 0.003%–0.007% to the amplitude of annual oscillations in the decay rate. There are no apparent indications for systematic oscillations at a level of weeks or months.

The hypothesis that seasonal changes in proximity to the Sun cause variation of decay constants at permille level has been tested for radionuclides disintegrating through electron capture and beta plus decay. Activity measurements of ²²Na, ⁵⁴Mn, ⁵⁵Fe, ⁵⁷Co, ⁶⁵Zn, ⁸²⁺⁸⁵Sr, ⁹⁰Sr, ¹⁰⁹Cd, ¹²⁴Sb, ¹³³Ba, ¹⁵²Eu, and ²⁰⁷Bi sources were repeated over periods from 200 d up to more than four decades at 14 laboratories across the globe. Residuals from the exponential nuclear decay curves were inspected for annual oscillations. Systematic deviations from a purely exponential decay curve differ from one data set to another and appear attributable to instabilities in the instrumentation and measurement conditions. Oscillations in phase with Earth's orbital distance to the sun could not be observed within 10⁻⁴–10⁻⁵ range precision. The most stable activity measurements of β⁺ and EC decaying sources set an upper limit of 0.006% or less to the amplitude of annual oscillations in the decay rate. There are no apparent indications for systematic oscillations at a level of weeks or months.

Some investigations have concluded that the diurnal pattern in the time comparison results of present two way satellite time and frequency transfer (TWSTFT) links may come mainly from Doppler dependent errors in the time of arrival (TOA) measurements made by the receivers of the TWSTFT modems. In this paper, several experiments were carried out to test if there is a Doppler dependent error in the 'delay' measurements of the receivers currently used. By simulating quantitative Doppler effects in the time transfer signal both on the carrier and the code, a type of Doppler sensitivity on the code was observed in the receivers, which has about  −0.49 ns offset in 'delay' measurement for a 1  ×  10⁻⁹ fractional Doppler shift. This sensitivity is basically the same for modems with different serial numbers from the same manufacture. We calculated this Doppler caused diurnal pattern in the time comparison results of the transatlantic TWSTFT link between NIST and PTB and found that it is very small and negligible, because the Doppler dependent error is almost identical in the NIST and PTB measurements and therefore it is nearly canceled in the TWSTFT difference.

Ensuring measurement trueness, compliance with regulations and conformity with standards are key tasks in metrology which are often considered at the time of an inspection. Current practice does not always verify quality after or between inspections, calibrations, laboratory comparisons, conformity assessments, etc. Statistical models describing behavior over time may ensure reliability, i.e. they may give the probability of functioning, compliance or survival until some future point in time.

It may not always be possible or economic to inspect a whole population of measuring devices or other units. Selecting a subset of the population according to statistical sampling plans and inspecting only these, allows conclusions about the quality of the whole population with a certain confidence.

Combining these issues of sampling and aging, raises questions such as: How many devices need to be inspected, and at least how many of them must conform, so that one can be sure, that more than $100p \%$ of the population will comply until the next inspection? This research is to raise awareness and offer a simple answer to such time- and sample-based quality statements in metrology and beyond.

Reliability demonstration methods, such as the prevailing Weibull binomial model, quantify the confidence in future reliability on the basis of a sample. We adapt the binomial model to be applicable to sampling without replacement and simplify the Weibull model so that sampling plans may be determined on the basis of existing ISO standards. Provided the model is suitable, no additional information and no software are needed; and yet, the consumer is protected against future failure.

We establish new sampling plans for utility meter surveillance, which are required by a recent modification of German law. These sampling plans are given in similar tables to the previous ones, which demonstrates their suitability for everyday use.

We present a measurement and analysis scheme for determining traceable thermodynamic temperature at cryogenic temperatures using Coulomb blockade thermometry. The uncertainty of the electrical measurement is improved by utilizing two sampling digital voltmeters instead of the traditional lock-in technique. The remaining uncertainty is dominated by that of the numerical analysis of the measurement data. Two analysis methods are demonstrated: numerical fitting of the full conductance curve and measuring the height of the conductance dip. The complete uncertainty analysis shows that using either analysis method the relative combined standard uncertainty (k  =  1) in determining the thermodynamic temperature in the temperature range from 20 mK to 200 mK is below 0.5%. In this temperature range, both analysis methods produced temperature estimates that deviated from 0.39% to 0.67% from the reference temperatures provided by a superconducting reference point device calibrated against the Provisional Low Temperature Scale of 2000.

In this paper we present the outcome of the first international comparison in the terahertz frequency range among three different kinds of spectrometers. A Fourier-Transform infrared spectrometer, a vector network analyzer and a time-domain spectrometer have been employed for measuring the complex refractive index of three travelling standards made of selected dielectric materials in order to offer a wide enough range of parameters to be measured. The three spectrometers have been compared in terms of measurement capability and uncertainty.

Recently, a scientific comparison of flatness measuring instruments at European National Metrology Institutes (NMIs) was performed in the framework of EURAMET. The specimen was a well-polished optical surface with a maximum measurement aperture of 150 mm in diameter. Here, we present an evaluation concept, which allows the determination of a mean flatness map taking into account different lateral resolutions of the instruments and different orientations of the specimen during measurement. We found that all measurements are in agreement with the mean flatness map within the uncertainty intervals stated by the participants. The aim of this scientific comparison is to specify an appropriate operation and evaluation procedure for future comparisons.

In this paper, the uncertainty of time transfer based on a bidirectional time division multiplexing transmission over a single fiber with the same wavelength (BTDM-SFSW) is investigated via theoretical models and experimental measurements. According to the principle and system architecture, the uncertainty evaluation schemes for BTDM-SFSW based time transfer are presented and the uncertainty sources are identified accordingly. In order to avoid the effect of the temperature-dependent fiber delay, the measured time intervals at two sites are regarded as a whole to obtain an overall uncertainty of time interval measurements. For the uncertainty of time transfer modem calibration, aside from the type A uncertainty obtained under the applied calibration scheme, the system reproducibility against practical operation and the contribution of optical power-dependent receiving delays are also included. A mathematical model considering fiber dispersion, polarization mode dispersion (PMD) and the Sagnac effect is established to evaluate the uncertainty from the fiber link. The characteristics of the uncertainty sources in a long-distance fiber-optic time transfer testbed are then explored in detail. The combined expanded uncertainties with a coverage factor of 2 are calculated and experimentally validated over various non-calibrated fiber extensions.

We report the absolute frequency measurement of the unperturbed transition ${{}^{1}}{{\text{S}}_{0}}$ – ${{}^{3}}{{\text{P}}_{0}}$ at 578 nm in ¹⁷¹Yb realized in an optical lattice frequency standard relative to a cryogenic caesium fountain. The measurement result is 518 295 836 590 863.59(31) Hz with a relative standard uncertainty of $5.9\times {{10}^{-16}}$. This value is in agreement with the ytterbium frequency recommended as a secondary representation of the second in the International System of Units.

A collaborative study to compare the long-term measurement performance between guarded-hot-plate facilities at the Laboratoire national de métrologie et d'essais (LNE) in France and the National Institute of Standards and Technology (NIST) in the United States is presented. Thermal conductivity data were compiled from three international comparisons organized from 1997 to 2014. Measurements were conducted in accordance with standardized test methods (ISO 8302 or ASTM C 177) over a temperature range from 280 K to 320 K. Nine thermal insulating materials (either mineral fiber or expanded polystyrene) were examined covering broad ranges of bulk densities (13 kg · m⁻³–200 kg · m⁻³) and thicknesses (13 mm–70 mm). A different set of specimens was utilized for each comparison. Results of this study indicate that, over a 17 year interval, the majority of test data from LNE and NIST agreed to within  ±1.0%, or less, for mineral fiber materials and to within  ±0.5%, or less, for expanded polystyrene. The long-term variability limit of 1% between the two laboratories is in good agreement with their current measurement uncertainties. Regression coefficients and their standard uncertainties for a straight-line model relating thermal conductivity to temperature from 280 K to 320 K were computed by material and laboratory. Graphical analysis of the data and corresponding fits exhibit consistent behavior by material type between the two laboratories. Sources of measurement variability are addressed.

In the second part of this two-part series on the development of a versatile reference instrument at the National Research Council of Canada (NRC), we have extended the characterization of the NRC Reference Goniospectrofluorimeter to high-accuracy fluorescence measurements in a sphere geometry (8:d) that is specified in standard test methods for many practical applications in colorimetry. This builds upon the work reported in part-one of this series which described in detail the design, characterization and validation of this new instrument for realizing a total spectral radiance factor scale in a bidirectional (45a:0) geometry. To extend the measurement capabilities to a sphere geometry, it was configured with a large diameter integrating sphere accessory. Preliminary results using a substitution-mode operating procedure showed large sphere errors that were characterized and corrected for. To improve this traceability, the sphere was modified to operate in comparison-mode and this effectively eliminated many of the sphere-related errors that typically limit the accuracy of sphere-based fluorescence measurements. The performance of the instrument configured for a sphere geometry (8:d) with this modified sphere design has been validated by means of comparison measurements of both non-fluorescent and fluorescent artifacts. The reflectance component has been validated using non-fluorescent comparison samples that have been calibrated under the same geometric conditions with traceability to the NRC Absolute Reflectometer (d:0 geometry). The fluorescent-only component has been validated using near-Lambertian fluorescent reflecting materials with traceability to the NRC Reference Spectrofluorimeter (45:0 geometry), under the assumption that this component is nearly the same for these two geometries. This work has enabled NRC to provide an uninterrupted link for improved traceability of fluorescence calibrations that specify a sphere geometry. These calibration requests include many important commercial applications, such as whiteness specification of fluorescently whitened paper and textile samples and color specification of fluorescent safety goods.

Recent thermodynamic temperature measurements at PTB with dielectric-constant gas thermometry were performed up to 140 K. Particular care was taken to check for possible systematic sources of errors by repeating experiments applying two new measuring capacitors and both helium and neon as working gases. The development of a new method for determining the effective compressibility of the new capacitors as a function of temperature has decreased the uncertainty significantly. The combination of the recently obtained results with former values of thermodynamic temperature, corrected using new thermodynamic input data, has yielded a consistent dataset in the range from 2.5 K to 38 K as well as at 84 K, 120 K, 130 K, and 140 K. This dataset is in good agreement with the newest results of acoustic gas thermometry, which has quite different sources of uncertainty compared with dielectric-constant gas thermometry.