Characterisation of dielectric coefficients of vegetable oils based on heat exchange and frequency of usage by using microwave rezonator based sensors

To measure oil parameters such as oil usage, viscosity, base number, refractive index, acid numbers, density, water substances etc. complex laboratory equipments are required. Instead of these complex laboratory equipments, such oil properties can also be characterized by microwave measurements, and microwave measurements are preferred for oil quality determination due to ease and clear results. There are two main parameters in microwave measurements: dielectric constant value e and loss tangent value tanδ. When the dielectric constant of the sunflower oil used as frying oil was measured, dielectric constant and loss tangent of used oil varied compared to unused pure sunflower oil. These differences were caused by the presence of pollutants, such as water and other particles, and by changes in the chemistry of the oil. In this study, two different sensor structures have been proposed to determine the amount of sunflower oil usage as frying oil. Firstly, to see the effects of heat on the dielectric parameter of the oil, pure sunflower oil was heated 16 times without using potato samples and a small amount of oil sample was taken after each heating process. Subsequently, pure sunflower oil was fried 16 times with the same amount of potato samples, and a small amount of oil sample was taken for each dielectric measurement. Then, the dielectric constant and loss tangents of the sunflower oil samples were measured by the vector network analyzer in the microwave laboratory. As a result of the characterization of sunflower oil, the dielectric constant has increased linearly from 2.9 to 3.3 between the 4 GHz and 8 GHz band, and this linear increase depends on the number of fries with the potato samples. This change in the dielectric constant is caused by water and other particles which are mixed with the oil from the potato samples during the frying process. In addition, the measured dielectric constant values were added to the microwave simulator which is based on the finite integration technique to design the sensor structures to determine the oil lifetime. In this paper, two different transmission line sensors are designed and presented. The first transmission line sensor has two advantages; resonance shift on S11 at about 5,45GHz and resonance shifts at 4,5GHz, 5,3GHz and 7,5GHz in transmission-S21. The second transmission line sensor has resonance shifts at about 5.4 GHz in S21. Finally, designed sensor structures were fabricated and experimental measurements were conducted by using the vector network analyzer. According to the experimental results, two sensor structures designed to determine the life of the frying oil sample are usable.