Under the steady state technique, the Parallel Plate Method per ASTM D5470 measures thermal impedance and calculates the apparent thermal conductivity of a material. The schematic here displays a side view of the test fixture, where the testing sample is compressed under a known pressure between a heat source and a cooling unit. Starting from the top of the schematic, there is first a guard heater and primary heater. This is where the heat flow (Q) is going to come from. Air currents around the heater cause convection, so it is necessary to have a guard heater to be sure the heat flow is uniform. Next, heat flows down the Upper Calorimeter, which in this case is a solid block of aluminum. The heat travels through the sample that is d meters thick, then down to the Lower Calorimeter (also a solid aluminum piece). Below the Lower Calorimeter is a cooling unit, typically cooled by water of a uniform temperature. The contact area A is calculated from the size of the Calorimeters, which is typically a 0.033 meter diameter disk. The Delta-T temperature gradient is taken in steady state from thermocouples placed just beyond the surface of the sample in each calorimeter. Thus using Fourier’s Law of heat conduction, the apparent thermal conductivity can be calculated.