An Improved Microwave Sensor for Qualitative Assessment of Recycled Cooking Oils

An improved planar microwave sensor comprising a resistor-embedded top microstrip line and a ground-etched complementary split-ring resonator (CSRR), designed on a 1.6-mm Taconic substrate (
$\varepsilon _{r}$
= 2.2), is presented to assess the quality of recycled cooking oils. The proposed resonant sensor exhibits an improved quality factor and a reasonably larger shift in the resonant parameters under loading conditions, thereby facilitating the detection of even slight variations in electrical and chemical properties of low-loss dielectric samples. The enhanced performance of the proposed sensor is attributed to incorporating a specific valued lumped resistor on the top side of a microstrip line, which helps in coupling the maximum electric field to the CSRR and providing an extreme improvement in the
${S} _{21}$
notch (−40 dB) near resonance. The proposed sensor is designed and simulated using the full-wave simulator, the Computer Simulation Technology (CST) Microwave Studio (MWS). The equivalent circuit model is developed using the Advanced Design System (ADS) to understand the behavior of the circuit in detail. An analytical formulation is implemented using MATLAB, which provides a definite relationship between the small value of the resistor connected on the top of the microstrip line and the
${S} _{21}$
value near resonance. Several fresh and recycled cooking oil samples are measured, and it is observed that their acid values (AVs) get degraded from 1.5 to even 9.4 indicating the toxic nature of the recycled oils and their adverse effect on human health. The AV of the oil samples in the present situation is obtained under a controlled chemical environment by developing an empirical relationship between this parameter and the normalized frequency and the
${S} _{21}$
notch. The fabricated sensor provides a substantially higher value of fractional sensitivity (~25 MHz/mgKOHgm
$^{-1}$
) and larger deviation in the
${S} _{21}$
notch level (~3.33 dB/mgKOHgm
$^{-1}$
), for a unit change in the oil rancidity.