Low-frequency dielectric measurements of complex fluids using high-frequency coaxial sample cells

A simple and low-cost interface for dielectric measurements with pulsed or time-harmonic excitation is described, covering the frequency range from 30 Hz to 15 MHz. It can be directly connected to the high-frequency coaxial sample cells used for time- or frequency-domain reflectometry (TDR/FDR). Therefore, low-frequency measurements of complex fluids can be made without disturbing the sample. Capacitances from 5 pF to 240 pF are measured with a single set of reference capacitors. One of the reference capacitors is installed as a plug-in element to provide different measurement ranges. A linear substitution technique is described to improve measurement accuracy. The measuring system achieves a standard uncertainty lower than 0.05 pF in the entire frequency range. Experimental results include measurements of lumped capacitors, a coaxial line, a RC circuit, cyclohexane and a ternary microemulsion (water/decane/surfactant).