DE2852212A1 - capacitor level sensor with dielectric constant compensation - has air and fluid filled compensation and measurement capacitors - Google Patents

Abstract

A capacitive level meter has a first measurement capacitor immersed in the fluid measured, an oscillator producing an electrical output corresp. to the fluid level, and a compensation circuit for different fluid dielectric constants. It gives extremely accurate level indications yet is very economical to produce. Two compensation capacitors of equal capacitance have the medium and air as dielectric respectively. A second measurement capacitor has the same capacitance as the first measurement capacitor when it is completely surrounded by air as the dielectric. The fluid height is the ratio of the second compensation and measurement capacitances multiplied by the ratio of the differences between the measurement and compensation capacitances. The capacitances are measured by charging them and then measuring the discharge time using pulsed signals.

Description

Device for capacitive level measurement

The invention relates to a device for capacitive level measurement with a first immersed into the medium to be measured according to the level Measuring capacitor, with an oscillator to create a level corresponding to the level Size and means to compensate for different dielectric constants of the medium.

In such a known device for capacitive level measurement a measuring capacitor is provided, the length of which is greater than the maximum filling height is. This more or less in the medium to be measured, usually a liquid, submerged measuring capacitor can be in the frequency-determining branch of the oscillator be arranged so as to change, for example, the frequency with which the oscillator oscillates when the capacitance of the measuring capacitor changes as a result of a level change changes. Since with this measuring method the capacitance of the measuring capacitor is not only on the fill level, but also on the dielectric constant, which in particular in the case of a fuel as the medium to be measured with different methanol content is true, it is desirable to examine the influence of the dielectric constant on the to compensate for the electrical quantity emitted by the device.

The present invention is therefore based on the object for precise measurement of the level of substances or media with different To create a device that measures the dielectric constant as precisely as possible, which, however, can be implemented in a manner that is as inexpensive as possible.

To solve this problem, the establishment of the at the beginning mentioned type by the following means for compensation: a first compensation capacitor with the medium as a dielectric, a second compensation capacitor of a capacitance, the same as that of the first compensation capacitor, but with air as the dielectric is a second measuring capacitor whose capacitance is equal to that of the first Measuring capacitor is when it is completely surrounded by air as a dielectric is, an evaluation circuit arrangement in which the capacitors are arranged in this way are that the relationship h = 1 KO (CM - CMO) CMO (CK - CKO) is formed, where is: h = height of the section of the first measuring capacitor immersed in the medium 1 = total height of the first measuring capacitor CK = capacitance of the first compensation capacitor CKO = capacitance of the second compensation capacitor CM = capacitance of the first Measuring capacitor CMO = capacitance of the second measuring capacitor.

According to this invention it is achieved that the height of the in the medium immersing section of the first measuring end sators or the fill level exactly independent of the dielectric constant of the medium to be measured the capacities of the first measuring capacitor, which is partially immersed in the medium, and its first compensation capacitor, which is completely immersed in the medium, in Form of an electrical quantity, namely a voltage, is formed.

The device is particularly expediently designed with the features that to determine the capacitances of the first and the second measuring capacitor as well as the first and second compensation capacitor at least one constant current source each, Means for charging the capacitors to a predetermined voltage, means for Discharge or reload of each capacitor and means for generating synchronized Pulses with a pulse duration dependent on the charging time are provided that to form the differences between the means determined in this way for carrying out a pulse duration subtraction the synchronized pulses are provided and that for dividing a capacity by the difference of two other capacities the capacity to one by one Control voltage controlled constant current source is switchable and the control voltage by means for averaging from one generated by the pulse duration subtraction Pulse is formed.

With this circuit a linear measurement of the capacitance is achieved, since the voltages of the capacitors rise at a rate reciprocal of their capacitance, so that a fixed predetermined voltage is reached after a period of time that is proportional the respective capacity. In addition, the evaluation circuit arrangement is used, that this period of time is reciprocal to the current with which the capacitor, its capacity Is to measure, is loaded. The differences in capacities are shown in the Evaluation circuit determined by subtracting the synchronized pulses, whose Pulse duration is proportional to the capacity and reciprocal to the current with which the Capacitor of this capacity is charged.

A constant current source is therefore used as a function to form the quitient controlled by the difference in capacities, according to the formula above is in the denominator. - This device with the evaluation circuit arrangement can with little costly resources, which for the most part have to be provided several times, in Realize less costly way. The facility still works exactly, especially without the disturbing influence of changes in the dielectric constant of the medium to be measured on the measurement result.

A particularly advantageous implementation of the above device has the features that an oscillator is provided in its frequency-determining Branch of the first compensation capacitor is arranged that three monostable multivibrators are provided in whose branch that determines the duration of a switching position (flop time) one each of the second compensation capacitor, the first and the second measuring capacitor are arranged that the oscillator for triggering each with one input of the three monostable multivibrators that an output of the oscillator is connected to the pulse of one of the capacitance of the first compensation capacitor proportional Pulse duration and proportional period of the pulse arise with a first The input of a first AND gate is connected to that an output of the first monostable Multivibrator, on which pulses one of the capacitance of the second compensation capacitor proportional pulse duration arise at a second input of the first AND element are connected that an output of the second monostable multivibrator to the pulses generated a pulse duration proportional to the first measuring capacitor with a first entrance a second AND element coupled is that an output of the third monostable multivibrator, at the pulses of a pulse duration proportional to the second measuring capacitor arise, to the second The input of the second AND element is connected, so that every constant current source, which can be connected to the first and to the second measuring capacitor, voltage controllable is that each control input of the constant current sources via a first smoothing stage is connected to the output of the first AND element and that to the output of the second AND gate, a second smoothing stage is connected, which is one of the level, but not dependent on the dielectric constant of the medium Gives off tension.

In this evaluation circuit arrangement, three are thus of the same type built-up monostable multivibrators, as well as an oscillator for triggering them Multivibrators used.- Particularly due to the similar structure of the three With monostable multivibrators, this device is not very expensive.

In detail, the facility can be designed in such a way that that the oscillator is designed as an astable multivibrator in which the first Compensation capacitor via a controlled switch to a first constant current source and can be connected to a second constant current source via a second controlled switch is, in which the first compensation capacitor is connected to a first input each first and a second comparator is connected, in which between the second Input of the first comparator and the second input of the second comparator There is a fixed voltage difference in which one output of the first and one of the second Comparator to a first and a second input a tilting stage is connected, and in which one output of the flip-flop to each control input of the first and second controlled switch is performed.

Semiconductor switches are preferably provided as controlled switches. The effort for this oscillator designed as an astable multivibrator is also low because it uses commercially available comparators and a flip-flop is formed, with similar components and assemblies for realizing the monostable multivibrators can be used.

For this purpose, the device is also advantageously designed with the features davss one capacitor of the second compensation capacitor, the first and the second measuring capacitor via a first controlled switch to a second, third and fourth constant current source can be connected and via a second switch it can be short-circuited that the capacitor is connected to a first input of a comparator is connected, the second input of which has a predetermined voltage applied to it and its output is a second input (reset input) of the flip-flop is connected, and that a first input (set input) of the flip-flop with the Output of the oscillator is connected.

The monostable multivibrators are thus implemented in a similar way like the oscillator identified above. These monostable multivibrators are triggered by the oscillator, i.e. at a predetermined point in time in an initial position brought them depending on the capacitance of the capacitor in it leave the frequency-determining branch and depending on the charging current, the The relationship mentioned at the beginning for the level h is determined by suitable dimensioning achieved the device, which is characterized in that all constant current sources of the astable multivibrator or

of the oscillator and the monostable multivibrator, in which the second Compensation capacitor is arranged, are dimensioned to deliver the same current, and that the voltage difference between the second inputs of the first and the second comparator of the astable multivibrator and the specified voltage at the second input of the comparator of all monostable multivibrators to the same The value is adjusted at which the multivibrator is out of its own by triggering generated switching position flips.

The device is also characterized by a particularly advantageous one Design of the first measuring capacitor and the first compensation capacitor such that the first measuring capacitor and the first compensation capacitor is formed by means of a suction pipe immersed in a fuel tank that in the suction pipe a first electrode completely flooded by the medium (fuel) is arranged, which together with the intake manifold the first compensation capacitor forms, and that outside the suction pipe in the fuel tank a second electrode is arranged, which is approximately as high as the clear height of the fuel tank and which, together with the intake manifold, forms the first measuring capacitor.

In this case, the fact that the intake manifold is constantly filled with fuel, so a separate and more expensive There is no construction of the first compensation capacitor.

The device is particularly advantageously characterized by a concentric arrangement of the suction pipe, of the first electrode in the suction tube and a tubular second electrode outside the suction tube.

This design of the capacitor arrangement is particularly compact and enables easy installation in the fuel tank.

The invention is explained below with reference to a drawing with three figures explained. It shows: FIG. 1 the arrangement of the first measuring capacitor and the first Compensation capacitor in a section of a fuel tank (cut), FIG. 2 shows a circuit diagram of the evaluation circuit arrangement and FIG. 3 shows diagrams of the pulses, which occur in the evaluation circuit arrangement according to FIG.

In Fig. 1, 1 denotes a fuel tank in which the level of the medium fuel 2 is to be measured. Immersed in the fuel tank Suction pipe 4 extending through a flange 3.

The inside of the suction tube is concentrically arranged on both sides enclosed open tube which forms a tubular electrode 5. The tubular Electrode 5 and suction pipe 4 are the electrodes of the first measuring capacitor.

The total height 1 of the measuring capacitor is greater than the height the maximum fill level. The height of the section immersed in the fuel medium of the first measuring capacitor is denoted by h.

Within the intake manifold and completely from the fuel medium enclosed, a first electrode 6 is arranged concentrically to the suction tube. Together with the suction pipe, this first electrode forms the first compensation capacitor.

A second compensation capacitor shown in FIG. 1 but in FIG. 2 a capacitance equal to that of the first compensation capacitor, if it is surrounded by air as a dielectric instead of the medium fuel is, is through a capacitor in the evaluation circuit arrangement outside the Realized fuel tank. The same applies to the second measuring capacitor to, whose capacitance is equal to that of the first measuring capacitor when this is completely surrounded by air as a dielectric.

In the evaluation circuit arrangement in FIG. 2 there is the first compensation capacitor with the medium fuel denoted as dielectric with 7, the second compensation capacitor with air as the dielectric with 8, the first measuring capacitor that is immersed in the medium, with 9 and the second measuring capacitor - with air as the dielectric, with 10.

The first compensation capacitor 7 is in an astable multivibrator arranged as an oscillator 11. The second compensation capacitor, the first and the second measuring capacitor are each in a monostable multivibrator 12, 13, 14 switched in such a way that 5s) 'the duration of the unstable switching state, the also known as the flop time.

In detail, the first compensation capacitor 7 is in the oscillator 11 can be connected to a first constant current source 16 via a first switch 15 and via a second controlled switch 17 to a second constant current source 18, the one of the first constant current source opposite the same current to the Capacitor 7 supplies. A control input of the first controlled switch is with 19 denotes a control input of the second controlled switch with 20. Die controlled Switches are implemented by semiconductors.

The oscillator 11 comprises a trigger stage 21 with a first input 22, a second input 23, a first output 24 and a second output 25. The first input 22 is connected to the output of a first comparator 26 the second input is connected to the output of a second comparator 27. A first input 28 or 29 aes the first and the second comparator is connected to the first compensation capacitor 7 connected.

In each case the second input 30, 31 of the first and the second comparator is via a voltage divider 32, 33, 34 to a constant voltage source on the Terminal 35 connected. The voltage divider forms between the second input 30 and the second input 31 a differential voltage a U1.

The output 24 of the flip-flop 21, which is the first input, the set input-22, is assigned, is with the Ste.1ereingang 19 of the first controlled switch in connection.

The second, inverse output 25 of the flip-flop is connected to the control input 20 of the second controlled switch connected.

The following describes the first monostable multivibrator 12, the one with the flip-flop 36 with a first set input 37 and a second reset input 38 and with a first output 39 and a second output 40 is constructed.

The first input 37 is connected to the output 24 of the oscillator 11 Triggering connected. The second input 38, the flip-flop 36 is with a Output of a comparator 41 in connection, with its first input 42 the second Compensation capacitor 8 is coupled. The second one corridor 43 of the comparator is steeper over a voltage 44, 45 with a constant voltage U2 applied.

The capacitor 8 can via a controlled switch 46 with the Control input 47 through a constant current source 48 connected to the constant current source 18 can be identical to be fed. The control input 47 of this controlled The switch is connected to the output 39 of the flip-flop 36. The condenser 8 can also have a control input via a second controlled switch 49 50 can be short-circuited. The control input 50 is connected to the second output 40 of the Tilting stage 36 coupled.

The second monostable multivibrator 13 is the same as the monostable multivibrator 12 described above is constructed. In the monostable Multivibrator 13 is a flip-flop with 51, a first control input, which is equal to the set input, with 52, a second control input, which is a reset input is, with 53, a first output with 54 and a second output with 55. During the the first input 52 is coupled to the output 24 of the oscillator, the second is available Input 53 connected to the output of a comparator 56, the first input of which 57 is connected to the first measuring capacitor 9, the second input 58 is fed with a predetermined voltage U4 via a voltage divider 59, 60. The first measuring capacitor 9 is via a first controlled switch 61 with a Control input 62 can be connected to a constant current source 63 and via a second one controlled switch 64 with a control input 65 can be short-circuited. The control inputs 62, 65 are in turn connected to the outputs 54, 55 of the trigger stage. A special feature the constant current source 63 is that it is controllable by a voltage that it is fed via a line 66.

The third monostable multivibrator 14 has a flip-flop 67 a first input 68, a second input aisle 69, a first Output 70 and a second output 71. A comparator 72 at input 69 of the flip-flop is connected to the second measuring capacitor at its first input 73 and is at its second input 74 via a voltage divider 75,76 with a given voltage U5 applied.

The voltages d Ul, U2, U4 and U5 are chosen to be equal.

The second measuring capacitor 10 can in turn via two controlled switches 77, 78 are alternately charged via a constant current source 79 or short-circuited will. For this purpose, there are control inputs 80, 81 with the outputs 70, 71 of the flip-flop 67 tied together.

The constant current source is voltage-controlled by a voltage on line 66.

To generate a voltage on the line 66, the outputs 24, 40 of the flip-flops 21, 36 each lead to an input of an AND element 82. To the AND element includes a smoothing stage 83 from one in the signal flow direction Resistor-capacitor combination and an amplifier 84.

To create a voltage that is a defined function of the level is, the output 54 of the flip-flop 51 and the output 71 of the flip-flop 67 are closed a second AND gate 85 is performed. The output voltage U (h) is taken from the output of the AND gate 85 is derived via a second smoothing stage 86 and an amplifier 87.

In the following, the function of the evaluation circuit arrangement is described in accordance with Fig. 2 explained on the basis of the pulse diagrams in Fig. 3: In the pulse diagrams are the voltages at points Pl to P5 and P7 to P11, which are marked in FIG are applied.

im It is assumed that / time zero is the trigger stage 21 of the oscillator 1 ° 1 is set, so that a pulse appears at the first output 24 at point P2. As a result, the first switch 15 is closed. The first compensation capacitor 7 is charged by a constant current from the constant current source 16. The voltage of the first compensation capacitor increases linearly, as in the diagram for P1 shown. This happens until the end of the period tck, at which the voltage reaches the value ZSU1 at point P1. In this case, the comparator 27 to the second input the reset input of the flip-flop 21 a signal so that the flip-flop 21 is switched from its first switch position to the second switch position. In order to the pulse at the output 24 or the point P2 disappears, while a pulse appears at the output 25, which closes the second controlled switch 17. As a result, the first compensation capacitor 7 is connected to the constant current source 18 unload. The voltage across the capacitor drops linearly until it reaches the Time TCk at which the flip-flop? 1 is set again via the comparator 26. The duration Tck thus corresponds to the period duration of the pulse, since now again the processes described at time zero begin again. The impulse duration tck and the period duration Tck are proportional to the capacitance of the first compensation capacitor 7 and dsmit proportional to the dielectric constant of the medium, its level should be measured.

The pulse at point P2 is used to trigger the monstable multivibrators 12, 13, 14, so that these all at time Tg and after a period has elapsed be set.

Accordingly, in the monostable multivibrator 12th the controlled switch 46 is closed when the flip-flop 36 is set, whereby the second compensation capacitor 8 is charged by the constant current source 48. This happens up to the time tcko, at which the capacitor voltage is the fixed predetermined Voltage U2 reached. The comparator 41 then sends a control signal a switch is sent to the second input, the reset input, 38 of the flip-flop 36 of the flip-flop until it is back by the oscillator after the period has elapsed 11 is set. The duration tcko, which is the flop time of the monostable multivibrator 12 is proportional to the capacitance of the second compensation capacitor. Of the Course of the pulse at the second output 40, which is an inverse output, is for Point P4 shown as a pulse diagram.

In the AND gate 82, the pulses of the points P2 and P4. With one another linked, - so that the output pulse of the AND element has the course as shown for P5 having.

Pulses of this pulse to P5 have a duration that is the difference of the The duration of the impulses to P2 minus the impulses to P4 is the same. This difference corresponds to thus also minus the difference in the capacity of the first compensation compensator the capacitance of the second compensation capacitor. After smoothing the pulse too P5 and gain appear at the output of the amplifier on line 66 a DC voltage, the level of which is proportional to the difference in the capacities of the two the aforementioned capacitors 7 and 8 is. With this tension the Constant current sources 83 and 79 of the monostable multivibrators 13 and 14 are controlled. Thus, the first measuring capacitor 9 and the second measuring capacitor 10 are each fed with a constant current if the associated controlled switch 6i resp. 77 is closed, the difference between the capacitances of the capacitors 7 and 8 and thus corresponds to the dielectric constant of the medium.

The switching processes in the monostable multivibrators 13 and 14 take place similar to the monostable multivibrator 12 and start triggering at the first inputs 52 and 68 of the flip-flops 51 and 67.

As a result, the voltage of the first measuring capacitor 9 increases across the Point P7 linear with a slope proportional to the current of the constant current source 63 is and is reciprocal of the current capacitance of the first measuring capacitor 9, which in turn depends on the level. This increase in voltage reaches the fixed value Voltage U4 at the comparator after the pulse duration tCM, the pulse this duration appears at the point P8 or at the output 54 of the flip-flop 51. This pulse duration is therefore proportional to the capacitance of the first measuring capacitor or the level and the reciprocal of the difference in the capacitance of the first compensation capacitor minus the capacitance of the second compensation capacitor.

In practically the same way in the monostable multivibrator 14 by the increase in the voltage of the second measuring capacitor 10 according to the Pulse diagram for P9 up to the fixed voltage U5 at the comparator the pulse duration tCMO elapse. The pulse duration tcMO corresponds to the duration of the Output 70 appearing impulse, while at the second output 71 or the measuring point P1O the inverse pulse arises, which is shown for P1O in the pulse diagram.

It is assumed at this point that the constant currents, which are output from the constant current sources 63 and 79 are each equal.

The pulse at the second output 71 of the flip-flop 67 becomes with the pulse linked at the first output 54 of the flip-flop 51 in the AND gate 85, so that at the output of the AND element in measuring point P11, which is shown in FIG. 3 for P11 Pulse arises. The impulses of this pulse to P 11 have a duration which is the difference between the pulse duration tCM minus the pulse duration tCM0.

This pulse for P11, smoothed in the smoothing stage 86, results in after Amplification in the amplifier 87 a voltage U (h)) which is proportional to the expression (CM - CMO) (CK - CKO) is. A voltage measuring device can thus be connected to terminal 88 connected, the current level independent of the dielectric constant of the measured medium. This device can, for example, be a display device or a limit value transmitter, both of which are not shown in FIG.

L e r s e i t e

Claims (8)

P a t e n t a n s p r u c h e 1. Device for capacitive level measurement with a first immersed into the medium to be measured according to the level Measuring capacitor, with an oscillator to form a level corresponding to the level electrical quantity as well as means to compensate for different dielectric constants of the medium, characterized by the following means of compensation: Ein.erster Kompensationskondensator (7) with the medium as a dielectric, a second compensation capacitor (8) one Capacity the same as that of the first compensation capacitor, but with air as a dielectric, is a second measuring capacitor (10) whose capacitance is the same that of the first measuring capacitor (9) when it is completely air is surrounded as a dielectric, an evaluation circuit arrangement (Fig. 2) in which the capacitors (7 to 10) are arranged such that the relationship h = l ÇKO (CM - CMO) CMO (CK ~ CKo) is formed, where: h = height of the immersed in the medium Section of the first measuring capacitor 1 = total height of the first measuring capacitor CK = capacitance of the first compensation capacitor CKO = capacitance of the second compensation capacitor CM = capacitance of the first measuring capacitor CMO = capacitance of the second measuring capacitor. 2. Device according to claim 1, characterized in that for determining the capacitances of the first and second measuring capacitors (9, 10) and of the first and second compensation capacitor (7, 8) at least one constant current source each (16, 18, 48, 63, 79) means for charging the capacitors to a predetermined one Voltage d Ul, U2, U4, U5, means for discharging resp. Reloading of each capacitor (controlled switches 15, 17, 46, 49, 61, 64, 77, 78), means for generating synchronized pulses with one of the Charging time-dependent pulse duration (oscillator 11, monostable multivibrators 12, 13, 14) are provided so that to form the differences between the capacities determined in this way Means for performing a pulse duration subtraction of the synchronized pulses (AND gates 82, 85) are provided and that for dividing a capacity by the difference between two other capacities increases the capacitance to one through a control voltage controlled constant current source (63 or 79) is switchable and the control voltage (on line 66) through means for averaging (line stages 83, 86) is formed from a pulse generated by the pulse duration subtraction. 3. Device according to claim 2, characterized in that an oscillator (11) is provided, in the frequency-determining branch of which the first compensation capacitor (7) is arranged that three monostable multivibrators (12, 13, 14) are provided are, in whose branch the duration of a switching position (flop time) is determined by one the second compensation capacitor (8) of the first and the second measuring capacitor (9, 10) are arranged that the oscillator (11) for triggering with one input each (37, 51, 68) of the monostable multivibrators in Connected, that an output (24), at the pulses of one of the capacitance of the first compensation capacitor (7) proportional pulse duration and proportional period duration of the pulse arise, is connected to a first input of a first AND element (82) that an output (40) of the first monostable multivibrator, on which pulses one of the capacitance of the second compensation capacitor 8 proportional pulse duration arise, appropriate second input of the first AND gate are connected that an output (54) of the second monostable multivibrator (13), on the pulses of one of the first measuring capacitor (9) Proportional pulse duration can be generated with a first input of a second AND gate (85) is coupled that an output of the third monostable multivibrator (14), on the pulses of a pulse duration proportional to the second measuring capacitor (10) is connected to the second input of the second AND element (85). that each constant current source (63, 79) connected to the first and to the second measuring capacitor can be switched on, it is voltage controllable that each one control input of these constant current sources via a first smoothing stage (83) to the output of the first AND element (82) in Connection is and that at the output of the second AND gate (85) a second Smoothing stage (86) is connected, which one of the level, but not of emits a voltage dependent on the dielectric constant of the medium. 4. Device according to claim 3, characterized in that the oscillator (11) is designed as an astable multivibrator in the deverste compensation capacitor (7) via a first controlled switch (15) to a first constant current source (16) and a second controlled switch (17) to a second Constant current source (18) can be connected, in which the first compensation capacitor (7) to a first input (28, 29) each of a first and a second comparator (26, 27) is connected, in which between the second input (30) of the first Comparator and the second input (31) of the second comparator a fixed voltage difference (AU1) prevails and in each of which one output of the flip-flop to a control input of the first and second controlled switch (15, 17) is performed. 5. Device according to claims 3 and 4, characterized in that that each one capacitor of the second compensation capacitor (8) of the first and of the second measuring capacitor (9, 10) via a first controlled switch (46, 61, 77) can be connected to a second, third and fourth constant current source (48, 63, 79) is and can be short-circuited via a second switch (49, 64, 78) that the capacitor to a first input (42, 47, 73) of a comparator tk (41, 57, 72), the second of which Input (43, 58, 74) is subjected to a predetermined voltage and its Output is connected to a second input (38, 53, 69) (reset input) of the flip-flop and that a first input (37, 52, 68) (set input) of the flip-flop with the output (24) of the oscillator (11) is connected. 6. Device according to claims 3, 4 and 5, characterized in that that all constant current aellen (16, 18) of the astable multivibrator and the monostable Multivibrator (12) in which the second compensation capacitor (8) is arranged, for the delivery of the same current and that the savings difference (U1) between the second inputs (D, 31) of the first and second comparators (26, 27) of the astable multivibrator and the specified voltage (U2, U4, U5) at the second input (43, 58, 74) of the comparator of all monostable multivibrators . is adjusted to the same value at which the multivibrator runs out of its the switching position (set position) generated by triggering flips. 7. Device according to claim 1, characterized in that the first Measuring capacitor and the first compensation capacitor by means of a in a fuel tank (1) submerged suction pipe (4) is formed that in the suction pipe one of the medium (Fuel 2) completely flooded first electrode (6) is arranged, which together forms the first compensation capacitor with the suction pipe, and that outside of the suction pipe in the fuel tank a second electrode (ring-shaped electrode 5) is arranged, which is approximately as high as the clear height of the fuel tank and which, together with the intake manifold, forms the first measuring capacitor. 8. Device according to claim 7, characterized by a concentric Arrangement of the suction tube (4) of the first electrode (6) in the suction tube and one tubular second electrode (5) outside the suction tube.