What is the Guarded Hot Plate Method?

The guarded hot plate method is a technique for measuring the thermal conductivity of a material under steady-state conditions. In this method, the material sample is subjected to controlled heating from one side through an electrically heated plate. This plate is integrated into a metal plate known as the guard, which is independently heated to match the temperature of the plate. On the opposite side of the sample, a cooled plate is employed to maintain temperature control. This configuration represents the one-plate method, as depicted in the left illustration. In the two-plate arrangement, the hot plate is positioned between two samples of identical size, as shown in the right figure. The electrically heated plate is once again situated within the guard, and both the guard and the hot plate can have square or circular shapes.

Hot plate scheme

A fixed heat rate is applied by the electric heater, generating a heat flow. Because the hot plate and the guard share the same temperature, heat produced by the plate is solely transmitted through the sample, preventing any heat loss from the sample and the heat source to the surroundings. Consequently, a unidirectional heat flow is established. Once a steady state is achieved, the heating and cooling plates stabilize at constant temperatures. Thermocouples are then employed to measure the resulting temperature difference across the sample.

The thermal conductivity of the material can be determined by considering the heat input, the sample thickness, the area of the heating plate, and the temperature difference across the sample.

Which properties are determined?

The Guarded Hot Plate (GHP) method allows for the direct measurement of thermal conductivity, rendering it an absolute measurement technique. However, a notable drawback of the GHP is the extended duration of measurements. This limitation can be addressed by employing the Heat Flow Meter (HFM), which utilizes a similar measurement setup as the GHP.

Which norms count for the Guarded Hot Plate Method?

Standards such as ASTM C177 and ISO 8302 provide guidelines for conducting steady-state heat flux measurements and determining thermal transmission properties using the Guarded-Hot-Plate Apparatus. These standards serve as benchmarks for ensuring the accuracy and reliability of thermal conductivity measurements in materials.

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