Method for determining a measurement variable of a medium

Abstract

The invention relates to a method for determining a measurement variable of a medium (4). In the method, a mechanically oscillatory unit (2) is brought into contact with the medium (4) in such a way that the medium covers at least a portion of the mechanically oscillatory unit (2). Subsequently, the mechanically oscillatory unit (2) is positioned in such a way that it is possible for the medium (4) to form drops, and the mechanically oscillatory unit is excited to perform mechanical oscillations. Measured values are determined for at least one characteristic variable of the mechanical oscillations at different points in time. A behavior over time resulting therefrom is analyzed with regard to the measurement variable.

Description

[0001] Method for determining a measured value of a medium

[0002] The invention relates to a method for determining a measured variable of a medium.

[0003] Vibration sensors are known in the art, for example, in the form of tuning forks or single-rod sensors. Such sensors have a mechanically oscillating unit that is excited to mechanical vibrations by a drive / receiver unit. The vibrations resulting from the interaction with a medium to be measured or monitored are received by the drive / receiver unit and sent for evaluation. For example, a measurement exploits the fact that the vibration frequency or amplitude changes when the mechanically oscillating unit transitions from an uncovered state to one covered by the medium. This allows, for example, the monitoring of the fill level of the medium in a container.

[0004] In DE 10 2021 126 826 A1, a tuning fork is used to detect different media based on a calibration measurement. In one embodiment, the calibration is performed while a medium adheres to the sensor.

[0005] DE 102020 121 610 A1 refers to a vibrating fork in which an optical distance sensor is used to monitor whether medium adheres to the fork.

[0006] DE 102014 115 693 A1 describes a vibrating fork whose paddles are optimized with regard to dripping behavior.

[0007] Intrinsic parameters of a medium, such as its viscosity, usually require very complex measuring equipment. The invention is based on the object of proposing a method with which even intrinsic parameters of a medium can be determined as simply as possible.

[0008] The object is achieved by a method for determining a measurand of a medium, wherein the method comprises at least the following steps: a) that a mechanically oscillatable unit is brought into contact with the medium in such a way that the medium covers at least part of the mechanically oscillatable unit, b) that the mechanically oscillatable unit is positioned in such a way that droplet formation of the medium on the mechanically oscillatable unit is possible, c) that the mechanically oscillatable unit is excited to mechanical oscillations at least during step b), d) that measured values ​​are determined for at least one characteristic of the mechanical oscillations at different times, and e) that a temporal behavior of the measured values, which results from the determined measured values, is evaluated with regard to the measurand.

[0009] In the method according to the invention, the mechanically oscillating unit is brought into contact with the medium whose measurement variable is to be determined in such a way that the medium at least partially covers the mechanically oscillating unit. The mechanically oscillating unit is then positioned in such a way that droplets of the medium can form on the mechanically oscillating unit. The medium is therefore in particular a flowable medium such as a liquid. In this state, with the medium at or on the mechanically oscillating unit, the mechanically oscillating unit is excited to oscillate. These oscillations depend on the medium that is at or on the mechanically oscillating unit. Therefore, the parameters that can be determined from the received mechanical oscillations also depend on the interaction with the medium. Parameters include, for example, frequency, amplitude, phase or quality factor.At least one characteristic is measured at different points in time to capture the temporal behavior of the medium at or on the mechanically oscillating unit. In particular, the formation or dripping of the medium is recorded in order to draw conclusions about the ultimately relevant measured variable of the medium. Whether or not droplets form also depends on the properties of the medium.

[0010] In one embodiment, a further variable of the medium is determined. This further variable of the medium is preferably its temperature. For this purpose, a temperature sensor, for example, is provided.

[0011] In addition or alternatively, at least one further characteristic of the vibrations is determined and used for the evaluation.

[0012] One embodiment of the method provides that in step a) the mechanically oscillating unit is at least partially immersed in a quantity of the medium in a container and then - preferably completely - withdrawn. The mechanically oscillating unit is thus at least partially covered with the medium by immersion. So that drops can subsequently form, the mechanically oscillating unit is withdrawn from the medium again. In a supplementary embodiment, the mechanically oscillating unit is completely withdrawn from the medium in the container by a relative movement between the mechanically oscillating unit and the container. In one embodiment, the mechanically oscillating unit is therefore withdrawn from the container. Alternatively, the container is moved away from the stationary mechanically oscillating unit. In a further embodiment, both of the aforementioned variants can be combined.

[0013] An alternative embodiment includes that the medium is applied to the mechanically oscillatable unit through a medium line or through a spray unit.

[0014] In one embodiment, the possibility of droplet formation is supported by aligning the mechanically oscillating unit at a predefined angle to the Earth's gravitational field in step b). Thus, in this embodiment, the mechanically oscillating unit is preferably arranged at an angle so that the medium can flow, for example, to the front end of the unit and form droplets there.

[0015] The following details are dedicated in particular to the evaluation of the parameters determined from the vibrations.

[0016] One embodiment involves identifying discontinuities in the temporal behavior in step e). If a droplet forms and detaches from the mechanically oscillating unit, this results in a sudden change in the influence of the mechanical oscillations, which results in a discontinuity in the characteristic oscillations. Assuming that droplet formation depends on the measured variable of interest, the discontinuities serve to determine the measured variable in this embodiment.

[0017] A further embodiment provides that in step e) the case in which a change in the measured value exceeds a predeterminable limit value within a predeterminable period of time is assessed as a dripping moment of the medium, that a statement about the dripping behavior of the medium is determined from the at least one dripping moment of the medium, and that the statement about the dripping behavior is compared with stored data in order to determine the measured variable. In this embodiment, it is determined whether the measured value of the characteristic variable of the vibrations changes by a certain amount within a certain time window. In one variant, this relates in particular to whether there is a sudden change. If such a situation of a strong change in the measured values ​​occurs, this is assessed as a dripping moment, i.e. a drop of the medium has detached itself from the mechanically vibrating unit within this time window.The dripping behavior of the medium itself is determined from at least one dripping moment, or preferably from several dripping moments. Based on the dripping behavior thus determined and using appropriately stored data, the measured variable of the medium is determined. In an alternative or supplementary variant, the course of the measured values ​​in the time window around the detected dripping moments is evaluated. This refers, for example, to the period before a dripping moment. Alternatively or additionally, one embodiment refers to a period between several dripping moments. Preferably, several variables are derived from the measured values ​​and the identified dripping moments and used for the evaluation.

[0018] The method is primarily used to determine a rheological parameter of the medium. This parameter is preferably the viscosity of the medium.

[0019] This is explained in more detail using the following figures.

[0020] Fig. 1 shows a schematic and not to scale measuring arrangement for applying the method according to the invention and

[0021] Fig. 2 shows the temporal development of the quality of the mechanically vibrating unit when wetted with water (a) and with silicone oils with three different viscosities (b) to (d).

[0022] Fig. 1 shows a so-called vibrating fork as a device 1 for determining the measured variable of the medium 4. The two fork tines of the mechanically vibrating unit 2 are excited to mechanical vibrations.

[0023] Here, it is indicated that during the execution of the method, the device 1 is positioned relative to the Earth's gravitational field such that the mechanically oscillating unit 2 is inclined at a predeterminable angle of inclination. This occurs, for example, after the mechanically oscillating unit 2 has been immersed in the medium 4 within the container 3 and withdrawn again. The inclined position particularly promotes the dripping of the medium 4, which at least partially covers the mechanically oscillating unit 2.

[0024] Additionally, a temperature sensor 5 is provided, which allows the medium temperature to be measured. The measurement signals from device 1 or data regarding the vibrations, as well as the measurement results from temperature sensor 5, are fed to an evaluation unit 6, which determines the measured variable of medium 4.

[0025] The temporal behavior of a parameter of the mechanical vibrations as a function of the covering medium is shown in Fig. 2 with media with four different viscosities.

[0026] Figures 2 a) to d) show the quality of the mechanically oscillating unit (y-axis) as a function of time (x-axis, t / s). For this purpose, the quality is determined at different points in time.

[0027] Fig. 2 a) shows the dripping behavior of the fork after it was immersed in water. Fig. 2 b) to d) show the dripping behavior after immersion in silicone oil with a viscosity of approximately 1,000 mm. 2 / s (Fig. 2 b)), 10 000 mm 2 / s (Fig. 2 c)) and 20 000 mm 2 / s (Fig. 2 d)). The mechanically oscillating unit used for the measurements, in the form of a tuning fork, has a quality factor of approximately 3,300 in air with a resonance frequency of approximately 1,130 Hz.

[0028] If the fork is immersed in the silicone oils (Fig. 2 b) to d)) or in water (Fig. 2 a)), the quality factor drops to a value that is determined by the density of the media. The quality factor, in combination with the frequency of the mechanical vibrations and the temperature of the medium, is used to determine the density of the medium, for example. If the fork is removed from the water, the frequency and the quality factor increase again. However, both values ​​do not immediately return to their original state, since a thin film of water forms on the fork when it is removed from the water, and a film of oil forms when it is removed from the silicone oils.

[0029] In the case of water (Fig. 2 a)), no drops detach and the water film evaporates after some time.

[0030] Individual droplets detach from the silicone oils (Fig. 2 b) to d)). If there is a low point in the quality followed immediately by a high point, this means that a droplet has detached. This is a "drip-off moment," at which the measured values ​​of the parameter change beyond a certain limit within a certain period of time.

[0031] After the droplet has detached, the quality decreases again because the medium flows from a location further up along the gravitational field on the mechanically oscillating unit towards the front side.

[0032] These specific points in the time course are marked with arrows in Fig. 2 b). These are (from left to right): the initial value after withdrawal from a reservoir containing the medium, a local maximum of the quality factor after the detachment of a droplet, and the sudden increase in the quality factor after the detachment of a droplet at a local minimum of the quality factor.

[0033] It is clear that a dripping behavior characteristic of the medium can be observed over time. Based on the dripping duration and the course of this dripping curve, or dripping characteristics, suitable algorithms can be used to determine the viscosity and density of the medium as measured variables.

[0034] List of reference symbols

[0035] 1 device

[0036] 2 mechanically vibrating unit 3 container

[0037] 4 Medium

[0038] 5 Temperature sensor

[0039] 6 Evaluation unit

Claims

Patent claims 1 . Method for determining a measurand of a medium (4), the method comprising at least the following steps: a) a mechanically oscillatable unit (2) is brought into contact with the medium (4) in such a way that the medium (4) covers at least part of the mechanically oscillatable unit (2), b) the mechanically oscillatable unit (2) is positioned in such a way that droplet formation of the medium (4) on the mechanically oscillatable unit (2) is enabled, c) the mechanically oscillatable unit (2) is excited to mechanical oscillations at least during step b), d) measured values ​​are determined for at least one characteristic of the mechanical oscillations at different times, and e) a temporal behavior of the measured values, which results from the determined measured values, is evaluated with regard to the measurand.

2. Method according to claim 1, wherein in step a) the mechanically oscillatable unit (2) is at least partially immersed in a quantity of the medium (4) in a container (3) and then - preferably completely - withdrawn.

3. Method according to claim 1 or 2, wherein in step b) the mechanically oscillatable unit (2) is aligned at a predeterminable angle of inclination to the Earth's gravitational field.

4. Method according to one of claims 1 to 3, wherein in step e) discontinuities in the temporal behavior are identified.

5. Method according to one of claims 1 to 4, wherein in step e) the case that within a predeterminable period of time a change in the measured value exceeds a predeterminable limit value is evaluated as a dripping moment of the medium (4), wherein from the at least one dripping moment of the medium (4) a statement about a dripping behavior of the medium (4) is determined, and The statement about the dripping behavior is compared with stored data in order to determine the rheological measurement value.

6. Method according to one of claims 1 to 5, wherein the viscosity of the medium (4) is determined as a rheological measurement variable.

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