DE2316437A1 - DEVICE FOR DISPLAYING THE SPEED OF A FLOW - Google Patents

Description

PATENT Attorney Phone: (0271) 32409 DIPL-ING. ERICH SCHUBERT I ^ TT

2 3 1 6 A 3 7 ^ΚδΙη IOÄ'31, Ej5 »n 20362

Bank accounts: From ,, Po-ntanwa »KpWng. SCHUBERT ^ Sieeen. Owner criminal * 227.

(RhW.)

73 025 Kü / Sch April 2, 1973

TTNIiDED KINGDOM AOJOMIO ENERGY AUTHORITY, 11, Charles II Street,

London, S.W.1 (England)

I ¹ The priorities of British patent application No. 15554/72 of April 4, 1972 and that of March 29, 1973 are claimed for this application.

Device for displaying the speed of a flow

middle flow

The invention relates to devices for displaying the Velocity of a fluid flow by "comparing the transit times of pressure wave pulses occurring in opposite Directions are transmitted over a predetermined path in the fluid.

The invention provides a device for measuring the speed or a component of the speed a fluid flow created, which is composed of first and second units, which are spaced apart

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are arranged on the other and emit pressure wave pulses and which travel a path through the fluid in the direction in which the velocity component is to be measured, from variable frequency oscillator devices, from a device which for this ensures that the pressure wave pulses from the first unit to the second unit and, separately, from the second unit to the first unit are transmitted from control means for controlling the frequency of the variable frequency oscillator means to generate a number SL of oscillations or pulses in a first period of time which corresponds to that which a pressure wave needs on its way from the first unit to the second unit, and a number N2 of Vibrations or pulses in a second period of time, that of those corresponds to which a pressure wave needs on its way from the second unit to the first unit, as well as from a comparator for displaying the frequency difference between the vibrations or pulses occurring in the first period of time and the vibrations or pulses generated in the second period.

Preferably EL = N ₂ = N.

The invention also provides a device for measuring the speed or a speed component of a fluid flow, which is composed of first and second, spaced apart units, which each transmit and receive pressure wave pulses, which via a path through the fluid in the Cover the direction in which the speed component is to be measured, from a first and a second variable frequency oscillator, from a device that transmits pressure wave pulses from the first unit to the second unit, from a first control device for controlling the frequency of the first Variable frequency oscillator, in such a way that it generates a number IL of oscillations ©, d © p. Pulses in time-SOS 841/0946 ·

duration generated, which a pressure wave on its way from the first unit to the second unit "needs, from a device, the pressure wave pulses can be transmitted from the second unit to the first unit, from a second control device to the Controlling the! Frequency. the second variable frequency oscillator, such that this is a number Np of oscillations or pulses generated in the time taken by a pressure wave on its way from the second unit to the first unit as well as a comparator for displaying the frequency difference between the first and second variable frequency oscillators. Again, preferably L = L = N.

Each control device expediently comprises a device which determines whether the Nth oscillation (or pulse) of its associated variable frequency oscillator appears before or after a signal which indicates the end of the corresponding period of time indicates, and a device for generating an error signal for corresponding ^ take or increase in frequency of the variable frequency oscillator, depending on whether the Nth oscillation (or pulse) appears before or after this signal, which indicates the end of the corresponding time period.

In one embodiment of the invention, each is a controller effective in such a way that it changes the frequency of its variable frequency oscillator by a set amount can change. It can take a long time before there is a coincidence between the appearance of a Process and the completion of N oscillations or pulses is achieved. It should be noted that the device thus tends to average out small changes in the time spans between successive events, and rapid cannot follow extensive changes in the duration.

With this arrangement, if there is a large variation in the initial setup, it may be an unacceptable one

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take a long time to approximate the coincidence he follows. To overcome this difficulty, a Device for the temporary · adjustment of the strength of the Frequency change with each actuation of the control device are provided. If the coincidence is approached, the device is returned to its normal setting, as it is operationally provided so that the desired Resolution results.

The invention will now be described with reference to the drawing, which shows it by way of example

iig. 1 is a block diagram of an embodiment, 2 shows a block diagram of a further embodiment, Iig. 3 is a block diagram in greater detail of a

Part of the construction according to Fig. 1, Fig. 4 different waveforms to illustrate the relative timings of different processes or

Operations while
FIG. 5 shows a block diagram of a modification of the construction of FIG.

In British Patent 1,285,175 there is a flow meter having an output frequency proportional to the fluid velocity. This output frequency is the difference frequency f d from "Upstream" and "downstream" "sing-around" pulse trains. With "sing-around" pulse trains are meant pulse trains that be generated by retriggering the transmitter when a pulse is received by the receiver.

The frequency of the "upstream" - ^ Q_y

Pulse chain f ^ ■ ΪΓ ~ »L

The frequency of the "downstream" - ^ ο V

Pulse chain .. ^ p "* 2"

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where V = velocity of the fluid C «. Speed of the sound in the fluid L »distance between the transmitters t * » "upstream" transit time - ie time of

Transmission of pulses from the transformer

or transmitter to receiver tg - ^M downstream "transit time,

From which follows

2V fd - f ₁ rv / f ₂ »L

The output time for such a flow meter is the time it takes from f to the required Measure accuracy. The long times required for this output or for this readout for low flow velocities may be required in some use cases mean a disadvantage.

The constructions of the apparatus of the present invention provide improved flow meters which employ variable frequency multiplying oscillators which can provide much faster readout or output.

In Fig. 1 are the essential components of a flow meter shown.

The output of this flow meter is the differential frequency of two voltage controlled variable frequency oscillators en VFO1 and VF02, whose frequencies exactly to U χ ^ - and

N χ r— can be set by control circuits, soft t “with -C ₂ 1

* VF01 '* VF02 ³ ^ ^η ^ ^ - ^e time spans of the two controlled oscillators.

The output frequency is proportional to the fluid velocity . 3 0 9841/0946

^f d ^{= f} VF01 ~ ^f VF02 ⁼ * <[~ * 2 ^{= L}

For similar water speed and transmitter spacing this method results in an output frequency that is proportional to a water speed of N times that of the method described in British Patent 1,285,175, so that a reading out in the jp times the Time can be done. N can be any convenient number, for example 100 or 1000.

In the arrangement according to FIG. 1, the transmitters are not set up in such a way that they generate a "sing-around", the transmitter is not retriggered by a received pulse, but by a pulse from a main oscillator 11 whose oscillation period is greater than t. or tp is.

The speed resolution of this flow meter is, within limits, determined or determined by the smallest time difference between t ^ and Nty ^ Q, or between t ^ and can be resolved.

• Electromechanical ultrasonic transducers 1 and 2 are spaced apart so as to transmit pressure wave pulses through the fluid along the direction in which the velocity component should be measured.

It seems appropriate to carry out the activity according to the order Fig. 1 for the one transmission direction to describe, the opposite direction atm is borrowed. -

The main oscillator 11 supplies a chain of /. "Start" = pulses, the period of which is expediently about 10 % or 20 % longer

as L is
ü

where Ii is the distance between transformers 1 and 2 and C is the velocity of the sound in the fluid

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Each "Starf" impulse enables a trigger impulse after the transmitter 1 via a drive circuit ΤΣΛ as soon as the next impulse from the oscillator VE ¹ OI appears after this "Starf" impulse. The resulting ultrasonic pulse from the transmitter 1 is propagated through the fluid to the transmitter 2.

A logic 1 circuit compares the transit time of the received pulse from the transmitter 2 via an amplifier BX1 with the time when the K-th pulse from the oscillator VE ¹ OI appears. Depending on whether the received pulse appears before or after the Nth VF01 pulse, a control 1 circuit is caused to make a small change in the control voltage to the oscillator VF01 in order to increase or decrease its frequency in each case. After a number of transmissions, the Nth VF01 pulse will be in time with the received pulse so that

1_ L

^f VFOi " ^Kxt i ^ t ₁ - Ü-V") is,

where t "j is the time that the ultrasonic pulse its way between the transformers.

Similar transmissions are made in the opposite direction from transmitter 2 to transmitter 1 to get a VF02 frequency of ίγρ _Ο 2 ^{= Wx:} fe ^t 2 ⁼ TOT ^ ^TO *

The individual parts for this process are the same as the corresponding, Individual parts described above are provided with reference numerals, but using the index number 2 instead the index number 1.

A differential frequency circuit 12 provides an output whose frequency is proportional to the speed of the flow

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means is between the transformers.

2NV
^f VF01 ~ ^f VF02 ⁼ "ΊΓ

A scanner 13 generates a pulse that follows each start pulse after a fixed time delay, which is followed by a delay circuit 14 is set. The sampling period is set up to be the expected time of arrival of a received pulse. If no received pulse arrives in this sampling time due to an obstacle in the fluid path, then this is indicated by the logic circuit recognized as an error condition, and no change in the VFO ■ Control voltage is applied. A temporary obstruction or disruption that affects only a few transmissions, can be admitted. A permanent malfunction is recognized and reading out is blocked.

Fig. 3 shows some parts of Fig. 1 in greater detail. Again, the process is only carried out in one transfer

considered direction. The VFO frequency must be exactly Nx ^ -, where ty. is the time it takes for an ultrasonic pulse to travel from the sending transmitter to the receiving transmitter.

The sending transmitter will be in time coincidence with triggered by a VFO pulse and a comparison is made between the time to the Nth VFO pulses and the time which elapses before a pulse is received at the receiving transmitter. Any difference between these times is generated an error voltage that controls the VFO frequency. After a sufficient number of transmissions. There will be a time match between the Nth VFO and the received pulse, i.e. fyjin is exactly equal to N χ t-.

Referring to Figure 3, the process is as follows:

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A start pulse (labeled RESET in Fig. 3) from Main oscillator 11 resets flip-flop circuits 15 and 16, resets the -r-N counter 17 and sets the start flip-flop 18 back.

At the end of the reset pulse, the -S ^ -N counter 17 begins to count on.

When a next VFO pulse (n = 1) the -r-N counter 17 is supplied, the "start" flip-flop circuit 18 is set, and its output triggers the transmitter drive TX, i.e. the transmission transmitter is in time coincidence with a VFO pulse triggered or controlled.

Either the Nth VFO pulse (output from the -r counter) set the flip-flop circuit 16, or the received Pulse RX can set the flip-flop circuit 15 (whichever appears first. The first flip-flop setting prevents the other.

Depending on whether the flip-flop circuit 16 or the flip-flop circuit 15 works, then the VFO requires a control voltage increase, to reduce or increase its frequency in each case.

When the flip-flop circuit 15 operates, this triggers a monostable circuit 19, which is via a level shift circuit 21 turns on an input current for an integrator 22, which increases its output by a small positive Increase changes. This change in the VFO control voltage increases something the VFO frequency.

On the other hand, when the flip-flop circuit 16 operates, its monostable circuit 23 supplies it through a level shift circuit 24 an input current of opposite polarity to in this way the integrator output to change a small negative increase and decrease the VFO frequency. 309841/0946

After a number of transmissions, the VFO frequency is adjusted very precisely to ΪΓ χ tT, and subsequent transmissions produce errors of opposite polarity, so that the VFO frequency alternates slightly lower and higher

1 '

is regulated as N χ τ—. '

The integrator 22 is set so that it has a sufficient small increase results when passing through the level shift circuit 21 or the level shift circuit 24 is triggered in order to achieve the best resolution that can be achieved with the required response time and the other sources of error in the system are compatible «, Such small increases can however, a long delay in achieving coincidence result in the initial setup if initially there is a widespread discrepancy between the arrival signs of the received EX pulse and the "Nth VFO 'pulse. The Integrator 22 is therefore in this embodiment with a manually operable control for increasing the size of the increases so that an approximate coincidence can be quickly achieved.

The smallest time difference between received EX- and "Nth VF0" pulses can be resolved is about 6 nanoseconds, and this gives a speed resolution that is better than that required for most applications.

The speed resolution actually achieved is always better than indicated from the consideration of the time resolution capability of the circuit. Even with stagnant fluids or very constant flow rates, there are always changes in the transmission distances of ultrasound to the average time around _o Although this time changes "lead around the true mean than if © a" to greater vibration of the VFO lrequenz bescliriebenen alternating positive and negative error ,, so. it is

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the average VFO frequency closer to this mean.

If there is an obstacle or a malfunction between the transmitters and no EX pulse is received, none will be issued Correction of the VFO frequency. This obstacle or this disturbance is recognized by a fault circuit (not shown), which blocks the operation of the flip-flop circuit 16. The integrator has a long decay time constant, so a small one Number of "miss" transmissions produced a negligible error. A glitch for more than a couple of transmissions is detected by the "fault" circuit, and the speed reading is prevented until the error is eliminated.

Fig. 2 shows another arrangement in which the speed resolution is improved at the expense of response time. In this procedure, again, two are voltage controlled Variable frequency oscillators VFO1 and VF02 used,

where their frequencies are controlled so that they N χ ττ—

1 1

and U χ are "τ— , but the control voltage is obtained by comparing Mt ^ and MNty- ^ and between Mt ₂ and MNtyj ^. M means how many times each channel is given the opportunity to have a" sing- " around "after each main oscillator pulse. The period of this oscillator must be greater than Mt. or Mt ~. In this way the speed resolution is improved by a factor of M over the system (as in FIG. 1), with multiplying VFO's without" sing-around "can be used. By selecting M accordingly, a satisfactory compromise between the speed resolution and the response time can easily be set for each selected measurement.

As with Fig. 1, it is useful to describe the operation for one transmission direction, with the opposite Direction is similar.

The main oscillator 25 supplies a train of pulses, the period of which is greater than ML.

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2 3 -S4

Each start pulse from the main oscillator 25 gives one Transmitter drive circuit TX1 the possibility of a drive pulse to be delivered after the transformer 1 as soon as the next pulse appears after this start pulse.

The resulting ultrasonic pulse travels through the fluid from the transmitter 1 to the. Transmitter 2. The pulse received at the amplifier SX1 output immediately triggers drive TX1 to generate another drive pulse for transmitter Λ ; ie the system carries out a "singaround". The M-th received pulse at the amplifier RX1 does not trigger the drive TX1 again, so that only M ^ 'sing-arounds "appear. In this exemplary embodiment, M is selected so that there are not as many" sing-arounds "as any one Probability of coincidence between sending and receiving at one of the transmitters is introduced.

compares the logic "1" circuit / arrival time of the M-th

received pulse with the time when the M χ Nth VFO1 pulse appears after the start pulse; ie there is a comparison between Mt and MNtyp _Q. instead of.

Depending on whether the received pulse appears before or after the MNth, there will be a small change in voltage executed by the control-1 circuit to set the VFOI frequency to increase or decrease in each case.

Subject to a number of these periods of the "sing-arounds", the MN-th VP01 pulse is in time coincidence with the M-th pulse from the amplifier ΒΣ1, so that, as in the previous method _f

" ^{N x} * 1 (* 1 * ^C " ^{V) is} ·

Similarly , transmissions are made in the opposite direction from transmitter 2 to transmitter 1 to

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^ '2 1 6437

= κ χ t «(t-, = G + V).

The flow _{meter output (} proportional to the water velocity, is the difference frequency between the "two VFOs".

^f D ^sf VF0i ~ ^f FV02 ^β * 1 2

It can be seen that the circuit of FIGS. 1 and 3 readily for this limited "sing-around" method suitable is. The operation is similar, except that after the main oscillator trigger pulse, a counter through MN must share, and the flip-flop circuits work at the MN-th VFO pulse and at the Kth received SX pulse.

For a given error in the VFO frequency, the time difference between the inputs after the flip-flop circuits is thus K times as large, which results in an effective time comparison between t> j and NtypQ ^ j, which corresponds to y times that of the one without a limited "sing-around" is possible. An increase in K in flow rate resolution is thus achieved.

Each trigger output after the drive circuit ΤΣ also generates a sampling pulse which is delayed so that a received pulse arrives within this sampling time. The trigger is blocked, except during the sampling pulse, in order to prevent a new triggering by unwanted signals as far as possible. If no pulse is received during the sampling pulse because of an obstruction or disturbance in the fluid path between the transmitters, this is determined to be a fault condition. A trigger pulse is generated for the drive circuit TX for each error condition in order to maintain the "sing-around", but the YPO control voltage is not changed.

3QSBt "'CS46

/ 21 ε

IB 43 7

A temporary or fleeting one. Fault that only a few Transfers, impaired, can be tolerated. A longer one Fault is recognized and the reading or output is prevented.

Since the transmitters do not receive pulses at the same time, is it possible to use a single amplifier as a replacement for that Amplifier EX1 AMP and BX2 AM? in Fig. 1 to use. One corresponding rearrangement of the circuit is shown in Fig. 5, which shows some additional improvements.

According to Fig. 5, a master oscillator 35 provides a chain of pulses whose period is greater than L / C, after, each the trigger TX1 and TX2, the pulses after the trigger TX2 in the time relative to the pulses after the trigger TXI are shifted, "tim a. Clash of reception and Avoid sending impulses to the transformers.

A pulse from the main oscillator 55 controls the TX ^ trigger, which triggers the drive circuit TX1 on the next VFO1 pulse. The output from the trigger ΤΧΐ. also resets the ■ T'-U counter 36 to η = Λ . _. ·. · ..,.

The resulting ultrasonic pulse travels through the Fluid through from the transfer 1 to the transfer, Bger 2. The received pulse is passed on to amplifier 37.

A precise time reference is received in this case constructed by a Hull passage detector 38. This is a feature which enables improved resolution.

The output from the Hull passage detector 3S will be above. a connected scanner and a gate 39 after, the logic 1 circuit and the control 1 circuit transmitted, which correspond in operation to the components that are provided with the same reference numerals in Fig., Λ. So the logical 1-

30 9841/094 6

Switching the time of arrival of the received signal from transmitter 2 with the time at which the Nth pulse appears from the oscillator VFOI. Depending on whether the received Pulse appears before or after the Nth VF01 pulse, the control 1 circuit is caused to make a small change in the control voltage for the oscillator VF01 in order to . to increase or decrease its frequency in each case. After a number of transmissions, the Kth VF01 pulse will be - are in time with the received pulse.

The signal from the main oscillator 35 also controls the switched sampler and the gate 39 via a delay device 4-1 and a sampler 42. The delayed sampling signal serves to both the response _/ except around the expected arrival time of a received pulse, as well as to block also to operate the switch to forward the received signal according to the logic 1 circuit.

The actuation for a transmission from the transmitter 2 takes place in parallel with that for the transformer 1, except that the received signal from the transmitter 1 via the switched scanner and the gate 39 after the logic 2 circuit is forwarded.

The outputs, which are labeled VF01 0 / P, corresponding to Nf _x ., And VF02 0 / P, corresponding to H _f p, are fed to a differential logic circuit 4-3. The flow rate is indicated by a differential output (Hf ^ / vif.). In this embodiment, the direction of flow is indicated by logic outputs that either Nf ,. Show> Nfp or Uf, <Hf2.

As with the arrangement of FIG. 1, the integrator has one sufficient time constant, whereby it does not mind if some impulses are obstructed. The received impulses are

30984 1/0946

monitored by the amplifier 37 and, as RX1 O / P and RX2 0 / P indicated, transmitted after a logic fault circuit 44. This circuit indicates an error and locks the reading out if less than a preset minimum percentage received by pulses. Typically, it has been found to be satisfactory Set the fault circuit so that it comes into operation when less than 10% of the transmitted pulses from one of the Transmitters are received. In practice it arises homely out that either all impulses are received or there is some interference if none are received will. A temporary, fleeting disturbance, such as a passing boat, where to monitor the river flow can interrupt some pulses but will generally not affect the reading.

It will be understood that the arrangement of FIG. 2 is modified can be used to use a single amplifier in a manner similar to FIG.

The device of the preceding exemplary embodiments is particularly suitable for measuring the total flow of rivers or large pipes advantageous, where the transformer over the whole width of the river or pipe are spaced, and on an inclined path to this way introduce a speed component along the way. With such large baselines, the response time of systems as shown, for example, in British Patent 1,285 are unacceptably slow.

However, the device according to the preceding exemplary embodiments is also suitable for measuring flow in small pipelines advantageous. Transmitters can be attached to the outlet side of the pipeline so that no Disturbance to the flow is introduced. The transformers are arranged axially offset and so angled and connected to the pipe

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line coupled that, taking into account refractions at the various intermediate surfaces, a corresponding path, inclined as steeply as possible to the radial direction, followed by the ultrasonic pulses in the flowing fluid will. A flow of about 1 cm / second in a pipe about 10 cm in diameter would give a response time in Minutes if require a "sing-around" system and a coincidence count be used. In the device according to the preceding embodiments, the response time can be without further reduced to a fraction of a second.

The invention is not restricted to details of the exemplary embodiments described above. For example, will it preferred to use two variable frequency oscillators and associated control circuitry so that the measurements can progress in parallel in opposite directions, the only condition being sufficient There is a difference in the timing between individual pulses in order to ensure that the transmission and reception coincide to avoid any transmitter with the following receiver blocking. The main advantages of this type of measurement are as follows:

(1) The measurement ultrasonic pulses are almost completely simultaneously on their way across the fluid path, so that there are any possible effects in this way on the transit speed measurement due to temperature? , Density or other changes in the fluid are minimized, and

(2) it is a straightforward and simple matter to display a difference frequency between two oscillators operating at the same time, and the output is a continuous display of the flow rate. Although changes in flow rate are not immediately followed, they are followed very quickly, especially in the embodiment of FIG. 1,

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However, if desired, it would be possible to use a single voltage controlled oscillator and. associated control circuit and successive determinations of initially the frequency required to fit Έ pulses into the length of time it takes for pressure waves to travel from transmitter 1 to transmitter 2 and, subsequently, the frequency that is required is so that "E pulses fit into the period of time that pressure waves need to get from transmitter 2 to transmitter 1" Such a system or installation would require a memory to hold the first frequency while the second frequency is determined after which the difference frequency can be found by subtraction.

In addition, it is undoubtedly extremely convenient and simple to set up the number N of pulses of the variable frequency oscillator so that it is the same within each time period, so that the difference frequency has a simple proportionality to the flow rate. For example, use SL pulses for upstream measurement and No pulses for downstream measurement. Would it just be necessary then _? adjust the differential output by a constant factor when calculating the flow velocity _o

The invention also relates to modifications of the embodiment outlined in the attached patent claim 1 and relates above all to all features of the invention ₉ which are disclosed individually - or in combination - in the entire description and drawing _{or the like} . ₍ "

Claims

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Claims (1)

Z316437 ~ 19 - PATENT Attorney Phone: (0271) 32409 DIPL-ING. ERICH SCHUBERT τ ** «· **., Μη *. Postal check accounts: Cologne 106931, Esien 203 «2 Bank accounts: FromJ. t Patent attorney Dipl.-Ine. SÄUBERT, 5f Siegen, Eiserner Strasse 227 ~ Deutsche Bank AG., P.O. Box 462 Branches in Siegen and Oberhauten (RhId.) 73 025 Patent claims \ 1 ·) Apparatus for measuring the speed or a speed component of a fluid flow, consisting of a first and a second unit, which are spaced from one another and are each capable of viewing and receiving pressure wave impulses which are via a path through the fluid run in the direction in which the velocity component is to be measured, from a variable frequency oscillator device, from a device which transmits pressure wave pulses from the first unit to the second unit and, separately, from the second unit to the first unit, characterized by a control means (controller 1; controller 2) for controlling the frequency of the variable frequency oscillator means (VFO1; VF02) for generating a number N ^, of vibrations or pulses in a first period corresponding to that which a pressure wave on its way from the first unit (transformer 1) after the second unit t (transmitter 2) needs, and a number Np of oscillations or pulses in a second time span, which corresponds to that which a pressure wave needs on its way from the second unit (transmitter 2) to the first unit (transmitter 1), as well as through a comparator (12i Nf ^, ~~ Nfρ, 45) for displaying the difference in frequency between the vibrations or pulses generated in the first period and the vibrations or pulses generated in the second period. 2. Apparatus according to claim 1, characterized in that N ^ -N ₂ -N. 30984 1 / Π946 J. Apparatus for measuring the velocity, or a component of velocity, of fluid flow, comprising first and second units spaced apart and each capable of transmitting and receiving pressure wave pulses traveling through a path through the fluid in run in the direction in which the velocity component is to be measured, further comprising a first and a second variable frequency oscillator, with a device that transmits pressure wave pulses from the first unit to the second unit, and with a device that transmits pressure wave pulses from the second Unit can be transmitted to the first unit, characterized by a first control device (controller 1) for controlling the frequency of the first variable frequency oscillator (VFO1) so that it generates a number N ^, of oscillations or pulses in the period of time that a pressure wave needs to get from the first unit (transmitter 1) to get to the second unit (transmitter 2) by a second control device (controller 2) for controlling the frequency of the second variable frequency oscillator (VS ¹ OE) ₉ so that it generates a number Np of oscillations or pulses in the period ₉ . which needs a pressure wave to get from the second unit (transmitter 2) to the first unit (transmitter 1), as well as "through a comparator (12; Nf ^^ H ^ p? 4-3) to display the difference in the Frequency between the first (VF01) and the second (VF02) variable frequency oscillator ο 4 ·. Apparatus according to claim 5, characterized in that N ₁ - N ₂ = N. 5. Device according to claim 3 or 4, characterized in that that the first control device (control 2) has a device (15 »16) for determining whether the N ^ -th oscillation (or pulse) of the first variable frequency oscillator (VF01) before or after the arrival of the pressure wave at the second unit (Transmitter 2) appears, and that a device (19 ,. 309841/0 946 21, 22, 23, 24) to generate an error signal for the respective Decrease or increase the frequency of the variable frequency oscillator (VFO1) is provided, depending on whether the iL-th oscillation (or pulse) is before or after the arrival the pressure wave appears on the second unit (transmitter 2). 6. Device according to one or more of claims 3-5 » characterized in that the second control device (control 2) contains a device (15, 16) which determines whether the N ^ th oscillation (or pulse) of the second variable frequency ossillator (VF02) appears before or after the arrival of the pressure wave at the first unit (transmitter 1), and that a device (.19 »21, 22, 23, 24) is provided, which will result in an error for the corresponding decrease or increase of the frequency of the variable frequency oscillator (VF02) generated, ~ ie after, whether the Np-th oscillation (or impulse) before or after the Arrival of the pressure wave at the first unit (transmitter * 1) appears. 7. Apparatus according to claim 5 or 6, characterized in that that the control device (control 1, control 2) is effective in such a way that it has the frequency of its assigned Variable frequency oscillator (VFOI, VF02) changes by a set amount. 8. Apparatus according to claim 7, characterized in that a device for temporarily adjusting the size of the Frequency change with each actuation of the control device (control 1, control 2) is provided. 9) Device according to one or more of claims 1-8, characterized in that a time reference on received pressure wave pulses by a zero crossing detector (38) is built. 309841/0946