DEVICE AND METHOD FOR CAPACITIVELY MEASURING A FILL LEVEL OF A FILLING MEDIUM

Abstract

Device (1) for capacitive measurement of a filling level of a filling medium (2) in a filling volume (3) that can be filled with a filling medium, comprising: a first measuring element (5), a second measuring element (6), wherein the second measuring element (6) is created such that a potential gradient forms between a first portion (6a) of the second measuring element (6), and a second portion (6b) of the second measuring element (6); a voltage generation means (7) that is associated with the second measuring element (6) and is designed to generate a first electrical voltage (U.sub.1) and a second electrical voltage (U.sub.2) that is optionally different from the first electrical voltage (U.sub.1), and to apply these to the second measuring element (6); a control means (8) that is associated with the voltage generation means (7) and is designed to control the operation of the voltage generation means (7) such that the first and second electrical voltage (U.sub.1, U.sub.2) is applied in an alternating manner to the first and second portion (6a, 6b) of the second measuring element (6).

Claims

1. Device (1) for capacitive measurement of a filling level of a filling medium (2) in a filling volume (3) that can be filled with a filling medium, comprising: a first measuring element (5), an arranged second measuring element (6), wherein the second measuring element (6) is created such that a potential gradient forms between a first portion (6a) of the second measuring element (6), and a second portion (6b) of the second measuring element (6); a voltage generation means (7) that is associated with the second measuring element (6) and is designed to generate a first electrical voltage (U.sub.1) and a second electrical voltage (U.sub.2) that is optionally different from the first electrical voltage (U.sub.1), and to apply these to the second measuring element (6); a control means (8) that is associated with the voltage generation means (7) and is designed to control the operation of the voltage generation means (7) such that the first and second electrical voltage (U.sub.1, U.sub.2) is applied in an alternating manner to the first and the second portion (6a, 6b) of the second measuring element (6).

2. Device according to claim 1, characterized by a measuring device (9) that is designed for measuring the electrical charge between the first and second measuring element (5, 6) during the alternating application of the first and second electrical voltage (U.sub.1, U.sub.2) to the first and second portion (6a, 6b) of the second measuring element (6).

3. Device according to claim 2, characterized by an evaluation device (10) that is associated with the measuring device (9) and is designed for evaluating the measured electrical charge during the alternating application of the first and second electrical voltage (U.sub.1, U.sub.2) to the first and second portion (6a, 6b) of the second measuring element (6), in view of the filling level of the filling medium (2) in the filling volume (3).

4. Device according to claim 1, characterized in that the first measuring element (5) and the second measuring element (6) are arranged side-by-side, wherein the first measuring element (5) and the second measuring element (6) form a capacitor assembly.

5. Device according to claim 1, characterized in that the first measuring element (5) is designed so as to be plate-like or planar, or tube-like or tubular, and the second measuring element (6) is designed so as to be plate-like or planar, or rod-like or rod-shaped.

6. Device according to claim 1, characterized in that the second measuring element (6) is designed as a measuring resistor or comprises at least one such resistor.

7. Device according to claim 6, characterized in that the measuring resistor is designed as a resistance element that is arranged or formed so as to extend continuously along a longitudinal axis of the second measuring element (6), in particular on a substrate element (18) of the second measuring element (6).

8. Device according to claim 6, characterized in that the measuring resistor is formed by a plurality of discrete resistance elements that are arranged or formed along a longitudinal axis of the second measuring element (6), on a substrate element (18) of the second measuring element (6).

9. Device according to claim 1, characterized in that the second measuring element (6) comprises, at least in portions, in particular fully, an insulation coating (17), made of an insulation material, which forms an electrical insulation of the second measuring element (6).

10. Device according to claim 1, characterized in that the first portion (6a) of the second measuring element (6) is formed in the region of a first free end of the second measuring element (6) or by a first free end of the second measuring element (6), and the second portion (6b) of the second measuring element (6) is formed in the region of a second free end of the second measuring element (6) or by a second free end of the second measuring element (6).

11. Device according to claim 1, characterized in that a first voltage application region of the second measuring element (6) is formed by or in the region of the first portion (6a) of the second measuring element (6), and a second voltage application region of the second measuring element (6) is formed by or in the region of the second portion (6b) of the second measuring element (6).

12. Measuring assembly (12) for capacitive measurement of a filling level of a filling medium (2) in a filling volume (3) that can be filled with a filling medium (2), in particular for a device (1) according to claim 1, comprising: a measuring element (6) which is created such that a potential gradient forms between a first portion (6a) of the measuring element (6) and a second portion (6b) of the measuring element (6); a voltage generation means (7) that is associated with the measuring element (6) and is designed to generate a first electrical voltage (U.sub.1) and a second electrical voltage (U.sub.2) that is optionally different from the first electrical voltage (U.sub.1), and to apply these to the measuring element (6); a control means (8) that is associated with the voltage generation means (7) and is designed to control the operation of the voltage generation means (7) such that the first and second electrical voltage (U.sub.1, U.sub.2) is applied in an alternating manner to the first and second portion (6a, 6b) of the second measuring element (6).

13. Method for capacitive measurement of a filling level of a filling medium (2) in a filling volume (3) that can be filled with a filling medium (2), comprising the steps of: providing a first measuring element (5), providing a second measuring element (6), wherein the second measuring element (6) is created such that a potential gradient forms between a first portion (6a) of the second measuring element (6), and a second portion (6b) of the second measuring element (6); providing or generating a first electrical voltage (U.sub.1) and a second electrical voltage (U.sub.2) that is optionally different from the first electrical voltage (U.sub.1); applying the generated first and second electrical voltage (U.sub.1, U.sub.2) alternately to the first and second portion (6a, 6b) of the second measuring element (6), measuring the electrical charge between the first and second measuring element (5, 6) during the alternating application or presence of the first and second electrical voltage (U.sub.1, U.sub.2) on the second measuring element (6), evaluating the measured charges during the alternating application or presence of the first and second electrical voltage (U.sub.1, U.sub.2) on the second measuring element (6), in view of the filling level of the filling medium (2) in the filling volume (3).

14. Method for capacitive measurement of a filling level of a filling medium (2) in a filling volume (3) that can be filled with a filling medium (2) in a device (1) according to claim 1, comprising the steps of: providing a first measuring element (5), providing a second measuring element (6), wherein the second measuring element (6) is created such that a potential gradient forms between a first portion (6a) of the second measuring element (6), and a second portion (6b) of the second measuring element (6); providing or generating a first electrical voltage (U.sub.1) and a second electrical voltage (U.sub.2) that is optionally different from the first electrical voltage (U.sub.1); applying the generated first and second electrical voltage (U.sub.1, U.sub.2) alternately to the first and second portion (6a, 6b) of the second measuring element (6), measuring the electrical charge between the first and second measuring element (5, 6) during the alternating application or presence of the first and second electrical voltage (U.sub.1, U.sub.2) on the second measuring element (6), evaluating the measured charges during the alternating application or presence of the first and second electrical voltage (U.sub.1, U.sub.2) on the second measuring element (6), in view of the filling level of the filling medium (2) in the filling volume (3).

Description

[0051] The invention will be explained in greater detail in the following, with reference to embodiments shown in the drawings, in which:

[0052] FIGS. 1 and 2 are each schematic views of a device for capacitive measurement of a filling level in a filling volume that can be or is filled with a filling medium, according to an embodiment; and

[0053] FIG. 3-6 are each schematic views of measuring elements according to a further embodiment.

[0054] FIGS. 1 and 2 are each schematic views of a device 1 for capacitive measurement of a filling level in a filling volume 3 that can be or is filled with a filling medium 2, according to an embodiment. The filling volume 3 is formed by the geometric/structural dimensions of a receptacle 4, i.e. in particular by the walls or wall portions 4a-4c of the receptacle 4.

[0055] In the embodiment, the filling medium 2 is a fluid, i.e. for example a fuel. Therefore, by way of example, in the embodiment the receptacle 4 is a tank. The walls of the receptacle 4 are denoted 4a-4c.

[0056] The device 1 comprises two measuring elements 5, 6 which are described in greater detail in the following.

[0057] In the embodiment, a first measuring element 5 which is also to be denoted or referred to as a first measuring electrode, is formed, by way of example, by a wall of a housing structure 14 of the device 1. The first measuring element 5 is electrically conductive or exhibits electrically conductive properties; accordingly, the first measuring element 5 is formed of an electrically conductive material, e.g. metal. The potential of the first measuring element 5 is denoted U.sub.0.

[0058] In the operating state of the device 1, shown in the figure, a second measuring element 6 which is also to be denoted or referred to as a second measuring electrode, is arranged so as to be adjacent to the first measuring element 5. Accordingly, the second measuring element 6 is received in or within a receiving space 15 formed by the housing structure 14 of the device 1.

[0059] The two measuring elements 5, 6 are each designed so as to be elongate and are arranged in parallel, i.e. the respective longitudinal axes (not shown) of the two measuring elements 5, 6 are oriented so as to be in parallel.

[0060] It is clear that the two measuring elements 5, 6 are arranged side-by-side, such that the two measuring elements 5, 6 form a capacitor assembly 16 or a capacitor. In this case, the first measuring element 5 forms a first electrode of the capacitor assembly 16, and in this case the second measuring element 6 forms a second electrode of the capacitor assembly 16. A clearance or gap (cf. receiving space 15) is formed between the two measuring elements 5, 6, which clearance or gap can be or is filled with the filling medium 2, according to the filling level, i.e. in a manner dependent on the filling level.

[0061] It is clear that the two measuring elements 5, 6 are arranged in the filling volume 3, or, depending on the filling level, immersed in the filling medium 2 at least in portions.

[0062] The second measuring element 6 is (significantly) less electrically conductive in contrast or in comparison to the first measuring element 5, or has (significantly) reduced electrically conductive properties in contrast to the first measuring element 5. Accordingly, in contrast or in comparison to the first measuring element 5, the second measuring element 6 is formed of a (significantly) less electrically conductive material or a (significantly) less electrically conductive material structure.

[0063] A shown in FIGS. 1 and 2, the second measuring element 6 can comprise an (optional) insulation coating 17 or be surrounded by such a coating. The insulation coating 17 can be omitted if the filling medium 2 is not electrically conductive; therefore, the insulation coating 17 is in principle optional.

[0064] The second measuring element 6 is created such that, during operation of the device 1, a potential gradient, i.e. a gradient of an electrical potential, is formed between a first portion 6a of the second measuring element 6, i.e. in particular a first free end of the second measuring element 6, and a second portion 6b of the second measuring element 6, i.e. in particular a second free end of the second measuring element 6 that is located opposite the first free end of the second measuring element 6. In the embodiment shown in the figures, the first portion 6a of the second measuring element 6 is formed in the region of the first free end of the second measuring element 6 or by the first free end of the second measuring element 6. In the embodiment shown in the figures, the second portion 6b of the second measuring element 6 is formed in the region of the second free end of the second measuring element 6 or by the second free end of the second measuring element 6. The potential gradient formed between said portions 6a, 6b of the second measuring element 6 is typically independent of the filling level of the filling medium 2. The measuring region of the second measuring element 6 extends between the two portions 6a, 6b.

[0065] The second measuring element 6 is designed as a measuring resistor. Accordingly, the formation of the potential gradient between said portions 6a, 6b of the second measuring element 6 is based on the second measuring element 6 being designed as a measuring resistor. The resistance (value) of the measuring resistor is sufficiently large that a corresponding potential gradient is formed between said portions 6a, 6b of the second measuring element 6.

[0066] The measuring resistor can be designed as a resistance element that is arranged or formed so as to extend continuously along the longitudinal axis of the second measuring element 6, in particular on a substrate element 18, i.e. for example a circuit board of the second measuring element 6. An equivalent circuit diagram for the first measuring element 5 and a corresponding second measuring element 6 that is designed as a measuring resistor or resistance element constitutes an RC chain with an “infinite” number of RC elements.

[0067] As is clear from below, in connection with the embodiment shown in FIG. 5, it is alternatively also conceivable for the measuring resistor to be formed by a plurality of discrete, electrically conductive, in particular metal, surface elements F.sub.1-F.sub.n that are arranged or formed along the longitudinal axis of the second measuring element 6 on a corresponding substrate element 18 of the second measuring element 6, and which surface elements are each connected to discrete resistance elements R.sub.1 -R.sub.n. An equivalent circuit diagram for a first measuring element 5 and a correspondingly designed second measuring element 6 displays a plurality of discrete capacitive surfaces that are connected by means of electrical resistors.

[0068] Whatever the specific design, the measuring resistor, and thus the second measuring element 6, cam be formed by coating the substrate element 18 at least in portions, and possibly completely, with a suitable material, i.e. for example a metal. The coating 11 can extend continuously or discontinuously between said portions 6a, 6b of the second measuring element 6 (in the longitudinal direction of the second measuring element). The coating 11 can be achieved by means of various, in particular chemical and/or physical, deposition or application or coating techniques which allow for material application that forms a coating 11. In the embodiment shown in FIGS. 1 and 2, by way of example the coating 11 that forms the measuring resistor is applied to the substrate element 18 by means of printing technology, in particular screen printing technology.

[0069] In the embodiment shown in FIGS. 1 and 2, the second measuring element 6 additionally comprises an insulation coating 17, made of a suitable insulation material, such as plastics material, which forms an electrical insulation of the second measuring element 6. The second measuring element 6 is insulated from the filling medium 2 in this manner; i.e. there is no electrical contact between the filling medium 2 and the second measuring element 6.

[0070] The device 1 further comprises a voltage generation means 7 that is associated with the second measuring element 6, i.e. in particular electrically connected to the second measuring element 6. The voltage generation means 7 is designed to provide or generate a first electrical voltage U.sub.1 and a second electrical voltage U.sub.2, and apply these to the second measuring element 6. In order to provide or generate respective electrical voltages U.sub.1, U.sub.2, the voltage generation means 7 can be connected to an internal or external voltage (supply) source (not shown), i.e. for example a voltage supply network. The first electrical voltage U.sub.1 can be the same as the second electrical voltage U.sub.2. The first electrical voltage U.sub.1 can, however, optionally also be different (in terms of magnitude) from the second electrical voltage U.sub.2, or vice versa. Therefore, the voltage generation means 7 can optionally be designed to generate at least two identical or different electrical voltages U.sub.1, U.sub.2.

[0071] In order to apply respective first and second electrical voltages U.sub.1, U.sub.2 to the second measuring element 6, the second measuring element 6 comprises separate voltage application regions, i.e. for example electrical contacts, at which the electrical voltages U.sub.1, U.sub.2 provided or generated by the voltage generation means 7 can be or are applied. A first voltage application region is formed by or arranged in the region of the above-mentioned first portion 6a of the second measuring element 6, and a second voltage application region is formed by or arranged in the region of the above-mentioned second portion 6b of the second measuring element 6.

[0072] The electrical voltages U.sub.1, U.sub.2 provided or generated by the voltage generation means 7 can each be provided or generated in the form of a voltage pulse that is defined, in particular in terms of magnitude and time. Accordingly, the voltage generation means 7 can be designed for providing or generating defined voltage pulses. The correspondingly designed voltage generation means 7 can for example be designed as a pulse generator. A first voltage pulse can be the same as a second voltage pulse. A first electrical voltage pulse can, however, optionally also be different (in terms of magnitude) from a second electrical voltage pulse, or vice versa. Therefore, the voltage generation means 7 can optionally be designed to generate at least two identical or different electrical voltage pulses.

[0073] The device 1 furthermore comprises a control means 8 that is implemented as hardware and/or software and is associated with the voltage generation means 7. The control means 8 is designed to control the operation of the voltage generation means 7 such that the first and second electrical voltage U.sub.1, U.sub.2 is applied in an alternating manner to the second measuring element 6, i.e. to the first and second voltage application regions of the second measuring element 6. Alternating application or presence of the first and second electrical voltage U.sub.1, U.sub.2 on the second measuring element 6, i.e. the respective voltage application regions of the second measuring element 6, is in particular to be understood to mean that electrical voltages U.sub.1, U.sub.2 or voltage pulses that are defined (in terms of magnitude) are applied to respective voltage application regions of the second measuring element 6 in an alternating manner. In this case, as shown in FIG. 1, either the first voltage U.sub.1 is applied to the first voltage application region of the second measuring element 6, while the second voltage U.sub.2 is applied to the second voltage application region of the second measuring element 6, or, as shown in FIG. 2, the second voltage U.sub.2 is applied to the first voltage application region of the second measuring element 6, while the first voltage U.sub.1 is applied to the second voltage application region of the second measuring element 6.

[0074] The control means 8 is accordingly designed to control the operation of the voltage generation means 7 such that the first electrical voltage U.sub.1 and the second electrical voltage U.sub.2 are applied in an alternating manner to the second measuring element 6, i.e. to respective voltage application regions of the second measuring element 6. Accordingly, in a first time interval (cf. FIG. 1) the first electrical voltage U.sub.1, and in a following second time interval (cf. FIG. 2) the second electrical voltage U.sub.2 is applied to the first voltage application region of the second measuring element 6, while in the first time interval (cf. FIG. 1) the second electrical voltage U.sub.2, and in the following second time interval (cf. FIG. 2) the first electrical voltage U.sub.1 is applied to the second voltage application region of the second measuring element 6. In a following third time interval the first electrical voltage U.sub.1 is again applied to the first voltage application region of the second measuring element 6, while the second electrical voltage U.sub.2 is again applied to the second voltage application region of the second measuring element 6, and so on.

[0075] The principle for capacitive filling level measurement that can be implemented using the device 1 is based in particular on detection and evaluation of the electrical charge or electrical capacitance between the first and the second measuring element 5, 6, which varies depending on the filling level of the filling medium 2, from which measurement the filling level of the filling medium 2 can be concluded and this can thus be determined.

[0076] In this case, the second measuring element 6 typically functions as a filling level-independent voltage divider which generates a potential gradient along the second measuring element 6 or along the measuring region of the second measuring element 6. The filling level-independent voltage division along the second measuring element 6 or along the measuring region of the second measuring element 6 results in a filling level-dependent weighting of the electrical charge or capacitance between the first and the second measuring element 5, 6. Accordingly, the electrical charge between the first and the second measuring element 5, 6 is in particular dependent on the filling level of the filling medium 2, the potential along the potential gradient, and the permittivity of the filling medium 2 which, however, is not relevant, and can therefore be disregarded, for the determination of the filling level that can be performed using the device 1. Accordingly, the determination of the filling level that can be performed using the device 1 is possible independently of the permittivity of the filling medium 2.

[0077] The device 1 furthermore comprises a measuring device 9 that is implemented as hardware and/or software. The measuring device 9 is designed for measuring the, possibly changing, electrical charge or electrical capacitance between the first and second measuring element 5, 6 during the alternating application or presence of the first and second electrical voltage U.sub.1, U.sub.2 on the voltage application regions of the second measuring element 6. The measuring device 9 is thus designed to measure a first electrical charge between the first and second measuring element 5, 6 during application of the first electrical voltage U.sub.1 to the first voltage application region of the second measuring element 6 and while the second electrical voltage U.sub.2 is applied to the second voltage application region of the second measuring element 6, and to measure a second electrical charge between the first and second measuring element 5, 6 during application of the second electrical voltage U.sub.2 to the first voltage application region of the second measuring element 6 and during application of the first electrical voltage U.sub.1 to the second voltage application region of the second measuring element 6.

[0078] The measuring device 9 is furthermore designed for generating a measuring signal that describes the measured charge between the first and the second measuring element 5, 6 during the alternating application or presence of the first and second electrical voltage U.sub.1, U.sub.2 on the voltage application regions of the second measuring element 6. The measuring signal can contain at least two partial signals, wherein a first partial signal describes the electrical charge between the first and second measuring element 5, 6 during application of the first electrical voltage U.sub.1 to the first voltage application region of the second measuring element 6 and during application of the second electrical voltage U.sub.2 to the second voltage application region of the second measuring element 6, and a second partial signal describes the electrical charge between the first and second measuring element 5, 6 during application of the second electrical voltage U.sub.2 to the first voltage application region of the second measuring element 6 and application of the first electrical voltage U.sub.1 to the second voltage application region of the second measuring element 6.

[0079] The measuring device 9 is associated with an evaluation device 10 that is implemented as hardware and/or software. The evaluation device 10 is designed for evaluating the measured charge during the alternating application or presence of the first and second electrical voltage U.sub.1, U.sub.2 on the second measuring element 6, i.e. in particular on the respective voltage application regions of the second measuring element 6, or for evaluating said measuring signal in view of the filling level of the filling medium 2 in the filling volume 3. In order to evaluate the measured charge or the measuring signal, the evaluation device 10 can comprise suitable evaluation logics or suitable evaluation algorithms.

[0080] FIGS. 1 and 2 furthermore show a measuring assembly 12 for capacitive measurement of a filling level of a filling medium 2 in a filling volume 3 that can be filled with a filling medium 2. The measuring assembly 12 comprises the second measuring element 6, the measuring element 6 forming the device 1, the voltage generation means 7 associated with the second measuring element 6, the control means 8, the measuring device 9, and the evaluation device 10.

[0081] As indicated by the box 13 shown in dashed lines, individual ones, a plurality, or all of the parts of the measuring assembly 12 can be combined, structurally, to form an assembly that can be handled separately, i.e. for example arranged or formed on or in a housing body (not shown) of the measuring assembly 12.

[0082] In the embodiment shown in FIGS. 1 and 2, the first measuring element 5 and the second measuring element 6 are designed in a plate-like or planar manner in each case. FIG. 3 is a cross-sectional view of the measuring elements 5, 6 or the arrangement thereof, in a schematic view of an embodiment of a corresponding capacitor assembly 16. Since it is optional, the first measuring element 5, which is at the bottom in FIG. 3, is shown in dashed lines.

[0083] FIG. 4-6 show further embodiments of first and second measuring elements 5, 6 or capacitor assemblies 16 formed thereby:

[0084] FIG. 4 is in turn a cross-sectional view, for example of a coaxial arrangement of the measuring elements 5, 6. The coaxial arrangement of the measuring elements 5, 6 results from a tube-like or tubular design of the first measuring element 5, and a rod-shaped or rod-like design of the second measuring element 6.

[0085] FIG. 5 (already mentioned) is a purely schematic view of the formation of the second measuring element 6 by a plurality of discrete, electrically conductive, in particular metal, surface elements F.sub.1 -F.sub.n that are arranged or formed along the longitudinal axis A1 of the second measuring element 6 on a corresponding substrate element 18 of the second measuring element 6, and which surface elements are each connected to discrete resistance elements R.sub.1-R.sub.n. It is clear that an alternating arrangement consisting of corresponding surface elements F.sub.1 -F.sub.n and resistance elements R.sub.1-R.sub.n results. It is accordingly visible, in FIG. 5, that the second measuring element 6 can also be designed so as to be segmented, i.e. can comprise a plurality of measuring element segments, specifically in the form of corresponding surface elements F.sub.1 -F.sub.n and resistance elements R.sub.1-R.sub.n, that form the second measuring element 6.

[0086] It is clear from the embodiment shown in FIG. 6, which is a plan view of a capacitor assembly 16, that the measuring elements 5, 6 can also be formed by specified more or less electrically conductive structures, on a substrate element 18 such as a circuit board. Electrically conductive structures (first electrically conductive structures 19) having higher electrical conductivity form the first measuring element 5, and electrically conductive structures (second electrically conductive structures 20) having a comparatively (significantly) lower electrical conductivity form the second measuring element 6. In this connection, what is known as an interdigital arrangement or structure is shown in FIG. 6. Accordingly, respective first and second electrically conductive structures 19, 20 are designed so as to be arranged in a manner so as to be mutually offset, in parallel, so as to engage in one another in a finger-like manner. In this case, the arrangement of respective first and second electrically conductive structures 19, 20 is selected such that respective first electrically conductive structures 19 are arranged or designed so as to engage in two gaps that are formed between two (directly) adjacent second electrically conductive structures 20, at least in regions.

[0087] A method for capacitive measurement of a filling level of a filling medium 2 in a filling volume 3 that can be or is filled with a filling medium 2 can be implemented using the device 1 or the measuring assembly 12 shown in the figures.

[0088] The method comprises in particular the following steps: [0089] providing a first measuring element 5, [0090] providing a second measuring element 6, wherein the second measuring element 6 is created such that a potential gradient forms between a first portion 6a of the second measuring element 6, and a second portion 6b of the second measuring element 6; [0091] providing or generating a first electrical voltage U.sub.1 and a second electrical voltage U.sub.2 that is optionally different from the first electrical voltage U.sub.1; [0092] applying the generated first and second electrical voltage U.sub.1, U.sub.2 alternately to the first and second portion 6a, 6b of the second measuring element 6, [0093] measuring the charge between the first and second measuring element 5, 6 during the alternating application or presence of the first and second electrical voltage U.sub.1, U.sub.2 on the second measuring element 6, [0094] evaluating the measured charges during the alternating application or presence of the first and second electrical voltage U.sub.1, U.sub.2 on the second measuring element 6, in view of the filling level of the filling medium 2 in the filling volume 3.