A level measurement system for conductive liquids The present invention relates to a level measurement system for conductive liquids. In particular, the present invention relates to a level measurement system for water in a storage container. Further, the present invention relates to a domestic appliance comprising at least one liquid container for supplying said do- mestic appliance with liquid.

Level measurement of water in storage containers is a substan ^¬ tial feature for any application requiring a feed of water. The information about the level status is relevant for the control of said application, e.g. for launching warning messages or triggering refill or drain actions by specific actuators.

Conductivity sensors are well known and used for a discrete lev ^¬ el measurement. Two electrodes are arranged inside a liquid con- tainer at different levels. If the liquid level is above both electrodes, then an electric circuit is closed and said liquid level above an upper electrode is detected. If the liquid level is below the upper electrode, then the electric circuit is open and said liquid level below the upper electrode is detected. This system allows only the determination, if the liquid level is above or below a defined level.

It is an object of the present invention to provide a level measurement system, which allows a more exact measuring of the liquid level by low complexity.

The object of the present invention is achieved by the level measurement system according to claim 1. According to the present invention the level measurement system for conductive liquids includes a liquid container and at least three electrodes arranged inside the liquid container at differ ^¬ ent levels, wherein

the electrodes are connected to a network comprising a plu ^¬ rality of resistor elements,

the network comprises at least two input terminals for apply ^¬ ing an external voltage, and

the network comprises at least two output terminals providing a voltage depending on the level of the conductive liquid in the liquid container,

so that the level measurement system is provided for detect ^¬ ing at least three different levels of conductive liquids in the liquid container and indicating said levels by one volt ^¬ age value.

Major aspects of the present invention refer to the at least three electrodes arranged inside the liquid container at differ ^¬ ent levels and the network of resistor elements connected to said electrodes. One voltage value is sufficient to indicate the at least three different levels of the conductive liquids in the liquid container. The electrodes and the resistor elements allow the level measurement system by low complexity.

According to a preferred embodiment of the present invention the level measurement system includes or corresponds with an analog- digital converter for processing the level of the conductive liquid in the liquid container.

In particular, the at least two output terminals of the network are connected or connectable to an input of the analog-digital converter. The output of the analog-digital converter may be processed by a control unit for further applications.

For example, the external voltage is a direct current voltage. Alternatively, the external voltage is an alternating current voltage. This avoids polarization effects on the electrodes and accumulations of substances on said electrodes. Further, the level measurement system may be provided for deter ^¬ mining the electric conductivity of the liquid.

In particular, the level measurement system may be provided for determining the hardness grade of water or liquids including wa- ter.

Moreover, predetermined voltage ranges of the voltage between the output terminals are defined, wherein each voltage range corresponds with one discrete level of the conductive liquid in the liquid container.

At last, the present invention relates to a domestic appliance comprising at least one liquid container for supplying said domestic appliance with liquid, wherein the domestic appliance comprises at least one level measurement system mentioned above.

Novel and inventive features of the present invention are set forth in the appended claims. The present invention will be described in further detail with reference to the drawings, in which

FIG 1 illustrates a schematic side view of a liquid container filled at three different liquid levels of a level meas- urement system according to a preferred embodiment of the present invention,

FIG 2 illustrates a schematic equivalent circuitry of the level measurement system according to a first embodiment of the present invention, FIG 3 illustrates a schematic equivalent circuitry of the level measurement system according to a second embodiment of the present invention, and

FIG 4 illustrates three voltage-time diagrams of the level

measurement system according to the preferred embodiment of the present invention.

FIG 1 illustrates a schematic side view of a liquid container 10 filled at three different liquid levels 12, 14 and 16 of a level measurement system according to a preferred embodiment of the present invention. The liquid container 10 is filled by an elec ^¬ trically conductive liquid, e.g. water.

The liquid container 10 is shown triply, wherein said container 10 is filled at an upper level 12, a medium level 14 and a lower level 16, respectively. The level measurement system includes an upper electrode 18, a medium electrode 20 and a lower electrode 22. The upper electrode 18, the medium electrode 20 and the low ^¬ er electrode 22 are arranged inside the liquid container 10. The upper level 12 is defined as one of the levels above the upper electrode 18. The medium level 14 is defined as one of the lev ^¬ els between the upper electrode 18 and the medium electrode 20. In a similar way, the lower level 16 is defined as one of the levels between the medium electrode 20 and the lower electrode 22.

A first terminal A is connected to the upper electrode 18. In a similar way, a second terminal B is connected to the medium electrode 20. At last, a third terminal C is connected to the lower electrode 22.

In FIG 1 the electric resistances of the liquid between the electrodes 18, 20 and 22 are represented by symbols RW1, RW2 and RW3. Said symbols RW1, RW2 and RW3 are pictured as the wiring symbols of resistor elements. RW1 represents the resistance be- tween the upper electrode 18 and the medium electrode 20. RW2 represents the resistance between the upper electrode 18 and the lower electrode 22. RW3 represents the resistance between the medium electrode 20 and the lower electrode 22.

If the liquid container 10 is filled at the upper level 12, then the electric resistances RWl, RW2 and RW3 of the liquid have measureable values. However, if the liquid container 10 is filled at the medium level 14, then only the electric resistance RW3 of the liquid has a measureable value, while the electric resistances RWl and RW2 are infinite. Moreover, if the liquid container 10 is filled at the lower level 16, then all three electric resistances RWl, RW2 and RW3 are infinite. FIG 2 illustrates a schematic equivalent circuitry of the level measurement system according to a first embodiment of the pre ^¬ sent invention. The level measurement system of the first embod ^¬ iment relates to the liquid container 10 shown in FIG 1. RWl represents the resistance of the liquid between the upper electrode 18 and the medium electrode 20. RW2 represents the re ^¬ sistance of the liquid between the upper electrode 18 and the lower electrode 22. RW3 represents the resistance of the liquid between the medium electrode 20 and the lower electrode 22. The upper electrode 18 is connected to the first terminal A. The me ^¬ dium electrode 20 is connected to the second terminal B. The lower electrode 22 is connected to the third terminal C.

The level measurement system further includes resistor elements Rl, RP1 and RP2. The resistor elements RP1 and RP2 are serially interconnected between a voltage terminal V+ and a ground termi ^¬ nal GND. The resistor elements RP1 and RP2 form a voltage divid ^¬ er. The first terminal A is connected to the voltage terminal V+ . The third terminal C is connected to a junction between the resistor elements RP1 and RP2. The resistor element Rl is inter- connected between the first terminal A and the second terminal B.

The resistor element Rl is dimensioned in such a way that the level measurement system allows a clear distinction between the detected measurements of the upper level 12 and the medium level 14. The voltage across the resistor element RP2 is considered for the measurement. The upper level 12, the medium level 14 and the lower level 16 correspond with a related voltage range across the resistor element RP2 in each case. The related volt ^¬ age ranges are adapted to different values of the water hard ^¬ ness, if the liquid is water or includes water. The resistance between the electrodes 18, 20 and/or 22 depends on the water hardness .

The resistor value Rl can be dimensioned preferably considering typical ranges of water resistivity with different water hard ^¬ nesses. Accordingly, it has been found in the present invention that a typical value range of 100 to 300 kOhm is suitable for many cases and is suitable even for a majority of cases. Conse ^¬ quently, dimensioning of the other resistors shall be prefera ^¬ bly:

- RP1 = RP2 ≥ 3*R1 The voltage across the resistor element RP2 is connectable to the input terminals of an analog-digital converter. Thus, a sin ^¬ gle analog-digital converter channel is sufficient to detect a plurality of levels 12, 14 and/or 16. FIG 3 illustrates a schematic equivalent circuitry of the level measurement system according to a second embodiment of the pre ^¬ sent invention. The level measurement system of the second em ^¬ bodiment relates also to the liquid container 10 shown in FIG 1. RWl represents the resistance of the liquid between the upper electrode 18 and the medium electrode 20. RW2 represents the re- sistance of the liquid between the upper electrode 18 and the lower electrode 22. RW3 represents the resistance of the liquid between the medium electrode 20 and the lower electrode 22. The level measurement system includes resistor elements R, Rl, R2, R3, RR1 and RR2. The resistor elements Rl and R2 are con ^¬ nected in series between the voltage terminal V+ and the ground terminal GND. In a similar way, the resistor elements R3 and R are connected in series between the voltage terminal V+ and the ground terminal GND.

The upper electrode 18 is connected to a resistor element RR2. The other terminal of said resistor element RR2 is connected to the junction between the resistor elements R3 and R. The medium electrode 20 is connected to a resistor element RR1. The other terminal of said resistor element RR1 is also connected to the junction between the resistor elements R3 and R. The lower electrode 22 is connected to the ground terminal GND. The resistor elements Rl, R2 are optional and allow an increased precision. The resistor elements R, Rl, R2, R3, RR1 and RR2 are dimensioned in order to allow a clear distinction between the detected measurements of the upper level 12 and the medium level 14. The voltage across the resistor element R is considered for the measurement. The upper level 12, the medium level 14 and the lower level 16 correspond with a related voltage range across the resistor element R in each case. The related voltage ranges are adapted to different values of the water hardness, if the liquid is water or includes water. The resistance of the water between the electrodes 18, 20 and/or 22 depends on the water hardness .

Preferably, the resistor Rl value can be the same as for the ex ^¬ ample circuitry previously explained, i.e. in a a value range of 100 to 300 kOhm. Dimensioning of the other resistors shall be preferably : - Rl ≥ R3 ≥ RR1

- R2 = Rl/10

- RR2 ≥ RRl/100

- R > 10*R3

The voltage across the resistor element R is connectable to the input terminals of the analog-digital converter. Therefore, the single analog-digital converter channel is sufficient to detect a plurality of levels 12, 14 and/or 16.

FIG 4 illustrates three voltage-time diagrams of the level meas ^¬ urement system according to the preferred embodiment of the pre ^¬ sent invention. The voltage-time diagrams relate to charges of water having a different hardness grade.

A first voltage-time diagram 24 relates to water having an hardness of 0.7 °F (French degrees) . A second voltage-time dia ^¬ gram 26 relates to water having an hardness of 35°F (French degrees) . A third voltage-time diagram 24 relates to a saturated salt solution.

If a voltage value upper than 3.5 V occurs, then the liquid in the liquid container 10 is at the lower level 16. If a voltage value between 2.25 V and 3.5 V occurs, then the liquid in the liquid container 10 is at the medium level 14. If a voltage val ^¬ ue below 2.25 V occurs, then the liquid in the liquid container 10 is at the upper level 12.

FIG 4 clarifies the relationship between the voltage ranges and the different hardness grades. Thus, the level measurement sys ^¬ tem of the present invention is also suitable for detecting the hardness grade of the liquid, in particular water.

In order to prevent polarization effects on the electrodes, the polarity of the applied voltage between the voltage terminal V+ and the ground terminal GND is inverted according to a predeter ^¬ mined scheme. The polarization effects may lead to accumulations of substances on the electrodes. For example, an alternating current may be applied between the voltage terminal V+ and the ground terminal GND.

A major advantage of the level measurement system according to the present invention refers to the detection of the plurality of liquid levels via one single analog-digital converter chan ^¬ nel. Additionally, said level measurement system allows the de ^¬ termination of the hardness grade of the liquid, in particular water. Further, the polarity of the applied voltage between the voltage terminal V+ and the ground terminal GND is inverted in order to avoid polarization effects on the electrodes.

Although two illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the present invention is not limited to these precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the in ^¬ vention. All such changes and modifications are intended to be included within the scope of the invention as defined by the ap ^¬ pended claims.

List of reference numerals

10 liquid container

12 upper level

14 medium level

16 lower level

18 upper electrode

20 medium electrode

22 lower electrode

24 first voltage-time diagram

26 second voltage-time diagram

28 third voltage-time diagram

A first terminal

B second terminal

C third terminal

V+ voltage -terminal

GND ground terminal

R resistor element

Rl resistor element

R2 resistor element

R3 resistor element

RP1 resistor element

RP2 resistor element

RR1 resistor element

RR2 resistor element

RW1 resistance of the

RW2 resistance of the

RW3 resistance of the