The main objective of this article is to demonstrate by performing experimental measurements how the equivalent capacitance C0 changes when a fluid sample such as water is in contact with the crystal and to relate this change with the relative permittivity of the fluid. These measurements were compared with simulations of traditional models like Butterworth-Van Dyke (BVD). To obtain the change of C0 when the crystal is in contact with water, the relation between the series resonance frequency (f_s) and frequency at minimum impedance with zero phase (f_r) is used. To know these values, a new way of finding principal parameters in quartz crystal resonator (QCR) sensors, such as series resonant frequency (f_s), half band half width (Γ) and maximum peak of conductance (G_m), is proposed; both for an unperturbed crystal and a crystal loaded with a liquid. The method consists in measuring the current (I) that flows through the crystal and the voltage (V) between electrodes at frequency values near to resonance (sweep frequency). Additionally, the susceptance |B| of the crystal is also measured, multiplying the 90 degrees shifted current of the crystal and its voltage, using a mixer. The DC component of this operation is proportional to the susceptance of the crystal. With the magnitudes of the admittance and susceptance, the real value of the conductance |G| is obtained for each frequency value in the sweep. The conductance and susceptance curves were fitted with a summation of Gaussian and sine functions respectively with the minor RMSE possible. The proposed method has been compared with simulations done in COMSOL Multiphysics in order to verify the experimental results with simulation data. MATLAB curve fitting toolbox was used to fit the experimental curves.

The main objective of this article is to demonstrate through experimental means the capacity of the quartz crystal resonator (QCR) to characterize biological samples of aqueous dilutions of hyaluronic acid according to their viscosity and how this capacity may be useful in the potential diagnosis of arthritic diseases. The synovial fluid is viscous due to the presence of hyaluronic acid, synthesized by synovial lining cells (type B), and secreted into the synovial fluid thus making the fluid viscous. In consequence, aqueous dilutions of hyaluronic acid may be used as samples to emulate the synovial fluid. Due to the viscoelastic and pseudo-plastic behavior of hyaluronic acid, it is necessary to use the Rouse model in order to obtain viscosity values comparable with viscometer measures. A Fungilab viscometer (rheometer) was used to obtain reference measures of the viscosity in each sample in order to compare them with the QCR prototype measures.

The main objective of this article is to demonstrate by experimental measurements the capability of the quartz crystal resonator (QCR) to characterize samples of liquor at different concentrations of adulteration with methanol, classify them according to their viscosity, and identify how to use this capability as a potential detection of fake liquor. According to the literature, the fake liquor is a common problem detected in several countries across the globe. The current strategy to detect the fake liquor is based on a piezoelectric biosensor that permits the classification of the samples according to its viscosity. The viscosity is a useful variable to determinate the degree of adulteration in the liquor samples because the viscosity value between ethanol (straight liquor) and methanol (most common fake liquor) is very different.