Thermal Conductivity – Thermal Effusivity on heterogeneous insulation materials with transient methods

There are a number of papers and spec sheets on insulators using short test times (1 to 5 seconds) from any transient thermal conductivity method. Although the measurement of thermal effusivity can be used to compare materials in terms of touch times, thermal conductivity whether measured directly or backed out of thermal effusivity would not be representative of the sample.

Typically, it takes several seconds for the transient heat to get into the bulk of the sample, thus test times shorter than that is required to penetrate the bulk properties should be viewed with suspicion. The time required to penetrate past contact resistance is not possible to be calibrated away.

Transient measurements where the penetration depth is known is best. Alternatively, steady-state measurement, which measures the full thickness is suitable as well.