DE3825422A1 - Device for measuring the density of fluids by means of acoustic signals - Google Patents

Abstract

The measurement of the density of fluids (gases and liquids) is carried out by means of detecting the transit time of an acoustic signal through an acoustic measuring path filled with fluid and converting it into the density. The transit time of the acoustic signal is measured by means of generating a series frequency, in that an acoustic signal on an electroacoustic receiving transducer having an electronic device triggers an acoustic signal to an electroacoustic transmitting transducer for transmitting through the acoustic measuring path. This can be carried out both with acoustic pulse signals and with acoustic continuous signals. In the latter case, the electroacoustic transmitting transducer, the acoustic measuring path and the electroacoustic receiving transducer are in the feedback network of an electronic broadband amplifier and thus in total form a self-exciting electroacoustic oscillator. In a further refinement, the electronic amplifier can be formed by a phase-locked loop (PLL), consisting of a phase detector, a low-pass filter and a voltage-controlled oscillator (VCO), the electroacoustic feedback network in this case forming the external feedback of the PLL. Having electronic dividers in the internal and external feedback of the PLL, the VCO works at a multiple of the series frequency, so that the frequency of the acoustic signal can be matched optimally to the electroacoustic properties of the transducers.

Description

The invention relates to a device for continuous Determine the density of flowable substances (fluids) by Measuring the frequency changes that occur as the density changes one exposed to the mass forces of the substance to be determined electro-acoustic ultrasonic running track that together an electro-acoustic with an electronic amplifier Oscillator to excite the resonance frequency of the electroacoustic Ultrasound running track forms.

The density measurement of fluids (gases and liquids) is from of great practical importance since the usual quantity measurement methods determine only the volume while for the installation an operating balance sheet, especially those on a defined condition, related mass is of interest.

To determine the density of fluids are mechanically vibrating Systems known in which the mass forces of the elements exposed to the respective fluids, such as strings, pendulums, Torsion bars, tuning forks or similar measurable frequency changes Experienced. A device for measuring the density of fluids with tuning forks is for example in the German patent 14 98 602.

The density is not substance-specific. Therefore it is preferred used for the analysis of binary substance mixtures; here it is then used to measure the concentration of a substance in one other suitable.  

It is known that the speed of sound c changes with the density of the sound-transmitting fluids. The following relationship applies to liquids according to L. Bergmann, Der Ultrasound, Hirzel-Verlag, Stuttgart (1954)

With

c = speed of sound,
D = density of the liquid and
k = apparatus constant.

This law can be used to measure density by measuring the speed of sound over a fixed distance. The measurement of the speed of sound is used, among other things, to measure the flow velocity of liquids in pipelines. Here, an ultrasound pulse is transmitted over a defined distance l . The speed of sound can be determined from the transit time t

d. H. the density results in

with an apparatus constant K.

The temperature T has a large disturbing influence on the speed of sound c . According to L. Bergmann, Der Ultrasound, Hirzel-Verlag (1954), page 373, there is roughly a quadratic relationship for water with a maximum at about 74 ° C

d. H. the relative change in the speed of sound with the Temperature is about

This means that the density changes due to the quadratic Also changes with temperature.

It follows from this that in a device for density measurement the influence of temperature can be taken into account with ultrasound must, d. H. the temperature must be recorded. Furthermore is a Influence of pressure on the speed of sound must also be taken into account. In practice you become one Relate density measurement to a specified state, for example for gases to the normal state.

In principle, the methods for direct transit time measurement require a relatively small amount of electronic means. Here but the term must be recorded very precisely, are considerable electronic Means for generating and receiving the ultrasound pulse necessary. The time of transmission and reception must be very be accurately detected. This is a very accurate counter required, the trigger point of the transmit and receive signal, that controls the start and stop time of a counter with high accuracy must be detected.  

When sending, transmitting and receiving the finite acoustic Vibration bundles are decisive in the transmitter, fluid and receiver physical transient processes involved. The amplitude and so that the intensity gradually increases from zero, i.e. H. out the noise over several vibrations up to a maximum. Differences in density can also be caused by streaks or suspended particles in the fluid, for example bubbles and solid particles Scatter or absorb the ultrasound beam in such a way that the Receive pulse is ground or that the receive converter no longer receives any signal. With the impulse method is the trigger point can only be reproduced sufficiently if it is so is placed,

• - That it is always above the thermal noise of the ultrasonic transducer and the electronic means,
• - that he is always aware of possible acoustic and electromagnetic Interference lies and
• - That it is in the steepest possible part of the vibration amplitude lies.

The main disadvantage of the pulse method, however, is that the measured pulse transit time due to the settling processes discussed above is longer than the actual pulse duration in the Ultrasound running track. This can be insufficient at the Evaluation by a constant amount can be taken into account.

Despite considerable electronic measures such as suppression obviously wrong measurements, can be sufficient Inadequate accuracy of measurement. The big disadvantage of Bubble or solid particles in the fluid are prone to failure in principle, however, not to be eliminated.

The present invention has for its object a Device for measuring the density or concentration of a homogeneous or inhomogeneous fluids with the help of ultrasonic vibrations of the type mentioned at the beginning, in which the Speed of sound is measured in the fluid, triggering the Received signal is not included in the measurement result and brief  Differences in density in the fluid are automatically averaged. This task is characterized by the characteristics of the Claim 1 solved.

The present invention is based on the knowledge that the Measurement of the speed of sound is suitable for density measurement, by changing the runtime indirectly by generating a repetition frequency can detect, an arriving in the receive converter Ultrasound signal at the transmitter transducer immediately the transmission triggers another ultrasound signal. The repetition rate is inversely proportional to the signal propagation time through the fluid. The big advantage of this repetition frequency method is the averaging in frequency measurement. A frequency measurement is usually carried out. a. by opening a gate for a defined measuring time and Counting the incoming signal periods. It will be a medium one Period formed over the measuring time. This has the big one Advantage that short-term interference in the ultrasound transmission do not enter the measurement due to bubbles or particles.

In a first embodiment of the measuring method, ultrasound pulse trains used as an ultrasonic signal. With this pulse repetition methods known per se become pulse-shaped Ultrasonic vibration beams from a transmitter transducer too transmitted to a receive converter. The recipient reports the Receipt of the ultrasound pulse by an electrical signal the transmitter electronics, the next ultrasound pulse at the transmitter transducer triggers etc. The distance between the individual pulses the transit time of the ultrasound pulses is equal to one another the fluid; the pulse repetition frequency is measured for this. The disadvantage of this pulse repetition frequency method is that The fact that the trigger point is caused by the transients in the transducers and in the fluid to extend the measured Lead impulse transit times.  

The knowledge lies in a further embodiment of the invention based on the fact that the advantages of the pulse repetition frequency method can maintain without its disadvantages having to take over if you keep the ultrasound signal going transmits. With continuous transmission of ultrasonic vibrations the fluid eliminates the need for triggering, since no finite vibration bundles are sent. At an interruption or disturbance of the ultrasonic vibrations the measurement falls due to the fluid due to bubbles or particles for this interruption time or is disturbed. At a However, continuous tone procedures immediately become correct again Measurements carried out.

This task is achieved with the continuous tone process solved that the ultrasonic route with constantly sent Ultrasonic vibrations of a single frequency the acoustic Part of the positive feedback branch of an electroacoustic oscillator forms and the frequency an integer multiple of the repetition frequency is. In this variant, the frequency of the ultrasonic vibrations is rigid with the repetition frequency of the electroacoustic Oscillator coupled.

An oscillator basically consists of an amplifier with the complex gain V and a positive feedback network with the complex coupling factor K. Such an arrangement is capable of oscillation if the general oscillation condition

V · K = 1

is satisfied. The frequency with which such an oscillator oscillates, ie the resonance frequency of the oscillator, depends on the properties of the positive feedback network and the amplifier. The period of the basic oscillation, ie the lowest resonance oscillation, is equal to the transit time of the signal through the loop, consisting of amplifier (transit time t _e ) and positive feedback network (transit time t) ; ie

t ′ = t + t _e .

In the electroacoustic oscillator according to the invention, the positive feedback network forms the transmitter transducer, the ultrasound travel path and the receiver transducer, so that only t is dependent on the speed of sound c and thus on the density d . The resonance frequency of this oscillator

with the acoustic coupling branch is therefore equal to the repetition frequency which is given by the time t of an acoustic signal through the fluid and the additional electronic signal time t _e by the necessary electronic means. In practical exemplary embodiments with water as a fluid and with transducer distances of a few centimeters to a few meters, the repetition frequencies are in the range from a few 100 kilohertz down to a few kilohertz. In these frequency ranges, electroacoustic transducers have relatively large dimensions and poor transmission properties. Furthermore, a measuring method in this frequency range is susceptible to interference due to possible acoustic external influences. For this reason, ultrasound vibrations of significantly higher frequencies are used for acoustic signal transmission in this invention. Piezoelectric ultrasonic transducers with usable dimensions and with suitable electroacoustic properties are available for frequencies above about 100 kilohertz up to a few megahertz. Since the electroacoustic transducers have a bandpass character, it is not the fundamental frequency that is excited, but a harmonic whose frequency is an integral multiple of the fundamental frequency.

In a first embodiment, the repetitive frequency oscillations of the electroacoustic oscillator between the output of the electronic amplifier and the transmitter converter are multiplied by a fixed factor N with an electronic multiplier and divided by this fixed factor N between the receive converter and input of the electronic amplifier with an electronic divider . In this way, a coupled amplifier loop is created, in which oscillation signals with the repetition frequency are generated at the amplifier input and at the amplifier output. The multiplied repetition frequency is particularly advantageous in that the measurement time for determining the N -fold frequency can be shorter by a factor N for the same accuracy of the measurement result than in the direct evaluation of the simple repetition frequency.

In a preferred embodiment of the invention, the electronic Amplifier of the electroacoustic oscillator replaced a phase locked loop (PLL) be, the phase locked loop in the input from a Phase detector, a downstream low-pass filter or PI controller and at the output from a voltage controlled oscillator (VCO - voltage controlled oscillator). The phase locked loop with an internal signal feedback from the output of the voltage controlled Oscillator on one input of the phase detector is regulated by the phase detector in such a way that the two Inputs of the phase detector under all conditions sets a constant phase position between the two input signals, by the frequency of the voltage controlled oscillator is adjusted. Frequency is again the term that determines the frequency through the ultrasonic path, consisting of a transmitter, Coaxial cable, acoustic route and reception converter. The great advantage of a phase locked loop is that the Freewheeling frequency of the voltage-controlled oscillator nearby the repetition frequency can be set. This way you reach a safe oscillation of the electroacoustic oscillator with the Repetition rate. Additional start-up aids are not necessary, d. H. even if the ultrasonic measuring section is briefly interrupted  through bubbles or particles is an immediate settling guaranteed to the repetition frequency.

In a further embodiment of the invention, the output frequency of the voltage-controlled oscillator is multiplied to N times the repetition frequency by a divider or divider in the internal feedback branch of the phase locked loop. In addition, a second electronic divider can be arranged in front of the second, external input of the phase detector. The great advantage of this arrangement is the relatively low electronic complexity, since a large selection of such electronic components is available as integrated circuits.

The repetition frequency also depends on the running times of the Signals through the electrical and electronic assemblies, such as the electroacoustic transmission transducer, the electroacoustic reception transducer and any existing divider, amplifier, etc. These additional electronic terms can be obtained through a electronic phase shifter in the internal signal feedback the phase-locked loop in the effect on the resulting Repetition rate can be compensated. The electronic signal runtime due to the internal signal feedback on the electronic Phase shifters should be set just as large as that electronic signal transit time through the external signal feedback via the electroacoustic transducers, coaxial cables, dividers and Amplifier, etc. This ensures that the repetition frequency is no longer dependent on the electronic signal transit times, but only from the signal runtime through the acoustic Measuring section with the density or sound velocity dependent Fluid. This is particularly advantageous since there are no additional ones electronic corrections when calculating the density from the Running time can be carried out over the acoustic sound measuring section have to. The electronic phase shifter should be the signal for all frequencies in the frequency working range of the voltage controlled Shift oscillator by a constant phase, or by one delay constant time.  

If the fluid moves in the ultrasonic measuring section, changes the speed of sound due to the entrainment effect of the moving transmission medium. Send the ultrasound signal both forward and backward the ultrasonic measuring section, so the fault added and subtracted once by the fluid movement, d. H. the forward frequency results

and the repetition rate in the reverse direction

If you add the two measured repetition frequencies in the forward direction and in the reverse direction, the interference is caused by the Fluid movement eliminated, i. H. it results

The present invention is described below with the aid of exemplary embodiments explained in more detail.

Fig. 1 shows the basic structure of a device for measuring the density-dependent speed of sound according to the pulse repetition frequency method.

Fig. 2 shows the basic structure of an electro-acoustic oscillator for frequency measurement result of the density-dependent velocity of sound with an electro-acoustic measurement path 1, 3, 2 as the positive feedback network of an electronic amplifier.

FIG. 2a shows the basic structure of an electroacoustic oscillator according to FIG. 2 with an electronic multiplier and an electronic divider for a frequency of the acoustic oscillations which is N times the repetition frequency.

FIG. 2b shows the basic structure of an electroacoustic oscillator according to FIG. 2 with one or two bandpass filters for a frequency of the acoustic vibrations which is N times the repetition frequency.

FIG. 3 shows the basic structure of an electroacoustic oscillator according to FIG. 2 with a phase locked loop as an electronic amplifier.

Fig. 3a shows the basic structure of an electro-acoustic oscillator of Fig. 3 with one or two electronic dividers and an electronic phase shifter.

Fig. 1 shows the basic structure of an embodiment for measuring the density or concentration-dependent speed of sound according to the pulse repetition frequency method with an electroacoustic measuring section. This electroacoustic measuring section consists of an electroacoustic transmission transducer 1 , which transmits ultrasonic vibrations over the length l of the acoustic measuring section 3 to an electroacoustic receiving transducer 2 . The electronic transmitter unit 4 uses the transmitter converter 1 to generate an ultrasound pulse, which is transmitted over the measuring section 3 with the density-dependent speed of sound to the electroacoustic receiver converter 2 in the transit time t . An electrical signal is formed in the reception converter 2 and passed into the electronic reception unit 5 , in which the time of reception is obtained by triggering the reception signal. At this trigger time, a trigger signal is generated in the reception unit 5 , which triggers a new transmission pulse via the line 6 in the transmission unit 4 . This transmit pulse runs through the structure shown in the same way.

The sequence of this pulse repetition frequency procedure is repeated continuously. The pulse repetition frequency is present on the connecting line 6 and is measured with a counter (not shown), the period of the pulse repetition frequency being equal to the pulse transit time t through the acoustic measuring path 3 and the electronic transit times t _e through the electroacoustic transducers 1 and 2 , the associated lines, the transmitter unit 4 and the receiver unit 5 .

In an evaluation unit, not shown, for example a microcomputer, the pulse repetition frequency is converted into one Numerical value for the speed of sound or for the associated one Density or converted for a concentration, possibly under Consideration of the electronic signal transit times.

The first pulse for generating the pulse repetition frequency must take place on line 6 with a pulse stage, not shown. The disadvantage of this pulse repetition frequency method is that the pulse train in the acoustic measuring section 3 can be easily interrupted by bubbles or particles. In this case too, the pulse train must be excited again with the pulse stage. For this purpose, an evaluation circuit (not shown) is required, which detects the presence of the regular pulse sequence and, in the absence of the pulse sequence, stimulates the pulse stage to emit a pulse. These interruptions in the pulse train can lead to considerable disturbances and measurement errors.

Fig. 2 shows the basic structure of an electro-acoustic oscillator for measuring the density or concentration-dependent speed of sound for the continuous repetition rate process with an electro-acoustic measurement path 1, 3, 2 as the positive feedback network of an electronic repetition frequency amplifier 7. This electroacoustic measuring section 1, 3, 2 has the structure and mode of operation according to FIG. 1. The input of the repetition frequency amplifier 7 is connected to the electroacoustic transducer 2 . The received signal from the transducer 2 is amplified in the amplifier 7 , passed to the electroacoustic transmitter transducer 1 and sent as acoustic vibrations via the acoustic measuring section 3 .

If the oscillation condition is met, this electroacoustic oscillator is automatically excited to oscillate at its resonance frequency, the period duration of which is given by the electroacoustic measuring section consisting of the acoustic measuring section 3 , the transducers 1 and 2 and the signal lines and by the amplifier 7 . Because of the bandpass behavior of transducers 1 and 2 , harmonics of this fundamental frequency can also be excited. This continuous tone repetition frequency is evaluated using the same means as in FIG. 1. If the acoustic measuring section 3 is interrupted by bubbles or particles, the resonance vibrations are only interrupted for this time. After the interruption has been eliminated, the electroacoustic oscillator starts up again without a starting aid; this is the great advantage over the device according to FIG. 1.

FIG. 2a shows the basic structure of an electroacoustic oscillator according to FIG. 2, in which the frequency of the ultrasonic vibrations is N times the repetition frequency. With an electronic multiplier 8 between the output of the electronic repetition rate amplifier 7 and the electroacoustic transmission converter 1 , the repetition rate is multiplied by a factor of N. Since the repetition frequency is generally in the lower kilohertz range, this multiplication means that ultrasound transducers in the upper kilohertz to lower megahertz range can be used. Ultrasonic transducers in the upper kilohertz to lower megahertz range have very good transmission properties and are relatively easy to manufacture. With an electronic divider 9 between the electroacoustic reception converter 2 and the input of the electronic amplifier 7 , the multiplied repetition frequency is divided by the sub-factor N , so that an alternating signal with the repetition frequency is again present at the input of the amplifier 7 . The factor N is chosen so that the multiplied repetition frequency lies in the optimal transmission frequency range of the converters 1 and 2 . The properties of the electroacoustic oscillator are the same as those according to FIG. 2.

By this construction according to Fig. 2a the frequency of the ultrasonic vibrations in the electro-acoustic measurement path 1, 3, 2 is rigidly coupled to the repetition frequency of the electro-acoustic oscillator.

FIG. 2b shows the basic structure of an electroacoustic oscillator according to FIG. 2, in which the resonance frequency of the electroacoustic oscillator is N times the repetition frequency. With a bandpass filter 10 or 10 ' , the frequency bandwidth is narrowed to such an extent that only N times the repetition frequency is transmitted via the electroacoustic measuring section 1, 3, 2 , so that the electroacoustic oscillator only oscillates at this frequency. The electronic broadband amplifier 7 must amplify this frequency so that the oscillation condition for the electroacoustic oscillator is fulfilled.

FIG. 3 shows the basic structure of an electroacoustic oscillator in which the electronic repetition frequency amplifier 7 according to FIG. 2 is formed by a phase locked loop consisting of a phase detector 11 , a low-pass filter or PI controller 12 and a voltage-controlled oscillator 13 . One input of the phase detector 11 is connected directly to the output of the voltage-controlled oscillator 13 via an internal feedback. The second input of the phase detector 11 is connected to the output of the voltage-controlled oscillator 13 via the external feedback as a positive feedback network of the electroacoustic oscillator. The positive feedback network consists, as in FIG. 2, of an electroacoustic transducer 1 , the acoustic measuring section 3 and the electroacoustic receiving transducer 2 . With this construction, a repetition frequency of the electroacoustic oscillator is set, which corresponds to the reciprocal of the signal transit time from the output of the voltage-controlled oscillator 13 via the electroacoustic measuring section 1, 3, 2 to the first input of the phase detector. The signal delay in the phase detector 11 , in the low-pass filter or PI controller 12 and in the voltage-controlled oscillator 13 is no longer included. Depending on the choice of electroacoustic transducers 1 and 2, the electroacoustic oscillator oscillates at the fundamental frequency or a harmonic. To compensate for the electrical signal propagation times in converters 1 and 2 and their electrical feed lines, an electrical or electronic phase shifter 14 can be inserted into the internal feedback. In this phase shifter 14 , the signal is delayed by the same signal transit time as the signal transit time in the converters 1 and 2 and their electrical leads. In the simplest case, coaxial cables or RC delay chains are suitable as passive phase shifters, whose frequency behavior corresponds to that of converters 1 and 2 and their electrical supply lines. The phase shifter 14 can also be inserted into the external feedback of the electroacoustic oscillator, then the phase of the phase shifter must be set so that the phase shift is supplemented by the electronic signal propagation times of the electroacoustic measuring path 1, 3, 2 to the detected phase of the phase detector.

FIG. 3a shows the basic structure of an electroacoustic oscillator according to FIG. 3, in which the frequency of the acoustic vibrations is N times the repetition frequency. The repetition frequency is multiplied by this factor N with an electronic divider 15 with the sub-factor N in the internal feedback of the phase-locked loop. This multiplication of the repetition frequency can also be achieved with an electronic divider 16 with the partial factor N in the external feedback of the electroacoustic oscillator immediately before the second input of the phase detector 11 . Both dividers 15 and 16 can also be present; this minimizes the phase jitter of the voltage controlled oscillator 13 . In these arrangements, the voltage-controlled oscillator 13 oscillates at N times the repetition frequency; the acoustic vibrations of the electroacoustic measuring section 1, 3, 2 are therefore N times the basic repetition frequency. The statements made under Fig. 2a apply accordingly to the selection of the ultrasonic transducers and the advantages of the arrangement. To compensate for the electronic signal propagation times in transducers 1 and 2 and their electrical feed lines, an electrical or electronic phase shifter 14 can be inserted into the internal or external feedback of the electroacoustic oscillator, as shown in FIG. 3. The embodiment according to Fig. 3a illustrates a preferred construction, since the transmission frequency can be optimally adapted to the electroacoustic transducer here and the repetition frequency or transmission frequency, that is, the N-fold repetition frequency, only on the acoustic measurement path 3, that is the speed of sound, and the length depends. All other device parameters are no longer included in the measurement.

Claims (13)

1. Device for measuring the density of fluids by measuring the speed of sound of the fluid, the relationship between the density and the speed of sound being known per se by determining the transit time of an acoustic signal over an electroacoustic measuring path which is obtained from an electroacoustic transmitter transducer 1 . there is an acoustic measuring section 3 with the fluid as the transmission medium and an electroacoustic receiving transducer 2 , characterized in that

• - That after the repetition frequency method, a signal repetition frequency is formed, which is inversely proportional to the signal transit time through the acoustic measuring section 3 , by an acoustic signal received at the receiving transducer 2 by means of electronic receiving and transmitting elements, an acoustic signal at a transmitting transducer 1 triggers again
• - That there are electronic means for measuring the repetition frequency and
• - That there are electronic means for calculating the speed of sound and the density.

2. Device according to claim 1, characterized in

• - That a switching device is present so that the acoustic signals are transmitted once from transducer 1 to transducer 2 and then from transducer 2 to transducer 1 to measure the speed of sound in both directions of transmission and
• - That the sum or the arithmetic mean of the two speeds of sound is used to calculate the density.

3. Device according to claim 1 or 2, characterized in that

• that, according to the pulse repetition frequency method, the acoustic pulse received at the receiving transducer 2 generates a trigger pulse with an electronic receiving and trigger unit 5 ,
• that the trigger pulse generates an electrical pulse via the trigger line 6 in the electronic transmission unit 4 ,
• - That the electrical pulse in the transmitter converter 1 triggers an acoustic pulse and
• - That the trigger pulses on the trigger line 6 represent the pulse repetition frequency.

4. The device according to claim 3, characterized in

• - That a pulse stage with a trigger pulse on the trigger line 6 excites the pulse train and
• - That a trigger pulse is generated when there is no regular pulse train.

5. The device according to claim 1 or 2, characterized in that the acoustic vibrations are transmitted as continuous signals. 6. The device according to claim 5, characterized in that the electroacoustic measuring section with the electroacoustic transmitter transducer 1 , the acoustic measuring section 3 and the electroacoustic receiving transducer 2 forms the positive feedback network of an electroacoustic oscillator. 7. The device according to claim 6, characterized in that the electroacoustic oscillator is formed from an electronic broadband amplifier 7 and the electroacoustic measuring section 1, 2 and 3 . 8. The device according to claim 7, characterized in that before the input of the amplifier 7, an electronic divider 9 with the sub-factor N and after the output of the amplifier 7, an electronic multiplier 8 is arranged with the same factor N , so that the frequency of the acoustic continuous signal is N times the repetition frequency of the electroacoustic oscillator. 9. The device according to claim 7, characterized in

• - That one or two bandpass filters 10 or 10 ' are inserted into the positive feedback network of the electroacoustic oscillator before the input or after the output of the electronic amplifier 7 , so that the electroacoustic measuring section transmits the N times the repetition frequency and
• - That the electronic broadband amplifier 7 amplifies the N- fold repetition frequency so that the oscillation condition is met.

10. The device according to claim 7, characterized in that the electronic broadband amplifier 7 is formed by a phase locked loop, the phase locked loop in the input from a phase detector 11 , a subsequent low-pass filter or PI controller 12 and in the output from a voltage controlled oscillator 13 and wherein one input of the phase detector 11 is connected as an internal feedback directly to the output of the voltage-controlled oscillator 13 and the second input of the phase detector 11 is connected as an external feedback or positive feedback network to the output of the voltage-controlled oscillator 13 . 11. The device according to claim 10, characterized in that an electronic divider 15 is inserted with the sub-factor N in the internal feedback of the phase locked loop, so that the ultrasonic vibrations are N times the repetition frequency. 12. The apparatus according to claim 11, characterized in that an electrical divider 16 with the divider factor N is inserted into the external feedback of the phase locked loop. 13. The apparatus according to claim 10, characterized in that an electronic phase shifter 14 is inserted in the internal feedback, so that the signal is delayed by the internal feedback by the same time period or phase as the signal by the external feedback due to the electrical and electronic assemblies - such as the electroacoustic transmission transducer 1 , the electroacoustic reception transducer 2 , the associated supply lines to the transducers and any existing electronic amplifiers and dividers, so that only the acoustic measuring section 3 determines the repetition frequency.