Magazine | Issue 1 2025 TRACEABILITY RESULTS FOR INTERNATIONAL AND NATIONAL REFERENCE STANDARDS AND COHERENT SI UNITS We’d all like to know the actual value of a measurement, but we have to make compromises. In the real world, we’re a long way from being able to provide an absolute True Value for a measurement. It’s comprised of an array of uncertainties that add up to what’s often referred to as accuracy; the technically correct term is Measurement Uncertainty. Within the Coherent International System of Units (SI), all measurement values are based on one or a combination of several of the seven defined Natural Constants. These natural constants are theoretic values that already have a specific error resulting from the limited number of places after the decimal point – this small, welldefined error is where uncertainty begins. The national metrological institutes from each country usually provide and run national standards, which are the most accurate local comparison solutions. These National Reference Standards have a defined uncertainty that is expressed in relation to the theoretical Natural Constant(s). International services like EURAMET or BIPM offer programs to compare National References. Ring comparisons compare results to ensure they’re similar and correct, to determine if drifting has occurred, if instruments have failed, or to detect bad measurement principles. Accredited labs such as OMICRON Calibrations can buy calibration services from these National Metrologic Institutes to ensure a guaranteed unbroken chain of traceability and that measurement and calibration equipment function properly. EURAMET started a cross-continental project related to partial discharge measurements, featuring a ring comparison with 14 worldwide participants (EURAMET.EM-S36). The process starts with selecting a stable test object that is analyzed precisely and calibrated (measured) before being sent to the participants. The participants then perform identical measurements and share the results (measurement value plus guaranteed uncertainty per test point) with the first NMI. Afterward, the test object is sent to the next participant, and the process repeats. Following this, the leading intercomparison laboratory calibrates the test object again. Finally, the comparison’s coordinator collects and shares the anonymized data with all participants. RISE, the Swedish National Metrologic Institute, was the coordinating lab, and VTT MIKES (National Metrological Institute of Finland), one of the leading labs for small partial discharge measurements, was the pilot lab. It was a truly unique process for a unique measurement. On tour with OMICRON’s CAL 542 PD calibrators VTT MIKES chose our CAL 542 A, B, C, and D to be their test objects, knowing from experience how reliable and stable they were. This was truly an Production process (e. g. acc. to ISO 9001) Reference test equipment NMI: National Metrologic Institute (BEV, PTB, NIST,...) SI Gremium Chain of traceability Certainty (accuracy) True Value Calibrated products (CMC 500, CPC 100, ...) Industry laboratory standards (internal reference) Laboratory standards calibrated at ISO/IEC 17025 accredited laboratory National and international reference standards Coherent SI unit based on natural constant s kg mol cd K A m SI Δν h NA Kcd k e c 47
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