Method for operating a flow meter and flow meter

Abstract

A method (17) for operating a flow meter (1) is described and illustrated, wherein the flow meter (1) has at least one measuring tube (3), wherein the measuring tube (3) is flowed through by a medium, wherein the flow meter (1) further comprises at least one sensor unit comprising at least two electrodes (5) for detecting a primary measured quantity from which the flow rate of the medium is determined, and a control and evaluation unit (6), wherein the electrodes (5) are arranged on the measuring tube (3) such that they are in contact with the medium during operation, wherein the control and evaluation unit (6) comprises a flow path (7), an impedance path (12) which includes an impedance unit (13) for generating an impedance signal and for evaluating an impedance measurement signal, and a computing unit (11), wherein the electrodes (5) are connected to both the flow path (7) and the impedance path (12).

Description

[0001] The invention relates to a method for operating a flow meter, wherein the flow meter has at least one measuring tube, wherein the measuring tube is flowed through by a medium, wherein the flow meter further comprises at least one sensor unit comprising at least two electrodes for detecting a primary measured variable from which the flow rate of the medium is determined, and a control and evaluation unit, wherein the electrodes are arranged on the measuring tube in such a way that they make contact with the medium, wherein the control and evaluation unit comprises a flow path, an impedance path which includes an impedance unit for generating an impedance signal and for evaluating an impedance measurement signal, and a computing unit, the electrodes are connected to both the flow path and the impedance path.

[0002] Furthermore, the invention relates to a flow meter for carrying out the method according to the invention. The flow meter comprises a measuring tube, and furthermore, the flow meter comprises at least one sensor unit comprising at least two electrodes for detecting a primary measured variable from which the flow rate of the medium is determined, and a control and evaluation unit. wherein the electrodes are arranged on the measuring tube in such a way that they are in contact with the medium during operation, wherein the control and evaluation unit comprises a flow path, an impedance path which includes an impedance unit for generating an impedance signal and for evaluating an impedance measurement signal, and a computing unit, the electrodes are connected to both the flow path and the impedance path.

[0003] Flow meters that measure both the flow rate of a flowing medium and the impedance of the medium are known from the prior art.

[0004] For example, publication WO 2008 / 113774 A1 describes a magnetic-inductive flowmeter, in which the conductivity of the medium is determined based on an impedance value measured between the measuring electrode and the reference potential.

[0005] The publication EP 4 071 447 A1 relates to a magnetic inductive flowmeter, wherein a first signal with a first frequency and a second signal with a second frequency are applied to the electrodes, and wherein the impedances are determined from the first frequency and the second frequency, and wherein the conductivity of the medium is further determined taking into account the two impedances.

[0006] Furthermore, the publication EP 3 594 635 B1 describes an electromagnetic flow meter, wherein the conductivity of a medium is measured by applying a square wave signal with a first frequency and a square wave signal with a second frequency between the electrode and an earthing ring, and wherein the conductivity of the medium is determined from the impedance.

[0007] Flow meters that already incorporate electrodes for measuring flow can be particularly advantageous for simultaneous impedance measurement. In particular, the flow meter according to the invention is designed as a magnetic-inductive flow meter or as a vortex flow meter.

[0008] A fundamental problem with measuring impedance using the aforementioned flow meters is that the evaluation electronics are not necessarily designed to acquire and process both a primary measurement signal for flow measurement and a higher-frequency impedance signal. Instead, evaluation electronics for flow measurement often exhibit low-pass filtering characteristics to optimize the evaluation of the lower-frequency measurement signals of the primary measured quantity. For example, the sampling rate for acquiring the electrode voltage signal as the primary measurement signal in magnetic-inductive flow meters is in the range of a few kHz. In comparison, the frequency of an impedance signal for measuring impedance can easily reach up to 10 MHz.

[0009] Furthermore, the amplitude of both the impedance signal applied to the electrodes and the amplitude of the impedance measurement signal acquired and transmitted by the electrodes is significantly larger than the amplitude of the voltage signal used to determine the flow rate. In this respect as well, the measurement signals compete with each other.

[0010] Overall, measuring impedance using a flow meter therefore requires electronics that go beyond the original requirements and, in particular, must also meet conflicting requirements.

[0011] Based on the prior art described above, the object of the present invention is therefore to provide a method for operating a flow meter that can detect both the flow rate of a flowing medium and the impedance of the medium particularly well. It is also an object of the invention to provide a corresponding flow meter.

[0012] According to a first teaching of the present invention, the aforementioned problem is solved by a method described at the outset, in that the method comprises the following steps: - Applying an impedance signal to the electrodes, - Detection of an impedance response signal by the electrodes, - Forwarding the impedance response signal as an impedance measurement signal to the impedance unit, - Determination of the total impedance Z from the impedance measurement signal by the impedance unit, - Detection of the primary measured quantity by the electrodes, - Forwarding the primary measured variable as the primary measurement signal to the flow path, - Determination of the flow rate through the processing unit from the primary measurement signal.

[0013] According to the invention, it has been found that by means of a flow meter comprising electrodes for flow measurement, both the flow rate of the medium and the total impedance Z between the electrodes, which also includes the impedance Z1 of the medium, can be determined if the evaluation unit is designed to also acquire and evaluate higher-frequency signals and / or signals that are superimposed on the signal for acquiring the primary measured quantity. For this purpose, an impedance path is provided according to the invention, which is configured to emit a higher-frequency impedance signal and to acquire and evaluate a higher-frequency impedance measurement signal. The previously defined steps for determining the flow rate and the total impedance Z are carried out in the described sequence according to one embodiment. As explained below, another sequence for determining the flow rate and the total impedance Z is also conceivable.

[0014] According to a preferred embodiment, the flow path and the impedance path are separate evaluation paths.

[0015] In principle, with such a combined evaluation unit, it must be ensured that the impedance measurement does not influence the flow measurement. This is particularly relevant when the impedance and flow are measured via the same electrodes. Such separation of the impedance signal and the primary measurement signal can be achieved, as described below, through the characteristics of the signals themselves—that is, the impedance signal or the impedance measurement signal and the primary measurement signal—and / or through the structural design of the flow path and / or the impedance path.

[0016] The total impedance Z to be measured, which is determined from the impedance measurement signal, always includes the impedance Z1 of the medium and the crossover impedances Z2 of the junction between the electrodes and the medium. Therefore, when an impedance signal is applied to the electrodes, only the combination of Z1 and Z2 can be determined. Within the scope of the present invention, the combination of the impedance Z1 of the medium and the crossover impedances Z2 is referred to as the total impedance Z. A problem in determining the total impedance Z is that unpredictable fluctuations in the constant phase element, which influences the crossover impedance Z2 of the junction between the electrodes and the medium, cause unpredictable fluctuations in the total impedance Z in certain ranges of the excitation frequency, i.e., in certain ranges of the impedance frequency.

[0017] If another target parameter, for example of the medium, is determined from the total impedance Z, unpredictable fluctuations also occur in these critical frequency ranges between the dependence of the total impedance Z and the target parameter to be determined.

[0018] Therefore, it is relevant to measure the total impedance Z in a frequency range where the fluctuations caused by the constant phase element are minimal or, ideally, non-existent. By carefully selecting the frequency of the impedance signal, the influence of the constant phase element can thus be advantageously made negligible.

[0019] According to one embodiment, the frequency of the impedance signal is selected such that the unpredictable influence of the electrochemistry at the interface between the electrodes and the medium on the impedance measurement signal can be reduced or essentially avoided. Particularly preferably, the frequency of the impedance signal is higher than a cutoff frequency, wherein the cutoff frequency depends on the ratio of the impedance Z1 of the medium and the interface impedance Z2 of the electrode-medium interface.

[0020] If this ratio is large, i.e., the impedance Z1 of the medium is much larger than the transition impedance Z2, then the frequency of the impedance signal can be, for example, in the low kHz range. In this case, the influence of the constant phase element ceases at low frequencies. For example, the frequency of the impedance signal is between 1 and 20 kHz, preferably between 5 and 15 kHz.

[0021] For example, if the ratio Z1 / Z2 is greater than 100, the relative error in determining the total impedance Z is at most 2%.

[0022] If the impedance Z1 of the medium is only slightly greater than the interface impedance Z2, the frequency of the impedance signal is higher to reduce the influence of the electrochemical effect at the interface between the electrodes and the medium. For example, the frequency of the impedance signal is between 1 and 10 MHz.

[0023] The method according to the invention relates in particular to the measurement of conductive media in which the impedance Z1 of the medium is only slightly larger, if at all, than the transition impedance Z2. The measurement of conductive media has the advantage that the frequency of the impedance signal, and thus the frequency of the impedance measurement signal, is significantly different from the frequency of the primary measurement signal, which simplifies the separation of these signals.

[0024] In principle, different forms of an impedance signal are suitable for impedance measurement.

[0025] According to an advantageous embodiment of the invention, the impedance signal is a periodic signal, in particular a sinusoidal signal or a square wave signal or a superposition of at least two sinusoidal signals.

[0026] For example, to determine the impedance, an alternating current signal is applied to the electrodes. The voltage that develops between the electrodes is measured as the impedance response signal and forwarded to the impedance unit as the impedance measurement signal. Alternatively, an alternating voltage can be applied to the electrodes, in which case the current flowing between the electrodes is measured as the impedance response signal and forwarded as the impedance measurement signal.

[0027] Preferably, the frequency of the impedance signal is varied within a previously defined frequency range to measure the medium.

[0028] According to one embodiment, the frequency of the impedance signal is between 1 kHz and 100 kHz. According to another embodiment, the frequency of the impedance signal is between 100 Hz and 10 MHz, preferably between 1 MHz and 10 MHz. These specified frequency ranges do not refer to the defined frequency range within which the frequency for determining the total impedance Z can be varied during a measurement. Rather, the aforementioned frequency values ​​merely indicate the general frequency range of the impedance signal.

[0029] Alternatively, the periodic impedance signal can also have multiple frequencies, whereby the periodic impedance signal can be designed as a superposition of several sinusoidal signals or as a periodic square wave signal.

[0030] It is particularly advantageous to determine both the real and imaginary parts of the total impedance Z. The real part of the total impedance determines the conductivity of the medium, and the imaginary part determines the permittivity of the medium. According to this advantageous embodiment, the medium to be measured can be characterized with respect to both its conductivity and its permittivity.

[0031] From the conductivity and permittivity values ​​of the medium, further properties of the medium, such as the concentration of a chemical product and / or the degree of crystallinity of a product in solution, can be determined. Particularly preferably, a relationship between the conductivity and permittivity of the medium and the desired concentration or degree of crystallinity is stored in the control and evaluation unit.

[0032] Alternatively, the impedance signal can be a non-periodic signal, in particular a pulse signal. For example, the pulse signal can be a square wave. For evaluation, the impedance measurement signal is transformed from the time domain to the frequency domain, and the total impedance Z is determined from the amplitude of at least one frequency.

[0033] A frequency analysis is preferably also performed when the impedance signal is designed as a periodic signal that has several frequencies, for example as a periodic square wave signal.

[0034] According to a further embodiment of the method according to the invention, the determination of the total impedance Z is carried out with a time delay compared to the acquisition of the primary measured quantity, preferably such that no impedance signal is present at the electrodes during the acquisition of the primary measured quantity. This embodiment prevents the impedance signal and / or the impedance measurement signal from superimposing on the primary measured quantity or the primary measurement signal. In this way, it is ensured that the impedance measurement does not influence the determination of the flow rate of the medium.

[0035] According to a further advantageous embodiment, the determination of the total impedance Z is carried out essentially simultaneously with the determination of the flow rate. In this embodiment as well, it must be ensured that the impedance measurement does not influence the flow rate measurement.

[0036] According to one embodiment, the flow path has a low-pass filter and / or low-pass filter characteristics, and the impedance signal has a frequency that is higher, preferably at least 5 times higher, and particularly preferably at least 10 times higher, than the cutoff frequency of the low-pass filter. This embodiment ensures that the impedance measurement signal is not introduced into the flow path. This virtually eliminates any influence on the flow rate determination. The preferred frequency offset from the cutoff frequency of the low-pass filter optionally depends on the quality factor or order of the low-pass filter. The required frequency offset from the cutoff frequency can be lower for higher-order low-pass filters. If the measurement signals for determining the flow rate are also averaged, frequency components of the impedance measurement signal that pass through the low-pass filter can be averaged out or at least attenuated.

[0037] According to this design, the measurement of the total impedance Z can, in principle, be carried out simultaneously or at a time delay with the acquisition of the primary measured quantity.

[0038] According to a further embodiment, an electronic switch, in particular a semiconductor relay or a JFET, is provided between the electrodes and the impedance path and / or between the electrodes and the flow path, wherein the electronic switch ensures that only the primary measurement signal is routed to the flow path and only the impedance measurement signal is routed to the impedance path. According to this embodiment, the total impedance Z is preferably measured with a time delay compared to the acquisition of the primary measurement.

[0039] Particularly preferably, the flow meter has an analog switch, in particular a multiplexer, between the electrodes and the impedance path and the flow path, wherein the analog switch connects the electrodes to the impedance path when an impedance measurement is performed and wherein the analog switch connects the electrodes to the flow path when the primary measured quantity is detected by the electrodes.

[0040] According to this design, the primary measurement is recorded with a time delay compared to the determination of the total impedance.

[0041] According to a further preferred embodiment, the flow path and the impedance path are identical, i.e., designed as a single component or a common path, into which both the primary measurement signal and the impedance measurement signal are fed as a combined measurement signal, in particular as a superposition of the primary measurement signal and the impedance measurement signal. The impedance unit is part of the processing unit, such that the processing unit determines the total impedance Z from at least one frequency of the combined measurement signal. Furthermore, the processing unit determines the DC offset of the combined measurement signal and calculates the flow rate of the medium from the DC offset. This embodiment has the advantage that a second path for determining the total impedance Z is not required.According to this design, the impedance unit and / or the processing unit and / or an existing amplifier have both a high sampling rate and a high resolution in order to be able to detect both the frequency of the combined measurement signal, which is determined by the frequency of the impedance signal, and the DC component.

[0042] The flow meter is particularly advantageously designed for installation in an at least partially closed circuit. The advantage of the invention is especially evident when installation space is limited, as both the flow rate and the total impedance Z can be reliably measured with a single, space-saving device.

[0043] For example, the flow meter is arranged in a measuring setup in which a chemical product achieves a higher degree of purity through crystallization.

[0044] According to a further embodiment of the method, the medium is a solution of a contaminated chemical product and a solvent, wherein the concentration of the chemical product is determined by measuring the total impedance Z, in particular by evaluating the real part and the imaginary part of the impedance, and wherein optionally the flow rate of the chemical product is determined from the measured flow rate of the solution.

[0045] This process is particularly preferred as part of a chemical product purification process. For this purpose, the product containing impurities is completely dissolved in a feed unit. In a crystallization unit, the product crystallizes, thereby increasing its purity. Subsequently, the crystallized solid is separated from the solution in a separation unit. The remaining solvent is returned to the feed unit. Recycling the remaining solvent, which also contains impurities, is efficient until the impurity concentration exceeds a certain limit. If the impurity level is too high, purification by crystallization no longer achieves a sufficient degree of purity.

[0046] Therefore, it is advantageous to monitor the concentration of the impurity in the solution at various stages of this process. It is also advantageous to monitor the concentration of the crystallized substance.

[0047] In a preparatory step, a relationship is established between the conductivity, permittivity, and concentration of the crystallized fraction of the chemical product or impurity in the solution using known samples, preferably taking the temperature of the medium into account. With this prior process knowledge, the crystallization process and / or the fraction of impurity in the recycled solution can be monitored by measuring the total impedance Z and evaluating the real and imaginary parts.

[0048] If both the concentration of the crystallized solid fraction and the flow rate are determined, the amount of the solid fraction can also be determined.

[0049] In a method for monitoring the crystallization process, where the concentration of the crystallized substance is the measured quantity of interest, the following steps are carried out: As preparation, the medium to be measured in the application is first characterized using known samples.

[0050] In a first preparatory step, the frequency range in which the total impedance Z will later be measured in the actual measurement process is determined, thus avoiding unpredictable influences from the electrochemistry of the interface between the electrodes and the medium. For this purpose, several samples with different concentrations of the crystallized substance are measured. Specifically, a Nyquist plot is created for each sample to identify a frequency range with a particularly high signal-to-noise ratio.

[0051] For this frequency range, a dependence between the complex-valued impedance and the concentration of the crystallized substance is determined by measuring the known samples, so that an impedance profile in the advantageous frequency range is known for different concentrations.

[0052] In this application, i.e., for monitoring the crystallization process, an impedance signal with a frequency from the defined frequency range is applied to the electrodes. Preferably, the frequency of the impedance signal is varied within the advantageous frequency range. For each impedance signal, the impedance response signal is recorded and forwarded as an impedance measurement signal to the impedance unit, which determines the total impedance Z, in particular the real and imaginary parts. From the total impedance Z, in particular from the real and imaginary parts, or from the impedance profile in the measured frequency window, the concentration of the crystallized substance can be determined according to the relationship established in the preparatory step.

[0053] According to one embodiment of the method, the flow meter performs self-optimization with respect to the frequency of the excitation impedance signal. For this purpose, the signal-to-noise ratio is determined at adjacent frequencies while maintaining a constant concentration range of the product being measured. The flow meter then automatically selects the frequency with the best signal-to-noise ratio as the new frequency of the impedance signal.

[0054] Various approaches (algorithms) are conceivable here. For example, the frequency of the impedance signal, starting from the original frequency, can be changed in small, preferably constant, steps in at least one direction (e.g., increasing the frequency) as long as an improvement in the signal-to-noise ratio occurs. Preferably, the frequency is then changed in the other direction (e.g., decreasing the frequency) as long as an improvement in the signal-to-noise ratio occurs compared to the original frequency. The flow meter then selects the frequency with the absolute best signal-to-noise ratio as the new frequency.

[0055] According to a further embodiment, the magnitude of the improvement in the signal-to-noise ratio can be incorporated into the step size of the frequency change. If the original frequency is first changed by a first step in one direction—for example, if it is increased—and the signal-to-noise ratio changes significantly in this step, then the second step of the frequency change in that direction is larger than the first step. For example, the second step could be twice as large as the first step. If the signal-to-noise ratio worsens after the first step or after the second step, the direction in which the frequency is changed is reversed; in particular, the frequency is then decreased. The signal-to-noise ratio measurement in the reverse direction is also performed with an adjusted step size until the signal-to-noise ratio deteriorates.The flow meter then selects the frequency with the absolute best signal-to-noise ratio. This method has the advantage that the optimum can be reached in fewer steps than with the previously described iterative method.

[0056] According to a further embodiment, the signal-to-noise ratio is measured at N points, i.e., for N frequencies, within a predefined frequency range. The N points can be chosen equidistantly. Alternatively, M points per decade can be chosen (logarithmic sampling). Preferably, N > 2, for example, 3. In this way, a frequency is determined at which the signal-to-noise ratio is maximal. This frequency is called the optimal frequency. Once the optimal frequency has been found, the two adjacent frequencies considered in the previous step define a frequency window around the optimal frequency. Within this new frequency window, the signal-to-noise ratio is measured again at X points, i.e., for X frequencies. The number X can correspond to the number N frequencies, or the number of frequencies at which the signal-to-noise ratio is measured can differ.The frequency within this frequency window that exhibits the maximum signal-to-noise ratio is the new optimal frequency. The frequencies adjacent to this new optimal frequency, which were considered in the previous step, define a new frequency window in which the signal-to-noise ratio is measured at Y points, i.e., for Y frequencies. The number Y of these points can correspond to the number N and / or X, or it can differ from the number N and / or X. This process is repeated until the difference between the signal-to-noise ratio of the optimal frequency and the signal-to-noise ratio of an adjacent frequency falls below an upper limit. For example, the difference between the signal-to-noise ratios might then be less than 1%. The advantage here is that the entire relevant frequency range can be sampled in just a few steps.

[0057] If a new frequency is selected for the impedance signal in a given application, this must be taken into account when determining the concentration of the crystallized substance. The relationship between the total impedance Z and the concentration of the crystallized product may change. For example, if the measured impedance signal exceeds a threshold, it can be assumed that the concentration of the crystallized substance has changed. In this case, according to a preferred embodiment, the concentration of the crystallized substance is determined again using the impedance signal frequency previously determined under laboratory conditions, where the dependence of the concentration on the total impedance Z is known.

[0058] According to a further embodiment of the method, it is also possible to determine a full, broadband impedance spectrum in the flow meter.

[0059] Determining the complete impedance spectrum, especially at high frequencies, offers a significant advantage. In the Nyquist plot, the graphical representation of the negative imaginary part versus the real part of the impedance, the points at high frequencies are often arranged in a semicircle. Knowing this, the center point and radius of the semicircle can be determined from at least three measurements. Therefore, only a few measurements are needed to determine the impedance profile. A point on this constructed semicircle has a lower statistical measurement uncertainty than the points from which the semicircle was constructed. The radius corresponds to half the ohmic resistance of the medium. Consequently, the impedance profile within a frequency window can be determined for a specific concentration of a product by measuring only a few frequencies.Determining a complete impedance spectrum can further improve the accuracy of concentration determination.

[0060] According to a second teaching of the present invention, the aforementioned problem is solved by a flow meter in that the control and evaluation unit is designed to carry out a method according to the invention.

[0061] According to a particularly preferred embodiment, the flow meter is a magnetic-inductive flow meter or a vortex flow meter.

[0062] Both flow meters have the advantage that electrodes are already present for detecting a primary measured variable, from which the flow rate of a medium flowing through a measuring tube can be determined. Therefore, it is not necessary to install additional electrodes in the flow meter, which simplifies the design and complexity of a flow meter used to measure both flow rate and total impedance Z.

[0063] The flow meter is particularly preferably designed according to at least one of the embodiments described above.

[0064] In particular, according to one embodiment, the flow path has a particularly high input and output impedance.

[0065] According to one embodiment, the impedance path and the flow path are separate evaluation paths.

[0066] Furthermore, it is advantageous if, alternatively or additionally, an electronic switch, in particular a semiconductor relay or a JFET, is provided, wherein the electronic switch ensures that only the primary measurement signal is directed to the flow path and only the impedance measurement signal is directed to the impedance path.

[0067] Particularly preferably, the flow meter has an analog switch, in particular a multiplexer, between the electrodes and the impedance path and the flow path, wherein the analog switch connects the electrodes to the impedance path when an impedance measurement is performed and wherein the analog switch connects the electrodes to the flow path when the primary measured quantity is detected by the electrodes.

[0068] According to a further preferred embodiment, the flow path and the impedance path are identical, i.e., configured as a single evaluation path into which both the primary measurement signal and the impedance measurement signal are fed as a combined measurement signal, in particular as a superposition of the two signals. The impedance unit is part of the processing unit, and the processing unit determines the total impedance Z from at least one frequency of the combined measurement signal. Furthermore, the processing unit determines the DC offset of the impedance signal and, from the DC offset, determines the flow rate of the medium. This embodiment has the advantage that no second path for determining the total impedance Z is required. According to this embodiment, the impedance unit and / or the processing unit have a high sampling rate in order to be able to acquire both the frequency of the impedance signal and the DC component.

[0069] According to a preferred embodiment, the flow meter has one or more of the structural features described above.

[0070] There are now numerous possibilities for designing and further developing the inventive method and the inventive flow meter. Reference is made to the claims subordinate to the independent claims and to the description of the following drawing. The drawing shows Fig. 1 a first embodiment of a flow meter, Fig. 2 a second embodiment of a flow meter, Fig. 3 a third embodiment of a flow meter, Fig. 4 another embodiment of a flow meter, Fig. 5 a first embodiment of a method for operating a flow meter and Fig. 6 an application according to the invention of the method and the flow meter according to the invention.

[0071] Fig. Figure 1 shows a first embodiment of a flow meter 1, which is designed both for measuring the flow rate of a medium flowing through the measuring tube 3 and for measuring the total impedance Z.

[0072] The illustrated flow meter 1 is a magnetic-inductive flow meter 2. To determine the velocity of the flowing medium, two magnetic coils 4 are provided, which, during operation, generate a magnetic field within the measuring tube 3. In the case of a conductive medium, this results in a charge separation within the medium, which can be measured as a voltage between the electrodes 5. The measured voltage is therefore, in this embodiment, the primary measured quantity, which is transmitted as the primary measurement signal from the electrodes 5.

[0073] The flow meter 1 has a control and evaluation unit 6 for evaluating the measured signal.

[0074] The control and evaluation unit 6 comprises two separate paths for evaluating the primary measured quantity and for determining the total impedance Z, which is composed of the impedance of the medium Z1 and the impedance Z2 of the transition between the electrodes 5 and the medium.

[0075] The primary measurement signal is fed into a flow path 7 for further evaluation. An intrinsically safe barrier 8 is initially present in the flow path 7, which limits the energy fed into the flow path 7. For example, the intrinsically safe barrier is a low-pass filter or has low-pass characteristics.

[0076] The primary measurement signal is then amplified in an amplifier 9 before being digitized in an AD converter 10 and fed to the evaluating processing unit 11.

[0077] In addition to the flow path 7, an impedance path 12 with an impedance unit 13 is provided, wherein the impedance unit 13 is designed to measure the total impedance Z. For this purpose, the impedance unit 13 is designed such that it applies a periodic or pulsed signal to the electrodes 5 during operation.

[0078] If the periodic signal is an alternating current, the voltage applied to the electrodes 5 is detected as the impedance response signal and forwarded as an impedance measurement signal to the impedance path 12. During operation, the total impedance Z is determined from the known alternating current and the measured voltage by the impedance unit 13.

[0079] In the computing unit 11, further properties of the medium can be determined from the total impedance Z, in particular from the real and imaginary parts. For this purpose, a relationship between the total impedance Z, especially between the real and imaginary parts, and the target variable to be determined is advantageously provided in the computing unit.

[0080] This illustrates a flow meter 1 which can advantageously determine both the flow rate and the total impedance Z, which comprises the impedance Z1 of the medium and the impedance Z2 of the junction between the electrodes 5 and the medium. The flow meter 1 has the significant advantage that the electrodes 5 already used for flow measurement can be used to determine the total impedance Z, resulting in a particularly space-saving design for the combined flow meter 1.

[0081] Fig. Figure 2 shows an embodiment of a flow meter 1, which is also designed for both flow measurement and impedance measurement. The flow meter 1 shown is a vortex flow meter 14. Electrodes 5 are provided to detect the pressure fluctuations of the vortices detaching from the obstruction 15.

[0082] The signal tapped at electrodes 5 is forwarded as the primary measurement signal to a flow path 7 for determining the flow rate. Here, the signal is first passed through an intrinsically safe barrier 8, then amplified and digitized in an amplifier stage 9.

[0083] Furthermore, an impedance path 12 with an impedance unit 13 is provided, the impedance unit 13 being designed to determine the total impedance Z. Specifically, the impedance unit 13 is configured such that, during operation, it applies an alternating current to the electrodes 5, and the impedance response signal is detected by means of the electrodes 5 and forwarded to the impedance path 12 as an impedance measurement signal. The total impedance Z is determined in the impedance unit 13 from the current and voltage values.

[0084] Furthermore, the ones in the Fig. 1 and Fig. The two flow meters 1 shown are designed such that the impedance measurement does not influence the flow measurement. For this purpose, the control and evaluation units 6 shown each have a separate flow path 7 and impedance path 12, respectively.

[0085] Fig. Figure 3 shows another embodiment of a flow meter 1, which is designed to measure both the flow rate and the total impedance Z.

[0086] In the illustrated embodiment, the flow path 7 and the impedance path 12 are configured as a single, combined path into which the primary measurement signal and the impedance measurement signal are introduced. The combined path 7, 12 includes a processing unit 11, which also incorporates the impedance unit 13. Thus, the flow and impedance path 7, 12 is designed for both measuring and determining the flow rate and measuring and determining the total impedance Z. During operation, an alternating current or voltage is applied to the electrodes 5 by means of the processing unit 11 to determine the impedance. Furthermore, the voltage across the electrodes 5 or the current flowing through the medium is measured. The processing unit 11 is configured to detect and evaluate both the phase and amplitude of the combined measurement signal as well as the DC offset.The processing unit determines the total impedance Z from the phase and amplitude. From the DC offset, the processing unit determines the flow rate of the medium. The illustrated embodiment has the significant advantage that both the total impedance Z and the flow rate of the medium can be determined simultaneously using a single evaluation path.

[0087] Fig. Figure 4 shows another embodiment of a flow meter 1 in the form of a magnetic-inductive flow meter 2, which enables integrated impedance measurement. As in Fig. The flow meter 1 shown has two separate evaluation paths in the control and evaluation unit 6, namely a flow path 7 and an impedance path 12.

[0088] To ensure that the impedance measurement does not affect the flow measurement, an analog switch 16 is also provided, which ensures that the primary measurement signal is routed to the flow path 7 and that the impedance measurement signal is routed to the impedance path 12.

[0089] With each of the in the Fig. The flow meters 1 shown in Figures 1 to 4 can be used to determine further properties, for example of the medium, via the measured total impedance Z. For this purpose, a relationship between the target variable to be determined and the measured total impedance Z is stored in the control and evaluation unit 6.

[0090] The relationship between the total impedance Z and the target variable can be empirically determined, for example, in preparatory measurement series.

[0091] In particular, if both the real part of the impedance Z and the imaginary part of the impedance Z are taken into account in the functional dependence on the target variable, the target variable can be determined with particular accuracy.

[0092] Fig. Figure 5 shows a first embodiment of a method 17 for operating a flow meter 1, wherein the flow meter 1 is operated as shown in Figure 5. Fig. 1 is shown, formed.

[0093] In a first step 18 of the method, a periodic impedance signal is applied to the electrodes 5. In the illustrated embodiment, the impedance signal is an alternating current whose frequency is varied.

[0094] In a subsequent step 19, the voltage that develops between the electrodes 5 is recorded at the electrodes 5 as an impedance response signal. In a subsequent step 20, this voltage is forwarded as an impedance measurement signal via the impedance path 12 to the impedance unit 13.

[0095] From the impedance measurement signal, the impedance unit 13 determines the total impedance Z, which comprises the impedance Z1 of the medium and the impedance Z2 of the interface. In particular, the impedance unit 13 determines both the real part and the imaginary part of the impedance Z.

[0096] Simultaneously or at different times, the electrodes 5 detect the voltage formed by charge separation in the medium as the primary measured quantity and transmit 23 this as the primary measurement signal into the flow path 7.

[0097] In a next step 24, the computing unit 11 determines the flow rate of the medium from the primary measurement signal; for example, the computing unit determines the flow velocity or the volumetric flow rate.

[0098] The method 17 shown has the advantage that both the flow rate of a medium and the total impedance Z, which includes the impedance Z1 of the medium, can be determined with one measuring device.

[0099] Determining the total impedance Z, in particular determining the real part and the imaginary part of the total impedance Z, can be advantageously used to determine a target variable that characterizes, for example, the medium, provided that a relationship between the total impedance Z and the target variable is stored in the control and evaluation unit.

[0100] For example, the determination of the total impedance Z can be used to monitor a crystallization process within a recrystallization purification procedure. For this purpose, the control and evaluation unit contains a relationship between the total impedance Z, in particular between the real and imaginary parts, and the concentration of a crystallizing substance.

[0101] In Fig.Figure 6 shows an embodiment of a measuring setup for such a cleaning process. The cleaning device comprises a feed unit 25 in which the chemical product to be cleaned is dissolved in a solvent, a crystallization unit 26 in which the product to be cleaned is recrystallized to achieve a higher degree of purity, a separation unit 27 in which the crystallized product is separated from the solvent still containing impurities, and a return unit 28 through which the remaining solvent is returned to the feed unit 25.

[0102] The flow meter 1 is arranged at the crystallization unit 26 to monitor the crystallization process. Furthermore, the control and evaluation unit 6 of the flow meter 1 contains a relationship between the measured total impedance Z and the concentration of the crystallized chemical product, which was empirically determined prior to the measurement.

[0103] In order to establish a relationship between the total impedance Z and the concentration of the crystallized chemical product, the total impedance of known samples of the medium to be measured is determined in preparatory measurements.

[0104] First, the frequency range that is advantageous for determining the total impedance Z is determined; that is, the frequency range in which unpredictable influences from the electrochemistry at the interface between the electrodes 5 and the medium are minimized. For this purpose, a Nyquist diagram is created for various concentrations. From the Nyquist diagram, the frequency range in which the highest possible signal-to-noise ratio is expected is identified.

[0105] For various frequencies within this frequency range, a relationship between the complex-valued total impedance Z and the concentration of the crystallized substance is subsequently determined by series of measurements. In particular, a relationship is established between the real and imaginary parts of the total impedance Z.

[0106] Since temperature affects the overall impedance Z, either temperature compensation must be implemented or the measuring point must be specifically temperature-controlled to prevent temperature fluctuations from distorting the measurement.

[0107] Using this established relationship, the concentration of the crystallized substance can then be monitored in the illustrated application by determining the total impedance Z. If the flow rate is also determined, the flow rate of the chemical product being measured can additionally be ascertained. Reference sign 1 flow meter 2 Magnetic Inductive Flow Meter 3 measuring tube 4 magnetic coil 5 electrode 6 Control and evaluation unit 7 Flow path 8 Intrinsically safe barrier 9 amplifiers 10 AD converters 11 Calculation unit 12 Impedance path 13 Impedance unit 14 Vortex flowmeter 15 interfering bodies 16 Analog Switches 17 methods for operating a flow meter 18 Applying an impedance signal to the electrodes 19. Acquiring an impedance measurement signal through the electrodes 20. Forwarding the impedance measurement signal to the impedance unit 21 Determination of the impedance Z by the impedance unit 22. Acquisition of the primary measurement signal by the electrodes 23. Forwarding the primary measurement signal to the flow path 24 Determination of the flow rate through the computing unit 25 Feed unit 26 Crystallization units 27 Separation unit 28 Return unit QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] WO 2008 / 113774 A1

[0004] EP 4 071 447 A1

[0005] EP 3 594 635 B1

[0006]

Claims

[1] Method (17) for operating a flow meter (1) wherein the flow meter (1) has at least one measuring tube (3) wherein the measuring tube (3) is through which a medium flows, wherein the flow meter (1) further comprises at least one sensor unit comprising at least two electrodes (5) for detecting a primary measured quantity from which the flow rate of the medium is determined, and a control and evaluation unit (6), wherein the electrodes (5) are arranged on the measuring tube (3) such that they are in contact with the medium during operation, wherein the control and evaluation unit (6) comprises a flow path (7), an impedance path (12) which includes an impedance unit (13) for generating an impedance signal and for evaluating an impedance measurement signal, and a computing unit (11), wherein the electrodes (5) are connected to both the flow path (7) and the impedance path (12), characterized by , that the procedure (17) comprises the following steps: - Applying (18) an impedance signal to the electrodes (5), - Detection (19) of an impedance response signal by the electrodes (5), - Forwarding (20) the impedance response signal as an impedance measurement signal to the impedance unit (13), - Determination (21) of the total impedance Z by the impedance unit (13), - Detection (22) of the primary measured quantity by the electrodes (5), - Forwarding (23) the primary measured quantity as the primary measurement signal to the flow path (7), - Determination (24) of the flow rate through the computing unit (11) from the primary measurement signal. [2] Method (17) according to claim 1, characterized by, that the frequency of the impedance signal is chosen such that the unpredictable influence of the electrochemistry at the interface between the electrodes (5) and the medium on the impedance measurement signal can be reduced or substantially avoided. [3] Method (17) according to claim 1 or 2, characterized by , that the impedance signal is a periodic signal, in particular a sinusoidal signal, a superposition of at least two sinusoidal signals or a square wave signal. [4] Method according to claim 3, wherein the impedance signal is a sinusoidal signal, characterized by , that the frequency of the impedance signal is varied within a defined frequency window. [5] Method (17) according to claim 1 or 2, characterized bythat the impedance signal is a non-periodic signal, in particular a pulse signal, wherein the impedance measurement signal is transformed from the time domain to the frequency domain and wherein the total impedance Z is determined from at least one characteristic frequency of the frequency spectrum. [6] Method (17) according to any one of claims 1 to 5, characterized by , that both the real part and the imaginary part of the total impedance Z are determined. [7] Method (17) according to any one of claims 1 to 6, characterized by , that the measurement of the total impedance Z is carried out with a time delay compared to the acquisition of the primary measured quantity, preferably in such a way that no impedance signal is present at the electrodes during the acquisition of the primary measured quantity. [8] Method (17) according to any one of claims 1 to 6, characterized by , that the measurement of the total impedance Z occurs essentially simultaneously with the acquisition of the primary measured quantity. [9] Method (17) according to any one of claims 1 to 8, characterized by , that the flow path (7) has a low-pass filter and that the impedance signal has a frequency that is larger, preferably at least 5 times larger, particularly preferably at least 10 times larger than the cutoff frequency of the low-pass filter. [10] Method (17) according to any one of claims 1 to 9, characterized by , that an electronic switch, in particular a semiconductor relay or a JFET, is provided between the electrodes (5) and the impedance path (12) and / or between the electrodes (5) and the flow path (7), wherein the electronic switch ensures that only the primary measurement signal is directed to the flow path (7) and only the impedance measurement signal is directed to the impedance path (12). [11] Method (17) according to any one of claims 1 to 10, characterized by, that an analog switch (16), in particular a multiplexer, is provided between the electrodes (5) and the impedance path (12) and the flow path (7), wherein the analog switch (16) connects the electrodes (5) to the impedance path (12) when an impedance measurement is performed and wherein the analog switch (16) connects the electrodes (5) to the flow path (7) when the primary measured quantity is detected by the electrodes (5). [12] Method (17) according to any one of claims 1 to 11, characterized by, that the flow path (7) and the impedance path (12) are designed as a common path into which both the primary measurement signal and the impedance measurement signal are fed as a combined measurement signal, wherein the impedance unit (13) is part of the computing unit (11) and wherein the computing unit (11) determines the total impedance Z from at least one frequency of the combined measurement signal and that the computing unit (11) determines the DC offset of the combined measurement signal and determines the flow rate of the medium from the DC offset. [13] Method (17) according to any one of claims 1 to 12, characterized by, that the medium is a solution of a contaminated chemical product and a solvent, wherein the chemical product is at least partially in crystalline form, wherein a relationship between the total impedance Z and the concentration of the crystallized chemical product, in particular between the real part and the imaginary part of the total impedance Z and the concentration of the crystallized product, is stored in the control and evaluation unit (6), wherein the concentration of the crystallized chemical product in the solution is determined by measuring the total impedance Z, in particular by evaluating the real part and the imaginary part of the total impedance Z, and optionally, the flow rate of the chemical product is determined from the measured flow rate of the solution. [14] Method according to any one of claims 1 to 13, characterized by, that the medium is a solution of a solvent containing impurities, that in the control and evaluation unit (6) a relationship between the total impedance Z and the concentration of the impurity, in particular between the real part and the imaginary part of the total impedance Z and the concentration of the impurity, is stored, wherein the concentration of the impurity in the solution is determined by measuring the total impedance Z, in particular by evaluating the real part and the imaginary part of the total impedance Z. [15] Method (17) according to any one of claims 1 to 14, characterized by, that the control and evaluation unit of the flow meter determines the signal-to-noise ratio at at least two frequencies adjacent to the frequency of the impedance signal for self-optimization with regard to the frequency of the excitation impedance signal at a constant concentration of the product to be measured, and that the control and evaluation unit selects the frequency with the maximum signal-to-noise ratio as the new frequency for the impedance signal. [16] Flow meter (1) for determining the flow rate of a medium flowing through a measuring tube (3), wherein the flow meter (1) has a measuring tube (3), wherein the flow meter (1) further comprises at least one sensor unit comprising at least two electrodes (5) for detecting a primary measured quantity from which the flow rate of the medium is determined, and a control and evaluation unit (6), wherein the electrodes (5) are arranged on the measuring tube (3) such that they are in contact with the medium during operation, wherein the control and evaluation unit (6) comprises a flow path (7), an impedance path (12) comprising an impedance unit (13) for generating an impedance signal and for evaluating an impedance measurement signal, and a computing unit (11), wherein the computing unit (11) determines the flow of the medium, wherein the electrodes (5) are connected to both the flow path (7) and the impedance path (12), characterized by , that the control and evaluation unit (6) is designed to carry out a method (17) according to one of claims 1 to 15. [17] Flow meter (1) according to claim 16, characterized by , that the flow meter (1) is a magnetic inductive flow meter (2) or a vortex flow meter (14). [18] Flow meter (1) according to one of claims 16 and 17, characterized by , that the flow meter is designed according to one of claims 1 to 15, insofar as the claims relate to the structural design of the flow meter.

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