Matko Vojko, Milanovič Miro
Faculty of Electrical Engineering and Computer Science, University of Maribor, Koroška c. 46, 2000 Maribor, Slovenia.
Sensors (Basel). 2021 May 20;21(10):3565. doi: 10.3390/s21103565.
A method to measure complex permittivity of liquids by using a capacitive-dependent quartz crystal and two quartz oscillators for temperature compensation in the frequency range of 4-10 MHz is described. Complex permittivity can be detected with high precision and sensitivity through a small change of capacitance and conductance, because a change in reactance in series with the quartz crystal impacts its resonant oscillation frequency. The temperature compensation in the range below 0.1 ppm is achieved by using two quartz oscillators that are made of elements of the same quality and have a temperature-frequency pair of quartz crystals. With the help of a reference oscillator, measurements of frequency are more accurate, because the frequency difference is in the kHz region, which also enables further processing of the signal by a microcontroller. With a proper calibration, the accuracy of this highly sensitive quartz crystal method is ±0.05%, which is an order of magnitude lower than that for a capacitance method without quartz crystals. The improved accuracy is of significant importance in the field of power engineering to monitor coolants and lubricants, oils, liquid fuels and other liquids, the dielectric properties of which are crucial for proper operation of devices.
描述了一种通过使用电容相关石英晶体和两个用于温度补偿的石英振荡器，在4 - 10 MHz频率范围内测量液体复介电常数的方法。由于与石英晶体串联的电抗变化会影响其谐振振荡频率，所以通过电容和电导的微小变化就能高精度、高灵敏度地检测复介电常数。通过使用由相同品质元件制成且具有温度 - 频率对的两个石英晶体，可实现低于0.1 ppm范围内的温度补偿。借助参考振荡器，频率测量更准确，因为频率差在kHz区域，这也使得微控制器能够对信号进行进一步处理。经过适当校准，这种高灵敏度石英晶体方法的精度为±0.05%，比无石英晶体的电容法低一个数量级。在电力工程领域，提高的精度对于监测冷却液、润滑剂、油、液体燃料及其他液体非常重要，这些液体的介电特性对设备的正常运行至关重要。
