On the calibration of DC resistance ratio bridges

Current comparator bridges are employed for the realization of the resistance scale from the quantum Hall effect in several National Metrology Institutes and calibration centers. Quantum resistance standards under development, based on novel materials and tabletop dry cryostats, make the more achievable DC current comparator bridges (DCCs) a viable alternative to the more accurate but more expensive cryogenic current comparator bridges (CCCs). A DCC ratios' calibration against a reference CCC is a straightforward way to improve the DCC's performances and the resistance scale overall accuracy.The paper reports the calibration results of two DCCs on the ratios employed in a 1 omega to 10 k omega resistance scale traceable to a 12.906 k omega quantized Hall resistance, showing a good reproducibility and stability of the DCC readings over the measurement period and supporting the possibility of a DCC errors' correction and of a realization of the primary resistance scale at the 10-8 level

Universality of transport properties of ultra-thin oxide films

We report low-temperature measurements of current-voltage characteristics for highly conductive Nb/Al-AlOx-Nb junctions with thicknesses of the Al interlayer ranging from 40 to 150 nm and ultra-thin barriers formed by diffusive oxidation of the Al surface. In the superconducting state these devices have revealed a strong subgap current leakage. Analyzing Cooper-pair and quasiparticle currents across the devices, we conclude that the strong suppression of the subgap resistance comparing with conventional tunnel junctions originates from a universal bimodal distribution of transparencies across the Al-oxide barrier proposed earlier by Schep and Bauer. We suggest a simple physical explanation of its source in the nanometer-thick oxide films relating it to strong local barrier-height fluctuations which are generated by oxygen vacancies in thin aluminum oxide tunnel barriers formed by thermal oxidation.Comment: revised text and a new figur

Metrological assessment of DC current comparator resistance bridges

Direct-current comparator bridges (DCC) are the working horses of primary resistance metrology in the intermediate resistance range. Having a ratio accuracy reaching 10^-7 or better, they allow the realization of resistance scales and the calibration of artifact standard resistors for customers. In this paper we compare the performances of three commercial DCC bridges, by performing measurements on resistors in decadal ratios (1 Ω to 10 kΩ) of very high stability in a thermostated environment. The results show that the three bridges give mutually compatible results within the manufacturer’s specifications, therefore mutually validating the bridges; nevertheless, the readings time series show quite different statistical behavior, with internal correlations, making an evaluation of the Type A measurement uncertainty not trivial

Supplementary comparison EURAMET.EM-S35: comparison of high-current ratio standard. Final report.

The primary scope of the comparison was the validation of CMCs for quantities related to dc high currents, for current values ⩾ 100 A. The travelling standards were two direct-current current transformers (DCCT). The measurand was the current ratio of the DCCT, having a nominal value R = 1:1500, to be measured at primary currents of 90 A (mandatory), 300 A and 600 A (optional). The measurement results could to be stated for positive and negative currents, and/or as an average of the ratios obtained with both current polarities. The comparison involved twelve participants (two pilot laboratories included), eleven of them measured the travelling standard, and ten provided results. Two participants withdrew due to technical problems. The comparison was organized into two loops, where the participants of each loop measured only one of the two travelling standards, while the pilot laboratories measured both standards, and making possible the linking of measurement results of the participants of loop 1 with those of loop 2. The circulation of the standards started in December 2012 and ended in June 2015. Concerning the measurement results, a good agreement was achieved, although the travelling standard instability of loop 1 represented a relevant contribution to the DOE uncertainty, which is significantly larger than the measurement uncertainty declared by some participants

Modelli matematici per la taratura di trasformatori di corrente continua mediante multimetri numerali, derivatori e trasformatori di corrente

I trasformatori di corrente per correnti in regime continuo (DCCT) sono tra i più diffusi sensori di forti correnti oggi commercialmente disponibili con ampia variabilità nell’accuratezza di misura, per cui è possibile utilizzare sistemi più o meno complessi ed accurati per effettuarne la taratura. Nel presente rapporto tecnico vengono presentati alcuni modelli matematici di misura per sistemi di taratura in corrente continua “semplificati” che utilizzano come campioni di riferimento multimetri numerali (DMM), derivatori di corrente (shunt) o altri DCCT.Direct-current current transformers (DCCTs) are nowaday the most popular high current sensors commercially available with a wide variability of measurement acuracy, therefore it is possible using more o less complex and accurate systems to carry out their calibration. In the present tecnical report some measurement mathematical models for “simplified” dc calibration systems are presented, which they use digital multimeters (DMMs), resistance shunts or other DCCTs as reference standards

Validazione del sistema di misura del ponte a comparatore di corrente Measurements International MI 6010B

Il presente rapporto tecnico descrive i risultati delle misure effettuate per la validazione del ponte a comparatore di corrente Measurements International MI 6010B, utilizzato per la taratura di campioni di resistenza di basso valore, coperta dalle capacità di misura (CMC) “2.1.1 DC, resistance and standards and source: low values”This technical report describes the results of the measurements performed for the validation of the current comparator bridge Measurements International MI 6010B, used for the calibration of low value resistance standards, supported by the Calibration and Measurement Capabilities (CMC) "2.1.1 DC, resistance and standards and source: low values

The practical realisation of the ohm at INRIM

This paper describes how the Istituto Nazionale di Ricerca Metrologica (INRIM) performs the practical realisation of the ohm in order to transfer the traceability to the Italian accredited calibration laboratories in the field of dc and ac electrical resistance and of capacitance. These quantities are afterwords disseminated by the accredited laboratories to the Italian industry through the calibration of electric standards and electronic instruments. The INRIM primary measurement setup, the transfer method and more than 20 years of calibrations results are here presented

Taratura di derivatori e resistori campioni nel campo da 1 μΩ a 1 mΩ, con correnti fino a 2000 A

Il presente rapporto tecnico descrive nel dettaglio il sistema INRIM di trasferimento della riferibilità a campioni di resistenza elettrica, impiegati come derivatori di corrente, nel campo di valori da 1 μΩ a 1 mΩ con correnti sino a 2 kA. Vengono anche riportate le migliori capacità di misura (CMC) dell’Istituto per questo tipo di tarature.This technical report describes in detail the INRIM transfer traceability system to electrical resistance standards, used as current shunts, in the range of values from 1 μΩ to 1 mΩ with currents up to 2 kA. The best measurement capabilities (CMC) of the Institute for this type of calibration are also reported

Simulated shunts as dc low-resistance travelling standards

We introduce a device that simulates a low-valued resistance standard (shunt) that can be employed as a metrology-grade travelling standard, for the traceability transfer of dc resistance in the low range, and in laboratory intercomparisons. The device is composed of a direct-current current comparator (DCCT) and a mid-range resistance standard. A prototype having a nominal resistance of 10m Ω has been constructed. Experiments show that it can be measured with high-accuracy commercial dc resistance bridges in their default operating configurations. The power coefficient is much lower than a corresponding shunt of the same nominal resistance. The expected long-term and transport stability is a few parts in 106 or better