What is the Guarded Hot Plate Method?
The guarded hot plate method is a steady state measurement method which determines the thermal conductivity of a material by using the electrical power output of a hot plate with guided heat conduction. The sample is heated from one side by an electrically heated plate. This plate is embedded in a metal plate, called guard, which is separately heated to the same temperature as the plate. The other side of the sample is also temperature controlled by a cooled plate. This is the setup of the one-plate method, which is illustrated schematically in the left figure. In the two-plate setup the hot plate is sandwiched between two samples of same size (see figure on the right). The electrically heated plate is again placed into the guard. The shape of these can be square or circular.
A heat flow is generated by the electric heater with a fixed heat rate. Due to the same temperature of the hot plate and the guard, the heat produced by the plate is only transferred through the sample and no heat loss from the sample and the heat source to the surrounding occurs. Thus, unidirectional heat flow is generated. After a steady-state is reached, the heating and cooling plates have stable temperatures. Then, the thermocouples measure the resulting temperature difference over the sample.
The thermal conductivity can be determined by using the heat input, the thickness of the sample, the area of the heating plate and the temperature difference through the sample:
Which properties are determined?
With the Guarded Hot Plate method, the thermal conductivity can be measured. Since the thermal conductivity is determined directly, the GHP is an absolute measurement method. The main disadvantage of the GHP are long measurement times, which can be overcome by using the Heat Flow Meter (HFM) which uses a similar measurement setup as the GHP.
Which norms count for the Guarded Hot Plate Method?
- ASTM C177 – Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus
- ISO 8302 – Thermal insulation; determination of steady-state thermal resistance and related properties; guarded hot plate apparatus