Wireless Sub-mm Liquid-Level Sensing by Noncontact Nested Split-Ring Resonators

A wireless, noncontact, and sub-mm liquid-level sensor is demonstrated. The design is based on the nested split-ring resonator (NSRR) structure, preferred for its high sensitivity and compact size. The concept of sensing is explained by an equivalent circuit model, the parameters of which are functions of the change in the level of the liquid that loads the NSRR. Sensing is demonstrated via full-wave electromagnetic simulations and measurements. Average sensitivity values of 12.9 MHz/mm for a range of 0–5 mm and 3.00 MHz/mm for a wider range of 0–50 mm are obtained by experiment when the liquid under test is water. The sensor is demonstrated to be able to detect sub-mm level changes in liquid height as well, where the resolution is obtained around 65-<inline-formula> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>m. Results of various measurements are presented by which the effects of the liquid type, holder material, and holder thickness on sensing performance are investigated. It is shown that the proposed sensor can be used to perform wireless noncontact level measurements of various nonconducting liquids such as pure water, ethanol, or oil.