Design and Performance Checks of the NPL Axial Heat Flow Apparatus

This paper describes the design and performance checks of the NPL axial heat flow apparatus developed at the National Physical Laboratory for measurement of thermal conductivity. This apparatus is based on an absolute steady-state technique and is suitable for measuring specimens with thermal conductivities in the range from \(10\,\hbox {W}{\cdot }\hbox {m}^{-1}{\cdot } \hbox {K}^{-1}\) to \(240\,\hbox {W}{\cdot } \hbox {m}^{-1}{\cdot } \hbox {K}^{-1}\) and at temperatures between \(50\,^{\circ }\hbox {C}\) and \(500\,^{\circ }\hbox {C}\). A uniform heat flow is induced in a cylindrical bar-shaped specimen that is firmly clamped between a guarded heater unit at the top and a water-cooled base. Heat is supplied at a known rate at the top end of the specimen by the heater unit and constrained to flow axially through the specimen by a surrounding edge-guard system, which is closely matched to the temperature gradient within the test specimen. The performance of this apparatus has been checked against existing NPL thermal-conductivity reference materials NPL 2S89 (based on Stainless Steel 310) and BSC Pure Iron (pure iron supplied by the British Steel Corporation with 99.96 % purity). The measured data produced by the newly designed NPL axial heat flow apparatus agree with the reference data for NPL 2S89 within 2 % and with that of BSC Pure Iron to within 3 % at temperatures from \(50\,^{\circ }\hbox {C}\) to \(500\,^{\circ }\hbox {C}\). This apparatus is being used to provide accurate measurements to industrial and academic organizations and has also been used to develop a new range of NPL reference materials for checking other experimental techniques and procedures for thermal-conductivity measurements.