A new guarded hot plate for thermal conductivity measurement of solid moderately conductive materials at high temperatures


J. Hameury1, V. Scoarnec1 and B. Hay1

1Laboratoire National de Métrologie et d’Essais (LNE), Trappes, France

Keywords: guarded hot plate, high temperatures
property: thermal conductivity
material: solid materials

The guarded hot plate is the most accurate technique for thermal conductivity measurement around room temperature for low and moderately thermal conductive materials. It has been used for about a century mainly for building insulating materials. Industry requires now thermal conductivity data for many homogeneous or non-homogeneous materials used at medium and high temperatures. For homogeneous opaque materials with thermal conductivity above 0.5 W m-1 K-1, transient techniques are suited to get thermal conductivity data, from low to very high temperatures, in a short time with relative uncertainties of a few percents. For insulating or non-homogeneous materials guarded hot plate is still the most accurate technique to measure the effective thermal conductivity. LNE has developed a new guarded hot plate for characterization of industrial moderately thermal conductive materials (0.2 to 5 W m-1 K-1) in the temperature range 50 to 400°C. The instrument has been designed to perform easily measurement on a large variety of industrial materials. The two specimens configuration is used; three heating plates built on the same design (separated metering and guard sections) are used. There are heated with metal-sheathed electrical resistors sandwiched between thick nickel sheets. The overall size of the plates is 308 X 308 mm (square shape), the metering section is square 100 X 100 mm. The specimens are square and have the size of the metering section, the square shape has been chosen for ease of shaping and the limited size allow to machine quite easily very flat surfaces. Type K metal sheathed thermocouples (1mm in diam.) are used for all temperature measurements (specimens gradients and temperature imbalances between metering and guard sections). The measurement of temperature gradients in specimens can be performed in different ways depending on thermal resistance of specimens. If specimens have a low thermal resistance and are thick enough thermocouples are embedded directly in holes drilled in specimens parallel to faces, with that arrangement thermal contact resistances between specimens and heating plates are not a source of error. If thermal resistance of specimens is high enough, thermocouples are placed in grooves machined on the faces of the heating plates. The technique has been validated on the certified CRM039 Pyrexã glass and by comparison to a low temperature guarded hot plate.


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