WO2020239329A1

WO2020239329A1 - Vibronic multisensor

Info

Publication number: WO2020239329A1
Application number: PCT/EP2020/061181
Authority: WO
Inventors: Tobias Brengartner; Sergey Lopatin; Dieter Waldhauser; Thomas Alber; Philipp Leufke; Markus Kilian; Rebecca Page; Alfred Rieder
Original assignee: Endress+Hauser SE+Co. KG
Priority date: 2019-05-27
Filing date: 2020-04-22
Publication date: 2020-12-03
Also published as: DE102019114174A1

Abstract

The invention relates to a method, in particular a computer-implemented method, for detecting gas bubbles in a liquid medium (M) using a sensor (1) with a mechanically vibratable unit (4), having the following steps: - exciting the vibratable unit (4) using a first excitation signal (I1) in order to generate mechanical vibrations of the vibratable unit (4) according to a first specifiable vibration mode (S1) of the vibratable unit (4) and receiving the mechanical vibrations from the mechanically vibratable unit (4) in the form of a first received signal (X1); - exciting the vibratable unit (4) using a second excitation signal (I2) in order to generate mechanical vibrations of the vibratable unit (4) according to a second specifiable vibration mode (S2) of the vibratable unit (4) and receiving the mechanical vibrations from the mechanically vibratable unit (4) in the form of a second received signal (X2); - determining a first and the second characteristic variable (f, A) of the first (X1) and second received signal (X2); - determining the ratio (V) of the first characteristic variable (f₁, A₁) to the second characteristic variable (f₂, A₂); and - generating a conclusion regarding the presence of gas bubbles using the ratio (V) of the first characteristic variable (f₁, A₁) to the second characteristic variable (f₁, A₁).

Description

Vibronic multi-sensor

The invention relates to a, in particular computer-implemented, method for the detection of gas bubbles in a liquid medium with a sensor with a mechanically oscillatable unit. The invention also relates to a computer program and a computer program product. The sensor is a device for determining and / or monitoring at least one process variable of the medium, in particular a vibronic sensor or a field device operating according to the Coriolis measuring principle. The medium is located in a container, for example in a container or in a pipeline. The process variable in turn is, for example, a fill level, in particular a specifiable fill level, a flow rate, the density or the viscosity of the medium.

Vibronic sensors are widely used in process and / or

Automation technology. In the case of fill level measuring devices, they have at least one mechanically oscillatable unit, such as an oscillating fork, a single rod or a membrane. During operation, this is excited to mechanical vibrations by means of a drive / receiver unit, often in the form of an electromechanical converter unit, which in turn can be, for example, a piezoelectric drive or an electromagnetic drive.

Corresponding field devices are produced by the applicant in great variety and

for example sold under the name LIQUIPHANT or SOLIPHANT. The

The underlying measurement principles are known in principle from a large number of publications. The drive / receiver unit stimulates the mechanically oscillatable unit to produce mechanical oscillations by means of an electrical excitation signal. Conversely, the drive / receiver unit can receive the mechanical vibrations of the mechanically vibratable unit and convert them into an electrical received signal. The drive / receiver unit is accordingly either a separate drive unit and a separate receiver unit, or a combined drive / receiver unit.

In many cases, the drive / receiver unit is part of a feedback electrical oscillating circuit, by means of which the mechanically oscillatable unit is excited into mechanical oscillations. For example, for a resonant oscillation, the resonant circuit condition, according to which the gain factor is> 1 and all phases occurring in the resonant circuit result in a multiple of 360 °, must be met. A certain phase shift between the excitation signal and the received signal must be guaranteed to excite and fulfill the resonant circuit condition. Therefore, a specifiable value for the phase shift, that is to say a setpoint value for the phase shift between the excitation signal and the received signal, is often set. For this purpose, the most varied from the prior art Solutions, both analog and digital methods, have become known, for example in the documents DE102006034105A1, DE102007013557A1, DE102005015547A1,

DE102009026685A1, DE102009028022A1, DE102010030982A1 or DE00102010030982A1.

Both the excitation signal and the received signal are characterized by their frequency w, amplitude A and / or phase F. Accordingly, changes in these variables are usually used to determine the respective process variable. The process variable can be, for example, a fill level, a predetermined fill level, or the density or viscosity of the medium, as well as the flow rate. With a vibronic

Level switch for liquids, for example, a distinction is made as to whether the oscillatable unit is covered by the liquid or whether it oscillates freely. These two states, the free state and the covered state, are for example different on the basis of this

Resonance frequencies, that is, based on a frequency shift, are differentiated.

The density and / or viscosity, in turn, can only be determined with such a measuring device if the oscillatable unit is covered by the medium. In connection with the determination of the density and / or viscosity, different possibilities have also become known from the prior art, such as those described in documents DE10050299A1,

DE10200704381 1 A1, DE10057974A1, DE 102006033819A1, DE102015102834A1 or

DE1020161 12743A1 disclosed.

In the case of measuring devices working according to the Coriolis measuring principle, which for example

To determine and / or monitor the mass flow rate or the density, a measuring tube which is held in a housing module and which is able to oscillate and which communicates with a pipeline is used as an oscillatable unit, which is at least temporarily closed

Vibrations around a static rest position, in particular bending vibrations, is excited. The underlying measuring principles are also known from a large number of publications and, for example, in US-A 47 93 191, US-A 48 23 614, * US-A 48 31 885, US-A 56 02 345, US- A 2007/0151368, US-A 2010/0050783, WO-A 96/08697, WO-A

2009/120222 or WO-A 2009/120223 described in detail and in detail.

This is a problem with sensors based on mechanical vibrations

Presence of gas bubbles in different media. Gas bubbles have a major influence on the viscoelastic properties of liquids. Accordingly, it can lead to an unwanted, unrelated to the process variable being considered, Changes in the oscillation frequency of the oscillatable unit and, as a result, falsified measured values for the respective process variable.

A wide variety of causes can be responsible for the formation of gas bubbles in liquid media, such as stirring or pumping in the process, outgassing of dissolved air after a pressure drop in the medium, or a change in the medium temperature. Gas bubbles are particularly common in fresh water or aqueous solutions. Gas bubbles that are distributed in the medium and that are also deposited on a surface of the respective sensor unit of the sensor which comprises the oscillatable unit play a role. .

From DE102015122661 A1, for example, a method for determining a physical parameter of a gas-laden liquid for a Coriolis flowmeter is known. The oscillatable unit is excited to mechanical oscillations in two different oscillation modes, which depend to varying degrees on gas bubbles present within the medium. From the ratio for the density and / or the

Mass flow calculated values in the two vibration modes, the influence of the gas bubbles on the measurement can be determined and corrected.

The present invention is based on the object of enabling reliable measuring operation of a sensor with an oscillatable unit in a simple manner in the event of gas bubble formation.

This object is achieved by the method according to claim 1, by the computer program according to claim 15 and the computer program product according to claim 16.

With regard to the method, the object is achieved by a method, in particular

Computer-implemented method for the detection of gas bubbles in a liquid medium with a sensor with a mechanically oscillatable unit, comprising the following

Process steps:

- Exciting the oscillatable unit with a first excitation signal for generating mechanical vibrations of the oscillatable unit according to a first predeterminable oscillation mode of the oscillatable unit, and receiving the mechanical vibrations from the mechanically oscillatable unit in the form of a first received signal,

- Exciting the oscillatable unit with a second excitation signal for generating mechanical vibrations of the oscillatable unit according to a second predeterminable oscillation mode of the oscillatable unit, and receiving the mechanical vibrations from the mechanically vibratable unit in the form of a second received signal,

- determining a first and second characteristic variable of the first and second received signal,

- determining a ratio of the first and second characteristic variables, and

- Generating a statement about the presence of gas bubbles based on the

Ratio of the first and second characteristic quantity.

The oscillatable unit is part of a sensor unit of the sensor for determining and / or monitoring at least one process variable of a medium. In the case of a vibronic sensor, the oscillatable unit is, for example, a single rod or an oscillating fork. In the case of a flow meter, on the other hand, the oscillatable unit is given by a measuring tube. The first and second excitation signals are each an electrical signal with a predeterminable first or second frequency, in particular a sinusoidal or a square-wave signal. Preferably it is mechanical

oscillatable unit excited at least temporarily to resonance oscillations. The mechanical vibrations are influenced by the medium surrounding the vibratable unit, so that conclusions can be drawn about various properties or process variables of the medium on the basis of a received signal representing the vibrations.

However, the first and second received signals are also influenced by the presence of gas bubbles. However, since the influence of the gas bubbles on the first and second received signal is different, the evaluation of the relationship between the two

characteristic quantities of the two received signals in the two different

A statement about the presence of gas bubbles can be made in accordance with vibration modes. The method is advantageously very easy to implement. No complex structural measures or additional sensor units are necessary. Rather, all that is required is the excitation of the oscillatable unit with two different excitation signals in order to generate two different oscillation modes. The two oscillation modes can be excited simultaneously, that is to say superimposed on one another, or alternately, in particular sequentially.

In one embodiment of the invention, the first oscillation mode is a fundamental oscillation mode of the oscillatable unit. In this regard, it is advantageous if the mass distribution, rigidity and / or geometry of the oscillatable unit is / are chosen such that the fundamental oscillation mode is at a frequency f 1 <1.5 kHz. A further particularly preferred embodiment includes that the second oscillation mode is selected in such a way that the oscillations of the oscillatable unit are influenced by the formation of gas bubbles in the area of the mechanically oscillatable unit. The higher oscillation mode is therefore deliberately selected with a view to detecting the gas bubbles. The respective process variable is then preferably determined and / or monitored on the basis of a different oscillation mode.

With regard to the second oscillation mode, it is advantageous if the higher oscillation mode is selected such that the frequency of the second oscillation mode lies in a frequency range in which a natural frequency of the gas bubbles lies. The natural frequency or the

The resonance frequency of gas bubbles depends, among other things, on the, in particular

physical and / or chemical properties of the medium and the bubble size.

If the sensor is operated at frequencies which are in the range of the resonance frequency of the gas bubbles occurring, the vibration energy of the sensor is absorbed by the gas bubbles and a resonance vibration of the vibratable unit is very strongly damped or no longer possible.

The second oscillation mode is preferably a first, higher one

Oscillation mode of the oscillatable unit.

Another embodiment of the invention includes that the first and second characteristic variable of the first and second received signal is a frequency, a

Amplitude, or a variable derived from at least the frequency or the amplitude.

A particularly preferred embodiment includes that in the event that the ratio of the first and second characteristic variable is zero or a slope of the ratio of the first and second characteristic variable as a function of time exceeds a predeterminable limit value, the presence of gas bubbles is concluded. In the case of the

In the presence of gas bubbles, it is no longer possible to excite the vibratable unit in a vibration mode influenced by the gas bubbles, for example the second vibration mode. There can be no frequency or amplitude for the corresponding second

Received signal can be detected more.

Another particularly preferred embodiment includes that in the event that the ratio of the first and second characteristic variable is greater than zero or a slope of the

Ratio of the first and second characteristic variable as a function of time The predeterminable limit value is fallen below, it is concluded that no gas bubbles are present.

Another embodiment of the method according to the invention includes that a process variable of the medium is determined on the basis of the first received signal on the basis of the first received signal. The process variable is preferably a fill level, in particular a predeterminable fill level, a flow rate, the density or the viscosity of the medium. especially the

Determining the density of the medium shows a sensitive dependence on the presence of gas bubbles in the respective liquid.

It is advantageous if the process variable is only determined when there are no gas bubbles.

It is also advantageous if, during a time interval in which gas bubbles are present, a value for the process variable that was last determined before the formation of the gas bubbles is output, and / or a current value in each case during a time interval in which there are no more gas bubbles determined value for the process variable is output.

Another particularly preferred embodiment includes that process monitoring is carried out on the basis of the ratio of the first and second characteristic variables. The presence of gas bubbles can also be desired in certain processes and monitored or verified by means of the method according to the invention.

For example, fermentation or a disinfection process can advantageously be monitored by means of the method according to the invention. The presence of gas bubbles during individual process steps is absolutely necessary for such processes. The method according to the invention can on the one hand provide information about the actual presence of the

Give gas bubbles. In addition, a point in time can also be determined at which there are no more gas bubbles. On the basis of this point in time, further process steps can then, for example, be initiated or process variables of the medium can be determined and / or monitored.

The object on which the invention is based is also achieved by a computer program for the detection of gas bubbles in a liquid medium with computer-readable ones

Program code elements which, when executed on a computer, cause the computer to execute at least one embodiment of the method according to the invention. The object on which the invention is based is also achieved by a

Computer program product with a computer program according to the invention and at least one computer-readable medium on which at least the computer program is stored.

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

Computer program and the computer program product according to the invention can be used and vice versa.

The invention is explained in more detail with reference to the following figures. It shows:

1 shows a schematic sketch of a (a) vibronic sensor and (b) a field device according to the prior art that works according to the Coriolis measuring principle,

2 shows a schematic representation of the influence of gas bubbles on the received signal,

3 shows diagrams of the frequency of the first and second received signal and the ratio of the first and second frequency as a function of time, and

4 shows diagrams of the amplitude of the first and second received signals and of the ratio of the first and second amplitudes as a function of time.

In the figures, the same elements are provided with the same reference numerals.

1a shows a vibronic sensor 1 with a sensor unit 2 with an oscillatable unit 4 in the form of a vibrating fork, which is used in particular to determine and / or monitor a, in particular predeterminable, fill level, the density and / or the viscosity of the medium. The mechanically oscillatable unit 4 is partially immersed in a medium M, which is located in a container 3, and is excited to mechanical oscillations by means of the excitation / receiving unit 5, which in turn can be, for example, a piezoelectric stack or bimorph drive. Other vibronic sensors have, for example

electromagnetic drive / receiving units 5. It is possible to use a single drive / receiving unit 5, which is used to excite the mechanical vibrations and to detect them. However, it is also conceivable to implement a drive unit and a receiving unit. Also shown in Fig. 1 is an electronics unit 6, by means of which the

Signal acquisition, evaluation and / or feeding takes place. In FIG. 1 b, in turn, a Coriolis measuring device 1 according to the prior art is shown, which has two measuring tubes 7a, 7b, a housing module 8 with a carrier 9 and a casing 10, and an inlet-side 11a and outlet-side process connection 11a . Other configurations of generic field devices have different numbers of

Measuring tubes 7. By means of the two process connections 11a, 11b, the field device 1 can be integrated into an existing pipeline, which is not shown here for the sake of simplicity. The carrier 4 is designed in the form of a laterally at least partially open, in particular tubular, support cylinder and is connected to the two measuring tubes 7a, 7b. The measuring tubes 7a, 7b are also surrounded by the casing 10. As a rule, a neck tube, not shown here, is also attached to the carrier 9 for connecting an electronic unit 6, which is used, for example, for signal acquisition, evaluation and supply.

In the area of the inlet-side 1 1 a and outlet-side process connection 1 1 b, an inlet-side and an outlet-side (not visible) distributor piece are integrated, which

Distributor pieces are mechanically connected to the carrier 9 and to the two measuring tubes 7a, 7b, and which distribute the flowing medium M from the pipeline (not visible) to the two measuring tubes 7a, 7b. The two measuring tubes 7a, 7b are further by means of several

Coupling elements (10; only one is marked here) mechanically coupled to one another.

Each of the two measuring tubes 9a, 9b performs mechanical vibrations during operation. Furthermore, at least one electromechanical, especially electrodynamic, exciter arrangement (not visible here) acting on at least one measuring tube 9a, 9b for generating and / or maintaining mechanical vibrations of the measuring tube 1 1a, 1 1 b is shown, as well as at least one for vibrations of the measuring tubes 1 1a, 1 1b reacting vibration sensor arrangement (also not visible) for generating at least one vibration measurement signal representing the vibrations of the measuring tubes.

Gas bubbles in a liquid medium have a great influence on the viscoelastic properties of the liquid. As a result, gas bubbles also have a great influence on that which characterizes the mechanical vibrations of the vibratable unit 4

Received signal X, as illustrated in Fig. 2 for the case of a vibronic sensor. The frequency change Af / fi is shown as a function of the frequency f for oscillations of the oscillatable unit 4, for example in the fundamental oscillation mode. The frequency T in the

The fundamental mode depends on the density p of the liquid M. In the case that it is a Newtonian liquid without gas bubbles, the following applies for the frequency change Af / fi _{, Vac,} for example: Here, fi _{, Vac is} the resonance frequency of the oscillatable unit 4 in the fundamental oscillation mode in a vacuum or in air, and S is the sensitivity of the oscillatable unit 4 that is dependent on the geometry of the oscillatable unit 4.

In the case of FIG. 2, the same oscillatable unit 4 now dips twice into the same medium M, gas bubbles being present in the medium M in a first case (squares) and in the second case no gas bubbles being present (triangles). In the example shown here, the medium M is water. While the frequency change is the same for both cases for frequencies f <f _P , there are significant deviations for frequencies f> f _P. The

The change in frequency for the case of the presence of gas bubbles is significantly greater than the change in frequency for the same medium M without gas bubbles. The frequency f _P describes a critical limit frequency, from which the vibration behavior of the vibratable unit 4 is influenced by the presence of the gas bubbles. If the sensor is operated at frequencies f> f _P , in particular at frequencies which are in the range of the natural frequency of the gas bubbles occurring, the vibration energy of the sensor 1 is absorbed by the gas bubbles and resonance vibration of the vibratable unit 4 is no longer possible.

In the case of media in the form of aqueous solutions, the natural frequency f _G of gas bubbles can be calculated, for example, according to the following equation:

Here a is the radius of the gas bubbles, g is the polytropic coefficient, p _{A is} the process pressure and p is the density of the liquid. For a pressure p _A = 1 bar, the gas bubble frequency is f _G = 6520 / D Hz, where D is the diameter of the gas bubbles in millimeters. If the sensor is operated at frequencies that are in the range of the resonance frequency of the gas bubbles occurring, the vibration energy of the sensor is absorbed by the gas bubbles and a

Resonance oscillation of the oscillatable unit is very weak or no longer possible.

The maximum gas bubble size in water depends on the Archimedean force that drives the bubbles out of the liquid and on the adhesion of the gas bubbles to the surface of the sensor unit 2, in particular the oscillatable unit 4. At a process pressure p _A = 1 bar, gas bubbles have usually a diameter d of 2-3mm before they are detached from the surface of the sensor unit 2. Operation of a sensor is therefore essential for this application 1 possible undisturbed at frequencies f <2kHz. For frequencies f> 2kHz, on the other hand, there is a considerable influence from the gas bubbles.

In the event that the density p of the medium M is to be determined by means of the sensor 1, the presence of gas bubbles is particularly critical. With a variance of the oscillation frequency f due to gas bubbles of 1-2%, there is already a gas bubble-induced measurement error of approx. 10%. However, such an order of magnitude for the measurement error is not acceptable in the area of density determination in process measurement technology.

Similar considerations apply to the case of a Coriolis measuring device 1 with a measuring tube 7 as an oscillatable unit

The method according to the invention now allows the reliable detection of gas bubbles in liquid media. For this purpose, the vibratable unit 4, 7 is excited by means of two different excitation signals 11 and I2 in two different vibration modes S1 and S2. A characteristic variable is then determined for each of the two received signals X1 and X2 and a statement is made about the presence of gas bubbles on the basis of the ratio V of the characteristic variables.

Two possible exemplary configurations for the method according to the invention are illustrated in FIGS. 3 and 4.

In the case of FIG. 3, the characteristic size of the received signals X1 and X2 is given by the frequency f1 and f2, respectively. The first oscillation mode S1 is therefore the fundamental oscillation mode with the frequency f1 and the second oscillation mode S2 is the first higher oscillation mode with the frequency f2.

For the example shown, the oscillatable unit 4, 7 is designed such that the

Fundamental oscillation mode is at a natural frequency fi, _V ac <1.5 kHz and the first is higher

Oscillation mode at a natural frequency f _{2, vac} ~ 9fi _{, vac} . In this way, gas bubbles with diameters d> 0.5mm can be reliably detected.

3a shows the first frequency f of the first received signal X1 as a function of time t, where fi, vac = 1000 Hz, and where the medium M is fresh fresh water. In the medium M, air bubbles are deposited on the oscillatable unit 4, 7 with increasing time.

When the air bubbles reach a certain size, they are removed from the surface of the

vibratable unit 4.7 replaced. This leads to a gas bubble-induced Reduction of the first frequency fi from 752Hz to 744Hz. After the gas bubbles have emerged from the medium M (approx. 14h), the first frequency fi rises again to the original value of 752 Hz, corresponding to the state immersed in the medium M, which it reaches after a period of At = 14h.

The course of the second frequency f2 of the second received signal X2 is also shown as a function of time t in FIG. 3b. The gas bubbles temporarily lead to a break in or to a strong damping of the vibrations in the second vibration mode. However, the mere presence of the second oscillation mode as a criterion for the presence of gas bubbles is not reliable. For the period At = 4h-14h, the oscillatable unit 4, 7 can indeed be excited in the second oscillation mode S2; however, there is one more at the same time

A reduction in the first frequency fi in the first oscillation mode S1 compared to the case without gas bubbles can be ascertained, so that it is not yet possible to reliably determine the respective process variable on the basis of the first oscillation mode S1. Rather, a reliable determination of a process variable is only possible after a period of time At = 14h.

A more reliable statement about the presence of gas bubbles is possible through

Consideration of the ratio V of the two frequencies fi and f ₂ , as shown in FIG. 3c. in the

Period At = 4h-14h, the ratio V of the first fi and the second frequency f _{2 is} initially zero and then has a considerable gradient. Only after a period of At = 14h does the gradient of the ratio V flatten and converge to a constant value V> 0.

The same considerations can be applied to the case that instead of the frequency f as

characteristic quantity the amplitude A used is transmitted. This case is shown as an example in FIG. 4 for the same application of a sensor 1 immersed in fresh fresh water.

4a shows the first amplitude Ai of the first received signal X1 as a function of time t. There is a temporary, gas bubble-induced reduction in the first amplitude Ai. After At = 14h, the first amplitude Ai has again reached its original value, which corresponds to the absence of gas bubbles. The second amplitude A _2, on the other hand, (FIG. 4b) temporarily collapses or is greatly attenuated. Gas bubbles in the medium M can only be reliably detected on the basis of the ratio V of the first Ai and the second amplitude A ₂ , as illustrated in FIG. 4c. In the time period At = 4h-14h, the ratio V of the first Ai and the second amplitude A _{2 is} initially zero and then has a considerable gradient. Only after a period of At = 14h does the slope of the

Ratio V decreases and converges to a constant value. In the case of FIG. 4, it is assumed that the amplitudes of the first and second excitation signals 11 and I2 are each kept constant. Alternatively, it would also be conceivable to keep the amplitudes Ai and A _{2 of} the two received signals X1 and X2 constant. In this case, an evaluation of the ratio V of the amplitudes of the two excitation signals 11 and I2 is advisable. Another possibility consists in evaluating a normalized amplitude A _n0 r = X / l, which results from the quotient of the amplitudes of the received signal X and the excitation signal I. The use of a normalized amplitude is advantageously independent of the amplitude of the excitation signals 11 and I2 used in each case.

List of reference symbols

1 vibronic sensor

2 sensor unit

3 containers

4 Vibratory unit of a vibronic sensor

5 Drive / receiver unit

6 electronics unit

7, 7a, 7b oscillatable unit of a Coriolis measuring device 8 housing module

9 carriers

10 casing

1 1a, 11b process connections

M medium

S1.S2 vibration modes

1.11, 12 excitation signals

XX1 _, X1 received signals

fi, f ₂ first, second frequency

Ai, A ₂ first, second amplitude

fG natural frequency of the gas bubbles

fp critical frequency

P density of the medium

V ratio

t time

s sensitivity

d diameter of the gas bubbles

a radius of the gas bubbles

Claims

1 . Method, in particular computer-implemented method, for the detection of gas bubbles in a liquid medium (M) with a sensor (1) with a mechanically oscillatable unit (4), comprising the following method steps,

- Exciting the oscillatable unit (4) with a first excitation signal (11) for

Generating mechanical vibrations of the vibratable unit (4) according to a first predeterminable vibration mode (S1) of the vibratable unit (4), and receiving the mechanical vibrations from the mechanically vibratable unit (4) in the form of a first received signal (X1),

- Exciting the oscillatable unit (4) with a second excitation signal (I2) for

Generation of mechanical oscillations of the oscillatable unit (4) according to a second predeterminable oscillation mode (S2) of the oscillatable unit (4), and receipt of the mechanical oscillations from the mechanically oscillatable unit (4) in the form of a second received signal (X2),

Determining a first and second characteristic variable (f, A) of the first (X1) and second received signal (X2),

Determining a ratio (V) of the first (fi, Ai) and second characteristic quantities (f2, A ₂ ), and

- Generating a statement about the presence of gas bubbles based on the

Ratio (V) of the first (fi, Ai) and second characteristic quantity (fi, Ai).

2. The method according to claim 1,

the first oscillation mode (S1) being a fundamental oscillation mode of the oscillatable unit (4).

3. The method according to claim 2,

the mass distribution, rigidity and / or geometry of the oscillatable unit (4) is / are selected such that the fundamental oscillation mode (S1) is at a frequency fi <1.5 kHz.

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

wherein the second oscillation mode (S2) is selected such that the oscillations of the oscillatable unit (4) are influenced by the formation of gas bubbles in the area of the mechanically oscillatable unit (4).

5. The method according to claim 4, wherein the second oscillation mode (S2) is selected such that the frequency (f ₂ ) of the second oscillation mode (S2) is in a frequency range in which a

Gas bubble natural frequency (f _G ) of the gas bubbles lies.

6. The method according to claim 4 or 5,

the second oscillation mode (S2) being a first higher one

Vibration mode of the vibratable unit (4) is.

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

where it is the characteristic size (f, A) of the first (X1) and second

Received signal (X2) is a frequency (f), an amplitude (A), or a variable derived from at least the frequency (f) or the amplitude (A).

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

where in the case that the ratio (V) of the first (fi, Ai) and second characteristic quantities (ί ₂ , A ₂ ) is zero, or a slope of the ratio (V) of the first (fi, Ai) and second characteristic quantities (f ₂ , A ₂ ) as a function of time (t) a specifiable

Exceeds the limit value, it is concluded that gas bubbles are present.

9. The method according to at least one of claims 1-7,

where in the case that the ratio (V) of the first (fi, Ai) and second characteristic quantities (f ₂ , A ₂ ) is greater than zero or a slope of the ratio (V) of the first (fi, Ai) and second characteristic Size (f ₂ , A ₂ ) as a function of time (t) falls below a predeterminable limit value, it is concluded that there are no gas bubbles.

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

wherein a process variable of the medium (M) is determined on the basis of the first received signal (X1).

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

The process variable is only determined in the event that no gas bubbles are present.

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

wherein during a time interval in which gas bubbles are present, a value for the process variable that was last determined before the formation of the gas bubbles is output, and / or a currently determined value for the process variable is output during a time interval in which there are no more gas bubbles.

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

where based on the ratio (V) of the first (fi, Ai) and second characteristic variable

(fi, Ai) process monitoring is carried out.

14. The method according to claim 12,

where a fermentation or a disinfection process is monitored.

15. Computer program for the detection of gas bubbles in a liquid medium with

Computer-readable program code elements which, when executed on a computer, cause the computer to carry out a method according to at least one of the preceding claims.

16. Computer program product with a computer program according to claim 14 and

at least one computer-readable medium on which at least the computer program is stored.