Report of the Italian inter-laboratories comparison of high dc resistance on the calibration of a and a resistors
Introduction
The interest in calibration of high value resistors in research and industrial fields recently increased due to the needs of traceable measurements in particular in companies with certified quality systems. An inter-laboratories comparison aims to verify if the measures of two or more Laboratories performed on the same measurand are compatible within the measurement uncertainties of the same Laboratories. As a matter of fact, the measurement compatibility is a concept strictly connected with measurement uncertainty and plays a central role in the comparison of the measurement results of the same measurand, in well-defined conditions, obtained through measurement processes based on different methods and instruments or performed by different Laboratories.
In past years the Italian Service of Accreditation of Calibration Laboratories (SIT) used to perform bilateral comparisons between the Italian Metrological Institutes (Istituto Elettrotecnico Nazionale “Galileo Ferraris” IEN, Istituto di Metrologia “Gustavo Colonnetti” IMGC) and the Secondary calibration Laboratories to evaluate their technical competence in the framework of their first accreditation or in the periodical renewal of their accreditation as SIT Centres elaborating the results according to the ISO Guide 43. Nowadays, as the number of accredited Laboratories grew significantly, the possibility to perform bilateral comparisons became quite difficult. A solution for this problem could be the possibility that National Institute of Metrological Research (INRIM) operate also as Inter-laboratories Comparisons (ILCs) provider [1], [2]. In this paper an example of the function that INRIM could also assume is presented. The Electromagnetic Division of INRIM organized and piloted a national comparison of high dc resistance at and level at which participated 8 SIT Centres. These Centres measured the two standard resistors of the comparison following their accredited procedures or following new procedures to be evaluated by SIT. The results of this comparison will be also utilized by SIT for the evaluation of the competence of the participating SIT Centres in the field of high dc resistance.
Section snippets
Participating laboratories
The participating Laboratories (SIT Centres) were the following (in alphabetical order):
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ARO SIT Centre no. 46;
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AVIATRONIK, SIT Centre no. 19;
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FIRENZE TECNOLOGIA SIT Centre no. 56;
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NEMKO SIT Centre no. 42;
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SIMAV (Naples) SIT Centre no. 14;
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SIMAV Milan) SIT Centre no. 04;
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SIMAV (Turin) SIT Centre no. 64.
Table 1 lists the participant Laboratories in chronological order (not corresponding to alphabetical order) along with the mean dates of their measurements and those of INRIM.
Vishay–Mann – Resistor
This standard Fig. 1a, Fig. 1b is a Vishay component with a metallic body fixed to an aluminium support inside an aluminium cylinder, in which were made some holes to facilitate the thermal exchange, while in the central body a groove allows to insert a thermometer. In the original definition it is a two terminal resistor, but a mechanical addition to it made at INRIM allows also its use as three terminals standard. The resistor was calibrated by all the Laboratories at a voltage of 10 V. Some
Measurement procedures
The Laboratories adopted their accredited (or submitted to accreditation) by SIT procedures and in particular for: The standard:
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reading directly to DMM: Centre 04;
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substitution method with a standard resistor and auxiliary DMM: Centres 42, 46, 64;
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reading directly to Multifunction Transfer Standard: Centres 19, 56;
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DMM and dc voltage calibrator method [4]: Centres 05, 14, 19, 42, 64. The standard:
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DMM and dc voltage calibrator method for all Centres.
Analysis of the results
To evaluate the measurement comparison the results of each measurement point were elaborated in the following way [5]:
The measurement values obtained by the Laboratories and by INRIM were defined respectively as:where are respectively the measurement values and the standard uncertainties declared by a Laboratory and by INRIM. A new measurand was defined for each measurement point, where , for the resistor, is the mean of the
Results
In Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9 the results of the inter-laboratories comparison are reported. In columns from 1 to 9 are reported respectively: the Laboratories, the dates of their measurements, the means or the interpolated values of the measurements of INRIM, the relative standard uncertainties of , the results of the Laboratories, the relative standard uncertainties of , the relative differences between the
Discussion
There are two typical situations in which an inter-laboratories-comparison can be performed: the first is among National Primary Metrological Laboratories belonging to different countries, the second among a National Primary Laboratory and some accredited Secondary Laboratories belonging to the same country. The comparison presented in this paper can be considered a typical example of the second situation. As a matter of fact, in this comparison, to determine the reference values were taken
Conclusions
From Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9 the and from Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10 the compatibility indexes CI (normalized errors) resulted less than 1 for all the examined measurement. This result demonstrates that the dissemination process or traceability transfer from INRIM to the SIT Centres and the accreditation criteria of the Italian Accreditation Service (SIT) in the field of high dc resistance are correctly assured.
Acknowledgment
The authors thank all the personnel of the 8 SIT Centres that participated at this comparison for their courtesy and technical competence.
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Metrological management of the high dc resistance scale at INRIM
Measurement
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