A sensor suitable for online monitoring of viscosity and density of glycerol-water mixtures is presented. The device is based on Lorentz force excitation and features an integrated piezoresistive readout. The core sensing element is a rectangular vibrating plate suspended by four beam springs. Two of the plate-carrying springs comprise piezoresistors. With two additional resistors on the silicon rim they form a half Wheatstone-bridge. Through the conductive layer of the beam springs a sinusoidal excitation current is driven. In the field of a permanent magnet, the Lorentz force excites plate vibrations resulting in a bridge unbalance. We recorded both the frequency response of the amplitude and the phase of the bridge output. By evaluating the properties of the resonant system, it is possible to extract the glycerol percentage and, hence, the viscosity and the mass density of the mixtures. This work is an extended version of the paper originally presented at SPIE
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We demonstrate that the thermal conductivity and diffusivity of fluids can be measured independently of their motion state utilizing a micromachined calorimetric sensor. The sensor membrane bears a heating resistor and two symmetrically arranged thermistors. By immersing the sensor into the laminarflowing sample fluid and applying an AC heating current, the frequency response of the thermistor temperatures can be exploited to evaluate the thermal properties of the fluid. We developed a novel analytical model to describe the conductive transfer in the micromachined sensor as well as the fully conjugated heat transfer in the fluid. The validity of this model was confirmed experimentally by a practical example using nitrogen as fluid flowing through a rectangular flow channel. Based on these results, a thermal parameter extraction procedure can be deduced. Moreover, with known thermal parameters, the arrangement can also be used for flow measurements.
Increasing demands for online monitoring of liquids have not only resuted in many new devices relying on well-established sensing parameters like shear viscosity but also initiated research on alternative parameters. Recently, the longitudinal viscosity has been evaluated as a promising candidate because the devices arising enable the bulk of the liquid to be probed rather than a thin surface layer. We report on a multi-purpose sensor which allows simultaneous measurement of the sound velocity and longitudinal viscosity of liquids. The device embodiment features a cube-shaped chamber containing the sample liquid, where one boundary surface carries a flush-mounted PZT transducer. In operation, the transducer induces standing, resonant pressure waves in the liquid under test. We studied the influences of sound velocity and longitudinal viscosity on the generated pressure waves by means of the Navier–Stokes equation for adiabatic compressible liquids and exploited both parameters as the basic sensing mechanism. Furthermore, a three-port network model describing the interaction of the transducer and sample liquid was developed in order to be applied for extracting the parameters of interest from the raw measurement data. Finally, we demonstrate the device and method by carrying out and discussing test measurements on glycerol–water solutions.
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