FLOWMETER AND METHOD FOR MEAUSURING THE FLOW OF A FLUID

Abstract

A flowmeter for measuring a flow of a fluid has a sensing element that has a pipeline for the fluid with a pipe wall, at least one phased array ultrasonic transducer unit, which can emit ultrasonic signals into different emission angles and can receive ultrasonic signals from different reception angles, a control and evaluation unit that is designed to control the ultrasonic transducer unit for emitting the ultrasonic signals along a measurement path and for evaluating the received ultrasonic signals and determining a flow using transit times of the ultrasonic signals. The sensing element has at least one reflector, which is designed to reflect the ultrasonic signals emitted by the ultrasonic transducer unit back to the same ultrasonic transducer unit. The ultrasonic signals pass through the measurement path from the ultrasonic transducer unit to the reflector and back to the ultrasonic transducer unit on at least partially different path sections.

Claims

1. A device for measuring a flow of a fluid (18), having a sensing element (12) having a pipeline (14) for the fluid (18) with a pipe wall (16), at least one phased array ultrasonic transducer unit (20) that can emit ultrasonic signals into different emission angles (γ) and can receive ultrasonic signals from different reception angles (ϕ) a control and evaluation unit (28) that is designed for controlling the ultrasonic transducer unit (20) for emitting the ultrasonic signals along a measurement path (34, 432, 434, 436, 64) and for evaluating the received ultrasonic signals and determining a flow using transit times of the ultrasonic signals, wherein the sensing element (12) has at least one reflector (30, 430, 50) that is designed to reflect the ultrasonic signals emitted by the ultrasonic transducer unit (20) back to the same ultrasonic transducer unit (20), wherein the ultrasonic signals pass through the measurement path (34, 432, 434, 436, 64) from the ultrasonic transducer unit (20) to the reflector (30, 50) and back to the ultrasonic transducer unit (20) on at least three different path sections (34a-c, 432a-c, 434a-c, 436a-c, 64a-c), characterized in that the measurement path (34, 432, 434, 436, 64) is a secant path not extending diametrically through a center axis (26) of the pipeline (14).

2. The device according to claim 1, characterized in that the control and evaluation unit (28) is designed to control the ultrasonic transducer unit (20) such that the ultrasonic signals in a first transit time measurement pass through the measurement path (34, 64) in a first direction (34.1, 64.1) and in a second transit time measurement in a second direction (34.2, 64.2) opposite to the first direction, and to determine a mean flow rate (v) of the fluid (18) from a difference in the transit time measurements.

3. The device according to claim 1, characterized in that the ultrasonic transducer unit (20) is integrated in the pipe wall (16).

4. The device according to claim 1, characterized in that the ultrasonic transducer unit (20) is designed as a two-dimensional array of ultrasonic transducers (22).

5. The device according to claim 1, characterized in that the ultrasonic transducer unit (20) is designed as a one-dimensional array, wherein a received ultrasonic signal has an angle with respect to a nominal transmitting and receiving plane of the ultrasonic transducer unit (20) that is at most as large as an acceptance angle of the ultrasonic transducer unit (20).

6. The device according to claim 5, wherein a magnitude of the acceptance angle is less than 10 degrees.

7. The device according to claim 1, characterized in that a path section (34c, 432c, 434c, 436c) of the measurement path (34, 432, 434, 436) lying between reflector (30) and ultrasonic transducer unit (20) extends at a path angle (β) of less than 20 degrees, preferably less than 15 degrees, to the center axis (26) of the pipeline (14).

8. The device according to claim 1, characterized in that the ultrasonic transducer unit (20) is oriented such that the ultrasonic signals are emitted at emission angles (γ) and incidence angles (ϕ) that are equal in magnitude and are received again after passing through the measurement path (34, 432, 434, 436).

9. The device according to claim 1, characterized in that the control and evaluation unit (28) is designed to control the ultrasonic transducer unit for tracking the emission angle (γ) as a function of the mean flow rate (v) of the fluid (18).

10. The device according to claim 1, characterized in that for at least one path section (34a-c), a ratio r/R is between 0.3 and 0.65, wherein R is the radius (R) of the pipeline (14) and r is the shortest distance (r) of the path section (34a-c) to the center axis (26) of the pipeline (14).

11. The device according to claim 10, characterized in that at least two path sections (34a-c) have a different ratio r/R.

12. A method for measuring a flow of a fluid (18) flowing in a pipeline (14), comprising the steps of: emitting ultrasonic signals along a measurement path (24, 34, 64) in the pipeline (14) with a phased array ultrasonic transducer unit (20) controlled by a control and evaluation unit (28) receiving the emitted ultrasonic signals with the same ultrasonic transducer unit (20) after passing through the measurement path (24, 34, 64). evaluating the received ultrasonic signals and determining a flow of the fluid (18) using transit times of the ultrasonic signals with the control and evaluation unit (28) wherein the ultrasonic signals emitted by the ultrasonic transducer unit (20) are reflected back by at least one reflector (30) to the ultrasonic transducer unit (20), wherein the ultrasonic signals pass through the measurement path (34, 432, 434, 436, 64) from the ultrasonic transducer unit (20) to the reflector (30, 430, 50) and back to the ultrasonic transducer unit (20) on at least three different path sections (34a-c, 432a-c, 434a-c, 436a-c, 64a-cc), characterized in that the measurement path (34, 432, 434, 436) is a secant path that is not diametrically extending through a center axis (26) of the pipeline (14).

13. The method according to claim 12, characterized by the further steps of: emitting the ultrasonic signals in a first measurement such that the ultrasonic signals pass through the measurement path (34, 64) in a first transit time measurement time in a first direction (34.1, 64.1) and in a second transit time measurement in a second direction (34.2, 64.2) opposite to the first direction, and determining a mean flow rate (v) of the fluid (18) from a difference of the first and second transit time measurements.

Description

[0034] The invention is explained in detail below by way of exemplary embodiments with reference to the drawing. In the drawings:

[0035] FIG. 1 shows a schematic illustration of a flowmeter;

[0036] FIG. 2a shows a schematic plan view of an ultrasonic transducer unit designed as a two-dimensional array;

[0037] FIG. 2b shows a schematic side view of an ultrasonic transducer unit designed as a two-dimensional array;

[0038] FIG. 3 shows a schematic illustration of a flowmeter according to the invention;

[0039] FIG. 4 shows a schematic perspective illustration of a flowmeter according to the invention;

[0040] FIG. 5 shows a schematic illustration of an alternative embodiment of a flowmeter according to the invention for multi-path measurement;

[0041] FIGS. 6a-6c show schematic illustrations of shields of a measurement path in a flowmeter according to the invention;

[0042] FIG. 7 shows a schematic illustration of a further embodiment of the flowmeter according to the invention;

[0043] FIG. 8 shows a schematic illustration of a flowmeter according to the prior art;

[0044] FIG. 8 is a flowmeter 110 according to the prior art for general explanation of the function of a generic flowmeter. The flowmeter 110 comprises a sensing element 112, which has a pipeline 114 for the fluid 118 with a pipe wall 116. The fluid flowing through the pipeline 114, a gas or a liquid, is illustrated in FIG. 8 with a wide arrow and flows in the z-direction along a center axis 126 of the pipeline 114.

[0045] Furthermore, the flowmeter 110 has two ultrasonic transducers 120 and 122, which define a measurement path 124 therebetween in the pipeline 114. The ultrasonic transducers 120 and 122 are arranged offset in the flow direction z, that is, spaced apart in the longitudinal direction along the center axis 126 of the pipeline 114. As a result, the measurement path 124 is not orthogonal to the flow direction z, but instead at a path angle α. Each of the ultrasonic transducer units 120 and 122 can operate as a transmitter or receiver and is controlled by a control and evaluation unit 128.

[0046] The path angle α and the pipe diameter D result in the length L of the measurement path 124 in the fluid medium. Ultrasonic signals which are emitted and received as ultrasonic wave packets on the measurement path 124 in opposite directions thus have one component once in the direction of the flow direction z and another time counter to the flow direction z, and are thus accelerated with the flow of the fluid 118, or decelerated, respectively, against the flow. The mean flow rate v of the fluid is calculated in this runtime method according to

[00001]$v=\frac{\frac{t_2-t_1}{2*t_2\star t_1}*L}{\cos \left(\alpha \right)}$

where t.sub.2 and t.sub.1 denote the sound transit times, which are required by the emitted ultrasonic wave signals to cover the measurement path 124 upstream or downstream, respectively, and are detected in the control and evaluation unit 128. With the pipe cross section and the mean flow rate v of the fluid 118, the flow can then be calculated.

[0047] The flowmeter 10 also operates according to this principle, which is illustrated very schematically in FIG. 1. It also has a sensor element 12 with a pipeline 14 and pipe wall 16 and a control and evaluation unit 28. The fluid 18 flowing through the pipeline 14, a gas or a liquid, is illustrated using a wide arrow and flows in the z-direction along a center axis 26 of the pipeline 14. The flowing fluid 18 has a flow profile 32, which has little influence on ultrasonic signals propagating along the pipe wall 16, for example due to a lower flow rate of the fluid 18 in the area of the pipe wall 16, compared to a flow rate in the area of the center axis 26.

[0048] In contrast to the flowmeter 110, from FIG. 8, the device for measuring a flow of a fluid in FIG. 1 has only one ultrasonic transducer unit 20 in the pipe wall 16. The ultrasonic transducer unit 20 is also not a “simple” ultrasonic transducer, but is designed as a phased array ultrasonic transducer unit 20. It can be designed as a one-dimensional linear array consisting of a row of at least two individually controllable ultrasonic transducers, whose orientation is parallel to the measurement path 24, or as shown in the schematic plan view of FIG. 2a, as a two-dimensional array of individually controllable ultrasonic transducers 22. The individual ultrasonic transducers 22 are controlled for emitting an ultrasonic signal by the control and evaluation unit 28, such that that they each have a phase offset from one another, wherein the phase offset is selected such that superposition of the resulting ultrasonic waves leads to an ultrasonic wave signal which leaves the ultrasonic transducer unit 20 at an emission angle γ perpendicular to a surface normal 40 of the ultrasonic transducer unit 20, as shown in FIG. 2b, in which the emitted ultrasonic wave signal is represented by a solid line 42. The ultrasonic transducer unit 20 can further be controlled such that ultrasonic wave signals which strike the ultrasonic transducer unit 20 at an incidence angle ϕ (illustrated by the dash-dotted line 44 in FIG. 2b) are detected. Emission angle γ and incidence angle ϕ can differ both in magnitude and direction. The mode of operation of such phased array ultrasonic transducer units is known from the prior art.

[0049] FIG. 2a illustrates an example of an array of four times four ultrasonic transducers. This limitation is essentially due to the fact that the drawing is to remain simple and clear. If 16 such individual ultrasonic transducers provide too low a signal level, the array can also have more ultrasonic transducers. Therefore, the array is preferably designed with more ultrasonic transducers in a manner not illustrated. The number of ultrasonic transducers is a compromise between signal strength, complexity and costs.

[0050] The ultrasonic transducer unit 20 thus transmits and receives ultrasonic signals which move along a measurement path 24 through the pipeline 14. As shown in FIG. 1, the measurement path 24 has multiple path sections 24a, 24b, 24c.

[0051] In a first measurement, the ultrasonic transducer unit 20 transmits the ultrasonic signals along a first path section 24a of the measurement path 24 from the ultrasonic transducer unit 20 to the pipe wall 16, wherein the transit direction of the ultrasonic signals in the first measurement is indicated by solid arrows 24.1 in FIG. 1. After reflection at the pipe wall 16, the ultrasonic signals arrive along a second path section 24b from the pipe wall 16 at a reflector 30 that reflects the ultrasonic signals back along a third path section 24c to the ultrasonic transducer unit 20. The reflector 30 is arranged downstream in the direction of flow 18 of the fluid, that is after the ultrasonic transducer unit 20, on or in the pipe wall, so that the flow of the fluid in the area between the ultrasonic transducer unit 20 and the reflector 30 is disturbed only slightly or not at all, in particular if the ultrasonic transducer unit 20 is integrated flush into the pipe wall 16 (not shown).

[0052] In a second measurement, the ultrasonic transducer unit 20 transmits the ultrasonic signals in the reverse transit direction, indicated by the dashed arrows 24.2, along the third path section 24c of the measurement path 24 in the direction of the reflector 30. After reflection at reflector 30, the ultrasonic signals arrive along the second path section 24b at the pipe wall 16, from which they are reflected back along the first path section 24a to the ultrasonic transducer unit 20.

[0053] The ultrasonic transducer unit 20 and the reflector 30 are arranged such that the third path section 24c of the measurement path 24 extends between the ultrasonic transducer unit 20 and the reflector 30 at a path angle β of less than 20 degrees, preferably less than 15 degrees to the center axis 26, wherein the path angle β is indicated between the center axis 26 and the third path section 24c, here in relation to a parallel 26.1 of the center axis 26. The third path section 24c therefore extends in an area as close as possible to the pipe wall 16. Due to the flow profile 32 in the pipeline 14, the ultrasonic signal is thus only slightly influenced by the fluid flow on the third path section 24c between the ultrasonic transducer unit 20 and the reflector 30.

[0054] On the other hand, on the first path section 24a between the ultrasonic transducer unit 20 and the pipe wall 16 and on the second path section 24b between the pipe wall 16 and the reflector 30, the ultrasonic signal is strongly influenced by the flow profile 32 and the velocity of the fluid in the pipeline. As a result, the transit time of the ultrasonic signals against the flow direction measured using the second measurement (measurement path illustrated by dashed arrows) is longer than the transit time with the flow direction measured using the first measurement (measurement path illustrated by solid arrows). This allows the mean flow rate of the medium to be calculated by the evaluation of the differential transit time of both measurements.

[0055] The mean flow rate v of the fluid is calculated in this transit time method according to

[00002]$\mathrm{v}=\frac{{\left(L_{24a}+L_{24b}+L_{24c}\right)}^2}{2.Math.\left(L_{24a}\cos {\alpha }_{24a}+L_{24b}\cos {\alpha }_{24b}+C_{\nu }{L}_{24c}\cos \beta \right)}.Math.\frac{t_{24.2}-t_{24.1}}{t_{24.1}{t}_{24.2}}$

[0056] Here

t.sub.24.1 and t.sub.24.2 denote the sound transit times required by the emitted ultrasonic signals to cover the measurement path 24 in the first transit direction 24.1 and in the reverse transit direction 24.2;
L.sub.24a, L.sub.24b, L.sub.24C denote the lengths of the path sections 24a, 24b, 24c,
α.sub.24a, α.sub.24b denote the path angle of the first path section 24a and the second path section 24b to the center axis 26;
β denotes the path angle of the third path section 24c to the center axis 26;
C.sub.v denotes a correction factor dependent on the flow profile and thus on the flow rate that can be determined by measurement, calibration or simulation;

[0057] With the pipe cross section and the mean flow rate v of the fluid 18, the flow can then be calculated.

[0058] The correction factor C.sub.v can be determined, for example, such that the sound transit times t.sub.24.1 and t.sub.24.2 are measured in a calibration process at one or more different predetermined mean flow rates v and the correction factor C.sub.v is calculated by rearranging the above equation. The measurement of several different flow rates is preferred, since the flow profile 32 can also depend on the flow rate. Alternatively, the correction factor C.sub.v can also be calculated by conventional simulation of the sound transit times t.sub.24.1 and t.sub.24.2 of the ultrasonic signals, taking into account a flow-rate-dependent flow profile which is likewise simulated in a conventional manner. The correction factor C.sub.v dependent on the flow profile and thus also on the flow rate can therefore be specified as a function of the sound transit times t.sub.24.1 and t.sub.24.2.

[0059] Since the ultrasonic transducer unit 20 is designed as a phased array, the emission angle γ can be changed by controlling the individual ultrasonic transducers 22 via the control and evaluation unit 28. This can counteract a drift effect, in particular at high flow rates. In fact, the emission angle γ can be readjusted such that the reflector 30 is always struck independent of the flow rate, and the emitted ultrasonic signals are again reflected back to the ultrasonic transducer unit 20.

[0060] Due to the configuration of the ultrasonic transducer unit 20 as a phased array, the emission angle γ depends on the set phase shift of the individual signals and on the speed of sound in the fluid. The speed of sound itself is dependent on ambient conditions such as temperature and pressure. Therefore, it is advantageous for the phase difference to be adapted as a function of the ambient conditions via the control of the individual ultrasonic transducers 22 by means of the control and evaluation unit 28 in such a way that the emission angle γ remains the same, even if the speed of sound changes. To determine the ambient conditions, an environment detection unit (not shown) can be provided, which detects, for example, temperature and/or pressure in the pipeline 14 and forwards it to the control and evaluation unit 28 in order to monitor the fluid properties and thus calculate the sound velocity and density. With this knowledge, the ultrasonic transducers 22 can be better controlled or evaluated. The density is necessary to calculate the mass flow and can be calculated from the properties of the medium as well as temperature and pressure. The sound velocity itself can be measured initially using known ambient conditions, fluid at rest and known length of the measurement path by measuring a transit time for both transit directions of the ultrasonic signal along the measurement path, determining a mean transit time therefrom and dividing the length of the measurement path by the mean transit time:

[00003]$c=\frac{L}{\frac{\left(t_1+t_2\right)}{2}}$

Where:

[0061] c=sound velocity
L=length of the measurement path
t.sub.1=transit time of the ultrasonic signal along the measurement path in the first direction
t.sub.2=transit time of the ultrasonic signal along the measurement path in the second direction

[0062] The measurement path 24 is illustrated in FIG. 1 as a diametrical measurement path extending through a center axis 26 of the pipeline 14. According to the invention, it can be designed as a secant path, as shown in FIG. 3.

[0063] FIG. 3 shows an embodiment of a flowmeter 310 according to the invention, wherein the pipeline 14 is illustrated in the flow direction. The ultrasonic transducer unit 20 emits ultrasonic signals on a measurement path 34, which now does not extend diametrically through the center axis 26 of the pipeline 14, but rather as a so-called secant path with the path sections 34a, 34b, 34c. In addition, as in the example of FIG. 1, the measurement path 34 does not extend orthogonally to the flow direction 18, that is, out of the drawing plane or into the drawing plane. The use of secant paths is advantageous for asymmetrical velocity distributions. Since the path sections 34a, 34b, 34c of the measurement path 34 do not extend through the center axis 26 of the pipeline 14, the center axis 26 of the pipeline 14 does not lie in a measurement plane spanned by the path sections 34a, 34b, 34c of the measurement path 34, but only intersects it at one point. In this way, higher accuracies are possible in the determination of the mean flow rate. To reasonably scan the flow profile, it can be true for at least one path section that the ratio r/R is between 0.3 and 0.65, wherein R is the radius of the pipeline and r is the shortest distance of the path section to the center axis of the pipeline, shown here for the path section 34a. Preferably, a multi-path measurement can take place, wherein the ultrasonic transducer unit 20 emits ultrasonic signals at different angles, so that the ultrasonic signals pass through the pipeline 14 on different measurement paths within a measurement plane, wherein a reflector 30 can be arranged on or in the pipe wall for each measurement path.

[0064] FIG. 4 shows a perspective view of an embodiment of a flowmeter 410 according to the invention with an ultrasonic transducer unit 20 having a two-dimensional array of individually controllable ultrasonic transducers and capable of transmitting and receiving ultrasonic signals in different measurement planes. By way of example, three different measurement paths 432, 434, 436 are illustrated with path sections 432a-c, 434a-c, 436a-c. The measurement paths 432, 434, 436 are secant paths that do not extend diametrically through the center axis 26 of the pipeline 14. The center axis 26 of the pipeline 14 therefore does not lie in the measurement planes spanned by the measurement paths 432, 434, 436, but only intersects each of them at one point. For the sake of clarity, the transit direction of the measurement signals is not illustrated along the measurement paths 432, 434, 436, but also here, as in the examples shown previously, the measurement signals each extend through the measurement paths 432, 434, 436 in both directions, that is, for example for the measurement path 432 in a first measurement, initially along path section 432a from the ultrasonic transducer unit 20 to the pipe wall 16, then along the path section 432b to the reflector 430 and from the reflector 430 along the path section 432c back to the ultrasonic transducer unit 20. In a second measurement, the measurement path 432 is then passed through in the reverse direction, that is, first along path section 432c from the ultrasonic transducer unit 20 to the reflector 430, then along the path section 432b to the pipe wall 16 and along path section 432a from the pipe wall 16 back to the ultrasonic transducer unit 20.

[0065] Instead of individual reflectors, this exemplary embodiment has a reflector 430 arranged in an arc shape on or in the pipe wall. Since the ultrasonic transducer unit 20 is designed as a two-dimensional ultrasonic transducer unit, it can emit and receive these ultrasonic signals in different measurement planes and can be controlled so that the ultrasonic signals strike the reflector 430 at different locations. As a result, different measurement paths and/or measurement planes can be used flexibly. The reflector 430 can also be designed circular, that is, it covers the entire inner circumference of the pipe wall, whereby the number of possible measurement paths is further increased.

[0066] In principle, the use of several ultrasonic transducer units is also possible for a multipath measurement, as shown in FIG. 5. In addition to a first ultrasonic transducer unit 20/1 that emits and receives ultrasonic signals along a measurement path 24/1, wherein the ultrasonic signals is reflected at a first reflector 30/1, the flowmeter 10 has a second ultrasonic transducer unit 20/2 that emits and receives ultrasonic signals along a second measurement path 24/2, wherein the ultrasonic signals are reflected at a second reflector 30/2. Depending on the accuracy requirement for the flow measurement, a plurality of N ultrasonic transducer units can be used that span N measurement paths. Instead of N reflectors, a reflector can then also be provided, for example, which is designed as a circumferential elevation or groove of the pipe wall 16.

[0067] In each of the exemplary embodiments shown in FIG. 3 and FIG. 4, a simple reflection of the ultrasonic signals occurs at the pipe wall 16. To further increase the measurement accuracy, the measurement paths 34, 432, 434, 436 can also be designed such that the ultrasonic signals between the ultrasonic transducer unit 20 and the reflector 30, 430 are reflected several times on the pipe wall 16. Preferably, the path sections 34c, 423c, 434c, 436c, on which the ultrasonic signals arrive directly, that is, without reflection at the pipe wall 16, from the reflector 30, 430 at the ultrasonic transducer unit 20 (or in the reverse transit direction from the ultrasonic transducer unit 20 directly at the reflector) as explained above, in an area close to the pipe wall 16.

[0068] To further reduce the influence of the fluid flow on the ultrasonic signals extending directly between ultrasonic transducer unit 20 and reflector 30, these can be shielded from the fluid flow by various embodiments, which are shown in the following FIGS. 6a-6c.

[0069] FIG. 6a shows a mechanical shield 40, 42, which essentially houses the area between the ultrasonic transducer unit 20 and the reflector 30, and has only openings for entry and exit of the ultrasonic signals.

[0070] FIG. 6b shows an alternative embodiment in which the ultrasonic transducer unit 20 and the reflector 30 are arranged in a recess 44 of the pipe wall.

[0071] For further shielding, the recess 44 can be closed, as shown in FIG. 5c, except for openings for entry and exit of the ultrasonic signals, comparable to the exemplary embodiment in FIG. 5a. The indentation 44 can also be configured as a measurement module that can be flanged to an opening in the pipe wall 16 and includes the ultrasonic transducer unit 20 and the reflector 30.

[0072] Another alternative embodiment of the invention is shown in FIG. 7. The ultrasonic transducer unit 20 emits and receives ultrasonic signals that move along a measurement path 64 through the pipeline 54. As in the previous exemplary embodiments, the measurement path 64 has a plurality of path sections 64a, 64b, 64c.

[0073] In contrast to the exemplary embodiments in FIG. 3, the reflector 60 is formed for reflecting back the ultrasonic signals through the pipe wall 56 of the pipeline 54 itself. The pipeline 54 has a u-shaped winding.

[0074] In a first measurement, the ultrasonic transducer unit 20 emits the ultrasonic signals along a first path section 64a of the measurement path 64, wherein the transit direction of the ultrasonic signals during the first measurement is denoted by solid arrows 64.1. After a reflection at the pipe wall 56, the ultrasonic signals pass along a second path section 64b to a reflector 60, which is formed by the pipe wall 56 and reflects the ultrasonic signals back along a third path section 64c to the ultrasonic transducer unit 20.

[0075] In a second measurement, the ultrasonic transducer unit 20 emits the ultrasonic signals in reverse transit direction, characterized by the dashed arrows 64.2, along the third path section 64c of the measurement path 64 in the direction of the reflector 60. After reflection at the reflector 60, the ultrasonic signals pass along the second path section 64b to the pipe wall 56, from which they are reflected back along the first path section 64a to the ultrasonic transducer unit 20.

[0076] Due to the u-shaped geometry of the pipeline 54, the measurement path 64 extends through the fluid 18 such that the third path section 64c extends essentially parallel to the flow of the fluid 18, while the other two path sections 64a, 64b extend essentially perpendicular to the flow of the fluid 18. Thus, the propagation speed of the ultrasonic signals on the first and second path sections 64a, 64b is influenced only slightly by the fluid flow.

[0077] On the third path section 64c between ultrasonic transducer unit 20 and reflector 60, the ultrasonic signal is strongly influenced by the fluid flow and the velocity of the fluid in the pipeline 54. As a result, the transit time of the ultrasonic signals against the flow direction measured using the second measurement (measurement path illustrated by dashed arrows) is longer than the transit time with the flow direction measured using the first measurement (measurement path illustrated by solid arrows). Thus, with this embodiment of the invention as well, it is possible to calculate the mean flow rate of the fluid 18 via the evaluation of the differential transit time of both measurements.