DE19841770A1 - Level gauge system for urea solution used in selective catalytic reduction of exhaust pollutants, includes sensors for interdependent magnitudes, to establish filling level reliably - Google Patents

Abstract

A reference sensor (20) determines a second value of a measured magnitude (C, L). It is arranged in the liquid (F) such that the second value is a function of other liquid parameter(s) (T, c, Fl). With the level (H) above a minimum (Hmin), this measurement is independent of the level. An Independent claim is included for the corresponding method of level measurement. Preferred Features: The parameter is a liquid: temperature (T) and/or composition (Fl) and/or concentration (c). The measured magnitude is electrical conductivity (L) and/or capacity (C). Main electrodes (12, 14) and a reference electrode (22) are included. The first electrode is common to the second- and reference electrodes. In a variant, the reference sensor element (20) is introduced through a base wall of the vessel. Reference sensor (20) and sensor element (10) are immediately adjacent, they may be included in a common measurement head or carrier body which includes a temperature sensor (60). The measurement head includes heating and cooling arrangements. Reference sensor element and sensor element are fastened at or form, an inlet or outlet unit for the fluid, e.g. they are formed by the corresponding pipes.

Description

The invention relates to a level measuring device for a Liquid in a container, especially for a urine substance solution in a urea storage container, with a Sen sensor element with which a first value is one of a level of the container and at least one parameter of the liquid speed-dependent measured variable is measurable. The invention relates also relates to a method for measuring the level a liquid in a container, especially a urine substance solution in a urea storage container, in which a first value one of the fill level of the container and of min least one parameter of the liquid-dependent measured variable will measure.

From the book "Dubbel: Taschenbuch für den Maschinenbau", Springer-Verlag, Berlin, 1995, pages W14 to W15, be it knows the level of a liquid in a container with a mechanical, an electrical, a hydraulic, a pneumatic, an acoustic or an optical Measure method. In mechanical processes Float or tactile plates used. Electrical procedures use, for example, liquid level-dependent changes change in electrical resistance or capacitance between two electrodes. Pneumatic processes are based on the manometric measurement of the liquid hydrostatic pressure at the bottom of the container. At optical processes, light barriers are used.

The mentioned methods for level measurement and the used for this. Devices share the common part on that the sensor element with which the respective The measuring principle on which the measuring principle is based is measured by Level of the container, but also of at least one para meter of the liquid is affected. So it is often un  possible when the output signal of the sensor changes elements clearly on a corresponding change of the Be container level to close because the change is also from a Change in the parameter of the liquid has been caused could be.

For example, an electrical level meter hangs solution with an electrical conductivity as the measured variable or an electrical capacity between two in the liquid immersion electrodes is measured, the measured variable ne ben the level of a temperature and / or egg concentration of a substance dissolved in the liquid from. Especially if the container is high temperature fluctuations is an exact level measurement impossible. Such large temperature fluctuations occur for example with containers used in automotive engineering can be used as a storage container for a fuel.

For example, for a method known per se for catalytic cleaning of an exhaust gas of a motor vehicle (SCR- Process, Selective Catalytic Reduction Technology) Taking a urea containing a urea solution storage container required on the motor vehicle. For one exact electronic regulation and control of the catalytic Exhaust gas purification may require a measure of that Control supplied and thus consumed amount of urea a decrease in the level in the urea storage container win. Because the urea carried on the motor vehicle advice container naturally strong temperature fluctuations under lies with the known methods and device level measurement an exact measurement of the level levels or an exact measurement of a change in the level not possible in the urea storage container.

The invention is based, both a method ren as well as a device for level measurement ben with which a level measurement more reliable than with  the known devices or methods possible is. The device and the method are intended to measure Solution of the level of the container unaffected by one or several disturbing influencing parameters of the liquid lichen.

The task related to the device is according to the Er invention solved by a reference sensor element for measurement a second value of the measured variable, which is so in the liquid speed is arranged that the second value depending on the or any parameter and at a level above one Minimum level is independent of the level.

For example, the sensor element and the reference sensor are immersed into the liquid at different depths. Ins special is the reference sensor element or a sensitive one Area of the reference sensor element only below of the minimum fill level and the sensor element or a sensitive area of the sensor element extends (also) above the minimum fill level.

By comparing the output signals of the sensor element and the reference sensor element, the level or determine a change in the level exactly and clearly. A change in the level only leads to a change in the Output signal of the sensor element. However, it changes Liquid parameters, this is reflected in a change change in the output signal of both the sensor element and of the reference sensor element. During a level measurement device without a reference sensor element could be from a ver Change in the output signal of the sensor element neither quali tativ still quantitatively clearly on an actual filling status changes are closed. With a level measurement device according to the invention, the output signal of the sensor element both qualitatively and quantitatively regarding a change in a parameter of the liquid kor rig. A quantitative correction is possible, for example  Lich, if the sensitivity of the sensor element and the reference sensor element, for example in each case flows through a sensor surface, to be chosen as known. It But it is also possible to have a calibration curve for the output signals of the sensor element and the reference sensor element in Depending on the level of the container and the Calibration curve for a later level measurement gene.

The level of the Liquid surface in the container understood, but a lot more generally a proportion of the container volume that flows with rivers liquid is filled. For example, a level means 100% that the container is full or operational cannot or should no longer be filled. At a Level of 0% is the container z. B. empty. Accordingly there is still some liquid in at the minimum level the container. The minimum level is, for example, be predetermined due to drive, z. B. because of the container for reasons operational safety should not be completely emptied.

The parameter of the liquid is, for example, a temperature rature and / or a composition and / or a concentration tion of the liquid.

The measured variable from the sensor element and from the reference sensor element is measurable, is in particular a physical Measured variable, preferably an electrical conductivity or a electrical capacity. In both cases the level is Measuring device according to the invention is particularly simple stable and reliably buildable.

The electrical conductivity usually depends on the spe specific electrical conductivity of the liquid electrical capacity usually from the relative Dielek Tricity of the liquid. The specific electrical  The conductivity and the relative dielectric constant are then dependent on at least one parameter of the liquid.

According to a preferred embodiment, the level measurement has device a first electrode, a second electrode and a first reference electrode.

The electrodes and / or the reference electrode can be used as Pipe or rod can be formed.

For example, the first electrode is longer than the second Electrode, so that the two electrodes - z. B. by a Ceiling wall of the container introduced - in different depths immerse the liquid. At one to below the minimum level immersed area of the first electrode can then the reference sensor element, in particular the first refe the reference electrode.

The first electrode and the second elec trode the sensor element, and the first electrode and he Most reference electrode assigned to the reference sensor element. The fact that the first electrode as a common counterelectr trode both the sensor element and the reference sensor element is assigned, is such a filling level measuring device is particularly simple and space-saving buildable.

As a counter electrode to the first reference electrode second reference electrode be present.

The electrodes and / or reference electrodes can, for example as a large area glued or attached to a support body be steaming.

According to another advantageous embodiment of the filling level measuring device is the reference sensor element by a Bottom wall of the container can be inserted into this. This is on  simple way ensures that the reference sensor element above a minimum fill level, always completely with rivers liquid is wetted.

The reference sensor element and the sensor element are preferred arranged immediately adjacent. This makes the To Reliability of the level measurement is increased because it is ensured is that the sensor element and the reference sensor element largely identical changes to a parameter of the rivers liquid, for example a temperature change gene.

For this purpose, it is also expedient that the reference sensor element and the sensor element forming a common measuring head are arranged. It also becomes an advantage Space saving and compact handling achieved. At for example, the entire level measuring device is as one a unit-forming compact component from the container removable or mountable in the container.

The measuring head can, for example, comprise a supporting body, which in particular up to the tube sheet in the container is feasible. On the support body, for. B. the reference sen sensor element and electrodes assigned to the sensor element be brought. The support body can, for. B. Any tubular body cross section.

According to a preferred development of the fill level measurement direction, the measuring head has a temperature sensor. There This allows statements about changes to be made in an advantageous manner changes in the liquid in both qualitative and meet in quantitative terms. If the measured variable, for example of a temperature, a concentration and an assignment composition of the liquid depends and the liquid be always the same in terms of their type and composition remains, can be compared by comparing the output signals of the reference sensor element and the temperature sensor to one  Close concentration change in the liquid. At for example, an identification of the liquid in the Container possible, so that with a urea storage container e.g. B. incorrect refueling, d. H. an unwanted container stop, is recognizable.

In the case of a urea solution, the concentration is exemplary show the urea concentration in the urea and Water composed liquid.

The temperature sensor is preferably in a support body for the sensor element and / or the reference sensor element inte freezes. This makes the level measurement compact direction further increased in an advantageous manner.

The temperature sensor can, for example, in the interior of a the tubular body forming the support body may be arranged.

In a further embodiment of the level measuring device the measuring head comprises a heating and / or cooling device. There through that the heating and / or cooling device on the measuring head is arranged, the temperature of the liquid in the Area of the sensor element and / or the reference sensor element quickly bring them into an area where the Level measurement is exact or even possible in the first place. For example, it is possible with frozen liquid that Liquid in a small area around the sensor element and thaw the reference sensor element so that the level measurement possible shortly after activating the heating device is.

According to a very particularly preferred embodiment of the filling level measuring device is the reference sensor element and / or the sensor element at least partially on an inlet and / or removal device for the liquid can be attached. The feed and / or removal device is not used only the inflow or withdrawal of liquid, but  rather also as a supporting body for the reference sensor element, so that the level measuring device in an advantageous manner is particularly space-saving and compact.

According to another particularly preferred embodiment of the Level measuring device is at least part of the Refe renzsensorelements and / or the sensor element as a Zu running or removal device for the liquid or as Part of such a trained. Also designed like this Level measuring device is particularly space-saving and also inexpensive.

The feed and / or removal device can, for example be an inlet or withdrawal pipe. The tube can be a dear, especially circular or rectangular, cross have cut.

For example, one of the sensor element or the Refe renzsensor element associated electrodes an inlet or a sampling tube or part of it.

The task related to the procedure is performed according to the Erfin tion solved in that a second value of the measured variable at egg ner reference measuring point is measured in the liquid which the measured variable depends on the or each parameter and at a level above a minimum level of Level is independent.

With such a method, a fill level or determine a level change that is unaffected from a parameter of the liquid, in particular from one Temperature, composition and / or concentration tion of the liquid. A level determined in this way or a level change determined in this way is accordingly far more reliable than a corresponding one with a ver drive without the reference measuring point.  

The reference measuring point is, for example, a point at which liquid is also below the minimum level det.

The method according to the invention is preferably with a Filling level measuring device according to the invention can be carried out.

Five exemplary embodiments of a fill level measuring device according to the invention are explained in more detail with reference to the schematic FIGS. 1 to 7. Show it:

Fig. 1 shows a first embodiment of a Füllstandsmeß device having two electrodes and two reference electrodes,

Fig. 2 shows a second embodiment of a filling level measuring device having two electrodes and two electrodes rence Refe,

Fig. 3 shows a third embodiment of a filling level measuring device according to the invention having a measuring head,

Fig. 4 shows a cross section through the measuring head of Fig. 3,

Fig. 5 shows a fourth embodiment of a filling level measuring device according to the invention having a measuring head,

Fig. 6 shows a cross section through the measuring head of FIG. 5,

Fig. 7 shows a fifth embodiment of a level measuring device according to the invention with two electrodes and a reference electrode.

Fig. 1 shows a container 1 with a top wall 1 A and a bottom wall 1 B. The container 1 is filled up to a liquid surface 3 with a liquid F corresponding to a level H.

Through the top wall 1 A is a sensor element 10 comprising a first electrode 12 and a second electrode 14, introduced into the container. 1 The first electrode 12 and the second electrode 14 dip into the liquid F up to the bottom wall 1 B. The first electrode 12 and the second electrode 14 are connected to one another via a first voltage source 16 and a current meter 18 . At the first voltage source 16 , an electrical AC voltage is generated in any curve shape (sine, square, sawtooth, exponential function, etc.). If the fill level H of the liquid F in the container 1 changes , the electrode surfaces immersed in the liquid F change and thus the conductivity of the space between the first electrode 12 and the second electrode 14 . This is manifested in a changed current flow that is detected by the first current measuring device 18 .

Instead of the first voltage source 16 , a constant (alternating) current source can also be present. A voltage measured with a voltage measuring device on the electrodes 12 , 14 is then used as a measure of the conductivity and the filling level H.

The embodiment of FIG. 1 also includes a reference sensor element 20 with which the conductivity at a reference measuring point 21 in the liquid F can be measured. The reference measuring point 21 is below a minimum fill level H _min . The reference sensor element 20 has a first reference electrode 22 and a second reference electrode 24 , which are connected to a second voltage source 26 and a second current measuring device 28 .

The first electrode 12 and second electrode 14 are formed as electrically conductive, elongated, tubular components which range from the top wall 1 A to the bottom wall 1 B of the container 1, in particular up to the minimum level H _min and beyond. In comparison, the first reference electrode 22 and the second reference electrode 24 are small, electrically conductive components which are attached to the end of a removal device 42 or to the end of an inlet device 40 and exclusively below the minimum fill level H _min . The inlet device 40 is a tube through which liquid F can be filled into the container 1 along an inlet direction 41 . Correspondingly, the removal device 42 is a further tube with which liquid F can be removed from the container 1 along a discharge direction 43 .

If the fill level H of the liquid F exceeds the minimum fill level H _min , then the reference sensor element 20 , ie the reference electrodes 22 , 24 , are always completely wetted with liquid F, so that a change in the fill level H does not change the measured by the second ammeter 28 Electricity.

The liquid F in the container 1 is a solution of urea in water. The conductivity measured by the electrodes 12 , 14 depends not only on the fill level H but also on the temperature, the urea concentration in the water and also on whether other constituents besides urea are dissolved in the water. It also depends on the physical state of the urea solution. If one of these parameters of the liquid F changes, it is only with knowledge of the conductivity measured by the first current measuring device 18 that neither qualitatively nor quantitatively identifiable whether and if so to what extent the change in the conductivity affects the level H or one of the parameters of the liquid F. is. Only through the reference sensor element 20 , in which a change in the parameters of the liquid F, but not a change in the fill level H, is reflected in a conductivity detected by the second ammeter 28 , is this possible.

Inside the second electrode 14 there is a temperature sensor 60 . This can be concluded with the help of the level measuring device of FIG. 1 also on a change in the urea content in the liquid F or on a changed composition of the liquid F.

The level measuring device also has a heating and / or cooling device 70 , with the aid of which, for example, the liquid F can be thawed around the electrodes 12 , 14 and the reference electrodes 22 , 24 , if it is frozen.

A fluid settler 80 , for example realized by a liquid-permeable ring which surrounds the electrodes 12 , 14 and the reference electrodes 22 , 24 , prevents excessive currents and fluctuations in the liquid surface in the vicinity of the reference sensor element 20 , which is particularly the case with a urea reservoir carried in a motor vehicle 1 is an advantage.

Fig. 2 shows a second embodiment of a level measuring device according to the invention, which differs from the embodiment shown in Fig. 1 in that the reference sensor element 20 through the bottom wall 1 B of the container 1 can be inserted therein. The reference electrodes 22 , 24 are not attached to the inlet device 40 or the removal device 42 , as in the exemplary embodiment in FIG. 1, but are formed by them. The inlet and the removal device 40 , 42 are formed by two pipe sockets, which are inserted through the bottom wall 1 B to below half the minimum fill level Hmin and simultaneously serve as the first reference electrode 22 and second reference electrode 24 .

Fig. 3 and Fig. 4 taken along a cross section through the line IV-IV a third embodiment showing a level measuring device according to the invention, in which the Re ference sensor element 20 and len, the sensor element 10 at a Hoh, electrically insulating support body 88, a common measuring head 90 forming, are arranged. In the interior 94 of the support body 88 , the temperature sensor 60 is arranged, on the long sides of the support body 88, the curved first electrode 12 and the curved second electrode 14 , which are essentially from the top wall 1 A to the bottom wall 1 B of the container 1 , in particular extend to about the minimum fill level H _min . The first electrode 12 and the second electrode 14 are circumferentially spaced around the supporting body 88 ( FIG. 4)

On the lower end face of the support body 88 , namely below half the minimum fill level H _min , the first reference electrode 22 and the second reference electrode 24 , forming the reference sensor element 20 , are attached.

For the sake of clarity, the electrical wiring of all electrodes 12 , 14 and reference electrodes 22 , 24 is omitted in FIG. 3 and in the following figures.

With the level measuring devices shown in FIGS . 1 to 4, an electrical conductivity L is preferably measured, which depends, among other things, on a specific electrical conductivity σ of a space between the respective electrodes and on the size of the electrode surface wetted with liquid F.

The fourth exemplary embodiment shown in FIG. 5 is particularly suitable for measuring a capacitance C. The capacitance C of an electrode arrangement depends, among other things, on the relative dielectric constant ε _{r of} the spatial region which is penetrated by the electric field generated by the electrodes.

In the embodiment of FIG. 5, the sensor element 10 and the reference sensor element 20 are attached to the support body 88 , forming the measuring head 90 . The preferably made of electrically nonconductive material Tragkör by 88 is a mounted on the top wall 1 A longitudinally-stretched out, closed bottom and at least liquid-tight at the bottom of the hollow body, the temperature sensor is located in the interior of the 60th On the inner wall of the Tragkör pers the large and curved second electrode 14 is fixed, which forms an approximately semicircle. On the outer wall, the large-area and curved first electrode 12 is fastened, the azimuthal extent of which is smaller than that of the second electrode 14 ( FIG. 6).

If an electric field is applied to the first electrode 12 and the second electrode 14 , then the electric field lines penetrate not only the immediate space filled by the wall of the support body 88 between the electrodes 12 , 14 ( FIG. 6), but they also penetrate into the liquid-filled room. This is supported by the fact that the second electrode 14 has a greater extent on the circumferential side on the support body 88 than the first electrode 12 , as can be seen from FIG. 6, which represents a section along the line VI-VI of FIG. 5. The reference sensor element 20 is formed by the first reference electrode 22 on the one hand and by the first electrode 12 on the other. The first electrode 12 is thus assigned to both the sensor element 10 and the reference sensor element 20 .

In the preferred for measuring the capacitance C NEN measuring apparatus of Fig pre see. 5 and 6, the support body 88 made of plastic or other electrically non leitfähi gen material is made.

In the fifth exemplary embodiment shown in FIG. 7, the sensor element 10 is configured essentially as in FIG. 1 or 2. In particular, the first electrode 12 and the second electrode 14 are designed as tubular components. In contrast to these figures, the two electrodes 12 , 14 are of different lengths. The first electrode 12 is closed at the bottom, dips below the minimum fill level H _min into the liquid F and carries the first reference electrode 22 of the reference sensor element 20 on its lower end face. The first reference electrode 22 is insulated with respect to the first electrode 12 and has the second electrode 14 as the counter electrode, the wiring of the reference sensor element 20 not being shown for reasons of clarity. He he reference electrode 22 can be realized by a flat electrical conductive body or by a wire. In the interior of the second electrode 14 , a temperature sensor 60 is attached.

Claims (16)

1. level measuring device for a liquid (F) in a container ( 1 ), in particular for a urea solution in a urea storage container,
with a sensor element ( 10 ) with which a first value of a measured variable (C, L) dependent on a fill level (H) of the container ( 1 ) and on at least one parameter (T, c, Fl) of the liquid (F) can be measured ,
characterized by a reference sensor element ( 20 ) for measuring a second value of the measured variable (C, L), which is arranged in the liquid (F) such that the second value depending on the or each parameter (T, c, Fl) and at a level (H) above a minimum level (H _min ) is independent of the level (H). 2. level measuring device according to claim 1, characterized in that the Parame ter a temperature (T) and / or a composition (Fl) and / or a concentration (c) of the liquid (F). 3. level measuring device according to claim 1 or 2, characterized in that the measuring size is an electrical conductivity (L). 4. level measuring device according to one of claims 1 to 3, characterized in that the measuring size is an electrical capacity (C). 5. Level measuring device according to one of claims 1 to 4, characterized by a first electrode ( 12 ), a second electrode ( 14 ) and a first reference electrode ( 22 ), the first electrode ( 12 ) and the second electrode ( 14 ) Sensor element ( 10 ) and the first electrode ( 12 ) and the first reference electrode ( 22 ) are assigned to the reference sensor element ( 20 ). 6. Level measuring device according to one of claims 1 to 5, characterized in that the reference sensor element ( 20 ) through a bottom wall ( 1 B) of the container ( 1 ) can be inserted therein. 7. Level measuring device according to one of claims 1 to 6, characterized in that the reference sensor element ( 20 ) and the sensor element ( 10 ) are arranged immediately adjacent. 8. Level measuring device according to one of claims 1 to 7, characterized in that the reference sensor element ( 20 ) and the sensor element ( 10 ) are arranged to form a common measuring head ( 90 ): are. 9. Level measuring device according to claim 8, characterized in that the measuring head ( 90 ) has a temperature sensor ( 60 ). 10. Level measuring device according to claim 9, characterized in that the temperature sensor ( 60 ) is integrated in a support body ( 88 ) for the sensor element ( 10 ) and / or the reference sensor element ( 20 ). 11. Level measuring device according to one of claims 8 to 10, characterized in that the measuring head ( 90 ) comprises a heating and / or cooling device ( 70 ). 12. Level measuring device according to one of claims 1 to 11, characterized in that the reference sensor element ( 20 ) and / or the sensor element ( 10 ) at least partially on an inlet and / or Entnahmeinrich device ( 40 ; 42 ) for the liquid ( F) is attachable. 13. Level measuring device according to one of claims 1 to 11, characterized in that at least a part of the reference sensor element ( 20 ) and / or the sensor element ( 10 ) as a supply or removal device ( 40 ; 42 ) for the liquid (F) or is formed as part of one. 14. Level measuring device according to claim 12 or 13, characterized in that the inlet and / or removal device ( 40 ; 42 ) is an inlet or Ent receiving tube. 15. A method for measuring the fill level (H) of a liquid (F) in a container ( 1 ), in particular a urea solution in a urea storage container, in which a first value of the fill level (H) of the container ( 1 ) and at least a parameter (T, c, Fl) of the liquid (F) is measured from the dependent measured variable (C, L), characterized in that a second value of the measured variable (C, L) at a reference measuring point ( 21 ) in the liquid (F ) is measured on which the measured variable (C, L) depends on the or each parameter (T, c, FL) and is independent of the fill level (H) at a fill level (H) above a minimum fill level (H _min ). 16. The method according to claim 15, characterized in that the reference reference measuring point ( 21 ) is a point at which liquid (F) is also below half the minimum fill level (H _min ).