International comparison on SST and Epstein measurements in grain-oriented Fe-Si sheet steel

We discuss the results of an international comparison regarding the measurement of magnetic power loss and apparent power between 20 Hz and 100 Hz at magnetic polarization values ranging between 1.3 T and 1.8 T in grain-oriented steel (GO) sheets using the Single Sheet Testing (SST) and Epstein methods. This exercise, carried out in the framework of the activity of Joint Working Group IEC 68/WG1-ISO/TC17/WG16, was aimed at a solid assessment of the degree of reproducibility of the SST method, under its prospective adoption in the industrial practice as a reference method for the specifications of grain-oriented alloys. Five different types of Fe-Si GO alloys were circulated among eleven different laboratories of metrological institutes and industry. The statistical analysis of the results shows close reproducibility properties of the SST and Epstein methods. It is obtained, in particular, that the power loss figures, measured by the participants at 50 Hz and the aforementioned peak polarization values are distributed around their reference value with relative standard deviation σ (P)SST = 0.88% and σ (P)Epst = 0.82% for SST and Epstein, respectively. It is obtained σ (S)SST = 2.20% and σ (S)Epst = 2.15% for the apparent power. For the industrially relevant case of polarization Jp = 1.7 T in the high-permeability P-type grades, these quantities amount to σ (P)SST = 0.48%, σ (P)Epst = 0.80%. By restricting the comparison to the European metrological laboratories (INRIM, NPL, PTB), the overall distributions narrow to σ (P)SST = 0.42%, σ (P)Epst = 0.55%, σ (S)SST = 1.19%, and σ (S)Epst = 0.82%. A qualitative interpretation of the main physical mechanisms and the measuring features underlying the lab-to-lab dispersion of the SST and Epstein results is discussed in this paper. It is concluded that the combination of good reproducibility and simple practical implementation make the SST method totally appropriate as a reference method for the definition of the material quality in the specification standards