Method for measuring the quality of a fuel and measuring device therefor

Abstract

The invention relates to a method for measuring the quality of a fuel transported via a fuel line by means of cavity resonance by way of microwaves coupled in a resonator (1) formed in the fuel line, wherein a resonant frequency established as a function of the fuel being transported through the resonator (1) is measured, characterized in that in the resonator (1) an electric field mode is excited which has a maximum in conjunction with substantially uniform field strength over the cross-section of the fuel line through which fuel flows, wherein a measure of the quality of the fuel or fuel mixture is derived from the resonant frequency established. The invention furthermore relates to a measuring device for carrying out the method according to claims 1 to 8 comprising a resonator (1) in the form of a housing (10) composed of conductive material, in which is formed a resonance chamber (2) with at least one radio-frequency connection (21, 22), the resonance chamber being widened relative to the fuel line, wherein a fuel supply line (15) and diametrically opposite that a fuel discharge line (16) are arranged on the resonance chamber (2) for the purpose of guiding the fuel through the resonator (1), characterized in that the resonance chamber (2) is filled with a nonconductive, heat-resistant, solid material, wherein for the purpose of guiding the fuel a drilled hole (20) is formed in the nonconductive, heat-resistant, solid material (23) with a diameter approximately equal to (+ / -10%) of the internal diameter of the fuel supply line (15) and / or the fuel discharge line (16), wherein the drilled hole (20) connects the fuel supply line (15) to the fuel discharge line (16) and is arranged in the region of maximum field strength within the resonance chamber.

Description

[0001] DESCRIPTION

[0002] Method for measuring the quality of a fuel and measuring device therefor

[0003] The invention relates to a method for measuring the quality of a fuel transported via a fuel line by means of cavity resonance via microwaves coupled into a resonator formed in the fuel line, wherein a resonance frequency that is established as a function of the fuel transported through the resonator is measured. Furthermore, the invention relates to a measuring device for carrying out the aforementioned method, comprising a resonator in the form of a housing made of conductive material, in which a resonance chamber is formed that is widened relative to the fuel line and has at least one high-frequency connection. A fuel supply line and a fuel discharge line are arranged diametrically opposite the resonance chamber for conveying the fuel through the resonator.

[0004] Internal combustion engines such as gas turbines and combustion engines are powered by liquid fuels in most mobile applications. State-of-the-art fuels include diesel, heating oil, gasoline, kerosene, alcohols, and possibly cryogenic liquids such as liquefied natural gas. The task of fuel systems is to supply fuel from a main reservoir (tank) to the internal combustion engine under the conditions required by the engine. These are usually requirements regarding pressure, temperature, and purity, particle loading and contamination with undesirable accompanying substances, and also viscosity. Pressure and temperature are usually continuously monitored at several points in the system, which is very simple in terms of measurement technology. Measuring viscosity is more complex but also state-of-the-art, for example in ship fuel systems.The measurement of purity cannot be described in general terms, as there are many different types of contaminants, each of which requires more or less complex measurement methods. A distinction must be made between continuous measurements, which are suitable for flow measurement, and measurements in which a sample is taken at a specific point and subsequently measured. For the qualitative and quantitative determination of contaminants, individual measurement methods are usually used depending on the substances or groups of substances to be determined. These methods are usually carried out in the laboratory and thus with a time delay from machine operation.

[0005] Currently, an additional challenge is emerging, particularly in the shipping sector: For environmental reasons, a variety of additional fuel types are now being considered for internal combustion engines, which previously played no role due to cost constraints. Operators are particularly interested in a) blending different fuels on board or b) switching between them during operation. Or c) receiving a ready-made blend from the fuel supplier prior to refueling.

[0006] Example for a): Ships usually have several tanks in which different fuel quantities and qualities can be stored. Since reduction targets for fossil greenhouse gases (especially carbon dioxide) are required in the coming years, it is planned to carry both fossil and climate-neutral liquid fuels on board, which will be mixed together during operation and fed to the combustion engines in accordance with the limits or reduction targets.

[0007] Example for b): Ships in special shipping areas, e.g., so-called Emission Control Areas, must use fuels with a sulfur content of <0.1% by mass, whereas in other global waters they may use fuels with a sulfur content of <0.5% by mass. For cost reasons, the fuel switches between, for example, low-sulfur heavy fuel oil and light diesel when crossing the protected area boundary.

[0008] Examples of c): 1. Aircraft must gradually reduce the proportion of fossil carbon in their fuel within certain timeframes. For cost reasons, it is expected that so-called "drop-in fuels" containing carbon from renewable sources in the required quantities will be mixed with conventional kerosene of fossil origin on the ground, and the aircraft will be refueled with this blend. 2. In road transport fuels, it is already common practice to blend limited amounts of biofuels with fossil diesel and gasoline.

[0009] Regardless of whether these scenarios involve mixing different fuels permanently (a), c)) or only temporarily (b)), undesirable effects can arise from the mixing. Examples include the chemically induced precipitation of solids and / or a lack of miscibility, resulting in bubbles, plumes, and two-phase mixtures instead of a homogeneous liquid. Such effects pose a risk to the fuel system (clogging of filters, etc.) as well as to the internal combustion engine (malfunctions in injection, mixture formation, combustion), which can lead to engine failures and increased emissions.

[0010] The device described above is known from DE 34 12 704 C2 for measuring the alcohol content in fuel mixtures. This describes a microwave resonance chamber in the form of an enlarged cylindrical cavity through which a continuous fuel line, for example in the form of a rubber hose, runs. The attenuation of the microwave signals transmitted into the resonance chamber is received and evaluated.

[0011] The disadvantage is that such a measuring device is only possible on fuel lines made of a material with a very low dielectric constant and the lowest possible dielectric loss factor. This measuring device is not suitable for fuel lines made of electrically conductive materials, especially steel pipe fuel lines.

[0012] US Pat. No. 10,753,302 B2 discloses a method for determining fuel components using a high-frequency sensor for motor vehicles. This method describes measurements in a vehicle fuel tank using patch antennas or a monopole antenna. The different fuel mixtures are determined based on a specific resonance frequency. This essentially results in a static measurement of the fuel quality in the tank in the vicinity of the measuring section between the antennas or in the immediate vicinity of the monopole antenna. This requires that the fuel contained in the tank is thoroughly mixed, meaning that bubbles, plumes, or inhomogeneities outside the sensitive area are not detectable with this method.

[0013] Furthermore, from DE 102008 044 383 A1 a method and device for determining a composition of a fuel mixture, in particular for determining an ethanol content and / or a water content in the fuel mixture, in which a first microwave radiation with at least two microwave frequencies is radiated into the fuel mixture, wherein at least one second microwave radiation is received from the fuel mixture, wherein the second microwave radiation is compared with the first microwave radiation and at least one characteristic variable is determined as a function of the microwave frequency of the first microwave radiation, wherein the composition of the fuel mixture is inferred from the course of the characteristic variable over the microwave frequency of the first microwave radiation.

[0014] A similar method and device is known from DE 10 2009 054 844 A1, in which a differential sensor concept is proposed, with which the change and the direction of the change in the fuel mixture, i.e. in this case the ethanol content, can be determined and a corresponding ignition angle adjustment can be controlled.

[0015] Furthermore, similar methods and devices are known from

[0016] DE 33 16 328 A1 , a microwave measuring device for the void fraction in a liquid flow;

[0017] DE 38 41 471 A1 , a method for determining the alcohol content and / or calorific value of fuels;

[0018] DE 10 2010 029 007 A1 , a device for determining a composition of a fuel mixture;

[0019] US 4 767 982 A, a method and apparatus for determining the concentration or proportion of the individual components of a two-component liquid;

[0020] EP 0 069 969 A1 , a microwave sensor for the alcohol content in fuels; and

[0021] EP 0 564 879 A1 , a device for determining material parameters by microwave measurements.

[0022] The object of the invention is to provide, starting from a measuring device according to DE 34 12 704 C2, a measuring method and a measuring device with which the quality of a fuel or fuel mixture transported via a fuel line is detected, whereby a high sensitivity is achieved for the occurrence of different fuel qualities, their varying mixing ratios, bubbles, plumes and / or two-phase mixtures.

[0023] This object is achieved with a measuring method according to claim 1 and a measuring device according to claim 9.

[0024] By exciting an electric field mode in the resonator, which exhibits a maximum at a substantially uniform field strength across the fuel line's cross-section through which the fuel flows, a measure of the quality of the fuel or fuel mixture is derived from the resulting resonance frequency. High sensitivity is achieved across the entire fuel line cross-section in determining the fuel or fuel mixture flowing through it, thus reliably determining the current quality. By covering the entire line cross-section, even bubbles or vapors are reliably detected.

[0025] In a first embodiment, the measurement is carried out continuously, which allows a temporal progression to be displayed in the flow and in real time, which can, for example, be introduced into a control loop with a sufficiently fast reaction, for example for mixing different fuel components.

[0026] Alternatively, the measurement can be carried out discretely by repeatedly performing the measurement at a sampling rate of 10 to 500 ms for a respective measurement duration of 1 to 400 ms. The recurring measurement must have sufficient resolution with fine quantization in order to be able to measure the fuel flowing through the line with the desired sensitivity. Sampling rates of less than or equal to 100 ms are preferred, particularly preferably between 10 ms and 50 ms. The sampling rate specifies how long the latency is between two value updates or how many processed measured values ​​are output per second (sampling frequency). The sampling rate must be greater than the measurement duration. In a practical implementation, for example, a measurement time of 10 ms can be used to acquire many values, which are then processed, converted, and output during the following 40 ms, which would enable a sampling rate of 50 ms.

[0027] By calculating the effective permittivity of the fuel currently present in the resonator from the resulting resonance frequency, which is a measure of the quality of the fuel or fuel mixture, a reliable quality determination of the fuel or fuel mixture can be achieved. For this purpose, the effective permittivity is determined compared to a reference measurement using a fuel with a known permittivity based on the shift of the measured resonance frequency, whereby the permittivity of the fuel is proportional to the inverse square of the measured resonance frequency. Thus, the effective permittivity can be determined depending on the polarizability of the fuel mixture inside the resonator, and from this, the quality of the fuel can be determined.

[0028] Furthermore, the quality and mixing ratio of fuel mixtures can be measured, whereby the mixing ratio of fuel mixtures is determined with known permittivities of the individual components.

[0029] In a further embodiment, it is preferred that the permittivities of individual components are measured and, after their mixing, the permittivity of the fuel mixture is measured, from which the mixing ratios are calculated. This makes it possible, for example, to detect changes in the quality of a fuel before it is mixed with another fuel and, if necessary, to adjust the mixing ratio or even add a third component. This makes it possible to monitor and control the mixture even without prior knowledge of the parameters of the individual fuels. Analogously, mixtures with more than two individual fuels can also be monitored by measuring the inflow of the individual fuel components and after individual mixing points.

[0030] If bubbles, plumes or inhomogeneities in the fuel are detected due to recorded sudden changes in measured values ​​that are greater than a specified limit value, such irregularities can be detected with correspondingly continuous or sufficiently fast measurements (low sampling rate) in order to be able to initiate countermeasures in the control of the downstream internal combustion engine and, for example, to avoid a disruption of the injection, mixture formation or combustion and thus engine failures and / or increasing emissions.

[0031] According to the device, the object mentioned at the outset is achieved with a generic measuring device in that the resonance chamber is filled with a non-conductive, heat-resistant, solid material, wherein a bore is formed in the non-conductive, heat-resistant, solid material with a diameter approximately equal to (+ / - 10%) of the inner diameter of the fuel supply line and / or the fuel discharge line for guiding the fuel, wherein the bore connects the fuel supply line to the fuel discharge line and is arranged within the resonance chamber in the region of the maximum field strength.This means that a homogeneous dielectric, namely the non-conductive, heat-resistant, and solid material, and the fuel / fuel mixture flowing through the bore in this material are located within the resonance chamber. Therefore, changes in the measured values ​​of the resonating electromagnetic waves in the resonance chamber can only result from changes in the fuel or fuel mixture flowing through the bore. Furthermore, since this bore is located within the resonance chamber in the area of ​​maximum field strength of the resulting field mode, maximum sensitivity for the fuel or fuel mixture located in this bore is achieved, thus achieving high measurement accuracy.

[0032] In principle, the housing of the resonator and thus the shape of the resonance chamber can be of any desired shape. Cuboid or, in particular, cylindrical designs are preferred. Particularly preferably, the resonance chamber is cylindrical, with the bore positioned on the cylinder axis. Thus, upon excitation of a resonance mode H o, a maximum is achieved precisely on the cylinder axis, thus achieving maximum sensitivity. Of course, it is also possible to use higher-order resonance modes for the measurement. Thus, even with a cylindrical resonance chamber, the fuel line and thus the corresponding bore in the dielectric would not necessarily have to be positioned in the center (cylinder axis).

[0033] If the diameter of the cylindrical resonance chamber is selected in relation to the diameter of the fuel supply line and the fuel outlet line such that the waveguides formed by the fuel supply line entering the resonator and the fuel outlet line exiting the resonator are below the cutoff frequency of this resonator mode with respect to the resonance frequency generated in the resonator, the electromagnetic energy coupled into the resonance chamber cannot leave the resonator through the fuel supply line or fuel outlet line, thus impairing the resonator's sensitivity. The cutoff frequency represents the lowest frequency at which an electromagnetic wave of the respective mode could still propagate in the tube.

[0034] PTFE has proven to be a particularly suitable dielectric because it is insensitive to many types of fuels, has a very low inherent permittivity and is also heat-resistant and easy to process.

[0035] A preferred embodiment of the invention is described in detail below with reference to the accompanying drawings.

[0036] It shows:

[0037] Fig. 1 shows a resonator in a first embodiment in a sectional side view,

[0038] Fig. 2 the resonator in spatial view,

[0039] Fig. 3 shows an electric field in the resonator in a sectional side view and

[0040] Fig. 4 in cross section through the resonator, Fig. 5 a graph of the damping over the excitation frequency with

[0041] Reflectance measurement results for three different samples and

[0042] Fig. 6 shows a graph of attenuation versus excitation frequency with transmission measurement results for three different samples.

[0043] Fig. 1 shows a sectional side view of an embodiment of the measuring device with a circular cylindrical resonator 1, to which instruments (not shown here) for determining the resonance frequency forming in the resonator 1 can be connected.

[0044] The housing 10 of the resonator 1 consists of a tube section 11 made of conductive material, which is closed by a first end face 12 on one side and by a second end face 13 on the other side. The end faces 12, 13 are also made of conductive material. In the embodiment shown here, the tube section 11 and the end faces 12, 13 are connected to each other via flange connections in a liquid-tight and pressure-tight manner. On the first end face 12, a

[0045] The fuel supply line 15 is shown on the cylinder axis of the resonator 1 in the form of a pressure-tight screw connection. The fuel discharge line 16 is also designed on the second end face 13 on the cylinder axis with a corresponding pressure-tight screw connection.

[0046] The free interior of the pipe section 11 forms a resonance chamber 2 of the resonator 1. The resonance chamber 2 is filled with a non-conductive, heat-resistant, solid material 23, a suitable dielectric with low dielectric losses, and a bore 20 is formed on the cylinder axis to guide the fuel from the fuel supply line 15 to the fuel outlet 16. In the exemplary embodiment shown here, the diameter of the bore 20 corresponds to the inner diameter of the fuel supply line 15 or the fuel outlet 16. This is advantageous for the most uninterrupted fuel flow in the fuel line. Deviations in the inner diameter of the bore 20 compared to the fuel supply line 15 or the fuel outlet 16 should not exceed 10%.The inner diameter of pipe section 11, which forms the expanded resonance chamber 2, is significantly larger than the inner diameter of the fuel supply line 15 or fuel discharge line 16. In the exemplary embodiment shown here, the inner diameter of pipe section 11 is approximately 2.5 times the inner diameter of the fuel lines 15, 16. The inner diameter of pipe section 11 is tuned to the desired resonance frequency. The inner diameters of the fuel lines 15, 16 and their openings in the two end faces 12, 13 are dimensioned such that only minimal portions of the resonating electromagnetic wave can escape from the resonance chamber 2 into the connected fuel lines.

[0047] To connect the resonator 1 to the instruments for measuring the resonance frequency and, of course, for coupling the high-frequency signal, corresponding high-frequency connectors (coaxial connectors) are provided at at least one, advantageously at least two, suitable positions on the circumference of the tube section 11 or the end faces 12, 13 of the resonator 1. In the exemplary embodiment shown here, a first high-frequency connector 21 is arranged centrally in the tube section 11, and a second high-frequency connector 22 is arranged opposite. These connectors penetrate the wall of the tube section 11 into the resonance chamber 2 and couple the high-frequency signal required for the measurement into the resonator 1 (into the resonance chamber 2).

[0048] In this case, either the reflection of the high-frequency signal, which varies depending on the medium under investigation, or the transmission between the two (or more) high-frequency ports 21, 22, or a combination of reflection and transmission, is measured. It is important that the interaction of the medium with the resonating electromagnetic wave is at its maximum, i.e., that the fuel guide, i.e., bore 20, is positioned in the area of ​​maximum field strength of the electromagnetic signal.

[0049] In the exemplary embodiment shown here, an electric field mode is excited via the high-frequency connections 21, 22 in the resonance chamber 2, which has a maximum in the region of the cylinder axis, namely a so-called Hm mode (see Fig. 3 and Fig. 4). Figures 3 and 4 show the electric field generated with the corresponding Hw field mode and its field lines, with the field strength maximum being in the region of the field lines that are closest to one another, i.e. in the middle in the side view of Fig. 3, approximately in the region of the opposing high-frequency connections 21, 22, and in Fig. 4 in the cross-section in the region of the bore 20 through which the fuel to be measured flows.

[0050] Depending on the polarizability of the fuel or fuel mixture flowing in the bore 20 of the resonance chamber 2, the resonance frequency of the resonator changes according to the relationship shown, as shown in Figs. 5 and 6. The high-frequency instrument measures the resonance frequency, from whose shift compared to a reference measurement, the effective permittivity of the fuel / fuel mixture currently present in the resonator can be determined. For fuel mixtures, the effective permittivity results from the individual permittivities of the fuels involved according to mixing rules. Of course, disruptive influences on the resonance frequency shift, such as those caused by temperature changes, can be computationally compensated in the result through calibration steps and / or additional measurements of the influencing parameters.

[0051] Fig. 5 shows the attenuation of the coupled high-frequency signal as a function of frequency by measuring the reflection (Sn). The three different samples have a permittivity of e r = 2.4 corresponding to 4.84 GHz resonance frequency, e r = 3.0 corresponding to 4.69 GHz and e r = 3.6 corresponding to 4.58 GHz. When measuring the transmission (S21) with both high-frequency connections (coupling probes) connected, maxima of the transmission are found at the corresponding resonance frequencies, so that for the first sample with e r = 2.4 a maximum at 4.84 GHz, for the second sample with e r = 3.0 at 4.69 GHz and for the third sample with e r = 3.6 a maximum is at 4.58 GHz.

[0052] By measuring the resonance frequency of the resonator, the formula fr,mess~fr,0 ' (fl' EKraftstoff b ' EDieilektrikumf^f

[0053] (where f r, mess represents the measured resonance frequency, f r ,o is the resonance frequency of the resonator when the entire resonator is filled with air, E K fuel is the permittivity of the fuel inside the resonator, £ dielectric is the permittivity of the dielectric 23 and a and b are the weighting factors of the influence of fuel and dielectric 23 respectively on the resonance frequency,) the permittivity of the fuel is measured.

[0054] Each fuel has its own permittivity value, which depends on its chemical composition. If the permittivity of two fuels in tanks A and B is determined, the mixing ratio of the two fuels can then be determined. The permittivity of the fuel mixture is

[0055] Efuel ~f A 'Efuel, A~^f B 'Efuelß,

[0056] (where f A and f Bthe mixture proportions of fuels A and B are (the following applies

[0057] The mixing parameters f A and f B with a single measurement. This allows the volume flow to be adjusted to achieve the exact desired mixing ratio. By measuring the resonance frequency with a sampling rate adjusted for the flow velocity, bubbles and precipitating reaction products, etc., can be detected by sudden (short-term) measured value changes that exceed a threshold value, allowing possible countermeasures to be taken.

[0058] In the overall configuration, the measurement method can be implemented in a first embodiment with a resonator, for example, at the fuel transfer point directly upstream of the internal combustion engine. This allows the quality of the fuel to be monitored with regard to delivery conditions (e.g., sulfur content, impurities, water content, etc.) as well as systemic treatment (viscosity, filtration, water separation), and the treatment steps to be controlled if necessary. Furthermore, subtle, undesirable effects, such as separator contamination, can be identified.

[0059] Assuming that at least the delivery conditions regarding certain parameters (e.g., water content, sulfur content, nominal viscosity, etc.) are known, this simplest design can also be used to monitor fuel mixtures. Knowing the relationships between the permittivity and the parameter to be determined, which can be linear or nonlinear, or knowing the permittivity of the individual components, the homogeneity and ratio of the blending components can be determined with just one measuring point.

[0060] In a second embodiment of the method, fuel mixtures consisting of two fuels can be monitored. This requires three measuring devices for permittivity measurement, namely one in the inlet to the mixing point and one in the outlet to the mixing point. This allows the mixture to be monitored and controlled even without prior knowledge of the parameters of the individual fuels. Analogously, mixtures containing more than two individual components can also be monitored by installing corresponding measuring devices in the inlet and at any number of locations after individual mixing points. This makes it possible, for example, to implement a measuring system on board a ship in which the mixing ratio of two (or more) fuels is monitored, even though the composition of the individual components is not fully known.

[0061] List of reference symbols

[0062] 1 resonator

[0063] 10 housings

[0064] 11 Pipe section

[0065] 12 first frontal surface

[0066] 13 second frontal surface

[0067] 14 Flange connection

[0068] 15 Fuel supply line

[0069] 16 Fuel drainage

[0070] 2 resonance chamber

[0071] 20 holes

[0072] 21 first high-frequency connection

[0073] 22 second high-frequency connection

[0074] 23 non-conductive, heat-resistant, solid material; dielectric

Claims

PATENT CLAIMS 1 . Method for measuring the quality of a fuel transported via a fuel line by means of cavity resonance via microwaves coupled into a resonator (1) formed in the fuel line, wherein a resonance frequency which is established as a function of the fuel transported through the resonator (1) is measured, characterized in that an electric field mode is excited in the resonator (1), which has a maximum at a substantially uniform field strength over the cross section of the fuel line through which the fuel flows, wherein a measure of the quality of the fuel or fuel mixture is derived from the resonance frequency which is established.

2. Method according to claim 1, characterized in that the measurement is carried out continuously.

3. Method according to claim 1, characterized in that the measurement is carried out repeatedly with a sampling rate of 10 to 500 ms for a respective measurement duration of 1 to 400 ms.

4. Method according to claims 1 to 3, characterized in that an effective permittivity of the fuel currently in the resonator (1) is calculated from the resulting resonance frequency, which is a measure of the quality of the fuel or fuel mixture.

5. Method according to claim 4, characterized in that the effective permittivity is determined compared to a reference measurement with a fuel with a known permittivity based on the shift of the measured resonance frequency, wherein the permittivity of the fuel is proportional to the inverse of the square of the measured resonance frequency.

6. Method according to claim 4 or 5, characterized in that the mixing ratio of fuel mixtures is determined with known permittivities of the individual components.

7. Method according to claim 6, characterized in that the permittivities of individual components are measured and, after their mixing, the permittivity of the fuel mixture is measured, from which the mixing ratios are calculated.

8. Method according to one of the preceding claims, characterized in that bubbles, plumes or inhomogeneities in the fuel are determined on the basis of detected sudden changes in measured values ​​which are greater than a predetermined limit value.

9. Measuring device for carrying out the method according to claims 1 to 8 with a resonator (1) in the form of a housing (10) made of conductive material, in which a resonance chamber (2) is formed which is widened relative to the fuel line and has at least one high-frequency connection (21, 22), wherein for the passage of the fuel through the resonator (1) a fuel supply line (15) and diametrically opposite a fuel discharge line (16) are arranged on the resonance chamber (2), characterized in that the resonance chamber (2) is filled with a non-conductive, heat-resistant, solid material (23), wherein for guiding the fuel a bore (20) is formed in the non-conductive, heat-resistant, solid material (23) with a diameter approximately equal to (+ / - 10%) of the inner diameter of the fuel supply line (15) and / or the fuel discharge line (16), wherein the bore (20) connects the fuel supply line (15) with the Fuel line (16) and is arranged within the resonance chamber in the area of ​​maximum field strength.

10. Measuring device according to claim 9, characterized in that the resonance chamber (2) has a cylindrical shape and the bore (20) is arranged on the cylinder axis.

11. Measuring device according to claim 10, characterized in that the diameter of the cylindrical resonance chamber (2) in relation to the diameter of the fuel supply line (15) and the Fuel discharge line (16) is selected such that the waveguides formed in the form of the fuel supply line (15) entering the resonator (1) and the exiting fuel discharge line (16) are below the cut-off frequency of this resonator mode with respect to the resonance frequency generated in the resonator.

12. Measuring device according to claim 9, 10 or 11, characterized in that the non-conductive material (23) is PTFE.

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