A6.2 - Density-Viscosity Measurement Cell for Electrodynamic-Acoustic Resonators

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
A6 - Ultrasonic Sensors I
F. Lucklum, B. Jakoby - Johannes Kepler University Linz (Austria)
141 - 146


Miniaturized physical sensors for precise density and viscosity analysis are required as supplement and replacement for complex and expensive laboratory instruments. Utilizing miniature mechanical resonators as transducers, one can greatly reduce the necessary liquid volume, measurement time, and complexity of the experimental setup.
Electromagnetic-acoustic transduction in mechanical resonators has been established as a low cost alternative to the ubiquitous piezoelectric or capacitive methods with unique advantages in terms of suitable materials, vibration mode flexibility, and simplicity of the excitation setup. These devices include suspended beams and plates, remotely excited silicon membranes and metal plates, and clamped polymer membranes.
In this work, Lorentz forces are generated by directly impressing an alternating current across suspended plate resonators made of different conductive substrates (nickel-silver, aluminum, copper, brass, stainless steel) with a superposed, perpendicular external magnetic field of 0.72 T flux density supplied by NdFeB magnets. The measurement cell was developed as a modular design for easy assembly and disassembly, milled into inexpensive PCB material, interspaced with Parafilm® or Teflon® sheets, and electrically contacted and fluidically sealed by tightly screwing the different modules together. This inexpensive design allows for quick cleaning and exchange of the sensor element. The suspended plate resonators have been fabricated by standard photolithography and etching, milling, and laser cutting of thin metallic sheets. The resonator itself is electrically operated as a two-port device, where the excitation can be applied to any two contact pads, while the other two are connected to a
network analyzer to measure the voltage induced by mechanical motion. The excitation current can thus flow across the plate in a horizontal, vertical and diagonal fashion, resulting in vertical, horizontal and diagonal forces, respectively.
Viscosity is a highly temperature-dependent property, thus we investigated the temperature behavior of the resonator in air due to changes in elasticity, expansion, boundary/clamping conditions, and also quality of the electrical contact. From the frequency spectrum, we can evaluate the quality factor and frequency shift in different liquids, which are a function of the density-viscosity product. Due to the small fluidic cell, only small amounts of liquids (on the order of 100 μl) are required.