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This study presents a resistive sensor concept based on Barium Iron Tantalate (BFT) to measure the oxygen stoichiometry in exhaust gases of combustion processes. The BFT sensor film was deposited on the substrate by the Powder Aerosol Deposition (PAD) method. In initial laboratory experiments, the sensitivity to pO2 in the gas phase was analyzed. The results agree with the defect chemical model of BFT materials that suggests the formation of holes h• by filling oxygen vacancies VO•• in the lattice at higher oxygen partial pressures pO2. The sensor signal was found to be sufficiently accurate and to have low time constants with changing oxygen stoichiometry. Further investigations on reproducibility and cross-sensitivities to typical exhaust gas species (CO2, H2O, CO, NO, …) confirmed a robust sensor signal that was hardly affected by other gas components. The sensor concept was also tested in real engine exhausts for the first time. The experimental data showed that the air-fuel ratio can be monitored by measuring the resistance of the sensor element, including partial and full-load operation modes. Furthermore, no signs of inactivation or aging during the test cycles were observed for the sensor film. Overall, a promising first data set was obtained in engine exhausts and therefore the BFT system is a possible cost-effective alternative concept to existing commercial sensors in the future. Moreover, the integration of other sensitive films for multi-gas sensor purposes might be an attractive field for future studies.

The quality of wood combustion processes can be effectively improved by achieving the automated control of the combustion air feed. For this purpose, continuous flue gas analysis using in situ sensors is essential. Besides the successfully introduced monitoring of the combustion temperature and the residual oxygen concentration, in this study, in addition, a planar gas sensor is suggested that utilizes the thermoelectric principle to measure the exothermic heat generated by the oxidation of unburnt reducing exhaust gas components such as carbon monoxide (CO) and hydrocarbons (CxHy). The robust design made of high-temperature stable materials is tailored to the needs of flue gas analysis and offers numerous optimization options. Sensor signals are compared to flue gas analysis data from FTIR measurements during wood log batch firing. In general, impressive correlations between both data were found. Discrepancies occur during the cold start combustion phase. They can be attributed to changes in the ambient conditions around the sensor housing.

Many studies are dealing with the behavior of mixed potential sensors. However, a quantitative description of the processes leading to the sensor signals has not yet been carried out. This paper describes a first approach to address the question to what extent a mixed potential sensor can be modelled in a finite element model. In 1D geometry, the electrochemical reactions that lead to signal formation, but also the gas phase reactions at the electrodes, were taken into account. Polarization curves, taken by a novel and for such research optimized setup, are used to determine the electrochemical parameters. Those are necessary to quantify the kinetics and electrical properties of the sensor system. It will be shown how to deduce other analyte concentrations and sensor temperatures from a single data set. In addition, the geometry of the electrode can be modified. In the model, the sensor signal is calculated for the analytes propene, hydrogen and carbon monoxide and compared with measured values. In particular, it shows the limitations of the conventionally used simplified mixed potential theory, since a complete Butler-Volmer equation has to be used for each analyte, especially for small analyte concentrations. This model serves as a basis for even more detailed studies that further elucidate the mechanisms behind mixed potential formation in mixtures or by varying the electrode configuration.

Recently, a laboratory setup for microwave-based characterization of powder samples at elevated temperatures and different gas atmospheres was presented. The setup is particularly interesting for operando investigations on typical materials for exhaust gas aftertreatment. By using the microwave cavity perturbation method, where the powder is placed inside a cavity resonator, the change of the resonant properties provides information about changes in the dielectric properties of the sample. However, determining the exact complex permittivity of the powder samples is not simple. Up to now, a simplified microwave cavity perturbation theory had been applied to estimate the bulk properties of the powders. In this study, an extended approach is presented which allows to determine the dielectric properties of the powder materials more correctly. It accounts for the electric field distribution in the resonator, the depolarization of the sample and the effect of the powder filling. The individual method combines findings from simulations and recognized analytical approaches and can be used for investigations on a wide range of materials and sample geometries. This work provides a more accurate evaluation of the dielectric powder properties and has the potential to enhance the understanding of the microwave behavior of storage materials for exhaust gas aftertreatment, especially with regard to the application of microwave-based catalyst state diagnosis.

In recent years, particulate filters have become mandatory in almost all gasoline-powered vehicles to comply with emission standards regarding particulate number. In contrast to diesel applications, monitoring gasoline particulate filters (GPFs) by differential pressure sensors is challenging due to lower soot masses to be deposited in the GPFs. A different approach to determine the soot loading of GPFs is a radio frequency-based sensor (RF sensor). To facilitate sensor development, in previous work, a simulation model was created to determine the RF signal at arbitrary engine operating points. To ensure accuracy, the exact dielectric properties of the soot need to be known. This work has shown how small samples of soot-loaded filter are sufficient to determine the dielectric properties of soot itself using the microwave cavity perturbation method. For this purpose, mixing rules were determined through simulation and measurement, allowing the air and substrate fraction of the sample to be considered. Due to the different geometry of filter substrates compared to crushed soot samples, a different mixing rule had to be derived to calculate the effective filter properties required for the simulation model. The accuracy of the determined mixing rules and the underlying simulation model could be verified by comparative measurements on an engine test bench.

In order to determine directly the quantity “conversion of a catalyst”, we developed a new YSZ solid-electrolyte based mixed-potential sensor that enables to compare electrochemically two gas compartments. Core of the sensor is a self-heated YSZ disc that provides in the center sufficiently high temperatures for sensing. At the sensor rim, the temperatures are low enough to allow for applying polymer sealings to separate both gas chambers gas tightly. In this study, the YSZ sensor disc compares two gas mixtures emulating the propene concentrations that occur up- and downstream of a diesel oxidation catalyst (DOC). At a temperature of 500 °C at the center of the disc, the changing propene concentrations on one side of the sensor, which emulate a changing propene conversion of the catalyst, are reflected by the sensor signal. Up to a conversion of 90 %, the sensor signals follow a theoretical equation that is derived from mixed-potential theory. For higher propene conversions, the signal differs from that theory. An initial explanation for this may be the catalytic activity of the platinum electrode at these high operation temperatures. This is supported by the fact that when reducing the catalytic activity, which is achieved by reducing the sensing temperature, the sensor signal is meeting more and more the theoretically derived equation. At 425 °C and below, there is no more dependency on the feed gas concentration, and all measured data fit exactly with the above-derived theory, i.e., the sensor signal depends only on the “conversion of the catalyst” and not on the propene concentration. As result, a sensor that measures directly the quantity “conversion” is obtained.

Catalyst materials can be characterized with a thermoelectric gas sensor. Screen-printed thermopiles measure the temperature difference between an inert part of the planar sensor and a part that is coated with the catalyst material to be analyzed. If the overall sensor temperature is modulated, the catalytic activity of the material can be varied. Exothermic reactions that occur at the catalyst layer cause a temperature increase that can then be measured as a sensor voltage due to the Seebeck coefficient of the thermopiles. This mechanism can also be employed at stationary conditions at constant sensor temperature to measure gas concentrations. Then, the sensor signal changes linearly with the analyte concentration. Many variables influence the sensing performance, for example, the offset voltage due to asymmetric inflow and the resulting inhomogeneous temperature distributions are an issue. For even better understanding of the whole sensing principle, it is simulated in this study by a 3-D finite element model. By coupling all influencing physical effects (fluid flow, gas diffusion, heat transfer, chemical reactions, and electrical properties) a model was set up that is able to mirror the sensor behavior precisely, as the comparison with experimental data shows. A challenging task was to mesh the geometry due to scaling problems regarding the resolution of the thin catalyst layer in the much larger gas tube. Therefore, a coupling of a 3-D and a 1-D geometry is shown. This enables to calculate the overall temperature distribution, fluid flow, and gas concentration distribution in the 3-D model, while a very accurate calculation of the chemical reactions is possible in a 1-D dimension. This work does not only give insight into the results at stationary conditions for varying feed gas concentrations and used substrate materials but shows also how various exhaust gas species behave under transient temperature modulation.

Solid electrolyte gas sensors following the mixed potential sensing principle are usually operated at several hundred °C. They are often characterized using half-cell setups where one side of the sensor faces a reference gas atmosphere whereas the other side faces the gas mixture to be analyzed. Since two gas compartments are compared, a high temperature sealing is needed. Typically, such setups require indirect heating, and often, the manufacturing process is complicated and the dismantling of sensor and sealing is only possible by destroying it. To overcome these and other drawbacks of the half-cell setup, a self-heatable, stand-alone ceramic sensor device is developed in this work. We choose high temperature co-firing ceramics technology for the sensor design that bases on a self-heated yttria-stabilized zirconia disc that is hot enough for sensing in the central region and cold enough at the outer radius for contacting and plastic sealing. This work shows how problems that occurred due to the high thermal stress were overcome by employing finite element simulations. After developing and manufacturing the novel device, it is demonstrated that the sensor is capable to compare two gas mixtures electrochemically. For that purpose, the device was used to compare the gas composition up- and downstream of an oxidation catalyst. The sensor signal response correlates well with a theoretically derived dependence on the catalyst conversion.

Particulate matter (PM) emissions of automotive exhausts are considered to cause serious health problems. Diesel particulate filters (DPF) are in serial use to trap soot from the exhaust. For on-board diagnostics, soot sensors are applied downstream of a DPF. They follow a conductometric principle: The current signal for a constant voltage applied between two interdigital electrodes (IDE) increases with increasing soot deposition after an initial blind time, at which the sensors show no signal. In the present study, a thin insulation layer – manufactured by the aerosol-deposition-method (ADM) – covers the electrode area of a conductometric device and forms a capacitive device. During soot deposition on the surface, the sensor capacitance increases as calculated by FEM simulations. The higher the soot concentration, the faster is the capacitance increase. By applying a dc voltage between the electrodes during soot collection, the sensor performance can be further improved. Compared to the conductometric device with a similar IDE geometry, a reduced blind time at the same dc voltage is achieved. Even more, the capacitive sensor can be operated at higher collection voltages, since the electrodes are insulated towards each other. This allows for a further increase of the applied dc voltage, which reduces even more the blind time. Measurements in an engine dynamometer showed that the blind time can be reduced by a factor of three, with a high potential for a further reduction by technological means.

Up to now, sensor applications have rarely used materials whose dielectric properties are a function of the gas concentration. A sensor principle, by which this material effect can be utilized, is based planar radio-frequency sensors. For the first time, such a sensor was equipped with an integrated heater and successfully operated at temperatures up to 700 ∘C. This makes it possible to apply materials that show gas-dependent changes in the dielectric properties only at higher temperatures. By coating the planar resonance structure with a zeolite, ammonia could be detected. The amount of ammonia stored in the sensitive layer can thereby be determined, since the resonant frequency of the sensor shifts with its ammonia loading. Desorption measurements showed a dependence of the storage behavior of the ammonia in the gas-sensitive layer on the operating temperature of the sensor. Thus, it was possible that by operating the sensor at 300 ∘C, it only shows a gas-concentration-dependent signal. At lower operating temperatures, on the other hand, the sensor could possibly be used for dosimetric determination of very low ammonia concentrations.

The present study is focused in two directions. In the first part, BaFe(1-x)-0.01Al0.01TaxO3−δ (BFATx) thick films with a Ta content between 0.1 and 0.4 were manufactured using the novel room temperature coating method “aerosol deposition” (ADM), and its material properties were characterized to find the best composition of BFATx for temperature-independent oxygen sensors. The material properties “Seebeck coefficient” and “conductivity” were determined between 600 and 800 ∘C at different oxygen partial pressures. BaFe0.69Al0.01Ta0.3O3−δ (BFAT30) was found out to be very promising due to the almost temperature-independent behavior of both the conductivity and the Seebeck coefficient. In the second part of this study, films of BFAT30 were prepared on a special transducer that includes a heater, equipotential layers, and special electrode structures so that a combined direct thermoelectric/resistive oxygen sensor of BFAT30 with almost temperature-independent characteristics of both measurands, Seebeck coefficient and conductance could be realized. At high oxygen partial pressures (pO2 > 10−5 bar), the electrical conductance of the sensor shows an oxygen sensitivity of m = 0.24 (with m being the slope in the logσ vs. logpO2 representation according to the behavior of σαpO2m), while the Seebeck coefficient changes with a slope of −38 µV K−1 per decade of pO2 at 700 ∘C. However, at low pO2 (pO2 −14 bar) the conductance and the Seebeck coefficient change with pO2, with a slope of m = −0.23 and −21.2 µV K−1 per decade pO2, respectively.

A manufacturing process for a planar binary lambda sensor is shown. By joining the heating and the sensing components via glass soldering with a joining temperature of 850 °C, a laboratory platform has been established that allows the manufacturing of two independent parts in high-temperature co-fired ceramics technology (HTCC) with electrodes that are post-processed at lower temperatures, as is required for mixed-potential sensors. The final device is compared to a commercial sensor with respect to its sensing performance. Important processes and possible origins of problems as well as their solutions during sensor development are shown, including heater design and joining process.

Almost all diesel engines are equipped with a diesel oxidation catalyst (DOC). It is one task of the DOC to oxidize hydrocarbons. Its conversion efficiency has to be monitored during operation (on-board diagnostics, OBD). In contrast to approaches that use one hydrocarbon sensor downstream of the DOC, we suggest a novel concept to determine in operando and directly the conversion rate of such a catalyst. This conversion rate sensor (CRS) bases on the mixed potential principle, and compares the gas atmospheres up- and downstream of the catalyst directly within one single device. To prove this concept, an initial laboratory setup was set up. It is shown that the sensor responds to the conversion rate as expected with a CRS characteristic that follows theoretical expectations.

Thermoelectric hydrocarbon sensors were fully manufactured in both alumina substrate-based thick-film technique and low temperature cofired ceramics (LTCC) tape technology. Au–Pt thermocouples were screen-printed on insulating sensor substrates. Porous alumina impregnated with one weight percent platinum was used as catalytic material that covered one side of the sensor. Due to the generated thermal exothermicity, a temperature difference between the catalytically active and the catalytically inactive (reference) side occurred. Additionally, the sensor was equipped with an internal platinum heating element to operate the sensor at temperatures above the light-off temperature of the catalyst material. Owing to the lower thermal conductivity of LTCC, the sensitivity could be improved essentially. To further increase the sensor signal, the number of thermocouples per sensor was enhanced by applying thermopiles of several Au–Pt thermocouples. Therefore, the layout of the heating element was optimized. Altogether, a sensitivity of 0.57 μV/ppm propene was achieved.

A planar microstrip ring resonator structure on alumina was developed using the commercial FEM software COMSOL. Design parameters were evaluated, eventually leading to an optimized design of a miniaturized microwave gas sensor. The sensor was covered with a zeolite film. The device was successfully operated at around 8.5 GHz at room temperature as a humidity sensor. In the next step, an additional planar heater will be included on the reverse side of the resonator structure to allow for testing of gas-sensitive materials under sensor conditions.

The gas sensing properties of resistive gas sensors of BaFe0.7Ta0.3O3-δ (BFT30) prepared by the so-called aerosol deposition method, a method to manufacture dense ceramic films at room temperature, were investigated. The electrical response of the films was investigated first under various oxygen concentrations and in a wide temperature range between 350 and 900 °C. Between 700 and 900 °C, the conductivity of BaFe0.7Ta0.3O3-δ (BFT30) depends on the oxygen concentration with a slope of almost 1/4 in the double-logarithmic plot vs. oxygen partial pressure. In addition, the sensor response is temperature independent. BFT30 responds fast and reproducibly to changing oxygen partial pressures even at 350 °C. The cross-sensitivity has been investigated in environments with various gases (C3H8, NO, NO2, H2, CO, CO2, and H2O) in synthetic air between 350 and 800 °C. BFT30 exhibits good sensing properties to NO between 350 and 400 °C in the range from 1.5 to 2000 ppm with a high selectivity to the other investigated gas species. Thus this semiconducting ceramic material is a good candidate for a temperature-independent oxygen sensor at high temperatures with the application in exhausts and for a selective nitrogen monoxide (NO) sensor at low temperatures for air quality monitoring.

The sensing properties for resistive oxygen sensors of BaFe1-xTaxO3-δ (BFT) were investigated as a function of the Tantalum content x (0.2 ≤ x ≤ 0.7). Bulk samples were produced by using the mixed-oxide route. X-ray diffraction pattern showed that the perovskite structure was obtained and no second phases were found up to x = 0.5. The electrical conductivity of sintered samples was determined in an oxygen concentration between 1% and 100% in a wide temperature range between 400 and 900 °C. Samples between x = 0.2 and 0.4 are p-type conducting perovskite oxides with almost zero thermal activation. The range, at which the temperature dependency is absolutely zero, shifts with x from 450 °C for x = 0.2 to about 800 to 900 °C for x = 0.4. Specimens of BaFe0.7Ta0.3O3-δ revealed the lowest overall temperature dependency in the entire temperature range. All samples up to x = 0.4 responded fast and reversibly to oxygen concentration changes, even at 400 °C.

Electroceramic functional materials often need to be characterized in a sensor-like setup close to application. We present a novel sensor transducer that allows four-wire impedance spectroscopy of planar thick-film materials. Fast and easy temperature adjustment is possible by an integrated heater element. The tool can be manufactured either with Au or with Pt electrodes. The measurement of the transducers idle capacitance and calibration measurements with water covering allowed us to define a geometry factor, which is useful to determine specific materials parameters directly from the four-wire measurements. Characterization of three different kinds of materials (an n-type semiconductor, an oxygen ion-conductor and a sodium ion-conductor material) demonstrates the applicability of planar four-wire impedance spectroscopy as a powerful tool to investigate functional materials close to application.

A study on the application of V(2)O(5)/WO(3)/TiO(2) (VWT) as the sensitive material for resistive-type SO(2) sensor was conducted, based on the fact that VWT is a well-known catalyst material for good selective catalytic nitrogen oxide reduction with a proven excellent durability in exhaust gases. The sensors fabricated in this study are planar ones with interdigitated electrodes of Au or Pt. The vanadium content of the utilized VWT is 1.5 or 3.0 wt%. The resistance of VWT decreases with an increasing SO(2) concentration in the range from 20 ppm to 5,000 ppm. The best sensor response to SO(2) occurs at 400 °C using Au electrodes. The sensor response value is independent on the amount of added vanadium but dependent on the electrode materials at 400 °C. These results are discussed and a sensing mechanism is discussed.

Recently, a novel approach for the characterization of the state of automotive catalysts has been proposed, viz., the direct electrical characterization of the catalyst material itself. This direct approach has the potential to do away with the complex models needed by indirect measurement methods to infer the catalyst state, for instance from the oxygen partial pressure outside of the catalyst determined by lambda probes. In this work, we compare two ways of directly observing the catalyst state. Firstly, the electrical conductivity of the catalyst material is determined by wirebound sensors mounted inside of the catalyst and fabricated from catalyst-formulation coated planar interdigital electrodes. Secondly, we employ a contactless method based on the perturbation of the partially filled microwave cavity resonator formed by the catalyst and its metallic housing. For a better understanding of the observed effects, the data obtained at microwave frequencies from engine tests are compared with the local electrical conductivity measurements. It is shown that both methods give a coherent picture of the oxygen loading state of an automotive three-way catalyst and that both methods enable one to follow the progress of reaction fronts inside the catalyst. Furthermore, it is demonstrated that the resonance frequency is strongly correlated with the oxygen load.

Several metal-organic framework (MOF) materials were under investigated to test their applicability as sensor materials for impedimetric gas sensors. The materials were tested in a temperature range of 120 °C - 240 °C with varying concentrations of O(2), CO(2), C(3)H(8), NO, H(2), ethanol and methanol in the gas atmosphere and under different test gas humidity conditions. Different sensor configurations were studied in a frequency range of 1 Hz -1 MHz and time-continuous measurements were performed at 1 Hz. The materials did not show any impedance response to O(2), CO(2), C(3)H(8), NO, or H(2) in the gas atmospheres, although for some materials a significant impedance decrease was induced by a change of the ethanol or methanol concentration in the gas phase. Moreover, pronounced promising and reversible changes in the electric properties of a special MOF material were monitored under varying humidity, with a linear response curve at 120 °C. Further investigations were carried out with differently doped MOF materials of this class, to evaluate the influence of special dopants on the sensor effect.

Due to increasing environmental concerns the need for inexpensive selective gas sensors is increasing. This work deals with transferring a novel zeolite-based impedimetric hydrocarbon gas sensor principle, which has been originally manufactured in a costly combination of photolithography, thin-film processes, and thick-film processes to a lowcost technology comprising only thick-film processes and one electroplating step. The sensing effect is based on a thin chromium oxide layer between the interdigital electrodes and a Pt-loaded ZSM-5 zeolite film. When hydrocarbons are present in the sensor ambient, the electrical sensor impedance increases strongly and selectively. In the present work, the chromium oxide film is electroplated on Au screen-printed interdigital electrodes and then oxidized to Cr2O3. The electrode area is covered with the screen-printed zeolite. The sensor device is self-heated utilizing a planar platinum heater on the backside. The best sensor performance is obtained at a frequency of 3 Hz at around 350 °C. The good selectivity of the original sensor setup could be confirmed, but a strong cross-sensitivity to ammonia occurs, which might prohibit its original intention for use in automotive exhausts.

Highly sensitive and selective hydrocarbon gas sensors were obtained by covering gold interdigital capacitor (IDC) electrodes with platinum-doped Na-ZSM-5 zeolite thick films. A chromium(III) oxide layer between gold and zeolite is responsible for the new outstanding sensor effect. Such Au/Cr2O3/zeolite interfaces might be formed by diffusion of the chromium bonding agent through the gold electrode during annealing of the zeolite layer and subsequent oxidation or depositing of additional Cr2O3 layers on top of the electrodes by thin-film techniques. Impedance spectra consist of high-frequency semicircles and low-frequency Warburg-like contributions. The impedance at a fixed frequency in the low-frequency region depends very selectively on the hydrocarbon concentration in wet oxygen/nitrogen mixtures. Very low or zero cross-sensitivities were found towards CO, H2, NO, and CO2 gases. Different substrate materials and thicknesses of the Cr2O3 layer were compared. A preliminary simple equivalent circuit model is presented.

Selective hydrocarbon (HC) gas sensors can help to further optimize on-board monitoring of catalysts in automotive exhaust gas after treatment systems. A highly selective hydrocarbon gas sensor is obtained by covering Au interdigital electrodes (IDE) with zeolite and chromium(III) oxide thick-films. As a novel arrangement, the zeolite does not cover the semiconducting metal oxide as a catalytically active filter layer. A Cr2O3-thick-film is applied on top of an ion-conducting Pt-doped ZSM-5 zeolite thick-film. Impedance spectra of such sensors show high-frequency semicircles, which are assigned to ionic mobility in the zeolite bulk material, a hydrocarbon dependent semicircle in the medium-frequency range and low-frequency contributions. Measurements at fixed frequency in the gas dependent range show high sensitivity to different hydrocarbon gases and no cross sensitivity toward CO and H2. This new sensing mechanism is attributed to the interface between zeolite and Cr2O3. A typical filter effect or the known resistive sensing mechanism of p-type Cr2O3 were ruled out as main effects.

Zeolites are promising materials in the field of gas sensors. In this technology-oriented paper, a planar setup for potentiometric hydrocarbon and hydrogen gas sensors using zeolites as ionic sodium conductors is presented, in which the Pt-loaded Na-ZSM-5 zeolite is applied using a thick-film technique between two interdigitated gold electrodes and one of them is selectively covered for the first time by an electroplated chromium oxide film. The influence of the sensor temperature, the type of hydrocarbons, the zeolite film thickness, and the chromium oxide film thickness is investigated. The influence of the zeolite on the sensor response is briefly discussed in the light of studies dealing with zeolites as selectivity-enhancing cover layers.

In order to reduce the tailpipe particulate matter emissions of Diesel engines, Diesel particulate filters (DPFs) are commonly used. Initial studies using a conductometric soot sensor to monitor their filtering efficiency, i.e., to detect a malfunction of the DPF, are presented. The sensors consist of a planar substrate equipped with electrodes on one side and with a heater on the other. It is shown that at constant speed-load points, the time until soot percolation occurs or the resistance itself are reproducible means that can be well correlated with the filtering efficiency of a DPF. It is suggested to use such a sensor setup for the detection of a DPF malfunction.

Impedometric zeolite hydrocarbon sensors with a chromium oxide intermediate layer show a very promising behavior with respect to sensitivity and selectivity. The underlying physico-chemical mechanism is under investigation at the moment. In order to verify that the effect occurs at the electrode and that zeolite bulk properties remain almost unaffected by hydrocarbons, a special planar setup was designed, which is very close to real sensor devices. It allows for conducting four-wire impedance spectroscopy as well as two-wire impedance spectroscopy. Using this setup, it could be clearly demonstrated that the sensing effect can be ascribed to an electrode impedance. Furthermore, by combining two- and four-wire impedance measurements at only one single frequency, the interference of the volume impedance can be suppressed and an easy signal evaluation is possible, without taking impedance data at different frequencies.