Method for operating an ultrasonic flow meter and corresponding ultrasonic flow meter

Abstract

A method (1) for operating an ultrasonic flow meter (2) for measuring the flow rate through a measuring tube (4) through which a medium (3) flows is presented and described, wherein the ultrasonic flow meter (2) comprises at least one transmitting ultrasonic transducer (5) for transmitting ultrasonic signals (6) and at least one receiving ultrasonic transducer (7) for receiving ultrasonic signals (6) and a control and evaluation unit (8), wherein the ultrasonic transducers (5, 7) are arranged such that they realize an ultrasonic measurement path (9) in the medium (3) and wherein the control and evaluation unit (8) controls the transmitting ultrasonic transducer (5) such that it emits the ultrasonic signal (6).The receiving ultrasonic transducer (7) receives the emitted ultrasonic signal (6), and the control and evaluation unit (8) determines, during measurement operation (14), at least an indirect value for the flow rate (Vp) of the medium (3) through the measuring tube (4) from a determined signal transit time (t_sig) of the ultrasonic signal (6) by evaluating the emitted and received ultrasonic signals (6). The method can react flexibly to changing measurement conditions by the control and evaluation unit (8) controlling the transmitting ultrasonic transducer (5) so that a broadband ultrasonic signal (USb,tx) is emitted, by determining an optimal measurement frequency range (M_opt) in an evaluation step (10), by frequency-filtering (11) the received broadband ultrasonic signal (USb,rx) into several measurement frequency ranges (M) of the received broadband ultrasonic signal (USb,rx), by furthermore calculating (12) at least one quality value (Q) from the frequency-filtered ultrasonic signal (USb,rx,f) of the corresponding measurement frequency range (M) for each of the measurement frequency ranges (M) and determining the measurement frequency range (M) that achieves the highest quality value (Q) as the optimal measurement frequency range (M_opt), and by using a signal propagation time (t_sig) when determining the at least indirect value for the flow rate (Vp).which was determined after frequency filtering (11) of the received broadband ultrasound signal (USb,rx) from the optimal measurement frequency range (M_opt).

Description

[0001] The invention relates to a method for operating an ultrasonic flow meter for measuring the flow rate through a measuring tube through which a medium flows, wherein the ultrasonic flow meter comprises at least one transmitting ultrasonic transducer for transmitting ultrasonic signals and at least one receiving ultrasonic transducer for receiving ultrasonic signals and a control and evaluation unit, wherein the ultrasonic transducers are arranged such that they realize an ultrasonic measuring path in the medium and wherein the control and evaluation unit controls the transmitting ultrasonic transducer such that it emits the ultrasonic signal.The receiving ultrasonic transducer receives the emitted ultrasonic signal, and the control and evaluation unit, during measurement operation, determines at least an indirect value for the flow rate of the medium through the measuring tube by evaluating the emitted and received ultrasonic signals from a determined signal transit time of the ultrasonic signal. Furthermore, the invention also relates to such an ultrasonic flow meter.

[0002] Flow measurement using ultrasonic waves has been known for a long time. Regardless of the specific measurement method employed (e.g., transit-time measurement, transit-time difference measurement (with and against the flow direction), frequency measurement / Doppler effect), flow measurement is always based on the propagation of ultrasonic waves in the medium flowing through the measuring tube, the flow velocity of which is to be determined. The (average) flow velocity of the medium along the ultrasonic measurement path can be deduced from the signal transit time of the ultrasonic signal, and thus indirectly also the flow rate of the medium through the measuring tube.

[0003] Typically, the transmitting ultrasonic transducers are excited with a very narrowband signal, preferably at a single, specific frequency. This offers advantages in terms of energy consumption (eigenvalues ​​of piezoelectric actuators / sensors in the ultrasonic transducer) and signal processing; by selecting a specific frequency, it is also possible to selectively influence, for example, the shape and attenuation of the transmitted ultrasonic signal. This approach becomes problematic if the boundary conditions under which the specific frequency was selected change, such as the speed of sound in the medium. It is also generally known to excite a broadband ultrasonic signal for ultrasonic flow measurement, for example, from US 2022 / 0120596 A1 and DE 198 15 199 A1.

[0004] The object of the present invention is to further develop the method for operating an ultrasonic flow meter and the ultrasonic flow meter described above in such a way that it is possible to react robustly to changing operating conditions during measurement.

[0005] In the previously described method, the derived problem is initially solved by the control and evaluation unit controlling the transmitting ultrasound transducer in such a way that a broadband ultrasound signal is emitted.

[0006] In an evaluation step, an optimal measurement frequency range is determined by frequency-filtering the received broadband ultrasound signal into several measurement frequency ranges. Furthermore, in this evaluation step, at least one quality value is calculated for each of the frequency-filtered ultrasound signals within the corresponding measurement frequency range, and the measurement frequency range that achieves the highest quality value is determined as the optimal measurement frequency range.

[0007] This procedural step clarifies what is meant by a broadband ultrasound signal. The broadband ultrasound signal must, in any case, contain frequency components over a certain frequency range – either continuously or distributed across specific areas – so that the required frequency filtering into multiple measurement frequency ranges is meaningfully possible, meaning that frequency components can be present in the areas under investigation.

[0008] When determining the at least indirect value for the flow rate, a signal transit time is used - i.e., in regular measurement operation - which has been determined from the optimal measurement frequency range after frequency filtering of the received broadband ultrasound signal.

[0009] Therefore, a broadband ultrasound signal is always used, both in the evaluation step, where the optimal measurement frequency range is determined, and in normal measurement operation, where the broadband ultrasound signal is frequency-filtered to the optimal measurement frequency range and the signal propagation time is then determined from the frequency-filtered ultrasound signal. By consistently using a broadband ultrasound signal, the emitted ultrasound signal always contains a suitable frequency, which is advantageous even under changing measurement conditions. Such changes in measurement conditions are not addressed by a modified ultrasound signal emitted by the transmitting ultrasound transducer, but rather simply by an adjusted frequency filtering of the received broadband ultrasound signal, because the optimal measurement frequency range has changed.

[0010] According to an advantageous embodiment of the method, the evaluation step for determining an optimal measurement frequency range is performed with each determination of the at least indirect value for the medium's flow rate. This allows for the fastest response to changing measurement conditions; however, the effort is considerable, both in terms of hardware (e.g., multiple frequency filters), computational effort, and therefore also energy consumption, which can be a problem, especially with two-wire devices. An alternative embodiment of the method therefore provides that the evaluation step for determining an optimal measurement frequency range is performed after a plurality of determinations of the at least indirect value for the medium's flow rate, or that the evaluation step for determining an optimal measurement frequency range is triggered by an external signal from the ultrasonic flowmeter.This allows for large evaluation intervals to be implemented, or the evaluation step can be triggered as needed, for example if there is a suspicion (changed process parameters) that the measurement situation has changed.

[0011] There are several ways to generate the broadband ultrasound signal. In one variant, a pulsed signal is generated (as close as possible to a Dirac impulse with respect to the corresponding frequency spectrum and its subsequent evaluation). In a preferred variant of the method, the broadband ultrasound signal is generated by exciting the transmitting ultrasound transducer with a superposition of several periodic time signals of different frequencies, in particular where the periodic time signals are harmonic signals. This method is advantageous because, in particular, harmonic periodic time signals can be generated very easily, and the measurement frequency ranges and the signal amplitudes within these ranges can be defined by selecting these periodic time signals.

[0012] In an alternative further development of the method, the broadband ultrasound signal is generated by exciting the transmitting ultrasound transducer with a square wave signal or with a periodic sequence of square waves, in particular where the fundamental frequency of the periodic square wave corresponds to a fundamental mode of an oscillation of the transmitting ultrasound transducer. This type of excitation is also easy to implement, and frequency components at the odd multiples of the frequency of the periodic square wave signal are automatically generated.

[0013] In a further preferred embodiment of the method, the received broadband ultrasound signal is frequency-filtered into several measurement frequency ranges by using at least one analog or one digital bandpass filter. If only one bandpass filter is used, its center frequency must be configurable so that the several measurement frequency ranges can be examined sequentially with successive broadband ultrasound signals during the evaluation step. If the signal processing is implemented digitally, i.e., as a sampling system, as is the case, for example, with modern digital signal processors, then implementing the bandpass filter as a finite impulse response (FIR) or infinite impulse response (IIR) filter is advantageous.

[0014] Within the scope of the invention, various types of quality values ​​or methods of determining a quality value for a measurement frequency range from the respective frequency-filtered ultrasound signal of the associated measurement frequency range have proven to be meaningful and therefore suitable.

[0015] A preferred refinement of the method is characterized by determining the quality value for a measurement frequency range by calculating the signal propagation time of the frequency-filtered ultrasound signal and determining its propagation time deviation from a reference propagation time, where a smaller propagation time deviation corresponds to a higher quality value. Depending on the application, the reference propagation time can be obtained in different ways. For example, the reference propagation time can be a signal propagation time from a predetermined measurement frequency range, or it can be determined as the average of signal propagation times from predetermined measurement frequency ranges, such as higher-frequency measurement frequency ranges, where smaller signal propagation time differences are expected. However, it can also be advantageous to calculate the average of signal propagation times from all measurement frequency ranges as the reference propagation time.

[0016] In another development of the method, the quality value for a frequency range of the received broadband ultrasound signal is determined by calculating a signal deviation within the considered time interval between the received, frequency-filtered broadband ultrasound signal and a corresponding reference signal, where a smaller signal deviation corresponds to a higher quality value. This criterion thus focuses on the similarity of the signal waveform. It has proven advantageous to use prominent points in the signal waveforms for comparison within the considered time interval, such as maximum peaks in harmonic signals—in other words, the points in the signal waveforms that would define an envelope. In this context, it is useful to calculate the signal deviation by comparing corresponding oscillation amplitudes.

[0017] In a further advantageous embodiment of the method, the quality value for a frequency range of the received broadband ultrasound signal is determined by calculating at least one frequency deviation in the considered time interval between the received, frequency-filtered broadband ultrasound signal and a corresponding reference signal, where a smaller frequency deviation corresponds to a higher quality value. For this purpose, it is necessary to perform a frequency analysis of the frequency-filtered ultrasound signal in the considered time interval. In a preferred embodiment of the method, it has proven advantageous to calculate the instantaneous signal frequencies of the compared signals in the time domain by applying the Hilbert transform to the signals, in particular where the quality value is the mean of several of the calculated frequency deviations in the considered time interval.

[0018] A further embodiment of the method is characterized in that the quality value for a frequency range of the received broadband ultrasound signal is determined by calculating at least one signal-to-noise ratio of the received broadband ultrasound signal in the considered time interval, where a higher signal-to-noise ratio corresponds to a higher quality value. In an advantageous implementation of the method, the signal-to-noise ratio is calculated from the peak-to-peak value of the pure noise signal and the peak-to-peak value of the noisy useful signal in the considered time interval.

[0019] The task is also solved by the previously described ultrasonic flow meter, whose control and evaluation unit is designed in such a way that the ultrasonic flow meter is able to carry out the previously described procedure during operation.

[0020] In detail, there are numerous possibilities for designing and further developing the inventive method for operating an ultrasonic flow meter and the corresponding ultrasonic flow meter. Reference is made, on the one hand, to the claims subordinate to the independent claims, and on the other hand, to the following description of exemplary embodiments in conjunction with the drawing. The drawing shows Fig. 1 schematically a method for operating an ultrasonic flow meter and an ultrasonic flow meter that performs this method, Fig. 2 schematically a method for operating an ultrasonic flow meter in which a quality value and from this an optimal measurement frequency range are determined from a broadband ultrasonic signal, Fig. 3 schematically a method for operating an ultrasonic flow meter in which a quality value is determined based on a signal transit time of the frequency-filtered ultrasonic signal, Fig. 4 schematically a method for operating an ultrasonic flow meter in which a quality value is determined based on a signal deviation in the considered time interval between the received, frequency-filtered broadband ultrasonic signal and a corresponding reference signal, Fig. 5 schematically a method for operating an ultrasonic flow meter in which a quality value is determined by calculating a frequency deviation in the considered time interval between the received, frequency-filtered broadband ultrasonic signal and a corresponding reference signal and Fig. 6 schematically a method for operating an ultrasonic flow meter in which a quality value is determined by calculating at least one signal / noise ratio of the received broadband ultrasonic signal in the considered time interval.

[0021] The figures schematically depict various aspects of method 1 for operating an ultrasonic flow meter 2, as well as of the ultrasonic flow meters 2 operating method 1.

[0022] Fig. Figure 1 shows a method 1 relating to the measuring operation 14 of an ultrasonic flowmeter 2, in which a flow rate Vp of a medium 3 is determined as it flows through a measuring tube 4 of the ultrasonic flowmeter 2. The ultrasonic flowmeter 2 has a transmitting ultrasonic transducer 5 for transmitting ultrasonic signals 6 and a receiving ultrasonic transducer 7 for receiving ultrasonic signals 6, as well as a control and evaluation unit 8. The ultrasonic transducers 5, 7 are arranged such that they create an ultrasonic measuring path 9 in the medium 3.The control and evaluation unit 8 controls the transmitting ultrasonic transducer 5 so that it emits the ultrasonic signal 6, the receiving ultrasonic transducer 7 receives the emitted ultrasonic signal 6, and the control and evaluation unit 8 determines, in the measurement operation 14 shown here, the value for the flow rate Vp of the medium 3 through the measuring tube 4 from a determined signal transit time t_sig of the ultrasonic signal 6 by evaluating emitted and received ultrasonic signals 6.

[0023] It is known to excite the ultrasound signal 6 with a very narrowband frequency or even a single frequency for the reasons mentioned in the general description. The excitation frequency is then selected and fixed, for example, depending on the properties of the medium, the speed of sound in the medium, the frequency-dependent attenuation of ultrasound signals in the medium, etc. This approach is inflexible with regard to significant changes in the measurement conditions, for example, due to a change in the medium and a corresponding change in the speed of sound in the medium, which can have a considerable influence on the measurement, especially the measurement quality.

[0024] In the Fig. Figures 2 to 6 show methods 1 for operating an ultrasonic flow meter 2 and corresponding ultrasonic flow meters 2 which make it possible to react flexibly and robustly to changed measurement conditions, and thus to maintain the quality of the measurement.

[0025] The methods 1 and ultrasonic flow meters 2 shown have in common that the control and evaluation unit 8 controls the transmitting ultrasonic transducer 5 in such a way that a broadband ultrasonic signal USb,tx is emitted ( Fig. 2).

[0026] In an evaluation step 10, an optimal measurement frequency range M_opt is determined by frequency-filtering the received broadband ultrasound signal USb,rx 11 into several measurement frequency ranges M1-M4 of the received broadband ultrasound signal USb,rx. For several of the measurement frequency ranges—in the illustrated case, for all measurement frequency ranges M1-M4—a quality value Q1-Q4 is calculated from the frequency-filtered ultrasound signal USb,rx,f of the corresponding measurement frequency range M1-M4 12. Of course, multiple quality values ​​can also be calculated from the frequency-filtered ultrasound signal USb,rx,f of the corresponding measurement frequency range M1-M4 12. The measurement frequency range M that achieves the highest quality value Q is determined as the optimal measurement frequency range M_opt; in the illustrated embodiment, this is the quality value Q2 = max(Qi), so that the corresponding measurement frequency range M2 is the optimal measurement frequency range M_opt.

[0027] When determining the value for the flow rate Vp, the following is then used in measurement operation 14 (lower section in Fig. 2) A signal propagation delay t_sig is used, which was determined after frequency filtering 11 of the received broadband ultrasound signal USb,rx from the optimal measurement frequency range M_opt = M2. Thus, a broadband ultrasound signal USb,tx is transmitted continuously, so a broadband ultrasound signal USb,rx is always received. This applies to evaluation step 10 as well as to measurement operation 14. Once the optimal measurement frequency range M_opt has been found, only this single frequency filtering 11 needs to be performed in measurement operation 14.

[0028] At the in Fig. In the two described methods 1 and ultrasonic flowmeter 2, after evaluation step 10 is performed to determine the optimal measurement frequency range M_opt, a plurality of flow measurements are carried out in measurement mode 14. In other variants of method 1, which are not shown here, evaluation step 10 is performed for each flow measurement in measurement mode 14, which is relatively complex and places high demands on signal processing.

[0029] Signal processing in Fig. 2. The process is digital: the received broadband ultrasound signal USb,rx is sampled at a high frequency, and the multitude of samples is then frequency-filtered 11 into the measurement frequency ranges M1-M4 using digital bandpass filters 13. A separate bandpass filter 13 is implemented for each measurement frequency range M1-M4, allowing the frequency filtering 11 to be performed simultaneously. The bandpass filters 13 are implemented as finite impulse response (FIR) filters. Due to the parallel execution of the bandpass filters 13, the implementation of the method is fast, but also technically complex.

[0030] In another implementation, not shown here, only a single bandpass filter 13 is used, the center frequency of which is adjustable within the measurement frequency ranges (M1-M4) to be covered. The frequency filtering 11 into the various measurement frequency ranges M1-M4 must then be performed sequentially, as must the associated calculation of the quality values ​​Q1-Q4. Either sequentially received ultrasound signals USb,rx are used for this purpose, or a sampled broadband ultrasound signal USb,rx is stored and processed multiple times in succession. This implementation is more time-consuming, but technically much simpler, especially since only a single bandpass filter 13 is required for frequency filtering 11 during measurement operation 14, namely for frequency filtering 11 into the optimal measurement frequency range M_opt.

[0031] In procedure 1 according to Fig. 2. The broadband ultrasound signal USb,tx is generated by exciting the transmitting ultrasound transducer 5 with a periodic square wave signal sequence 15, wherein the fundamental frequency of the periodic square wave signal sequence 15 corresponds to a fundamental mode of an oscillation of the transmitting ultrasound transducer 5. The ultrasound signal 6 thus generated contains frequency components with frequencies that are odd multiples of the fundamental frequency of the periodic square wave signal sequence 15 and is therefore clearly broadband.

[0032] In embodiments of method 1 not shown here, the broadband ultrasound signal USb,tx is generated by exciting the transmitting ultrasound transducer 5 with a superposition of several periodic time signals of different frequencies, preferably harmonic signals. The method is easy to implement and signals with defined frequencies and independently determinable amplitudes can be generated.

[0033] Fig. Figure 3 shows a method 1 and an ultrasonic flowmeter 2, in which the quality value Q for a measurement frequency range M is determined by determining a signal propagation delay t_sig,i (for i = 1...4) of the frequency-filtered ultrasonic signal USb,rx,f and a propagation delay deviation Δt_sig,i from a reference propagation delay t_sig,ref, where a smaller propagation delay deviation Δt_sig,i corresponds to a higher quality value Qi. In this case, the reference propagation delay t_sig,ref is calculated as the mean value mean(t_sig,i) of signal propagation delays t_sig,i from all measurement frequency ranges M1-M4. In the case shown, the quality value Q3 of the measurement frequency range M3 is the highest of all quality values ​​Q, so M3 is the optimal measurement frequency range Mopt.

[0034] Fig. Figure 4 shows a method 1 in which the quality value Q for a measurement frequency range M of the received broadband ultrasound signal USb,rx is determined by calculating a signal deviation ΔUSb in the considered time interval, namely between the received, frequency-filtered broadband ultrasound signal USb,rx,f (solid line) and a corresponding reference signal USref (dashed line), where a smaller signal deviation ΔUSb corresponds to a higher quality value Q. The waveforms of the reference signal USref were recorded here during the calibration of the ultrasonic flowmeter 2 at the factory under controlled reference conditions.

[0035] For the two measurement frequency ranges M1 and M2, corresponding curves are shown, namely, on the one hand, the frequency-filtered curve profiles with several oscillations in the considered time domain, and on the other hand, the corresponding envelopes 16 of the higher-frequency curve profiles mentioned first.

[0036] The quality values ​​Q1 and Q2 were calculated using a signal deviation delta_USb by comparing corresponding vibration amplitudes A, B, C, D, and E of the received, frequency-filtered broadband ultrasound signal USb,rx,f (solid line) and the corresponding reference signal USref (dashed line). In the case shown, the quality value Q is calculated for each measurement frequency range (M1, M2) according to the following equation:

[0037] N amplitudes are considered (A, B, C, D, E), and the respective waveforms of the received, frequency-filtered broadband ultrasound signal USb,rx,f and the corresponding reference signal USref are normalized to their maximum peaks. The greater the deviations, the lower the calculated quality value Q in the respective measurement frequency range M.

[0038] The amplitude values ​​labeled A, B, C, D, and E in the measurement frequency range M1 are 0.508, 0.840, 1.00, 0.865, and 0.587 for the received, frequency-filtered broadband ultrasound signal USb,rx,f and 0.308, 0.725, 1.00, 0.864, and 0.561 for the corresponding reference signal USref. This results in a quality value Q1 of 0.65.

[0039] The amplitude values ​​labeled A, B, C, D, and E in the measurement frequency range M2 are 0.430, 0.843, 1.00, 0.766, and 0.554 for the received, frequency-filtered broadband ultrasound signal USb,rx,f and 0.397, 0.828, 1.00, 0.756, and 0.526 for the corresponding reference signal USref. This results in a quality value Q2 of 0.91.

[0040] Numerous mathematical evaluation measures are conceivable that quantitatively capture the similarity of the signal forms. One example of another measure is the deviation of the in Fig. The four plotted envelopes show the relationship between the received ultrasound signal USb,rx,f and the reference signal USref (difference area). Other classes of measures can be correlations or statistical distance measures. Q=1−∑i=1nabs(ampl,i(USb,rx,f)max(USb,rx,f)−ampl,i(USref)max(USref))

[0041] The in Fig. The methods 1 and ultrasonic flowmeter 2 described above are characterized by the fact that the quality value Q for a measurement frequency range M of the received broadband and frequency-filtered ultrasonic signal USb,rx,f is determined by calculating a frequency deviation Δf in the considered time interval between the received, frequency-filtered broadband ultrasonic signal USb,rx,f and a corresponding reference signal USref, where a smaller absolute value of frequency deviation Δf corresponds to a higher quality value Q. The reference signal USref is, as already mentioned, Fig. 4 described, recorded under reference conditions during the factory calibration of the ultrasonic flowmeter 2. Fig. Figure 5 does not show the different frequencies for the received, frequency-filtered broadband ultrasound signal USb,rx,f and the corresponding reference signal USref, but rather shows the frequency deviations delta_f, which were calculated at different times k = A, B, C, D, E of the signal waveforms according to the following equation: delta_f,k=(freq,k(USref)−freq,k(USb,rx,f)) / freq,k(USref)

[0042] The quality value Qi in the measurement frequency range Mi was calculated as the mean value of the calculated instantaneous frequency deviations delta_f,k, where k goes through all frequency deviations: Qi=mean(delta_f,k(Mi)).

[0043] For the displayed frequency deviations delta_f in the measurement frequency ranges M1 and M2, a quality value of Q1 = -2.7 is obtained for measurement frequency range M1, and a quality value of Q2 = +0.4 is obtained for measurement frequency range M2. Therefore, in this example, measurement frequency range M2 is the optimal measurement frequency range M_opt. The instantaneous signal frequencies of the compared signals in the time domain at times A, B, C, D, and E are calculated by applying the Hilbert transform to the signals USb, rx, f, and USref.

[0044] Finally, in Fig. Section 6 shows the calculation of another quality value Q. The quality value Q is shown in Fig.6 for a frequency range M of the received broadband ultrasound signal USb,rx is determined by calculating a signal-to-noise ratio SNR of the received broadband and frequency-filtered ultrasound signal USb,rx,f in the considered time interval, where a larger signal-to-noise ratio SNR corresponds to a higher quality value Q.

[0045] In the illustrated embodiment, method 1 is carried out such that the signal-to-noise ratio (SNR) is calculated from the peak-to-peak value of the pure noise signal (Noise) and the peak-to-peak value of the noisy useful signal (Signal+Noise) in the considered time interval, according to the following equation: SNRi=20∗log10(ampl(Signal+Noise) / ampl(Noise))(dB).

[0046] Under the conditions shown, this results in a quality value of Q1 = 18.1 dB in the measurement frequency range M1 and a quality value of Q2 = 7.6 dB in the measurement frequency range 2. It follows that the measurement frequency range M1 is the optimal measurement frequency range M_opt.

[0047] In a further development of the method 1 and the corresponding ultrasonic flowmeter 2, several types of the presented quality values ​​Q are determined and evaluated to determine the optimal measurement frequency range M_opt.

[0048] Preferably, a lower acceptance threshold is defined for each type of quality value Q (latency, signal similarity, frequency deviation, signal-to-noise ratio), below which a corresponding measurement frequency range M is categorically ruled out as the optimal measurement frequency range M_opt. This prevents the applied quality criteria Q from falling below a minimum value.

[0049] If two quality values ​​Q of one type of quality value within a tolerance range at different measurement frequency ranges M are considered equivalent, another type of quality value in the measurement frequency ranges M determines the optimal measurement frequency range M_opt.

[0050] Preferably, for each type of quality value Q (latency, signal similarity, frequency deviation, signal-to-noise ratio), an upper acceptance threshold is defined, and if this threshold is exceeded, a corresponding measurement frequency range M is categorically selected as the optimal measurement frequency range. This allows applied quality criteria Q to outperform other types of quality values ​​Q if they achieve a superior rating. Reference sign 1 Procedure 2 Ultrasonic flow meters 3 Medium 4 measuring tube 5 transmitting ultrasonic transducers 6 Ultrasound signal 7 receiving ultrasound transducer 8 Control and evaluation unit 9 Ultrasound measurement path 10 Evaluation step 11 Frequency filtering 12. Calculation of the quality score 13 bandpass filters 14 Measurement operation 15 Rectangle signal sequence 16 envelopes t_sig signal propagation time delta_t_sig runtime deviation t_sig,ref Comparison runtime USB,tx emitted broadband ultrasound signal USB,rx received broadband ultrasound signal USB,rx,f received broadband and frequency-filtered ultrasound signal USref Reference Signal M Measurement frequency range M_opt optimal measurement frequency range Q quality value

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