Permittivity Characterization of Conductive and Corrosive LiBr Water Solutions, Method Validation up to 9 GHz Using a Low-Cost SMA Probe

TL;DR

The paper introduces a low-cost method to measure the permittivity of corrosive and conductive LiBr water solutions up to 9 GHz.

Contribution

A low-cost SMA probe method is proposed for accurate permittivity characterization of high-conductivity and corrosive solutions.

Findings

The method was validated on saline solutions and applied to LiBr water solutions with concentrations up to 54%.

The imaginary part of permittivity showed consistent results correlated with concentration, while the real part was underestimated.

An inversion in permittivity variation direction was observed for LiBr concentrations exceeding 35%.

Abstract

In this article, we present a method for extracting the complex permittivity of high-conductivity solutions up to 9 GHz. Microwave measurements were performed using a low-cost SMA connector, employed as an open-circuit coaxial probe, which was subsequently brought into contact with the liquids under characterization. Compared to state-of-the-art techniques, this method offers the advantage of good accuracy while remaining simple to implement with a low-cost sensor. The affordability of the sensor is crucial because the sensor must operate in a corrosive environment. The use of existing but expensive commercial solutions is prohibitive. Therefore, sensor replacement must be straightforward and inexpensive in case of damage. Two permittivity extraction methods were studied, both relying on a straightforward experimental approach and knowledge of the complex permittivity of reference liquids (deionized water, ethanol, methanol). The technique was initially validated on saline solutions (NaCl) known from the literature before being applied to aqueous lithium bromide (LiBr water) solutions. Eight LiBr water solutions, known to be highly corrosive, were measured for LiBr mass concentrations ranging from 1% to 54% and for conductivities up to 14 S/m. The high conductivity of these solutions brings challenges to extract the real part of the permittivity, which is underestimated by both methods. In contrast, the imaginary part exhibits consistent results with variations strongly correlated to the concentration. Notably, an inversion of the direction of variation was observed for mass concentration in LiBr exceeding 35% aligning with the conductivity curve.

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