LIQUID VOLUME MEASUREMENT DEVICE

Abstract

The present invention relates to a measurement device (10) for measuring a volume of liquid contained in a vessel, comprising: —a main vessel (12) for receiving a liquid; —a reference vessel (14) which has a smaller volume than the main vessel (12), wherein the main vessel (12) and the reference vessel (14) are fluidly connectable to each other as communicating vessels; —a pump (16) which is connected to the main vessel (12) and the reference vessel (14) for extracting liquid from the main vessel (12) and the reference vessel (14); —a minimum level sensor (26) which is configured to send a reference signal if a minimum liquid level within the reference vessel (14) is reached; —a first valve (20, 20′) via which the reference vessel (14) is fluidly connected to the pump (16), wherein the first valve (20, 20′) is configured to be closed off if the minimum liquid level within the reference vessel (14) is reached; and —a control unit (30) which is connected to the minimum level sensor (26, 26′) and the pump (16) and configured to measure a reference time between an activation of the pump (16) and a receipt of the reference signal, wherein the control unit (30) is further configured to calculate, based on the reference time and a flow rate of the pump (16), a liquid volume within the main vessel (12) and/or a total liquid volume within the main vessel (12) and the reference vessel (14) together.

Claims

1. A measurement device for measuring a volume of liquid contained in a vessel, comprising: a main vessel for receiving a liquid; a reference vessel which has a smaller volume than the main vessel, wherein the main vessel and the reference vessel are fluidly connectable to each other as communicating vessels; a pump which is connected to the main vessel and the reference vessel for extracting liquid from the main vessel and the reference vessel; a minimum level sensor which is configured to send a reference signal if a minimum liquid level within the reference vessel is reached; a first valve via which the reference vessel is fluidly connected to the pump, wherein the first valve is configured to be closed off if the minimum liquid level within the reference vessel is reached; and a control unit which is connected to the minimum level sensor and the pump and configured to measure a reference time between an activation of the pump and a receipt of the reference signal, wherein the control unit is further configured to calculate, based on the reference time and a flow rate of the pump, a liquid volume within the main vessel and/or a total liquid volume within the main vessel and the reference vessel together.

2. The measurement device according to claim 1, further comprising a selector for selecting an amount of liquid to be extracted from the main vessel and the reference vessel by means of the pump during an extraction event, wherein an extraction event denotes the event between an activation and a subsequent deactivation of the pump.

3. The measurement device according to claim 2, wherein the control unit is configured to calculate the liquid volume within the main vessel and/or the total liquid volume additionally based on the selected amount of liquid.

4. The measurement device according to claim 1, wherein the control unit is configured to calculate the liquid volume within the main vessel and/or the total liquid volume additionally based on a geometrical dimension of the main vessel and the reference vessel.

5. The measurement device according to claim 2, wherein the control unit is configured to calculate the liquid volume within the main vessel and/or the total liquid volume based on: (i) the reference time measured during a first extraction event, (ii) the reference time measured during a second extraction event following the first extraction event, (iii) the flow rate of the pump, (iv) the amount of liquid selected for the first extraction event, and (v) the amount of liquid selected for the second extraction event.

6. The measurement device according to claim 1, further comprising a memory unit which is connected to the control unit, wherein the control unit is configured to store the measured reference time, the calculated liquid volume of the main vessel and/or the calculated total liquid volume in the memory unit.

7. The measurement device according to claim 1, further comprising a restriction element for restricting a liquid flow from the main vessel to the pump.

8. The measurement device according to claim 7, wherein the minimum level sensor comprises a float which is configured to close off the first valve if the minimum liquid level within the reference vessel is reached.

9. The measurement device according to claim 8, wherein the minimum level sensor comprises a first contact sensor which is arranged at or near the first valve and configured to detect a contact of the float with the first valve.

10. The measurement device according to claim 8, further comprising: (i) a stopper which is arranged within the reference vessel and configured to prevent the float from floating over a predetermined height within the reference vessel; and (ii) a second contact sensor which is arranged at or near the stopper and configured to detect a contact of the float with the stopper.

11. The measurement device according to claim 1, further comprising a second valve, wherein the main vessel is connected to the pump via the second valve wherein the first valve is configured to be closed off as soon as the minimum level sensor detects that a minimum liquid level within the reference vessel is reached, and wherein the second valve is configured to be opened as soon as the minimum level sensor detects that the minimum liquid level within the reference vessel is reached.

12. The measurement device according to claim 1, wherein the pump is, upon activation, configured to generate a liquid flow rate which is constant over time.

13. The measurement device according to claim 1, further comprising a flow meter for measuring the flow rate of the liquid extracted from the main vessel and the reference vessel.

14. Beverage dispensing machine comprising the measurement device as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the following drawings

[0054] FIG. 1 shows a first embodiment of the measurement device according to the present invention, wherein FIG. 1A shows the measurement device in a first operating position, FIG. 1B shows the measurement device in a second operating position, FIG. 1C shows the measurement device in a third operating position, and FIG. 1D shows the measurement device in a fourth operating position;

[0055] FIG. 2 shows a second embodiment of the measurement device according to the present invention; and

[0056] FIG. 3 shows an example of a look-up table stored within a memory unit of the measurement device according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0057] FIG. 1 shows a first embodiment of a measurement device according to the present invention. The measurement device is therein in its entirety denoted by reference numeral 10.

[0058] The measurement device 10 comprises a main vessel 12, a reference vessel 14 and a pump 16. If the measurement device 10 is applied in a beverage dispensing machine, the main vessel 12 is realized by the main liquid tank of the machine and the reference vessel 14 may be realized as a riser pipe. Both the main vessel 12 and the reference vessel 14 may have any arbitrary design and shape. The only thing that is important is that the reference vessel 14 has a smaller volume than the main vessel 12. In a typical arrangement the volume of the reference vessel 14 will be in the order of 10% or less, or even in the order of 5% or less of the volume of the main vessel 12.

[0059] It is furthermore important that the main vessel 12 and the reference vessel 14 are fluidly connected to each other as two communicating vessels. As long as the same pressure (atmospheric pressure) is applied to the liquid within the main vessel 12 and the liquid within the reference vessel 14, the liquid will balance out to the same level h.sub.1 in both vessels 12, 14.

[0060] The pump 16 is fluidly connected to both vessels 12, 14. The pump 16 may thus extract liquid from both vessels 12, 14. A customary pump may generally be used for the pump 16. However, it is preferred to use a pump 16 with a constant flow rate, since this facilitates the measurement of the measurement device 10, as it will be outlined in detail below.

[0061] According to the first embodiment shown in FIG. 1, the main vessel 12 is connected to the pump 16 via a restriction element 18. This restriction element 18 is configured to restrict the liquid flow from the main vessel 12 to the pump 16. However, it shall be noted that the restriction element 18 has to be configured to only partly restrict the liquid flow, since the functional principle of two communicating vessels 12, 14 would otherwise be impeded. A too strong restriction would on the other hand also impede the extraction of liquid from the main vessel 12 by means of the pump 16. The restriction element 18 may, for example, be realized as a constriction within the tube or hose that connects the main vessel 12 to the pump 16. However, the restriction element 18 may also comprise a valve that restricts or stops the liquid flow from the main vessel 12 to the pump 16 until the reference vessel 14 is empty and then opens up the connection between the main vessel 12 and the pump 16 again.

[0062] The reference vessel 14 is according to the first embodiment connected to the pump 16 via a valve 20. The valve 20 is arranged within the reference vessel 14. According to the exemplary embodiment shown in FIG. 1, this valve 20 is realized as a float operated valve. It comprises a float 22 which is configured to float within the liquid contained in the reference vessel 14. The float 22 should thus be made of a material having a density which is smaller than the density of the liquid that is used within the measurement device 10 (if the measurement device 10 is applied in a beverage dispensing machine, the liquid is usually water). The valve 20 in this example furthermore comprises a receiving element 24 that is configured to receive the float 22. This receiving element 24 may be realized as a kind of bowl that is adapted to the shape of the float 22. The receiving element 24 is preferably arranged at or near a lower end of the reference vessel 14, i.e. in the area of a minimum liquid level within the reference vessel 14.

[0063] The exemplarily shown float operated valve 20 works as follows: If the liquid contained in the reference vessel 14 is above the minimum liquid level, the float 22 will float within the liquid such that it is not connected to the receiving element 24. As soon as the liquid level within the reference vessel 14 falls below the minimum liquid level, the float 22 will connect to the receiving element 24 and thereby close the valve 20, i.e. close off the connection between the reference vessel 14 and the pump 16. In the latter case, the valve 20 also closes off the connection between the main vessel 12 and the reference vessel 14. The main vessel 12 and the reference vessel 14 are thus only communicating with each other as long as the valve 20 is opened.

[0064] The measurement device 10 further comprises a minimum level sensor 26 that detects a minimum liquid level within the reference vessel 14. It may, for example, detect if the reference vessel 14 is empty or not. According to the exemplary embodiment shown in FIG. 1, said minimum level sensor 26 is combined with the valve 20. The minimum level sensor 26 may comprise a contact sensor 28 that is arranged at or near the receiving element 24 of the valve 20. Said contact sensor 28 is configured to detect a contact of the float 22 with the valve 20/the receiving element 24. As soon as a contact of the float 22 with the valve 20 is detected by means of the contact sensor 28, the minimum level sensor 26 will generate a signal which is herein denoted as reference signal.

[0065] It shall be noted that the present invention is not limited to the usage of a float operated valve and a minimum level sensor comprising a contact sensor as shown in the exemplary embodiment of FIG. 1. The same functional principle could also be achieved by means of an electronically actuated valve 20 and a minimum level sensor 26 that optically, mechanically, inductively or capacitively detects a minimum liquid level within the reference vessel 14.

[0066] The presented measurement device 10 further comprises a control unit 30 for controlling the operation of the device 10. This control unit 30 may be realized as a processor or microcontroller having software stored thereon for controlling the various elements of the measurement device 10. The control unit 30 is preferably connected to the pump 16 and to the minimum level sensor 26, as this is indicated in FIG. 1 by means of the dotted lines. The connections between the control unit 30, the pump 16 and the minimum level sensor 26 may be either realized as a wireless connection or as a hard-wired connection.

[0067] The operation of the measurement device 10 shall now be explained in detail with reference to FIGS. 1A-1D:

[0068] FIG. 1A shows the initial situation in which liquid has been filled into the main vessel 12 and the liquid has been balanced out between the main vessel 12 and the reference vessel 14. The liquid within both vessels 12, 14 is thus at the same height h.sub.1. The valve 20 is in its open position such that the pump 16 may extract liquid from both the main vessel 12 and the reference vessel 14. A user may now activate the pump 16 to start the liquid extraction. This could be done by means of a user interface 32 which comprises a display 34 and a selector 36. In an application within a beverage dispensing machine, the user may e.g. select an amount of liquid V.sub.dose that shall be extracted on total from the main vessel 12 and the reference vessel 14. The selector 36 may thus comprise a mechanical bottom or a touchscreen allowing the user to select said amount of liquid V.sub.dose. It shall be noted, however, that users of beverage dispensing machines do not always directly select a certain amount of liquid to be extracted V.sub.dose, but rather select a type of predefined recipe (e.g. an espresso, a small coffee or a large coffee). The control unit 30 in such cases calculates V.sub.dose or takes it from a look-up table that is stored in a memory unit 38 connected to the control unit 30. The control unit 30 will then also calculate the time t.sub.pump the pump 16 needs to be activated in order to extract the amount V.sub.dose. This information may also be stored in a look-up table in the memory unit 38, such that it does not necessarily need to be calculated every time based on the flow rate θ.sub.pump of the pump 16.

[0069] Upon activation, the pump 16 will then begin extracting liquid from the main vessel 12 and the reference vessel 14. Due to the restriction element 18, most of the liquid will be drawn in the beginning from the reference vessel 14. The reference vessel 14 will thus empty first. Depending on the liquid level within the reference vessel 14, this will take a certain amount of time and will be noticed by means of the minimum level sensor 26. As soon as the liquid level within the reference vessel 14 reaches the minimum liquid level, the float 22 contacts the valve 20 and thereby closes it (see FIG. 1B). The contact sensor 28 will detect this contact and send the reference signal to the control unit 30. The connection between the reference vessel 14 and the pump 16 is then closed off so that the pump 16 will continue to extract liquid from the main vessel 12 only (see FIG. 1C). The pump 16 will continue to extract liquid until the selected amount V.sub.dose is extracted on total. Even though the liquid is then extracted through the restriction element 18, the pump 16 is configured to keep the flow rate at a constant level. The pump 16 may be, for example, configured to adapt this restriction change automatically. As soon as the selected amount V.sub.dose is extracted on total, the control unit 30 will stops/deactivates the pump 16 and thereby finalizes the extraction event. An extraction event denotes the event between the activation and the subsequent deactivation of the pump 16, i.e. a full cycle for extracting the selected amount of liquid V.sub.dose from the system. As soon as the pump 16 is deactivated by the control unit 30, the valve 20 opens up automatically, since the float 22 will rise within the reference vessel 14. The main vessel 12 and the reference vessel 14 may then communicate with each other again, such that the liquid level within both vessels 12, 14 will balance out again (see FIG. 1D). The liquid level within both vessels 12, 14 will then be at a height h.sub.2, wherein h.sub.2<h.sub.1 (compare FIG. 1A and FIG. 1D).

[0070] During the extraction event, the control unit 30 may measure the time (herein denoted as reference time) between the activation of the pump 16 and the receipt of the reference signal. After the (first) extraction event, the control unit 30 may then calculate the liquid volume V.sub.main(t1) that the main vessel 12 contained before the extraction event. The control unit 30 may also calculate the liquid volume V.sub.main(t2) that the main vessel 12 contained after the extraction event. This calculation may be based on the reference time t.sub.ref and the flow rate θ.sub.pump of the pump 16.

[0071] An easy way of calculating V.sub.main is by additionally taking into account the amount of liquid selected by the user V.sub.dose and the geometrical dimensions of the main vessel 12 and the reference vessel 14. An example for such a calculation is given below:

Example 1

[0072] A main vessel 12 is used which has a cross-sectional area A.sub.main=3925 mm.sup.2, wherein said cross-sectional area A.sub.main is constant along the height of the main vessel 12. A reference vessel 14 is used which has a cross-sectional area A.sub.ref=200 mm.sup.2 that is also constant along the height. A reference time t.sub.ref of 10 s has been measured during the extraction event. The pump 16 has a constant flow rate θ.sub.pump of 4 ml/s (4000 mm.sup.3/s). Taking equation 5, which has been mentioned in the introductory portion of the description, the volume within the main vessel 12 may then be calculated as follows:

[00004]$\begin{array}{c}{V}_{\mathrm{main}}=.Math.\frac{V_{\mathrm{ref}}}{A_{\mathrm{ref}}}\times A_{\mathrm{main}}\\ =.Math.\frac{{\theta }_{\mathrm{pump}}\times t_{\mathrm{ref}}}{A_{\mathrm{ref}}}\times A_{\mathrm{main}}\\ =.Math.\frac{4000\times 10}{200}\times 3925.Math..Math.{\mathrm{mm}}^3\\ =.Math.785.000.Math..Math.{\mathrm{mm}}^3\\ =.Math.785.Math..Math.\mathrm{ml}\end{array}$

[0073] It is clear that V.sub.main in the above-mentioned example indicates the liquid volume within the main vessel 12 before the extraction event (V.sub.main(t1)). However, if the selected amount of liquid extracted on total V.sub.dose is known as well, the total liquid volume V.sub.total(t2) that is within the main vessel 12 and the reference vessel 14 together after the extraction event may be easily calculated as follows:

V.sub.total(t2)=V.sub.total(t1)−V.sub.dose=V.sub.main(t1)+V.sub.ref(t1)−V.sub.dose

[0074] It is also clear that the above-mentioned calculation only becomes exact if it is assumed that all the liquid is in the beginning extracted from the reference vessel 14 and no leakages occur at that time at the restriction element 18. It should be also noted that the above-mentioned exemplary calculation is only possible if the geometrical dimensions A.sub.main and A.sub.ref of the main vessel 12 and the reference vessel 14 are known.

[0075] If the dimensions A.sub.main and A.sub.ref are not known, the control unit 30 may calculate V.sub.main in another way. This will be shown in Example 2 given below. The calculation in Example 2 is still based on the measured reference time t.sub.ref and the flow rate of the pump θ.sub.pump. However, V.sub.main is now calculated after two subsequent extraction events, e.g. after the user has withdrawn two drinks from the system. V.sub.main is then calculated based on the consideration that the change of the total volume within the appliance ΔV.sub.total divided by the total volume before the extraction V.sub.total(t1) equals the volume change within the reference vessel ΔV.sub.ref divided by the liquid volume within the reference vessel before the extraction V.sub.ref(t1) (see equations 6 to 8 indicated in the introductory portion of the description).

Example 2

[0076] The amount of extracted liquid V.sub.dose(t1) selected by the user during the first extraction event is V.sub.dose(t1)=120 ml. The average flow rate θ.sub.pump is 4 ml/s. The reference time t.sub.ref(t1) measured during the first extraction event is t.sub.ref(t1)=3.1 s. The liquid volume that the reference vessel 14 contained before the extraction event may thus be calculated (:

V.sub.ref(t1)=θ.sub.pump×t.sub.ref(t1)=4 ml/s×3.1 s=12.4 ml.

[0077] In the second extraction step, the user selects an amount of liquid V.sub.dose(t2) of 100 ml. The reference time t.sub.ref(t2) measured by the control unit 30 during the second extraction event is 2.8 s. θ.sub.pump remains the same (4 ml/s). The reference vessel 14 thus contained a volume V.sub.ref(t2) before the second extraction event of:

V.sub.ref(t2)=θ.sub.pump×t.sub.ref(t2)=4 ml/s×2.8 s=11.2 ml.

[0078] The total liquid volume V.sub.total and/or the liquid volume V.sub.main within the main vessel 12 before the first extraction event (V.sub.total(t1) and/or V.sub.main(t1)), after the first extraction event (V.sub.total(t2) and/or V.sub.main(t2)), and after the second extraction event (V.sub.total(t3) and/or V.sub.main(t3)) may then be calculated by means of the above-mentioned equations 8 to 11:

[00005]$\begin{array}{c}{V}_{\mathrm{total}\left(t.Math..Math.1\right)}=.Math.\frac{V_{\mathrm{ref}\left(t.Math..Math.1\right)}}{V_{\mathrm{ref}\left(t.Math..Math.1\right)}-V_{\mathrm{ref}\left(t.Math..Math.2\right)}}\times V_{\mathrm{dose}\left(t.Math..Math.1\right)}\\ =.Math.\frac{12.4}{12.4-11.2}\times 120.Math..Math.\mathrm{ml}\\ =.Math.1240.Math..Math.\mathrm{ml}\end{array}$$.Math.V_{\mathrm{main}\left(t.Math..Math.1\right)=}.Math.V_{\mathrm{total}\left(t.Math..Math.1\right)}-V_{\mathrm{ref}\left(t.Math..Math.1\right)}=1240.Math..Math.\mathrm{ml}-12.4.Math..Math.\mathrm{ml}=1227.6.Math..Math.\mathrm{ml}$$.Math.V_{\mathrm{main}\left(t.Math..Math.2\right)}=V_{\mathrm{main}\left(t.Math..Math.1\right)}-V_{\mathrm{dose}\left(t.Math..Math.1\right)}=1227.6.Math..Math.\mathrm{ml}-120.Math..Math.\mathrm{ml}=1107.6.Math..Math.\mathrm{ml}$$.Math.V_{\mathrm{main}\left(t.Math..Math.3\right)}=V_{\mathrm{main}\left(t.Math..Math.2\right)}-V_{\mathrm{dose}\left(t.Math..Math.2\right)}=1107.6.Math..Math.\mathrm{ml}-100.Math..Math.\mathrm{ml}=1007.6.Math..Math.\mathrm{ml}$

[0079] It is clear that the liquid volume V.sub.total(t1), which was present within the main vessel 12 and the reference vessel 14 before the first extraction event, may also be calculated by means of equation 12 instead of equation 8:

[00006]$\begin{array}{c}{V}_{\mathrm{total}\left(t.Math..Math.1\right)}=.Math.\frac{V_{\mathrm{dose}\left(t.Math..Math.1\right)}}{t_{\mathrm{ref}\left(t.Math..Math.1\right)}-t_{\mathrm{ref}\left(t.Math..Math.2\right)}}\times t_{\mathrm{ref}\left(t.Math..Math.1\right)}\\ =.Math.\frac{120}{3.1-2.8}\times 3.1.Math..Math.\mathrm{ml}\\ =.Math.1240.Math..Math.\mathrm{ml}\end{array}$

[0080] The volume within the main vessel 12 may thus be also calculated if the geometrical dimensions of the main vessel 12 and the reference vessel 14 are not known. The calculated liquid volumes V.sub.total and/or V.sub.main may be shown to the user on the display 34. However, the calculation according to Example 2 requires two extraction events (two drink extractions) which might be disadvantageous, since users usually want to see directly if enough liquid is left in the device.

[0081] The presented measurement device 10 may be improved in a way that such a direct indication of the liquid volume on the display 34 is possible as well. The control unit 30 may thereto store the measured reference time t.sub.ref together with the calculated volumes V.sub.total and/or V.sub.main in a look-up table within the memory unit 38 after each extraction event. If this is done a plurality of times, i.e. after a plurality of extraction events, the look-up table within the memory unit 38 contains enough data. After this initialization phase, the control unit 30 does then no longer have to calculate V.sub.total and/or V.sub.main each time in one of the above-mentioned exemplary ways, but may look up V.sub.total and/or V.sub.main directly from the look-up table stored within the memory unit 38. An example of such a look-up table is schematically shown in FIG. 3. The look-up table of shown in FIG. 3 only shows the correlation of V.sub.main and t.sub.ref. However, it is clear that V.sub.total may be included in the look-up table as well.

[0082] The above-mentioned look-up table thus accelerates the measurement and also provides the advantage that the measurements become more and more accurate after each extraction event. If a measured reference time t.sub.ref is not already included in said look-up table, V.sub.total and/or V.sub.main can be calculated in one of the above-mentioned ways and a new entry in the look-up table may be made. Alternatively, the control unit 30 can be configured to interpolate between the two next closest reference times t.sub.ref that are already included in the memory unit 38. This will of course require a substantially linear relationship between t.sub.ref and V.sub.total and/or a linear relationship between t.sub.ref and V.sub.main.

[0083] The measurement device 10 may still comprise some further improvements: According to the exemplary embodiment of the measurement device 10 shown in FIG. 1, the measurement device 10 further comprises a stopper 40 which is arranged within the reference vessel 14. This stopper 40 is configured to prevent the float 22 from floating over a predetermined height within the reference vessel 14. It may be realized by a simple mechanical stopper element that projects into the reference vessel 14 at a certain height. The measurement device 10 may further comprise a second contact sensor 42 which is arranged at or near the stopper 40 and configured to detect a contact of the float 22 with the stopper 40.

[0084] The inclusion of such a stopper 40 and contact sensor 42 provides the following advantages: 1.) The stopper 40 prevents the float 22 from unintentionally falling out of the reference vessel 40 if the user manually empties the two vessels 12, 14. 2.) The second contact sensor 42 may be used to detect the presence of the main vessel 12. The main vessel 12 may, for example, be separately detachable from the device 10. If the user detaches the main vessel 12, e.g. to refill it, the liquid level within the reference vessel 14 will decline, so that the float 22 will no longer be pushed against the stopper 40. This may be detected by the contact sensor 42. 3.) The stopper 40 and the second contact sensor 42 may also be used as a minimum level detector in order to detect whether the liquid level within the main vessel 12 is above a certain minimum threshold.

[0085] It is also possible to combine the first and second contact sensors 26, 42 and the minimum level sensor 26 with each other. The stopper 40 has to be thereto arranged near the minimum level sensor 26, i.e. in close proximity to the minimum liquid level within the reference vessel 14. In this case the first and second contact sensors 26, 42 may be combined in only one sensor.

[0086] FIG. 2 shows a second embodiment of the measurement device 10. The functional principle remains the same as explained before with reference to the first embodiment shown in FIG. 1. However, some modifications are made therein. Instead of the flow operated valve 20, a valve 20′ is provided that may be electronically actuated by the control unit 30. In this case, the control unit 30 is configured to close off the valve 20′ upon receipt of the reference signal. Instead of the restriction element 18, a second valve 44 is provided between the pump 16 and the main vessel 12. This second valve 44 is preferably realized as an electronically actuatable valve as well. The control unit 30 may thus control the valves 20′ and 44 as follows: In the beginning both valves 20′, 44 will be opened, such that the main vessel 12 and the reference vessel 14 communicate with each other and balance out their liquid levels. As soon as the pump 16 is started, the first valve 20′ is opened and the second valve 44 is closed off. This ensures that the pump 16 extracts in the beginning liquid only from the reference vessel 14. As soon as the reference vessel 14 gets empty (detected by the minimum level sensor 26′), the first valve 20′ will be closed off and the second valve 44 will be opened. The first valve 20′ will be kept closed until the end of the extraction event and then opened up again, so that the two vessels 12, 14 may communicate with each other again.

[0087] The measurement may be still further improved by the inclusion of a means for measuring the flow rate of the liquid extracted from the main vessel 12 and the reference vessel 14. This may be realized by a flow meter 46 that is either arranged before or after the pump 16. This flow meter 46 may also be connected to the control unit 30.

[0088] It shall be noted that the modified components of the second embodiment may be also implemented within the measurement device 10 according to the first embodiment either separately or altogether. The first embodiment could, for example, be modified by simply replacing the restriction element 18 by the second valve 44. It could also be modified by simply adding the flow meter 46 and/or by replacing the float operated valve 20 by the electronically actuated valve 20′.

[0089] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

[0090] In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0091] Any reference signs in the claims should not be construed as limiting the scope.