Monitoring Physical Fluid Properties Using a Piezoelectric Tuning Fork Resonant Sensor

Die Bestimmung von Fluidparametern, beispielsweise in der Ölzustandsüberwachung, zur Messung der Treibstoffqualität oder von Gaskonzentrationen ist ein Anwendungsgebiet, in dem resonante Sensoren vielfältige Vorteile aufweisen. In den letzten Jahren konnten durch verschiedene Entwicklungen im Zusammenhang mit der Auswertung solcher Sensoren signifikante Verbesserungen in Messgeschwindigkeit und Messgenauigkeit und eine deutliche Reduktion von potentiellen Querempfindlichkeiten erzielt werden. Besonders bei der Auswertung stark gedämpfter Resonatoren, wie bei der Messung von Flüssigkeitseigenschaften, ergeben sich dadurch neue Anwendungsfelder für diese Sensorfamilie. In diesem Beitrag wird die Leistungsfähigkeit dieser Technologie am Beispiel eines miniaturisierten Quarz-Resonators gezeigt und mit einem hochwertigen kommerziellen Analysegerät verglichen.For fluid analysis applications, such as oil condition monitoring, fuel quality, or gas concentration measurements, resonant sensors deliver an outstanding performance when signal processing is optimized and the fluid-mechanical model of the electromechanical resonator is suitable and accurate for the particular resonator. By combining recent advancements, significant improvements in accuracy, measurement speed, dynamic range, and suppression of cross-sensitivities could be achieved. These features enable the development of new solutions for a variety of measurement issues in industry and bio technology. In this contribution the performance of a highly universal evaluation system is demonstrated using a commercially available quartz crystal tuning fork resonator as sensing element for liquid viscosity and mass density. The obtained results are quantified with respect to an accurate lab bench viscosity and mass density meter. A significant advantage of this system is that it operates reliably and accurately even for strongly damped resonators. Therefore, the sensor elements can be used in a larger viscosity range than with alternative evaluation methods.(VLID)342643

Active magnetic levitation and 3-D position measurement for a ball viscometer

We present a new technique for 3-D position sensing and active magnetic levitation of a steel ball for use in a levitating ball viscometer. In order to achieve a stable levitation, a very sensitive positioning measurement system is mandatory. For this task the differential transformer principle was chosen to realize a 3-D position measurement. This leads to a purely magnetic sensor and actuator system without the need for other transducer types such as optical readout. The actuation utilizes power efficient switch-mode electronic circuitry which opens the possibility of upscaling the device, if demanded, for future applications. It is shown that this switch-mode actuation can be combined directly with the position measurement when special switching patterns are applied. A position resolution of  ∼  100 µm in all three axial directions at a sample rate of 476.19 Hz is achieved. For viscosity sensing, the steel ball is magnetically driven to orbital movements of variable revolution frequency of up to 2.5 Hz within a fluid chamber. The frequency response is analyzed and related to the shear viscosity of the fluid under test. As a proof of concept, measurements in various viscous liquids were performed with the prototype, showing promising results in the range of 1–10 mPa s. The principle may also be of interest for applications beyond viscosity sensing, such as fluid mixers, or as actuators in microfluidic devices

Balanced torsionally oscillating pipe used as a viscosity sensor

We present a robust viscosity measurement system based on a torsionally oscillating pipe. The sensitive surface of the sensor performs periodic movements in the fluid to be sensed, generating a shear wave that penetrates the fluid. Due to this interaction, the resonance characteristic of the structure is affected, in particular the quality factor decreases with increasing viscosity. The pipe is mounted at its center where it features a nodal point of the preferred resonant mode, reducing temperature issues while simultaneously enabling high quality factors. A mathematical model is presented illustrating how different parameters influence the sensitivity of the sensor. Long-term measurements were performed to demonstrate the time stability of the sensor setup.(VLID)341185