DE4302368C1 - Ultrasonic measuring method for fluid flow velocity - Google Patents

Abstract

The measuring system has ultrasonic transmission and reception transducers (W1,W2) at opposite ends of an ultrasonic measuring path within the pipeline flow, coupled to 2 respective phase comparators (Comp 1, Comp 2) controlling a digital counter (C).The transducers are supplied with different oscillator frequencies (F1,F2), exhibiting a frequency difference which is between 40 -4 and 10 -7 times one of the transmission frequencies.The counter records a value corresponding to the difference in the propagation times for the signals propagated in the upstream and downstream direction. The transmission frequencies are of the order of 32 KHz, allowing measurement in the nanosecond range.

Description

The invention relates to a method and an apparatus for Ultrasonic measurement of the flow velocity of fluids in pipelines according to the preamble of claims 1 and 9.

If an ultrasound signal penetrates a flowing fluid, see the speed of sound is equal to the vector sum of Speed of sound c in the fluid at rest and the Flow velocity v of the fluid. Usually takes one measurement upstream and one downstream over the same Length of measuring section l before. You get the two Sound travel times t₁ and t₂

t₁ = l / (c + v), (1)

t₂ = l / (c - v). (2)

If you solve equations (1) and (2) for v, you get

v = l (1 / t₁ - 1 / t₂) / 2. (3)

The advantage of the approach according to equation (3) is that it is independent of the speed of sound c; this must for reaching the flow velocity v not be known.

If one wants to determine the flow velocity v from the transit time difference t _D , the equation is obtained

T _D = t₂ - t₁ = 2 lv / (c² - v²). (4)

Since c »v applies in most fields of application, the Simplify equation (4) with a good approximation  

v = t _D c² / 2 l. (5)

Usually the measuring line is against the angle beta the flow axis of the fluid is inclined so as to over the Pipe cross section mean flow velocity v to be able to determine. The following relationship is then obtained

v = t _D c² / 2 l cos (beta). (6)

Equations (5) and (6) each contain the square of the speed of sound c; As a result, even small changes in the speed of sound c, for example due to changes in temperature, pressure or density, affect the accuracy of the measurement. For this reason, several options are already known for eliminating this dependence on the speed of sound c. They are essentially based on also forming the total term t _S = t₂ + t₁ or the term product t _p = t₁ · t₂ of the two sound propagation times t₁ and t₂ and appropriately combining them with the transit time difference t _D. The following relationship is then obtained

v = t _D 2 l / t _S ² cos (beta). (7)

Equation (7) does not contain the speed of sound c more.

In the measurement method after the carryover effect a distinction is made between phase difference measurement (e.g. DE 26 51 142 A1, EP 0 262 441 A1, EP 0 262 461 A2), the Frequency difference measurement (e.g. DE-A 23 22 749, US 34 20 102), the lambda locked loop method (e.g. DE 28 28 937 A1), the transit time difference method (e.g. DE 34 38 976 A1) and the beam deflection measuring method. Everyone  these methods have in common that due to the high Velocity of propagation of the ultrasonic signals in the fluid - in water the speed of sound is c = 1500 m / s - Phase and transit time differences in the nanosecond range occur. To measure this with sufficient accuracy electronics is in the gigahertz range required. Electronic circuits in this Frequency ranges are extremely expensive, however, provided they are are available at all, and have a high Power consumption. You are therefore excluded from practice, especially when these electronic circuits over are to be fed from a battery for a long time.

To make accurate measurements anyway with these procedures to perform, they became statistical Measuring method extended by measuring over a variety of measuring cycles or over correspondingly extended measuring cycles is carried out. These are typical examples Sing-around process and that Runtime duty cycle method, also ping-pong method called. With the sing-around method (e.g. US 29 12 856) the receive pulse is electronically transmitted to the transmitter returned so that this again a sound pulse sends out. Both upstream and downstream will be the same Number of pulse runs started. The sum of the terms is registered in separate time counters. The difference the meter readings are proportional to the Flow velocity v, which is the sum of the meter readings inversely proportional to the speed of sound c. The However, the accuracy of this measurement method is determined by the Trigger times of the measuring electronics are impaired since there are none Possibility to determine whether and if so how how much it has changed over the years. Furthermore the two sing-around oscillators, one for each Measurement upstream and measurement downstream, be highly stable.  

These difficulties also exist with the others statistical measurement method, the one already mentioned Ping pong or the Sing-around frequency difference measurement method.  

It should only be mentioned here briefly that in addition to the Measuring method taking advantage of the entrainment effect too Measuring method using the Doppler effect and Correlation measurement method there. You are under the However, the present invention has no meaning.

Another way of obtaining phase information at "stretched time scale" is from DE-A 26 51 142 known. This ultrasonic flow meter has one highly stable quartz oscillator with the relatively high frequency of 5 MHz. Through digital reduction it becomes one Main measurement frequency of 2 kHz obtained. The keyed with 2 kHz 5 MHz signal is sent upstream and downstream over the Ultrasonic measuring section sent. To get out of the reception signals to be able to win the phase difference will Receive signals first with a 5 MHz quartz filter filtered, mixed with an auxiliary signal of 5002 kHz, each filtered through a 2 kHz filter and finally on given two zero crossing detectors. The two Zero crossing detectors open or close a pulse gate over which a counting frequency, in this case the 5- MHz base frequency, on a phase difference counter is switched. Its counter reading at the end of a measuring cycle is proportional to the difference between the upstream phase and the phase downstream.

A similar method emerges from the two Receive signals upstream or downstream the sum of the Runtimes determined upstream and downstream, so how explained at the beginning, there is the possibility of the current Speed of sound calculated from the measurement results eliminate.

The disadvantage of the known measuring device Need a high frequency oscillator and that  extensive electronics, because of this a high power consumption connected is. The known measuring device is therefore for a battery supply is not suitable. One also analyzes the principle underlying the known device, so one finds that despite measuring periods of approx. 1 Second a minimum resolution of just 250 nanoseconds enables. It is therefore completely unsuitable for small ones Fluid flow rates.

The present invention is based on the object To specify methods and an apparatus that are suitable Ultrasound measurements of small Flow rates, d. H. in the nanosecond range, perform with great accuracy, and the very are low in energy and therefore preferably for years over five years, operated from a single battery can be.

This problem is solved by a generic Method with the features according to the characterizing part of the claim 1 or by a generic device with the Features according to the characterizing part of claim 9.

The main advantage of the present invention is in that it is possible to use frequencies in Ultrasound range, preferably at 32 kHz, minimal Resolve measurement times of one nanosecond. Highly precise Oscillators in this frequency range are commercially available; they are used for example in quartz wristwatches, are extremely energy efficient and inexpensive and both short-term and long-term stable. The evaluation electronics in this frequency range is also extremely inexpensive and economical and easily available. The measuring times are typically at a second, so correspond to that Measuring times of the known statistical measuring methods.  

The frequency difference is advantageously between the two transmission signals 0.2 to 5 Hz, typically 1 Hz The appropriate value does not only depend on the desired one minimum resolution, but also from the absolute Frequency values of the transmission signals. The higher the frequency of the Transmission signals, the larger the same resolution Frequency difference or the same with the same frequency difference Resolution.

To calculate the speed of sound in the fluid to be able to eliminate the terms of the Signals measured through the ultrasonic path. To do this the inventive method for measuring the Term difference can only be slightly modified, as is disclose the characterizing features of claim 2.

As already mentioned, the invention is particularly intended for battery operated ultrasonic flowmeters. To the German calibration law is the typical operating time five years. During this long time the Frequency difference between the two oscillators change, whereby the measuring accuracy suffers. Around Here To remedy this, a of the two transmission frequencies controlled changed. To this The purpose of the transmission frequencies is in a frequency counter counted, and a phase comparison is made immediately between the broadcast signals themselves Actual counter reading difference that corresponds to the actual value of the Frequency difference corresponds. This actual value is shown with a stored in a suitable memory, the Target frequency difference compared corresponding target value. In the event of a deviation, the one oscillator frequency becomes so long changed until the meter reading deviation is sufficient is small. Procedure for the targeted change of  Oscillator frequencies are state of the art.

If the ultrasonic transducers used require  the transmission signals can be modulated onto suitable carriers become.

As already mentioned, are the subject of the present Invention also devices for performing the Ultrasonic flow measurement. Are generic Measuring circuits that cross the fluid Ultrasonic measuring section with transmitter and receiver transducers, two transmit frequency oscillators, signal switches, with whose help the transmission signals downstream and upstream via the Sent ultrasonic measuring section and just like that respective received signals to two phase comparators be switched, and one of the phase comparators start and stop digital counter. The The device according to the invention is defined by the Features of claim 9.

Not just the term differences, but the terms the ultrasonic signals upstream and downstream immediately to be able to measure, for example, around the To be able to mathematically eliminate the speed of sound, a slight change in the invention is sufficient Device as in the characteristic features of the Claim 10 is defined.

To the frequency difference between the oscillators on the To be able to adjust the setpoint is preferably one of the frequency controlled by both oscillators. Also are additional components are provided, as in the characterizing features of claim 11 are defined. This is the digitally saved one Target counter reading difference value alone decisive for the accuracy of the measurement. Since be assumed can that this stored setpoint during the The lifespan of the measuring device remains unchanged Accuracy of measurement also the same over the entire lifetime.  

In addition, the use of extreme is unnecessary long-term stable, expensive analog circuit components. Devices for the targeted change of Oscillator frequencies are state of the art.

The invention will be explained in more detail with reference to the drawing become. It shows

Fig. 1 leading fluid in a schematic representation an ultrasound measuring section in a pipe,

Fig. 2 is a timing diagram for explaining the measuring principle according to the invention,

Fig. 3 is a block diagram of an embodiment of measuring electronics and

Fig. 4 is a timing diagram to explain the processes in determining the transit time difference.

Fig. 1 shows a typical ultrasonic measuring section US, each with an ultrasonic transducer W₁, W₂ at the two ends. The ultrasound measuring section US is inclined at an angle beta against the pipe R through which fluid flows. The fluid flows through the pipe R from left to right, which is represented by the arrow v, which symbolizes the flow velocity of the fluid. The ultrasonic measuring section US is irradiated both upstream and downstream with transmit signals f 1, f 2, the transducers W 1, W 2 being switched alternately as transmit and receive converters and delivering delayed receive signals f 1 ′, f 2 ′ to the downstream electronics in accordance with the entrainment effect.

The measuring principle on which the invention is based will be explained with reference to FIG. 2. Rectangular transmit signals f 1, f 2, whose frequencies fr 1, fr 2 differ by f _D are shown one above the other. Without restricting generality, it is assumed that

fr₂ <fr₁, (8)

fr₂ = fr₁ - f _D. (9)

The following applies to the respective period

and for their difference

dt = T₂ - T₁. (12)

Also shown is a cycle period T, the beginning of which is determined in that the transmission signals f₁, f₂ are in phase and their end by the beginning next time is determined at which the Transmitted signals f₁, f₂ are in phase again. Within the Cycle period limps the rising edge of the transmission signal f₂ that of the transmission signal f₁ by a certain, well-defined measure according to time, namely according to T₂, d. H. after the first pulse of f₂ by German n₂-th impulse of f₂ the relationship

(n₂ - 1)

The size dt can be determined with sufficient accuracy for the measuring method according to the invention by considering the cycle period T within which a certain number n _1max periods (in the example in FIG. 2: ten) of the transmission signal _{f 1} and n _2max periods (in the example in FIG . 2: nine of the transmission signal are f₂). The following therefore applies:

T = n _1maxT₁ = n _2maxT₂ (13)
u

or with equation (12)

n _1maxT₁ = n _2max (T₁ + dt) (14)

and resolved after dt

Expressed with the frequencies fr₁, fr₂ one obtains

Approximately applies to fr₁ approximately equal to fr₂ sufficient accuracy

It can therefore be derived by counting n _1max and n _2max and with a sufficiently well-known period T₁ from the transmission signals f₁, f₂ further signals that are shifted from each other by an extremely small but known time, although the frequencies fr₁, fr₂ of the transmission signals f₁, f₂ are relatively low-frequency.

For example, if fr₁ = 32,000 Hz and fr₂ = 31,999 Hz, then the cycle period is T = 1 sec and dt = 0.97 · 10 ^-9 sec, so that processes in the nanosecond range can be measured.

The determination of n _1max and n _2max is done with the aid of a phase comparator and two counters: If the transmission signals _{f 1, f 2 are given} to the input of a phase comparator known _{per se} , the pulse train shown in FIG. 2 at the bottom appears at its output Ph. As soon as the phase of the transmission signals f 1, f 2 is identical, the phase signal jumps to high, which is referred to in the diagram as Ph0. As soon as the two transmission signals f 1, f 2 are in antiphase, the output of the phase comparator Ph goes low and at the next phase equality goes back to high, which is referred to as Ph1. During the time between Ph0 and Ph1, the counting frequency pulses of the transmission signals f₁, f₂ are counted in two counters, the final counter readings n _1max , n _2max - as shown above - being used to determine dt and a measure of the frequency difference f _{D of} the transmission signals represent f₁, f₂.

The two transmission signals f 1, f 2 therefore form a type of "time vernier" and can advantageously be used in the exemplary embodiment shown in FIG. 3 of measuring electronics for ultrasound measurement of the flow velocity v of fluids in pipelines.

One can see in Fig. 3 two oscillators O1, O2, the transmit signals f₁, f₂ emit. The transmit signals f₁, f₂ are fed to the inputs 22, 23 of a second phase comparator Comp2. The output signal 13 of this phase comparator Comp2 controls via a control logic L the start and the stop of the two counters C1, C2, which are acted upon by the transmission signals f 1, f 2 as counting pulses, such that the counting pulses present between the times Ph0 and Ph1 according to FIG , ie for the duration of one cycle period T counted. 2nd The counter values n _1max , n _{2max of} the counters C1, C2 then reached are fed via lines 15, 16 to a control and processing unit SV and are processed there, among other things, to determine dt according to equation ( 15 ).

In addition, in the control and processing unit SV from the above counter readings n _1max and N _2max a signal 17 corresponding to the counter reading _{difference is} formed, which detunes the oscillator O2 in its frequency fr₂ via a frequency control amplifier A as signal AFC in such a way that the frequency difference f _D between the frequencies fr₁, fr₂ of the oscillators O1, O2 in a desired range, preferably between 10 ^-4 and 10 ^{-7 of} the oscillator frequencies themselves.

The transmit signals f₁, f₂ of the two oscillators O1, O2 also pass through the switches S1.1a and S1.2a to the respective ultrasonic transducers W₁, W₂. The emitted transmission signals pass through the ultrasonic measuring section US, namely the signal f₂ downstream and the signal f₁ upstream. They are picked up by the ultrasonic transducer W₁ after the sound propagation time t₂ and by the ultrasonic transducer W₂ after the sound propagation time t₁ and pass through the switches S1.1b, S1.2b and a receiving amplifier 10, 11 in the form of the received signals f₂ ', f₁' via the second switch S2.1b, S2.2b as input signals to the two inputs 20, 21 of a first phase comparator Comp1.

The counting process of the pulse counter C is started by the output signal 13 of the above-mentioned phase comparator Comp2. The output signal 12 of the phase comparator Comp1 stops the counting process of the pulse counter C, which counts pulses with a counting frequency fr _z - preferably with the frequency fr₂ of the transmission signal f₂. The resultant counter reading 14 of the pulse counter C is fed to the control and processing unit SV and processed there to a value for the flow rate v, which, if desired, can be displayed on the display Dis.

In addition, in the control and processing unit SV from the signals f 1 and f 2 suitable control signals s1.1, s1.2, s2.1, s2.2 for the changeover switches S1.1, S1.2, S2.1, S2.2 formed and fed this.

The operation of the measuring electronics shown in Fig. 3 is to be explained for the transit time difference measurement dt of the ultrasonic signals: The measurement takes place within the cycle period T according to Fig. 2, beginning with the beginning of the cycle period in itself with the period duration T 1 or T 2 repeating cycles , which are divided into phases I and II. Phase I begins at the beginning of the cycle and preferably lasts about 50% of the period T₁ or T₂; during phase I of each cycle, the switches S1.1 and S1.2 are in position a, so that the transmission signals f 1, f 2 are emitted via the corresponding ultrasound transducers W 1, W 2 into the medium, the flow of which is to be measured . Phase II begins at the end of phase I and ends at the end of the periods T₁, T₂; during phase II of each cycle, the switches S1.1 and S1.2 are in position b, so that, on the one hand, the signal f₂ emitted downstream during phase I from the ultrasonic transducer W₂ is received after the transit time t₂ by the transducer W₁ and from the receiving amplifier 10 processed as a received signal f₂ 'and on the other hand received during the phase I from the ultrasonic transducer W₁ upstream emitted signal f₁ after the transit time t₁ from the transducer W₂ and processed by the receive amplifier 11 as a received signal f₁'. For the transit time difference measurement dt of the ultrasonic signals, the changeover switches S2.1 and S2.2 are in the position b during the entire cycle period T, so that the above-mentioned received signals f 1 ′, f 2 ′ are fed to the inputs 20, 21 of the first phase comparator Comp1.

For the cycles at the beginning of the cycle period T at a fluid flow velocity v <0, the reception signal f₂ 'lead the reception signal f₁' in phase, since the sound propagation time t₂ downstream is less than the sound propagation time t₁ upstream. But since the transmission signal f₂ is per cycle "delayed" by dt more than the transmission signal f₁, as shown in Fig. 2, after n₂ cycles of f₂, the two reception signals f₁ ', f₂' without phase shift at the inputs 20, 21 of the phase comparator Comp1.

Fig. 4 shows a temporal section around the n₂-th pulse of the transmission signals f₁, f₂ and the temporally shifted by the sound propagation time t₁ associated reception signal f₁ 'and the temporally shifted by the sound propagation time t₂ associated reception signal f₂'. At the n₂-th pulse of the transmission signal f₂, this is shifted in time by (n₂-1) _* dt relative to the transmission signal f₁ in accordance with what has been said for FIG. 2; the corresponding reception signals f₁ 'and f₂' but after the respective sound propagation times t₁, t₂ at the same time, ie without a time shift against each other. So the difference tD of the sound propagation times is equal to the time shift of the transmission signals f₁, f₂ to each other:

t _D = t₁ - t₂ = (n₂ - 1) _* dt.

Since the output signal 12 of the phase comparator Comp1 stops the counting operation of the counter C when the phase shift of the received signals f₁ ', f₂' at its inputs 20, 21 becomes zero, and since the counter C is started at the beginning of the cycle period by the output signal 13 of the phase comparator Comp2 was, the counter reading of the counter C is a measure for (n₂-1) and thus for the transit time difference tD. The counter reading 14 is fed to the control and processing unit and processed there to a flow value in accordance with the above-described calculation processes.

The inventive method can be both with Perform square-wave pulses, as they are known 32 kHz oscillators are delivered as well sinusoidal signals what is dashed by the drawn frequency filter F₁, F₂ is indicated. Especially when using continuous transmission signals it is recommended to use sinusoidal signals.

With the help of the circuit of Fig. 3, the transit times of the ultrasonic signals can also directly upstream and thus the compensation is known per se be measured downstream, to perform the temperature, pressure and density dependence of the sound velocity. To determine the sound propagation time t₂ downstream, the changeover switches S1.1 in position b, S1.2 in position a, S2.1 in position b and S2.2 in position a are switched during the entire cycle period T, so that the phase comparator Comp1 on its first input 20, the reception signal f₂ 'and at its second input 21, the transmission signal f₁ is supplied. From the counter reading 14 of the counter C, the signal transit time t 2 can then be determined in the control and processing unit SV and, if desired, can be displayed on the display Dis. Likewise, to determine the sound propagation time t 1 upstream during the entire cycle period T, the changeover switches S1.1 in position a, S1.2 in position b, S2.1 in position a and S2.2 in position b, so that the phase comparator Comp1 is switched on its input 20, the transmission signal f₂ and at its input 21, the reception signal f₁ 'are supplied. From the counter reading 14 of the counter C, the signal transit time t 1 can then be determined in the control and processing unit SV and, if desired, can be displayed on the display Dis.

Claims (13)

1. Method for ultrasound measurement of the flow velocity (v) of fluids in pipelines (R), ultrasound-frequency transmission signals (f 1, f 2) the fluid downstream (USv) and upstream (USr) along a ultrasound measuring path (US) traversing the fluid run through, due to the carry-along effect receive signals (f₁ ', f₂') with a time delay to the transmission signals (f₁, f₂), and wherein the phases of the signals (f₁, f₁ ', f₂, f₂') compared with each other and for starting and Stopping a digital counter (C) are used, characterized in that the difference (f _D = | fr₁-fr₂ |) of the frequencies (fr₁, fr₂) of the two transmission signals (f₁, f₂) is 10 ^-4 - to 10 ^-7 times the first transmission frequency (fr₁) itself is that phase comparisons on the one hand between the transmission signals (f₁, f₂) and on the other hand between the received signals delayed after passing through the respective ultrasonic measuring section (USv; USr) len (f₁ ';f₂') and that when a phase equality (Ph) occurs between the transmission signals (f₁, f₂) the counter (C) is started and when the next phase equality (Ph1) occurs between the reception signals (f₁ ', f₂ ') Is stopped again. 2. The method according to claim 1, characterized in that the phase comparisons on the one hand between the transmission signals (f₁, f₂) and on the other hand between a transmission signal (f₁, f₂) and the other received signal (f₂ ′, f₁ ′) be made and that when a Phase equality (Ph0) between the transmission signals (f₁, f₂) the counter (C) started and when the next occurs Phase equality (Ph1) between the transmit and Received signals (f₁, f₂ '; f₂, f₁') is stopped. 3. The method according to claim 1 or 2, characterized  characterized in that the first transmission signal (f₁) a frequency (fr₁) of approximately 32 kHz. 4. The method according to any one of claims 1 to 3, characterized in that the differential frequency (f _D ) is 0.2 to 5 Hz. 5. The method according to any one of claims 1 to 4, characterized in that the first transmission frequency (fr₁) is used as the counting frequency (fr _z ). 6. The method according to any one of claims 1 to 4, characterized in that the second transmission frequency (fr₂) is used as the counting frequency (fr _z ). 7. The method according to any one of claims 1 to 6, characterized in that one of the two transmission frequencies (fr₂) is changed in a controlled manner before the start of a measuring cycle, that for this purpose the transmission frequencies (fr₁, fr₂) each in a frequency counter (C1, C2) be counted that a phase comparison only takes place between the transmission signals (f₁, f₂) itself and that if the actual difference of the counter readings deviates from a target difference of the counter readings corresponding to the setpoint value of the difference frequency (f _D ), this one transmission frequency (fr₂) is changed until the target-actual deviation is sufficiently small. 8. The method according to any one of claims 1 to 7, characterized characterized in that the transmission signals (f₁, f₂) on one high-frequency carriers are modulated. 9. Device for ultrasound measurement of the flow velocity (v) of fluids in pipelines (R), comprising an ultrasonic measuring section (US) crossing the fluid (US) with transmitting and receiving transducers (W 1, W 2), two transmitting frequencies (fr 1, fr 2) oscillators (O₁, O₂), signal switch (S1.1, S1.2, S2.1, S2.2), with the help of which the transmission signals (f₁, f₂) downstream (USv) and upstream (USr) via the ultrasonic measuring section ( US) sent and just like the respective received signals (f₁ ', f₂') are switched to two phase comparators (Comp1, Comp2), and one of these phase comparators (Comp1, Comp2) started and stopped digital counter (C), characterized in that the of the oscillators (O₁, O₂) generated transmission frequencies (fr₁, fr₂) differ by a difference (f _D = | fr₁-fr₂ |) from 10 ^-4 - to 10 ^-7 times the first transmission frequency (fr₁) that the two after passing through the respective ultrasound measuring section (USv, USr) h delayed received signals (f₁ ', f₂') on one phase comparator (Comp1) and the two transmission signals (f₁, f₂) on the other phase comparator (Comp2) are performed and that between the occurrence of a phase equality (Ph0) between the transmission signals (f₁ , f₂) and the occurrence of the next phase equality (Ph1) between the received signals (f₁ ', f₂') a pulse gate is open and counting frequencies ( _{for z} ) pulses are counted in the counter (C). 10. The device according to claim 9, characterized in that in each case a transmission signal (f₁, f₂) and the other reception signal (f₂ ', f₁') on one phase comparator (Comp1) and the two transmission signals (f₁, f₂) on the other Phase comparators (Comp2) are performed, and that between the occurrence of a phase equality (Ph0) between the transmission signals (f₁, f₂) and the occurrence of the next phase equality (Ph1) between the transmission and reception signals (f₁, f₂ ′; f₂, f₁ ′ ) the pulse gate is open and counting frequencies ( _{for z} ) pulses are counted in the counter (C). 11. The device according to claim 9 or 10, characterized in that one of the two oscillators (O₂) is frequency controllable in that the phase comparator (Comp2) for the two transmission signals (f₁, f₂) when a phase equality (Ph0) occurs two frequency counters (C1, C2) - one for both transmission frequencies (fr₁, fr₂) - starts and stops when the next phase equality (Ph1) occurs, that the actual meter reading difference is compared with the stored, the target difference frequency (f _D ) corresponding meter reading difference and that a control amplifier (A) is provided which generates a frequency control signal (AFC) proportional to the target-actual deviation, with the aid of which the frequency (fr₂) of the frequency-controllable oscillator (O₂) is adjusted until the counter reading difference is sufficiently small. 12. The device according to one of claims 9 to 11, characterized characterized in that at least one oscillator (O₁, O₂) 32 kHz quartz oscillator. 13. Device according to one of claims 9 to 12, characterized characterized in that the oscillators (O₁, O₂) frequency filters (F₁, F₂) are connected in the form of low or bandpass.