Precision measurement of humidity dependent permittivity of foams

Permittivity measurement of foams and other soft materials poses several difficulties. E.g., the permittivity of such material is usually low in value and is strongly dependent on the compression-ratio, atmospheric humidity, etc. Furthermore, the thermal expansion of the materials used to construct the probe, i.e., the temperature dependency can affect the measurement. In this paper we present a measurement setup for permittivity measurement of foams in the frequency range of up to several MHz. Results for a test material are reported and analyzed with respect to temperature, humidity and compression-ratio dependency. The measurement uncertainty of the probe has been investigated by means of a theoretical analysis within a finite element simulation framework.