Title of Invention: Apparatus and method for measuring a quantity of milk yielded by an animal during a milking process Abstract: An apparatus for measuring a quantity of milk yielded by an animal during a milking process comprises a container, a supply unit, a discharge unit, a filling level measuring device, a control unit and an evaluation unit. The supply unit is configured to feed milk into the container and can be connected to the milking unit of a milking machine. The discharge unit is configured to discharge milk from the container and can be connected to the milking line of the milking machine, which is adapted to have applied thereto a milking vacuum. The discharge unit comprises the means for changing the size of a discharge opening through which the milk flows when it is being discharged from the container. The discharge opening allowing milk to flow through the discharge opening can be adjusted to at least two sizes. The filling level measuring device is configured to measure a filling level of the milk in the container. The control unit is configured to control the means for changing the size of the discharge opening in dependence on the milk level in the container determined by the filling level measuring device, in order to adjust a size of the discharge opening such that the filling level remains within a predetermined range. The evaluation unit is configured to calculate a milk flow into the container on the basis of the set size of the discharge opening and of the filling level measured by the filling level measuring device. Detailed Description: The present invention relates to an apparatus and a method for measuring a quantity of milk yielded by an animal during a milking process. In dairy farming, it can be an advantage to measure the quantity of milk yielded during a milking process by an animal, for example a cow, a sheep or a goat. Thereby, the performance of each single animal can be monitored, and the composition of the herd as well as the feeding of the animals can be adjusted in order to obtain a milk yield as high as possible. An apparatus for determining the milk performance of cows according to the state of the art is described, for example, in DE 3118865 A1. Hereinafter, this apparatus will be described in detail with reference to FIG. 1. The apparatus 100 comprises a conduit 111 through which the milk is sucked in together with air. The milk is collected in a collection container 113 into which the conduit 111 leads. At the bottom of collection container 113, a discharge conduit 114 is provided. The outlet line 114 leads into a collection line 112. Shortly before the mouth of the outlet line 114, a magnet valve 115 is located. At the upper side of the collection container 113, a tubing 116 is connected which transfers the vacuum in the collection line 112 via the collection container 113 to the suck-off conduit 111. During operation of the apparatus 100, it is measured via electrodes 117, 119 whether the filling level in the collection container is above the height of the electrode 119 or below the height of the electrode 117. Once the milk reaches the electrode 119, the solenoid valve 115 is opened by a control unit 120. Once the electrode 117 is exposed, the solenoid valve 115 is closed. The time during which the solenoid valve 115 is open is measured, and from the measured time a quantity of milk yielded by the milked animal is calculated. The line 114 of apparatus 100 is configured such that the milk flow passing through the discharge line 114 when the solenoid valve 115 is opened is greater than the maximum milk flow occurring during the milking process. Cows can produce a milk flow of up to 12 kg/min. If the milk flow is considerably smaller, the solenoid valve is opened only during very short time periods, and during a milking process only few opening and closing operations of the solenoid valve 115 may take place. Therefore, metering low milk flows may be difficult. This can be particularly the case if animals yielding considerably lower milk flows than 12 kg/min, as for example sheep or goats, are milked. Considerably smaller milk flows than during an intermediate stage may also occur at the beginning and at the end of the milking session. With cows, the limit between milking and over-milking (milking without substantially gaining milk therefrom) may be considered to be 200 g/min. This means that with cows, measuring small milk flows of down to 0.2 kg/min may be considered as being reasonable. Moreover, the apparatus 100 only determines the total amount of milk yielded by the animal. The apparatus 100, due to its discontinuous operating mode, is not suitable for measuring variations of the milk flow during the milking process. In the apparatus 100, the discharge line 114 is made relatively long in order to keep variations of the milk flow through the discharge line 114, caused by fluctuations of the filling level of the collection container 113, at a low level. This results in a relatively large structural height of apparatus 100, thereby limiting the transportability of apparatus 100. Further prior at milk quantity measuring apparatusses are described in U.S. Pat. No. 3,919,975 A, U.S. Pat. No. 6,497,143 B1, DE (925)286-7721 A1 and EP 0 382 852 A1. It is an object of the invention to provide an apparatus and a method enabling a measurement of strongly varying milk flows. A further object of the invention is to provide an apparatus and a method enabling measuring milk flows which occur during the milking process. A further object of the invention is to provide a transportable apparatus for measuring a quantity of milk yielded by an animal during a milking process. An apparatus for measuring a quantity of milk yielded by an animal during a milking process comprises a container, a supply unit, a discharge unit, a filling level measuring device, a control unit and an evaluation unit. The supply unit is configured to supply milk to the container and is connectable to a milking unit of a milking machine. The discharge unit is configured to discharge milk from the container and is connectable to a milking conduit of the milking machine which is adapted to have applied a milking vacuum thereto. The discharge unit comprises the means for changing a size of the discharge opening through which milk flows when it is discharged from the container. The discharge opening, which allows milk to flow therethrough when it is discharged from the container, is adjustable to at least two sizes. The filling level measuring device is configured to control the means for changing the size of the discharge opening in dependence on the filling level of milk in the container, determined by the filling level measuring device, in order to set a size of the discharge opening such that the filling level remains within a predetermined range. The evaluation unit is configured to calculate a milk flow into the container on the basis of the set size of the discharge opening and of the filling level measured by the filling level measuring device. By setting the filling level in the predetermined range, which can be achieved by means of the control unit, the filling level measuring device and the means for changing the size of the discharge opening, the apparatus can be used for varying milk flows under similar operating conditions. Thus, the apparatus can not only be used in milking cows, where milk flows of up to 12 kg/min may occur, but also in milking other animals such as sheep or goats, where relatively small flows of 20 to 50 g/min at the milking end may occur. The discharge from the container depends substantially on the hydrostatic pressure which the milk has at the discharge opening, said pressure being in turn a function of the filling level, and on the size of the discharge opening, and can be calculated on the basis of these values and/or by means of calibration. The milk flow into the container is substantially equal to a sum of the discharge from the container and the rise of the milk amount in the container per unit of time. The rise of the amount of milk in the container, which may also be negative if the filling level decreases, can be calculated from the temporal change of the filling level, if the form of the container is known. Consequently, the milk flow into the container during a milking process can be measured. In some embodiments, the control unit is adapted to enlarge the discharge opening if the filling level of the container exceeds a predetermined upper threshold value, and to reduce the size of the discharge opening if the filling level is lower than a lower threshold value. The control unit can be adapted to determine, on the basis of the milk flow calculated by the evaluation unit, when the upper threshold value is exceeded and/or the lower threshold value is not reached, that in one of the at least two sizes of the discharge opening a balance between the milk flow into the container and the milk flow out of the container is established and, if this is the case, to adjust this size of the discharge opening. By enlarging the discharge opening when the upper threshold value is exceeded and by reducing the discharge opening when the lower threshold value is underachieved, the filling level can be kept in a range of values in which exact measurements can be carried out. By setting a size of the discharge opening such that a balance between the milk flow into the container and the milk flow out of the container is established, a number of necessary adjusting operations of the size of the discharged opening can be reduced. In some embodiments of the invention, the means for changing the size of the discharge opening can be configured such that three or more different sizes of the discharge opening, which allow milk to flow through the discharge opening, are adjustable. By providing two or more possible sizes of the discharge opening, it is easier to establish a balance between the milk flow into the container and the milk flow out of the container, since there exist more possibilities to adapt the size of the discharge opening to the momentary milk flow into the container. In some embodiments of the invention, at least one or all of the adjustable sizes of the discharge opening allowing the milk to flow through the discharge opening can be configured such that, at a milk flow into the container lying in a sub-range of a range of 0.5 kg/min to 12 kg/min, a balance is established between the milk flow into the container and the milk flow out of the container when the filling level lies in the predetermined range. A measurement of milk flows in a range of 0.5 kg/min to 12 kg/min is often required in measuring the amount of milk yielded by a cow. By providing one or more adjustable sizes of the discharge opening allowing to establish a balance in a sub-range of this range particularly relevant for cows, it can be achieved that during the measuring process of the quantity of milk yielded by a cow over relatively long time periods the process can be performed while maintaining a balance between the supply into the container and the discharge from the container, thus requiring a very small number of switching operations for changing the size of the discharge opening. Thereby, in the case of an electrically operated apparatus, the power consumption of the apparatus can be reduced. Furthermore, the evaluation unit can be adapted to determine the total amount of milk yielded by an animal during the milking process by temporal integration of a milk flow from the container, calculated on the basis of the size of the discharge opening and the filling level. Since in some embodiments the milk flow from the container can be determined with higher precision than the milk flow into the container, a particularly accurate measurement of the total amount of milk yielded by the animal can be obtained. In some embodiments, the filling level measuring device comprises a pitot tube, a gas supply and a pressure measuring device. The pitot tube comprises a bottom end which is located in the interior of the container, below a minimum filling level of the container. The gas supply is adapted to feed a gas, in particular air, into the pitot tube so that the gas is discharged from the pitot tube at its bottom end. The pressure measuring device is adapted for measuring a differential pressure between the interior of the pitot tube and an area of the container above a maximum filling level. When the gas is discharged from the bottom end of the pitot tube, it must not only overcome the pressure being present in the upper area of the container, which can be lower than the ambient air pressure due to the connection of the container to the milking conduit to which the milking vacuum is applied, but also the hydrostatic pressure of the milk at the bottom end of the pitot tube. The higher the filling level of the container, the higher is this pressure. Thus, in the pitot tube a pressure is established which is substantially equal to the sum of the pressure in the interior of the container and the hydrostatic pressure. However, in the area of the container above the maximum filling level where there is no milk, only the interior pressure of the container prevails. Thus, by measuring the differential pressure, the hydrostatic pressure of the milk can be determined, said pressure being a measure for the filling level of the container. The differential pressure is measured between two gases so that the pressure measuring device must not enter into contact with the milk. Thereby, a caseation and/or a calcification of the pressure measuring device, which could occur if the pressure measuring device would come into contact with the milk, can be prevented. Furthermore, by separating the pressure measuring device from those parts of the apparatus that come into contact with milk, the risk of a mechanical damage to the pressure measuring device can be reduced. The gas supply can comprise an opening of the pitot tube which is in contact with the surroundings of the apparatus so that air from the surroundings is sucked into the pitot tube by the milking vacuum. Thus, the gas supply can be secured with a reduced complexity of the apparatus. The energy necessary for the gas supply is provided by the milking machine so that the apparatus does not need to have an energy source of its own for the gas supply. The pitot tube can include at least one notch on an edge of its open bottom end. The gas can flow through the notch out of the pitot tube and can rise upward through the milk in the form of regular little bubbles. Thereby, irregular formations of bubbles, which could lead to pressure fluctuations in the pitot tube, can be avoided. In some embodiments, the pressure measuring device comprises a differential pressure sensor adapted to measure a pressure gradient between a first portion of the differential pressure sensor and a second portion of the differential pressure sensor, a first conduit connecting the interior of the pitot tube to the first portion of the differential pressure sensor, and a second conduit connecting the area of the container above the maximum filling level to the second portion of the differential pressure sensor. In comparison to a measurement of the differential pressure with two pressure sensors that are independent from each other, the differential pressure sensor provides the possibility to prevent or at least reduce corruption of the measuring results due to slight differences between the pressure sensors, for example with regard to the linearity, the offset and the temperature course. In some embodiments, an end of the first conduit in the interior of the pitot tube and/or an end of the second conduit in the interior of the container may have a drip projection. Thereby, a penetration of milk or cleaning liquids into the conduits, which might cause corruption of the pressure measurements and damage to the differential pressure sensor, can be prevented. Between the first conduit and the differential pressure sensor and/or between the second conduit and the differential pressure sensor, a diaphragm made of a material which is permeable to gas and impermeable to liquids can be disposed. Thereby, the electronics of the differential pressure sensor can be protected against moisture, for example in the form of milky fog, cleaning liquid and/or condensation water. Additionally, the apparatus can comprise a heating device for heating the differential pressure sensor. Thereby, the pressure measuring sensor can be kept at a temperature which is a little higher than the ambient temperature, in order to suppress the formation of condensation water on the pressure measuring sensor. In some embodiments, the means for changing the size of the discharge opening can comprise a disk with at least two openings of a different size. The disk is disposed in front of an opening of the discharge unit and is movable relative to the opening of the discharge unit such that each of the at least two openings of the disk can be placed in front of the opening of the discharge unit by moving the disk so that the milk, when it is drained from the container, flows through one of the at least two openings of the disk disposed in front of the opening of the discharge unit. Furthermore, a disk driving means for moving the disk relative to the opening of the discharge unit can be provided. Thus, the size of the discharge opening can be adjusted by placing one of the at least two openings, which has a desired size, in front of the discharge unit. Thereby, the size of the discharge opening can be set in discrete intervals, thus achieving a high precision of the adjusted size. In some embodiments, the disk is rotatable about an axis perpendicular to a bottom face of the disk. The at least two openings of the disk are disposed around the axis, and the bottom face of the disk is in contact with a rim of the opening of the discharge unit. Since a relatively small amount of energy is necessary for rotating the disk, in particular if milk, which has relatively good lubrication properties, is present at the bottom face of the disk and on the rim of the opening, the power consumption of the apparatus can be kept low, enabling a battery operation with small and lightweight batteries, and a relatively weak and therefore lightweight disk driving means can be used. Thereby, an easy-to-use and robust apparatus can be achieved. The container can have a vertical direction. At least in an area of the container between a minimum filling level and a maximum filling level, a cross-sectional area of an interior space of the container in each plane which is perpendicular to the vertical direction and which intersects the container within the area can be constant, the axis about which the disk is rotatable being inclined towards the vertical direction of the container. Due to the vertical direction of the container which is defined by the constancy of the cross-sectional area between the minimum and the maximum filling level and which, for example in the case of a cylindrical container, can be the axis of the cylinder, a standard orientation of the container during operation of the apparatus, wherein the vertical direction is perpendicular, is determined. By the inclination of the disk, a volume in the vicinity of the discharge opening can be provided in which only a small residual volume of milk can accumulate. Thus, a complete evacuation of the container at the end of the milking process can be facilitated. In some embodiments, the axis about which the disk is rotatable, can be perpendicular to the vertical direction of the container. Thereby, the discharge unit can be mounted at the side wall of the container, thus enabling a reduction of the structural height of the apparatus. In some embodiments, the disk comprises an area where no opening exists, and the disk is movable relative to the opening of the discharge unit such that the area of the disk, in which no opening exists, can be brought in front of the opening of the discharge unit by moving the disk, in order to close the opening of the discharge unit. Thereby, the discharge of liquid from the container can substantially be completely prevented. This can be used to measure relatively small milk flows which may occur, for example, at the end of the milking process with sheep and goats. With small milk flows, the drainage from the container can substantially be completely prevented, and the milk flow can be determined on the basis of the rise of the filling level in the container. When milking animals which yield a total quantity of milk in the milking process that is smaller than the volume of the container of the apparatus, the discharge opening can also remain closed during the whole milking process. At small milk flows, an improved measuring precision can be achieved by measuring with a closed discharge opening, as compared to measuring with an open discharge opening. The possibility to substantially completely close the opening of the discharge unit, can also be used when cleaning the apparatus, in order to alternately fill the container substantially completely with a cleaning liquid, e.g. water, the discharge opening being closed, and to empty the same by opening the discharge opening. Thereby, even with relatively small quantities of water, an effective cleaning of the apparatus can be achieved. The area of the disk in which no opening exists can be disposed near the largest of the at least two openings. Thereby, switching between a closed discharge opening and a maximally opened discharge opening can be done quickly, which allows the container to be evacuated rapidly after finishing the milking procedure and when cleaning the apparatus. In some embodiments, the disk has a plurality of openings whose size is configured such that flow rates of the milk through two adjacent openings, respectively, at a predetermined filling level of the container, differ from each other by a predetermined flow rate difference. The flow rate difference between two adjacent openings has an influence on a reaction time, within which the decision has to be taken if the momentary size of the discharge opening is maintained or if a larger or smaller size of the discharge opening is to be set, by moving the disk such that an opening adjacent to the momentarily used opening of the disk is place in front of the opening of the discharge unit. When the difference of the flow rates between adjacent openings has the same value, the reaction time is independent from the milk flow into the container. Thereby, the control of the apparatus can be simplified. A relation between a difference of a quantity of milk in the container at a maximum filling level and a quantity of milk in the container at a minimum filling level on the one hand and the flow rate difference on the other hand can be greater than approximately 20 seconds. When at a minimum filling level of the container the size of the discharge opening is reduced by one stage, the time until the maximum filling level is reached, at which the switch-over to a larger opening is carried out in order to keep the filling level within a measurable range, is determined at least approximately by the relation indicated above. Accordingly, the time until the minimum filling level is reached is also at least approximately determined by this relation at the switch-over to a larger opening at a maximum filling level. When the relation is greater than approximately 20 seconds, the time between two successive switching operations can be greater than approximately 20 seconds so that a fast switch-over, which could imply a high power consumption of the apparatus, can be prevented. In some embodiments, the means for changing the size of the discharge opening can comprise a shut-off means with an adjustable opening. Thereby, a substantially continuous adjustment of the size of the discharge opening is possible. In some embodiments, the means for changing the size of the discharge opening can comprise two or more openings of the container and two or more closing mechanisms. Each of the closing mechanisms is adapted to close and expose an opening of the container allocated to the closing mechanism. The discharge opening is formed by the entirety of the exposed openings of the container. The control unit is adapted to control the closing mechanisms in order to adjust the size of the discharge opening by closing and/or exposing one or a plurality of the openings of the container. In some embodiments, the means for changing the size of the discharge opening can comprise a disk. The disk is rotatable about an axis perpendicular to the lower face of the disk, which is disposed near an opening of the discharge unit. A radius of the disk from the axis to the rim of the disk increases as a function of an angle about the axis from a first value, which is smaller than or equal to a distance from the axis to a rim of the opening of the discharge unit at the side facing the axis, to a second value, which is greater or equal to a distance from the axis to a rim of the opening of the discharge unit at the side facing away from the axis. Depending on the position of the disk, the disk can expose or totally or partly cover the opening of the discharge unit. The larger the part of the opening covered by the disk is, the more the milk flow through the opening is restricted by the disk. The part of the opening of the discharge unit not covered by the disk forms a discharge opening whose size can be changed by rotating the disk about the axis. The apparatus can comprise an inclination sensor, and the evaluation unit can be adapted to carry out a correction of the calculated milk flow on the basis of the inclination measured by the inclination sensor. Thereby, corruption of the measurement of the milk flow by holding the apparatus obliquely can be reduced. The apparatus can additionally comprise a centrifuge head for separating milk and transport air, wherein the inlet of the centrifuge head is connectable to the milking cluster of the milking machine, and a milk outlet of the centrifuge head leads into the container. Furthermore, the apparatus can comprise a bypass line directing the air from the centrifuge head, at a side of the discharge opening facing away from the container, into the discharge unit. Thereby, corruptions of the measurement through admixtures of air in the milk and/or undesired pressure differences upstream and downstream of the discharge opening can be reduced. Furthermore, in this manner flow losses caused by the apparatus can be reduced, thereby improving the milking speed and/or the quality of the milk. In some embodiments, the discharge unit can comprise a collection chamber which comprises a first inlet which is connected to the discharge opening, a second inlet which is connected to the bypass opening, and an outlet connectable to the milking line of the milking machine. Furthermore, the apparatus can comprise a means for closing the outlet. By closing the outlet of the collection chamber, the flow of air and milk through the apparatus can be interrupted, thus finishing the milking process. The termination of the milking process can thus be performed by means of the apparatus for measuring the quantity of milk yielded by the animal during milking whereby additional equipment, such as for example a vacuum shut-off valve or a pneumatic hose clamp, are no longer needed. In some embodiments, the evaluation unit can be adapted to determine, for calculating the milk flow into the container, a temporal change of the quantity of milk in the container on the basis of a temporal change of the filling level in the container, to determine a discharge rate from the container on the basis of the size of the discharge opening and of the filling level, and to calculate a sum of the temporal change of the quantity of milk in the container and of the discharge rate. Thereby, a precise determination of the momentary milk flow into the container can be performed. A method according to the invention for measuring a quantity of milk yielded by an animal during a milking process comprises supplying the milk yielded by the animal during the milking process into a container. The filling level of the milk in the container is measured. The size of a discharge opening, through which the milk can flow out of the container, is changed in dependence on the measured filling level of the milk in the container. To this purpose, a discharge opening is used, which is configured such that at least two different sizes of the discharge opening which allow milk to flow through the discharge opening are adjustable. The size of the discharge opening is set such that the filling level of the milk in the container remains within a predetermined range. The milk flow into the container is calculated on the basis of the set size of the discharge opening and of the filling level measured by the filling level measuring device. In some embodiments, the size of the discharge opening is enlarged if the filling level of the container exceeds a predetermined upper threshold value, and the size of the discharge opening is reduced if the filling level of the container drops below a predetermined lower threshold value. If the upper threshold value is exceeded and/or the lower threshold value is not reached, it is determined on the basis of the calculated milk flow into the container if at one of the at least two sizes of the discharge opening, a balance between the milk flow into the container and the milk flow out of the container is established. If this is the case, this size of the discharge opening is set. The discharge opening can be configured such that three or more different sizes of the discharge opening which allow milk to flow through the discharge opening can be set. At least one or all the adjustable sizes of the discharge opening, which allow milk to flow through the discharge opening, can in some embodiments be configured such that, at a milk flow into the container in a sub-range of a range from 0.5 kg/min to 12 kg/min, a balance between the milk flow into the container and the milk flow from the container is established, a filling level being in the predetermined range. Furthermore, by temporal integration of a milk milk flow from the container, calculated on the basis of the size of the discharge opening and of the filling level, a total quantity of milk yielded by the animal during the milking process can be determined. The measurement of the filling level can be performed by means of a pitot tube having an open bottom end which is located below the minimum filling level in the interior of the container. A gas, in particular air, can be supplied to the interior of the pitot tube, so that the gas flows out of the pitot tube at a lower end thereof, and a differential pressure between the interior of the pitot tube and an area of the container above a maximum filling level can be measured. On the basis of the differential pressure, the filling level can be calculated. The changing of the size of the discharge opening can comprise moving a disk with two or more openings of different size. The disk is mounted in front of an opening of the discharge unit. One of the openings of the disk is placed in front of the opening of the discharge unit, so that the milk can flow through the opening of the disk out of the container. In some embodiments, the container can comprise two or more openings. The process of changing the size of the discharge opening can comprise closing and/or exposing one or a plurality of the openings of the container. In such embodiments, the discharge opening is constituted by the entirety of the exposed openings of the container. In some embodiments, changing the size of the discharge opening comprises rotating a disk, which is rotatable about an axis perpendicular to the lower face of the disk. The axis is disposed near an opening of the discharge unit. A radius of the disk from the axis to the rim of the disk increases as a function of an angle about the axis from a first value, which is smaller than or equal to a distance from the axis to a rim of the opening of the discharge unit at the side facing the axis, to a second value, which is greater than or equal to a distance from the axis to a rim of the opening of the discharge unit at the side facing away from the axis. The calculation of the milk flow can comprise determining a temporal change of the quantity of milk in the container on the basis of a temporal change of the filling level, and determining a discharge rate from the container on the basis of the size of the discharge opening and of the filling level, and a sum of the temporal change of the quantity of milk in the container and of the discharge rate can be calculated. The inclination of the container can be measured, and a correction of the measured milk flow on the basis of the measured inclination can be carried out. Embodiments of the invention will be described with reference to the accompanying drawings, in which: FIG. 1 is a schematic outline drawing of an apparatus for determining the milk yield of cows according to prior art; FIG. 2 is a schematic cross-sectional view of an apparatus for measuring the quantity of milk yielded by an animal during a milking process according to an embodiment of the present invention; FIG. 3 is a schematic perspective view of an apparatus for measuring the quantity of milk yielded by an animal during a milking process according to an embodiment of the present invention; FIG. 4 is a schematic outline drawing of a disk which can be used with means for changing a size of a discharge opening in an apparatus according to the present invention; FIG. 5 is a schematic cross-sectional view of an apparatus for measuring the quantity of milk yielded by an animal during a milking process according to a further embodiment of the present invention; FIG. 6a shows a schematic top view of means for changing a size of a discharge opening in an apparatus according to an embodiment of the present invention; FIGS. 6b and 6c show schematic cross-sectional views of the means represented in FIG. 6a for changing the size of a discharge opening; and FIG. 7 shows a schematic top view of means for changing the size of a discharge opening in an apparatus according to an embodiment of the present invention. FIG. 2 shows a schematic cross-sectional view of an apparatus 200 for measuring the quantity of milk yielded by an animal during a milking process according to an embodiment of the present invention. A schematic perspective view of the apparatus 200 is shown in FIG. 3. The apparatus 200 comprises a container 201 adapted to receive milk therein. Furthermore, the apparatus 200 comprises a supply unit 202 configured to feed milk into the container 201. The supply unit 202 is connectable to a milking unit of a milking machine via a port 205. Thereby, a mixture of milk and air can be supplied from the milking machine to the supply unit 202. In some embodiments, the supply unit 202 can comprise a centrifuge head 250, suited for separating the air feeded in by the milking machine from the milk. The centrifuge head 250 comprises a cup 203 which can have a substantially rotationally symmetric form with a symmetry axis 251. The port 205 ends in a inlet opening 204 in the upper part of the cups 203. At a lower end of the cups 203, there is located a milk discharge opening 206 through which the milk from the cup 203 can enter into the container 201. The cup 203 is tapered from the upper part, in which the inlet opening 204 is located, to the lower end, where the milk discharge opening 206 is located. The inlet opening 204 is disposed such that the mixture of milk and air from the milking machine enters in a tangential direction, which can be substantially perpendicular to the symmetry axis 251 and be substantially parallel to an inner wall of the cups 203, into the interior of the centrifuge head 250. Thereby, a rotational movement of the mixture of milk and air about the symmetry axis 251 is obtained. During the rotational movement, the milk is pressed onto the inner wall of the cups 203 while the air, which has a lower density than milk, moves towards the symmetry axis 251. The milk can flow through the milk discharge opening 206 into the container 201, as soon as its rotational speed, due to friction with the inner wall of the cups 203, has slowed down to such an extent that the gravitational force which draws the milk downwards, prevails over the centrifugal force which pulls the milk, due to the conical form of the cup 203, upwards. The air can enter into an opening 208 at the upper end of a bypass line 207 which can be disposed such that the symmetry axis 251 extends through the opening 208. Due to an annular structure 209 which is disposed around the opening 208, it can be additionally prevented that milk droplets enter into the opening 208. Thus, since the milk is supplied to the container 201 while the air flows into the bypass line 207, a separation of milk and air can be effected. In some embodiments can below the milk discharge opening 206 in the container 201, a distribution plate and/or a sieve grid (for the sake of clarity not represented in FIG. 2) can be provided. Features of the distribution plate and of the sieve grids are explained in more detail below with reference to FIG. 5. The apparatus 200 further comprises a discharge unit 211, configured for discharging milk from the container 201. The discharge unit 211 can be connected via a port 212 with a milk line of the milking machine which, during the operation of the milking machine, has a milking vacuum applied thereto. The discharge unit 211 comprises a collection chamber 240. A lower end 210 of the bypass line 207 leads into the collection chamber 240. The bypass line 207 can, in some embodiments, extend substantially vertically through the milk discharge opening 206 of the centrifuge head 250 and through the container 201. In the proximity of the milk discharge opening 206 the bypass line 207 can be narrowing. Thereby, a limitation of the milk flow through the milk discharge opening 206 caused by the bypass line 207 can be reduced. In other embodiments, the bypass line 207 can be disposed differently, which will be explained in more detail below with reference to FIG. 5. The discharge unit 211 further comprises an opening 215 which leads at the bottom of the container 201 into the interior of the container 201. Milk from the interior of the container 201 can enter into the collection chamber 240 via the opening 215. Thereby, the milk and the air which have been separated in the centrifuge head 250 are brought together again in the collection chamber 240. The collection chamber 240 is connected via the bypass line 207, the interior space of the centrifuge head 250 and the milk discharge opening 206 to the upper part of the container 201, so that a pressure balance between the collection chamber 240 and the container 201 can take place. When the port 212 is connected with a milking line of the milking machine, to which a milking vacuum is applied, the upper part of the container 201 is exposed to a pressure that substantially equals the milking vacuum. When milk, which enters via the milk inlet opening 206 into the container 201, collects in the interior of container 201, the hydrostatic pressure exerted by milk is effective at the bottom of the container 201, additionally to the milking vacuum which prevails in the upper part of the container 201 above the surface of the milk. The higher the level of the milk in the container 201, the greater is the hydrostatic pressure. Since the milking vacuum is applied to the collection chamber 240, a differential pressure exists between the side of the opening 215 in the interior of the container 201 and the side of the opening 215 in the collection chamber 240, said differential pressure being equal to the hydrostatic pressure of the milk in the container 20, irrespective of how strong the milking vacuum is. The pressure which drives the milk through the opening 215 is therefore substantially independent from the exact value of the milking vacuum, whereby effects of fluctuations of the milking vacuum on the milk flow through the opening 215 can be prevented. The discharge unit 211 comprises means 213 for changing the size of a discharge opening through which the milk, when it is being discharged from the container 201, flows into the collection chamber 240. In the embodiment shown in FIG. 2, the means 213 comprise a disk 214. A view of the disk 214 is shown in FIG. 4. The disk 214 has a circular shape and comprises openings 401 to 407 disposed around the centre of disk 214. In the centre of disk 214, a borehole 409 can be provided through which the axis 217 on which the disk 214 is mounted and about which the disk 214 is rotatable, can be passed. The openings 401 to 407 have different sizes, wherein the size of the openings 401 to 407 decreases from the opening 401 to the opening 407 in a direction opposite to the clockwise direction. In other embodiments, the size of the openings 401 to 417 can also decrease in a clockwise direction. The openings 401 to 407 can be located, respectively, in a sector of the disk 214. In the embodiment represented in FIG. 4, in seven out of eight sectors of substantially equal size of the disk 214, one of the openings 401 to 407 is located, whereas in the eighth sector 408, no opening is present. The eighth sector 408 forms an area of the disk where no opening is provided and which is disposed between the smallest opening 407 and the largest opening 401. The disk must not comprise seven openings. In other embodiments, a larger or smaller number of openings can also be provided, wherein the number of openings can be more than or equal to two. In some embodiments, three or more openings can be provided. Likewise, the disk 214 must not be circular. In other embodiments it can, for example, have a polygonal shape. The smaller ones among the openings 401 to 407, for example the openings 405, 406 and 407, can be substantially circular whereas the larger openings 401 to 404 can have an elongated shape tapering towards the centre of the disk 214. Thereby, within the limits of the sectors of disk 214 a larger cross-sectional area among the openings 401 to 417 can be provided than would be possible with the same number of sectors and the same radius of the disk 214 with circular openings. In other embodiments, however, all the openings 401 to 407 can be circular or have another shape. The disk 214 is disposed in front of the opening 215. The lower face of the disk can be in contact with a rim of the opening 215, whereby it can be prevented that milk flows through the space between the disk and the rim of the opening 215 out of the container 201 and into the collection chamber 240. The opening 215 can, in some embodiments, be disposed at the end of a protrusion 216 at the bottom of the container 201. Thereby, in comparison to embodiments in which the disk 214 rests on the bottom of the container 201, a contact surface between the disk 214 and the rim 216 of the opening 215 can be reduced, and consequently a friction between the disk 214 and the rim of the opening 215 can be reduced. Furthermore, a hygienically critical development of a lubricant film between the disk 214 and the rim 216 of the opening 215 can be substantially prevented or at least reduced, since a relatively great part of the side of the disk 214 which faces the opening 215 can be flushed by a liquid, and due to the rotation of the disk 214, liquid between the disk 214 and the rim 216 of the opening 216 can be exchanged. The disk 214 can be rotated about an axis 217 perpendicular to a lower face of disk 214 by means of a disk driving means 218. The axis 217 can be mounted near the opening 215, wherein the distance between the axis 217 and the rim of the opening 215 facing the axis 217 can be smaller than the radius of the disk 214. Thereby, by rotating the disk 214, one of the openings 401 to 407 of the disk can be placed in front of the opening 215 of the discharge unit 211. The opening 215 can be shaped such that it can be completely covered by one of the sectors of disk 214, so that milk can flow through exactly one of the openings 401 to 407 which is being placed in front of opening 215, out of container 201 into collection chamber 240. In some embodiments, the opening 215 can have the shape of a sector of a circle whose centre coincides with the position of the axis 217, and whose opening angle can be equal to a relation between an angle of 360° and the number of the sectors of the disk 214. The one of the openings 401 to 407 of the disk that is being placed in front of the opening 215 of the discharge unit 211 forms a discharge opening through which the milk can flow out of the container 201. The size of the discharge opening can be varied by rotating the disk 214 and by positioning another one of the openings 401 to 407 in front of the opening 215 of the discharge unit 211. The drainage of milk from the container 201 can be stopped by positioning the sector 408 without opening in front of the opening 215 of the discharge unit 211, so that the latter is substantially completely covered by the disk 214. The disk driving means 218 can, in some embodiments, comprise a stepping motor wherein, in some embodiments, a gearing for providing a reduction between the rotational speed of the stepping motor and the rotational speed of the disk 214 can be provided. The disk driving means 218 can be configured such that the time period needed for positioning an opening adjacent to the opening momentarily disposed in front of the opening 215 of the disk 214 in front of opening 215 lies in the range of approximately 0.1 seconds to approximately 0.3 seconds. Due to the relatively good lubrication properties of milk and the relatively low pressure force on disk 214, which at most can be equal to the hydrostatic pressure on a sector of disk 214, and due to the relatively low inertia moment of disk 214, which in some embodiments can be made of stainless steel, can have a diameter of approximately 60 mm and a thickness of approximately 0.6 mm, this can also be achieved with relatively low-powered and light-weight stepping motors. Due to the differential pressure between the milk at the bottom of the container 201 and the collection chamber 240 which, as explained above, is substantially equal to the hydrostatic pressure of the milk in the container 201, the milk flows through the discharge opening. Since the hydrostatic pressure increases when the filling level of the milk in the container 201 rises, the milk flow through the discharge opening increases
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