Fuel tank unit for a motor vehicle

Abstract

Tank unit (1) for a motor vehicle, comprising a liquid tank (2) having a tank wall (3), characterized in that the liquid tank (2) has at least one strain gauge fiber (4) designed as an optical fiber, which is connected to the tank wall (3) at least sectionally in a strain-transmitting manner, wherein at least one optical property of the strain gauge fiber (4) is dependent on its strain, wherein the tank unit (1) has a measuring unit (10) which is coupled to the at least one strain gauge fiber (4) and is configured to send an optical signal into the respective strain gauge fiber (4), to receive a reflected signal from the strain gauge fiber (4) and to evaluate this signal to determine a strain of the strain gauge fiber (4) and to determine a quantity of liquid within the liquid tank (2) based on the reflected signal from at least one strain gauge fiber (4).

Description

[0001] The invention relates to a tank unit for a motor vehicle having the features of the preamble of claim 1.

[0002] Motor vehicles with internal combustion engines require fuel, usually petrol or diesel, which is carried in a fuel tank. However, fuels can also be gases, including liquefied gases. In order to estimate the remaining range of the motor vehicle, it is desirable to be able to measure the amount of fuel remaining in the fuel tank as accurately as possible. This is made difficult, among other things, by the often complex shape of the fuel tank. An inclination of the vehicle or dynamic acceleration forces can also distort the measurement. Methods known in the prior art are based, for example, on a float that floats on the surface of the fuel and whose position is registered by a sensor, or, for example, on a capacitance measurement that makes use of the different electrical properties of the fuel on the one hand and of the gases contained in the tank on the other.These methods work in principle, but leave something to be desired in terms of accuracy.

[0003] US 9,057,637 B2 discloses a method for optically determining state variables inside a container for liquefied gases. Light emanating from a lighting unit travels through a light guide to a contact point with the contents of the container, is partially reflected there, and the luminous intensity of the reflected light is measured to determine a state variable from this and the luminous intensity of the light emanating from the light source. Several light guides are routed to contact points distributed inside the container. From the measured values ​​obtained, spatially attributable state variables of the container contents are determined.

[0004] A sensor for measuring fuel levels in paragliders and the like is known from "A Polymer Optical Fiber Fuel Level Sensor: Application to Paramotoring and Powered Paragliding", D. Sanchez Montero et al., Sensors 2012, 12, pages 6186-6197. A strain gauge fiber is mounted in a meandering fashion on a metal bracket and arranged in a fuel tank, so that curved sections of the fiber are arranged one above the other. A light signal is fed into the fiber, and the reflected light quantity is analyzed. In the curved sections, where the fiber is surrounded by fuel, increased light is coupled out of the fiber, so that the fuel level can be estimated based on the reflected light quantity.

[0005] US 6,703,635 B2 discloses a method for measuring a liquid level, wherein a strain-measuring fiber is connected at one end to a float movably disposed in a liquid. A change in the force acting on the float, based on a change in the liquid level, is recorded as a changing strain within the strain-measuring fiber. This is done by means of a fiber strain gauge connected to the other end of the strain-measuring fiber.

[0006] US 6,668,645 B1 shows an optical sensor for determining a fuel level, which is designed to provide an electronic signal corresponding to the fuel level within a fuel tank. The sensor comprises an optical waveguide, a light source, and a light receiver. The waveguide has a shape that tapers upwards and downwards within the tank and has a plurality of angled facets. Light is coupled into the waveguide from the light source and is essentially reflected many times by the facets, provided the respective facet is surrounded by gas. If the facet is surrounded by fuel, light is coupled out, which affects the total amount of light reflected back to the light receiver. It is intended to calibrate the sensor with known fuel quantities.

[0007] FR 2 791 131 A1 discloses a device for indicating a fuel level in a fuel tank. The device comprises a plurality of optical sensors arranged one above the other in the fuel tank. Each optical sensor has a light source, a receiver, and a light deflection device. The latter reflects the light from the light source to the receiver when the sensor is surrounded by gas. When the sensor is surrounded by fuel, light is coupled into the environment and thus does not reach the receiver.

[0008] US Pat. No. 8,146,419 B2 discloses a device for temperature-compensated measurement of the liquid level in a tank. The device comprises a transducer for transmitting and receiving acoustic signals, a waveguide connected to the transducer and extending into the liquid, and a structure for directing a flow of liquid from the tank along an outer side of a portion of the waveguide that, in operation, is located above the liquid surface. During operation, the transducer emits acoustic signals that are directed by the waveguide to the liquid surface and reflected from there back to the transducer. The liquid level can be calculated from the propagation time.

[0009] GB 1 518 492 A discloses a fluid level warning device. This device comprises a light source and fiber optic light guides that direct light onto a polished surface of a block of transparent material. The light is totally reflected when the surface is surrounded by air and is decoupled when the surface is surrounded by fluid. Reflected light is directed to a receiver by further fiber optics. The block can be arranged, for example, in a reservoir containing brake fluid or engine oil to detect a critical fluid level.

[0010] From the subsequently published DE 10 2016 125 871 A1, an optical fiber multi-measurement system with an optical fiber extending at least partially within a container wall is known. This system is used, among other things, to measure the mechanical stress and fill level in the container. The section of the optical fiber arranged in the container wall is an integral part of the container wall.

[0011] In view of the state of the art shown, the determination of the amount of liquid in a liquid tank, in particular a fuel tank, still offers room for improvement.

[0012] The invention is based on the object of enabling a reliable determination of the amount of liquid in a liquid tank.

[0013] According to the invention, the object is achieved by a tank unit having the features of claim 1, wherein the subclaims relate to advantageous embodiments of the invention.

[0014] It should be noted that the features and measures listed individually in the following description can be combined with one another in any technically reasonable manner and demonstrate further embodiments of the invention. The description further characterizes and specifies the invention, particularly in conjunction with the figures.

[0015] The invention provides a tank unit for a motor vehicle. The motor vehicle can be, for example, a car or a truck. The tank unit has a liquid tank which has a tank wall. The liquid tank is of course intended to hold a liquid when the motor vehicle is in operation. In particular, it can be a fuel tank, e.g. for gasoline or diesel, or even for liquefied gases. In principle, however, it could also be a tank for another liquid, e.g. a coolant, a lubricant, windshield wiper fluid or the like. The tank wall serves to enclose the liquid and delimits the liquid tank to the outside. Of course, the tank wall normally has an opening for filling the liquid and another opening for removing the liquid.There are basically no restrictions regarding the material used for the tank wall. It can be a single material or a composite material, e.g., with a multi-layer structure. In particular, the tank wall can be made of plastic.

[0016] According to the invention, the liquid tank has at least one strain-measuring fiber designed as an optical fiber, which is connected at least partially to the tank wall in a strain-transmitting manner, wherein at least one optical property of the strain-measuring fiber is dependent on its strain. An optical fiber is defined as a fiber that is suitable as a waveguide for optical signals, which includes in particular visible light, and in a broader sense also infrared and ultraviolet light. Such an optical fiber can in particular be designed as a glass fiber, but optionally also as a polymer optical fiber. The strain-measuring fiber is connected at least partially to the tank wall in a strain-transmitting manner, i.e., at least individual sections of the strain-measuring fiber are connected to the tank wall in such a way that a strain is transmitted between the tank wall and the strain-measuring fiber.Where the corresponding connection exists, a strain in the tank wall is at least partially transmitted to the strain-measuring fiber. The term "strain" here includes not only the actual (positive) strain but also compression, i.e., negative strain. Thus, a strain or compression of the tank wall is transmitted to the strain-measuring fiber, at least in part.

[0017] At least one optical property of the strain-measuring fiber depends on the strain of the strain-measuring fiber. This means that this optical property changes depending on the strain. The optical property therefore changes, at least in the areas where the above-mentioned connection to the tank wall exists, depending on the strain of the tank wall. The optical property that depends on the strain of the strain-measuring fiber can preferably be a reflection behavior with regard to an optical signal, i.e. the extent to which an optical signal traveling within the fiber is reflected instead of being transmitted. In particular, this reflection behavior can be frequency-dependent, so that in a certain frequency range or wavelength range a reflection occurs, while other frequencies or wavelengths are transmitted. This can be the case, for example.This can be achieved by having the strain-measuring fiber include at least one fiber Bragg grating (FBG), normally a plurality of FBGs. Such an FBG selectively reflects optical signals within a limited wavelength range that depends on the grating period of the FBG. The period of the FBG, in turn, varies with the strain of the strain-measuring fiber. For example, a plurality of such FBGs can create a plurality of measurement points along the strain-measuring fiber, at which the reflection behavior can be varied depending on the strain. However, the invention is not limited to this. For example, a continuous strain measurement could also be realized by evaluating a reflected signal based on Rayleigh scattering within the strain-measuring fiber. The reflected signal can be evaluated using an optical time-domain reflectometer (OTDR).It is also possible to evaluate a reflected signal based on Brillouin scattering.

[0018] The invention is based on the finding that the expansion of the tank wall is significantly influenced by the liquid present in the tank. Due to its weight and occasional accelerations, the liquid exerts a force on the tank wall, leading to its deformation. Due to the connection to the respective strain-measuring fiber, which exists at least in sections, this deformation causes an expansion (or compression) of the same, which in turn affects the optical properties. It is possible to record the expansion either at a plurality of discrete points along the strain-measuring fiber or even continuously along the entire length of the strain-measuring fiber, even without individual, potentially costly or complicated sensors.Although the relationship between the fluid content and the fiber's strain profile is generally difficult to determine theoretically, it is readily possible to determine reference values ​​by calibrating with different known test quantities, with which current measured values ​​can be compared. This allows for a precise determination of the fluid volume. This applies even if the fluid changes position due to a tilt of the vehicle or acceleration. Such effects can also be simulated during calibration.

[0019] However, the capabilities of the tank unit according to the invention are not limited to adjusting the fuel quantity during vehicle operation. It is also possible to measure the static and dynamic loads on the liquid tank, for example, to optimize its connection to the vehicle body.

[0020] In general, the at least one expansion fiber can be arranged on the underside, on the top side and / or on at least one side surface of the liquid tank, relative to the intended installation position of the liquid tank.

[0021] In principle, it is possible for the respective strain-measuring fiber to be connected to the tank wall only in sections, for example, in a plurality of spaced-apart, possibly short sections. However, with a view to transmitting the strain of the tank wall to the strain-measuring fiber as directly and precisely as possible, it is preferred that at least one strain-measuring fiber be connected to the tank wall at least predominantly continuously. In particular, the corresponding strain-measuring fiber can be connected completely continuously to the tank wall.

[0022] Effective strain transmission could potentially be achieved through a positive and / or non-positive connection of the strain gauge fiber to the tank wall. However, this generally entails the risk of an unintentional change in position, such as slippage, which would result in strain in the tank wall not being transmitted or being transmitted incompletely to the strain gauge fiber. For this reason, it is preferred that at least one strain gauge fiber be firmly bonded to the tank wall. Such a firmly bonded connection, which can be created, for example, by gluing it to the tank wall, leads to a very direct transmission of strain forces to the strain gauge fiber, thereby minimizing measurement inaccuracies. It is possible, for example, to bond the strain gauge fiber to an inner surface of the tank wall or to an outer surface of the tank wall, or to integrate it directly into the tank wall, e.g.for plastic tanks.

[0023] In particular, at least one strain-measuring fiber can be embedded in the tank wall. The strain-measuring fiber can be fully or partially embedded. This can be achieved, for example, with a tank wall made of plastic if the corresponding strain-measuring fiber is embedded directly during the primary shaping (or reshaping) of the tank. Typically, fuel tanks, for example, are manufactured either by injection molding or blow molding. In both processes, it is possible to place an optical fiber either on the surface of the plastic material or in the plastic material itself. In both blow molding and injection molding, the strain-measuring fiber can be placed inside the mold before the plastic material is formed, where it is then fully or partially enclosed by the plastic material during the primary shaping or reshaping.Even in the case of at least partial embedding, different positions of the strain gauge fiber can be realized, for example, inside the wall, on the inner surface, or on the outer surface. In the latter cases, embedding is usually only partially achieved. Depending on the position of the strain gauge fiber, its sensitivity to deformation of the tank wall can be influenced, as can the mechanical protection of the strain gauge fiber by the tank wall.

[0024] In addition to a single strain-measuring fiber, the tank can also have a plurality of strain-measuring fibers. Such a plurality of strain-measuring fibers is preferably arranged at least predominantly at a distance from one another. This means that the strain-measuring fibers are not arranged as fiber bundles, but rather at a distance from one another and thus distributed along the tank wall. In this way, measured values ​​can be obtained along a larger area of ​​the tank wall, which generally leads to greater precision in determining the strain profile and thus the amount of liquid contained. Strain-measuring fibers can run offset next to one another. Likewise, two strain-measuring fibers can cross each other. A wide variety of arrangements are conceivable, for example such that the strain-measuring fibers form a type of grid along the tank wall.In general, the most extensive coverage of the entire tank wall possible, i.e., the densest possible network of measuring points, leads to improved measurement precision. In addition to or as an alternative to a plurality of strain-measuring fibers, at least one strain-measuring fiber can preferably be arranged in a meandering pattern. This means that the corresponding strain-measuring fiber is routed along the tank wall with an alternating, changing path.

[0025] The tank unit can further comprise a measuring unit that is coupled to the at least one strain-measuring fiber and is configured to transmit an optical signal into the respective strain-measuring fiber, receive a reflected signal from the strain-measuring fiber, and evaluate this signal to determine a strain of the fiber. The measuring unit can, under certain circumstances, comprise several spatially spaced components. At least parts of its functions can be implemented in software. In the operating state, the measuring unit transmits an optical signal into each strain-measuring fiber. This can, for example, be an optical signal distributed over a larger wavelength range or frequency range, e.g., white light. When such a signal strikes an FBG, a limited sub-range of the spectrum is selectively reflected there, with the center wavelength of the sub-range depending on the strain of the fiber.By measuring the time of flight, the measuring unit can assign the reflected portion to a specific point within the strain-measuring fiber. Even with other measuring mechanisms, which may be based, for example, on the aforementioned Rayleigh scattering or Brillouin scattering, the strain can be determined spatially resolved by evaluating the reflected signal from the strain-measuring fiber. The measuring unit does not have to explicitly determine the corresponding strain, but can, if necessary, determine values ​​from which this strain can be determined or which correspond to a specific strain.

[0026] The measuring unit is preferably designed to determine a quantity of liquid within the liquid tank based on the reflected signal from at least one strain measuring fiber. The quantity of liquid can be a mass, a volume or even a volume fraction within the tank. Normally, the quantity of liquid cannot be determined on the basis of a theoretical model. Rather, it is expedient to determine comparison measured values ​​in advance through tests or calibration, using a known quantity of liquid. The comparison measured values ​​can then be stored in a memory of the measuring unit and compared with current measured values. As already mentioned, it is not necessary for the measuring unit to explicitly determine the strain as an intermediate result. In principle, the measuring unit can use any values ​​that correspond to a strain orfrom which a strain could be determined. In the case of an FBG, for example, the measuring unit could determine the average wavelength of the light reflected by a specific FBG. By comparing this wavelength with the wavelength that would be expected without any strain, the strain in the area of ​​the corresponding FBG can be determined. The above-mentioned comparison measurements could either refer to a strain or, for example, to the average wavelength. In either case, the amount of fluid can be determined by comparing it with the currently obtained measured values.

[0027] In general, it should be noted that the characteristics of the reflected signal can also depend on the temperature in the strain gauge fiber or in the area of ​​the respective measuring point. For example, strain in the strain gauge fiber can be caused by strain transfer, i.e., mechanical stress from the tank wall, or by temperature-induced expansion (or contraction). Similarly, Rayleigh scattering and Brillouin scattering can be influenced by both mechanical stress and temperature. Therefore, the reflected signal can be influenced by the respective temperature, regardless of the amount of liquid in the tank. This could falsify the measurement results. For this reason, the measuring unit is preferably configured to include a temperature measurement when evaluating the reflected signal.The measuring unit itself can have at least one temperature sensor, or it can receive measured values ​​from at least one temperature sensor. Here, too, the temperature-related correction of the result does not have to be based on a mathematical model; instead, different temperatures can be set during calibration and the temperature-dependent measured values ​​can be recorded.

[0028] One possibility for carrying out the temperature measurement is the use of an optical fiber and the fact that its optical properties can also change depending on the temperature, as described above. According to a corresponding embodiment, the liquid tank has at least one temperature measuring fiber designed as an optical fiber, wherein at least one optical property of the temperature measuring fiber is dependent on its temperature. The temperature measuring fiber can be made of the same material as the at least one strain measuring fiber, but it can also be a different material. It can be connected to the tank wall at least point-wise or in sections, although a strain-transmitting connection is neither necessary nor desirable.Since the temperature measuring fiber is used for temperature measurement, it is advantageous if it does not undergo any mechanical strain, so that changes in its optical properties, in particular its reflection behavior, are based exclusively or predominantly on temperature changes. As described above with reference to the strain measuring fiber, the temperature measuring fiber can also have one or more FBGs, or Rayleigh or Brillouin scattering can be used for temperature measurement. A plurality of temperature measuring fibers can also be provided, arranged, for example, in a network-like manner. Likewise, a temperature measuring fiber can be arranged in a meandering manner. A temperature measuring fiber can be arranged at a distance from a strain measuring fiber or even at its borders, so that the two measuring fibers are laid out as a fiber pair, so to speak.Of course, the measuring unit can send an optical signal into the temperature measuring fiber in a similar way to the strain measuring fiber and evaluate a reflected signal to determine the temperature.

[0029] As an alternative to a separate temperature measurement fiber, it is also possible to perform temperature measurements using a strain-measuring fiber. For example, the measurement technique known as Distributed Temperature and Strain Sensing (DTSS) makes it possible to determine both the temperature and strain within an optical fiber from a reflected signal.

[0030] Further advantageous details and effects of the invention are explained in more detail below with reference to exemplary embodiments illustrated in the figures. They show: Fig. 1 is a schematic representation of a side view of a first embodiment of a tank unit according to the invention in partial section; Fig. 2 an enlarged sectional view along the line II-II in Fig. 1; Fig. 3 a bottom view of a tank of the tank unit according to the viewing direction III in Fig. 1; Fig. 4 a bottom view of a tank according to a second embodiment; Fig. 5 a bottom view of a tank according to a third embodiment; and Fig. 6 a bottom view of a tank according to a fourth embodiment.

[0031] In the different figures, identical parts are always provided with the same reference symbols, which is why they are usually only described once.

[0032] Fig. Figure 1 shows a first embodiment of a tank unit 1 according to the invention, which can be used, for example, in a passenger car. It is a side view in partial section, which has been highly schematic for clarity. A fuel tank 2 with a tank wall 3 made of plastic is visible, which is partially filled with fuel 20, for example, gasoline or diesel. The geometric shape of the fuel tank 2 is particularly Fig. 1 is shown in a simplified manner and is usually more complicated in reality. Furthermore, a measuring unit 10 is shown schematically, which is coupled to several strain-measuring fibers 4. The strain-measuring fibers 4 are optical fibers, e.g., glass fibers. These are predominantly embedded throughout the tank wall 3, e.g., by placing the strain-measuring fibers 4 in the mold before the primary shaping of the fuel tank 2 and subsequently enclosing them in the plastic material during the manufacture of the fuel tank 2 by injection molding. As shown in the greatly enlarged sectional view in Fig. As can be seen in Figure 2, the respective strain-measuring fiber 4 is arranged approximately centrally between an inner surface 3.1 of the tank wall and an outer surface 3.2. Alternatively, it could also be arranged closer to one of the surfaces 3.1, 3.2 or even protrude from one of them. Alternatively, it would also be possible, for example, to glue the strain-measuring fibers 4 to the tank wall 3 after the fuel tank 2 has been manufactured.

[0033] Fig. 1 shows, by way of example, a first strain-measuring fiber 4 extending downwards from a side surface 2.2 of the fuel tank 2 and along a bottom side 3 thereof, as well as a second strain-measuring fiber 4 extending upwards from the side surface 2.2 and along a top side 2.1 of the fuel tank 2. The illustration is to be understood purely as an example, and the tank unit could also have only a single strain-measuring fiber 4 or three or more strain-measuring fibers 4. Because the strain-measuring fibers 4 are embedded in the tank wall 3, a materially bonded connection is created, through which an expansion or compression of the tank wall 3 is transmitted to the respective strain-measuring fiber 4.The strain of the tank wall 3 - and thus also that of the enclosed strain measuring fiber 4 - in turn depends on the amount of fuel 20, as well as on the orientation of the tank 3 relative to gravity and on any dynamic acceleration forces that may occur.

[0034] The measuring unit 10 sends an optical signal into the respective strain-measuring fiber 4 and receives a reflected signal. The latter always depends on the strain of the strain-measuring fiber 4. Different measuring principles can be used here. For example, the strain-measuring fiber 4 can have a plurality of FBGs, each of which reflects light in a narrow range around a specific center wavelength, whereby the center wavelength depends on the (local) strain. Alternatively, FBGs could be omitted and mechanisms such as Rayleigh scattering or Brillouin scattering could be used. The measuring unit 10 determines the quantity of fuel 20 contained in the fuel tank 2 based on the reflected signal. The directly obtained measured values ​​(e.g., the respective center wavelength in the case of FBGs) can be compared with reference measured values ​​obtained in a calibration process with a known amount of fuel.Alternatively, the measuring unit 10 can first determine the local strain of the strain-measuring fiber 4 from the measured values ​​and then compare these with reference values ​​for the local strain. During the calibration process, certain situations such as an inclined position of the tank 2 or the effect of dynamic acceleration forces can also be simulated. Under these circumstances, it is possible to determine the fuel quantity even in situations where conventional measurement methods fail.

[0035] Under certain circumstances, it may be desirable or necessary to determine the temperature in the area of ​​the tank wall 3. If the measuring unit 10 uses, for example, DTSS to determine the strain, the temperature can be determined simultaneously using the strain measuring fibers 4. If the strain measurement is based, for example, on FBGs, the measured values ​​obtained are temperature-dependent or are distorted by different temperatures. To correct this, an independent temperature measurement is necessary, which can be carried out, for example, using a temperature measuring fiber 5, which is located in Fig. 2 and Fig. 3. The temperature measuring fiber 5 is laid parallel to the strain measuring fiber 4, in a sense parallel to it. Since a transfer of strain forces from the tank wall 3 to the temperature measuring fiber 5 is unnecessary and may even be undesirable, the temperature measuring fiber 5 does not have to be embedded in the tank wall 3 as shown, but could, for example, be laid on its inner surface 3.1. For effective correction of temperature-related effects, different temperatures can be set, for example, during calibration, so that separate comparison measured values ​​are available for each temperature.

[0036] Fig. 3 to 6 each show bottom views of the fuel tank 2 according to different embodiments, wherein Fig. 3 of the first embodiment Fig. 1 and Fig. 2. The strain measuring fiber 4 and the temperature measuring fiber 5 run straight and parallel to each other. Fig. 4 shows a second embodiment in which the strain measuring fiber 4 is arranged at an angle. Fig. 5 shows a third embodiment with a total of six strain measuring fibers 4, three of which run parallel to each other and cross each other. Fig. Figure 6 shows a fourth embodiment in which a single strain gauge fiber 4 is arranged in a meandering manner. In particular, the embodiments in Fig. 5 and Fig. 6 allow the expansion of the tank 2 to be recorded over a relatively large area, which generally improves the precision of determining the fuel quantity. It is understood that combinations of the Fig. 3 to 5 are conceivable, for example, in such a way that two meandering strain gauge fibers 4 cross each other or that several meandering strain gauge fibers run parallel to each other. In addition, each of the embodiments could be Fig. 4 to 6 can be supplemented by one or more temperature measuring fibers 5. List of reference symbols: 1 tank unit 2 fuel tank 2.1 Top 2.2 Side surface 2.3 Bottom 3 Tank wall 3.1 inner surface 3.2 outer surface 4 strain gauge fiber 5 temperature measuring fiber 10 measuring units 20 Fuel

Claims

[1] Tank unit (1) for a motor vehicle, comprising a liquid tank (2) having a tank wall (3), characterized by in that the liquid tank (2) has at least one strain-measuring fiber (4) designed as an optical fiber, which is connected to the tank wall (3) at least in sections in a strain-transmitting manner, wherein at least one optical property of the strain-measuring fiber (4) is dependent on its strain, wherein the tank unit (1) has a measuring unit (10) which is coupled to the at least one strain-measuring fiber (4) and is designed to send an optical signal into the respective strain-measuring fiber (4), to receive a reflected signal from the strain-measuring fiber (4) and to evaluate this signal to determine a strain of the strain-measuring fiber (4), and to determine a quantity of liquid within the liquid tank (2) based on the reflected signal from at least one strain-measuring fiber (4). [2] Tank unit according to claim 1, characterized by that at least one of the strain measuring fibers (4) is at least predominantly continuously connected to the tank wall (3). [3] Tank unit according to claim 1 or 2, characterized by that at least one of the strain measuring fibers (4) is integrally connected to the tank wall (3). [4] Tank unit according to one of the preceding claims, characterized by that at least one of the strain measuring fibers (4) is embedded in the tank wall (3). [5] Tank unit according to one of the preceding claims, characterized by that a plurality of strain measuring fibers (4) are arranged at least predominantly at a distance from one another. [6] Tank unit according to one of the preceding claims, characterized by that at least one of the strain measuring fibers (4) is arranged in a meandering manner. [7] Tank unit according to one of the preceding claims, characterized bythat the measuring unit (10) is designed to include a temperature measurement in the evaluation of the reflected signal. [8] Tank unit according to one of the preceding claims, characterized by that the liquid tank (2) has at least one temperature measuring fiber (5) designed as an optical fiber, wherein at least one optical property of the temperature measuring fiber (5) is dependent on its temperature.

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