Axial flow thermal conductivity methods measure the temperature at one end of the sample. Precise knowledge of the source producing the heat will provide absolute measurement. A comparative method is more widely used. The comparative method takes the measurement of a known sample and compares it with the unknown sample. A common comparison method is to sandwich the unknown sample between two known samples. This helps to control for known heat losses that may be present.
The heated flow meter method is suitable for measuring the conductivity of large cross sections of insulators. In this method, one or more specimens are sandwiched between heaters and surrounded by guard heater sections to prevent escape of heat. In the center, a metered area measures the thermal conductivity of the cross sections. Direct measurement rather than comparative methods provide the results, making this a form of absolute measurement.
This hot wire and probe method uses a needle-shaped probe inserted into the specimen. Placing the specimen in a furnace controls the outside temperature. This method of thermal conductivity measurement passes a specified electrical current through the probe as the heat source. The heat from the probe passes through the sample at a constant rate. When the heat reaches the surface of the sample, the heat will be lost to outside environment. Knowing the temperature change in a specified amount of time will allow you to calculate the thermal conductivity of the specimen. This method can be used in porous samples such as powders.
A variation of the heat flow method works to test samples that have thermal conductivity similar to air. Rather than surrounding the specimen with guards, the heat is allowed to flow to the surrounding air. The rationale for this approach is that guards in common usage will not surround these materials. Initial use of such prototypes indicates that the resulting measurements are sufficiently accurate. Computer models based upon this approach support the experimental accuracy. Those testing the material can use the data from initial experiments to refine this approach.