EP3652508A1

EP3652508A1 - Capacitive measuring method, and filling level measuring device

Info

Publication number: EP3652508A1
Application number: EP18732758.0A
Authority: EP
Inventors: Anna Klara SCHNEIDER; Raphael KUHNEN; Armin Wernet
Original assignee: Endress and Hauser SE and Co KG
Current assignee: Endress and Hauser SE and Co KG
Priority date: 2017-07-11
Filing date: 2018-06-19
Publication date: 2020-05-20
Also published as: CN110869720A; WO2019011595A1; US20200141789A1; DE102017115516A1

Abstract

The present invention relates to a method for capacitively determining and/or monitoring at least one process variable of a medium (4) and to a corresponding apparatus. According to the invention, at least the following method steps are carried out: - applying at least a first electrical excitation signal (A₁) at at least a first predefinable frequency (f₁) to a probe electrode (5), - receiving a first electrical reception signal (E₁) from the probe electrode (5), - determining a measurement capacitance (C_mess) of the probe electrode (5) or the measurement capacitance (C_mess) and a medium/batch resistance (R_M,A) of the probe electrode (5) at least on the basis of the first reception signal (E₁), and - determining the at least one process variable on the basis of the value for the measurement capacitance (C_mess).

Description

CAPACITIVE MEASURING METHOD AND LEVEL MEASURING DEVICE

The present invention relates to a device for the capacitive determination and / or monitoring of at least one process variable of a medium in a container. The process variable is, for example, a fill level of the medium in the container, the electrical conductivity of the medium or else the permittivity of the medium. In the case of a level measurement can be both a continuous

Level determination as well as the detection of a predeterminable level. Field devices based on the capacitive measuring principle are known per se from the prior art and are manufactured by the Applicant in many different embodiments and sold for example under the names Liquicap, Solicap or Liquipoint.

Capacitive level gauges generally have a substantially cylindrical sensor unit with at least one sensor electrode, which is at least partially insertable into a container. On the one hand, in particular for continuous level measurement, vertically extending into the container rod-shaped sensor units are widely used. However, in order to detect a limit level, sensor units which can be introduced into the side wall of a respective container have become known.

During the measurement operation, the sensor unit is supplied with a start signal, usually in the form of an alternating current signal. The fill level can then be determined from the response signal received by the sensor unit. This is dependent on the capacitance of the capacitor formed by the sensor electrode and the wall of the container, or by the sensor electrode and a second electrode. Depending on the conductivity of the medium, either the medium itself or an insulation of the sensor electrode forms the dielectric of this capacitor.

To evaluate the response signal received by the sensor unit with respect to the level, either the so-called apparent current measurement or the admittance measurement can be carried out. With an apparent current measurement the amount of the by the

Sensor unit flowing apparent current measured. However, since the apparent current in itself has an active and a reactive component, in the case of an admittance measurement in addition to the

Apparent current of the phase angle between the apparent current and the voltage applied to the sensor unit voltage measured. The additional determination of the phase angle also makes it possible to make statements about a possible formation of deposits, as has become known, for example, from DE102004008125A1.

To select the frequency of the excitation signal, various factors must be considered. Firstly, the frequency of the applied AC voltage is due to Resonance effects to choose the lower the longer the sensor unit is designed. On the other hand, however, for all sensor units, the influence of deposit formation, in particular the approach of a conductive medium, decreases in principle with increasing frequency. In addition, there are influences of the electrical conductivity of the respective medium.

On the one hand, capacitive field devices which are suitable for operation at one or a few selected constant frequencies are known from the prior art. The frequencies are chosen so that the respective frequency represents the best possible compromise with respect to the above mentioned opposing tendencies. Furthermore, it is from the

DE10201 1003158A1 become known to act on the sensor unit with a pickup signal of variable frequency in the form of a frequency sweep and from the to the

Responding to different frequencies response signals for each application (medium, design of the sensor unit, etc.) most suitable frequency to select.

A well-known in the prior art problem in the context of capacitive field devices is the formation of approach in the field of sensor unit, which can significantly falsify the respective measurement results. On the one hand, the highest possible frequency for the excitation signal can be selected to avoid an approach since fundamentally the falsifying influence of an approach decreases with increasing frequency of the excitation signal. However, designing an electronics of a corresponding high frequency field device appropriately involves, on the one hand, an increased degree of complexity. In addition, the additional cost factor for the components required in each case is not negligible. An alternative to avoid formation of deposits on the sensor electrode is the use of an additional electrode, in particular a so-called guard electrode, as described for example in DE3212434C2. The guard electrode is arranged coaxially around the respective sensor electrode and electrically separated from it by an insulation. It is also at the same potential as the sensor electrode. The gain on

Measuring accuracy by an additional guard electrode, however, depends on the one hand on the thickness of a shoulder layer, as well as on the conductivity of the approach. Particularly in the case of conductive approaches, resistive components of the approach dominate for lower frequencies of the starting signal the high-impedance measuring impedance determined on the basis of the received signal, by means of which the respective process variable is usually determined. In addition, the effect of the guard electrode is limited by the comparatively high impedance of an insulation capacity of the respective measuring probe. It can therefore be achieved by the guard electrode in principle no constant measurement accuracy regardless of the particular medium and its tendency to form approach, if you want to forego high frequencies for the excitation signal. Based on the prior art, the present invention is therefore based on the object to be able to make a capacitive determination of a process variable as independent as possible from the respective medium with high accuracy. This object is achieved by the method according to claim 1, as well as by the device according to claim 13.

With regard to the method, the object on which the invention is based is achieved by a method for the capacitive determination and / or monitoring of at least one process variable of a medium, comprising the following method steps:

Applying a probe electrode at least with a first electrical

Start signal with at least a first predetermined frequency,

Receiving a first received electrical signal from the probe electrode

Determining a measuring capacity of the probe electrode or the measuring capacity and a media / Ansatz resistance of the probe electrode, at least based on the first

Received signal, and

Determining the at least one process variable based on the value for the measuring capacity.

The probe electrode of a capacitive level gauge according to the invention is described by the measuring capacity and the media / Ansatzwiderstand. In a usual

Apparent current measurement or admittance measurement, the respective process variable based on the received signal, which has the form of an alternating current determined. In contrast, the respective process variable is determined according to the present invention on the basis of the measuring capacity. The influence of the approach present in the area of the probe electrode on the measuring capacity is advantageously negligible, so that a determination of the respective process variable based on the measuring capacity has a significantly lower sensitivity with regard to the presence of the batch. Thus, influences can be eliminated or minimized by the presence of an approach. Due to the significantly reduced sensitivity of the respective measuring device compared to the formation of a batch leads to a significantly improved accuracy, regardless of the medium

The method according to the invention can be applied to all types of measuring probes which are suitable for the capacitive measuring method. The probe can have both a single probe electrode, wherein a wall of the container has a second

Represents electrode, or via at least two electrodes. In the latter case, one of the further electrodes may, for example, be a guard electrode.

The measuring capacity reflects the capacitance between the probe electrode and another electrode or the wall of the container. This measuring capacity is thus in principle the size dependent on the respective process variable. The media / neck resistance includes again ohmic contributions of the medium and possibly contributions of an approach, if available. In the event that the probe electrode is not covered with medium, the probe electrode is either surrounded by air if there is no attachment. Otherwise, the surrounds

Probe Electrode A media layer formed into the neck followed by air and media / neck resistance is composed of these two components. In contrast, in the case where the probe electrode is essentially completely covered by the respective medium, a contribution through the attachment usually does not matter since the probe is already covered with the medium. The influence of the approach present in the area of the probe electrode on the measuring capacity is advantageously negligible, so that a determination of the respective process variable based on the measuring capacity has a significantly lower sensitivity with regard to the formation of the batch. This leads to a significantly improved

Measurement accuracy independent of the respective medium.

An embodiment of the method includes that the measurement capacitance and / or the attachment / media resistance is determined by means of an equivalent circuit of the probe electrode comprising at least one parallel connection of the measurement capacitance and the media / attack resistance. By way of example, determination equations for the measuring capacity and / or the batch media resistance can then be determined on the basis of the equivalent circuit diagram. Preferably, a determination equation for determining the measurement capacity does not depend on the batch / media resistance and vice versa.

An alternative embodiment of the method includes that the measuring capacitance and / or the approach / media resistance is determined by means of an equivalent circuit of the probe electrode comprising a series connection of an insulation capacitance and the parallel connection of the measuring capacitance and the media / neck resistance. The consideration of a

Insulation capacity of the probe electrode leads to a further improvement of

Measurement accuracy. The insulation capacity can be assumed to be known for calculating the measurement capacity and / or the approach / media resistance. For example, this can be determined once during the manufacture of the sensor or at its delivery, and stored in a memory. The memory can be assigned to the measuring device, in particular an electronic unit of the measuring device, or also to an external unit.

A particularly preferred embodiment of the method according to the invention includes that the probe electrode with at least the first excitation signal and with a second

Pickup signal is applied to a second predetermined frequency, wherein the first received signal and a second received signal are received and wherein the

Measuring capacity and / or the media / approach resistance based on the first and second received signal is / are determined. It is advantageous if at least one amplitude and / or one phase of at least the first received signal is / are determined, and wherein the measuring capacity and / or the media / batch resistance is / are determined on the basis of the first and second received signals. For example, in the case of a single first excitation signal, the measurement capacity and / or the media / batch resistance can be determined on the basis of the amplitude and phase of the first received signal. The same applies to a second start signal with a second frequency and the corresponding second receive signal. Alternatively, for example, the amplitudes or phases of at least the first and second received signal can be used.

In one embodiment of the method, the at least one process variable is a fill level of the medium in a container. It can also be a predefinable fill level, ie a limit level. Alternatively, the process variable may also be the electrical conductivity of the medium or the permittivity of the medium.

In a further refinement, a conductivity of the medium and / or a permittivity of the medium is / are determined on the basis of the media / neck resistance. On the basis of the permittivity, in turn, a dielectric constant of the

Medium can be specified. From the conductivity and / or the permittivity or

Dielectric constants of the medium can extract additional information, for example about the process, the nature and strength of an approach and many more. It is advantageous that a determination of the conductivity of the medium without an electrically conductive connection to the medium is possible by means of the method according to the invention.

A preferred embodiment includes that based on the measurement capacity, the media / approach resistance and / or at least one of at least the measurement capacity and / or the media / approach resistance derived size on the presence of approach in at least a portion of the probe electrode is closed. It is therefore not only possible to state with the method according to the invention that the approach is present, but also, if necessary, what kind of approach is or which medium forms the approach, or how much approach has formed.

A further preferred embodiment includes monitoring compliance with a recipe of a process taking place in the container on the basis of the measuring capacity, the media / batch resistance and / or at least one variable derived from at least the measuring capacity and / or the media / batch resistance becomes.

Yet another preferred embodiment includes that based on the measurement capacity, the media / approach resistance and / or at least one of at least the measurement capacity and / or the median batch resistance derived quantity, a mixing of at least a first and a second medium in the container is monitored.

Yet another preferred embodiment includes that a cleaning process in the container is monitored on the basis of the measuring capacity, the media / batch resistance and / or a variable derived from at least the measuring capacity and / or the media / batch resistance.

In addition to the determination and / or monitoring of the respective process variable, a process monitoring of a process taking place in the respective container can thus additionally be undertaken.

In one embodiment of the method, a degree of coverage of the probe electrode is determined. The degree of coverage is defined as the ratio of a current that can be tapped off from the probe electrode and a current that can be tapped off at a guard electrode of the respective measuring device.

The object underlying the invention is also achieved by a device for capacitive determination and / or monitoring at least one process variable of a medium in a container comprising

a sensor unit with at least one probe electrode, and

an electronic unit, which electronic unit is designed to carry out at least one method according to at least one of the preceding claims. In one embodiment of the device, the sensor unit comprises at least two electrodes. For example, it may be a device with two probe electrodes, or with a probe electrode and a ground electrode.

A further embodiment includes that one of the electrodes is a

Guard electrode acts.

It should be noted that the embodiments described in connection with the method according to the invention mutatis mutandis on the inventive

Apply device and vice versa.

The invention will now be described in more detail with reference to the following Figures 1 to 3. It shows:

1 is a schematic representation of a capacitive level measuring device according to the prior art, 2 is an exemplary electrical equivalent circuit diagram for describing the probe electrode on the basis of the measurement capacity and based on the media / approach resistance,

3 shows two diagrams for illustrating the influence of an approach on (a) the measuring capacity and (b) the amplitude of the received signal, in each case as a function of the conductivity of the medium,

4 shows two diagrams for illustrating the dependence of the measurement capacity and the attachment / media resistance on a projection in the region of the probe electrode, FIG. 5 shows two diagrams for illustrating the dependence of the measurement capacity and the attachment / media resistance on a process taking place in the container, and

6 shows a diagram of the dielectric constants and the electrical conductivities of various common media.

In Fig. 1 is a schematic drawing of a typical based on the capacitive measuring principle field device 1 according to the prior art. The example shows a

Sensor unit 2 with two cylindrically shaped electrodes 5, 6, which projects from the starting via a process connection 3a from the top in a partially filled with medium 4 container 3. It goes without saying, however, that numerous

Embodiments for a capacitive measuring device with a different number of electrodes are known, all of which fall under the present invention. Besides such

Measuring devices, in which the sensor unit 2, as shown in Fig. 1, protrude from above into the container, the present invention is also on front flush sensor units, which substantially complete the Bewandung the container 3 or such sensor units 3, which via a side wall of the container 3 are introduced into this applicable.

The sensor unit 2 itself is composed in the present example of a probe electrode 5 and a sensor electrode 5 coaxially surrounding and insulated from this guard electrode 6 together. Both electrodes 5, 6 are electrically connected to an electronic unit 7, which is responsible for signal acquisition, evaluation and / or supply. In particular, the electronic unit 7 determines and / or monitors the level of the medium 4 in the container 3 on the basis of the response signal received by the sensor unit 2. An additional guard electrode 6 is by no means necessary for the purposes of the present invention.

To determine the respective process variable, at least the probe electrode 5 is acted upon by a start signal A and the process variable is determined on the basis of the receive signal E received by the probe electrode 5, which is usually in the form of a

AC has. The guard electrode 6 is preferably, as described for example in DE 32 12 434 C2, operated at the same potential as the sensor electrode 5. It is thus the case that, irrespective of the use of a guard electrode 6, different components contribute to the received signal E and not only the component of the capacitor formed by the probe electrode 5 and a wall of the container 3 or a second electrode, which among other things depends on the level of the medium 4 in FIG Container 3 depends. Rather, ohmic resistance and numerous other influences also play a role. For example, an approach that forms at least in the region of the probe electrode 5 also contributes to the received signal E, which can lead to a reduction of the measuring accuracy. In the worst case, for example, a level of the medium 4 in the container 3 can no longer be reliably determined and / or monitored.

According to the invention, therefore, not the received signal E, itself, but the measuring capacitance Cmess of the at least one probe electrode 5 is evaluated. In an electric

Substitute circuit diagram, the probe electrode 5, for example, by a series circuit of an insulation capacitance Ci _SO and a parallel circuit of the measuring capacitance Cmess and the

Media / batch resistances RM, A are shown as shown in FIG. It should be noted that the equivalent circuit shown is merely a possible example. Many other possibilities are conceivable and also fall under the present invention. For example, in another embodiment, the isolation capacity Ci _{S0 can} also be dispensed with.

Many different possibilities are conceivable for determining the measuring capacitance Cmess and / or the media / contact resistance RM, A, all of which are within the scope of the present invention. If the sensor unit 3 is acted upon by a first frequency fi with a single first excitation signal Ai, and if a first receive signal Ei is accordingly received, it is advisable, for example, to set the measurement capacitance Cmess and / or the media

/ Insertion resistance RM, A based on an amplitude a and / or a phase Φ of the first

Receiving signal egg to determine. Alternatively, it is also possible for the measuring probe 3 to be acted upon by at least a first Ai and a second starting signal A ₂ having at least a first fi and a second frequency f ₂ . In this case, the measuring capacitance Cmess and / or the media / contact resistance RM, A can be determined on the basis of the at least first Ei and second received signal E ₂ , for example based on the first a and second

Amplitude a ₂ .

The measuring capacitance Cmess is a measure of the capacitance between the probe electrode 5 and a further electrode or the wall of the container 3 again and, consequently, a measure of the respective process variable. Ohmic influences of the medium 4 or a possibly existing approach layer in the area of the probe electrode 5, on the other hand, are taken into account by the media / neck resistance RM, A. In the case that the probe electrode 5 is not covered with medium 4, the probe electrode is either surrounded by air when no projection is present. Otherwise, the probe electrode 5 surrounds a residue formed of media Boundary layer followed by air and media / neck resistance RM, A is composed of these two components. In contrast, in the case where the probe electrode 5 is substantially completely covered by the respective medium, a contribution through the attachment usually does not matter since the probe electrode 5 is already covered with the medium 4. The influence of the approach present in the region of the probe electrode 5 on the measuring capacitance Cmess is advantageously negligible, so that a determination of the respective process variable can be made on the basis of the

Measuring capacity Cmess has a significantly lower sensitivity to the presence of approach. This leads to a significantly improved measurement accuracy regardless of the respective medium. 4

These relationships are illustrated with reference to FIG. 3. Here, Fig. 3a refers to the measured capacitance CMESS and Fig. 3b to the reception signal E. are the measured capacitance Cmess.o or the reception signal E ₀ for an empty container 4 in the case that no recognition is available, the measured capacitance C _m ess, o, A or the received signal E ₀ , A for an empty container 3 in the event that the probe electrode 5 is covered by an approximately 1 mm thick approach layer, and the measuring capacitance Cmess.i or the received signal Ei for a complete with Medium 4 filled container 3 each as a function of the conductivity σ of the medium 4. On the y-axis is in each case the ratio of the proportion of the contribution by approach C _m ess, o, A and E ₀ , A ZUITI total signal Cmess, 1 or Egg applied in percent. In the case that the measuring capacity Cmess is evaluated, the proportion of a 1 mm thick layer of coating for a typical

Conductivity range σ of common media 4 less than 25%. In the case of an evaluation of the received signal E with respect to the respective process variable, the contribution increases through the

Starting layer continuously with the conductivity σ on. With a conductivity of σ = 800μ8 / ιτι can no longer be distinguished between a fully covered probe electrode 5 and covered with a 1 mm thick layer approach probe electrode 5.

As can easily be seen, the influence of a projection in the area of the probe electrode 5 on the respective process variable can be evaluated by an evaluation of the measuring capacitance Cmess instead of the

Receiving signal significantly reduced and possibly almost completely eliminated.

4, the measuring capacitance Cmess and the media / batch resistance RM, A are each shown as a function of time in arbitrary units in the event that a projection forms in the region of the probe electrode 5 with increasing time. The measuring capacitance Cmess shown in FIG. 4a remains essentially constant, regardless of the presence of a projection. This again illustrates the increased measurement accuracy, which can be achieved by evaluating the measurement capacity C measurement. The media attachment resistance RM, A is significantly influenced by the formation of a shoulder layer and decreases with increasing approach. By evaluating the measuring capacitance Cmess and / or the media / contact resistance RM, A, additional statements can therefore be made about the existence of an approach. Alternatively, it is equally possible to use one of the measuring capacitance Cmess and / or the Medier approach resistances RM, A dependent size, for example, a ratio of the measuring capacitance C measurement and the media / approach resistance RM, A evaluate.

On the basis of an evaluation of the measuring capacitance Cmess and / or the media / approach resistor RM, A statements can be made about the respective medium in the container 3 4. It can therefore be made in principle a monitoring of a running in the container 3 process. Analogous considerations apply in the event that a cleaning process of the container 3 is to be monitored. This is illustrated on the basis of the measuring capacitance Cmess shown in FIG. 5 and the media / batch resistance RM, A as a function of time in arbitrary units in the event that the medium 4 in the container 3 changes at the time t3 , Both the measuring capacitance Cmess shown in FIG. 5a and the media contact resistance RM, A shown in FIG. 5b show a clear dependence on the medium 4 respectively located in the container 3. By an evaluation of the measuring capacitance Cmess and / or the media / Approach resistance RM, A so additional statements can be generated the respective process. Alternatively, as in the case of FIG. 4, it is equally possible to use a variable dependent on the measuring capacitance Cmess and / or the media / batch resistance RM, A, for example a ratio of the measuring capacitance Cmess and the media / batch resistor RM "Evaluate.

Finally, in FIG. 6, the conductivities σ and the dielectric constants ε _{Γ are} for

various common media 4 shown. With the aid of an evaluation of the measuring capacity Cmess and the media / batch resistance RM, A, a statement about the medium 4 located in the container can be made in a further embodiment of the present invention. For example, to determine the dielectric constant ε _{Γ of} a medium 4, in the first case that the container 3 is empty, the measuring capacity Cmess can be determined. For an empty container 3, it holds that ε _Γ «1. If, in addition, the measuring capacity Cmess is additionally determined in the case of a container 3 completely filled with the medium 4, it is possible to deduce the dielectric constant Sr of the medium 3. In an analogous manner, the conductivity σ of a medium 3 can also be determined.

reference numeral

1 capacitive level gauge

2 sensor unit

3 containers

3a process connection of the container

4 medium

5 sensor electrode

6 guard electrode

7 electronics unit,

8 housing of the field device measuring measuring capacity

RM, A media / neck resistance

Ciso Insulation capacity of the probe electrode σ Conductivity of the medium

ε _Γ Dielectric constant of the medium

A start signal

E received signal

a amplitude

Phase

Claims

claims

1 . Method for the capacitive determination and / or monitoring of at least one process variable of a medium (4),

comprising the following method steps:

Applying a probe electrode (5) to at least a first electrical stimulation signal (Ai) having at least a first predeterminable frequency (fi),

Receiving a first electrical received signal (Ei) from the probe electrode (5), determining a measuring capacity (Cmess) of the probe electrode (5) or the measuring capacitance (Cmess) and a media / batch resistance (RM, A) of the probe electrode (5) at least based on the first received signal (Ei), and

Determining the at least one process variable based on the value for the measuring capacity

(CMESS).

2. The method according to claim 1,

wherein the measurement capacitance (C measurement) and / or the attachment / media resistance (RM, A) are determined by means of an equivalent circuit of the probe electrode (5) comprising at least one

Parallel connection of the measuring capacity (Cmess) and the media / batch resistance (RM, A) is determined.

3. The method according to claim 1,

wherein the measurement capacitance (Cmess) and / or the attachment / media resistance (RM, A) are determined by means of an equivalent circuit of the probe electrode (5) comprising a series connection of an insulation capacitance (Ci _S0 ) and the parallel connection of the measurement capacitance (Cmess) and the medium / Neck resistance (RM, A) is determined.

4. The method according to at least one of the preceding claims,

wherein the probe electrode (5) is acted on by at least the first excitation signal (Ai) and by a second excitation signal (A ₂ ) having a second presettable frequency (f ₂ ), wherein the first received signal (Ei) and a second received signal (E ₂ ) and wherein the measuring capacity (Cmess) and / or the media / charge resistance (RM, A) is / are determined on the basis of the first (Ei) and second received signal (E ₂ ). 5. The method according to at least one of claims 1 -4,

wherein at least one amplitude (a) and / or a phase (Φ) of at least the first received signal (Ei) is / are determined, and wherein the measuring capacitance (Cmess) and / or the media / batch resistance (RM, A) the amplitude (a) and / or phase (Φ) is / are determined. 6. The method according to at least one of the preceding claims,

wherein the at least one process variable is a fill level of the medium

(4) in a container (3). 7. The method according to at least one of the preceding claims,

wherein based on the media / approach resistance (RM, A) a conductivity (σ) of the medium (4) and / or on the basis of the measurement capacity (Cmess) a permittivity (ε _Γ ) of the medium (4) is determined / are. 8. The method according to at least one of the preceding claims,

wherein based on the measured capacitance (Cmess), the media / approach resistance (RM, A) and / or at least one of at least the measurement capacity (Cmess), and / or the media / Ansatz resistance (RM, A) derived size is closed on the presence of approach in at least a portion of the probe electrode (5).

9. The method according to at least one of the preceding claims,

wherein based on the measured capacitance (Cmess), the media / approach resistance (RM, A) and / or at least one of at least the measuring capacitance (Cmess) and / or the media / Ansatz resistance (RM, A) derived size the Compliance with a recipe of a running in the container (3) process is monitored.

10. The method according to at least one of the preceding claims,

wherein on the basis of the measuring capacity (Cmess), the media / batch resistance (RM, A) and / or at least one of at least the measuring capacitance (Cmess) and / or the

Media / batch resistance (RM, A) derived size, a mixing of at least a first and a second medium (4) in the container (3) is monitored.

1 1. Method according to at least one of the preceding claims,

wherein based on the measurement capacity (Cmess), the media / approach resistance (RM, A) and / or a size derived from at least the measurement capacitance (Cmess) and / or the media / neck resistance (RM, A), a cleaning process in the container (3) is monitored.

12. The method according to at least one of the preceding claims,

wherein a degree of coverage of the probe electrode (5) is determined.

13. Device for capacitive determination and / or monitoring at least one process variable of a medium (4) in a container (3) comprising

- A sensor unit (3) with at least one probe electrode (5), and - An electronic unit (7), which electronic unit (7) is adapted to perform at least one method according to at least one of the preceding claims.

14. Device according to claim 13,

wherein the sensor unit (3) comprises at least two electrodes.

15. Device according to claim 14,

wherein one of the electrodes is a guard electrode (6).