DE10062875B4 - Flowmeter - Google Patents

Abstract

flowmeter for liquid media with a measuring chamber, in the at least one transmitting / receiving unit (1) for transmission and for receiving ultrasound signals (2) representing the measuring chamber with and against the flow direction enforce, is arranged, wherein the transmitting / receiving unit (1) at least one ultrasonic transducer (3, 4) for generating sound, characterized characterized in that additionally at least a sound pressure sensor (5; 6) is provided, the sound pressure sensor (5; 6) within the sound path L of the ultrasonic transducer (3; 4) is positioned, with the sound pressure sensor (5; 6) only sound-wave-dependent measuring signals can be generated and only the latter used for sound propagation be, wherein the sound pressure sensor (5; 6) has a defined starting point for the Sound transit time determination forms.

Description

The The invention relates to a flow meter for liquid media with the others Features of the preamble of claim 1.

For one measurement technology perfect transmission and reception operation of a Ultraschalldurchflußmessung is independence of disturbances a necessary Requirement. In practice, however, all components have one Ultrasonic measuring line one more or less strong dependency across from the environmental and medium field sizes. Around the differential transit time of the emitted by a transmitting / receiving unit and received ultrasonic signals, the measuring chamber with and against the flow direction enforce, determine, it is imperative that at the flow rate 0 of the medium, the respective signal propagation times of the opposing ultrasonic signals equal are. In the previously known solutions This signal equality, the so-called reciprocity principle, through as possible realized slight deviations in the transmitting and receiving circuit. The hitherto known transmitting / receiving units are constructed in such a way that that only certain elements, e.g. which send the ultrasonic signals Ultrasonic transducers (e.g., piezoelectric elements) of both Be traversed signal paths. The remaining parts of the signal path or the components arranged there are only passed through from one side, so that all field influences Create differences in the phase position. The components of the transmission and Receiving electronics also prove to be extremely sensitive, in which Couplings of parasitic capacities strong measurement deviations can cause. Furthermore, there are delays in the processing time by analog components of 1 nsec per component (so-called gate running time), which also to the above-described violation of the principle of equality to lead can. Measurable is thus far only the real Schallaufzeit, what with the disadvantage is connected that z. B. neither temperature, density and viscosity measurements with the ultrasonic measurement possible are. The temperature measurement is previously feasible only with a temperature sensor, the Density measurement is carried out via the concentration distribution. To carry out these measurements must be in the ultrasonic counter the respective measuring devices be integrated, resulting in a relatively complex structure of the ultrasonic counter.

Out US 4,389,899 a flow meter is known, in which determine in the medium to be measured two electro-acoustic transducers a sound path, within which the sound propagation time takes place. The sound transit time determination is adversely affected in this known embodiment by transformer-related disturbances.

The US 4,860,593 describes an ultrasonic measuring device for measuring the flow rate of fluid in a channel in which sound waves are transmitted from a transmitter to a first receiver and a second receiver. The signals of the two receivers are then further processed and correlated with the signal of a likewise located in the passage pressure transducer.

From the US 5,513,535 A , of the US 5,421,212 A and WO 93/20411 A1 discloses a method and a device for measuring flow velocities of gases within a pipeline. The flow velocity of the gas is detected by a polarity correlation of signals of two sensors positioned at a distance L from each other along a tube.

The WO 93/14382 A1 relates to a device and a method for Measurement of the flow rate a gas / liquid fraction in tubes. For this are on one side of the tube a transducer as well as on the opposite Side of the tube three Provided pressure sensors. Of these, each in different Distance or arranged on different side pressure sensors the sound wave emitted by the transducer is recorded and closed based on the measured differences on the flow rate.

The DE 84 22 080 U1 describes a device for determining the mass flow rate of gases and / or liquids by means of the travel time measurement method with acoustic waves which propagate within the medium between sound sensors. The two sound sensors are symmetrical to the sound transmitter, so that the flow information is formed by the displacement of the two received signals.

From the DE 196 05 164 C2 is an ultrasonic flowmeter for liquid or gaseous media with a flow-through measuring chamber and ultrasonic transducers for emitting and receiving ultrasonic signals known, the ultrasonic signals pass through the measuring chamber with and against the flow direction.

In the DE 42 13 170 C2 an ultrasonic flowmeter for fluid media is described, in which a Ultraschallmeßstrecke is arranged in a flowed through by the fluid medium measuring channel. As a transmitter and / or receiver ultrasonic transducers are provided. About an evaluation of the flow of the fluid medium is determined due to the transit time or phase difference of an ultrasonic signal. The measuring channel is wound helical or helical.

Of the Invention is the object of an ultrasonic meter with The features of the preamble of claim 1 form such that a completely reciprocal working measuring method is created and thus the Schallaufzeit more accurate than previously determined is.

These Task is solved by the entire teaching of claim 1. advantageous Further developments of the flow meter emerge from the dependent claims 2 - 23.

Of the has flowmeter according to the invention additionally at least one sound pressure sensor, the sound pressure sensor is positioned within the sound path L of the ultrasonic transducer, with the sound pressure sensor are exclusive sound wave dependent measuring signals producible and only the latter are used for determining the sound propagation time, wherein the sound pressure sensor forms a defined starting point for the Schalllaufzeitermittlung. By means of this Schallaufzeitermittlung is the measurement of deviations z. B. in the arrangement of the ultrasonic transducer independently. The Ultrasonic transducers are only responsible for the generation of sound, if the measurement is exclusive done by means of the pressure sensors. Due to the independence of the arrangement of the ultrasonic transducer is thus a completely reciprocal working measuring method created.

Out The determined sound transit time can be the temperature, density or viscosity of the be determined measuring medium.

Of the Pressure sensor thus absorbs the sound signal before it for the differential transit time used measuring section has passed through, and thus determines a defined start time at a defined measuring point.

The Schallaufzeitermittlung can over a first pressure sensor and a second pressure sensor take place. Thus, the Schallaufzeitermittlung exclusively by means of the pressure sensors and as explained above, it is a completely reciprocal working measuring method possible.

advantageously, can a first and a second ultrasonic transducer may be provided which define a sound path L Within the sound path L are a first and a second pressure sensor, which also are arranged at a predetermined distance M to each other. Of the first pressure sensor can be the first ultrasonic transducer and the second pressure sensor directly associated with the second ultrasonic transducer be. However, it is also possible that the Pressure sensors at a different angle or with different intervals are arranged to the ultrasonic transducers. By this arrangement the pressure sensors it is possible in the above constellations that this both the generated sound signals from that of the respective pressure sensor associated ultrasonic transducer come, as well as after passing received by the medium to be measured sound signals in electrical Convert signals. The Schallaufzeit is only between the Pressure sensors determines and is therefore of the arrangement of the ultrasonic transducer or other components in the transmitting / receiving unit independently.

alternative to the above described sound transit time determination between the two Pressure sensors it is also possible that the Schallaufzeitermittlung between at least one ultrasonic transducer and a pressure sensor.

In In this case, then the received signal (after the traversed measuring distance) taken up by the ultrasonic transducer. There is the possibility that only an ultrasonic transducer and a pressure sensor are provided, wherein the ultrasonic signals the flow enforce the medium to be measured. Thereby the time of the ultrasonic signals becomes between the ultrasonic transducer and the pressure sensor and back measured from which the differential transit time can be determined.

A another possibility the Schallaufzeitermittlung provides that the pressure sensor in a defined distance (e.g., exactly in the middle) to the two ultrasonic transducers is arranged. The pressure sensor is used in this arrangement, the recording the signal emitted by the respective ultrasonic transducer, wherein the ultrasonic transducers of the recording of the flow passing through Serve signal. In this embodiment must either the distance between pressure sensor and ultrasonic transducer known or the pressure sensor exactly halfway between the two transducers be arranged so that the two opposing ones Ultrasound signals the same measuring path return, what kind of the exact Schallaufzeitermittlung is necessary.

Of the Pressure sensor can be attached to an ultrasonic reflector, in the range of reflecting surface or arranged on a wall irradiated by the ultrasonic signal be so that the Ultrasound signals can pass through the medium unhindered.

However, if the pressure sensors are arranged in the area of the sound path, they can be sound-permeable at least partially so that the sound signal emitted by the ultrasound transducer can propagate unhindered in spite of the pressure sensor. So that the pressure sensor for the ultrasound is permeable, the Außenabmes tions of the pressure sensors at most equal to three times the wavelength of the ultrasonic signal.

Advantageous can the arrangement of the pressure sensors in the flow region of the medium to be measured be as with sound trajectories that are perpendicular to the flow or without directional flow (e.g. Totwasserräume, Nooks with vortex formation and areas with unsteady or unstable ones Flow conditions) or by other media than the medium to be measured (eg by a Schallleitkörper) run, the sound behavior of the emitted and to the measurement do not influence the ultrasound used. As above briefly mentioned, can be between the ultrasonic transducer and the ultrasonic transducer assigned Durcksensor a Schallleitkörper are located so that the from Ultrasonic transducer emitted ultrasonic signals directly into a good measurable Flow of Medium be initiated.

Especially in the case where only one ultrasonic transducer and one pressure sensor is provided, the pressure sensor should have a very fast transient response have, since already the time of the emitted ultrasonic signal between ultrasonic transducer and pressure sensor is measured.

Further it is possible, the existence Ultrasonic transducer pair formed as only a piezoceramic component which emits ultrasonic signals on both sides. By the thus created send / receive unit, the entire flow meter extremely compact accomplished become.

In the arrangement of two pressure sensors, the sound transit time t _{sound 1} in one direction (from the first to the second pressure sensor) and the sound transit time t _{sound 2} in the opposite direction (from the second to the first pressure sensor) can be determined. Based on the transit times, the transit time difference dt = t _{sound 1} -t _{sound 2} can then be determined. By the respective completely traversed signal paths of the two ultrasonic signals, which run in different directions, the absolute sound propagation time can be determined and the actual transit time difference dt can be determined. By means of the knowledge of the absolute sound propagation time, the temperature, density and viscosity can also be determined.

If, on the other hand, only one pressure sensor and one ultrasonic transducer are provided, the sound transit time from the ultrasonic transducer to the pressure sensor and back is determined, from which the transit time difference dt = t _{sound 1} -t _{sound 2} can then also be determined. If two ultrasonic transducers are provided, which are arranged at a defined distance from the pressure sensor, the transit time from the pressure sensor to the second ultrasonic transducer and from the pressure sensor to the first ultrasound transducer is determined and from this the transit time difference is determined.

The Ultrasonic transducers can with an oscillator for generating an oscillator frequency over at least a pulse generator in connection. Appropriately, everyone is here Ultrasonic transducer associated with a pulse generator by the oscillator to be started.

With two pressure sensors, the time measurement can take place in such a way that the sound propagation time t _{sound 1 is} determined by the formula t _{sound 1} = (T _g1s2 -T _g1s1 ) + m ₁ * T _OS after a start-up behavior of a wave train. T _{g1s1 is} the time between a reference signal and the digitized received signal at the first pressure sensor, T _g1s2 is the time between the reference signal after m oscillations and the digitized received signal at the second pressure sensor. m ₁ is a natural number and marks the number of oscillations between the first and second pressure sensor. This number is determined by the sound propagation time expected by the measuring system. T _OS is the period of the oscillator and is for example 1 μsec. The sound propagation time t _{sound 2} is correspondingly reversed by the formula t _{sound 2} = (T _g2s1 - T _g2s2 ) + m2 · T _OS can be determined. The transit time difference dt can be determined by the formula dt = ((T _g1s2 -T _g1s1 ) + m ₁ * T _OS ) - ((T _g2s1 -T _g2s2 ) + m ₂ * T _OS ). Thus, an unambiguous determination of dt is possible if the period of the oscillator T _OS (eg 1 μsec) is greater than the transit time difference dt (eg 600 nsec), since then m the range of values of - 2 <(m ₁ - m ₂ ) does not exceed <2. For m ₁ = m ₂ the delay difference is therefore dt = T _g1s2 - T _g1s1 - T _g2s1 + T _g2s2 . The advantage of the transit time difference measurement therefore consists in the fact that phase shifts, which can arise due to the pressure sensors, cancel each other out by virtue of the subtraction. The dynamic transient response of the piezoceramic of the ultrasonic transducer no longer needs to be fully considered. This makes it possible to measure in the signal burst at the very beginning, when no echo signals from the test section have been superimposed on the useful signal.

Corresponding the time is measured with only one pressure sensor, but here the reception delay time the / of the ultrasonic transducer in the differential transit time determination miteingeht.

through The oscillator can also generate a second oscillator frequency be with which m is determinable.

The pressure sensors may be z. B. piezo elements, capacitive silicon-based sensors or strain gauges act. The respective pressure sensor can be designed as a differential pressure sensor and thus in a simple manner both the record both transmitted and received ultrasonic signals and convert them into electrical signals.

Of the Pressure sensor can advantageously be used as a composite in the ultrasonic transducer be integrated, what additional Saves assembly steps and signal feeds.

The Invention is based on advantageous embodiments in the drawing figures explained in more detail. These demonstrate:

1 a basic structure of the transmitting / receiving unit,

2 a schematic representation of an alternative construction of the transmitting / receiving unit,

3 a schematic representation of Schallaufzeitermittlung from the pressure sensor 1 to the pressure sensor 2 such as

4 a further alternative embodiment of a basic structure of a transmitting / receiving unit,

5 a further alternative embodiment of a basic structure of a transmitting / receiving unit and

6 an arrangement of a transmitting / receiving unit in a Wirbelmeßkammer.

1 shows the send / receive unit 1 a flow meter for liquid or gaseous media with a measuring chamber, which is not shown in detail in the drawing figure. The send / receive unit 1 is used to send and receive ultrasound signals 2 , which pass through the measuring chamber with and against the flow direction of the medium, which is indicated by the arrow. The transmitting / receiving unit comprises two ultrasonic transducers 3 . 4 for generating sound, which are designed in particular as piezoelectric elements. Furthermore, pressure sensors 5 . 6 provided with which sound-wave-dependent measurement signals for Schallaufzeitermittlung can be generated. The ultrasonic transducers 3 . 4 Accordingly, only the sound generation, the pressure sensors 5 . 6 only the recording of the ultrasound signals emitted by the ultrasound transducers at the beginning of the sound path and the ultrasound signals passing through the sound path and conversion into electrical measuring signals. Consequently, the ultrasonic measurement is not dependent on disturbances in the range of the ultrasonic transducers and the sound path distance between the pressure sensor and the ultrasonic transducer, and a completely reciprocal measuring method can take place. For the conversion of the analogue electrical measuring signals of the pressure sensors 5 . 6 in digital signals is eg an A / D converter 10 intended.

The first 3 and the second ultrasonic transducer 4 are according to 1 arranged such that they define a measuring path L, wherein within the measuring path L also at a predetermined distance M to each other a first 5 and a second pressure sensor 6 are located. The first pressure sensor 5 is the first ultrasonic transducer 3 and the second pressure sensor 6 is the second ultrasonic transducer 4 assigned. The current with and opposite to the direction of flow ultrasonic signals 2 thus enforce the identical signal path, which is defined by the distance M, so that non-linearities outside this measurement path in the transit time determination of the ultrasonic waves play no role.

In the alternative construction of the transmitting / receiving unit 1 according to 2 are the pressure sensors 5 . 6 not directly to the ultrasonic transducers 3 . 4 but are located in the directed flow while the ultrasonic transducers 3 . 4 are arranged in lateral niches of the flow channel. Also in this case, the measurement of irregularities in the field of ultrasonic transducers 3 . 4 or the distance between the respective ultrasonic transducer and the pressure sensor independently, since only the measuring path M between the pressure sensors for the measurement 5 . 6 is significant, which is interspersed in each case by the two opposing ultrasonic waves.

By means of these arrangements, the sound propagation time t _{Sound 1} in the one direction from the first pressure sensor 5 to the second pressure sensor 6 and the sound propagation t _{sound 2} in the opposite direction from the second pressure sensor 6 to the first pressure sensor 5 be determined. Based on the transit times, the transit time difference dt = t _{sound 1} - t _{sound 2} can then be determined. Accordingly, in the differential transit time determination directly the two measurement signals of the pressure sensors 5 . 6 used. The phase shift between the respective ultrasonic transducer 3 . 4 and the respective associated pressure sensor 5 . 6 occur are not taken into account in the time measurement.

An exemplary Schallaufzeitermittlung with the arrangement according to 1 or 2 comes from 3 out. T ₀ denotes the starting point at which the oscillator 7 (O _S ) the first pulse generator 8th starts and by means of the first ultrasonic transducer 3 generates a sound burst signal, which with the first pressure sensor 5 (S1) is detected. A wave train in the sound burst signal is denoted by reference numeral 11 Mistake. After the transient response n · f _g1 (g1 denotes the first pulse generator 8th ), the time between a reference signal and the first digitized received signal E1 is determined (T _g1s1 ). n is an arbitrary integer number of wave trains up to the steady state. After another m oscillations of Os, the time between the reference signal and the digitized received signal E2 (T _g1s2 ) at the second pressure sensor 6 (S2) measured. The sound propagation t _{sound 1} can now be determined by the formula t _{sound 1} = (T _g1s2 -T _g1s1 ) + m ₁ * T _OS T _OS is the period of the oscillator 7 , The sound propagation time t _{sound 2} is correspondingly _reversible according to the formula t _{sound 2} = (T _g2s1 - T _g2g2 ) + m ₂ · T _OS can be determined.

The differential transit time, ie the transit time difference dt, can be determined by the formula dt = ((T _g1s2 - T _g1s1 ) + nx ₁ T _OS - ((T _g2s1 - T _g2s2 ) + nx ₂ T _OS ), the period of the oscillator T _OS is greater than the transit-time difference dt, since then m does not exceed the value range - 2 <(m _{1 -m 2} ) <2. For m ₁ = m ₂ the transit-time difference dt = T _g1s2 -T _g1s1 -T _g2s1 + T _g2s2 . Thus, the timing of phase shifts and dynamic time delays of the pulse generators 8th . 9 and the ultrasonic transducer 3 . 4 as well as the number m independently.

As the 4 . 5 and 6 can clarify, even only a pressure sensor 5 in combination with one or two ultrasonic transducers 3 . 4 be provided for Schallaufzeitermittlung. According to 4 the Schallaufzeitermittlung between an ultrasonic transducer 3 and a pressure sensor 5 which is at a reflection surface 13 is arranged. In this arrangement, the transit time of the ultrasonic transducer 3 to the pressure sensor 5 and vice versa from the pressure sensor 5 to the ultrasonic transducer 3 measured and determines the differential transit time. The pressure sensor 5 has a very fast transient response.

In 5 is a pressure sensor 5 right in the middle between the two ultrasonic transducers 3 . 4 arranged. The differential transit time results from the sound transit time between the pressure sensor 5 and the ultrasonic transducer 4 and between the pressure sensor 5 and the ultrasonic transducer 3 , Alternatively, there is also the possibility that the pressure sensor 5 at a different defined distance to the two ultrasonic transducers 3 . 4 is arranged. By knowing the distances from the pressure sensor 5 to the respective ultrasonic transducers 3 . 4 then the differential transit time can also be determined. According to the arrangement in the 4 and 5 are the pressure sensors 5 . 6 in the area of a reflection surface 13 arranged. In the 2 and 6 are the pressure sensors on an ultrasonic reflector 12 arranged.

The pressure sensors 5 . 6 are in particular in the arrangement according to 1 At least partially sound-permeable, so that from the ultrasonic transducers 3 . 4 emitted ultrasonic signals 2 can spread unhindered. For this purpose, the outer dimensions of the pressure sensors at most equal to three times the wavelength of the ultrasonic signal 2 ,

The pressure sensors 5 . 6 are in the 1 and 2 arranged in the flow region of the medium to be measured so that sound paths perpendicular to the flow (see 2 ) or sound trajectories without directional flow (eg niches with vortex formation) do not affect the sound behavior of the sound used for the measurement. Furthermore, between the ultrasonic transducer 3 . 4 and the ultrasonic transducer 3 . 4 associated pressure sensor 5 . 6 There is a Schallleitkörper, which forwards the ultrasound without obstruction.

In the 6 illustrated Wirbelmeßkammer 14 includes an ultrasonic transducer 3 , which is formed as a piezoceramic component and ultrasonic signals on both sides 2 sending out. Alternatively, two ultrasonic transducers 3 . 4 summarized, which in each case in one direction the ultrasonic signals 2 send out. The pressure sensor 5 takes the emitted ultrasonic signals 2 on, while the sound path traversed ultrasonic signals from the ultrasonic transducer 3 and or 4 be recorded.

By means of the oscillator 7 can also be generated a second oscillator frequency. Thus, the number of vibrations m can be determined and you get the respective exact time t _{sound 1} or t _{sound 2} .

The pressure sensors 5 . 6 can be designed as differential pressure sensors or as ultrasonic microphones. In a particularly advantageous embodiment variant, the pressure sensors 5 . 6 as a composite in the ultrasonic transducer 3 . 4 be integrated and thus used as a component in the flow meter, which greatly facilitates the assembly.

1

Transmitter / receiver unit

2

ultrasonic signals

3

first ultrasound transducer

4

second ultrasound transducer

5

first pressure sensor

6

second pressure sensor

7

oscillator

8th

first pulse generator

9

second pulse generator

10

A / D converter

11

wave

12

Ultrasonic Reflector

13

reflecting surface

14

Wirbelmeßkammer

Claims (23)

Flow meter for liquid media with a measuring chamber in which at least one transmitting / receiving unit ( 1 ) for transmitting and receiving ultrasound signals ( 2 ), which pass through the measuring chamber with and against the flow direction, is arranged, wherein the transmitting / receiving unit ( 1 ) at least one ultrasonic transducer ( 3 . 4 ) for generating sound, characterized in that in addition at least one sound pressure sensor ( 5 ; 6 ), the sound pressure sensor ( 5 ; 6 ) within the sound path L of the ultrasonic transducer ( 3 ; 4 ) is positioned with the sound pressure sensor ( 5 ; 6 ) only sound-wave-dependent measuring signals can be generated and only the latter are used for sound propagation time determination, the sound pressure sensor ( 5 ; 6 ) forms a defined starting point for the sound transit time determination. flowmeter according to claim 1, characterized in that from the determined sound transit time the temperature, density or viscosity of the medium to be measured is determinable. Flow meter according to one of the preceding claims, characterized in that the sound transit time determination via a first pressure sensor ( 5 ) and a second pressure sensor ( 6 ) he follows. Flow meter according to one of the preceding claims, characterized in that a first ( 3 ) and a second ultrasonic transducer ( 4 ) are provided which define a sound path L, and within the sound path L, also at a fixed distance M to each other, the first ( 5 ) and the second pressure sensor ( 6 ) are located. Flowmeter according to claim 4, characterized in that the first pressure sensor ( 5 ) the first ultrasonic transducer ( 3 ) and the second pressure sensor ( 6 ) the second ultrasonic transducer ( 4 ) assigned. Flow meter according to one of claims 1 or 2, characterized in that the sound transit time determination between at least one ultrasonic transducer ( 3 . 4 ) and a pressure sensor ( 5 . 6 ) he follows. Flow meter according to one of the preceding claims, characterized in that the pressure sensor ( 5 . 6 ) at a defined distance to the two ultrasonic transducers ( 3 . 4 ) is arranged. Flow meter according to one of the preceding claims, characterized in that the pressure sensor ( 5 . 6 ) on an ultrasonic reflector ( 12 ) is arranged. Flow meter according to one of the preceding claims, characterized in that the pressure sensor ( 5 . 6 ) in the region of a reflection surface ( 13 ) is arranged. Flow meter according to one of the preceding claims, characterized in that the pressure sensors ( 5 . 6 ) are at least partially sound permeable. Flowmeter according to one of the preceding claims, characterized in that the external dimensions of the pressure sensors ( 5 . 6 ) at most three times the wavelength of the ultrasonic signal ( 2 ) correspond. Flowmeter according to one of the preceding claims, characterized in that the pressure sensors ( 5 . 6 ) are arranged in the flow region of the medium to be measured. Flowmeter according to one of the preceding claims, characterized in that between the ultrasonic transducer ( 3 . 4 ) and the ultrasonic transducer ( 3 . 4 ) associated pressure sensor ( 5 . 6 ) is a Schalleitkörper located. Flow meter according to one of the preceding claims, characterized in that the pressure sensor ( 5 . 6 ) has a very fast transient response. Flow meter according to one of the preceding claims, characterized in that the ultrasonic transducer ( 3 . 4 ) is formed as a piezoceramic component which has ultrasonic signals ( 2 ). Flow meter according to one of the preceding claims, characterized in that the sound propagation time T _{sound 1} with the flow direction and the sound propagation time t _{sound 2} counter to the flow direction can be determined and based on the transit times of the transit time difference dt = t Sound 1 - t Sound 2 can be determined. Flow meter according to one of the preceding claims, characterized in that the ultrasonic transducers ( 3 . 4 ) with an oscillator ( 7 ) for generating an oscillator frequency via at least one pulse generator ( 8th . 9 ) keep in touch. Flowmeter according to one of the preceding claims, characterized in that, after an oscillation behavior of a wave train ( 11 ) starting from the first ultrasonic transducer ( 3 ) the sound transit time t _{sound 1} through the formula t Sound 1 = (T g1s2 - T g1s1 ) + m 1 * T OS can be determined, wherein T _g1s2 the time of the digitized received signal at the second pressure sensor ( 6 ), T _g1s1 the time of the digitized received signal at the first pressure sensor ( 5 ), m _{1 is} the number of oscillations between the first and second pressure sensors and T _{OS is} the period of the oscillator ( 7 ), and the sound transit time t _{sound 2} can be determined correspondingly inversely. Flowmeter according to claim 18, characterized in that the transit time difference dt can be determined by the formula dt = ((T g1s2 - T g1s1 ) + m 1 * T OS - ((T. g2s1 - T g2s2 ) + m 2 * T OS ) wherein the period of time of the oscillator T _{OS is} greater than the delay difference dt and at m ₁ = m _2, the delay difference dt = T g1s2 - T g1s1 - T g2s1 + T g2s2 is. flowmeter according to one of the preceding claims, characterized that at least a second oscillator frequency can be generated. Flow meter according to one of the preceding claims, characterized in that at least one of the pressure sensors ( 5 . 6 ) is a differential pressure sensor. Flow meter according to one of the preceding claims, characterized in that at least one of the pressure sensors ( 5 . 6 ) is designed as an ultrasonic microphone. Flow meter according to one of the preceding claims, characterized in that at least one of the pressure sensors ( 5 . 6 ) as a composite in the respective ultrasonic transducer ( 3 . 4 ) is integrated.