Conformance probability in the assessment of Calibration and Measurement Capabilities

We argue that the assessment of the Calibration and Measurement Capabilities, CMCs, by means of the results of a Key Comparison is a bona fide exercise of conformity assessment, and as such should be treated, using the appropriate tools, including risk assessment. This position contrasts with the current practice, in which acceptance or rejection of a CMC claim are based on the normalised error. We show that, behind this seemingly unique acceptance criterion, different decision rules - guarded acceptance, simple acceptance and guarded rejection - exist in reality, depending on the characteristics of the comparison. This variety of decision rules impairs the fairness of the current equivalence arrangement. We suggest that the conformance probability should be the key parameter to be considered in the assessment of a CMC claim. Using a suitable Probability Density Function, PDF, for the measurand, we calculate the conformance probability for the possible scenarios, and show that using the current acceptance criterion the conformance probability can attain unacceptably low values. Therefore, we maintain that the current acceptance criterion is ambiguous and inadequate, and suggest to rather adopt a criterion based on the calculation of the conformance probability and the establishment of a minimum threshold for acceptance. We demonstrate our proposal by applying it to a practical case and to a fictitious example in mass metrology

Air–vacuum transfer; establishing traceability to the new kilogram

The redefinition of the kilogram, along with another three of the base units of the International System of Units (SI), is scheduled for 2018. The current definition of the SI unit of mass assigns a mass of exactly one kilogram to the International Prototype of the kilogram, which is maintained in air and from which the unit is disseminated. The new definition, which will be from the Planck constant, involves the realisation of the mass unit in vacuum by the watt balance or Avogadro experiments. Thus, for the effective dissemination of the mass unit from the primary realisation experiments to end users, traceability of mass standards transferred between vacuum and air needs to be established and the associated uncertainties well understood. This paper describes a means of achieving the link between a unit realised in vacuum and standards used in air, and the ways in which their use can be optimised. It also investigates the likely uncertainty contribution introduced by the vacuum air transfer process

EURAMET Key Comparison EURAMET.M.D-K4.2020: Hydrometer calibration comparison from 600 kg/m3 to 2000 kg/m3

This report presents the results of the key comparison EURAMET.M.D-K4.2020 (EURAMET 1496) that covered the calibration of high-resolution hydrometers for liquid in the density range 600 kg/m3 to 2 000 kg/m3 at the reference temperature of 20 °C. Nine laboratories participated in this key comparison, eight European national metrology institutes and one from Asia Pacific metrology: NIM from China. The participating laboratories were initially divided into two petals linked by two density laboratories: INRIM and PTB. In each petal, four similar transfer standards, hydrometers in the density range 600 kg/m3 to 2 000 kg/m3 are used. The equipment used by the participants was found to have very similar characteristics. Most of the declared relative uncertainties were within the 5×10-5 . The results of all participated laboratories have been linked to the CIPM key comparison CCM.D-K4 by INRIM and PTB. Discrepant results have been reported for three laboratories, which, however, do not affect their CMCs

Buoyancy contribution to uncertainty of mass, conventional mass and force

The conventional mass is a useful concept introduced to reduce the impact of the buoyancy correction in everyday mass measurements, thus avoiding in most cases its accurate determination, necessary in measurements of ‘true’ mass. Although usage of conventional mass is universal and standardized, the concept is considered as a sort of second-choice tool, to be avoided in high-accuracy applications. In this paper we show that this is a false belief, by elucidating the role played by covariances between volume and mass and between volume and conventional mass at the various stages of the dissemination chain and in the relationship between the uncertainties of mass and conventional mass. We arrive at somewhat counter-intuitive results: the volume of the transfer standard plays a comparatively minor role in the uncertainty budget of the standard under calibration. In addition, conventional mass is preferable to mass in normal, in-air operation, as its uncertainty is smaller than that of mass, if covariance terms are properly taken into account, and the uncertainty over-stating (typically) resulting from neglecting them is less severe than that (always) occurring with mass. The same considerations hold for force. In this respect, we show that the associated uncertainty is the same using mass or conventional mass, and, again, that the latter is preferable if covariance terms are neglected

La taratura dei campioni di massa di riferimento dell'INRIM

L'INRIM possiede due prototipi del kilogrammo di Platino Iridio, il N.62 (Campione Nazionale), e il N.76, dai quali viene realizzata la così detta "scala di massa". Tale realizzazione avviene attraverso una serie di pesiere da cui si ottengono i multipli e sottomultipli del kilogrammo. Il metodo di taratura è quello del confronto, secondo schemi di pesata prestabiliti. I confronti sono realizzati mediante sette comparatori di massa, la maggior parte automatici. Il rapporto descrive il metodo di taratura dei campioni di riferimento, i quali a loro volta sono usati per la taratura dei campioni di lavoro che a loro volta vengono utilizzati per la taratura dei campioni dei clienti (vedi procedura tecnica PT-M.1.1-01 "Determinazione della massa e del valore convenzionale dei campioni di massa")