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Two main topics are presented in this work which enable more efficient use of oil condition monitoring systems based on resonant fluid sensing. A new fluid model for a recently introduced compact measurement unit for oil condition monitoring based on simultaneous measurement of viscosity and density is discussed. It is shown that a new fluid model allows achieving higher accuracies, which is demonstrated by comparison to earlier models. The second topic deals with measuring fluid parameters over varying temperatures and thus providing additional monitoring parameters and enhanced data consistency. We propose an alternative representation of the Vogel model using transformed parameters having a clear physical meaning and which are more stable in presence of measurement noise.

An online condition monitoring system based on the measurement of viscosity and density of liquids is presented and applied to three different measuring tasks relevant for industrial and automotive applications. One topic is oil characterization in hydraulic systems. It is shown that by measuring over varying temperature and pressure, additional physical properties can be made available for online condition monitoring, which are difficult to measure otherwise. These include, for example, the coefficient of thermal expansion and the bulk modulus, which is also related to the proportion of dissolved air. In the second application we investigate the efficiency of a passive oil defoamer and estimate the percentage of free air. Finally, the suitability of the measurement system for the determination of the diesel fraction in the engine oil as caused by the regeneration cycles of the diesel particulate filter is demonstrated.

In this work, a resonant quartz tuning fork sensor based measurement system is employed to study the settlement of small particles in suspension as they pass the sensing element. The resonator features varying sensitivity to fluid parameter changes along its length, with the tip being most sensitive. The spatial averaging effect is analyzed theoretically and measured by passing the sensor through a fluid–fluid interface. Subsequently, the averaging function is used for particle distribution determination from recorded resonance frequency and quality factor measurements. The associated inverse problem is of Fredholm type and is solved numerically. Based on the system specifications, detection limits, required temperature stability and expected precision are assessed and particle distribution functions of defined test dispersions are measured.

We investigated the vibration characteristics of the torsional oscillation mode of an earlier proposed resonant viscosity sensor based on vibrating pipes. To do so, the sensor housing is opened, so that half of the resonator is freely accessible. The actuation and readout of the sensor is realized with electromagnetic coils on the remaining part of the housing and the oscillation velocity is measured on different positions on the uncovered sensor surface. We demonstrate the feasibility and suitability of the electromagnetic readout principle and identified an additional spurious oscillation. All measurements were performed with an air filled resonator to minimize the damping and maximize the oscillation amplitudes.

We provide an overview of recent achievements using quartz tuning forks for sensing liquid viscosity and density. The benefits of using quartz crystal tuning forks (QTFs) over other sensors are discussed on the basis of physical arguments and issues arising in real world applications. The path to highly accurate and robust measurement systems is described and a recently devised system considering these findings is presented. The performance of the system is analyzed for applications such as the mixing ratio measurement of fuels, diesel-soot contamination for engine oil condition monitoring, and particle size characterization in suspensions. It is concluded that using properly designed systems enables a variety of applications in industry and research.

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.

In modern light-duty Diesel vehicles the particulate filters in the exhaust system require periodic regeneration to prevent plugging. The regeneration procedure is a major cause of dilution of engine oil with diesel fuel. In order to investigate the effect of the operation conditions of the engine on the fuel content in the oil, a precise and fast in-line measurement system is required. In this contribution a resonant sensor system for determination of diesel content by measuring the viscosity of the oil is tested. A commercial off-the-shelf quartz tuning fork resonator is used as sensor element and evaluated using a universal analyzer for resonant sensors developed for fast and precise acquisition of resonant sensor responses. The experiments presented in this paper show that fuel concentration in the range of 0.2 % can be determined within seconds. The obtained results are compared to that obtained with an accurate lab bench viscosity and mass density meter.

This paper presents the design of a viscometer principle using a magnetically levitated ball. A steel ball is submerged in a fluid and is supported by magnetic forces only. By superposing different magnetic fields it is possible to excite the ball to perform an orbiting movement so that a viscous force acting on the ball is generated. The ball's trajectory is recorded and evaluated for several excitation frequencies, resulting in a transfer function. First experimental measurement results show that there is a clear relationship between the extracted transfer function and the fluid's shear viscosity.

In this paper we present a new balanced flowthrough viscosity sensor working with a torsionally resonating pipe. The whole system is machined in aluminum and is designed to be placed in the pipes of a chemical production plant. The quality factor Q and the resonance frequency fr are used as measures for the viscosity and density. We describe the sensor setup, its working principle for different resonant modes, the actuation and the readout of the sensor and present first measurement results for liquids featuring different viscosities.

The performance of a recently introduced method to estimate the resonance parameters from complex spectral data and the error propagation for viscosity and density parameters of liquids are examined. The method is known to produce excellent results when used for piezoelectric and Lorentz force actuated sensors for single port or two-port devices. The method is also suited for very low quality factors (< 10) which extends the usable measurement range of many sensor concepts, especially for fluid sensing applications. Generally valid expressions for the measurement accuracy are stated and compared to measurement results obtained with a piezoelectric tuning fork and a Lorentz force actuated and inductively read out platelet sensor.

Resonating sensors are commonly used for various measurement tasks, e.g., as microbalances as well as for viscos-ity and density measurements of viscoelastic media. The monitored parameters usually are the resonance frequency and the quality (Q) factor of the mechanical resonance which is measured electrically. The measurement is affected by parasitic effects induced by the sensor design but also by the measurement setup. Accurate calculation of reso-nance parameters from measured sensor data then is one of the most crucial points affecting measurement precision of such sensor systems. In this contribution we demonstrate the performance of a recently proposed algorithm used to accurately determine resonance frequency and Q–factor from sensor data that is significantly disturbed by parasitic sensor effects.

Resonant mechanical sensors are powerful tools for measuring, e.g., physical properties of fluids. Suitable systems for real-time monitoring of a sensor’s resonance behavior are, e.g., oscillator circuits and phase-locked loop circuits. In this contribution, we present a novel specific digital phase-locked loop circuit with the ability to determine damping, as well as resonance frequency, and providing automatic compensation of spurious parallel capacitances of the sensor. Instead of a phase-frequency detector, the system measures phase and amplitude of the sensor signal with a 16-bit ADC and uses this information to control a numerically-controlled oscillator. Due to the used 4-channel digital synthesizer, the sensor can be excited with up to three frequencies at the same time facilitating enhanced performance in terms of simultaneous measurement of higher order resonances and compensation of spurious capacitances.

Resonating sensors are commonly used for various measurement tasks, e.g., as microbalances as well as for viscosity and density measurements of viscous or viscoelastic media. The resonance frequency and the quality (Q) factor of these sensors is typically determined electrically and can be disturbed by parasitic effects caused by the sensor or the interface circuit. To achieve high accuracy the resonator behavior is recorded in vicinity of the resonant frequency and the desired parameters are calculated from this spectral information. For the determination of resonance frequency and Q–factor the use of estimator algorithms which are insensitive to parasitic effects is preferable. We demonstrate advantageous estimator strategies and assess the performance of the specific estimators by evaluating the spectral data of a strongly damped resonant sensor evaluated with two different analyzer instruments.

We investigate a gas density sensor based on quartz crystal resonators for high pressure ( 2.5 × 1 0 6 Pa ) and high temperature ( 340 K ) application. The suitability of tuning fork resonators for density measurement in pure gases has been demonstrated. Furthermore, thickness shear-mode resonators are commonly used for viscosity and density measurement in liquids. In our contribution, these devices are compared with respect to their sensitivity to gas density for the target application in a polymerization reactor, involving high ambient pressures. It was confirmed, that tuning fork resonators are mainly sensitive to density, whereas shear-mode resonators are more suitable for viscosity measurement. Moreover, a density sensor for density measurements in unknown gases and gas mixtures, based on two tuning forks, is presented. At high pressures, an additional pressure-dependent frequency shift has to be considered.

Zusammenfassung Präzise Impedanzanalysatoren werden in jüngerer Vergangenheit vermehrt zur Auswertung resonanter Sensoren eingesetzt. In diesem Beitrag wird dazu ein Schaltungskonzept vorgestellt und der geplante Einsatz, ein Viskositätssensor basierend auf einem Quarz-Schwinger zur Überwachung eines Zeolith-Kristallisationsexperimentes auf der Internationalen Raumstation (ISS), präsentiert.

This paper reviews our recent work on vibrating sensors for the physical properties of fluids, particularly viscosity and density. Several device designs and the associated properties, specifically with respect to the sensed rheological domain and the onset of non-Newtonian behavior, are discussed.

Resonant mechanical sensors are powerful tools for measuring, e.g., physical properties of fluids. Suitable systems for real-time monitoring of a sensor’s resonance behavior are, e.g., oscillator circuits and phase-locked-loop circuits. In this contribution, we present a novel specific digital phase-locked-loop circuit with the ability to determine damping, as well as resonance frequency, and providing automatic compensation of spurious parallel capacitances of the sensor. Instead of a phase-frequency detector, the system measures phase and amplitude of the sensor signal with a 16-bit ADC and uses this information to control a numerically-controlled oscillator. Due to the used 4-channel digital synthesizer, the sensor can be excited with up to three frequencies at the same time facilitating enhanced performance in terms of simultaneous measurement of higher order resonances and compensation of spurious capacitances.

In recent years, compact and precise impedance measurement electronics have proved to be reasonable alternatives for dedicated measurement tasks like impedance measurement of resonant sensors. In this contribution, a novel design of a compact impedance analyzer is discussed, and the target application, a quartz crystal resonator (QCR) viscosity sensor for conductive liquids, as intended for the monitoring of a zeolite synthesis process aboard the International Space Station (ISS), is presented.

In recent years, compact and precise impedance measurement electronics have proved to be reasonable alternatives for dedicated measurement tasks like impedance measurement of resonant sensors. In this contribution, the design of a compact subsampling impedance analyzer is discussed. Furthermore the target application, a quartz crystal resonator (QCR) viscosity sensor for conductive liquids, as intended for the monitoring of a zeolite synthesis process aboard the International Space Station (ISS), is presented. In particular, accuracy aspects of the involved signal processing stage are examined.

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.

A performance estimation method for resonant viscosity and density sensors is described. The ultimate sensor precision and the separability of density and viscosity from measured frequency spectra are estimated. The approach relies on using recently introduced models which are flexible enough to describe a large class of sensors. It is therefore not required to develop a specific sensor model and a parameter extraction method for each newly devised sensor. With the method introduced in this contribution, it is sufficient to adjust and validate a general model using at least three test liquids. Then, the error propagation and the error bounds for viscosity and density can be estimated and consequently the required signal-to-noise ratio (SNR) or the amount of required data points can be determined. This method enables direct comparison of different sensor setups as will be demonstrated for piezoelectric tuning fork sensors, electrodynamic platelet sensors and steel tuning forks.

In this contribution a concept for a torsional oscillators which is electromagnetically driven and read-out is presented. The aim is to experimentally investigate a principle feasibility for a torsional resonator with application for viscosity and mass density sensing. Such a device is particularly interesting as cylindrical torsional resonators for fluid sensing applications are hardly reported but unlike other devices, would yield pure shear wave excitation in the liquid. The design of first conceptual prototypes and measurement results obtained with these devices are presented.

Miniaturized viscosity sensors operating at frequencies in the KHz-range offer portability and results comparable to existing lab viscometers. In this paper we will briefly describe a viscosity measurement cell utilizing electrodynamic-acoustic resonator sensors and provide a modified design with interchangeable resonator cards, which allow for ease of maintenance of the device and provides higher measurement repeatability. Some basic experiments made with the new resonator cards are presented, which show the clear dependence of the resonance frequency and Q factor on viscosity. In addition, an approach is taken to optimize the magnetic circuit for the measurement cell using finite element simulations and experiments. A temperature dependence comparison is made between straight beam resonators and S shaped (Meander) beam resonators the result of which is the adoption of the S shaped beam design for further investigation. Finally the data processing approach using a curve fitting function designed for low quality factors is briefly described.

Quartz tuning fork resonators are suitable sensors for the measurement of density and viscosity of a surrounding gas. For the continuous measurement with these sensors, a digital reactance-locked loop is proposed in this contribution. Furthermore, insight on the theoretical background of density and viscosity measurement with tuning fork resonators is given and the application of the system for simultaneous density and viscosity measurement in gases with a tuning fork resonator is presented.

Resonant sensors are used in a wide range of applications, e.g. as microbalances, chemical sensors in liquid and gaseous environments, and for physical property sensing of liquid and viscoelastic media. The signals obtained from these sensors are most often afflicted with unwanted or spurious components which, depending on sensor type and operating conditions, can be caused by parasitic sensor effects or by the implemented sensor interface circuit. Common ways of handling these effects by post-processing the sensor data usually reduce the dynamic range of the measurement interface due to the fact that a certain percentage of the input range is occupied by the unwanted signal components. In this contribution, a method of analog compensation for parasitic signal components is presented, which is implemented in a largely digital impedance analyzer circuit for resonating sensors.

The utilization of piezoelectric discs excited by lateral fields (LFE) for liquid phase sensing was first proposed by Vig in 1998. Since then, experimental investigations showed that LFE resonators show unique differences compared to common thickness field excited (TFE) resonators when used for liquid sensing. Due to the various modes of oscillation excited by LFE, the impedance spectrum cannot be properly calculated by one dimensional analysis, mostly sufficient for TFE. In our contribution, the interaction of piezoelectric disc and sample liquid, which is not fully manageable in an analytical way or too comprehensive for FEM, is modeled in a full-wave fashion by means of a semi-numerical method in the spectral domain (wavenumbers). The simulation results — shown for AT cut quartz and lithium niobate — enable a detailed analysis of the associated modes and their interaction with the liquid and qualitatively reproduce experimental data given in the literature.

A resonating double membrane based sensor for viscosity and mass density facilitating measurements at different frequencies and two adjustable modes of vibration is presented. The sensor is designed to be suitable, e.g., for low cost handheld devices with inline capabilities and disposable sensor elements. In the sensor, a sample liquid is subjected to time harmonic shear stress induced by two opposed vibrating polymer membranes, placed in an external static magnetic field. These membranes carry two conductive paths each. The first path is used to actuate the membranes by means of Lorentz forces while the second acts as a pick-up coil providing an induced voltage representing the movement of the membrane. From the measured frequency response the liquid's viscosity and mass density can be deduced. This double membrane based setup allows to examine the test liquid at adjustable frequencies in the low kilohertz range from 1 kHz to 15 kHz. Two different designs are presented and the results obtained by experimental investigation are compared to previous theoretical findings.

Resonating sensors are used for measurement tasks in a wide range of applications. Depending on sensor type and operating conditions, the signals of these sensors are often afflicted with unwanted components induced by parasitic sensor effects or the implemented sensor interface. Especially when using digitizing interface circuits it can be advantageous concerning the utilizable resolution of the system to compensate these parasitic signals before the analog to digital conversion. In this contribution, a method of analog compensation for parasitic signal components is presented, that is implemented in an largely digital impedance analyzer circuit for resonating sensors.

Resonant sensors are used in a wide range of applications, e.g. as microbalances, chemical sensors in liquid and gaseous environments, and for physical property sensing of liquid and viscoelastic media [1]. Our sensor electronics [2] will be used for the readout of resonant sensors for liquid properties, which means that the Q–factor of such devices is much lower than that of resonators operating in vacuum or gaseous environments. Hence oscillator circuits are not preferable evaluation circuits for this application. Moreover, the damping may be frequency dependent, so measurement over a wide frequency range is mandatory. Besides the common method of using precise laboratory instruments for the measurement of the impedance spectra of these sensors, various approaches have been reported [3–8]. For the interpretation of the measurement results not only the amplitude but also the phase angle of the sensor impedance is of interest allowing a more accurate characterization of the resonance behavior. In this paper we introduce the system concept and some measurement results in comparison to results obtained with a commercial lock–in amplifier.

For many applications, resonating sensors can be designed which exhibit excellent sensitivity of the resonant parameters (resonant frequency and quality factor) to the wanted physical parameters. When the resonant parameters are to be derived from measured data, the utilized signal processing algorithm significantly affects the precision of the obtained results, in particular, when the resonance is impaired with spurious contributions. In particular, for systems with low Q-factors (e.g., electromechanical resonators in viscous liquids with Q < 100), the measurement precision suffers from various unwanted spectral components induced by parasitic effects of the resonator and measurement errors. In order to separate the adverse effects from the ideal second-order characteristics, three related methods for estimating the parameters of second-order resonant systems are introduced in this paper extending established methods. To this end, the spectrum is separated into a component induced by a second-order resonant system and an unknown background spectrum.

An optimum freeze-drying cycle was developed for use with industrial equipment in the processing in vials of a product degraded by traces of moisture. The relative humidity of the headspace gas of the vials, which contained only 5 microgram of active ingredient, was monitored by a GC method and was then correlated with product stability.

Many sensor signals are afflicted with unwanted components induced by parasitic but well known sensor effects. With utilization of digitizing interface circuits the suppression of these components is typically performed in the digital domain. Hence the effective resolution of the system is reduced, especially when unwanted signal components are dominating the signal’s magnitude. In this contribution, a method of parasitic signal compensation as implemented in an impedance analyzer circuit for a quartz crystal resonating sensor is presented.

Quartz crystal tuning fork resonators are suitable sensors for separable, simultaneous measurement of the density and the viscosity of a surrounding fluid. In this contribution we present the application of a frequency tracking interface for measurement of gas density and viscosity with a single quartz crystal tuning fork resonator. By frequency tracking, we obtain equal accuracy for the density and viscosity estimation as by evaluation of the admittance spectrum in the vicinity of the resonance frequency (which is typically used) but with a significantly reduced measurement time (one measurement per second compared to about 50 seconds per admittance spectrum). Furthermore, sophisticated fitting algorithms are not required.

This report describes the isolation, derivative formation, GLC, and quantitation of unmetabolized 7-chloro-5,11-dihydrodibenz[b,e][1,4]oxazepine-5-carboxamide (I) in blood serum or plasma. Carbamazepine is used as the internal standard to compensate for losses of I during extraction and handling. Essentially complete recovery (100 +/- 6%) was demonstrated over a concentration range of 1-30 mug of I/ml of serum.