DT-900 – Porosity Analyzer
Porosity meter DT-900 applies a high frequency (3 MHz) AC electric field for measuring porosity. It can perform these measurements for porous particles either in dispersion or in sediment, as well as for porous monoliths.
The porous material is assumed as being saturated with a conducting aqueous solution, (usually 0.1 KCl solution). The motion of ions under the influence of the applied electric field generates an electric current, which in turn depends on the value of the electric conductivity. Measurement of this current for a known applied electric field yields information on electric conductivity. A higher conductivity indicates that more ions are present in the pores of the porous body. This can be used for monitoring the amount of space that is available for the water carrying these ions. The ratio of this space to the total volume of the porous body corresponds to the desired porosity value.
Typically there are several problems in using electric field for characterizing porosity, especially if it is in DC mode. First, conductivity measurement in a DC field is complicated by possible electrode polarization and electrochemical reactions. Secondly, only pores that percolate from one electrode to another would contribute to the conductivity of the porous body. Therefore, the DC method is not applicable for porous bodies with intricate pore structure including closed pores.
The application of a high frequency AC electric fields allows for easy resolution of both of these problems. Electrode polarization becomes negligible for MHz frequencies. This allows for construction of the probe with a very simple flat geometry, as shown in the picture presenting this instrument. Additionally, due to capacitive coupling high frequency electric fields penetrate all pores, even closed pores. This means that conductivity measured at high frequency would reflect the motion of ions in all pores of the porous body where saturating liquid could penetrate.
There is a well-known Maxwell-Wagner theory that predicts the difference between the conductivity of a heterogeneous system Ks and that of an equilibrium supernate Km. It yields the surprisingly simple expression for the ratio of these conductivities: Ks / Km = 2P / (3-P), where P is porosity. This expression can be used for calculating porosity from the measured conductivities of the porous material and equilibrium liquid that it is saturated with.