The invention is related to an apparatus and a method for measuring the flow velocity of a fluid in a pipe according to the preamble of claims 1 and 1 1 .

Ultrasonic flow meters are commonly used for measuring the volume flow of different fluid media in many industrial applications. Especially clamp-on ultrasonic flow meters are very convenient and flexible to use because they can be mounted without opening the pipe and without interruption of the process.

For mounting clamp-on ultrasonic flow meters, two transducers for emitting and receiving ultrasonic signals are mounted on outer surface of the pipe. The transducers are emitting ultrasonic signals under a fixed angle through the pipe. To do so, the transducers are arranged on the same side of the pipe at such a distance to each other that the second transducer receives the signal emitted by the first transducer and reflected at the opposite pipe wall and vice versa. Also variants exist were the transducers are arranged on opposite sides of a pipe but in a certain distance along the pipe axis and the signal transmitted straight through the pipe is detected. This avoids signal losses due to the reflection but generates a higher installation effort. The measurement principle for both configurations is the same.

Due to the emitting of the sound under a fixed angle, the transducers have to be ar- ranged in a specific way which is determined by the emitted beam angle and the dimension of the pipe, in particular the pipe diameter and the wall thickness. These parameters are varying strongly from pipe to pipe and are not known a priori. Therefore, the transducers cannot be arranged in fixed installations already during the production process but have to be manually adjusted on-site by the user. This generates a high effort for the user during the sensor installation. Also a wrong mounting of the transducers can result in significant measurement errors of the sensor or even a fail- ure of the sensor operation. Due to these requirements and problems during the sensor installation, clamp-on flow meters gained a low reputation although they provide the advantage of a non-intrusive mounting and operation of the sensor.

To gain a strong signal of an ultrasonic flow meter which is of great importance for obtaining a high accuracy of the measurements carried out afterwards, it is of advantage to focus the ultrasonic beam as good as possible onto the second transducer, in order to lose as less as possible of the signal. However, a precise focusing of the transmitter and receiver requires an even higher accuracy of the positioning of the transmitter and receiver during sensor installation.

Moreover, a further problem of the known prior art clamp-on flow meters can be seen in that the beam angle is changed by different flow rates of the medium flowing inside the pipe. Therefore, the focusing of the beam is limited to an area which is large enough to allow the flow sensor to operate under all conditions. In other words, the focusing area of the known clamp-on fixed beam angle flow meters has to be large enough to cover all flow rates speeds which the medium flowing through the pipe may have.

To avoid the change of the beam angle by the flow rate, EP 3 1 15 753 A1 of the applicant proposes an ultrasonic transmitter/transducer for measuring the filling level in a vessel which allows the emitted beam angle to be electronically controlled, in order to adapt the emitter/transducer to different conditions. To do so, the ultrasonic transducers, e.g. known piezo transmitters, are mounted at the outer surface of the pipe by means of plastic wedges which cause the wall material of the pipe which is con- tacted by the associated surfaces of the wedges to emit ultrasonic waves at an angle which strongly depends on the excitation frequency of the emitter/transducer. By tuning the excitation frequency, the beam angle of the wave package generated in and emitted from the pipe wall to the inside of the pipe can be altered.

Although EP 3 1 15 753 A1 discloses transmitters/transducers for generating an ultrasonic beam which is emitted at a variable beam angle into a vessel, the document is silent about measuring the height of a liquid or the volume flow of a liquid flowing through a pipe having a circular cross section with a high precision in a simple way without knowing the diameter and the wall thickness of the pipe, as well as the properties of the liquid.

Accordingly, it is a problem of the present invention to provide an apparatus which allows a non-intrusive measurement of the flow velocity, the volume flow and the filling height of a fluid running through a pipe having an unknown diameter.

This problem is solved by an apparatus as claimed in claim 1 .

Moreover, it is a further problem of the present invention to provide for a method which allows a quick and easy non-intrusive determination of the flow velocity, the diameter and the volume flow as well as the filling height of a fluid in a pipe having an unknown diameter or radius.

This problem is solved by a method as claimed in claim 1 1 .

Further objects of the present invention are included in the dependent claims.

The invention is hereinafter described with reference to the accompanying drawings. In the drawings

Fig. 1 is a schematic side view of a completely filled pipe in which a fluid is streaming with a clamp-on apparatus according to the present invention in which first ultrasonic pulses are emitted by a first transducer which are received by a second transducer,

Fig. 2 is the pipe of Fig. 1 in which second ultrasonic pulses are emitted by the second transducer which are received by the first transducer, in order to illustrate the general principle of measuring the speed of the fluid and the volume flow with the apparatus,

Fig. 3 is a schematic view of the pipe of the arrangement of Fig. 1 and 2, in which the first transducer generates lamb waves running inside the pipe wall which are received by the second transducer, and alternatively by the first transducer, in order to determine the sonic speed of the lamb waves in the wall material and the perimeter and average diameter of the pipe, is a schematic view of a further measuring configuration similar to the one of Fig. 3 in which the apparatus according to the invention is mounted to the bottom of a partially filled pipe when measuring the son ic speed of lamb waves in the wall material and afterwards the perimeter of the pipe, is a schematic illustration of the configuration of Fig. 4 when measuring the height of the fluid in the pipe on basis of reflections of ultrasonic pulses emitted by one of the transducers in parallel to the diameter of the pipe and reflected at the fluid surface, is a schematic view of the configuration of Fig. 5 when measuring the inner diameter of the pipe on basis of reflections of ultrasonic pulses emitted by one of the transducers in parallel to the diameter of the pipe which are reflected at the opposite inner surface of the pipe, is a schematic more detailed view of the working principle of the transducers used in the apparatus according to the present invention, is a schematic side view of a transmitter/receiver unit in which the sound transmitting element is configured as a mechanical grating, is a schematic side view of a transmitter/receiver unit in which the sound transmitting element is configured as an electronical grating which is arranged on the piezo electric element of the transmitter/receiver unit,

Fig. 9b is a schematic top view of a transmitter/receiver unit of Fig. 9a, and Fig. 10 is a schematic side view of a further embodiment of a transmitter/receiver unit in which the sound transmitting element is configured as an arrangement of opposing electrodes which are independently connected to separate signal generators which can be driven so as to generate ultrasonic waves of substantially the same frequency having a phase shift.

As it is shown in Figs. 1 to 6, an apparatus 1 for measuring the flow velocity V _me d of a fluid 5 in a pipe 2 which is herein after also called flow meter 1 , comprises a housing 3 in which a first ultrasonic transducer TD1 and a second ultrasonic transducer TD2 are arranged at a predefined distance L to each other. The first ultrasonic transducer TD1 includes a first sound transmitting element 4a and a transmitter/receiver unit 6a which are adapted to emit first ultrasonic pulses P1 at different angles CM . The second ultrasonic transducer TD2 is adapted to receive said first ultrasonic pulses P1 and generate a first electronic output signal. In the same way, the second ultrasonic transducer TD2 includes a second sound transmitting element 4b and a second transmitter/receiver unit 6b mounted thereto which are adapted to emit second ultrasonic pulses P2 at different angles a ₂ .The first ultrasonic transducer TD1 is adapted to receive said second ultrasonic pulses P2 and generate a second electronic output signal. The first and second transducer TD1 and TD2 are connected to a control and evaluation unit 8 which drives both transducers TD1 and TD2 when operated in a transmitting mode and also evaluates the signals received by the transducers, when operated in a receiving mode.

As it is shown in Fig. 1 , the control and evaluation unit 8 is adapted to vary the angle CM of the ultrasonic pulses P1 which are emitted by the first transducer TD1 when operated in the transmitting mode to an angel which generates a first electronic output signal in the second transducer TD2 that has a maximum amplitude.

In the same way, the control and evaluation unit 8 is adapted to vary the angle a ₂ of the second ultrasonic pulses which are emitted by the second transducer TD2 when operated in a transmitting mode to an angle which generates an electronic output signal in the first transducer TD1 that has a maximum amplitude. As it is shown in Figs. 1 , 2 and 7, the first transducer TD1 of the preferred embodiment of the invention comprises a first transmitter/receiver unit 6a, which is preferably a known piezo actuator and a first wedge shaped element 4a on which the transmitter/receiver unit 6a is mounted at an angle to the outer surface of the pipe. In the same way, the second transducer TD2 includes a second transmitter/receiver unit 6b which is preferably a known piezo actuator and a second wedge shaped element 4b, on which the second transmitter/receiver unit 6a is mounted at an angle inverse to the one of the first wedge shaped element 4a to the outer surface of the pipe wall 12. In the preferred embodiment of the invention, the first and second transducers (TD1 , TD2) are identical.

In the preferred embodiment of the invention, the first sound transmitting element and/or the second sound transmitting element are wedge shaped elements 4a, 4b as described herein before. The wedge shaped elements are preferably plastic wedges or prisms made of e.g. polyurethane, polyamid, polyetherimid or another known plas- tic material which is able to conduct ultrasonic waves from the transmitter/receiver units 6a, 6b to a contacting area in which the wedge shaped elements 4a, 4b are in contact with the wall 12 of the pipe 2 as shown in Figs 1 and 2. In order to improve the transmission of sound ultrasonic vibrations between the wedge shaped elements 4a, 4b and the wall 12 of the pipe 2, a known ultrasonic gel or a layer of soft plastics, preferably silicone rubber, may be arranged between the wedge shaped elements 4a, 4b and the outer wall surface.

As it is further indicated in Fig. 7, the ultrasonic waves generated by the transmitter/receiver units 6a, 6b of the first and second transducer TD1 , TD2, when operated as an emitter, are diffracted in the sound transmitting elements 4a, 4b and generate vibrations in the wall 12 of the pipe 2 which in turn generate an ultrasonic wave front which is emitted at an angle CM , <¾ to the inner surface 2a of the pipe 2. Due to the frequency dependency of the sonic speed in the wall material of the pipe, which is usually metal/steel, the angle CM , a ₂ strongly depends on the frequency f1 , f2 of the ultrasonic waves which are emitted by the transmitter/receiver units 6a, 6b, so that by tuning the frequency f1 , f2 of the transducers (TD1 , TD2) over a certain range, e.g. from 100 kHz to 1 MHz, the angel αι , <¾ of the ultrasonic waves/pulses P1 , P2 emitted from the transducers TD1 , TD2 into the fluid 5 can be changed. The vibrations generated in the wall are typically Lamb waves which can exist in different wave modes having different properties in the interaction with the liquid. A preferable mode to be used for the generation of the emitted beam into the fluid is the fundamental asymmetric Lamb wave mode in the wall which is also called A0- mode. This asymmetric AO Lamb wave mode provides for the advantage that it shows a strong interaction with the surrounding liquid and thus emits most of its acoustical energy already along a short travel path into the liquid. Alternatively, also other known Lamb wave modes with a strong interaction with the liquid may be used.

According to an alternative embodiment of the invention, the transmitter/receiver units 6a, 6b of the first and second transducer TD1 , TD2 can comprise a simple piezoelectric element which is mounted to the wall 12 by means of a sound transmitting element which is configured as a mechanical grating 4 ^' that is located in between the piezoelectric element 6a, 6b and the wall 12, as it is shown in Fig. 8. This grating preferably consists of two different materials, e.g. a first sound-transmitting material like metal and a second non-sound transmitting material like air or plastic, which are arranged in a 1 -dimensional or 2-dimensional periodic order. Due to the grating 4 ^' the waves/ultrasonic pulses P1 , P2 which are generated by the transmitter/receiver units (piezoelectric elements) 6a, 6b can only propagate in one of the materials, e.g. the metal, and are blocked in the other, e.g. the air or plastic, or they propagate with different propagation velocities so that a phase shift of the ultrasonic waves which form the pulses P1 , P2 occurs at the wall 12. As applicant has found, this results in a periodical excitation of the acoustic waves in the wall 12, so that only one specific wave front or pulse is emitted at a specific angle CM, <¾. By tuning the frequency f1 , f2 of the transducers (TD1 , TD2) over a certain frequency range, the angle CM, a ₂ under which the first and second ultrasonic pulses P1 , P2 are emitted from the grating 4 ^' into the liquid can be varied, and the angle can be tuned to an angle at which the amplitude of the signal received by the receiving transducer TD2, TD1 has a maximum, as it was set forth herein before with reference to the embodiment in which the sound transmitting elements are configured as wedges 4a, 4b. In this alternative embodiment, the periodicity is preferably selected in such a way that it fits to the wavelength of the required ultrasonic mode in the wall 12, so that as a result preferably only this mode is excited. As a further alternative embodiment to the mechanical grating 4 ^' , an electronic grating 104 on the piezoelectric element of the transmitter/receiver units 6a, 6b can be used, in which a periodic electrode structure which comprises two or more nested sets of comb shaped electrodes 105, 106 extending into each other is printed on the outer surface of the piezoelectric element of the transmitter/receiver units 6a, 6b as it is shown in Fig. 9a and 9b. The two different sets of electrodes 105, 106 are connected to a frequency tunable signal generator 108, e.g. an AC-power supply which is part of the control and evaluation unit and which can be tuned to a frequency that generates lamb waves in the material of the wall 12. As applicant has found, the comb shaped arrangement of electrodes 105, 106 also leads to a periodic excitation of acoustic waves in the wall 12 which finally results in ultrasonic pulses P1 , P2 that are emitted into the liquid under an angle CM , <¾ which strongly depends on the frequency of the signals fed to the first and second transmitter/receiver units 6a, 6b. As an even further alternative embodiment for a transducer TD1 , TD2 which can generate first and second ultrasonic pulses P1 , P2 at different emitting angles CM , <¾, a plurality of pairs of opposing electrodes 204a, 206b; 204b, 206b; 204c, 206c may be provided on the surface of the piezo electric elements of the transmitter/receiver units 6a, 6b, as it is shown in the exemplary embodiment of Fig. 9 in which 3 pairs of electrodes 204, 206 are shown for demonstrative purposes only. Each pair of electrodes 204, 206 is fed by a separate signal generator 208a to 208c with an individual periodic signal having the same frequency but comprising a phase shift. By tuning the frequency of the signal generators 208a to 208c to a desired lamb frequency and afterwards varying the phase shift between adjacent sets of electrodes, the emitting angle CM, a ₂ of the superposed wave front which forms the ultrasonic pulses P1 , P2 can be varied at a fixed frequency, as it is e.g. known from ultrasound scanning devices for scanning the human body. This embodiment provides for the advantage that the frequency which is fed to the electrodes of the ultrasonic emitter/receiver units 6a, 6b can be set to a desired lamb frequency which fits best for a specific material, and the angle CM , c^ can afterwards be varied until the received pulses P1 , P2 have a maximum amplitude by altering the phase shift between the pairs of electrodes 204, 206. In the afore described embodiments of Figs. 9 and 10, the sound transmitting element may be a known ultrasonic gel or a flat layer of soft plastics 109, preferably silicone rubber as mentioned herein before.

A further possible variant of a transmitter which is not shown in the drawings may comprise an electromagnetic acoustical transducer (EMAT) in which a more static magnetic field in combination with eddy currents is applied and generated inside the wall material. The interaction between the eddy currents and the magnetic field results in mechanical movements inside the wall which in turn generate ultrasonic waves/pulses propagating in the wall. With this alternative embodiment, a contactless excitation of mechanical waves can be obtained.

Now the embodiments of the method and apparatus of the present invention are described in more detail with regard to Figs. 1 to 7. In order to measure the flight time T1 of the first pulses P1 emitted by the first transducer TD1 the angle ai of the emitted first pulses P1 is altered until a maximum amplitude signal in transducer TD2 is obtained and the flight time of the pulses P altered is measured by the control and evaluation unit 8. In the embodiments of the invention using a transmitter/receiver unit 6a, 6b as shown in Figs. 1 to 9, the angel ai at which the amplitude of the received signal in the receiving transducer TD2 becomes a maximum is determined by tuning the frequency of the first emitter/receiver unit 6a to a frequency f1 max which generates a maximum signal in the second transducer TD2. In the embodiment of Fig. 10, the angle ai at which the amplitude of the received signal in the receiving transducer TD2 becomes a maximum is determined by tuning the phase between the signals fed to the pairs of electrodes 204a, 206a; 204b, 206b and 204c, 206c by the separate signal generators 208a, 208b and 208c until the signal received by the second Transducer TD2 becomes a maximum.

Afterwards, the apparatus is operated the other way round as shown in Fig. 2, in order to measure the flight time T2 of ultrasonic pulses P2 which are emitted by the second transducer TD2 and received by the first transducer TD1 . From the known fixed distance between the first transducer TD2 and the second transducer TD2 and the measured flight times T1 and T2 which are different if the fluids 5 is streaming through the pipe 2 with a velocity V _me d , the control and evaluations unit 8, which may include a known micro controller and adapted control software, calculates the speed V _me d of the fluid 5 in the pipe 2 as

. , _ L * (Γ2-Π)

Vmed - _{2 (rl+r2)2}

In order to account for different travelling lengths of the sonic pulses P1 , P2 in the wedge shaped elements 4a, 4b, the fixed distance L may be obtained by an initial calibration of the apparatus 1 mounted to a known pipe 2 in which a known fluid is streaming with a known speed V _me d . After calculating and storing the corresponding value L for the (effective distance) from the afore-mentioned mathematical relation in a memory of the micro controller of the control and evaluation unit 8, this effective distance value L may be used for all further applications of the apparatus 1 .

According to a further embodiment of the invention which is shown in Fig. 3, the first transducer TD1 is driven by the control and evaluation unit 8 to generate lamb waves 10 within the wall material of said pipe 2 having a first frequency f1 i _am b■ The lamb waves, which are only schematically indicated in Fig. 3 for illustration purposes propagate inside the wall material and are received by the second transducer TD2. The second transducer TD2 which is operated in a receiving mode receives the lamb waves 10 and generates an electronic signal having a frequency f 1 iam which is transmitted to the control and evaluation unit 8. From the flight time Tfij _g t TDi -TD2 of the lamb waves 10 travelling in the wall 12 from the first transducer TD1 to the second transducer TD2, the control and evaluation unit 8 calculates the sonic speed C(f1 iamb) of said lamb waves 10 to

C(f1 iamb) = L / Tfiight lamb TD1-TD2 which is preferably stored in the memory of the control and evaluation unit 8 in the same way as all other parameters measured. According to another preferred embodiment of the invention, the first transducer TD1 is further driven in a transmitting/receiving mode in which the transmitter/receiver unit 6a is first operated as a transmitter and transmits a short pulse P1 of the aforementioned lamb waves 10 of a known first frequency f 1 iam ■ Immediately after sending out the short Pulse P1 , the first transducer TD1 is switched to the receiving mode in which the transmitter/receiver unit 6a is operated as a receiver which receives the lamb waves 10 which are travelling around the perimeter U of the pipe 2 in the pipe wall 12 .

From the flight time T _f iig _ht iam _b TDi-TDi of the lamb waves 10 emitted from and received by the first transducer TD1 and preferably the stored value of the sonic speed

C(f1 iam _b ) of said lamb waves 10 measured before, the control and evaluation unit 8 calculates the perimeter U of said pipe 2 as

U = C(fl |amb) * (Tf|ight lamb TD1-TDl )- and preferably also the diameter D of the pipe as

D = U / TT, the values of which are preferably stored in the memory of the control and evaluation unit 8. In this respect, it should be noted that the perimeter U and the diameter D are not the exact perimeter /diameter values of the pipe, but are averaged values of the perimeter/diameter measured at about half way of the wall thickness. According to a yet further embodiment of the invention the control and evaluation unit 8 determines the volume flow J of the fluid 5 running through the pipe 2 from the measured and stored values of the speed V _med of the fluid 5 and diameter D of the pipe 2 as

J = Vmed * π ^* D ² / 4. According to another object of the present invention, the control and evaluation unit 8 is further adapted to determine the sonic speed C _me d in the fluid 5 which might be used to identify an unknown fluid or alternatively to determine if the composition of a mixture of two or more different fluids 5 running through the pipe 2 has changed or not. To do so, the control and evaluation unit 8 calculates preferably from measured and stored values of the flight times T1 and T2 and diameter or perimeter the sonic speed to or

Although the perimeter U and the diameter D may also be known values for a pipe, the apparatus according to the invention has the advantage that all of the aforementioned parameters can be measured without knowing any details about the pipe parameters itself which makes the apparatus highly flexible and allows the device to be used as a mobile, preferably also hand held clamp-on device for different kind of measuring applications.

For these described operation modes an emitting of the acoustical energy into the liquid is, in contrast to the flow speed measurement, of disadvantage. Thus, preferably lamb wave modes with a low interaction with the surrounded liquid are used as e.g. the fundamental symmetric Lamb wave mode (so called SO-mode).

According to a further embodiment of the invention which is illustrated in Fig. 4, the housing 3 of the apparatus 1 is adapted to be preferably releasable mounted at the bottom surface of a pipe 2, e.g. by means of holding flanges and straps (not shown) wrapped around the pipe or by magnets or releasable glue or the like. In this application, the apparatus 1 is operated similar to the embodiment of Fig. 3 for measuring the diameter D of the pipe 2 which is completely filled. As it is further shown in Fig. 5, the first transducer TD1 may be configured and controlled by the control and evaluation unit 8 to generate first ultrasonic pulses P1 parallel which are emitted in a direction parallel to the diameter D of the pipe 2 and receive reflections of the emitted ultrasonic pulses P1 parallel which are reflected at an adjacent inner wall surface 2a (not shown in detail) and/or at an upper surface 5a of the fluid 5 in a partially filled pipe 2 and/or at the inner wall surface 2a of the pipe 2 which is located opposite to said first transducer TD1 , as shown in Fig. 6.

In this measuring application, the control and evaluation unit 8 is further adapted to calculate from the flight time T1 ^' , T1 ^" and T1 ^{" '} of the reflected ultrasonic pulses P1 parallel which are received by said first transducer TD1 the wall thickness d _wa ii of the pipe 2 and/or the filling height Hfi _ui d of the fluid 5 in a partially filled pipe 2 and/or the inner diameter D, of the pipe 2 according to the following relations: dwall = Cwall ^* T1 ^'

Hfiuid = ½ * Cmed ^* T1 ^" ; and

Di = ½ * Cmed ^* T1 ^{" '} ; wherein

T1 ^' is the flight time of the first ultrasonic pulses P1 _par aiiei emitted and received by the first transducer TD1 which are reflected at the inner surface 2a of the pipe wall on the side of the pipe 2 which is opposite of the first transducer TD1 , T1 ^" is the flight time of the first ultrasonic pulses P1 _par aiiei emitted and received by the first transducer TD1 which are reflected at a surface 5a of the fluid 5 in a partially filled pipe 2, is the flight time of the first ultrasonic pulses P1 paraiiei emitted and received by the first transducer TD1 which are reflected at the adjacent inner surface 2a of the pipe wall,

Di is the inner diameter of the pipe 2, Cmed is the sonic speed of the first ultrasonic pulses P1 parallel in the fluid 5 which are determined similar to the embodiment of Fig. 3 and 4 as or

and

Cwaii is the sonic speed of the first ultrasonic pulses P1 parallel in the wall material

which are emitted by the first transducer TD1 in a direction parallel to the diameter D of the pipe 2 which is previously measured as set forth above with regard to the embodiment of Fig. 3.

With regard to the embodiments described herein before, it should be in the scope of the invention that instead of using the first transducer TD1 for generating the lamb waves 10 and ultrasonic pulses P1 parallel, also the second ultrasonic transducer TD2 may be employed. This may also include that the ultrasonic pulses P1 parallel are generated and received by the first transducer TD1 and the lamb waves 10 running through the wall 2 for measuring the sonic speed in the wall material are generated in the same configuration by the second transducer TD2 and vice versa. This provides for the advantage of an increased measuring speed and a shortened measuring time or even an improved measuring accuracy when repeating the measurements several time and calculating an average value from a series of measured single values.

According to another object of the present invention, a method of measuring the flow velocity V _me d of a fluid 5 in a pipe 2 comprises the following steps: - attaching an apparatus 1 as described herein before to the outer surface of a linear section of the pipe and aligning the housing 3 such that the predefined distance L between said first and second transducer TD1 , TD2 extends in parallel to the longitudinal axis of a linear section of the pipe 2,

- tuning the first transducer TD1 to an angle which generates a second electronic output signal of a maximum amplitude in said second transducer TD2, preferably by tuning the frequency of the first transducer TDI and measuring the amplitude of the output signal of the second transducer TD2,

- measuring the flight time T1 of first ultrasonic pulses P1 travelling from the first transducer TD1 to the second transducer TD2 and measuring the flight time T2 of second ultrasonic pulses P2 travelling from the second transducer TD2 to the first transducer TD1 , and

- calculating the speed V _me d of the fluid 5 running through the pipe 2 as

. , _ L * (Γ2-Π)

V ^{med _} 2 (Γ2+Γ1)2

In accordance with another embodiment of the invention the method is characterized by the further method steps of

- tuning said first transducer TD1 to a first frequency which generates lamb waves 10 within the wall material of said pipe 2 having a first frequency f1 i _am b,

- measuring the flight time Tfi _igh t TDi-TD2 of the lamb waves 10 between the first transducer TD1 and the second transducer TD2 and calculating the sonic speed C(fl iamb) of said lamb waves 10 within said wall material as

C(f1 iamb) = L / Tfiight lamb TD1-TD2

Pursuant to an even further object of the invention, the method comprises the further method steps of

- tuning said first transducer TD1 to generate lamb waves 10 having said first frequency fl i _amb>

- interrupting the generation of said lamb waves 10,

- running said first transducer TD1 in a receiver mode and measuring the flight time fiight iamb TDi-TDi of the lamb waves 10 emitted from and received by said first transducer TD1 and calculating the perimeter of the pipe as

U - C(f1 |amb) * (Tf|ight lamb TD1 -TDl ) and/or calculating the diameter D of said pipe 2 as

D - C(fl |amb) * (Tf|ight lamb TD1-TDl ) / π and/or calculating the speed of said fluid 5 in said pipe 2 as

Moreover, the above-described method may comprise the further method steps of - mounting the housing 3 at the bottom of the pipe 2,

- tuning the first transducer TD1 to generate first ultrasonic pulses P1 _pa raiiei which are emitted in a direction parallel to the diameter D of the pipe 2,

- interrupting the generation of the ultrasonic pulses P1 parallel,

- running the first transducer TD1 in a receiver mode and measuring the flight time T1 ^' of ultrasonic pulses P1 parallel which are reflected at an inner wall surface 2a of the pipe 2 which is adjacent the first transducer TD1 and

- calculating the wall thickness of the pipe 2 as

- running the first transducer TD1 in a receiver mode and measuring the flight time T1 ^" of ultrasonic pulses P1 parallel which are reflected at an upper surface 5a of the fluid 5 in a partially filled pipe 2 and calculating the filling height of the fluid as

Hfiuid = ½ Cmed * T1 ^" and/or with a higher precision as

Hfiuid = ½ Cmed * T1 ^" - Cwaii * T1 ^' and/or - running the first transducer TD1 in a receiver mode and measuring the flight time T1 ^{" '} of ultrasonic pulses P1 parallel which are reflected at said inner wall surface 2a of said pipe 2 which is located opposite to said first transducer TD1 and calculating the inner diameter D, of said pipe as

Di = ½ * C _m ed ^* T1 ^{" '} and/or

Di = ½ * Cmed ^* T1 ^{' "} - Cwall ^* T1 ^' .

Moreover, in the embodiment of the invention in which the apparatus 1 is mounted at the bottom of the pipe 2, the diameter obtained from a runtime measurement of lamb waves 10 running in the wall 12 of the pipe may be used to calculate whether a pipe 2 is completely or only partially filled with a liquid medium 5. To do so, the control and evaluation unit 8 calculates if the measured diameter D or Di is equal to H _f | _Uid or not. If the measured value of Hfi _ui d is smaller than D or D, the pipe 2 is only partially filled.

The values and parameters which can be measured with the apparatus 1 may be displayed on a display mounted to or included in the housing 3 and/or stored in the memory of the control and evaluation unit 8 and/or may be transmitted to a remote server or central control unit (not shown), e.g. via a known data communication network.

Listing of reference numerals

1 Apparatus according to the invention

2 pipe

2a inner surface of pipe

3 housing

4a first sound transmitting element

4b second sound transmitting element

4 ^' mechanical grating

5 fluid

5a surface of the fluid

6a transmitter/receiver unit of first transducer

6b transmitter/receiver unit of second transducer

8 control and evaluation unit

10 lamb waves

12 pipe wall

104 electronic grating

105 comb shaped electrode,

106 comb shaped electrode

108 frequency tunable signal generator

109 sound transmitting element

204a, b,c electrodes in embodiment of Fig. 9

206a, b,c electrodes in embodiment of Fig. 9

208a signal generator connected to first pair of electrodes 204a, 206a 208b signal generator connected to second pair of electrodes 204b, 206b

208c signal generator connected to third pair of electrodes 204c, 206c

J volume flow

TD1 first ultrasonic transducer

TD2 second ultrasonic transducer

L predefined distance

P1 first ultrasonic pulses

P2 second ultrasonic pulses

en angel at which first pules are emitted

<¾ angel at which second pules are emitted f1 max tuning frequency of first transducer for maximum pulses in second transducer

f2 _max tuning frequency of second transducer for maximum pulses in first transducer

Vmed speed of fluid in pipe

T1 flight time of first ultrasonic pulses P1

T2 flight time of second ultrasonic pulses P2

f1 iamb frequency of lamb waves emitted by first transducer

C(f1 iamb) sonic speed of lamb waves in wall

fNght TD1 - TD1 flight time of lamb waves emitted and received by first transducer TfNght TD1 - TD2 flight time of lamb waves between first and second transducer

U perimeter of pipe

D diameter of pipe

P1 parallel ultrasonic pulses emitted by first transducer parallel to diameter

Di inner diameter of pipe

dwaii thickness of wall

Hfiuid height of fluid in partially filled pipe

T1 ^' flight time of first ultrasonic pulses P1 _par aiiei reflected at inner surface of pipe wall

T1 ^" flight time of ultrasonic pulses P 1 parallel reflected at surface of fluid

T1 ^{" '} flight time of pulses P 1 parallel reflected at opposite pipe wall

Cmed sonic speed of pulses P1 parallel in fluid

Cwaii sonic speed of pulses P1 parallel in wall