Thermal conductivity of insulations using guarded hot plates, including recent developments and sources of reference materials

摘要

The hot plate technique for measuring the thermal conductivities (the exact term for the quantity measured is thermal transmission, which, depending on the material being measured, can have components of convective, radiative and conductive heat transfer; it is commonly referred to as the effective or apparent thermal conductivity) of insulating materials has been in existence in various forms since 1898. A brief historical survey of the early development of the experimental technique is followed by a brief description of the basic principles of the method of measurement. The technique has since become very well established and is documented in the written standard ISO8302: 1991. It is now unarguably recognized as the most accurate technique for determining the thermal conductivity of insulations, having an uncertainty of about 1.5% over a limited temperature range near ambient. Details of two guarded hot plate apparatuses designed and constructed at NPL over the last decade or so, one to measure insulations up to 250 mm thick at or around room temperature and the other to measure insulations and refractories at temperatures up to 850 ℃, are given. Finally, there is a section on certified reference materials required for validating the performance of newly built guarded hot plate apparatus and for calibrating heat flow meter apparatus, a type of hot plate apparatus commonly used for quality control purposes in insulation manufacturing plant. A brief overview of these reference materials includes details of their availability, thermal conductivities and temperature ranges.