Characterisation of thermal conductivity reference material using the NPL axial heat flow apparatus
Jiyu Wu, Roger Morrell, Lindsay Chapman, Jamie Clark and Peter Quested
The National Physical Laboratory (NPL) has developed a new thermal conductivity reference material, NPL code 2S09 that is based on Stainless Steel 304. This batch of material has been thermally and electrically characterised over the temperature range 100 °C to 500 °C. The primary thermal conductivity measurements used to generate the certified values used the NPL Axial Heat Flow apparatus, which is based on a steadystate absolute technique and is suitable for measuring specimens with thermal conductivities in the range 10 W m–1 K–1 to 240 W m1 K–1. Comparisons were made between the reference thermal conductivity values and derived thermal conductivity values obtained from two indirect methods based upon electrical resistivity and thermal diffusivity measurements respectively. The reference thermal conductivity values of NPL 2S09 compare with those calculated from the measurements of electrical resistivity within 2 % and those calculated from the measurements of thermal diffusivity, specific heat capacity, density and thermal expansion better than 4 %.
The new reference material NPL 2S09 is available from NPL and can be used to calibrate or check apparatuses that provide measurements of thermal conductivity from 16 W m–1 K–1 to 22 W m–1 K–1 covering the temperature range 100 °C to 500 °C. The overall uncertainty on the certified values is estimated to be within ±4.8 %, based on a standard uncertainty multiplied by a coverage factor k = 2, providing a level of confidence of approximately 95 %.
Keywords: Axial Heat Flow Apparatus, Thermal Conductivity, Reference Material, Thermal Diffusivity, Specific Heat, Electrical Resistivity