Method for determining and / or monitoring the viscosity of a medium, and vibration sensor

Abstract

The invention relates to a method for determining and / or monitoring the viscosity of a medium with a vibration sensor (10). The oscillations of a unit (11), which can be mechanically oscillated, of the vibration sensor (10) are used to determine whether the medium covers the unit (11) which can be mechanically oscillated. If it does, subsequent oscillations are used to determine whether the medium continues to cover the unit (11). If it does not, a development over time of the oscillation quality is used to ascertain a statement about the viscosity of the medium. The invention also relates to a vibration sensor (10).

Description

[0001] Method for determining and / or monitoring the viscosity of a medium and vibration sensor

[0002] The invention relates to a method for determining and / or monitoring the viscosity of a medium. Furthermore, the invention relates to a vibration sensor.

[0003] Mechanically vibrating units - e.g. tuning forks - of vibration sensors (see, for example, DE 10 2012 101 667 A1) are used, among other things, to determine the limit level of a medium in a tank.

[0004] Typically, a measured oscillation frequency is used to determine whether the oscillating unit is touched by a medium or not.

[0005] For example, it is also possible to monitor the condition of the mechanically vibrating unit itself. In DE 10 2011 075 113 A1, a damping factor is determined from the difference between excitation frequencies in order to detect buildup or corrosion during vibrations in the uncovered state.

[0006] In addition to the fill level, there are other process variables or properties of a medium that need to be monitored during a process. For example, the viscosity of the medium, which is usually measured using very complex measuring equipment.

[0007] The object of the invention is to propose a method and a vibration sensor carrying out the method for determining or monitoring the viscosity of a medium.

[0008] The object is achieved by a method for determining and / or monitoring the viscosity of a medium with a vibration sensor, wherein the method comprises at least the following steps: that a mechanically oscillating unit of the vibration sensor is excited to mechanical vibrations, that the mechanical vibrations of the mechanically oscillating unit are received, that it is determined from at least one measured value of a vibration variable of the received mechanical vibrations whether the medium covers the mechanically oscillating unit, that in the case that the medium covers the mechanically oscillating unit, it is determined from at least one measured value of a vibration variable of the subsequently received mechanical vibrations whether the medium continues to cover the mechanically oscillating unit, that in the case that the mechanically oscillating unit is free from coverage by the medium,from at least one measured value of a vibration quantity of the subsequently received mechanical vibrations, measured values ​​of a vibration quality are determined, and that a statement about the viscosity of the medium is determined from a temporal development of the measured values ​​of the vibration quality.

[0009] In the method according to the invention, the mechanical vibrations of the mechanically oscillating unit of the vibration sensor are evaluated. In this case, the mechanical vibrations are converted into electrical signals. This occurs, for example, using piezo elements as part of a transducer device. Measured values ​​of vibration variables are then determined from the signals. The vibration variables are, for example, the amplitude, frequency, or phase of the vibrations. In addition or alternatively, these can also be resulting variables such as the vibration quality. The measured values ​​of the vibration variables are then, in turn, used to determine the measured values ​​of the process variables of interest, such as coverage by the medium or viscosity. For processing the measured values ​​of the vibration variables, stored formulas and / or data sets such as reference values ​​are used, for example.

[0010] The process for determining and / or monitoring viscosity takes place in several consecutive steps or phases. It is assumed that the mechanically oscillating unit initially oscillates uncovered by the medium, for example, in air. If the fill level of the medium changes and the medium covers the previously freely oscillating mechanically oscillating unit, this can be detected based on the characteristics of the oscillations. For this purpose, at least one measured value of a vibration variable is evaluated. From the oscillations that follow the time of coverage, it is determined in a second phase whether coverage is still present or whether the medium no longer covers the mechanically oscillating unit, i.e. whether the oscillating unit is once again oscillating freely and uncovered.If this is the case, a measurement value for the vibration quality is determined as a vibration quantity from the vibrations that follow in the third phase. The vibration quality then allows a change in viscosity to be detected and / or the viscosity value of the medium to be determined. This takes advantage of the fact that after the oscillating unit has been covered by the medium and subsequently released, the medium continues to adhere to the oscillating unit and only detaches after a certain time. For the oscillating unit, the transition to completely free vibration is therefore delayed. Therefore, the viscosity can be deduced from the detachment behavior based on the vibration quality.

[0011] The sequence of the three phases is described again in other words:

[0012] Phase 1 : The oscillating unit oscillates in the uncovered state.

[0013] Phase 2: If the oscillating unit is covered by the medium, this can be determined from at least one vibration variable. Thus, it is recorded whether a change from "uncovered" to "covered" has occurred.

[0014] Phase 3: When the medium coverage ends, this is also detected based on the oscillation magnitude. The viscosity is determined from the subsequent behavior of the oscillation magnitude from the moment "coverage ends" until the return to free oscillation. Thus, the process of returning to the normal state of free oscillation in the air is evaluated.

[0015] It is therefore necessary for the medium to cover the oscillating unit and then to cease covering it. If a portion of the medium remains on the mechanically oscillating unit, there is no coverage by the medium itself. Coverage by the medium means, in particular, that the medium reaches a fill level such that the mechanically oscillating unit, due to its dimensions and arrangement, is below this fill level. If the coverage ceases, the fill level of the medium drops below the position of the oscillating unit. The process then monitors how the medium drips off the mechanically oscillating unit.One embodiment of the method includes determining vibration quality values ​​from the at least one measured value of the vibration magnitude of the subsequently received mechanical vibrations if the mechanically vibratable unit was covered by the medium for at least a predetermined coverage time and / or at least during a predetermined number of mechanical vibrations. In this embodiment, care is taken to ensure that the mechanically vibratable unit has been in the medium for a sufficiently long time and has performed mechanical vibrations. This is ensured by specifying a period of time or a number of vibrations to be completed.The vibrations precondition the medium, so to speak, by subjecting the medium, which is preferably a liquid, to shear stress as a result of the vibration of the oscillating unit. In this embodiment, the medium is thus moved in the second phase and thus conditioned to enable reproducible measurements.

[0016] One embodiment of the method provides that the vibration quantity used to determine whether the medium covers the mechanically oscillatable unit is a frequency of the received vibrations. In this embodiment, the frequency of the received vibrations is used to determine whether the medium covers the oscillatable unit.

[0017] In an alternative or supplementary embodiment, the frequency is used to determine whether the medium continues to cover the oscillating unit.

[0018] One embodiment of the method involves determining a statement about the viscosity of the medium based on the temporal development of the measured values ​​of the vibration quality up to a starting value. In this embodiment, the behavior of the vibration quality after the expiration of the covering and non-covering phases in relation to a starting value is evaluated. For example, it is evaluated how the quality adapts back to the starting value. The aforementioned starting value is predetermined in one embodiment. In an alternative embodiment, the measured value of the starting value of the vibration quality is determined from the vibrations in the uncovered state. This starting value is therefore also suitable for the respective measuring conditions.

[0019] According to a supplementary embodiment of the method, at least one measured value of another vibration variable is used to determine the viscosity of the medium. The additional vibration variable is preferably the frequency of the mechanical vibrations. In this variant, the temporal progression of the frequency is evaluated in comparison to the free, uncovered, oscillating unit. This determines, for example, how quickly the initial value of the vibration frequency is reached. Alternatively or additionally, the gradient with which the frequency approaches the starting value of the uncovered, free vibrations is determined. For example, it has been shown that a transition in the frequency curve from the covered to the free state allows a conclusion to be drawn about the viscosity. A sharp-edged transition is an indication of lower viscosity, and a rounded transition is a sign of higher viscosity.

[0020] One embodiment of the method provides that the determined statement relates to the viscosity value of the medium. Alternatively or additionally, the determined statement relates to the value range within which the viscosity of the medium lies. Also alternatively or additionally, the determined statement relates to whether the viscosity of the medium has changed beyond a specified tolerance range.

[0021] In one embodiment, a change in viscosity is detected, so the vibration sensor acts as a type of viscosity switch: A change in viscosity is indicated. Alternatively, a value or at least a range of values ​​for the viscosity of the medium is determined. Thus, values ​​or ranges of values ​​are determined and output.

[0022] One embodiment of the method includes the fact that, if the mechanically oscillating unit is free of medium coverage, the received mechanical oscillations are part of the decay behavior of the oscillations of the mechanically oscillating unit. In this embodiment, after the end of the medium coverage, the mechanically oscillating unit is no longer excited to oscillate, but only oscillations are received and evaluated.

[0023] The object is additionally achieved by a vibration sensor with a mechanically oscillatable unit (e.g., a tuning fork, a single rod, or a membrane) and a control device for operating the mechanically oscillatable unit and for detecting vibrations of the oscillatable unit, wherein the control device (e.g., at least comprising a transducer device, which preferably has at least one piezo element, and an electronic device for processing electrical signals and for executing measurement sequences) is designed to carry out the method according to one of the preceding or following embodiments. The vibration sensor is therefore capable of implementing the method. Alternatively, only the measurements are carried out by the vibration sensor, and the evaluation is carried out in a unit that is, for example, separate from the process.

[0024] In the following, the invention is described using exemplary embodiments.

[0025] Fig. 1 shows an exemplary vibration sensor,

[0026] Fig. 2 shows the course of the frequency of the oscillations and the quality of the oscillating circuit in a coating-free oscillating unit in the case of covering with water,

[0027] Fig. 3 shows the course of frequency and quality for an oscillating unit loaded with a coating in the case of covering with water,

[0028] Fig. 4 shows the course of frequency and quality for a coating-free oscillating unit in the case of covering with a highly viscous liquid and

[0029] Fig. 5 shows the frequency and quality curve for an oscillating unit loaded with a coating when covered with a highly viscous liquid. Fig. 1 shows an exemplary vibration sensor 10, which has a mechanically oscillating unit 11, which is designed here as a tuning fork, and a control device 12. A medium - not shown here - influences the vibration properties of the oscillating unit 11 as soon as it comes into contact with the oscillating unit 11. The mechanically oscillating unit 11 is excited to vibrate by the control device 12. The control device 12 thus comprises a transducer device such as a piezo element, as well as electronics that executes a measuring program and also evaluates the received vibrations and the resulting electrical signals.

[0030] Due to repeated wetting and drying due to level fluctuations, a coating of media residues can form over time on the oscillating unit 11. This coating manifests itself, among other things, in a shift in the vibration frequencies towards lower ranges. However, this effect usually becomes apparent slowly, i.e., with more pronounced deposits. This speaks to the robustness of the measurement method.

[0031] Figures 2 to 5 show how the method allows the viscosity of a preferably liquid or flowable medium to be monitored or determined. Shown are exemplary temporal profiles of oscillation frequency (dashed line in each case) and quality factor (solid line in each case) of a mechanically oscillating unit 11 in different states (with or without a coating on it) influenced by various media (water or a highly viscous medium). Time is plotted in seconds on the x-axis. The frequency in Hertz is plotted on the left y-axis, and the unitless quality factor is plotted on the right y-axis.

[0032] Fig. 2 shows the temporal course of vibration frequency and vibration quality of a mechanically vibrating unit 11 without a coating.

[0033] The oscillating unit 11 oscillates in a dry first phase P1, e.g., in air, is immersed in water as a medium in an immersion phase P2, and then dries in the third phase P3. Upon separation of the water from the oscillating unit 11, i.e., during the transition from the covered to the uncovered state, the frequency almost immediately assumes the oscillation frequency of the dry state of P1 of slightly over 1100 Hz, whereas the quality factor requires approximately 1000 seconds to reach an initial state with an oscillation quality factor of slightly over 120 as a starting value. The quality factor is sensitive to the presence of a water film that drains away drop by drop (gradations over time).

[0034] Fig. 3 shows the curves of vibration frequency and vibration quality when immersed in water and in the presence of a coating, for example made of cement, on the mechanically vibratable unit 11.

[0035] After immersion in the medium (immersion may involve changing the position of the oscillating unit or changing the fill level of the medium), the frequency very quickly reaches values ​​above 1100 Hz in phase P2, whereas the quality factor remains at a value around 70. It turns out that the quality factor is a good measure for deposit detection.

[0036] In both cases (Fig. 2 and Fig. 3) a similar quality curve is shown: there is initially a strong increase, followed by a section with a relatively constant value.

[0037] Fig. 4 shows time courses of vibration frequency and quality of a mechanically vibrating unit 11 without coating when immersed in a viscous medium with a viscosity of approximately 1000 square millimeters per second.

[0038] As in the case of Fig. 2 with water as the medium, a rapid return of the oscillation frequency to values ​​in the range of the unwetted state is observed. However, it should be noted that the transition is rounded and not as sharp-edged as with water. Furthermore, the oscillation frequency approaches the starting value more slowly than in the case of Fig. 2.

[0039] The vibration quality, however, shows an oscillation between values ​​in the range between 50 and 80 after the medium 21 has been separated from the oscillating unit 11. This oscillation is caused by the medium 21 dripping and running on the oscillating unit 11. At the same time, it can be seen that the quality allows a conclusion to be drawn about the viscosity of the medium. In the case of water, Fig. 2 shows an almost adiabatic increase in the quality, which approaches a starting value. In contrast, Fig. 4 shows an oscillating behavior with valleys that become longer over time between the individual deflections. Therefore, a statement about the viscosity of the medium can be derived from the quality or the temporal behavior.

[0040] Fig. 5 shows the curves of vibration frequency and vibration quality when immersed in the medium 21 and in the presence of a coating on the mechanically vibrating unit 11 - again by cement as an example.

[0041] In contrast to the graph in Fig. 4, the vibration quality remains in the range of 40 over a long period of time, significantly below the vibration quality in the case without cement coating. Nevertheless, a significantly different behavior is observed compared to Fig. 3, which allows conclusions to be drawn about the viscosity.

[0042] In general, it should be noted that the shape and progression of the curves may depend on the properties of the mechanically vibrating unit. This relates, for example, to the geometry, the type of coating (if present), or the material of the mechanically vibrating unit. It may also depend on the type and design of the transducer unit and / or the type of vibration excitation.

Claims

Patent claims 1. A method for determining and / or monitoring the viscosity of a medium using a vibration sensor (10), the method comprising at least the following steps: a mechanically oscillatable unit (11) of the vibration sensor (10) is excited to mechanical vibrations, the mechanical vibrations of the mechanically oscillatable unit (11) are received, whether the medium covers the mechanically oscillatable unit (11) from at least one measured value of a vibration variable of the received mechanical vibrations, in the event that the medium covers the mechanically oscillatable unit (11), it is determined from at least one measured value of a vibration variable of the subsequently received mechanical vibrations whether the medium continues to cover the mechanically oscillatable unit (11), in the event that the mechanically oscillatable unit (11) is free of coverage by the medium,from at least one measured value of a vibration quantity of the subsequently received mechanical vibrations, measured values ​​of a vibration quality are determined, and that a statement about the viscosity of the medium is determined from a temporal development of the measured values ​​of the vibration quality.

2. Method according to claim 1, wherein, in the event that the mechanically oscillatable unit (11) is free from coverage by the medium, measured values ​​of a vibration quality are only determined from the at least one measured value of the vibration quantity of the subsequently received mechanical vibrations if the mechanically oscillatable unit (11) was covered by the medium at least for a predetermined coverage time and / or at least during a predetermined number of mechanical vibrations.

3. Method according to claim 1 or 2, wherein the vibration quantity from which it is determined whether the medium covers the mechanically vibratable unit (11) is a frequency and / or an amplitude of the received vibrations.

4. Method according to one of claims 1 to 3, wherein a statement about the viscosity of the medium is determined at least on the basis of the temporal development of the measured values ​​of the vibration quality up to a starting value.

5. Method according to claim 4, wherein at least one measured value of a further vibration variable - preferably the frequency - is used to determine the statement about the viscosity of the medium.

6. Method according to one of claims 1 to 5, wherein the determined statement relates to the value of the viscosity of the medium or the value range in which the viscosity of the medium lies or whether the viscosity of the medium has changed beyond a predeterminable tolerance range.

7. Method according to one of claims 1 to 6, wherein in the case that the mechanically oscillatable unit (11) is free from coverage by the medium, the received mechanical oscillations belong to a decay behavior of the oscillations of the mechanically oscillatable unit (11).

8. Vibration sensor (10) with a mechanically oscillatable unit (11) and a control device (12) for operating the mechanically oscillatable unit (11) and for detecting vibrations of the oscillatable unit (11), wherein the control device (12) is designed to carry out the method according to one of claims 1 to 7.

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