Risk calculations for conformity assessment in practice

In 2012, the Joint Committee for Guides in Metrology (JCGM) published novel guidance on the consideration of measurement uncertainty for decision-making in conformity assessment (JCGM 106:2012). The two situations of making a wrong decision are considered: the risk of accepting a non-conforming item, denoted as the customer risk, and the risk of rejecting a conforming item, denoted as the producer risk. In 2017, the revision of ISO 17025 obliged calibration and testing laboratories to “document the decision rule employed, taking into account the level of risk (such as false accept and false reject and statistical assumptions) associated with the decision rule employed, and apply the decision rule” in the context of the decision made about the conformity of an item. However, JCGM 106:2012 can in some cases be perceived as quite difficult to apply for non-statisticians as it mainly relies on calculations involving probability distributions. In order to facilitate uptake of the methodology of JCGM 106:2012, EURAMET is funding the project EMPIR 17SIP05 “CASoft” (2018 – 2020), involving the National Measurement Institutes from France, Sweden and the UK. The objective is to make the methodology accessible to organisations involved in decision-making in conformity assessment: calibration and testing laboratories, industrialists and regulation authorities. Where the customer or producer are concerned, there are two kinds of risks arising from measurement uncertainty: specific risk which concerns the risk of an incorrect decision for a particular item and global risk which is the risk of an incorrect decision for any item chosen at random. Both kinds of risk may involve prior information, taken into account through a so-called prior probability distribution, introducing the concept of a Bayesian evaluation of the risks. If a calibration and testing laboratory performing the measurement has difficulty accessing prior information, it is likely that the industrialist in control of production processes will have some idea of the quality of the items produced. In this paper, the two problems of estimating the specific and global risks are addressed. The consideration of prior information is also discussed through a practical example as well as the use of software implementing the methodology, which will be made publically available at the end of the project.

Determination of the coefficient of expansion of a carbon tube and its assembly for thermal compensation of metrological structures

Composite materials are increasingly used in 3D metrology devices. Their use is justified by their interesting mechanical properties including their low density and good rigidity but especially their low coefficient of thermal expansion. In fact, in order to improve production efficiency, companies nowadays integrate more and more control equipment directly in situ. These are then subject to thermal variations. The use of composite materials is an interesting approach. However, in some cases, the lack of knowledge of their coefficient of thermal expansion and their behavior might increase measurement uncertainties. The objective is to study the thermal behavior of a carbon tube alone and the same tube with aluminium fixing elements at its extremities, in order to determine the coefficients of expansion of the carbon alone and to quantify the influence of the fixation with aluminium elements. This experiment makes it possible to directly compensate the dimensional variations of the metrological structure depending on the temperature variations and thus to limit measurement uncertainties. The thermal expansion coefficients of the carbon tube and its assembly are determined by measuring relative variations in height with a ZERODUR® reference bar. The whole is positioned in a climatic chamber. *

Experimental and numerical investigations on the shape and roughness of cylindrical critical flow venturi nozzles (CFVN)

This paper describes a part of a research project aimed at reducing the uncertainty of the determination of the CFVN discharge coefficient. In the current state of the work, there are two parallel studies: an experimental and a numerical. Experiments have been carried out to determine the impact of roughness on the CD and to demonstrate the need for dimension parameters, such as diameter and roughness coefficient characterisation. Numerical simulations are used in the same CFVN geometry with a smooth surface to determine the main parameters which can influence the numerical prediction of the discharge coefficient and the flow behaviour. The results show that an increase of roughness may lead to a significant decrease of the discharge coefficient, particularly in the high Reynolds number range.

Study of non-invasive instruments for the measurement of pulsed X-ray high voltage tube

Non-invasive instruments (kVp meters) are widely used in radiology with diagnostic and guidance systems. Placed in the x-ray beam, they combine detectors and filters, to determine X-ray tube voltage and exposure time, which are the most important quantities in radiology and diagnostic quality control. Calibration of these instruments were limited by reference bench capabilities. General Electric Medical System France and LNE (The national metrology institute in France) have developed a reference bench for the characterization of kVp meters. The set up includes a fast high voltage generator associated with its internal measuring systems and an X-ray tube. The measurements are compared with an invasive reference standard. The set-up is installed in a Faraday cage, precautions have been taken in order to carry out accurate measurements and special adaptations have been made to avoid stray capacitances, which affect the dynamic performance of the generator. Results have shown a good agreement with the internal measuring system of the generator but the kVp meters have shown both good and bad results depending the exposure time, the current and the positioning.

Metrology for biomethane conformity assessment: measure trace gas impurities in biomethane

To facilitate the use of biomethane in existing transmission and distribution infrastructures, CEN/TC 408 developed specifications (EN 16723) for injecting biomethane into the natural gas grids and using it as a transport fuel. Currently, the test methods cited in EN 16723 lack metrological aspects and have not been specifically developed for biomethane. To address this need, ISO/TC193/SC1/WG25 “Biomethane” has been created to work on standardized methods. To assess conformity of biomethane with the specification and to provide essential input to WG25, test methods are being developed in this research for a group of impurities such as siloxanes, halogenated volatile organic compounds, hydrogen chloride, hydrogen fluoride. A further objective of this research is to develop fit-for-purpose measurement standards for these parameters, to enable SI-traceable calibration and facilitate accurate measurement results. An overview of the progress made with respect to the development of measurement standards and test methods for trace level concentrations of impurities is presented, with a focus on static measurement standards of siloxanes and halogenated volatile organic compounds, dynamic gas standards of HCl and HF, as well as corresponding test methods based on gas chromatography and spectroscopic techniques. The work presented is pivotal for the development of metrological infrastructure for biomethane conformity assessment.

Investigation in optimisation of accuracy with non-contact systems by influencing variable processes

The use of 3D scanning systems is becoming increasingly popular and an essential tool for manufacturers for inspection and measurement. With such systems being utilised on the manufacturing shop floor due to their portability and ease of use, it is no doubt that such systems are designed to address a variety of users whom, with minimal training can operate the equipment. Due to continuing demands of high-quality products there is the need for manufacturers of 3D scanning systems to develop technologies that deliver fast and accurate information. However, one of the key challenges lies not in the training of people to use the equipment, but to develop engineers who can produce traceable, accurate and precise results with a declared statement of confidence quantifying the quality of the measurement. This statement of the quality of the output results relies on employing a set of workflow actions that involve planning, capture, processing and analysis, and finally output. This paper sets out to show how the results from a set of workflow actions from different categories of 3D scanning devices affects the quality of output.

Inter-laboratory comparison on the calibration of measurements photometric and radiometric sensors

The competences of the test and calibration laboratories are provided by two complementary methods. The first of these methods is the on-site audit carried out by the administrative and technical committee established by TURKAK (Turkish Accreditation Institution) according to the requirements of TS EN ISO / IEC 17025 [1]. The other technique is interlaboratory comparison and proficiency testing to evaluate laboratory performance and ensure the quality of results. Investing in the right equipment, training personnel, defining methods, documenting, calculating uncertainty, even performing internal verifications do not guarantee reliability or accuracy. It cannot answer the question of whether we can produce the same results as the same testers in the world. It is necessary to prove that the laboratory can actually produce accurate results externally by going through comparison tests with other national/international laboratories.

Practical Precision Electrical Impedance Measurement for the 21st Century – EMPIR Project 17RPT04 VersICal

The core objective of EMPIR project 17RPT04 VersICaL is to improve the European measurement infrastructure for electrical impedance, with particular emphasis on the capabilities of developing NMIs and calibration centres. The project will seek to exploit the results of existing research on digital impedance bridges (DIBs) by designing, constructing and validating simple, affordable versions suitable to realise the impedance scale in the range 1 nF to 10 μF and 1 mH to 10 H with relative uncertainties in the range 10-5 to 10-6. The first results of the research project, including the bridge designs and details of a polyphase digitally synthesized multichannel source capable of providing voltage outputs of precise ratio and phase are presented.

Calibration and measurement capability (CMC) and customer technical qualification

In the context of metrological confirmation, calibration is an essential process in all quality assurance efforts. Several organizations choose to outsource this activity to accredited laboratories in accordance with the requirements set forth in ISO/IEC 17025: 2017. Companies understand that accredited laboratory has formal recognition of its technical competence to perform the services within its scope of accreditation. The document ILAC P14: 2013 sets out guidelines for the presentation of Calibration and Measurement Capability (CMC). However, when analysing the scope of accredited laboratories in some national calibration bodies, it is possible to observe that, for the same instrument and the same measuring range, different values are attributed to CMC. If the CMC should result from normal calibration operations on the best existing device, what causes this dispersion? How can the customer make effective use of the information contained in accreditation scopes? In order to further standardize the presentation of CMC in accreditation scopes, calibration methods adopted by laboratories should be required to meet the maximum permissible errors established by manufacturers or normative documents. Companies can outsource calibration activities. But the selection of service provider and the interpretation of the results remains a customer assignment. The paper presents an analysis of accreditation scopes of different national calibration bodies and discusses the qualification of those in charge of metrology management, regarding the knowledge and skills required for activity.

The role of magnetic sensors in the hybridisation and electrification of vehicles

Electrical currents need to be measured in a wide variety of different applications in the field of power electronics. However, the requirements for these measurement devices are becoming steadily more demanding regarding accuracy, size and especially bandwidth. In order to increase the power density of power electronics, as particularly important in the field of electromobility, there is a clear causal chain. Soft switching leads to higher efficiency and higher frequencies, which enable smaller dimensions for a given power output. Higher switching frequencies allow the size of magnetic components to be reduced significantly, resulting in more compact and lighter designs. This trend is now being reinforced by use of new wide bandgap semiconductor materials like silicon carbide (SiC) and gallium nitride (GaN), as their low on-resistances and low parasitic capacitances reduce switching losses. Conventional current sensor solutions, e.g. hallor shunt based sensors exhibit a limited bandwidth, typically less than 250 kHz. Other current sensors, like those based on the Rogowski-Coil, are capable of highly dynamic current measurement, but are significantly more expensive, larger and hence not suitable for large series applications. Furthermore, Rogowski-Coils are only capable of measuring alternating currents (AC), which prevents their use in applications where DC currents must also be measured. In order to meet the above mentioned requirements, magnetoresistive (MR) current sensors are ideally suited due to the fact that the bandwidth of the magnetoresistive effect extends up into the GHz-range. This paper describes the principle of operation and main performance characteristics of highly integrated MR current sensors and describes their benefits compared to other types of current sensor, in particular with regard to applications in the hybridisation and electrification of vehicles.