EP2108843A1 - Sensor for turning a pump on or off - Google Patents

Abstract

The sensors (14,16) have two electrodes, an electronic circuit that has a voltage supply connected to former electrode and designed for issuing short voltage pulses for charging former electrode, and evaluating circuit. The evaluating circuit is designed such that it measures current between the electrodes during a voltage increase when the former electrode is charged or during a voltage drop when the former electrode is discharged. An independent claim is included for a pump for supplying fluids.

Description

The invention relates to a sensor for switching on and / or off a pump, in particular a submersible pump or basement pump.

Submersible pumps such as sewage pumps often have sensors or switches, which turn on the pump when exceeding a predetermined water level and turn off when falling below a generally lower second water level, the pump again. For this, e.g. mechanical float switch known. However, there is a risk that they will be blocked in their movement, which leads to errors when switching on and off the pump.

In addition, electronic sensors such as capacitive sensors for switching the pump on and off as a function of a fluid or water level are known. In these known capacitive sensors, a high-frequency oscillator is provided, which is connected to the water. The change in the capacity formed by the water is determined by the power consumption of the oscillator. These electronic circuits require a high frequency signal generator and a very sensitive circuit for detecting power consumption. This makes such circuits consuming and expensive.

It is therefore an object of the invention to provide an improved sensor for switching on and / or off a pump, which after a capacitive measuring principle works, however, is easier and less expensive to build.

This object is achieved by a sensor for switching on and / or off a pump having the features specified in claim 1 and by a pump having the features specified in claim 9. Preferred embodiments will become apparent from the appended subclaims, the following description and the attached figures.

The sensor according to the invention is for switching on and / or off a pump, in particular a submersible pump or basement pump, as used for example for basement drainage provided. The sensor operates on a capacitive measuring principle and has for this purpose a first and a second electrode, which form a capacitor. The condenser is arranged so that its capacity is influenced by the fluid to be delivered. That depending on the height of the liquid level or level, the capacity changes. The two extremes are defined by the condition in which there is no water between the electrodes, and the condition in which both electrodes are completely contained in the fluid, i. preferably below the water level. Further, an electronic circuit is provided, which is connected to the electrodes and the signal evaluation of the changing capacitance between the electrodes is used to generate an on and / or off signal for a pump.

According to the invention, the electronic circuit has a voltage supply connected to the first electrode. This power supply is intended to electrically charge the first electrode with respect to the environment and the second electrode. For this purpose, the power supply is designed so that it can deliver short voltage pulses for charging the first electrode. Preferred is the electronic Circuit configured such that a plurality of voltage pulses of the electrode, for example between three and forty pulses, more preferably between five and twenty pulses are delivered to charge the first electrode. These short voltage pulses prevent electrolysis between the electrodes and wear of the electrodes. Preferably, a very short on-time <1% of the total charging time is selected.

According to the invention, the electronic circuit further comprises an evaluation circuit, which is designed to detect and evaluate capacitance changes between the electrodes in order to generate an on and / or off signal for the pump. This evaluation circuit is designed such that it detects the current between the electrodes during a voltage increase during charging and / or a voltage drop when discharging the electrode and outputs an on and / or off signal depending on the detected current. The current flowing between the electrodes during charging and discharging is proportional to the capacitance between the electrodes. In this respect can be determined from the current, whether the electrodes are in the water or not.

The electronic circuit according to the invention is much simpler and cheaper to build than known capacitive sensors, as can be dispensed with a high-frequency signal generator. The detection of the current during charging and / or discharging is quite easy to accomplish and for charging only a pulse generator for generating the voltage pulses is required, but not a signal generator which generates a specific high-frequency signal.

Preferably, the electronic circuit is designed such that when charging the electrode and / or when discharging the electrode, the temporal waveform of the voltage U at least in one section has a predetermined slope. That is, in the range of this predetermined slope dU / dt is known. In knowledge of this slope can be determined on detection or measurement of the discharge current I _c, the capacitance C according to the formula $$I_C=C\cdot \frac{dU}{dt}\mathrm{,}$$

The capacity depends on whether there is fluid between the electrodes or not. In this way, thus knowing the charge or discharge curve by measuring the current capacity can be determined.

Further, it is preferable that the predetermined slope is steep, preferably steeper than 5V / μs. Such rapid charging or discharging of the capacitor formed by the electrodes reduces or eliminates the influence of the electrical resistance between the electrodes on the charging or discharging process. With slower charging or discharging, when there is water between the electrodes, a current would flow between the electrodes, causing a discharge. In this state, therefore, no defined charging or discharging curve with a previously known gradient could be achieved. As a result of the very rapid charging or preferably discharging via corresponding components in the electronic circuit, the discharge of the electrodes via the fluid located between the electrodes is largely minimized or eliminated. In order to be able to discharge the charged electrode in a defined manner, the electronic circuit preferably has an unloading device which effects the unloading process with the defined pitch. The slope of the voltage curve during charging or the negative slope during discharge is more preferably> 100V / μs, in particular> 500V / μs.

According to a preferred embodiment, the electronic circuit is designed such that a cyclically repeating charging and discharging the electrode takes place with detection of the current during charging and / or discharging. In this way, a continuous monitoring process is performed to determine whether there is fluid between the electrodes or not. In this way, the capacitive sensor can be used as a sensor for switching on a pump. Also, such a sensor can be used to turn off such a pump, wherein the switch-off is recognized by the fact that less or no fluid between the electrodes is present, ie the pump has the environment to the required level emptied or dry.

More preferably, the electronic circuit is designed such that the electrode is first charged by a plurality of voltage pulses of the power supply and then discharged, the evaluation circuit detects the current during discharge and outputs an input and / or off depending on the detected current. In this case, the detected current is representative or proportional to the capacitance between the electrodes, which in turn depends on whether there is fluid between the electrodes or not. Thus, the current measurement and thereby the capacity determination during a defined discharge process preferably takes place. This discharge can be initiated and performed by an unloading device provided in the electronic circuit, so that a discharging operation can be performed with a very steep discharge curve, as described above. Particularly preferably, this discharge curve is linear in the region in which the current measurement is carried out. By charging the electrode by means of very short voltage pulses, as described, an electrolysis in the fluid is prevented. Due to the fast discharge process, the influence of the electrical resistance can be reduced or eliminated.

The current measured during discharging can also be used to calculate the capacitance of the capacitor formed by the electrodes. If there is the fluid to be pumped between the electrodes, the capacity is significantly greater than if there is no fluid between the electrodes, ie air. In the case of water as fluid, the capacity is about eighty times greater than air, due to the greater relative permittivity of water (ε _R = 80) to air (ε _R = 1). The arrangement of the electrodes determines whether the on and / or off point of the sensor is determined by them. Basically, one sensor is sufficient to determine on and off point. Thus, a switch-on signal for switching on the pump can be issued if due to the larger capacity fluid between the electrodes of the evaluation circuit is detected. If again a lower capacity is detected by the evaluation circuit due to the lower discharge current, it can be concluded that there is no more fluid between the electrodes and a switch-off signal for switching off the pump is delivered. Alternatively, it is possible to arrange two sensors at different vertical levels and to turn on the pump by a turn-on signal of the upper sensor, this turn-on signal is generated by the evaluation circuit, when water is detected by the electrodes of this upper sensor. The pump can then be switched off by a switch-off signal of the second lower sensor, which is output by its evaluation device when no water, ie air between the electrodes is detected.

According to a further preferred embodiment, the electronic circuit is designed such that the evaluation circuit additionally determines the electrical resistance between the two electrodes and outputs an on and / or off signal depending on the detected current and the resistance. In the case where a conductive fluid such as water is located between the electrodes, this Electrodes do not form an ideal capacitance can be achieved by additional consideration of the electrical resistance of the medium, ie fluid between the electrodes greater accuracy.

The power supply preferably has a voltage source with one of these downstream electrical resistance and one of these parallel-connected capacitance. By this arrangement, the circuit can be made short-circuit proof.

To generate a charging and / or discharging voltage with a defined signal curve, the voltage supply preferably has a signal generator. This signal generator generates the defined and at least partially very steep voltage curve when loading and particularly preferably during unloading. Thus, the capacitance formed by the electrodes is discharged with a defined voltage curve over time. This voltage curve during discharging is specified by the signal generator.

Subject of the invention is further a pump for conveying a fluid with an electric drive motor and a control device for switching on and off of the drive motor. The pump according to the invention is designed such that its control device has at least one sensor according to the preceding description, which serves to turn on and / or off the pump as a function of the fluid level. The sensor, which generates the switch-on signal in cooperation with the evaluation device, is arranged at a vertical level, which is the switch-on level. That is, when the fluid level reaches this switch-on level, the pump is switched on. The sensor is arranged so that at this level of fluid its capacity is changed so that it is determined by the evaluation device via the discharge current and correspondingly a turn-on signal is emitted. To turn off either the same or another sensor arranged at a level below which the pump is to be switched off again by the fluid level. In this case, the shutdown takes place when the capacity changes so that it corresponds to the capacity of air between the electrodes. However, it is not absolutely necessary that the switching off of the pump is also caused by such a sensor as described above.

Preferably, one of the electrodes is formed by the housing of the pump and the second electrode is isolated from the housing. This is particularly useful when the pump housing is made of metal. Alternatively, it is also possible to provide on the outside of the housing two spaced-apart and mutually insulated electrodes. Preferably, the electrodes are in direct contact with the surrounding fluid, i. they are not covered by further layers of material to the pump outside.

As described, the at least one sensor is arranged to generate a switch-on signal for the drive motor at a predetermined fluid level. This sensor is preferably seconded in the vertically upper region of the pump.

According to a preferred embodiment, a shut-off device is provided for the pump, which has at least one detection means for detecting at least one electrical parameter of the drive motor and is designed such that on the basis of this electrical parameter, a dry run of the pump is detectable, and when dry run detected a shutdown signal for generates the drive motor. The dry run can be detected, for example, due to a phase shift in the electrical operating voltage supplied to the drive motor. The drive motor is preferably provided with a frequency converter for speed control. It can Means or functions of the existing frequency converter can be used to detect this phase shift and thus the dry run. However, other parameters, such as the electric current, may be used to detect dry run. The detection means is then designed accordingly.

According to a further preferred embodiment, a protective electrode is arranged on the pump, which shields the first electrode of the sensor against electrical components in the interior of the pump. For this purpose, this protective electrode is arranged in or on the housing further inwardly behind the first electrode, so that the protective electrode is located between electronic components in the interior of the housing and the first electrode.

Fig. 1

den Spannungsverlauf beim Entladen des Sensors,

Fig. 2

den Stromverlauf beim Entladen des Sensors,

Fig. 3

ein Modellschaltbild zweier Elektroden in dem zu fördernden Fluid,

Fig. 4

ein Blockschaltbild einer Pumpe mit einem erfindungsgemäßen Sensor,

Fig. 5

ein Blockschaltbild eines erfindungsgemäßen Sensors,

Fig. 6

schematisch die Anordnung einer Sensorelektrode am Gehäuse der Pumpe,

Fig. 7

schematisch die Anordnung einer Sensorelektrode im Gehäuse einer Pumpe unter Verwendung einer Schutzelektrode,

Fig. 8

schematisch die Anordnung einer Sensorelektrode an dem Pumpengehäuse,

Fig. 9

schematisch eine Anordnung einer Pumpe mit einem erfindungsgemäßen Sensor,

Fig. 10

schematisch die Anordnung einer Pumpe mit einem erfindungsgemäßen Sensor gemäß einer weiteren Ausführungsform,

Fig. 11

eine Anordnung ähnlich der Anordnung in Fig. 9 mit zwei Sensoren, und

Fig. 12

einen beispielhaften Aufbau der Sensorelektronik.

The invention will now be described by way of example with reference to the accompanying drawings. In these shows:

Fig. 1

the voltage curve when unloading the sensor,

Fig. 2

the current flow when unloading the sensor,

Fig. 3

a model circuit diagram of two electrodes in the fluid to be pumped,

Fig. 4

a block diagram of a pump with a sensor according to the invention,

Fig. 5

a block diagram of a sensor according to the invention,

Fig. 6

schematically the arrangement of a sensor electrode on the housing of the pump,

Fig. 7

schematically the arrangement of a sensor electrode in the housing of a pump using a protective electrode,

Fig. 8

schematically the arrangement of a sensor electrode on the pump housing,

Fig. 9

schematically an arrangement of a pump with a sensor according to the invention,

Fig. 10

FIG. 2 schematically the arrangement of a pump with a sensor according to the invention according to a further embodiment, FIG.

Fig. 11

an arrangement similar to the arrangement in Fig. 9 with two sensors, and

Fig. 12

an exemplary structure of the sensor electronics.

The sensor according to the invention is a capacitive sensor, ie on and / or off timing for a pump as a function of a fluid level are determined on the basis of a changing capacitance between two electrodes 2 and 4. For this purpose, the electrodes 2 and 4 are spaced from each other and electrically isolated from each other so arranged that the fluid to be delivered, the fluid level to be detected, affects the capacitance of the capacitor formed by the electrodes 2 and 4. This happens because in the case that fluid, for example water, is located between the electrodes 2 and 4, the capacitance changes markedly compared to a state in which there is air between the two electrodes 2 and 4. This results from the very different Permittivity of water and air. Fig. 3 shows a model or equivalent circuit diagram for the arrangement of the electrodes 2 and 4 in the environment in which either air or the fluid to be delivered is located. In particular, when the fluid to be delivered is water coming into contact with the two electrodes 2 and 4, the arrangement of the electrodes 2 and 4 does not behave like an ideal capacitor. This is in the equivalent circuit diagram Fig. 3 considered, there is parallel to the capacitance C, an electrical resistance R shown. This is the electrical resistance R of the medium between the electrodes 2 and 4. If there is air between the electrodes 2 and 4, this is very large. If there is water between the electrodes 2 and 4, this resistance R can become very small.

However, a detection of the fluid only on the basis of the electrical resistance is problematic, as even a thin film of water on the housing or the sensor array or for example a wet piece of paper covering both electrodes would reduce the resistance as if the fluid level were correspondingly high would have risen. However, capacity is not affected by such short circuits.

The measurement or detection of the capacitance between the electrodes 2 and 4 is carried out such that initially the electrodes 2 and 4 are slowly charged with low current. For this purpose, a charge can be applied to one of the electrodes 2, 4. The charging is preferably carried out by a plurality of very short voltage pulses. This has the advantage that no or only a small current flow occurs between the electrodes 2 and 4, so that an electrolysis between the electrodes 2 and 4, which could lead to damage of the electrodes, is avoided.

The voltage curve during charging is in Fig. 1 shown. The charging process takes place until time T at which the maximum charge is reached. As in Fig. 2 At time T, the capacitor C formed by the electrodes 2 and 4 is discharged very quickly, ie the voltage drops, as in Fig. 1 shown is steep. This leads to a high discharge current, as in Fig. 2 is shown. This discharge current during the discharging process is measured. The magnitude of the discharge current is proportional to the capacitance C between the electrodes 2 and 4.

Essential in the discharge process is that the discharge of the electrodes 2 and 4 takes place with a defined previously known very steep voltage curve. As in Fig. 1 can be seen, in the region 10, the slope of the voltage curve dU / dt is constant. In addition, this slope is previously known and is specified by an electronic circuit during unloading. Knowing this slope, the capacitance C can be calculated based on the relationship I _C = C * dU / dt on the basis of the measured discharge current I _C. The advantage of this measuring principle is that it can be realized very simply and inexpensively, because complex frequency generators can be dispensed with.

Fig. 4 schematically shows in a block diagram an inventive pump unit with a sensor which operates according to the previously described measuring principle. The pump unit has a power supply 11, for example in the form of a connection plug for connection to the power grid, and an electric drive motor M and a control device 12, which is responsible for switching on and off of the drive motor. In the example shown, two sensors 14 and 16 are provided, which each have two electrodes 2, 4, as described above. A sensor 14 is provided for switching on the pump, the second sensor 16 is for switching off provided the pump. For this purpose, the sensors 14 and 16 are arranged in two vertically spaced-apart positions. When the fluid level reaches the sensor 14, ie the upper sensor, the pump is turned on. The pump or the drive motor M is switched off when the fluid level falls below the lower sensor 16, and the lower sensor 16 thus detects air between the electrodes 2 and 4.

The control device 12 has a power supply 18 for the control device 12, a controller 20 and a power switch 22. The controller 20 controls the charging and discharging of the electrodes 24 of the sensors 14 and 16 in the manner described above and the current measurement and takes over the evaluation during discharge. When the electronic circuit detects a state in which the motor is to be turned on or off, the controller correspondingly drives the power switch 22 to turn on and off the motor. The controller 20 preferably carries out a continuous monitoring process in which the electrodes of the sensors 14 and 16 are periodically charged and then discharged again, with the described current measurement being carried out to detect the capacitance during each discharge process. It is conceivable that the discharge cycle and the next charge cycle are spaced in time. However, this time interval should not be too long to be able to detect the achievement of the on and off level of the fluid as soon as possible. Especially when switching off this is important to avoid prolonged dry running of the pump.

Fig. 5 shows in a block diagram the schematic structure of a sensor device with sensor electrodes 2 and 4 and the associated control and evaluation circuit, which will now be described in more detail. In addition to the power supply 18 and the controller 20, which the Whole operation of the sensor assembly and the evaluation of the sensor signals takes over, the electronic circuit as essential further components a pulse generator 24 and a current sensor 26. On the output side of the pulse generator 24, pulse shaper 28 and an output stage 30 are connected. The output stage 30 serves to buffer the signal in order to be able to detect even highly conductive fluids with the sensor according to the invention. The output stage 30 is connected via a capacitor 32 to the first electrode 2. The pulse generator generates for charging the electrode 2 a plurality or a plurality of very short voltage pulses, with which the sensor electrode is charged. To discharge the pulse generator 24 generates together with the pulse shaper 28, the steep predetermined discharge curve described above. When discharging, the current sensor 26 detects the discharge current between the electrodes 2 and 4. The output of the current sensor 26 is supplied to a sample and hold circuit 34 which stores the peak value of the discharge current and outputs a proportional voltage as an output signal. This output signal is fed to the microcontroller 20 which, knowing the discharge curve, determines the capacitance between the electrodes 2 and 4 and carries out an evaluation as to whether there is fluid or water between the electrodes 2 and 4. The microcontroller 20 also drives the pulse generator 24 and specifies the charge and discharge cycles.

Also, the electrode 4 is coupled via a capacitor 36. The coupling of the electrodes 2 and 4 via capacitors 32 and 36 isolates the electrodes 2, 4 from the electronics, so that a direct contact of a person with the electrodes 2 and 4 is harmless.

Fig. 6 shows a possible arrangement of the electrodes 2 and 4 in the pump unit. In this example, the electrode 4 is formed by the metallic pump and / or motor housing. The electrode 2 is arranged separately and via an insulator 38 to the housing 4th connected so that the electrodes 2 and 4 are electrically isolated from each other. As described above, the electrodes 2 and 4 are connected via capacitors 32 and 36 to the transmitter 40. The transmitter 40 comprises, as based on Fig. 5 explained, power supply 18, controller 20, pulse generator 24, current sensor 26, pulse shaper 28, power amplifier 30 and sample and hold circuit 34. However, the transmitter 40 can also be deviating in other suitable manner to implement the measuring principle of the invention.

Fig. 7 shows a further possible arrangement of the electrodes 2 and 4 in the pump unit, which substantially the arrangement in Fig. 6 equivalent. In addition, a protective electrode 42 is arranged between the housing which forms the second electrode 4 and the first electrode 2. The guard electrode 42 is connected to an active protection circuit 44. Protective electrode 42 and protective circuit 44 serve to shield electric fields which occur on the back side of the electrode 2 in the interior of the housing by the electrical or electronic components arranged there against the electrode 2, so that the electrode 2 detects only electric fields outside the housing, such as indicated by the field lines 46.

Fig. 8 shows again in a schematic plan view of the pump unit, the housing serves as a second electrode 4. The first electrode is arranged so as to be electrically insulated from the housing and thus from the second electrode, so that a capacitance C is present between the electrodes 2 and 4 as a function of the surrounding medium or fluid.

Fig. 9 shows a possible arrangement of a pump 48 with a sensor 50, which, as in Fig. 5 is constructed described. This sensor 50 is not integrated in the pump unit 48 but in the electrical Supply line between the power supply 11 and the pump unit 48 is arranged. The sensor 50 has, as in Fig. 10 shown, two sensor electrodes 2 and 4, which form a capacitor in the manner described above with the environment. The sensor 50 is located near the bottom 52. If the water or fluid level rises so high that the electrodes 2 and 4 of the sensor 50 are in the water, that is detected by the sensor and it turns on the power supply to the pump 48 so that it promotes fluid or water. When the fluid level drops below the level of the sensor electrodes 2 and 4, the capacitance of the electrodes 2 and 4 changes significantly, which is detected in the manner described above, and the sensor 50 then connects, via a power switch, the line between the power supply 11 and the pump unit 48 and thus switch off the pump set.

Fig. 11 shows an arrangement similar to the arrangement in Fig. 9 with the difference that two sensors 50 and 54 are provided. With two sensors 50 and 54, the pump unit 48 is operated such that when the fluid level reaches the upper sensor 54 and thus its electrodes 2 and 4 are in fluid, the pump 48 is turned on. The pump unit 48 is switched off when the lower sensor 50 detects air between its electrodes 2 and 4, ie, the fluid level has fallen below the vertical level of the sensor 50.

Turning off the pump according to the invention can also be initiated in other ways. For example, the motor control for the pump motor detects the dry running of the pump. This is recognizable from electrical parameters of the motor, for example based on a phase shift of the supply voltage.

In Fig. 12 an exemplary circuit diagram of the sensor electronics is shown, the essential components are described below. VCC is the input voltage for the capacitive sensor. C ₁ is a bypass capacitor and C ₂ is the capacitor that is charged to provide a certain amount of energy to the sensor. When the sensor is activated, the power supply VCC is interrupted and the sensor electrodes 2, 4 are supplied via the output A ₁ alone with the voltage from the capacitor C ₂ . The stored energy in the capacitor C ₂ is released by the capacity or the conductivity of the water. Consequently, at the end of the measurement, a residual amount of energy remains in the capacitor C ₂ , so that the conductivity of the water can be determined by the remaining voltage across the capacitor C ₂ .

U1 is a pulse shaper in the form of a Schmitt trigger. Via the input E ₂ , which represents the input of the Schmitt trigger, the pulses for activating the sensor are fed to the pulse shaper U1.

Through the resistor R ₂ and the capacitor C ₅ , the discharge curve or discharge rate dU / dt is specified for the sensor. The transistors Q ₁ and Q ₂ serve to supply a higher current to the sensor output A1. The diode D ₁ and the resistor R ₁ serve to protect the transistor Q ₁ and reduced the charging speed dU / dt. The capacitors C ₄ and C ₆ are isolating capacitors which serve to protect persons who come into contact with the electrodes 2, 4.

The resistor R ₃ serves to detect the current flowing here between the electrodes 2, 4 and the earth, that is, the current which is proportional to the capacitance to be measured between the sensor electrodes 2, 4.

The capacitor C ₈ is a decoupling capacitor, which allows the peak detector formed by the diode D ₃ and the capacitor C ₉ in conjunction with the bias circuit formed from the resistors R ₄ and R ₅ and the diode D ₄ offset Has error near zero.

The capacitor C ₉ serves to hold the voltage corresponding to the detected capacitance and to perform a slow digitization of the voltage via, for example, an analog-to-digital converter at the output A ₂ .

The capacitor C ₂₈ serves to eliminate interference or spurious oscillations.

LIST OF REFERENCE NUMBERS

2, 4 -

electrodes

10 -

Range of the voltage curve

11 -

power supply

12 -

control device

14, 16 -

sensor

18 -

power supply

20 -

controller

22 -

breakers

24 -

pulse generator

26 -

current sensor

28 -

pulse shaper

30 -

final stage

32 -

capacitor

34 -

Sample and hold circuit

36 -

capacitor

38 -

insulator

40 -

evaluation system

42 -

guard electrode

44 -

protection circuit

46 -

field lines

48 -

pump unit

50 -

sensor

52 -

ground

54 -

sensors

R -

resistance

C -

capacity

U -

tension

t -

Time

I _C -

electricity

T -

time

VCC -

input voltage

E ₂ -

entrance

A ₁ , A ₂ -

outputs

U ₁ -

pulse shaper

C ₁ , C ₂ , C ₃ , C ₄ , C ₅ , C ₆ , C ₇ ,

C ₈ , C ₉ , C ₂₈ -

capacitors

D ₁ , D ₂ , D ₃ ,

D ₅ -

diodes

R ₁ , R ₂ , R ₃ ,

R ₄ , R ₅ -

resistors

Q ₁ , Q ₂ , -

transistors

Claims (13)

Sensor for switching on and / or off a pump having at least a first (2) and a second (4) electrode, which form an influenceable by the fluid to be conveyed capacity (C), and one with the electrodes (2, 4) electronic circuit (40),
characterized in that the electronic circuit (40) has a voltage supply (24) connected to the first electrode (2), which is designed to deliver short voltage pulses for charging the first electrode (2), and an evaluation circuit (20, 26, 34 ), which is designed such that during a voltage increase during charging and / or a voltage drop during discharge of the electrode (2) detects the current (I _C ) between the electrodes (2, 4) and an on and / or off signal depending on the detected current (I _C ) outputs. Sensor according to claim 1, characterized in that the electronic circuit (46) is designed such that during charging of the electrode (2) and / or during discharge of the electrode (2) the temporal waveform of the voltage (U) at least in one section has predetermined slope (dU / dt). Sensor according to Claim 2, characterized in that the predetermined gradient (dU / dt) is steep, preferably steeper than 5V / μs. Sensor according to one of the preceding claims, characterized in that the electronic circuit (40) is designed such that a cyclically repeating charging and discharging the electrode (2) takes place with detection of the current (I _C ) during charging and / or discharging. Sensor according to one of the preceding claims, characterized in that the electronic circuit (40) is designed such that the electrode (2) is initially charged by a plurality of voltage pulses of the power supply (24) and then discharged, wherein the evaluation circuit (20, 26, 34) detects the current (I _C ) during discharging and outputs an on and / or off signal depending on the detected current (I _C ). Sensor according to one of the preceding claims, characterized in that the electronic circuit (40) is designed such that the evaluation circuit (20, 26, 34) additionally determines the electrical resistance (R) between the two electrodes (2, 4) and a On and / or off signal depending on the detected current (I _C ) and the resistor (R) outputs. Sensor according to one of the preceding claims, characterized in that the voltage supply has a voltage source (18) with a resistor connected downstream thereof and one of these capacitors connected in parallel. Sensor according to one of the preceding claims, characterized in that the voltage supply comprises a signal generator (24) for generating a charging and / or discharging voltage with a defined signal waveform. Pump for conveying a fluid with an electric drive motor and a control device (12) for switching on and off of the drive motor (M), characterized in that the control device (12) has at least one sensor (14, 16, 36) according to one of the preceding claims for switching on and / or off the pump as a function of a fluid level. Pump according to claim 9, characterized in that one of the electrodes (4) is formed by the housing of the pump and the second electrode (2) is arranged insulated from the housing. Pump according to claim 9 or 10, characterized in that the sensor (14, 16, 50) is arranged to generate a switch-on signal for the drive motor at a predetermined fluid level. Pump according to one of claims 9 to 11, characterized in that a shut-off device is provided for the pump, which comprises at least one detecting means for detecting at least one electrical parameter of the drive motor (M) and is designed such that on the basis of this electrical parameter, a dry run the pump is detectable, and when a dry run detected a shutdown signal for the drive motor (M) is generated. Pump according to one of claims 9 to 12, characterized in that a protective electrode (42) is arranged, which shields the first electrode (2) of the sensor against electrical components in the interior of the pump.

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