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Nomenclature A Specimen metering surface area normal to heat flow direction, m 2 ANOVA Analysis of variance BIPM Bureau International des Poids et Mesures b Half-side or radius of meter plate, m Metrologia Collaborative guarded-hot-plate tests between the Abstract A collaborative study to compare the long-term measurement performance between guardedhot-plate facilities at the Laboratoire national de métrologie et d'essais (LNE) in France and the National Institute of Standards and Technology (NIST) in the United States is presented.Thermal conductivity data were compiled from three international comparisons organized from 1997 to 2014. Measurements were conducted in accordance with standardized test methods (ISO 8302 or ASTM C 177) over a temperature range from 280 K to 320 K. Nine thermal insulating materials (either mineral fiber or expanded polystyrene) were examined covering broad ranges of bulk densities (13 kg · m −3 -200 kg · m −3 ) and thicknesses (13 mm-70 mm). A different set of specimens was utilized for each comparison. Results of this study indicate that, over a 17 year interval, the majority of test data from LNE and NIST agreed to within ±1.0%, or less, for mineral fiber materials and to within ±0.5%, or less, for expanded polystyrene. The long-term variability limit of 1% between the two laboratories is in good agreement with their current measurement uncertainties. Regression coefficients and their standard uncertainties for a straight-line model relating thermal conductivity to temperature from 280 K to 320 K were computed by material and laboratory. Graphical analysis of the data and corresponding fits exhibit consistent behavior by material type between the two laboratories. Sources of measurement variability are addressed.

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