Method for determining the transit time of an ultrasound signal in a medium

Abstract

The invention relates to a method for determining the transit time of an ultrasonic signal in a medium using an ultrasonic clamp-on flowmeter, comprising: - providing an ultrasonic clamp-on flowmeter (1) with a first ultrasonic transducer (10), a second ultrasonic transducer (20), a third ultrasonic transducer (30) and a computing unit (40), as well as a medium-carrying pipe (2) with a pipe axis (X), an outer surface (3) and an inner surface (4), - attaching the first ultrasonic transducer (10) and the second ultrasonic transducer (20) to the outer surface (3) at a first distance (A1) from each other along the pipe axis (X) such that a first ultrasonic path (UP1) with a reflection at the inner surface (4) of the pipe (2) can be formed between the first ultrasonic transducer (10) and the second ultrasonic transducer (20),- Attaching the third ultrasound transducer (30) to the outer surface (3) at a second distance (A2) along the pipe axis (X) to the first ultrasound transducer (10) and to the second ultrasound transducer (20) such that a second ultrasound path (UP2) can be formed between the first ultrasound transducer (10) and the third ultrasound transducer (30).

Description

The invention relates to a method for determining the transit time of an ultrasound signal in a medium. In many industrial applications, uninterrupted monitoring of process media is crucial for maintaining optimal plant productivity. Ultrasonic clamp-on flowmeters are particularly well-suited for such processes because they can be mounted externally onto a pipe to monitor the flow of a medium without interrupting the flow. Ultrasonic clamp-on flowmeters, for example, operate using the transit-time difference measurement method. In this method, acoustic signals (ultrasound) are transmitted between two sensors. The signals are transmitted bidirectionally, meaning each sensor functions as both a sound transmitter and a sound receiver. Since the propagation speed of sound waves is lower against the flow direction of the medium than in the flow direction, a difference in transit time arises. This difference in transit time is directly proportional to the flow velocity. The measuring system calculates the volumetric flow rate of the fluid from the measured transit time difference between the determined transit time in the medium and the pipe cross-section. The determined transit time in the medium allows its speed of sound to be derived. This additional measurement parameter enables the differentiation of various fluids or the monitoring of fluid quality. The problem, however, is that ultrasonic flow meters require information about the pipeline through which the ultrasonic signal is traversed. This means that the exact properties of the pipelines, such as material, wall thickness, etc., must be provided to the ultrasonic flow meter in order for it to measure accurately. In other words, if the pipe properties are specified differently, the expected transit time of the ultrasonic signal between the ultrasonic sensors will be incorrectly determined, which in turn leads to incorrect measurements regarding the derived measured quantities. To determine the transit time of an ultrasound signal, the time interval between the transmitted and received signal must be calculated. Typically, the start time of the received ultrasound signal is used for this purpose. This signal start becomes increasingly difficult to detect the worse the signal-to-noise ratio is. Ultimately, this leads to an inaccurate determination of the ultrasound signal's transit time and, consequently, to an inaccurate determination of the resulting measurements. It is therefore an object of the invention to provide a method which enables a simple, reliable and accurate determination of transit times of an ultrasonic signal in clamp-on ultrasonic sensors. This problem is solved according to the invention by a method for determining the transit time of an ultrasonic signal in a medium using an ultrasonic clamp-on flowmeter according to claim 1. The method according to the invention comprises: - providing an ultrasonic clamp-on flowmeter with a first ultrasonic transducer, a second ultrasonic transducer, a third ultrasonic transducer and a computing unit, as well as a medium-carrying pipeline with a pipe axis, an outer surface and an inner surface, - attaching the first ultrasonic transducer and the second ultrasonic transducer to the outer surface at a first distance from each other along the pipe axis such that a first ultrasonic path with a reflection at the inner surface of the pipeline can be formed between the first ultrasonic transducer and the second ultrasonic transducer, - attaching the third ultrasonic transducer to the outer surface at a second distance along the pipe axis from the first ultrasonic transducer and the second ultrasonic transducer such that a second ultrasonic path can be formed between the first ultrasonic transducer and the third ultrasonic transducer.- Emitting a first ultrasound pulse from the first ultrasound transducer to the second ultrasound transducer and to the third ultrasound transducer, - Detecting a first ultrasound reflection signal at the second ultrasound transducer, wherein the first ultrasound reflection signal is based on the first ultrasound pulse propagated along the first ultrasound path, - Detecting a first ultrasound direct signal at the third ultrasound transducer, wherein the first ultrasound direct signal is based on the first ultrasound pulse propagated along the second ultrasound path, - Determining a first transit time difference based on the first ultrasound reflection signal and the first ultrasound direct signal by the processing unit, - Outputting a media transit time based on the first transit time difference. The method according to the invention makes it possible to accurately determine the transit time of the ultrasonic signal, even under difficult operating conditions. This increases the precision of the ultrasonic clamp-on flowmeter, particularly for applications with a poor signal-to-noise ratio. Generally speaking, the method improves the precision of the measured quantities in all application areas. Thanks to the method, reliable detection of the relevant signal is possible even with a very poor signal-to-noise ratio. A rough estimate of the transit time of the ultrasonic signal is no longer necessary. According to one embodiment of the invention, a cross-correlation of the first ultrasound reflection signal and the first ultrasound direct signal is used to determine the first transit time difference. According to a further embodiment of the invention, when determining the first transit time difference, a subtraction of a first direct ultrasound transit time from a first ultrasound reflection transit time is performed, wherein the first direct ultrasound transit time is determined using a maximum, a zero crossing or a threshold value of the first direct ultrasound signal, and wherein the first ultrasound reflection transit time is determined using a maximum, a zero crossing or a threshold value of the first ultrasound reflection signal. According to one embodiment of the invention, the method further comprises: - Emitting a second ultrasound pulse from the second ultrasound transducer to the first ultrasound transducer, - Detecting a second ultrasound reflection signal at the first ultrasound transducer, wherein the second ultrasound reflection signal is based on the second ultrasound pulse propagated along the first ultrasound path, - Emitting a third ultrasound pulse from the third ultrasound transducer to the first ultrasound transducer, - Detecting a second ultrasound direct signal at the first ultrasound transducer, wherein the second ultrasound direct signal is based on the third ultrasound pulse propagated along the second ultrasound path, - Determining a second transit-time difference based on the second ultrasound reflection signal and the second ultrasound direct signal by the processing unit, wherein, in the output step, the media transit time is also based on the second transit-time difference.so that the media runtime is an average of the first runtime difference and the second runtime difference. The invention is explained in more detail with reference to the following description of the figures. They show: - Fig. 1: an exemplary representation of a clamp-on ultrasonic flowmeter mounted on a pipeline for carrying out the method according to the invention, - Fig. 2: a schematic representation of the ultrasonic clamp-on flowmeter shown in Fig. 1 with a first ultrasonic pulse, - Fig. 3: a schematic representation of the ultrasonic clamp-on flowmeter shown in Fig. 1 with a second ultrasonic pulse, - Fig. 4: a schematic representation of another exemplary embodiment of an ultrasonic clamp-on flowmeter, - Fig. 5: a schematic representation of an ultrasonic signal, - Fig. 6: a schematic representation of a displacement-time diagram of an ultrasonic pulse. An ultrasonic clamp-on flowmeter 1 for carrying out the method according to the invention comprises a first ultrasonic transducer 10, a second ultrasonic transducer 20, a third ultrasonic transducer 30 and a computing unit 40. The computing unit 40 is connected to the first ultrasonic transducer 10, the second ultrasonic transducer 20, and the third ultrasonic transducer 30. Naturally, a pipeline 2 is also necessary to carry out the process. Fig. 1 shows an example of the ultrasonic clamp-on flowmeter 1, which is detachably or permanently mounted on the pipeline 2. A coupling medium, for example a coupling mat or coupling gel, is preferably applied between the pipeline 2 and the ultrasonic transducers. Preferably, the ultrasonic clamp-on flowmeter 1 can be attached to the pipeline 2 with clamps. Of course, other detachable fastening means for attaching the ultrasonic clamp-on flowmeter are also possible. It is also possible, for example, to glue or weld the two ultrasonic transducers to the pipeline 2. The first ultrasonic transducer 10 is attached to the pipeline 2 by a first bracket 12, the second ultrasonic transducer 20 by a second bracket 22, and the third ultrasonic transducer 30 by a third bracket 32 ​​(see Fig. 1). Pipeline 2 is made of a metal, plastic, or other material and extends along a pipe axis X. Pipeline 2 has an outer surface 3, an inner surface 4, and a pipe wall 5. The outer surface 3 defines the pipe wall 5 radially away from the pipe axis X. The inner surface 4 defines the pipe wall 5 radially towards the pipe axis X. Pipeline 2 is suitable for conveying a medium, such as a liquid or a gas. The inventive method for correcting a transit time difference of a useful sound signal from the ultrasonic clamp-on flow meter is described below. In a first implicit step, the ultrasonic clamp-on flow meter 1 described above and the pipeline 2 described above are provided. Next, the first ultrasonic transducer 10 and the second ultrasonic transducer 20 are attached to the outer surface 3 of the pipe axis X. As mentioned above, a screw-on clamp, for example made of stainless steel, is preferably used for this purpose. The attachment is thus detachable. As mentioned above, it is also possible to choose a non-detachable type of attachment in this step. The first ultrasonic transducer 10 and the second ultrasonic transducer 20 are attached at a first distance A1 from each other along the pipe axis X (see Fig. 2). The reference point for the first distance A1 is the point where the ultrasonic signal exits the respective ultrasonic transducer. The fastening is carried out in such a way that a first ultrasonic path UP1 with at least one reflection on the inner surface 4 of the pipe 2 can be formed between the first ultrasonic transducer 10 and the second ultrasonic transducer 20. The third ultrasonic transducer 30 is then attached to the outer surface 3 at a second distance A2 along the pipe axis X from the first ultrasonic transducer 10 and the second ultrasonic transducer 20, such that a second ultrasonic path UP2 can be formed between the first ultrasonic transducer 10 and the third ultrasonic transducer 30. The second distance A2 preferably comprises a primary distance A2' and a secondary distance A2'', wherein the primary distance A2' is preferably half the first distance A1. The primary distance A2' thus corresponds to a distance extending parallel to the pipe axis X, from the point of exit of the ultrasonic pulse at the first ultrasonic transducer 10 to a second transition R2.The second transition R2 is the point where the ultrasonic pulse is refracted at the inner surface 4 of the pipe wall 5, allowing the third ultrasonic transducer 30 to detect the refracted ultrasonic pulse. The secondary spacer A2'' thus corresponds to a distance extending parallel to the pipe axis X, from the second transition R2 to the detection point at the third ultrasonic transducer 30. In other words, the third ultrasonic transducer 30 is located precisely at the point that enables the detection of a signal from the first ultrasonic transducer 10. Naturally, it is also possible for the first ultrasound path UP1 to have more than just the one reflection shown in Fig. 2. For example, it is possible to form a W-shaped first ultrasound path UP1 instead of the V-shaped one. Fig. 4 shows a selection of different possible arrangements of the second ultrasound transducer 20, 20', 20'' and the third ultrasound transducer 30, 30', 30''. The first distance A1, A1', A1'' defines the distance between the first ultrasound transducer 10 and the second ultrasound transducer 20, 20', 20''. The second distance A2, A2', A2'' defines the distance between the first ultrasound transducer 10 and the third ultrasound transducer 30, 30', 30''. Then, a first ultrasonic pulse UI1 is emitted from the first ultrasonic transducer 10 to the second ultrasonic transducer 20. As shown in Fig. 2, the first ultrasonic pulse UI1 passes through a first transition R1 at a first time T1. The first transition R1 is a material transition on the inner surface 4 of the pipe wall 5 towards the pipe axis X, i.e., between the pipe wall 5 and the medium in the pipeline 2. The first ultrasonic pulse UI1 then travels through the medium, i.e., the interior of the pipeline 2. Afterward, the first ultrasonic pulse UI1 passes through the second transition R2 at a second time T2. The second transition R2 is a material transition on the inner surface 4 of the pipe wall 5 away from the pipe axis X, i.e., between the medium in the pipeline 2 and the pipe wall 5. At the second transition R2 between the interior of the pipeline 2 and the pipe wall 5, the pulses split.The first ultrasound pulse UI1 is partially refracted and partially reflected. Reflection causes the first ultrasound pulse UI1 to propagate along the first ultrasound path UP1 as a first ultrasound reflection signal UR1 and, through refraction, along the second ultrasound path UP2 as a first ultrasound direct signal UD1. The first ultrasound reflection signal UR1 thus propagates to the second ultrasound transducer 20, and the first ultrasound direct signal UD1 propagates to the third ultrasound transducer 30, as shown in Fig. 2. The first ultrasound reflection signal UR1 travels through the medium to a third interface R3 at a third time point T3 before the first ultrasound reflection signal UR1 is detected by the second ultrasound transducer 20. The third interface R3 is a material transition on the inner surface 4 of the pipe wall 5 away from the pipe axis X, i.e., between the medium arranged in the pipeline 2 and the pipe wall 5. It should be noted that the term "direct ultrasonic signal" in Fig. 2 refers to a direct path, i.e., one without reflections, between the first ultrasonic transducer 10 and the third ultrasonic transducer 30. However, this term should not be interpreted restrictively. As explained above and illustrated in Fig. 4, it is quite possible to create paths between the first ultrasonic transducer 10 and the third ultrasonic transducer 30 where the first direct ultrasonic signal UD1 travels along a path that includes reflections from the inner surface 4 of the pipe wall 5. Furthermore, the first direct ultrasound signal UD1 is detected at the third ultrasound transducer 30. As explained above, the first direct ultrasound signal UD1 is based on the first ultrasound pulse UI1 propagated along the second ultrasound path UP2. Then, the first ultrasound reflection signal UR1 is detected at the second ultrasound transducer 20. As explained above, the first ultrasound reflection signal UR1 is based on the first ultrasound pulse UI1 propagated along the first ultrasound path UP1. Subsequently, the computing unit 40 determines a first transit time difference LZ1 based on the first ultrasound reflection signal UR1 and the first ultrasound direct signal UD1. Preferably, a cross-correlation of the first ultrasound reflection signal UR1 and the first ultrasound direct signal UD1 is used to determine the first transit time difference LZ1. According to one embodiment of the invention, as an alternative to using cross-correlation when determining the first transit-time difference LZ1, a subtraction of a first ultrasound direct transit time UDLZ1 from a first The ultrasonic reflection transit time URLZ1 was measured (see Fig. 6). The first transit time difference LZ1 represents precisely the transit time of the first ultrasonic pulse UI1 in the medium, i.e., inside the pipe 2. This is particularly evident in Fig. 6. The first transit time difference LZ1 is identical to the medium transit time LZM. The medium transit time LZM is the time the first ultrasonic pulse UI1 needs to traverse the medium along a medium path UPM. The medium transit time LZM occurs between the second time point T2 and the third time point T3. These times are marked in Fig. 2. The first direct ultrasound transit time UDLZ1 is determined using a maximum, a zero crossing, or a threshold value of the first direct ultrasound signal UD1. More precisely, for example, a parabola is used as the envelope of the first direct ultrasound signal UD1, and its first intersection with the time axis is defined as the so-called time-of-flight, i.e., as the first direct ultrasound transit time UDLZ1 (see Fig. 5). Fig. 5 shows the amplitude A of the exemplary first direct ultrasound signal UD1 or the exemplary first ultrasound reflection signal UR1 over time t. The same applies to the first ultrasound reflection transit time URLZ1. This is also determined using a maximum, a zero crossing or a threshold value of the first ultrasound reflection signal UR1. Finally, a first sound velocity SG1 is determined based on the pipe's inner diameter RID, the pipe wall thickness RWS, the second distance A2, and the first transit-time difference LZ1. Those skilled in the art know how this calculation is performed. Preferably, a refractive index of the material used for the pipeline 2 and the refractive index of the medium contained in the pipeline 2 are used. The first sound velocity SG1 determined in this way is the sound velocity of the first ultrasonic pulse UI1 in the medium, i.e. inside the pipeline 2. It should be noted that the first sound velocity SG1 was only determined in one direction through the medium (here from the third ultrasonic transducer 30 to the second ultrasonic transducer 20). Therefore, if the medium is stationary, the first sound velocity SG1 can be used to determine, for example, the medium flow rate. In a flowing medium, for example, with a flow from the first ultrasonic transducer 10 to the second ultrasonic transducer 20, the first speed of sound SG1 would be greater than in a stationary medium due to the flow. In this case, a second speed of sound SG2 must also be determined in the opposite direction of the flow, i.e., from the third ultrasonic transducer 30 to the first ultrasonic transducer 10, in order to determine the speed of sound of the corresponding stationary medium. If the speed of sound of a flowing medium is to be determined, the procedure further comprises: Emitting a second ultrasound pulse UI2 from the second ultrasound transducer 20 to the first ultrasound transducer 10. The second ultrasound pulse UI2 propagates along the first ultrasound path UP1 to the first ultrasound transducer 10. If the third ultrasound transducer 30 is capable of receiving signals from two directions, i.e., a bidirectional ultrasound transducer, the procedure described above could be modified accordingly. However, ultrasound transducers are generally only suitable for sending and receiving signals from one direction. Specifically, this means that in the present case, the third ultrasound transducer 30 can only receive a signal from the first ultrasound transducer 10 (Fig. 2) and can only send a signal to the first ultrasound transducer 10 (Fig. 3). Fig. 4 also shows arrangements of ultrasonic transducers 100, 200, 200', 200'', 300, 300', 300'', in which the ultrasonic transducers 10, 20, 20', 20'', 30, 30', 30'' described above are arranged in a direction opposite to that described above, with respect to the tube axis X. For clarity, these ultrasonic transducers are labeled with reference numbers that are one order of magnitude higher than those of the ultrasonic transducers described above. This allows for a simpler functional correlation between the ultrasonic transducers 100, 200, 200', 200'', 300, 300', 30'' and the ultrasonic transducers 10, 20, 20', 20'', 30, 30', 30'' described above. Subsequently, a second ultrasound reflection signal UR2 is detected at the first ultrasound transducer 10. As explained above, the second ultrasound reflection signal UR2 is based on the second ultrasound pulse UI2 propagated along the first ultrasound path UP1. Then a third ultrasound pulse UI3 is emitted from the third ultrasound transducer 30 to the first ultrasound transducer 10 (see Fig. 3 ). Next, a second direct ultrasound signal UD2 is detected at the first ultrasound transducer 10. The second direct ultrasound signal UD2 is based on the third ultrasound pulse UI3 propagated along the second ultrasound path UP2. Now, a second transit time difference LZ2 is determined based on the second ultrasound reflection signal UR2 and the second ultrasound direct signal UD2 by the processing unit 40. The same principles apply here as described above for the first transit time difference LZ1. Then a second sound velocity SG2 is determined based on the pipe inner diameter RID, the pipe wall thickness RWS, the second distance A2 and the second transit time difference LZ2. Finally, according to this embodiment, a mean speed of sound SG3 is determined based on the first speed of sound SG1 and the second speed of sound SG2. When determining the mean speed of sound SG3, it is preferably calculated as the average of the first speed of sound SG1 and the second speed of sound SG2. Alternatively, it is also possible to first subtract the first speed of sound SG1 from the second speed of sound SG2 to determine the difference in speed of sound, and then add half of this difference to the first speed of sound SG1 to arrive at the third speed of sound SG3. The third speed of sound SG3 thus represents the speed of sound of the medium flowing through the pipe 2, compensating for the influence of the flow. It is also possible to determine the mean speed of sound SG3 using a previously determined mean transit time, the pipe's inner diameter RID, and the second distance A2. In this case, one step involves determining the mean transit time based on the average of the medium's outbound transit time and return transit time. The outbound transit time is calculated by subtracting the first ultrasonic reflection transit time URLZ1 from the first direct ultrasonic transit time UDLZ1. The return transit time is calculated by subtracting the second ultrasonic reflection transit time URLZ2 from the second direct ultrasonic transit time UDLZ2. The second ultrasonic reflection transit time URLZ2 is based on the second ultrasonic reflection signal UR2. As explained above, the second ultrasonic reflection signal UR2 is based on the second ultrasonic pulse UI2 propagated along the first ultrasonic path UP1.Whereby the second ultrasound pulse UI2 is propagated from the second ultrasound transducer 20 to the first ultrasound transducer 10. The second ultrasound direct transit time UDLZ2 is based on the second ultrasound direct signal UD2. The second ultrasound direct signal UD2 is based on the third ultrasound pulse UI3 propagated along the second ultrasound path UP2. The third ultrasound pulse UI3 propagates from the third ultrasound transducer 30 to the first ultrasound transducer 10. Reference symbol list 1 Ultrasonic clamp-on flowmeter 2 Pipeline 3 Outer surface 4 Inner surface 5 Pipe wall 10 First ultrasonic transducer 12 First bracket 20 Second ultrasonic transducer 22 Second bracket 30 Third ultrasonic transducer 31 Third computing unit 32 Third bracket 40 Computing unit A1, A1', A1" First spacing A2, A2',A2" second distance A2' primary distance part A2" secondary distance part LZ1 first transit time difference LZ2 second transit time difference LZM medium transit time R1 first transition R2 first transition R3 first transition T1 first time point T2 first time point T3 first time point RID pipe inner diameter RWS pipe wall thickness SG1 first speed of sound SG2 second speed of sound SG3 third speed of sound UR1 first ultrasonic reflection signal UD1 first ultrasonic direct signal UR2 second ultrasonic reflection signal UD2 second ultrasonic direct signal UDLZ1 first ultrasonic direct transit time UDLZ2 second ultrasonic direct transit time URLZ1 first ultrasonic reflection transit time URLZ2 second ultrasonic reflection transit time UPM medium path UP1 first ultrasonic path UP2 second ultrasonic path UI1 first ultrasonic pulse UI2 second ultrasonic pulse UI3 third ultrasonic pulse X pipe axis,

Claims

Method for determining the transit time of an ultrasonic signal in a medium using an ultrasonic clamp-on flowmeter, comprising: - Providing an ultrasonic clamp-on flowmeter (1) with a first ultrasonic transducer (10), a second ultrasonic transducer (20), a third ultrasonic transducer (30) and a computing unit (40), as well as a medium-carrying pipe (2) with a pipe axis (X), an outer surface (3) and an inner surface (4), - Attaching the first ultrasonic transducer (10) and the second ultrasonic transducer (20) to the outer surface (3) at a first distance (A1) from each other along the pipe axis (X) such that a first ultrasonic path (UP1) with a reflection at the inner surface (4) of the pipe (2) can be formed between the first ultrasonic transducer (10) and the second ultrasonic transducer (20),- Attaching the third ultrasound transducer (30) to the outer surface (3) at a second distance (A2) along the tube axis (X) to the first ultrasound transducer (10) and to the second ultrasound transducer (20) such that a second ultrasound path (UP2) can be formed between the first ultrasound transducer (10) and the third ultrasound transducer (30), - Emitting a first ultrasound pulse (UI1) from the first ultrasound transducer (10) to the second ultrasound transducer (20) and to the third ultrasound transducer (30), - Detecting a first ultrasound reflection signal (UR1) at the second ultrasound transducer (20), wherein the first ultrasound reflection signal (UR1) is based on the first ultrasound pulse (UI1) propagated along the first ultrasound path (UP1), - Detecting a first ultrasound direct signal (UD1) at the third ultrasound transducer (30),wherein the first direct ultrasound signal (UD1) is based on the first ultrasound pulse (UI1) propagated along the second ultrasound path (UP2),- Determining a first transit-time difference (LZ1) based on the first ultrasound reflection signal (UR1) and the first direct ultrasound signal (UD1) by the processing unit (40),- Outputting a media transit-time (LZM) based on the first transit-time difference (LZ1). Method according to claim 1, wherein a cross-correlation of the first ultrasound reflection signal (UR1) and the first ultrasound direct signal (UD1) is used to determine the first transit time difference (LZ1). Method according to claim 1, wherein determining the first transit time difference (LZ1) involves subtracting a first direct ultrasound transit time (UDLZ1) from a first ultrasound reflection transit time (URLZ1), wherein the first direct ultrasound transit time (UDLZ1) is determined using a maximum, a zero crossing or a threshold of the first direct ultrasound signal (UD1), and wherein the first ultrasound reflection transit time (URLZ1) is determined using a maximum, a zero crossing or a threshold of the first ultrasound reflection signal (UR1). A method according to any one of the preceding claims, wherein the method further comprises: - emitting a second ultrasound pulse (UI2) from the second ultrasound transducer (20) to the first ultrasound transducer (10), - detecting a second ultrasound reflection signal (UR2) at the first ultrasound transducer (10), wherein the second ultrasound reflection signal (UR2) is based on the second ultrasound pulse (UI2) propagated along the first ultrasound path (UP1), - emitting a third ultrasound pulse (UI3) from the third ultrasound transducer (30) to the first ultrasound transducer (10), - detecting a second ultrasound direct signal (UD2) at the first ultrasound transducer (10), wherein the second ultrasound direct signal (UD2) is based on the third ultrasound pulse (UI3) propagated along the second ultrasound path (UP2).- Determining a second transit-time difference (LZ2) based on the second ultrasound reflection signal (UR2) and the second ultrasound direct signal (UD2) by the processing unit (40), wherein in the output step the media transit-time (LZM) is also based on the second transit-time difference (LZ2), so that the media transit-time (LZM) is an average of the first transit-time difference (LZ1) and the second transit-time difference (LZ2).

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