DEVICE FOR QUALITY DETERMINATION, TANK DEVICE

Description

[0001] The invention relates to a device for determining the quality of a liquid, in particular an exhaust aftertreatment agent, with at least one test unit that can be arranged in a tank containing the liquid, which has at least one ultrasonic transducer for emitting and receiving an ultrasonic signal and at least two ultrasonic reflector surfaces for reflecting the emitted ultrasonic signal back to the at least one ultrasonic transducer.

[0002] The invention further relates to a tank device, in particular a reducing agent tank device for an exhaust aftertreatment system of a motor vehicle, comprising a tank for storing and providing a liquid, in particular an exhaust aftertreatment agent, and a device for determining the quality of the liquid, wherein the device has at least one test unit arranged in the tank. State of the art

[0003] Devices and tank systems of the type mentioned above are known from the prior art. To comply with increasingly stringent legal requirements for exhaust emissions from motor vehicles with combustion engines, it is necessary, particularly in diesel engines, to reduce nitrogen oxides (NOx) in the exhaust gas. Exhaust aftertreatment systems are used for this purpose, selectively reducing nitrogen oxides to nitrogen using ammonia. Selective catalytic reduction (SCR) has proven to be particularly efficient in this regard. An aqueous urea solution is used as the reducing agent, which is introduced into the exhaust stream upstream of an SCR catalyst. The urea solution forms ammonia before and / or within the catalyst, which ultimately leads to the desired reduction of nitrogen oxides in the exhaust gas with the help of the SCR catalyst.

[0004] To ensure the safe operation of the exhaust aftertreatment system, the reducing agent must be provided in the tank in sufficient quantity and quality. If the quality is too low, particularly due to an insufficient concentration of the reducing agent in the liquid, the efficiency of the exhaust aftertreatment system decreases significantly. If the urea content or concentration is too high, the excess ammonia produced will not react with the nitrogen oxides in the exhaust gas, and ammonia will be emitted. It is therefore crucial to be able to monitor the quality and concentration of the reducing agent, especially the urea content, to detect any unauthorized refilling of the tank with water instead of reducing agents.

[0005] Various devices are already known in the art that monitor the quality or concentration of a reducing agent without contact using ultrasonic signals. For example, German patent application US 2016 / 041024 A discloses a device of the type mentioned above. This device features a piezoelectric transducer as an ultrasonic transducer, which is simultaneously a transmitter and receiver. The ultrasonic transducer first emits an ultrasonic signal into the liquid in the tank. The ultrasonic signal is reflected back to the transducer by an ultrasonic reflector and then passes through the liquid again. From the measured transit time of the ultrasonic signal between emission and reception, and the known length of the path, the speed of sound in the liquid, and thus the concentration of the reducing agent in the liquid, is determined.However, temperature-related, mechanically induced, and aging-related changes in the materials used in the system can lead to alterations in the actual measuring distance, which can affect the measurement result. To account for such changes, German patent application DE 10 2015 212 622 A1 proposes physically modeling thermal deformations of the reflector and incorporating them into the evaluation of the measured transit time using a temperature-dependent correction factor.

[0006] From German patent application DE 10 2013 219 643 A1, it is also known to provide two reflectors in order to reduce thermally and age-related changes in the measuring section that negatively affect the accuracy of the test unit. The reflectors are each designed as a reflector cylinder, the outer surface of which forms a reflector surface, such that the respective reflector surface is arranged on a separate support element, with the reflector cylinders being attached to or held on a separate base plate. Furthermore, the concentration determination is not based on a time-of-flight measurement of an ultrasonic signal, but on the evaluation of the measured difference in travel time between the two reflector cylinders, which are arranged at different distances from the transducer.Another device for determining quality is also known from German patent application DE 10 2011 086 774 A1, in which a reflector is arranged at an angle to the sound transducer.

[0007] Furthermore, utility model DE 20 2017 104 156 U1 discloses a device for measuring the concentration of a reducing agent with a tank, wherein a reflector component is arranged inside the tank in the liquid and a sensor device is arranged outside the tank which is in operative communication with the reflector component. Disclosure of the invention

[0008] The device according to the invention, with the features of claim 1, has the advantage of being inexpensive to manufacture and significantly improving the measurement accuracy of the test unit. An advantageous arrangement and design of at least two reflector surfaces optimizes the measurement accuracy and provides precise measurement results, particularly when the temperature in the measuring medium and / or the environment changes. Mechanical stresses that may arise from manufacturing and / or assembly tolerances, as well as from material-related aging effects, are also taken into account by the advantageous device. The invention is based on the idea of ​​utilizing two sound propagation paths with a difference in travel distance between these two paths, which is determined by the reflector geometry.According to the invention, the test unit has at least two ultrasonic reflector surfaces that are directly attached to a common, integrally formed support element at different distances from the ultrasonic transducer. The reflector unit, or reflector component, consisting of the support element and the at least two ultrasonic reflector surfaces, thus constitutes a single assembly in which the arrangement of the reflector surfaces relative to each other is predetermined during manufacturing or pre-assembly outside the tank. Due to the integrally formed support element, differing coefficients of thermal expansion play no role, or only a minor one, in the measurement of transit-time differences of the ultrasonic signals. The material of the support element is preferably selected to exhibit the lowest possible thermal expansion over the intended operating temperature range.The remaining influence of thermal expansion can be systematically corrected by the advantageous design. The preferred use of a differential measurement principle further minimizes the influence of positional or alignment tolerances during assembly. The absolute distance between the ultrasonic transducer and the respective ultrasonic reflector surface is not included in the calculation of the propagation speed and therefore does not need to be known or constant throughout the operating range. The actual measuring distance is defined solely by the reflector component with its integrally formed support element. The support element is manufactured, for example, using a cost-effective production process such as stamping, forming, or extrusion. This enables cost-effective mass production.The design of the reflector component is not limited to the presence of two ultrasonic reflector surfaces; rather, more than two ultrasonic reflector surfaces can be formed or arranged on the support element.

[0009] According to a non-inventive embodiment, the ultrasonic reflector surfaces are designed to reflect the ultrasonic signal directly back to the ultrasonic transducer. This means that the ultrasonic reflector surfaces are arranged opposite the ultrasonic transducer to ensure direct back reflection. This results in a particularly simple design of the test unit, in which the time-of-flight difference to be measured is directly derived from the distance between the ultrasonic reflector surfaces in the direction of the ultrasonic transducer.

[0010] According to the invention, at least one of the ultrasound reflector surfaces is oriented obliquely relative to the ultrasound transducer and the other ultrasound reflector surface and is designed to deflect the ultrasound signal at least once before it is reflected back to the ultrasound transducer. In this case, the ultrasound signal from at least one of the ultrasound reflector surfaces is therefore not reflected directly back to the ultrasound transducer, but is first directed or reflected to at least one further reflective surface, or, according to the invention, to at least one further ultrasound reflector, from which the ultrasound signal is directed / reflected back to the ultrasound transducer. The multiple deflections increase the transit time difference of the reflected ultrasound signals.The transit time difference between the two reflected ultrasound signals must be large enough to allow for unambiguous temporal separation of the two signals. Larger transit time differences also increase the accuracy of the media characterization due to the greater differences in path length. The advantageous design described above facilitates a simple increase in path length differences. While the present example assumes that two ultrasound signals are reflected back to the ultrasound transducer, it is also possible to reflect more than two signals back, for example, by using at least three or more ultrasound reflectors opposite the transducer, reflecting the emitted ultrasound signal back to the transducer directly or indirectly in different ways.

[0011] According to a preferred embodiment of the invention, at least one of the reflector surfaces has a curvature or is curved, in particular concave or convex. This improves the signal-to-noise ratio of the ultrasound signal. Depending on the curvature of the ultrasound reflector surface, sound field focusing is also possible. For this purpose, the ultrasound reflector surface in question is, for example, shaped like a paraboloid, a spherical segment, or a cylindrical segment.

[0012] Furthermore, according to the invention, at least one of the reflector surfaces is arranged next to, and in particular offset in the direction of propagation of the ultrasound signal from, the ultrasound transducer and opposite another of the ultrasound reflector surfaces. Thus, one ultrasound reflector surface is located next to the ultrasound transducer, but in particular offset in the direction of propagation from it, so that the signals do not have the same propagation time between the two ultrasound reflector surfaces on the one hand and between the ultrasound transducer and the other ultrasound reflector surface on the other. This allows for unambiguous evaluation of the received ultrasound signals and, in particular, makes it easy to distinguish between multiple reflections. One ultrasound transducer then reflects the detected signal back to the other ultrasound transducer, which subsequently reflects the multiple-reflected ultrasound signal back to the first ultrasound transducer.This results in a particularly large difference in transit time between the two ultrasound signals detected by the ultrasound transducer.

[0013] Preferably, one or both ultrasound reflector surfaces have a curved reflective surface, while the other or one ultrasound reflector surface has a flat or planar surface. The choice of curvature depends, in particular, on whether the ultrasound transducer emits a diverging ultrasound wave or a plane wave. If the ultrasound transducer emits a diverging wave as its ultrasound signal, the opposite ultrasound reflector surface is preferably curved such that the reflected ultrasound wave is planar. This ensures that part of the reflected ultrasound signal strikes the ultrasound transducer and another part strikes the adjacent ultrasound reflector. The part striking the ultrasound transducer is detected as the first echo.The other part is reflected back at the ultrasound reflector surface opposite the ultrasound transducer, strikes the curved reflector surface there again, and is reflected back to the ultrasound transducer, where it is received as a second echo. If, however, the ultrasound transducer emits a plane wave as the ultrasound signal, the ultrasound reflector opposite the ultrasound transducer is advantageously curved or flat in such a way that the reflected ultrasound signal or wave is curved and thus strikes both the ultrasound transducer and the adjacent ultrasound reflector.

[0014] Furthermore, it is preferably provided that the support element has at least one recess in a reflector-free area. This reduces the weight of the support element and also decreases the number of surfaces that can reflect the ultrasound signal. This advantageously reduces interference signals and signal noise.

[0015] According to a preferred embodiment of the invention, at least one of the ultrasonic reflector surfaces is formed as a coating on the carrier element. This ensures particularly simple and precise positioning of the respective ultrasonic reflector surface on the carrier element. Materials for the respective ultrasonic reflector are preferably metals, ceramics, or other materials with high acoustic impedance. In applications with aggressive media / fluids, stainless steel or another metal, optionally with a surface coating, is preferably used. Filled plastics are also known, with thermosets or thermoplastics being examples of suitable plastics. In particular, the respective ultrasonic reflector surface is manufactured by an additive manufacturing process. This results in a highly precise arrangement of the respective ultrasonic reflector surface on the carrier element.Together with the one-piece design of the support element, this results in a particularly high level of precision in carrying out the quality determination.

[0016] Preferably, the ultrasound reflector surfaces are arranged such that the ultrasound signal is reflected multiple times at at least one of the ultrasound reflector surfaces. This multiple reflection, for example, allows the path of the ultrasound signal to be lengthened to enable improved signal analysis.

[0017] Furthermore, the device preferably includes a control unit configured to control the test unit, in particular the ultrasonic transducer, to generate and receive burst signals as ultrasonic signals in order to determine the mass concentration of the liquid as a function of the transit-time difference of the ultrasonic signals reflected back from the ultrasonic reflector surfaces. Preferably, the frequency range of the burst signals is between 0.5 MHz and 10 MHz, more preferably between 1 MHz and 2 MHz. Higher frequencies allow for better spatial focusing of the ultrasonic field and a more precise determination of the transit time. However, with increasing frequency, the absorption of sound energy in the propagation medium also increases. The control unit is also configured to evaluate the ultrasonic signals or echoes received by the ultrasonic transducer in order to perform the quality determination.In particular, the control unit is designed to determine the transit time between the electrical excitation of the ultrasonic transducer, especially the piezoceramic, and the received echo by means of a simple threshold determination of the voltage amplitude of the received signal (echoes) or via zero-point detection or the determination of the arrival of the maximum amplitude after an envelope determination or the use of correlation methods or other known methods for transit time determination from time signals.

[0018] The tank device according to the invention, comprising the features of claim 6, is characterized by the inventive design of the device. This results in the advantages already mentioned.

[0019] Further advantages and preferred features and combinations of features will become apparent in particular from the foregoing and from the claims. The invention will now be explained in more detail with reference to the drawing. To this end, we show... Figure 1 shows an advantageous tank device in a simplified side view, Figure 2 shows a first embodiment of an advantageous test unit of the tank device, Figure 3 shows a second embodiment of the test unit, Figure 4 shows a third embodiment of the test unit, Figure 5 shows a fourth embodiment of the test unit, Figure 6 shows a fifth embodiment of the test unit, Figure 7 shows a sixth embodiment of the test unit, Figure 8 shows a seventh embodiment of the test unit and Figure 9 shows an eighth embodiment of the test unit, each in a simplified representation.

[0020] Figure 1Figure 1 shows a simplified representation of an advantageous tank device 1 for an exhaust aftertreatment system of a motor vehicle. The tank device 1 comprises a tank 2 in which a liquid exhaust aftertreatment agent, in particular a reducing agent 3, is stored. A dispensing module 4 is arranged in the tank 2, the dispensing module 4 being located, in particular, on the bottom of the tank 2. The dispensing module carries a conveying device 5 by means of which the reducing agent can be drawn from the tank 2 and conveyed via a line 6 to, for example, a metering valve or injection valve. The dispensing module 4 further comprises, in particular, a filter that is arranged upstream of the conveying device 5, and optionally a heating device to thaw any frozen exhaust aftertreatment agent.The reducing agent is, in particular, an aqueous urea solution used to generate ammonia, which reacts with the exhaust gas of a motor vehicle's internal combustion engine to reduce nitrogen oxides. A test unit 7 is provided for determining the urea concentration in the liquid; this unit is also assigned to the sampling module 4. Alternatively, the test unit 7 is located separately in tank 2. Together with a control unit 8, the test unit 7 forms a device 9 for determining the quality of the liquid in tank 2 by measuring its concentration.

[0021] Based on the Figures 2 to 9In the following, different embodiments of the test unit 7 will be explained in more detail. These embodiments have in common that the test unit 7 each comprises an ultrasonic transducer and at least two ultrasonic reflector surfaces, wherein the ultrasonic reflector surfaces are arranged or formed on a one-piece carrier element.

[0022] Figure 2 Figure 1 shows a first embodiment of the test unit 7 in a simplified representation. As mentioned previously, the test unit 7 has an ultrasonic transducer 10, which can be controlled by the control unit 8 and is designed, in particular, as a piezo-ceramic ultrasonic transducer. Opposite the ultrasonic transducer are two ultrasonic reflector surfaces 11 and 12, which are arranged on a common, integrally formed support element 13.

[0023] The use of ultrasonic transducers to determine fluid properties is a known technique. The measurement principle is based on measuring the transit time of a sound signal in the liquid medium over a known propagation distance between the transducer and the receiver. Various media properties can be determined from the resulting propagation speed of the sound signal in the fluid and, if necessary, other measured parameters such as the fluid temperature. For example, such an ultrasonic transducer is used to determine and monitor the mass concentration of the urea-water solution in the tank. The ultrasonic transducer converts an electrical signal into a sound wave and emits it into the fluid, specifically into the liquid in tank 2.A receiver converts the ultrasound signal emitted by the ultrasound transducer into an electrical signal that can then be evaluated. Known ultrasound transducers function as both transmitters and receivers, utilizing the piezoelectric effect to convert between sound and electrical signals. In a so-called pulse-echo operation, at least one reflector is required, which reflects the sound wave emitted by the ultrasound transducer 10 back to it via a known propagation path. The transient response of the ultrasound transducer 10 must be taken into account. This results in a predetermined minimum length for the measuring path, ensuring that the mechanical vibrations of the ultrasound transducer 10 generated during the transmission of the ultrasound signal have decayed sufficiently to allow for a clear separation between the transmitted and received signals when the backscattered echo (reflected ultrasound signal) is received.

[0024] Through the in Figure 2 In the advantageous design of the test unit 7 shown, a measuring distance ΔL results which is not determined by the distance of the reflector surfaces to the ultrasonic transducer 10, but by the distance of the reflector surfaces 11, 12 to each other.

[0025] Both ultrasonic reflector surfaces 11, 12 are arranged opposite the ultrasonic transducer 10. The support element 13 is stepped, with reflector surface 11 on a first step and reflector surface 12 on a second step. The second step is further away from the ultrasonic transducer 10 than the first step with reflector surface 11. Thus, different path lengths L1 and L2 result between the respective ultrasonic reflector surfaces 11, 12 and the ultrasonic transducer 10, the difference of which determines the measuring distance ΔL = L2 - L1. The measuring distance is therefore determined by the single-piece support element 13. The propagation speed of the ultrasonic signal cF is then calculated from the time difference Δt and the path difference ΔL as follows: cF = ΔL / Δt.

[0026] The absolute distance between the ultrasonic transducer 10 and the ultrasonic reflector surfaces 11, 12 is therefore not included in the calculation and thus does not need to be known or constant. Because the support element 13 is a single piece, there is little to no thermal expansion over the intended operating temperature range, which then affects both ultrasonic reflector surfaces 11, 12 equally. The time-of-flight difference between the two echoes received by the ultrasonic transducer 10, or reflected ultrasonic signals, is at least large enough to clearly separate the two ultrasonic signals in time. Longer time-of-flight differences due to greater differences in propagation path increase the accuracy of the media characterization. Furthermore, more than two propagation paths can be implemented, and thus more than two echoes can be used for evaluation, as discussed in more detail below.

[0027] According to the present embodiment of Figure 2 The reflectors 11, 12 are designed as planar ultrasound reflector surfaces 11, 12 or as ultrasound reflector surfaces 11, 12 with a planar / flat reflective surface. Suitable reflector materials are selected to achieve the highest possible signal amplitude, particularly of the reflected ultrasound signal. Furthermore, a curved shape of the reflector geometry of the respective ultrasound reflector surface 11, 12 allows for focused back-reflection to the ultrasound transducer 11.

[0028] When selecting the reflector materials, the acoustic impedance ZR of the respective ultrasonic reflector surface 11, 12 is chosen to be as high as possible. The acoustic impedance of the medium results from the product of the specific density of the medium and the speed of sound propagation in the medium. Assuming the incident of a plane wave from the liquid onto the respective ultrasonic reflector 11, 12, the amplitude reflection factor R is obtained from the acoustic impedances ZF of the liquid in the tank 2 and the respective ultrasonic reflector surface 11, 12 ZR: R = (ZR - ZF ) / (ZR + ZF ).

[0029] For a given acoustic impedance ZF of the fluid to be characterized, high backscattering occurs when the impedance of the reflector ZR is significantly greater than that of the fluid: ZR >> ZF. Suitable reflector materials include metals, ceramics, or other materials with high acoustic impedance. In applications involving aggressive liquids, stainless steel or other metals, optionally with a surface coating for protection, are particularly suitable. Other alternatives include filled plastics, such as thermosets or thermoplastics, filled with metal or ceramic powder. The respective ultrasonic reflector 11, 12 is preferably manufactured directly on the support element 13 using additive manufacturing processes.

[0030] Figure 3Figure 1 shows a second embodiment of the test unit 7, which differs from the previous embodiment in that the transit-time difference is increased by multiple reflections. Unlike the previous embodiment, the second reflector surface 12 is not parallel, but oriented obliquely to the ultrasonic transducer 10 and thus also to the ultrasonic reflector surface 11, so that the ultrasonic signal is reflected obliquely back from the reflector surface 12. A third reflector surface 14 is arranged on the support element 13, onto which the signal from the reflector surface 12 is reflected, as shown by the dashed arrows. The ultrasonic reflector surface 14 is arranged such that it, in turn, reflects the reflected ultrasonic signal back to the ultrasonic transducer 10. This increases the transit time of the ultrasonic signal reflected from the ultrasonic reflector surface 12 and optimizes the measurement.In the present case, the third ultrasound reflector surface 14 is also designed as a flat reflector surface.

[0031] Figure 4Figure 1 shows a third embodiment of the test unit 7, which differs from the second embodiment in that several recesses 15 are formed in the support element 13. The recesses 15 are each formed in reflector-free areas of the support element 13 and serve, on the one hand, to reduce weight and, on the other hand, to minimize disruptive back reflections from surfaces of the support element that are not part of the intended propagation paths of the ultrasound signals. In particular, only the surfaces of the support element required for reflection and mechanical stability are made of a material with high acoustic impedance, while other areas of the support element consist of a material with lower acoustic impedance, for example, plastic. Specifically, the support element as a whole consists of plastic, as previously discussed, and the reflector surfaces 11, 12, and 14 are made of a material with high impedance.

[0032] Figure 5 Figure 7 shows a fourth embodiment of the test unit 7, which differs from the previous embodiment in that the propagation path, which is deflected multiple times, has an even longer distance. According to the embodiment shown in Figure 7, the following applies: Figure 5 The first reflector 11 is arranged closer to the ultrasound transducer 10. For this purpose, the reflector 11 is arranged on a projecting surface of the support element 13. Furthermore, in this embodiment, the second ultrasound reflector 12 is not flat but curved to achieve a focused back reflection of the ultrasound signal to the third ultrasound reflector surface 14 and the ultrasound transducer 10. For this purpose, the curvature of the ultrasound reflector 12 is, in particular, paraboloid, segment-shaped, or segment-shaped.

[0033] Figure 6shows a further embodiment of the test unit 7, which differs in particular from the embodiment of Figure 2 The support element 13 differs in that it is cylindrical overall, comprising a first section 16 with a diameter D1 and a second section 17 with a diameter D2, where the diameter D1 is significantly larger than the diameter D2, resulting in the reflector 12 as an annular reflector surface on section 16 and the reflector 11 on the end face of section 17. This results in a rotationally symmetrical reflector component that is cost-effective and space-saving.

[0034] Figure 7 Figure 6 shows a sixth embodiment of the test unit 7, which differs from the embodiment of Figure 7. Figure 5This differs in that multiple reflections take place at the reflector surface 12. For this purpose, the second reflector surface 12 and the third reflector surface 14 are arranged relative to each other such that the ultrasound signal reflected or directed from reflector surface 12 to reflector surface 14 is directed from reflector surface 14 back to reflector surface 12 and from there reflected back to the ultrasound transducer 10. For this purpose, an angle of α = 45° is provided by the planar reflector surfaces 12 and 14.

[0035] Figure 8Figure 7 shows a seventh embodiment of the test unit 7, which differs from the previous embodiments in that the second reflector 12 is arranged next to and offset in the direction of travel of the ultrasonic signal relative to the ultrasonic transducer 10. In particular, the ultrasonic transducer 10 is located inside the reflector 12, which thus encloses the ultrasonic transducer 10 on at least two sides. The reflector surface 11 is located opposite the ultrasonic transducer 10 as before, but has an area that essentially corresponds to the area of ​​the reflector surface 12. According to the present embodiment, the ultrasonic transducer 10 is configured to emit diverging ultrasonic waves, as indicated by solid lines in Figure 1. Figure 8The transmitted ultrasound signal strikes the ultrasound reflector surface 11, which itself has a curvature chosen such that the ultrasound waves reflected by it are reflected back as plane waves to the ultrasound transducer 10 and the reflector surface 12, as shown by the dashed lines. These waves are detected by the ultrasound transducer 10 as the first echo. Since the waves also strike the ultrasound reflector surface 12, they are reflected back from it to the reflector surface 11, from where they are reflected back to the ultrasound transducer 10 as the second echo.

[0036] Figure 9 shows an eighth embodiment, which differs from the embodiment of Figure 8This differs in that the ultrasound transducer 10 is designed to emit plane ultrasound signals or waves, as shown by solid lines. Due to the curvature of the first reflector surface 11, diverging ultrasound waves are reflected back. These waves strike the ultrasound transducer 10 as the first echo and the second ultrasound reflector surface 12, from where they are reflected back to the first reflector surface 11, which in turn reflects them back to the ultrasound transducer 10 as the second echo. The curvature of the reflector surface 11 is chosen such that a diverging wave is reflected back towards the ultrasound transducer 10.Due to the beam expansion, part of this wave also hits the second reflector surface 12, which according to the present embodiment is also curved, so that wave focusing occurs, whereby the ultrasound signal is focused back from the reflector surface 12 to the reflector surface 11, so that the waves again hit the curved reflector surface 11.

Claims

1. Device (9) for determining the quality of a liquid, in particular an exhaust gas aftertreatment agent, having at least one test unit (7) which can be arranged in a tank (2) storing the liquid and has at least one ultrasonic transducer (10) for emitting and receiving an ultrasonic signal and at least two ultrasonic reflector surfaces (11, 12, 14) for reflecting the emitted ultrasonic signal to the at least one ultrasonic transducer (10), wherein the at least two ultrasonic reflector surfaces (11, 12, 14) are formed or arranged directly on a common and one-piece carrier element (13) at different distances from the ultrasonic transducer (10), characterized in that one of the ultrasonic reflector surfaces (12) is oriented obliquely to the ultrasonic transducer (10) and to the other ultrasonic reflector surface (11) and is designed to deflect the ultrasonic signal at least once, before it is reflected back to the ultrasonic transducer (10), wherein here the ultrasonic signal is directed to at least one further ultrasonic reflector surface (14) which is also arranged on the carrier element (13) and from which the ultrasonic signal is reflected back to the ultrasonic transducer (10).

2. Device according to Claim 1, characterized in that at least one of the ultrasonic reflector surfaces (11, 12, 14) has a curvature.

3. Device according to either of the preceding claims, characterized in that the carrier element (13) has at least one cutout (15) in a reflector-free region.

4. Device according to any of the preceding claims, characterized in that at least one of the ultrasonic reflector surfaces (11, 12, 14) is in the form of a coating on the carrier element (13).

5. Device according to any of the preceding claims, characterized by a controller (8), which is designed to actuate the test unit (7), in particular the ultrasonic transducer (10), to generate and to receive at least one burst signal as the ultrasonic signal in order to determine a mass concentration in the liquid as a function of a transit time difference of the ultrasonic signals reflected back by the ultrasonic reflector surfaces (11, 12, 14).

6. Tank device (1), in particular reducing agent tank device for an exhaust gas aftertreatment system of a motor vehicle, having a tank for storing and providing a liquid (3), in particular an exhaust gas aftertreatment agent, and having a device (9) for determining the quality of the liquid, wherein the device (9) has at least one test unit (7) which can be arranged in the tank (2), characterized by the design of the device (9) according to any of Claims 1 to 5.