Non-Invasive, Non-Contact Conductivity Measurement Using Radiofrequency Probe Loading

Accurate measurement of electrical conductivity in water-based samples is essential for applications ranging from water quality monitoring to biological sensing. However, direct-contact methods are prone to electrode degradation, while indirect-contact methods often suffer from undefined current paths. This study introduces a non-invasive, non-contact technique for conductivity measurement in water-based samples using radiofrequency (RF) probe loading, leveraging inductive losses induced by eddy currents in conductive media. Changes in the quality factor of the RF probe are analyzed to establish a direct proportionality between inductive losses and sample conductivity. This relationship is validated across solenoidal, loop-gap resonator, and surface coil designs with cylindrical and planar samples. Theoretical equations, derived from classical electromagnetism and the principle of reciprocity, closely align with experimental results, revealing the critical role of probe geometry and resonant frequency in measurement sensitivity. The findings demonstrate robust performance across diverse ionic solutions including samples with flow, achieving cost-effective measurement with minimal hardware. This method allows differentiation between samples with differing conductivity, even when absolute conductivity values cannot be determined. This makes it especially suited for analyzing complex, heterogeneous materials like foods or biological samples. The radio frequency probe loading approach offers a versatile alternative to conventional conductivity measurement methods, with potential for real-time, non-invasive monitoring in dynamic systems.