CA2712055A1

CA2712055A1 - Method for measuring the volume flow of electrically conductive liquids through a vessel

Info

Publication number: CA2712055A1
Application number: CA2712055A
Authority: CA
Inventors: Colin Henderson; Uwe Lang; Stefan Hother; Steven Bookbinder; Thomas Koehler
Original assignee: Individual
Current assignee: Brita SE
Priority date: 2008-03-28
Filing date: 2009-03-27
Publication date: 2009-10-01
Also published as: CN101981415B; US20110011184A1; RU2488779C2; BRPI0910117A2; MX2010007963A; EP2265905A1; PL2265905T3; JP5559141B2; RU2010144054A; US8171802B2; JP2011519020A; ES2387188T3; TW200946881A; EP2265905B1; CN101981415A; WO2009118402A1

Abstract

it is desc.pi.bed 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 vol-umes of liquids and the alteration of volume have to be determined. The measurements of filling heights are usually done by electrodes, which immerse at least 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 exhaus-tion of filter cartridges, which are used in gravitation driven filtration de-vices.

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 BI which comprises at least three electrodes, counting means and timers. The signals meas-ured 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

2 PCT/EP2009/053644 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 conductiv-ity of the water being treated.

In case of consideration a partial filling of the vessel numerous elec-trodes 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 mathe-matical formula.

Most of the measuring devices ignore vessel shape so that the determi-nation 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 conduc-tive 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.

3 DE 10 2005 035 045 Al 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 con-ductivity and the absolute value of the liquid level in the vessel need not to be known, if there is an exponential correlation between the measur-ing 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. D
This objective is solved by a method, which is characterized in that the measured values x are measured in time intervals and that the re-spective filling volumes V0 are determined by comparison of the respec-tive measured values x with calibration measured values xR of at least one reference table comprising at least calibration measured values xR
and filling volumes Vo belonging to them, and that the volume VD of the rate of flow is determined from the filling volumes VD over a time period, wherein the at least one reference table is constructed by means of calibration measurements using several liquid samples, which have dif-ferent p-values and different filling heights h in the vessel. J~
The time period, in which the filling volumes Vo 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 pe-riod 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

4 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 least one reference table.

The calibration measured values XR 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 nec-essary 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 measure-ments 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 Vo and the shape of the vessel. In these cases the correlation between x and h and therefore between x and Vo 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 VO. All these facts result in ambiguous val-ues, if only one table is used. This problem can be overcome by con-struction 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 x1 is measured at least once during said time period.

This first reference measured value x, is used to determine at least one of the parameters p of the liquid, which f. e. can be the hardness of wa-ter. It is further preferred that the first reference measured value x1 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 elec-trodes with the liquid results in the measurement of the first reference measured value x1. After this measurement the measuring device switches into the status "height measurement" so that all following measured values are classified as measured values x.
D
The first reference measured value x, 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 em-bodiment 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 x1 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 pro-cedure and the following measurements. This kind of measurement is more precise however it needs a reference electrode. The first refer-ence 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 X1R, which are corre-sponding to the first reference measured value x,, and the respective values of parameter p belonging to them. It is also preferred to con-struct a second reference table which at least contains the calibration measured values xR, 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 con-tains the filling heights h and the respective filling volumes Vo belonging to them.

It is preferred to determine the value of parameter p at least from the first reference measured value x, 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 Vo 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 VD.

It is preferred to use a first calibrated value l1 which is a function of x and x1 instead of x only. Therefore, in the reference tables I and 2 XR is replaced by the corresponding first calibrated value 1i. Preferably, the first calibrated value 11 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 be-ginning 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 de-vice.

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 xs to the second reference measured value x2. Preferably, such a D
second calibrated value 12 is 12 = x2/xl.

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

Therefore it is preferred to introduce 12 into the first reference table, which contains I1, 12 and the parameter p. From both values I1 and 12 the parameter p can be determined in a more precise manner.

Although 11 = x, I x and 12 = x2 / xt, both values can be multiplied by a suitable factor to achieve figures which can be handled easier. It is pre-ferred to achieve values without decimal point.

The values of the parameter p can be determined from the values 11 and 12 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 li 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. Its possible to use another property of the liquid as parameter p, f. e. the pollution of the water.

In a preferred embodiment the measured values x, x, 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 sec-ond.

It is preferred to measure not only the measured value x but also x, and x2 and to calculate l1 and 12. This can be done by an appropriate elec-tronic device which is part of the electrical conductivity measuring de-vice.

In a preferred embodiment the changes AV of the filling volumes Vo are determined and the volume VD of the flow rate is determined from the volume changes AV.

It is preferred to determine the volume VD of the flow rate from the re-spective 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 VD of the flow rate is compared with a volume Vim,., 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 de-vice contains at least one filter medium. The exhaustion of the filter me-dium is indicated, when Vmax is reached.

The maximum volume Vmax depends on the at least one parameter p, for example on the hardness H in case of water. Therefore, a fourth ref-erence table is recommended which contains the respective volume Vmax for various values of parameter p. Vm,, 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 possibility 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 car-tridge 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 VD of the flow rate of electrical conduc-tive 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 evalua-tion 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 elec-trodes characterized in that the evaluation unit is configured for the deposition of at least one reference table comprising at least calibration measured values XR and filling volumes Vo belonging to them and for comparison of the measured values x of the conductivity measuring device with the calibration measured values XR of the at least one refer-ence table and for the determination of the volume Vo of the flow rate from the filling volumes Vo.

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 elec-trode is provided which is arranged near both measuring electrodes.
This reference electrode is preferably shielded with the exception of its lower surface.

. CA 02712055 2010-07-12 The electrodes can comprise a constant cross-section along the total length. The benefit of these simple electrodes is the fact that the elec-trodes 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 illus-trated in connection with the claimed method the charging and/or dis-charging time of the capacitor means is the measured value x.

The evaluation unit preferably comprises a reference circuit having a J
reference resistor Ro.

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 elec-trodes 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.

In 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 em-bodiment) 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 em-bodiment 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 tug 2.

In fig. 4 the three electrodes 22, 24 and 26 are connected to an electri-cal 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 ca-pacitor means is charged until 1.5 Volts are reached and then it is dis-charged 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 ID
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 wall 6b and a side wall 6a com-prises 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-linear correlation between height h and volume V because the vol-ume increases in a non-linear manner when the water level rises.

The correlation between h and VO 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 measure-ment can be done. Since the measuring device is in the status "waiting for water" the first measurement is the measurement of the first refer-ence measured value xi. After this measurement all further measure-ments 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 pS/cm, the hardness value H is 3. This value x, is stored in the memory of the measuring device and during the further filling proc-ess 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 measure-ments can be done.

One first measurement concerns the measurement of x between elec-trodes 22, 24 and another first measurement concerns the measure-ment of x1 between f. e. electrode 22 and the reference electrode 26.
The hardness value H is determined by comparison x, with the values of table 1.

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

However, the measured value x might be falsified by various parame-ters. Therefore it is recommended to normalize the measured value x by the reference measurement of the reference electrode 26. The first calibrated value 11 = 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 x, by a measurement of the reference resis-tor R0 located in the reference circuit 15 in order to determine the sec-ond reference measured value x2. This second calibration results in the second calibrated value 12, which is 12 = x2/x..

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

If for example 12 = 2500, this value can be found in different rows of ta-ble 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 hard-ness H and the 11 - values, h = 50 mm can be found.

If the calibration is not conducted and therefore 11 is not determined, table 2 contains the measured values x instead of 11.

Since the shape and the volume of the vessel doesn't correlate in a lin-ear manner with the filling height, it is necessary to look into a third ref-erence table (table 3, figure 10), where the corresponding volume value Vo can be found. Since the measurement of value x starts from the be-ginning of the filling procedure, the difference volumes AV have to be added. When reaching h = 50 mm the total volume is 1,2 1, which is the sum of the difference values AV 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 Vmax = 1201.

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 vessel 6a side wall 6b bottom wall 7a inlet 7b outlet 8 water to be filtered 9 filtered water measuring device 12 evaluation unit 14 indication unit reference circuit 16 capacitor means 17 reference resistor 18 switch 19 switch measuring stick D
22 measuring electrode 24 measuring electrode 26 reference electrode 27 shield 28 lower surface electrical connection 32 electrical connection 33 electrical connection 40 water level 40' water level 50 filter cartridge x measured value XR calibration measured value (in the table) x1 first reference measured value x3R calibration first reference measured value x2 second reference measured value x2R calibration second reference measured value VO filling volume VD volume of the flow-rate of the electrical conductive liquid I1 first calibrated value 12 second calibrated value h filling height Vmax maximum volume of the liquid characterized by a parame-ter p that is allowed to flow through a filter device

Claims

1. Method of measurement of the volume V D 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 least one measured value x, which is meas-ured 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 con-stantly changing, characterized in that the measured values x are measured in time intervals and that the respective filling volumes V0 are determined by compari-son of the respective measured values x with calibration meas-ured 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 V D of the rate of flow is determined from the fill-ing 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 ves-sel.

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 ref-erence measured value x1 is measured at least once during said time period.

4. Method according to claim 3, characterized in that said first ref-erence measured value x1 is measured only once at the begin-ning 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 x1 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 ref-erence measured value x1 is measured every time when the measured value x is measured.

7. Method according claim 6, characterized in that the first refer-ence measured value x1 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 x1R and values of parameter p be-longing to them, that a second reference table is constructed which contains at least the calibration measured values x R1 the values of parameter p and the filling heights h belonging to them and that a third reference table is constructed which takes into ac-count 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 meas-ured value x1 by comparison with the values of first reference ta-ble, 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 ta-ble.

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 cali-brated values l1 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 be-ginning 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 de-vice.

16. Method according to one of claims 11 to 15, characterized in that the first reference measured value x, is referred to the second reference measured value X2 in order to determine a second cali-brated value l2.

17. Method according to claim 16, characterized in that the calibrated values l2 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 l1 and l2 by comparison with the values of the first refer-ence 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 l1 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 meas-ured 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 .DELTA.V of the filling volume V0 are determined and that the volume V D of the flow rate is determined from the volume changes .DELTA.V.

27. Method according to claim 26, characterized in that the volume V D of the flow rate is determined from the respective volume in-crease.

28. Method according to one of claims 26 or 27, characterized in that the volume V D of the flow rate is compared with a volume V max, wherein V max designates the maximum volume of the liquid char-acterized by at least one parameter p and is allowed to flow through a filter device which is arranged downstream to the ves-sel 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 re-spective volumes V max dependent from various values of parame-ter 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 car-tridge is located in the outlet of the vessel.

34. Measuring device (10) for the determination of the volume V D 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 evalua-tion unit (12) and at (east 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 config-ured for the deposition of at least one reference table comprising at least calibration measured values X R and filling volumes V0 be-longing 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 determi-nation of the volume V D of the flow rate from the filling volumes V0.

36. 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, character-ized 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, char-acterized 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, char-acterized 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, char-acterized 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, char-acterized 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, character-ized 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, character-ized in that the indication unit (14) indicates the time of change of the filter cartridge (50).