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

Abstract

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 • C, 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.

Thermal conductivity of cementitious composites containing microencapsulated phase change materials

“…Fourier's law, based on the imposed heat flux and the measured temperature gradient across the sample [21,22]. The hot plate method features simple measurement and analysis [21,22].…”

Thermal conductivity of refractory glass fibres

“…However, these methods are well known for their unidirectional heat flow which allows for high-accuracy measurements, even for nonisotropic materials, and panel calorimeter methods offer the highest precision in measurement amongst these methods [21]. The guarded hot plate method [45] has the capability to measure thermal conductivities at temperatures below 273 K. It is normally limited to upper measurement temperatures of 773 K, but a few instruments equipped with alumina heaters can conduct measurements at temperatures of up to 1273 K [45]. In this study, thermal conductivity measurements have been taken using laser flash and hot wire transient methods and a panel calorimeter steady-state method.…”

Characterisation of Ground Thermal and Thermo-Mechanical Behaviour for Shallow Geothermal Energy Applications

“…They should also have high emissivity surfaces, particularly when one is measuring low thermal conductivity materials. The temperature balance between the guards and the metering area must also be maintained within close limits (about 0.01 °C) to give confidence of negligible lateral heat exchange [31]. The sample tested needs to be relatively large (in the cm scale) and needs to be prepared in a standard circular or rectangular shape.…”

“…The absolute technique is recognised as the most accurate technique for determining the thermal conductivity of insulation materials, having an uncertainty of about 1.5% over a limited, near ambient temperature range [31]. However, testing soils is more challenging since moisture migration in unsaturated soils can occur when carrying out long duration steady-state tests.…”