Validation of a Complex Permittivity Meter for Porous Media Operating Between 1 and 20 MHz

This paper describes the procedure to qualify an admittance meter as a permittivity meter and a conductivity meter σ sensor through validation with liquids over a range of relative permittivity ε r values from 1 to 80 and with operating frequencies f between 1 and 20 MHz. The sensor is a capacitor which consists of two parallel cylinders of 10-cm typical dimension. A circuit-based model of the sensor was previously calibrated against accurate electronic components. A discrepancy of 10% is found between the calibration with components and the validation with liquids. All potential errors have been carefully examined: parasitic impedances of the electronic circuit and leads, modifications of liquid permittivity ε r liq due to temperature influence, water contamination or relaxation effects, and possible fringing effects with the help of numeric simulations. Forty percent of the discrepancy results from the finite dimensions of the liquid-filled recipient. The fringing effects due to insulating rings at each end of the capacitor seem to be discarded. The analysis of the uncertainty on ε r sensor and σ sensor shows a relative uncertainty of 3%-5% due in large part to the numerical acquisition of high frequency.