Level sensor with operating procedure and waste water tank

Abstract

Method for operating a fill level sensor (6), wherein the fill level sensor (6) has a measuring tip (14) with a first (16a) and a second electrode (16b), wherein the measuring tip (14) is arranged in a measuring chamber (8) which can be filled with a liquid (10), so that the measuring tip (14) is immersed in the liquid (10) from a certain limit fill level (SG) of the measuring chamber (8), characterized in that - an alternating electrical voltage (U) of a first frequency (f1) is applied between the first (16a) and the second electrode (16b) and a first impedance value (X1) is determined between the first (16a) and the second electrode (16b), - an alternating electrical voltage (U) of a second frequency (f2) is applied between the first (16a) and the second electrode (16b) and a second impedance value (X2) is determined between the first (16a) and the second electrode (16b), - where the impedance is a scalar value in the form of the magnitude of the complex-valued impedance (X), - a correction value (WK) is determined using a correction rule from the difference between the first (X1) and second impedance value (X2), - a result value (XE) is determined as the sum of the first impedance value (X1) and the correction value (WK), - the level sensor (6) outputs a yes value (J) if the result value has at least a predetermined limit value (XG), and - otherwise returns a No value (N).

Description

[0001] The invention relates to a method for operating a fill level sensor and a fill level sensor, as well as a waste water tank for an aircraft.

[0002] Level sensors are used primarily in the form of tank-full sensors for waste water tanks in aircraft. Such sensors are subject to high reliability requirements to prevent the toilet system from shutting down accidentally. The main problem with reliability is deposits on the sensor, which lead to erroneous signals.

[0003] It is known from practice to use low-frequency capacitive sensors in aircraft wastewater tanks, sometimes in redundant configurations, to detect faults. High-frequency impedance sensors in industrial environments are known, for example, from DE 10 2008 027 921 B4.

[0004] From DE 10 2016 214 387 A1 it is known to measure the complex impedance Z(f) at at least one frequency f1 in a method for operating a capacitive fill level limit switch for liquid media in a container made of a non-conductive material and a measuring electrode provided on the outer wall of the container.

[0005] The object of the invention is to propose improvements with regard to level sensors and in particular waste water tanks for aircraft.

[0006] This object is achieved by a method according to claim 1 for operating a fill level sensor, wherein the fill level sensor has a measuring tip with a first and a second electrode. Preferred or advantageous embodiments of the invention and other categories of the invention emerge from the further claims, the following description, and the accompanying figures.

[0007] During the execution of the method, the measuring tip is arranged in a measuring chamber which can be filled with a liquid, so that the measuring tip is immersed in the liquid from a certain limit filling level of the measuring chamber.

[0008] First, an alternating voltage of a first frequency is applied between the first and second electrodes, and a first impedance value (including the medium in front of the measuring tip or between the electrodes) is determined between the first and second electrodes. Subsequently, an alternating voltage of a second frequency is applied between the first and second electrodes, and a second impedance value is determined between the first and second electrodes.

[0009] A correction value is then determined from the difference between the first and second impedance values ​​using a correction rule. A result value is then calculated as the sum of the first impedance value and the correction value. Finally, the level sensor outputs a "Yes" value if the result value is at least within a specified limit. Otherwise, the level sensor outputs a "No" value.

[0010] The first and second frequencies are different. The impedance includes the impedance of the environment surrounding the measuring tip, i.e., the liquid surrounding the sensor, such as water, or the empty measuring space surrounding the sensor, such as air, possibly with contaminants, especially in the liquid and / or adhering to the sensor.

[0011] The "yes" value means that the fill level is greater than or equal to the limit fill level. The "no" value means that the fill level is less than the limit fill level. The condition that the result value is at least equal to the limit value is to be understood as meaning that the limit value is exceeded in a positive or negative direction, i.e., it can be greater than or equal to the limit value. According to the invention, the impedance is a scalar value, namely the magnitude of the complex-valued impedance, in particular a digital value.

[0012] The measuring space is in particular the area surrounding the measuring tip which has a measurable or noticeable influence on the measured impedance values ​​during an impedance measurement.

[0013] In aircraft, in particular, there are frequency ranges with more stringent EMC requirements in certain frequency ranges than in others. One such critical range is, for example, the frequencies around 108-152 MHz and 960-1680 MHz. According to the invention, the two measurement frequencies can always be set so that they lie outside this range. In particular, one of the measurement frequencies can be selected below and the other above the critical EMC frequency range, for example, at 90 and 340 MHz.

[0014] The invention thus results in a tank-full sensor with high contamination tolerance. The result is a tank-full sensor based on a high-frequency impedance measurement, in which contamination is compensated using a second measurement frequency.

[0015] The invention is based on the idea of ​​performing a measurement at two frequencies with a significant difference (e.g., a factor between 2 and 4) and generating a contamination signal from the two measured values. Two frequencies lead to different measurement volumes or penetration depths into the medium being measured, and to different complex AC resistances, as well as to different tangent delta values. The loss factor of the dielectric constant (DK) of the medium is also frequency-dependent. This compensates for the different frequency dependence of any buildup on the sensor. A measurement value correction is generated from the difference signal of the two frequencies.

[0016] In a preferred embodiment of the method, the second frequency is selected to be at most 108 MHz, in particular at most 100 MHz, in particular at most 95 MHz, in particular at most 90 MHz. In particular, the specified frequency is selected as the second frequency. In particular, a frequency below a predetermined EMC-critical frequency range is selected. The corresponding frequencies are generally less critical with regard to EMC, particularly in an aircraft, so that higher signal amplitudes can be used for the measurement than for EMC-critical frequencies.

[0017] In a preferred embodiment, the first frequency is selected to be at least a factor of 2, in particular at least a factor of 4, in particular at least a factor of 8, higher than the second frequency. In particular, the first frequency is selected at precisely this factor. In particular, the factor is 34 / 9. This results in frequencies of 90 (second) and 340 MHz (first), in particular. This ensures that the first frequency lies above an EMC-critical frequency range when the second frequency lies below this range.

[0018] In a preferred embodiment of the method, the impedance values ​​are determined by standardizing each measured raw impedance value using measured standard impedance values ​​for air and water. The measurements are performed using the same or identically typed level sensors. In particular, the impedance values ​​are then percentage values ​​standardized to 0% for air and 100% for water. The limit value in this case is also a percentage value, in particular between 0% and 100%. Thus, uniform impedance values ​​are created as a basis for implementing the further method, regardless of the actually available raw impedance values.

[0019] In a preferred variant of the method, when standardized to 0% air and 100% water, a limit value between 60% and 90%, in particular between 70% and 80%, in particular between 75% and 73%, is selected. These values ​​have proven particularly good in practice.

[0020] In a preferred embodiment of the method, the correction value is determined as a correction rule in such a way that the difference is raised to an exponent and the exponentiated difference is multiplied by a correction factor. The exponent lies in particular in a range between 0 and 3, in particular between 1 and 2, and is in particular 1 or 2. The correction factor lies in particular in a range between -8 and +8, in particular between -5 and +5. In particular, the correction factor for an exponent of 1 lies between -2 and +2 and for an exponent of 2 between -5 and +5. Corresponding values ​​have proven particularly good in practice.

[0021] In a preferred embodiment of the method, the first frequency and / or the second frequency and / or the limit value are determined empirically based on a predefined test series. This also applies alternatively or additionally to the above-mentioned method variants, i.e., if available, also to the exponent and / or the correction factor. Through appropriate test series, in particular using typical and / or model contamination levels in a wastewater tank, the parameters used in the method can be optimally adjusted to the expected contamination levels in reality, making the method particularly efficient.

[0022] In particular, a test is conducted for level sensors of the same type: fill levels are generated below (sensor in air) and above the sensor (sensor in liquid), respective contaminants are applied to the sensor, and each is checked for correct yes or no values. The reference is generated without contaminants. Water of different conductivities or with different additives can also be used. Examples of test media for deliberately contaminating the sensor include toothpaste, spreads, various types of paper, building materials, or pet food. The measurements are carried out in air (target result: no) and water (target result: yes), particularly with different conductivities or with other additives.

[0023] According to a preferred variant of this embodiment, various predetermined materials are applied to the measuring tip as a series of tests, and a respective result value is determined for air and water. This means that the coated / contaminated sensor is immersed in air on the one hand and water on the other. The correct result should then be "no" for air and "yes" for water.

[0024] According to a preferred variant of this embodiment, the empirical determination is carried out by optimizing the parameters to be determined in such a way that as many of the tests in the test series as possible yield correct yes or no values ​​and / or the distances of the result values ​​from the limit value are maximized for the correct yes and no values. Any suitable optimization method can be used here, which generally involves varying the parameters and evaluating the corresponding results, in particular approximation or error minimization methods. This allows the method to be executed particularly reliably with optimized parameters.

[0025] In a preferred embodiment, the method is carried out on a fill level sensor of a wastewater tank on board an aircraft. In such a wastewater tank, determining the fill level is particularly critical and can be carried out particularly well using the method according to the invention. In addition to wastewater tanks for aircraft, there are other similar applications in which the method is advantageous. Further applications can be found, for example, in the processing of pasty and adhesive media, e.g., in food technology, process technology, and automation technology, although this list is not exhaustive.

[0026] The object of the invention is also achieved by a fill level sensor according to patent claim 11. The fill level sensor contains a measuring tip with a first and a second electrode, wherein the measuring tip can be arranged in a measuring chamber that can be filled with a liquid, so that the measuring tip is immersed in the liquid above a certain limit fill level in the measuring chamber. The fill level sensor contains a generator module for generating an alternating voltage of at least a first and a second frequency. The voltage is then applied between the first and the second electrode. The fill level sensor contains a measuring module for determining an impedance value of an impedance present between the first and second electrodes. The fill level sensor contains an output for outputting a yes value or a no value. The fill level sensor contains an execution module for carrying out the method according to the invention.

[0027] The fill level sensor and at least some of its embodiments as well as the respective advantages have already been explained in connection with the method according to the invention.

[0028] According to a preferred embodiment, the generator module contains a frequency multiplier or a frequency multiplier function for generating the higher of the first or second frequencies from the lower of these frequencies. The frequency multiplier or the frequency multiplier function is controllable, in particular with regard to the multiplication (multiplication factor)—in particular for doubling—and is particularly a frequency doubler. This allows the higher frequency to be generated particularly easily. The frequency multiplier or the frequency multiplier function can be implemented with a discrete multiplier or with an RF generator that allows the frequency range in the desired bandwidth, or with multiple generators that can be switched on / off.

[0029] According to a preferred embodiment, the level sensor contains an RF generator for generating the alternating voltage and at least one control and evaluation unit for controlling the RF generator and determining the impedance values. The control and evaluation unit is, in particular, a microcontroller. In particular, the generator module and / or measuring module and / or execution module, or parts thereof, are then partially integrated into one another in the form of a common microcontroller, since the microcontroller performs tasks in the corresponding modules. This results in a reduction in components in the level sensor.

[0030] In a preferred variant of this embodiment, the RF generator has separate frequency and amplitude control inputs, each of which is controlled separately and independently of the other by correspondingly separate outputs of the control and evaluation unit. This allows the AC voltage to be generated with particular precision.

[0031] In a preferred variant of this embodiment, the control and evaluation unit has a measuring input each for a voltage or potential applied to the measuring tip and for a voltage or corresponding potential applied to or generated by the HF generator. In particular, the potentials are referenced to a common reference potential (e.g. ground). By interposing a shunt resistor between the generator and the measuring tip, the voltage drop across the shunt resistor and therefore the current flowing through the measuring tip can be determined by evaluating the two voltages or potentials. The measuring inputs are in particular ADC inputs (analog-digital converters). The shunt resistor is in particular the only resistance element arranged between the voltage source and the measuring tip. This results in a particularly simple design of the level sensor.

[0032] The object of the invention is also achieved by a wastewater tank according to claim 16 for an aircraft. The wastewater tank contains at least one fill level sensor. At least one of the fill level sensors is a fill level sensor according to the invention. Alternatively or additionally, at least one of the fill level sensors is operated according to the method according to the invention during operation of the wastewater tank.

[0033] The waste water tank and at least some of its embodiments as well as the respective advantages have already been explained in connection with the method according to the invention and / or the fill level sensor according to the invention.

[0034] The invention is based on the following findings, observations, and considerations and also includes the following embodiments. These embodiments are sometimes referred to as "the invention" for simplicity. These embodiments may also contain parts or combinations of the above-mentioned embodiments or correspond to them and / or may also include previously unmentioned embodiments.

[0035] The invention is based on the discovery that deposits form in aircraft wastewater tanks, which can also deposit on tank-full sensors and complicate the measurement of the tank's fullness. Since food waste from the galley can also be disposed of in aircraft wastewater tanks, a wide range of contamination can be expected.

[0036] The invention is based on the fundamental idea of ​​using a high-frequency capacitive / impedance measurement as the selected sensor principle. Typical measurement frequencies are between 50 and 200 MHz or a frequency sweep of approximately 50 and 200 MHz. However, this frequency range is particularly critical with regard to EMC in aircraft. The invention is based on the observation that this measurement principle has the best contamination tolerance of all point level switches, but commercially available sensors are not yet suitable for aircraft wastewater tanks.

[0037] Further features, effects, and advantages of the invention will become apparent from the following description of a preferred embodiment of the invention and the accompanying figures. These show a schematic diagram: Fig. 1 a waste water tank in an aircraft with level sensor, Fig. 2 an electrical circuit during operation of the level sensor.

[0038] Fig. 1 shows a wastewater tank 2 on board an aircraft (not shown in detail). The wastewater tank 2 has a wall 4 in which a fill level sensor 6 is installed. A measuring chamber 8 for the fill level sensor 6 is formed by the interior of the wastewater tank 2, which is delimited by the wall 4. During operation of the aircraft, i.e., during a flight, the wastewater tank 2 is gradually filled with wastewater in the form of a liquid 10. The remainder of the wastewater tank 2 is filled with air 12. Two fill levels S1 and S2 for the liquid 10 are symbolically indicated.

[0039] The level sensor 6 has a measuring tip 14 with a first electrode 16a and a second electrode 16b. Starting at a limit level SG, the measuring tip 14 is immersed in the liquid 10.

[0040] The level sensor 6 contains a generator module 18. This serves to generate an alternating voltage U of at least two different frequencies f1 and f2 between the first electrode 16a and the second electrode 16b.

[0041] The level sensor 6 further contains a measuring module 20. This serves to determine an impedance value X1,2 or a raw impedance value XR1,2 of an impedance X present between the first 16a and second electrode 16b in the measuring chamber 8.

[0042] The level sensor 6 further includes an output 22 for outputting a yes value J or a no value N. The values ​​J,N are determined according to a method which is executed in an execution module 24 of the level sensor 6.

[0043] Fig. Figure 2 shows an electrical circuit during operation of the fill level sensor 6 and its further details. The fill level sensor 6 contains an RF generator 26, which serves to generate the alternating voltage U. In the example, this is an adjustable RF generator for frequencies in the range 40-400 MHz, which has an internal resistance RI. The fill level sensor 6 also contains a control and evaluation unit 28, here a microcontroller, which serves to control the RF generator 26 and to determine the impedance values ​​XR1,2 and X1,2, respectively, and to execute the remaining method according to the invention. The control and evaluation unit 28 therefore assumes tasks of the measuring module 20, the generator module 18 and the execution module 24. The control and evaluation unit 28 has separate outputs, i.e. an output 30a for a frequency control 32a and an output 30b for an amplitude control 32b and an output 30c for a frequency multiplier control 32c of the RF generator 26.The RF generator contains a frequency multiplier 37 controlled by output 30c. The multiplication is alternatively controlled as a doubling. The frequency and amplitude of the voltage U can therefore be adjusted independently and separately. The outputs 30a and b are each connected via separate lines to correspondingly separate inputs of the RF generator 26.

[0044] The control and evaluation unit 28 also has a measuring input 34a for a voltage or potential (measured value voltage UM) present at the measuring tip 14, here the electrode 16b, and a measuring input 34b for the voltage U or potential (generator voltage UG) generated by the RF generator 26. The measuring inputs 34a,b lead to respective ADCs 35a,b (analog-to-digital converters) for the measured value voltage UM and the generator voltage UG.

[0045] Since in this case a shunt resistor 36 is connected between the RF generator 26 and the measuring tip 14 to measure the current I through the measuring tip 14, the voltage or potential difference between the measuring inputs 34a,b and thus between UM and UG is correlated with the current I.

[0046] In Fig. 2, the impedance X in the measuring chamber 8 or the medium surrounding the measuring tip 14, i.e., liquid 10 or air 12 including any contaminants, is symbolically represented by the parallel connection of a capacitance CM of the medium (liquid 10, air 12) and a resistance RM of the medium. The measuring tip 14 is symbolized by a triangle.

[0047] The execution module 24, here also implemented by the control and evaluation unit 28, now executes the following procedure: The control and evaluation unit 28 controls the frequency, here f1 or f2, and the amplitude of the alternating voltage U and thus the frequency and output voltage of the RF generator 26 via two separate digital-to-analog converters, here in the form of the frequency controller 32a and the amplitude controller 32b, and alternatively via the multiplier controller 32c. The generator output voltage UG is recorded by the ADC 35b. If the voltage (amplitude) deviates from a setpoint, the control and evaluation unit 28 (microcontroller) can adjust the voltage U using a special algorithm via the DAC (digital-to-analog converter) of the amplitude controller 32b. The setpoint is selected to ensure the highest possible signal-to-noise ratio while still allowing for acceptable EMC emissions from the entire arrangement.

[0048] Depending on the medium surrounding the measuring tip 14 and any contamination of the measuring tip, the capacitance CM and the resistance RM change and an impedance X and thus the respective measured value voltage UM are established. The current I flowing through the measuring tip 14 is determined using the known shunt resistor 36 and the difference between the measured value voltage UM and the generator voltage UG. The frequency generator 26 (or alternatively several frequency generators) are designed in such a way that at least two measuring frequencies with a large frequency separation can be generated, e.g. in the form of the first frequency f1 being more than 2 or 8 times or even an even greater multiple of the second frequency f2.

[0049] In level sensor 6, the currently measured impedance X is initially determined in the form of raw impedance values ​​XR1,2, initially for a known uncontaminated sensor with uncontaminated air and uncontaminated water. The following table shows an example measurement: Medium XR2 (90MHz) XR1 (340MHz) X2 (90MHz) X1 (340 MHz) Luft 3057 3156 0% 0 % Wasser 1281 320 100 % 100 %

[0050] The measured impedance raw values ​​XR1 and XR2 represent the respective impedance standard values ​​XN for air and water, respectively, to which the impedance values ​​X1,2 are standardized.

[0051] Then a correction value WK with an exponent Y and a correction factor FK is calculated to WK=(X1-X2) Y * FK is determined. In the example, Y=1 and FK=2. A result value XE is then determined as XE=X1+WK. The result value XE is then compared with a limit value XG, and the "yes" value J is output if XE >= XG, or the "no" value N is output if XE < XG. The limit value XG is thus a switching point for level sensor 6.

[0052] The fill level sensor 6 is coated with toothpaste of varying thicknesses in a controlled manner. In addition to the respective result value XE according to the invention, an evaluation is performed for comparison purposes, which would be based solely on impedance values ​​X1 at frequency f1 and X2 at frequency f2. The limit values ​​XG (switching point) are, for example, 77% for X2, 88% for X1, and 73% for XE.

[0053] A negative difference between the result and the switching point means that the switching point has been exceeded in the wrong direction and thus produces an incorrect result (J instead of N or N instead of J). As a result, the measurement with the second frequency or the measured values ​​X2 allows, for example, adhesion of up to 4 mm of toothpaste; the measurement with the first frequency and the measured values ​​X1 allows adhesion of up to 6 mm; the measurement with the measured value correction according to the invention and the measured values ​​XE allows adhesion of up to 9 mm of toothpaste.

[0054] In an empirical method for parameter determination, as described above, various contaminants are applied to the sensor instead of toothpaste, and respective measurements are performed to determine the raw impedance values ​​XR1,2 and, from these, the resulting values ​​XE. Parameter variations are then performed and optimized until the respective distances between the resulting values ​​XE and the switching points XG are maximized or as many correct results as possible are possible. The measurement frequencies can also be varied. List of reference symbols 2 waste water tank 4 wall 6 Level sensor 8 Measuring room 10 liquid 12 Air 14 Measuring tip 16a,b Electrode 18 Generator module 20 measuring module 22 Exit 24 Execution module 26 RF Generator 28 Control and evaluation unit 30a,b,c Exit 32a,b,c Frequency / amplitude / multiplier control 34a,b measuring input 35a,b ADC 36 Shunt resistance 37 Frequency multiplier f1.2 frequency S1,2 fill level SG limit level U alternating voltage X Impedance X1,2 impedance value XR1,2 impedance raw value XE result value XG limit XN Impedance standard value RI internal resistance I Current CM Capacity of the medium RM resistance of the medium UM measured voltage UG generator voltage WK correction value FK correction factor Y exponent Y Yes value N No value

Claims

[1] Method for operating a fill level sensor (6), wherein the fill level sensor (6) has a measuring tip (14) with a first (16a) and a second electrode (16b), wherein the measuring tip (14) is arranged in a measuring chamber (8) which can be filled with a liquid (10), so that the measuring tip (14) is immersed in the liquid (10) from a certain limit fill level (SG) of the measuring chamber (8), characterized by , that - an alternating electrical voltage (U) of a first frequency (f1) is applied between the first (16a) and the second electrode (16b) and a first impedance value (X1) is determined between the first (16a) and the second electrode (16b), - an alternating electrical voltage (U) of a second frequency (f2) is applied between the first (16a) and the second electrode (16b) and a second impedance value (X2) is determined between the first (16a) and the second electrode (16b), - where the impedance is a scalar value in the form of the magnitude of the complex-valued impedance (X), - a correction value (WK) is determined using a correction rule from the difference between the first (X1) and second impedance value (X2), - a result value (XE) is determined as the sum of the first impedance value (X1) and the correction value (WK), - the level sensor (6) outputs a yes value (J) if the result value has at least a predetermined limit value (XG), and - otherwise returns a No value (N). [2] Method according to claim 1, characterized by that the second frequency (f2) is chosen to be no higher than 108 MHz. [3] Method according to one of the preceding claims, characterized by that the first frequency (f1) is chosen to be at least a factor of 2 greater than the second frequency (f2). [4] Method according to one of the preceding claims, characterized bythat the impedance values ​​(X1,2) are determined in such a way that a measured impedance raw value (XR1,2) is standardized to the impedance value (X1,2) using measured impedance standard values ​​(XN) for air and for water. [5] Method according to claim 4, characterized by that the standardization is carried out to 0% air and 100% water and that a value between 60% and 90% is chosen as the limit value (XG). [6] Method according to one of the preceding claims, characterized by that the correction value (WK) is determined as a correction rule in such a way that the difference is raised to the power of an exponent (Y) and the exponentiated difference is multiplied by a correction factor (FK). [7] Method according to one of the preceding claims, characterized bythat the first frequency (f1) and / or the second frequency (f2) and / or the limit value (XG) and / or - if present - the exponent (Y) and / or the correction factor (FK) are determined empirically on the basis of a given series of tests. [8] Method according to claim 7, characterized by that various predetermined materials are applied to the measuring tip (14) as a test series and a respective result value (XE) is determined for air and water. [9] Method according to claim 7 or 8, characterized by that the empirical determination is carried out on the basis of an optimization in such a way that as many of the tests in the test series as possible provide correct yes (Y) and no values ​​(N) and / or for the correct values ​​(Y,N) the distances of the result values ​​(XE) from the limit value (XG) are maximized. [10] Method according to one of the preceding claims, characterized bythat it is carried out on a level sensor (6) of a waste water tank (2) on board an aircraft. [11] Level sensor (6), - with a measuring tip (14) with a first (16a) and a second electrode (16b), wherein the measuring tip (14) can be arranged in a measuring chamber (8) which can be filled with a liquid (10), so that the measuring tip (14) is immersed in the liquid (10) from a certain limit fill level (FG) of the measuring chamber (8), characterized by that the level sensor (6) contains: - a generator module (18) for generating an alternating voltage (U) of at least a first (f1) and a second frequency (f2) between the first (16a) and second electrode (16b), - a measuring module (20) for determining an impedance value (X1,2) of an impedance (X) present between the first (16a) and second electrode (16b) in the measuring space (8), - where the impedance is a scalar value in the form of the magnitude of the complex-valued impedance (X), - an output (22) for outputting a yes value (Y) or a no value (N), - and an execution module (24) for executing the method according to one of the preceding claims. [12] Level sensor (6) according to claim 11, characterized by that the generator module (18) contains a frequency multiplier (37) for generating the higher of the frequencies (f1,2) from the lower of the frequencies (f1,2). [13] Level sensor (6) according to claim 11 or 12, characterized by that the fill level sensor (6) contains an HF generator (26) for generating the alternating voltage (U) and at least one control and evaluation unit (28) for controlling the HF generator (26) and for determining the impedance values ​​(X1,2). [14] Level sensor (6) according to claim 13, characterized bythat the RF generator (26) has separate frequency and amplitude control inputs which are controlled by correspondingly separate outputs (30a,b) of the control and evaluation unit (28). [15] Level sensor (6) according to one of claims 13 to 14, characterized by that the control and evaluation unit (28) has a measuring input (34a,b) each for a voltage (UM) applied to the measuring tip (14) and a voltage (UG) applied to the HF generator (26). [16] Waste water tank (2) for an aircraft, with at least one level sensor, characterized by that at least one of the fill level sensors is a fill level sensor (6) according to one of claims 11 to 15 and / or at least one of the fill level sensors (6) is operated during operation of the waste water tank (2) according to a method according to claims 1-10.

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