FIELD OF THE INVENTION 
       [0001]    The present invention relates to a device and method for providing a compensated measurement of the level of a liquid in a tank. 
       BACKGROUND OF THE INVENTION 
       [0002]    An example of a device for providing a compensated measurement of the level of a liquid in a tank is disclosed in the international patent application publication no. WO2005038415. The device in WO2005038415 comprises a transducer for transmitting and receiving acoustic signals, and a waveguide connected to the transducer and extending into the liquid. The liquid level is basically determined based on the speed of sound and the transit time of the acoustic signal from the transducer to a surface of the liquid (where it is reflected) and back to the transducer again. Since the speed of sound depends on gas composition and temperature which may vary from time to time, the current speed of sound is used in determining the liquid level, which current speed of sound in turn is determined by means of a reference system in the waveguide. Further, since the gas composition and temperature in the reference system may differ from the gas composition and temperature in the rest of the waveguide above the liquid, for instance if the liquid level is low, a fuel pump associated with the tank is used to feed a flow of fluid (namely fuel) originating from the tank into a portion of the waveguide which in use is located above the liquid. The flow of fuel levels the temperature and gas composition, and consequently the speed of sound, throughout the waveguide above the liquid, which in turn allows for a more accurate measurement of the liquid level. 
         [0003]    However, the device in WO2005038415 relies on the fuel pump to provide the flow of fluid, which may require extensive and costly modifications of the fuel pump and the associated tank when installing the measurement device. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the present invention to at least partly overcome this problem, and to provide an improved liquid level measurement device, which device in particular is relatively inexpensive to realise and implement and does not require interaction with a fuel pump or other external part. 
         [0005]    These and other objects that will become apparent from the following description are achieved by a measurement device and method according to the appended claims. 
         [0006]    According to an aspect of the present invention, there is provided a device for providing a compensated measurement of the level of a liquid in a tank, the device comprising a transducer adapted to transmit and receive acoustic signals; a waveguide connected to the transducer and adapted to extend into the liquid; collecting means for collecting free-moving portions of said liquid; and directing means for directing fluid originating from liquid collected by the collecting means into or along a portion of the waveguide which during operation is located above the liquid level. 
         [0007]    When a tank for instance is installed in a vehicle such as a car or boat, the liquid contained in the tank is seldom still. Instead, it moves and splashes around in the tank when the vehicle is in motion. Also, when the tank is refuelled, there is a flow of moving liquid coming for the tank. Hence, the present invention is based on the understanding that at least part of such free-moving liquid can be collected and directed into or along the waveguide, to allow the compensated more accurate measurement. Namely, the collecting and directing means are arranged such that the composition of gas and/or the temperature and thereby the speed of sound is levelled throughout the waveguide above the liquid level. 
         [0008]    Preferably, the device further comprises at least one reference element arranged in the waveguide, wherein the portion of the waveguide between the transducer and the reference element is defined as a reference part, and wherein the directing means is adapted to direct fluid originating from liquid collected by the collecting means into or along said reference part. 
         [0009]    It should be noted that US patent no.  5 , 471 , 872  (Cummings) discloses an acoustic liquid level detector system comprising a first waveguide, a transmitter and receiver operably associated with the first waveguide, a second waveguide, and a secondary reflector arranged at an angled juncture between the first and second waveguides. Further, the second waveguide is provided with a plurality of orifices to allegedly facilitate ingress of liquid and flow of a cleaning purge gas outwardly of the second waveguide. Hence, any free-moving liquid entering the second waveguide via said orifices will not level the composition of gas and/or the temperature all the way through the waveguide above the liquid level, only (possibly) in the lower portion of the second waveguide between the orifices and the liquid surface. In particular, no collected liquid is directed into or along the first waveguide between the transmitter/receiver and the secondary reflector. 
         [0010]    In one embodiment, the collecting means has a liquid catching surface or opening, the directing means has an opening into the waveguide, and the liquid catching surface or opening of the collecting means has a larger (cross-sectional) area than the opening of the directing means. That is, the collecting means and the directing means together form a control volume representing a nozzle. 
         [0011]    In one embodiment, the collecting means is further adapted to flowingly discharge collected liquid (i.e. liquid collected by the collecting means). This facilitates the transportation of collected liquid by the directing means since the liquid is already in motion. The collecting means may for instance comprise a funnel-shaped structure. When using a funnel-shaped collecting structure, the liquid caught therein may be the flowingly directed out therefrom at the smaller end of the funnel-shaped collecting structure, allowing the collected liquid to be transported into or along the waveguide. The height of the funnel is preferably vertically aligned, to fully benefit from the gravitational forces to capture liquid in the funnel and direct a downward flow of liquid, but the funnel may alternatively be tilted to collect splashes or splatter or other free-moving liquid corning from the side. Instead of a funnel-shaped collecting structure, a (slightly) tilted collecting channel or trough (at the lower end of which a discharge flow of collected liquid may be provided) could be used. As directing means, a hole or aperture or conduit or branched conduit between the collecting means and the waveguide to direct fluid originating from the collected liquid into the waveguide at one or several locations could here be used. The fluid lead by the directing means may generally be collected liquid itself or gas evaporating from the collected liquid. Alternatively, the directing means could be a pipe or tube arranged adjacent to or in contact or integrated with at least a part of the waveguide to direct the collected liquid along the exterior of the waveguide. When directing the collected liquid into the waveguide, both gas composition and temperature may be accounted for. On the other hand, when directing gas from the collected liquid into the waveguide, mainly gas composition may be accounted for, and when directing the collected liquid along the exterior of the waveguide, only temperature may be accounted for, which may be sufficient for diesel which do not evaporate to the same extent as petrol or gasoline. 
         [0012]    In one embodiment, the directing means is further adapted to transport collected liquid by means of capillary attraction. This allows transportation of non-flowing collected liquid, which may simplify the construction of the collecting means. Also, it allows “upwards” transportation of collected liquid, increasing the design options of the measurement device. To this end, the directing means may be a tube or tubes so narrow that collected liquid is “automatically” transported therein by capillary attraction. Alternatively, the directing means may comprise a piece of absorbing material also benefiting from capillary attraction or action. As collecting means, a simple collecting cup or a piece of absorbing material, such as porous felt or cloth, which piece of absorbing material effectively may take up splashes of liquid in the tank coming from several or all directions, could here be used. In one embodiment, a single piece of absorbing material may beneficially act as both collecting and directing means, which facilitates the construction of the device. 
         [0013]    The present device is relatively inexpensive and easy to realise and implement. For instance, it does not require modifications of the fuel pump and the associated tank as the above prior art solution. Yet the present device effectively allows the provision of an accurate, compensated liquid level measurement. 
         [0014]    In one embodiment, the device further comprises at least one retaining member for at least temporarily retaining liquid in the device during operation. The liquid is preferably retained at a predetermined position in the device. The retained liquid may slowly evaporate and level the gas composition in the device where the acoustic signal travels, with a more accurate measurement of the liquid level as a result. The retained liquid is preferably liquid originating from the tank, for example it may be a portion of the liquid collected by the collecting means and entered into the device by the directing means. Further, the at least one retaining member may be arranged in the waveguide, or in case a portion of the waveguide is accommodated in or formed by a housing of the device, in association with the housing. The retaining member may for instance be a recess in the waveguide or a cup in the housing, grooves transversely provided on the floor of the waveguide in order to slow down and retain liquid flowing across the floor, or a piece of absorbing material (e.g. a spongy or porous material or felt or the like) adapted to absorb liquid. Further, the at least one retaining member is preferably embedded in the waveguide so as to be in level with a waveguide wall, at least when containing liquid during operation, in order to not significantly affect the acoustic signals travelling in the waveguide. A cup may for instance be embedded in an inner wall of the waveguide as a recess, so that when the recess is filled with liquid, the surface of the liquid is substantially in level with the inner wall of the waveguide, creating a smooth surface for the acoustic waves to pass without being disturbed (a rough surface may cause unwanted reflections of the acoustic signals, which in turn may lead to erroneous measurements of the tank liquid level). Likewise, the piece of absorbing material may be embedded in a size-matched recess or hole in an inner wall of the waveguide, so that the top surface of the piece of absorbing material is substantially in level with the inner wall of the waveguide. Due to its substantially fixed size, the piece of absorbing material can be made not to affect the acoustic signals in the waveguide even when it does not retain liquid, which beneficially serves to reduce unwanted reflections. 
         [0015]    In one embodiment, the device further comprises a valve having an inlet adapted to receive liquid, two outlets which each may be in fluid communication with the inlet, and a spherical element moveable in response to gravitational and/or centrifugal forces for automatically closing one of the outlets when the device is tilted. The three-way valve (one inlet, two outlets) of the present embodiment is advantageously used to automatically distribute liquid or fluid to different portions of the waveguide, especially to an upper portion when the device is tilted, the upper portion which otherwise usually is drained when the device is tilted. The three-way valve is also useful for automatically distributing liquid to a portion of the waveguide drained due to centrifugal forces, which may occur when the measurement device is quickly moved from side to side, as for instance when installed in a car. The liquid received at the inlet is for example the liquid collected by the collecting means. 
         [0016]    In one embodiment, a portion of the waveguide is formed by a first part with a surface having a recess defining the waveguide portion and a protrusion with a groove running along the recess, and a second part with a surface closing the recess of the first part to form the waveguide portion and further having a recess matching and engaging the protrusion of the first part so that a channel running along the waveguide portion is formed by the groove in the protrusion of the first part and the recess of the second part, whereby any small clearance between the channel and the waveguide portion is sealed by liquid when liquid during operation travels between the channel and the waveguide portion. The structure of the present embodiment allows liquid (e.g. liquid collected by the collecting means) to be delivered to the waveguide portion on various locations along the waveguide portion via the small clearances (to create the levelled atmosphere), while at the same time the means for delivering the liquid from the channel into the waveguide part (e.g. the small clearances) are sealed so that they do not affect or disturb the acoustic signal travelling in the waveguide. The device may further comprising protrusions, e.g. small pins or the like, between the channel and the waveguide portion to (intentionally) create the small clearances between the channel and the waveguide portion through which liquid in the channel during operation can pass to the waveguide portion, in order to create the levelled atmosphere. 
         [0017]    According to another aspect of the present invention, there is provided a method for providing a compensated measurement of the level of a liquid in a tank, the method comprising the steps of transmitting an acoustic signal from a transducer into a waveguide adapted to extend into the liquid; receiving a reflected acoustic signal to the transducer from the waveguide; collecting free-moving portions of said liquid; and directing fluid originating from liquid collected in the previous collecting step into or along a portion of the waveguide which during operation is located above the liquid level. In one embodiment, the method further comprises the step of at least temporarily retaining liquid in the device during operation. This aspect exhibits similar advantages and may exhibit similar features as the previously discussed aspect of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention. 
           [0019]      FIG. 1  is a schematic at least partly cross-sectional side view illustrating a liquid level measurement device arranged in a tank according to an embodiment of the present invention. 
           [0020]      FIGS. 2   a - 2   g  are schematic at least partly cross-sectional views illustrating details of variants of the device of  FIG. 1 . 
           [0021]      FIGS. 3   a - 3   c  are schematic cross-sectional side views illustrating retaining members according to embodiments of the present invention. 
           [0022]      FIGS. 4   a - 4   c  are schematic cross-sectional side views illustrating an automatic three-way distribution valve according to an embodiment of the present invention. 
           [0023]      FIGS. 5   a - 5   b  are cross-sectional views of a liquid level measurement device housing according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 1  is a schematic at least partly cross-sectional side view illustrating a liquid level measurement device  10  arranged in a tank  12  according to an embodiment of the present invention. 
         [0025]    The measurement device  10  is generally adapted to detect or measure the level of a liquid  14  in the tank  12 . The tank  12  may for instance be a fuel tank for a vehicle, such as a car, truck or boat. 
         [0026]    The measurement device  10  comprises a transducer  16 . The transducer  16  is an electro-acoustic transducer generally adapted to convert electrical signals to acoustic signals or sound pulses (exemplary frequency of about 3.4-17 kHz), and vice versa. The transducer  16  may for instance comprise a double-acting piezoelectric component, or comprise of a separate sound transmitter and sound receiver. In  FIG. 1 , the transducer  16  is placed inside at the top of the tank  12 , but it could alternatively be placed outside the tank  12 . 
         [0027]    The measurement device  10  further comprises an electronic unit  18  connected to the transducer  16  for supplying electrical signal thereto and to evaluate electrical signals therefrom. The electronic unit  18  is preferably arranged outside the tank  12 . 
         [0028]    The measurement device  10  further comprises a waveguide  20  generally adapted to guide acoustic signals (illustrated by          ). The waveguide may for instance be a tube or pipe in which acoustic signals can be guided. One end  22   a  of the waveguide  22  is connected to the transducer  16  to guide the acoustic signals to and from the transducer  16 , and an opposite open end  22   b  is adapted to be extended or submersed into the liquid  14 . 
         [0029]    The measurement device  10  further comprises at least one reference element  24  arranged in the waveguide  20 . The reference element  24  may for instance be a small protrusion such as a pin or tap or ring adapted to partly reflect an acoustic signal travelling in the direction away from the transducer  16  in the waveguide  20 . The reference element  24  is preferably arranged a known distance from the transducer  16  above a predetermined point in the wave guide  20  up to which liquid generally is allowed to reach. The portion of the waveguide between the transducer  16  and the reference element  22  is defined as a reference part  26 . At least a portion of the reference part  26  may have a helical or flat spiral shape (not shown in  FIG. 1 ) in order to save space, and at least a portion of the reference part  26  could also be placed outside the tank  12 . 
         [0030]    The measurement device  10  further comprises a funnel  28  generally shaped like hollow cone and having a larger (top) catching end (liquid catching opening)  30  and a smaller (bottom) discharge end  32  which is fluidly connected to the waveguide  20  via a conduit  34 , the catching end  30  having a larger diameter than the diameter of the conduit  34 . Namely, the conduit  34  leads to the reference part  26  of the waveguide  20 , preferably close to the transducer  16 . The funnel  28  is preferably so arranged in relation to the rest of the measurement device  10  that any liquid collected in the funnel  28  is flowingly provided out from the discharge end  32  via the conduit  34  and into the reference part  26  of the waveguide  20  due to gravitational forces. The height of the funnel  28  may for instance be vertically aligned as in  FIG. 1 . 
         [0031]    An exemplary operation of the present measurement device  10  when in use in the tank  12  will now be described. First, an electrical signal or pulse is provided by the electronic unit  18  to the transducer  16  causing the transducer  16  to transmit a corresponding acoustic signal or sound pulse. The transmitted acoustic signal is guided by the waveguide  20  towards a surface  36  of the liquid  14 . Part of the acoustic signal is reflected back towards the transducer  16  by the reference element  24 , while the rest of the acoustic signal continues towards the surface  36  where it is reflected and then returned to the transducer  16  via the waveguide  20 . In response to the two returning acoustic signals, the transducer  16  generates corresponding electric signals. The electronic unit  18  first uses the transit time for the signal reflected by the reference element  24  together with the known distance between the transducer  16  and reference element  24  to calculate the current speed of sound. The electronic unit  18  then calculates the level of liquid in the tank  12  based on the current speed of sound and the transit time for the signal reflected by the liquid surface  36 . 
         [0032]    Concurrently, when a vehicle in which the tank  12  is installed is in motion, the liquid  14  in the tank  12  moves and splashes around in the tank  12 . The resulting splashes or splatter of liquid (illustrated as dashed arrows) are at least partly collected by the funnel  28 . The liquid collected in the funnel  28  is then discharged as a flow of liquid through the discharge end  32  of the funnel and is directed via the conduit  34  into the reference part  26  of the waveguide  20 . The collected liquid introduced into the waveguide  20  may then return to the tank  12  via the waveguide itself or via drainage holes (not shown in  FIG. 1 ) in the waveguide. When collected liquid (especially gasoline or petrol in case the tank  12  is a fuel tank) is flowing in the waveguide  20  and evaporates, the temperature and gas composition, and consequently the speed of sound depending on temperature and gas composition, is levelled throughout the waveguide  20  above the liquid (i.e. in the reference part  26  and the portion of the waveguide  20  between the reference part  26  and the liquid surface  36 ), which in turn allows for a more accurate measurement of the liquid level, since the speed of sound in the portion of the waveguide  20  between the reference part  26  and the liquid surface  36  becomes substantially the same as the current speed of sound calculated with the reference measurement. For this, the funnel  28  and directing means (conduit  34  in  FIG. 1 ) should be suitably dimensioned and positioned so as to provide a flow of liquid in the waveguide  20  which is small enough to allow the acoustic signals to travel in the waveguide  20 , but at the same is time allows steam to be released from the collected liquid for the above levelling and compensation. 
         [0033]    Variants of the device illustrated in  FIG. 1  include replacing the conduit  34  by a hole or aperture  38  directly connecting the discharge end  32  of the funnel  28  with the waveguide  20  ( FIG. 2   a ). Omitting the conduit  34  allows a construction with fewer parts. Further, the conduit  34  may be replaced with a branched conduit  40  or the like to direct liquid from the funnel  26  into the waveguide  20  at several locations ( FIG. 2   b ). Directing liquid into the waveguide at several locations (using the branched conduit  40 ) may enhance the levelling of the atmosphere in the waveguide  20 . Further, the conduit  34  may be replaced by a tube or pipe  42  arranged adjacent to or in contact or integrated with at least a portion of the waveguide  20  to direct the collected liquid along the exterior of the waveguide  20  ( FIG. 2   c ). Here, only temperature may be accounted for, which may be sufficient in case the liquid is diesel which do not evaporate to the same extent as petrol or gasoline. At the same time, no opening for directing the liquid into the waveguide as well as drainage facilities are required, which is beneficial from a construction and cost point of view. Further, the funnel  28  may be tilted so as to catch splashes or splatter coming from the side ( FIG. 2   d ). Here, the angle of an inner wall of the funnel  28  as seen from a horizontal plane is preferably larger than a maximum tilt angle of the measurement device  10  to allow collected splashes to run into the conduit  32 . Nevertheless, splashes could force themselves into the conduit  32  due to their kinetic energy. Further, the funnel  28  may be replaced by a tilted collecting channel or trough  44  ( FIG. 2   e ), the channel  44  having a liquid catching surface  45  with a larger area than the cross sectional area of the opening  38 . Further, the funnel  28  may be replaced by a simple collecting cup or open container  46  from which liquid is lead up into the waveguide  20  via at least one tube  48  which is so narrow that liquid from the cup  46  is transported into the waveguide  20  by means of capillary attraction ( FIG. 2   f ). Further, the funnel  28  may be replaced by a piece of absorbing material  50  which may take up splashes of liquid in the tank  12  from all directions ( FIG. 2   g ) by means of its liquid catching surface  51  which is the exposed area of the piece of absorbing material  50 . The absorbing material  50  may extend ( 50 ′) into the waveguide  20  so that liquid collected by it is transported by capillary attraction into the waveguide  20 . Preferably, the liquid catching surface  51  is larger than the opening into the waveguide  20  for the extension  50 ′. Further, the absorbing material  50 ′ could be used as means for directing liquid into the waveguide from the collecting cup  46 , and the above narrow tube(s)  48  could be used as means for directing liquid into the waveguide from the absorbing material  50 . Also, several funnels  28  or other collecting structures or means having a liquid catching surface can be provided for a single measurement device to collect a desired amount of liquid (not shown). 
         [0034]    Further, except for catching splashes, the above various collecting means can catch other free-moving liquid outside or inside the tank, which moving liquid is from refuelling of the tank  12 , for example. 
         [0035]      FIGS. 3   a - 3   c  are schematic cross-sectional side views illustrating retaining members according to embodiments of the present invention. For instance in the above measurement device  10 , retaining members may be introduced to keep the collected liquid entered into the waveguide  20  some time in the waveguide  20  to allow enough liquid to evaporate, especially in case the device  10  is tilted and the liquid otherwise would quickly drain off. 
         [0036]    In  FIG. 3   a , the retaining member is an open container or recess  52  embedded in a bottom inner wall or the floor  54  of the waveguide  20 . When liquid flows over the floor  54  through the waveguide  20 , the recess  52  is filled with liquid  14 . Thus, should the flow of liquid along the interior of the waveguide  20  (temporarily) cease, there is still some liquid in the waveguide, namely in the retaining recess  52 , which retained liquid may serve to balance the gas composition and temperature in the waveguide  20 . Further, when the recess  52  in the substantially horizontal floor  54  is more or less completely filled with liquid  14 , the surface of the liquid  14  is substantially in level with the floor  54  of the waveguide  20 , creating a smooth surface for the acoustic waves to pass without being disturbed. In case the retaining recess  52  is arranged in an inner wall of the waveguide  20  which inner wall is not generally horizontal, the recess  52  can be filled with a liquid absorbing material, see also  FIG. 3   c.    
         [0037]    In  FIG. 3   b , the retaining member is transversal grooves  56  arranged on the floor  54  of the waveguide  20 , in order to slow down and retain liquid  14  flowing across the floor  54  transversal to the grooves  56 . 
         [0038]    In  FIG. 3   c , the retaining member is a piece of liquid absorbing material  58 , for instance a spongy or porous material or felt or the like adapted to retain liquid  14  running through the waveguide  20 . The piece of absorbing material  58  is preferably embedded in the waveguide  20  so as to not disturb (reflect) any incoming acoustic signal or sound pulse. The piece of absorbing material  58  can be size-matched to and placed in a recess  60  in the waveguide  20  (like the recess  52  of  FIG. 3   a ), to create a smooth surface for the acoustic signals to pass. The piece of absorbing material  58  (with optional recess  60 ) can advantageously be arranged horizontally as well as vertically or inclined. 
         [0039]    It should be noted that the above retaining members could as an alternative or complement be arranged in a housing (not shown in  FIGS. 3   a - 3   c ) accommodating or forming a portion of the waveguide  20 . Also, the measurement device may comprise several of the above retaining members as well as combinations of the above retaining members. Also, the above retaining members may be used with means or devices adapted to feed a flow of liquid into the waveguide other than the above collecting and directing means, or in other applications. 
         [0040]      FIGS. 4   a - 4   c  are schematic cross-sectional side views illustrating an automatic three-way distribution valve  62  according to an embodiment of the present invention. The valve  62  comprises a housing  64  having one inlet  66  in fluid communication with each of two outlets  68   a,    68   b.  The inlet  66  and outlets  68   a,    68   b  are here apertures. The valve  62  further comprises a weighted, at least partially spherical element, such as a ball  70 , placed in the housing  64 . The valve  62  is so adapted that when it is not substantially tilted or inclined or subjected to centrifugal forces, the ball  70  is in an intermediate position allowing any liquid or fluid from the inlet  66  to pass to both the outlets  68   a,    68   b  ( FIG. 4   a ). The floor of the housing  64  may for instance be concave with one outlet  68   a  ( 68   b ) on each side of the central lowest point (when the valve  62  is substantially horizontal), the ball  70  resting at said central lowest point in the intermediate position. However, when the valve  62  is tilted or inclined or subjected to centrifugal forces, the ball  70  due to its own weight is moved in relation to the rest of the valve  62  to close one of the outlets  68   a  ( 68   b ) so that liquid from the inlet  66  only can pass to the other outlet  68   b  ( 68   a ) ( FIGS. 4   b ,  4   c ). The ball  70  should be sized so as to close the respective outlet apertures  68   a,    68   b  and remain there for predetermined tilt angles and/or centrifugal forces (by resting in the aperture ( FIGS. 4 ) or against some other appropriately positioned structure (not shown) of the valve housing). Further, the inlet  66  should be positioned so that it is not closed by the ball  70  when in the intermediate position. For instance, the inlet  66  may be placed in the top of the housing  64  as shown in  FIGS. 4   a - 4   c.    
         [0041]    The valve  62  is advantageously used in the above measurement device  10  to automatically distribute collected liquid to different portions of the waveguide  20 , especially to a portion which usually is drained when the device  10  is tilted or inclined or subjected to centrifugal forces. The valve  62  can for instance form part of the above branched conduit or be otherwise arranged between the above collecting means and the waveguide. It should however be noted that the valve  62  may be used with means or devices adapted to feed a flow of liquid into the waveguide other than the above collecting and directing means, or in other applications. 
         [0042]      FIGS. 5   a - 5   b  are cross-sectional views of a liquid level measurement device housing  72  according to an embodiment of the present invention, which housing  72  (among other things) forms a portion  74  of a waveguide. The waveguide may for instance be the above waveguide  20 , the portion  74  may correspond to at least a portion of the above reference part  26 , and the housing  72  may in turn form part of the above measurement device  10 .  FIG. 5   a  is a top cross-sectional view of the housing  72 , and  FIG. 5   b  is a side cross-sectional view of the housing  72  along the cut I-I in  FIG. 5   a.    
         [0043]    The housing  72  comprises a first (top) part  76  with a surface  78  having a recess  80  defining the waveguide portion  74 . The recess  80  runs in a spiral pattern from one edge of the housing  72  to a more central position of the housing  72 . Along with the recess  80  in essentially the same spiral pattern runs a protrusion  82 . In the tip of the protrusion  82 , there is provided a groove  84 , which consequently also essentially runs along the recess  76 . Facing the surface  78  of the first part  76 , there is provided a surface  86  of a second (bottom) part  88  comprised in the housing  72 . The surface  86  of the second part  88  has a recess  90  matching and at least partially engaging (when assembled) the protrusion  82  of the first part  76 , while portions of the surface  86  between the recess  90  of the second part  88  substantially closes the recess  80  of the first part  76  forming said waveguide portion  74 . The waveguide portion  74  thus runs from the one edge to the more central portion of the housing  72 . Further, a channel  92  running along the waveguide portion  74 /recess  80  from the one end to the more central position of the housing  72  is formed by the groove  84  in the protrusion  82  of the first part  76  and the recess  90  of the second part  88  when the first and second parts  76  and  88  are assembled. Due to mismatching dimensions or through holes or by intentionally placing small pins  94  between the waveguide portion  74  and the channel  92  along the portion  74 /channel  92 , small clearances  96  are created between the waveguide portion  74  and the channel  92 . Further, the second part  88  is advantageously made somewhat flexible, which facilitates sealing between the channel  92  and waveguide portion  74 , and makes assembly of the first and second parts  76  and  88  easier. The profile of the second part  88  may for instance follow the recessed surface  86  thereof as in  FIG. 5   b.    
         [0044]    In the context of the above measurement device  10 , a purpose of the waveguide portion  74  is to guide therein acoustic signals to and from the transducer  16  which suitably is arranged at the end of the waveguide portion  74  at the more central position in the housing  72 . An additional waveguide portion  98  forming part of the waveguide  20  may be arranged at the end of the waveguide portion  74  at the one edge of the housing  72  to further guide the acoustic signals to and from the liquid surface. Further, a purpose of the channel  92  is to lead therein liquid from the tank in which the measurement device  10  is mounted, for instance liquid collected by the funnel  28 . The funnel  28  is preferably connected to the end of the channel  92  at the one end of the housing  72 , for instance via the conduit  34 . Additionally, the funnel  28  (or another funnel or other collecting means) may be fluidly connected to the channel  92  also at other locations in order to provide liquid into the channel  92  to other portions of the channel  92 . The distribution of liquid to various portions of the channel  92  (and thus to various portions of the waveguide portion  74  as will be explained next) may for instance be controlled by using the above automatic three-way distribution valve  62  installed between the funnel  28  and channel  92 . The liquid in the channel  92  may sip through clearances  96  and into the waveguide portion  74  at various locations in order to create the levelled atmosphere in the waveguide portion  74 . At the same time, the clearances  96  are sealed by the liquid “contained” in them so that they do not affect or disturb the acoustic signals travelling in the waveguide portion  74  to and from the transducer  16 . Thus, present housing  72  allows delivery of liquid into the waveguide portion  74  without the means for delivery significantly affecting the acoustic signal propagation. Overall, a more accurate measurement of the tank liquid level may be obtained. 
         [0045]    It should be noted that the above housing may be used with means or devices adapted to feed a flow of liquid into the waveguide other than the above collecting and directing means. Further, any of the above retaining members may be used in or built into the housing. Also, drainage holes  100  may be provided in the floor of the waveguide portion  74  to avoid the waveguide portion  74  from being flooded. Suitable positions of the drainage holes  100  include the curved parts of the waveguide portion  74  (as shown in  FIG. 5   a ), where liquid otherwise tends to accumulate when the housing  52  is tilted or subjected to centrifugal forces or the like. Also, the housing may  72  comprise more pins  94  than the exemplary ones shown. 
         [0046]    The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, although acoustic pulses have been used in the described embodiments, the inventive measurement device may also be used with other measurement modes such as standing wave measurement. Also, the above embodiments and variants may be combined in several ways.
At least one embodiment of the present invention relates to a device for providing a compensated measurement of the level of a liquid in a tank. In at least one embodiment, the device includes a transducer adapted to transmit and receive acoustic signals; a waveguide connected to the transducer and adapted to extend into the liquid; at least one collecting device for collecting free-moving portions of said liquid; and at least one directing device for directing fluid originating from liquid collected by the collecting device into or along a portion of the waveguide which during operation is located above the liquid level. At least one embodiment of the present invention also relates to a corresponding method.