WO2009118402A1

WO2009118402A1 - Method for measuring the volume flow of electrically conductive liquids through a vessel

Info

Publication number: WO2009118402A1
Application number: PCT/EP2009/053644
Authority: WO
Inventors: Colin Henderson; Uwe Lang; Stefan Hother; Steven Bookbinder; Thomas Köhler
Original assignee: Brita Gmbh
Priority date: 2008-03-28
Filing date: 2009-03-27
Publication date: 2009-10-01
Also published as: ES2387188T3; TW200946881A; MX2010007963A; CA2712055A1; CN101981415B; RU2488779C2; JP5559141B2; US8171802B2; BRPI0910117A2; EP2265905A1; JP2011519020A; PL2265905T3; EP2265905B1; RU2010144054A; CN101981415A; US20110011184A1

Abstract

it is descπbed a method of measurement of the volume V_D of the flow-rate of electrically conductive liquids the conductivity of which is at least codetermined by at least one parameter p, wherein the liquid flows through a vessel having a predetermined shape, and wherein the respective filling volume V_o is determined by at least one measured value x, which is measured by an electrical conductivity measuring device comprising electrodes, wherein the vessel is filled in succession and then is emptied through its outlet, through which the filling heights h are constantly changing. At least one reference table comprising calibration measured values x_R and filling volumes V_o belonging to them is constructed by means of calibration measurements using several liquid samples, which have different p-values and different filling heights h in the vessel. The measured values x are measured in time intervals and the respective filling volumes V_o are determined by comparison of the respective measured values x with values x_R of the table, and the volume V_D of the rate of flow is determined from the filling volumes V_o over a time period. The measuring device (10) for the determination of the volume V_D of flow rate of electrically conductive liquids through a vessel (5) comprises at least two measuring electrodes (22, 24).

Description

METHOD FOR MEASURING THE VOLUME FLOW OF ELECTRICALLY CONDUCTIVE LIQUIDS THROUGH A VESSEL

Description

The invention refers to a method of measurement of the volume of rate of flow of electrical conductive liquids through a vessel according to claim 1. The invention also refers to a respective measuring device.

The measurements of filling heights are conducted wherever the volumes of liquids and the alteration of volume have to be determined. The measurements of filling heights are usually done by electrodes, which immerse at ieast partially into the liquid. The electrical conductivity or the resistance of the liquid, which is proportional to the filling height or the volume of the liquid, is measured by a suitable measuring device.

Such measurements are necessary in order to determine the exhaustion of filter cartridges, which are used in gravitation driven filtration devices.

WO 02/27280 A discloses a device using three electrodes one of which is used as reference electrode. The electrodes for level measurement are configured in such a way that a measurement value sharply changes when certain limits of the level are exceeded or fallen short of. These leaps of the measurement values can be reliably recognized without high demands on the accuracy of measurement.

A similar device is known from EP 1 484 097 B1 which comprises at least three electrodes, counting means and timers. The signals measured by these components are fed to an input of a microprocessor that, on the basis of a resident programme, elaborates important data on the life-span of the cartridge according to the amount of time passed since its first activation and the amount of water treated identified in terms of closure considered important by circuit between the electrodes, and by the ionic concentration of the pollutants, identified in terms of conductivity of the water being treated.

In case of consideration a partial filling of the vessel numerous electrodes are located on growing levels in a compensation chamber within the vessel.

This device is expensive and never even takes into consideration the design and shape of the vessel. Exact measurements of volume require intermediate measurements of the filling height taking into account the vessel shape. Usually, the vessel has any design so that the correlation between filling height and liquid volume doesn't follow a simple mathematical formula.

Most of the measuring devices ignore vessel shape so that the determination of life-span of the filter cartridge is not as precise as it should be.

US 4,724,705 A relates to a fuel measurement device and particularly a device for determining for quantity of a fuel in a fuel tank. The fuel level indicator includes a hollow housing, a coded wafer, a short circuit wafer including a wafer substrate, a buoyant member and a continuity bridge. The coded wafer is made of a dielectric, ceramic material and extends along the interior length of the hollow housing. An electrically conductive wire strand having a known resistance per unit length is wound about the coded wafer to define a "pattern of resistance" representative of the contour of the interior wall of the fuel tank. The manufacturing of the fuel level indicator is quite an effort, in particular for manufacturing of the coded wafer. DE 10 2005 035 045 A1 discloses a measuring device comprising a measuring element that includes at least one electrode the area of which increases in an exponential manner from one and to the other. The benefit of this invention is the fact that the value of electrical conductivity and the absolute value of the liquid level in the vessel need not to be known, if there is an exponential correlation between the measuring values and the volume of the liquid in the vessel.

The objective of the invention is to provide a method and a measuring device which allows to measure the volume of rate of flow through a vessel in a more precise and easy manner.

This objective is solved by a method, which is characterized in that the measured values x are measured in time intervals and that the respective filling volumes Vo are determined by comparison of the respective measured values x with calibration measured values x_R of at least one reference table comprising at least calibration measured values x_R and filling volumes V₀ belonging to them, and that the volume V₀ of the rate of flow is determined from the filling volumes V₀ over a time period, wherein the at least one reference table is constructed by means of calibration measurements using several liquid samples, which have different p-values and different filling heights h in the vessel.

The time period, in which the filling volumes V₀ are measured, can be a predetermined time period. In case that the method of measurement is applied f. e. to a filtration device, the starting time can be the time when f. e. a new filter cartridge is put into the device. In this case the time period is limited f. e. by the life time of the cartridge or the time period until the cartridge is replaced.

The benefit of the invention is that simple electrodes can be used and that the parameter p and the shape of the vessel, which both influence the results of the measurements of the filling height h and therefore of the filling volume V₀, can be taken into consideration by constructing at feast one reference table.

The calibration measured values x_R contained in this reference table are constructed for each shape of the vessel and are deposited in the memory of the electrical conductivity measuring device. The mechanical features of the measuring device, in particular the shape and technical details of the electrodes, need not to be adapted to the shape of the vessel when one type of measuring device is used in different vessels. It is only necessary to provide the respective table or tables containing the specific values which reflect the shape and the different types of liquid flowing through the vessel. If the vessels are mass-products only, the construction of at least of one table for each type of vessel is necessary and one and the same measuring device can be used without mechanical adaption.

The values of the liquid volume in the vessel can be measured in a very precise manner, because not only parameter p but also the shape of the vessel are taken into consideration when the calibration measurements are conducted.

The at least one reference table can be deposited in a memory of the measuring device.

One reference table can be sufficient, if for example the influence of parameter p on the measurement of the filling height h is less or not significant and/or there is for example a linear relationship between V₀ and the shape of the vessel. In these cases the correlation between x and h and therefore between x and V₀ can be unique. However, in cases, where the influence of parameter p or where more than one parameter p is getting significant on the result, more than one reference table is needed. The same is true when there is a non-linear correlation between x and V₀. All these facts result in ambiguous values, if only one table is used. This problem can be overcome by construction of more than one table, for example two or three reference tables in order to get unique and precise results.

It is preferred that a first reference measured value Xi is measured at least once during said time period.

This first reference measured value Xi is used to determine at least one of the parameters p of the liquid, which f. e. can be the hardness of water. It is further preferred that the first reference measured value Xi is measured only once at the beginning of a filling procedure starting from an empty vessel. Before the beginning of filling the measuring device is in the status "waiting for water" so that at the first contact of the electrodes with the liquid results in the measurement of the first reference measured value xi. After this measurement the measuring device switches into the status "height measurement" so that all following measured values are classified as measured values x.

The first reference measured value X₁ is stored and can be used for the calibration of the measured values x until the vessel is empty again and the next filling of the vessel has been started. According to this embodiment it is preferred that said first reference measured value xi is measured by the same two measuring electrodes which are used for the measuring of measured values x.

According to another embodiment it is preferred that this first reference measured value xi is measured every time when the measured value x is measured. In this case the measuring device does not distinguish between the very first measurement at the beginning of the filling procedure and the following measurements. This kind of measurement is more precise however it needs a reference electrode. The first reference measured value X₁ is measured by this reference electrode and one of the measuring electrodes which are used for the measurements of measured values x.

As illustrated in connection with the measuring device, this electrode is shielded with the exception of the lower surface.

It is preferred that a first reference table is constructed which contains the calibration first reference measured values X-_IR, which are corresponding to the first reference measured value Xi, and the respective values of parameter p belonging to them, it is also preferred to construct a second reference table which at least contains the calibration measured values x_R, the values of parameter p and the respective filling heights h belonging to them and to construct a third reference table which takes into consideration the shape of the vessel and which contains the filling heights h and the respective filling volumes V₀ belonging to them.

It is preferred to determine the value of parameter p at least from the first reference measured value Xi by comparison with the first reference table.

it is also preferred to determine the filling height h at least from the measured value x and the values of the parameter p by comparison with the values of the second reference table.

The respective filling volume V₀ can be determined from the filling height h by comparison with the values of the third reference table. Starting with the measurement of the measured values x it is a step by step procedure to achieve the filling volume V₀.

It is preferred to use a first calibrated value I₁ which is a function of x and x-i instead of x only. Therefore, in the reference tables 1 and 2 x_R is replaced by the corresponding first calibrated value I₁. Preferably, the first calibrated value I₁ is I₁ = Xi / x.

It is preferred that a second reference measured value X₂ is measured at least once during said time period.

This second reference measured value X₂ can be measured at the beginning of the filling procedure started from an empty vessel or it can be measured every time when the measured value x is measured.

This second reference measured value X₂ is preferably measured by means of a reference circuit of the electrical conductivity measuring device.

In order to consider temperature drifts of the electronic part of the measuring device it is preferred to refer and therefore to calibrate the value X₁ to the second reference measured value X₂. Preferably, such a second calibrated value I₂ is I₂ = X₂Zx₁.

This step contributes to the improvement of the precision of the volume measurement.

Therefore it is preferred to introduce I₂ into the first reference table, which contains I₁, I₂ and the parameter p. From both values I₁ and I₂ the parameter p can be determined in a more precise manner. Although I₁ = Xi / x and I₂ = X2 / X₁, both values can be multiplied by a suitable factor to achieve figures which can be handled easier. It is preferred to achieve values without decimal point.

The values of the parameter p can be determined from the values I₁ and I₂ by comparison with values of the first reference table.

The filling height h can be determined from the values of parameter p and the first calibrated value I₁ by comparison with values of the second reference table.

Although the claimed method can be used to measure the volume of the flow rate of various liquids, the measurement of water is preferred. In case of water, the parameter p is the hardness H, which is the most important property of water that affects the electrical conductivity, it's possible to use another property of the ϋquid as parameter p, f. e. the pollution of the water.

In a preferred embodiment the measured values x, X₁ and/or X₂ are a time values.

The electrical conductivity measuring device comprises an electrical circuit which preferably comprises a capacitor means. The charging and/or the discharging time of this capacitor means can be used as measured values x, because it depends from the filling height of the liquid in the vessel.

The measured values x are measured at least once per second. It is preferred to measure the measured values x at least five times per second. ft is preferred to measure not only the measured value x but also X₁ and X₂ and to calculate I₁ and I₂. This can be done by an appropriate electronic device which is part of the electrical conductivity measuring device.

In a preferred embodiment the changes ΔV of the filling volumes V₀ are determined and the volume V₀ of the flow rate is determined from the volume changes ΔV.

It is preferred to determine the volume V₀ of the flow rate from the respective volume increase. This embodiment is preferred if the filling of the vessel happens more rapidly than the draining off of the liquid, f. e. faster by a factor of at least 10. It is assumed that the amount of liquid which is filled in is equivalent to the amount that is drained off.

The volume V₀ of the flow rate is compared with a volume V_max, which is the maximum volume of the liquid, that is characterized by the at least one parameter p and which volume is allowed to flow through the filter device which is arranged downstream of the vessel. This filter device contains at least one filter medium. The exhaustion of the filter medium is indicated, when V_maχ is reached.

The maximum volume V_max depends on the at least one parameter p, for example on the hardness H in case of water. Therefore, a fourth reference table is recommended which contains the respective volume Vmax for various values of parameter p. V_max can be determined by comparison of the values of the parameter p with the corresponding values deposited in the fourth reference table.

The exhaustion of the filter medium can be indicated acoustically and/or optically. It is another possibiiity to indicate remaining volumes acoustically and/or optically until the exhaustion of the filter medium is reached.

It is preferred to use a filter cartridge as a filtering device. This filter cartridge can be arranged in the outlet of the vessel.

The objective of the invention is also solved with a measuring device for the determination of the volume V₀ of the flow rate of electrical conductive liquids through a vessel wherein the filling heights h are changing in the vertical direction and wherein the vessel comprises an inlet and an outlet, and a conductivity measuring device which comprises an evaluation unit and at least two measuring electrodes wherein the measuring electrodes are located in the vessel and are connected to the evaluation unit, wherein at least one measured value x is measured by the electrodes characterized in that the evaluation unit is configured for the deposition of at least one reference table comprising at least calibration measured values x_R and filling volumes V₀ belonging to them and for comparison of the measured values x of the conductivity measuring device with the calibration measured values x_R of the at least one reference table and for the determination of the volume V_D of the flow rate from the filling volumes V₀.

Both measuring electrodes preferably extend over the total filling height of the vessel wherein these measuring electrodes are not shielded over the total filling height.

As explained in connection with the claimed method, a reference electrode is provided which is arranged near both measuring electrodes. This reference electrode is preferably shielded with the exception of its lower surface. The electrodes can comprise a constant cross-section along the total length. The benefit of these simple electrodes is the fact that the electrodes can be cut from a long wire in order to adapt the electrodes to the height of the vessel. It is not necessary to manufacture specific electrodes for each type of vessel.

The evaluation unit preferably comprises a capacitor means. As illustrated in connection with the claimed method the charging and/or discharging time of the capacitor means is the measured value x.

The evaluation unit preferably comprises a reference circuit having a reference resistor R₀.

Furthermore, it is preferred that the measuring device encompasses an indicator unit which can be an optical or an acoustical unit.

In order to simplify the manufacturing of the measuring device, the electrodes can be combined to a measuring stick. It is preferred that the measuring stick is integrated into the wall of the vessel.

The vessel can be a feeding hopper of a water filtration device.

A preferred use of the measuring device is the exhaustion measuring device for filter cartridges.

The indicator unit can preferably indicate the time of change of the filter cartridge.

Preferred embodiments are illustrated in connection with the following drawings:

Fig. 1 shows a schematic view of a measuring device, Fig. 2 shows the measuring stick comprising three electrodes,

Fig. 3 shows a vertical cross-section of a jug containing a vessel and measuring device,

Fig. 4 shows the electrical circuit of the measuring device,

Fig. 5 shows a diagram how the measured value x is calculated from the charging and discharging time of the capacitor means,

Fig. 6 shows a vertical cross section of a vessel

Fig. 7 table 1 ,

Fig. 8 table 1 a,

Fig. 9 table 2,

Fig. 10 table 3 and

Fig. 11 table 4.

[n fig. 1 there is shown a simplified vessel 5 which is filled with water up to the water level 40. The vessel comprises a bottom wall 6b and a sidewall 5 having an inlet 7a and an outlet 7b. Inside the vessel there is located a measuring stick 20 which is approximately 5 mm above the bottom wall 6b of vessel 5.

The measuring stick comprises two measuring electrodes 22, 24 (first embodiment) and an additional reference electrode 26 (second embodiment) which is located between the measuring electrodes 22 and 24. The three electrodes are connected via electrical connections 30, 32, 33 to an evaluation unit 12 which is connected to an indication unit 14. If the water level 40 rises up to water level 40', the volume change is measured by the measuring device.

In fig. 2 and in the following fig. 3 and 4 it is illustrated the second embodiment wherein the measuring electrodes 22 and 24 are unshielded and the reference electrode 26 is shielded by a shield 27 whereas the lower surface 28 is unshielded. In fig. 3 a water filtration device 1 is shown which comprises a jug 2 having a grip 3 and a feeding hopper which forms the vessel 5. In the outlet of vessel 5 there is located a filter cartridge 50. The measuring device 10 is located inside vessel 5 and the electrodes are connected to the evaluation unit and to the indication unit which are arranged in the lid 4. Water to be filtered 8 is filled into vessel 5. After the filtration by the filter cartridge 50 the filtered water 9 flows into and is collected in the jug 2.

In fig. 4 the three electrodes 22, 24 and 26 are connected to an electrical circuit which contains a reference circuit 15 in which a reference resistor 17 is arranged. Furthermore, there is a capacitor means 16 which is charged and discharged by switching the switches 18 and 19.

In fig. 5 the diagram that corresponds to the charging and discharging of the capacitor means 16 is shown. In a first step the capacitor means is brought to a well defined voltage value by charging and discharging it. After time T₃ is reached, the measuring procedure is started. The capacitor means is charged until 1.5 Volts are reached and then it is discharged until the starting value of 0.75 Volts is reached. The sum of the charging time T₄ and the discharging time T₅ is used as measured value x.

Example:

The method of determination of rate of flow of water is depicted in detail in connection with figures 6 to 11.

A vessel 5 (figure 6) having a bottom wail 6b and a side wall 6a comprises an inlet 7a and an outlet 7b, wherein the outlet is located in the bottom wall 6b. The vessel 5 is open at the upper side which forms the inlet 7a.

The shape of the vessel is defined by side wall 6a which are inclined upwards like a cone. At the left hand sight of vessel 5 there is indicated the height h in mm and the corresponding filling volume V₀. There is a non-finear correlation between height h and volume V because the volume increases in a non-linear manner when the water level rises.

The correlation between h and V₀ is deposited in table 3 {figure 10).

In order to measure the hardness value H there are two options.

According to the first embodiment (only two electrodes) both measuring electrodes 22, 24 are used to measure the hardness value.

When the vessel 5 is empty and the water is filled in, the rising water level contacts the lower tips of both electrodes so that a first measurement can be done. Since the measuring device is in the status "waiting for water" the first measurement is the measurement of the first reference measured value Xi. After this measurement all further measurements concern the measurement of the measured values x.

This single first reference measured value X₁ is used to determine the hardness value H by comparison with the values of table 1 (first table). If xi = 20 μS/cm, the hardness value H is 3. This value X₁ is stored in the memory of the measuring device and during the further filling process only values x are measured.

According to the second embodiment (two measuring electrodes and a reference electrode) only one measuring electrode 22 or 24 and the reference electrode 26 are used to measure the hardness value. When the vessel 5 is empty and the water is filled in, the rising water level contacts the lower tips of both electrodes so that first measurements can be done.

One first measurement concerns the measurement of x between electrodes 22, 24 and another first measurement concerns the measurement of X₁ between f. e. electrode 22 and the reference electrode 26. The hardness value H is determined by comparison Xi with the values of table 1.

During the following filling process always both values x and Xi are measured, whereby a change of the hardness value can be detected by a change of the values x-j.

However, the measured value x might be falsified by various parameters. Therefore it is recommended to normalize the measured value x by the reference measurement of the reference electrode 26. The first calibrated value U = Xi/x is for example 15.

However, the electronic components of the evaluation unit 12 might also falsify the measured values. Therefore, it is recommended the first reference measured value Xi by a measurement of the reference resistor Ro located in the reference circuit 15 in order to determine the second reference measured value X₂. This second calibration results in the second calibrated value b, which is b = X₂/X₁.

An improved first reference table 1 a is shown in figure 8.

If for example I₂ = 2500, this value can be found in different rows of table 1 a. However I₁ = 15 is known so that the corresponding hardness value H must be 3. In the next step the actual height h has to be found which corresponds to the measured value x.

In a second reference table (table 2, figure 9) which contains the hardness H and the I₁ - values, h = 50 mm can be found.

If the calibration is not conducted and therefore I₁ is not determined, table 2 contains the measured values x instead of I₁.

Since the shape and the volume of the vessel doesn't correlate in a linear manner with the filling height, it is necessary to look into a third reference table (table 3, figure 10), where the corresponding volume value V₀ can be found. Since the measurement of value x starts from the beginning of the filling procedure, the difference volumes ΔV have to be added. When reaching h = 50 mm the total volume is 1 ,2 I, which is the sum of the difference values ΔV in table 3 up to the height value h = 50 mm.

In order to determine the life-span of the filter cartridge, a fourth table (table 4, figure 11 ) is used. The hardness value is 3 which corresponds to V_max = 120 l.

All tables have been prepared for a specific filtration device and have been deposited in the memory in the measuring device.

It is preferred to determine the volume values and to compare them with the Vmax value every time when the value x is measured. The value x is preferably measured five times a second, so that a high precision can be achieved. List of reference numbers

1 water filtration device

2 jug

3 grip

4 lid

5 vessel

6a side wall b bottom wall

7a inlet b outlet water to be filtered filtered water

10 measuring device

12 evaluation unit 4 indication unit 5 reference circuit 6 capacitor means 7 reference resistor 8 switch 9 switch 0 measuring stick 2 measuring electrode 4 measuring electrode 6 reference electrode 7 shield 8 lower surface 0 electrical connection 2 electrical connection 3 electrical connection 40 water ievel

40^' water level

50 filter cartridge

x measured value

XR calibration measured value (in the table)

Xi first reference measured value

XIR calibration first reference measured value

X2 second reference measured value

X2R calibration second reference measured value

V₀ filling volume

V₀ volume of the flow-rate of the electrical conductive liquid h first calibrated value h second calibrated value h filling height

V_max maximum volume of the liquid characterized by a parameter p that is allowed to flow through a filter device

Claims

1. Method of measurement of the volume V₀ of the flow-rate of electrical conductive liquids the conductivity of which is at least codetermined by at least one parameter p,

wherein the liquid flows through a vessel having a predetermined shape, and wherein the respective filling volume V₀ in the vessel is determined by at ⁽east one measured value x, which is measured by an electrical conductivity measuring device comprising at least two measuring electrodes,

wherein the vessel is filled in succession and then is emptied through its outlet, through which the filling heights h are constantly changing, characterized in

that the measured values x are measured in time intervals and that the respective filling volumes V₀ are determined by comparison of the respective measured values x with calibration measured values x_R of at least one reference table comprising at least calibration measured values XR and filling volumes V₀ belonging to them, and

that the volume V₀ of the rate of flow is determined from the filling volumes V₀ over a time period,

wherein the at least one reference table is constructed by means of calibration measurements using several liquid samples, which have different p-values and different filling heights h in the vessel.

2. Method according to claim 1 , characterized in that the at least one reference table is deposited in a memory of the measuring device.

3. Method according to claim 1 or 2, characterized in that a first reference measured value Xi is measured at least once during said time period.

4. Method according to claim 3, characterized in that said first reference measured value x-i is measured only once at the beginning of a filling procedure starting from an empty vessel.

5. Method according to one of claims 3 or 4, characterized in that said first reference measured value Xi is measured by the same two measuring electrodes which are used for the measurement of the measured value x.

6. Method according to claim 3, characterized in that said first reference measured value X₁ is measured every time when the measured value x is measured.

7. Method according claim 6, characterized in that the first reference measured value Xi is measured by a reference electrode and one of the measuring electrodes which are used for the measurement of the measured value x.

8. Method according to one of claims 3 to 7, characterized in that a first reference table is constructed, which contains calibration first reference measured values X_IR and values of parameter p belonging to them, that a second reference table is constructed which contains at least the calibration measured values XR, the values of parameter p and the filling heights h belonging to them and

that a third reference table is constructed which takes into account the shape of the vessel and which contains the filling heights h and the volumes V₀ belonging to them.

9. Method according to claim 8, characterized in that the value of parameter p is determined at least from the first reference measured value X₁ by comparison with the values of first reference table,

that the filling height h is determined from at least the measured value x and the values of parameter p by comparison with the values of the second reference table and

that the respective filling volume V₀ is determined from the filling height h by comparison with the values of the third reference table.

10. Method according to one of claims 3 to 9, characterized in that the measured value x is referred to the first reference measured value X₁ in order to determine a first calibrated value I₁.

11. Method according to claim 10, characterized in that the first calibrated values I₁ are deposited in the first and/or in the second reference table.

12. Method according to one of the claims 1 to 11 , characterized in that a second reference measured value X₂ is measured at least once during said time period.

13. Method according to claim 12, characterized in that said second reference measured value X₂ is measured only once at the beginning of a filling procedure starting from an empty vessel.

14. Method according to one of claims 12 or 13, characterized in that said second reference measured value X₂ is measured every time when the measured value x is measured.

15. Method according to one of claims 12 to 14, characterized in that said second reference measured value X₂ is measured by means of a reference circuit of the electrical conducting measuring device.

16. Method according to one of claims 11 to 15, characterized in that the first reference measured value X₁ is referred to the second reference measured value X₂ in order to determine a second calibrated value I₂.

17. Method according to claim 16, characterized in that the calibrated values I₂ are deposited in the first reference table.

18. Method according to one of claims 11 to 17, characterized in that the values of the parameter p are determined from the calibrated values U and I₂ by comparison with the values of the first reference table.

19. Method according to one of claims 11 to 18, characterized in that the filling height h is determined from the values of parameter p and the first calibrated value h by comparison with the values of the second reference table.

20. Method according to one of claims 1 to 19, characterized in that the filling volume V₀ of water is determined.

21. Method according to one of claims 1 to 20, characterized in that the hardness H of water is determined as parameter p.

22. Method according to one of claims 1 to 21 , characterized in that at least the measured values x, X₁ and/or X₂ are time values.

23. Method according to claim 22, characterized in that the charging and/or discharging time of a capacitor means in a circuit of the electrical conductivity measuring device is used as time value.

24. Method according to claim 22 or 23, characterized in that at least the measured values x are measured at least once per second.

25. Method according to claim 24, characterized in that the measured values x are measured at least five times per second.

26. Method according to one of claims 1 to 25, characterized in that the changes ΔV of the filling volume V₀ are determined and that the volume V₀ of the flow rate is determined from the volume changes ΔV.

27. Method according to claim 26, characterized in that the volume V₀ of the flow rate is determined from the respective volume increase.

28. Method according to one of claims 26 or 27, characterized in that the volume V₀ of the flow rate is compared with a volume V_ma_X) wherein V_max designates the maximum volume of the liquid characterized by at least one parameter p and is allowed to flow through a filter device which is arranged downstream to the vessel and which contains at least one filter medium and

that the exhaustion of the filter medium is indicated, when V_max is reached.

29. Method according to claim 19, characterized in that the volume V_max is determined from the value of parameter p by comparison with the values of a fourth reference table, which contains the respective volumes V_max dependent from various values of parameter p.

30. Method according to one of the claims 28 or 29, characterized in that the exhaustion is indicated acoustically and/or optically.

31. Method according to one of claims 28 to 30, characterized in that the remaining volumes are indicated acoustically and/or optically before the exhaustion of the filter medium is reached.

32. Method according to one of claims 28 to 31 , characterized in that a filter cartridge is used as filtering device.

33. Method according to claim 32, characterized in that the filter cartridge is located in the outlet of the vessel.

34. Measuring device (10) for the determination of the volume V₀ of the flow-rate of electrical conductive liquids through a vessel (5) wherein the filling heights h are changing in the vertical direction and wherein the vessel (5) comprises an inlet (7a), an outlet (7b) and a conductivity measuring device which comprises an evaluation unit (12) and at least two measuring electrodes (22, 24) wherein the measuring electrodes (22, 24) are located in the vessel (5) and are connected to the evaluation unit (12), wherein at least one measured value x is measured by the measuring electrodes characterized in that the evaluation unit (12) is configured for the deposition of at least one reference table comprising at least calibration measured values XR and filling volumes V₀ belonging to them and for comparison of the measured values x of the conductivity measuring device with the calibration measured values X_R of the at least one reference table and for the determination of the volume V₀ of the flow rate from the filling volumes V₀.

35. Measuring device according to claim 34, characterized in that both measuring electrodes (22, 24) extend over the total filling height of the vessel (5) and that both measuring electrodes (22, 24) are unshielded.

36. Measuring device according to claims 34 or 35, characterized in that a reference electrode (26) is provided which is located near the measuring electrodes (22, 24).

37. Measuring device according to claim 36, characterized in that the reference electrode (26) is shielded with the exception of its lower surface (28).

38. Measuring device according to one of claims 34 to 37, characterized in that the electrodes (22, 24, 26) have a constant cross- section along their length.

39. Measuring device according to one of the claims 34 to 38, characterized in that the evaluation unit (12) comprises a capacitor means (16) and that the charging and/or discharging time of the capacitor means (16) is the measured value x.

40. Measuring device according to one of the claims 34 to 39, characterized in that the evaluation unit (12) comprises a reference circuit (15) including a reference resistor (17).

41. Measuring device according to one of the claims 34 to 40, characterized in that an indication unit (14) is provided.

42. Measuring device according to claim 41 , characterized in that the indication unit (14) is an optical and/or acoustical unit.

.

43 Measuring device according to one of the claims 34 to 42, characterized in that the electrodes (22, 24, 26) are combined in a measuring stick (20).

44. Measuring device according to claim 43, characterized in that the measuring stick (20) is integrated into a wall (6a) of the vessel (5).

45. Measuring device according to one of claims 35 to 44, characterized in that the vessel (5) is a feeding hopper of a water filtration device (1 ).

46. Use of the measuring device according to one of the claims 34 to 45 as exhaustion measuring device for filter cartridges (50).

47. Use of the measuring device according to claim 46, characterized in that the indication unit (14) indicates the time of change of the filter cartridge (50).