Sensitivity and Accuracy of Dielectric Measurements of Liquids Significantly Improved by Coupled Capacitive-Dependent Quartz Crystals

Vojko Matko,Miro Milanovič

doi:10.3390/s21103565

Abstract

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.

Highlights

Agnieszka SzypłowskaCharacterization of the dielectric properties of materials, such as relative permittivity, conductivity and loss tangent, are of a great importance for a variety of applications.Accurate measurements of these characteristics can provide scientists with valuable information on the suitability of individual materials for the intended use and an enhanced product quality control
We show that by the improved method presented in this article, the measurement uncertainty is reduced to ±0.05% for the relative permittivity, with no significant change in the uncertainty of the loss tangent
The experimental setup for the measurement of complex permittivity consists of a glass test tube with capacitance, C01, and quartz crystal oscillator

Summary

Introduction

Agnieszka SzypłowskaCharacterization of the dielectric properties of materials, such as relative permittivity, conductivity and loss tangent, are of a great importance for a variety of applications.Accurate measurements of these characteristics can provide scientists with valuable information on the suitability of individual materials for the intended use and an enhanced product quality control. Characterization of the dielectric properties of materials, such as relative permittivity, conductivity and loss tangent, are of a great importance for a variety of applications. To the molecular structure of the material, the complex relative permittivity depends on frequency, temperature, humidity and pressure This is why numerous methods for measurements of the dielectric properties in various liquids, powders and solid materials in different frequency ranges [1,2,3,4,5] have been developed. Measurements of conductivity of liquids present an important tool for the analysis of binary liquids/electrolyte mixtures and to determine critical points of various liquid mixtures It is widely used for determination of the pollution of oils, fuels and lubricants in power engineering [6,7] and for measurements of dielectric properties in bio-applications [1,5,8]

Methods

Results

Discussion

Conclusion