MXPA06008246A - Fluid metering with a disposable membrane type pump unit - Google Patents

Abstract

A disposable pump unit (100) for receiving and metering a predetermined volume of fluid has a body with a surface at which opens the mouth of a cavity (7) formed in the body. An inlet port (4) for the fluid opens at the surface adjacent to the mouth of the cavity (7) and a plurality of outlets (9) are provided in the cavity (7) that open to a fluid flow passageway extending through the body and connecting the cavity (7) to an outlet port (10). A flexible membrane (8) overlies the cavity (7), the inlet port (4) and the outlet port (10) and is sealingly secured at its periphery to the surface. A re-usatile pump actuating unit (200) is co-operable with the pump unit (100) to position the membrane (8) between the cavity (7) in the pump unit (100) and a matching cavity (21) in the actuating unit (200). The membrane (8) is towards and away from the cavity (7) by connecting the cavity (21) to a source of positive and negative fluid pressure via a port (18). The actuating unit (200) has an armature (19) operable to displace the membrane (8) to close the inlet port (4) when the outlet port (10) is open and an armature (20) operable to displace the membrane (8) to close the outlet port (10) when the inlet port (4) is open. In use, the pump unit (100) draws fluid into the cavity (7) when the inlet port (4) is open and a negative fluid pressure is applied to the chamber (21) and pumps the fluid out of the cavity (7), when the inlet port (4) is closed and a positive fluid pressure is applied to the chamber (21).

Description

FLUID MEASUREMENT WITH A DISPOSABLE MEMBRANE TYPE PUMP UNIT This invention relates to improvements in the measurement of fluid and especially, but not exclusively, to the sanitary measurement of highly viscous fluids, for example beverage concentrates. In the food industry, for example, there is often a requirement to distribute a measured quantity of liquid food product for use as such or in admixture with a diluent such as water. The importance of having the ability to distribute fluids in a sanitary manner where there is no possibility of external contamination of the liquid food product or where the companies wish to have the ability to ensure the integrity of the fluid that is distributed has increased. Commonly, the food product is supplied in a disposable container. To meet the sanitary requirements, it is desirable that the fluid has contact only with the disposable parts of the system, including the pump used to distribute the fluid. For this to be economically viable, the pumping method must be simple and therefore relatively inexpensive to produce. In the food supply, and particularly in the beverage industry, there is a common requirement to distribute an exact logometric mixture of a concentrate and a diluent. Common technologies involve measuring the flow of the concentrate and then varying the flow of diluent to achieve the correct mixture. This has the disadvantage that it involves the measurement of the fluid flow, in particular, of the concentrate. Common methods include the use of turbine flowmeters and differential pressure flowmeters. These techniques, however, are not effective in measuring the flow of a highly viscous concentrate such as an orange juice concentrate. The common methods to handle these viscous fluids are to measure them using peristatic pumps or diaphragm pumps integrated in distributors. Although these methods work well for many relatively viscous fluids, they do not work well for highly viscous fluids. For example, peristatic type pumps typically become less effective for fluids having a viscosity that exceeds approximately 5000 centipoise. Non-disposable diaphragm pumps can be effective for dispensing highly viscous fluids but distributor valve installations for disposable sanitary diaphragm pumps are commonly such that the resistance to fluid advancement makes them unsuitable for use with highly viscous fluids . Examples of disposable sanitary diaphragm pump designs are proposed in the U.S. Patent. No. 6,485,263.

However, these designs use somewhat tortuous fluid inlet paths and additionally require simultaneous control of different pressures for the three pump parts, inlet valve, pump chamber and outlet valve (eg, as shown in Figure 10) or alternatively use solenoid valves (eg, as shown in Figure 1) that are intended to be disposed of with the other disposable components of the pump. The tortuous entry path shown in the proposal in Figure 10 would create a considerable resistance to advance in a highly viscous fluid and would obstruct the performance of the pump resulting in a very limited pump in its speed of operation. This problem could be solved by pressurizing the source of fluid that is distributed by applying pressure internally to the fluid reservoir or by having a flexible reservoir and applying pressure externally. However, in sanitary systems, the application of internal pressure is not acceptable and the application of pressure in an external way implies a much more complex and expensive system. The use of solenoid valves in the waste pump unit, as shown in Figure 1, has the disadvantage that they are expensive. The purpose of the present invention is to provide an improved sanitary fluid measurement device that incorporates a disposable pump arrangement that is relatively cost effective and capable of measuring a highly viscous fluid. In accordance with the first aspect of the present invention, a disposable pump unit is provided for receiving and measuring a predetermined volume of fluid, the pump comprising a body having a surface on which the inlet of a cavity formed in the body is opened, an inlet port for the fluid that it opens on the surface adjacent to the entrance of the cavity, whereby, when the inlet port is opened, the fluid can flow from the inlet port into the cavity through the entrance thereof, a first flexible membrane secured in a sealed manner on its periphery to the surface and superimposed on the cavity and the inlet port, an outlet port for the fluid, there being a passage of fluid flow that extends through the body connecting the cavity to the outlet port , and a second flexible membrane secured in a sealed manner on its periphery and superimposed on the outlet port, serving the portions of the first and second flexible membranes, where they overlap the portals. input and output respectively, as closures for the ports. The outlet port, preferably, also opens on the aforementioned surface, in which case the first and second flexible membranes can be integral with each other. Preferably, at least the first flexible membrane, ie, the membrane that overlaps the cavity and the inlet port formed in the body, is substantially non-stretchable and is dimensioned so that, during the fluid measurement step, it can be driven by the driven fluid in contact with substantially the entire surface of the wall of the cavity whereby substantially all of the fluid withdrawn from the reservoir during the fluid filling stage is pumped out during the fluid measurement step. For that purpose, the first flexible membrane (and the second flexible membrane where the two are integral with each other), for example, may comprise a polyamide film material. In a preferred embodiment, at least the first flexible membrane comprises a flexible sheet or sheet of an integrated laminate comprising a non-stretchable polymer, for example a polyamide, and an underlying heat-weldable polymer, for example, a food-grade polyethylene, the latter having been fused with heat to the body in order to be sealed in a sealed manner thereto as mentioned above with the body comprising a mold in a food-grade compatible polymer, for example a polyethylene. An example of a suitable laminate is the S77 available from Amcor Flexibles Baricol. Preferably, the lamination is made in a configuration substantially similar to the configuration of the surface of the pump cavity, so that it substantially completely evacuates the cavity without the need to stretch the membrane material. Preferably, during storage and transportation, the preformed shape of the laminate remains level with the concavely curved surface of the pump cavity thereby reducing the susceptibility of the membrane to damage during transit. In use, and in accordance with a second aspect of the invention, the disposable pump unit of the invention is removably coupled to a reusable pump actuator, sealingly connected to said surface of the pump actuator, comprising a source of positive and negative pressure driving fluid, preferably air, and first and second valve actuation means associated respectively with closing the inlet port and closing the outlet port, the arrangement being such that, when the The outer surface of the first flexible membrane (which acts as a pumping membrane) is exposed to the negative pressure fluid source, with the inlet port open and the outlet port closed, leaving the body of the disposable pump whereby the fluid, such as concentrate for beverages, it leaves from a reservoir thereof through the inlet port in substantially all of the space defined by the cavity and the first flexible membrane (the "fluid filling stage"), then, with the inlet port closed, and the open outlet port, when the positive pressure fluid is applied to the outer surface of the first flexible membrane, the membrane is pushed back out and into the cavity and pumps the fluid from the cavity through said passage to the output port ("the fluid measurement stage"). There may be a downstream flow restrictor to allow the achievement of the same fluid measurement ratio for different fluid viscosities with the application of the same positive fluid pressure of Alternatively, the fluid pressure could simply vary or a combination of variable flow downstream and variable fluid pressure could be used. it is aforementioned variables and / or the pressure allows the outflow of the pumped fluid to vary, or alternatively, they allow the outflow to remain substantially constant while changing the properties of the fluid, for example the temperature. The first valve actuation means with the inlet port closure is preferably an axially movable armature that extends towards the volume subject to positive and negative pressure and a seal is provided wherein the armature splices in that volume; preferably the seal is a rotating seal type diaphragm. The second valve actuating means, similarly, is preferably an axially movable armature and a similar seal can also be provided. The provision of these seals further prevents the ingress of dust into the valve actuators and allows the use of cleaning fluids without the danger of fluids affecting the valve actuators, which can be electrically activated. Preferably, the disposable pump unit of the invention comprises a body having a plurality of cavities each having respective inlet ports, outlet ports and flexible membranes whereby, when coupled to a pump actuator, the fluid can be introduced into the pump. at least one of the cavities while simultaneously being pumped from another. In this way, the unit, if required, can be used to measure variable predetermined volumes of fluid in a substantially continuous and efficient manner. Preferably, the body has a pair of cavities. When there is a plurality of cavities, the simultaneous steps of fluid filling and fluid measurement can be of different duration so that when one stage of distribution is completed from one of the cavities, the other is ready to start, or already has begun, its distribution stage; this can be achieved, for example, by suitably adjusting the negative and positive pressures of the drive fluid by means of adjustable pressure regulators. Preferably, in the disposable pump unit of the invention, the fluid flow passage extending through the body and connecting the cavity to the outlet port terminates at one end in a generally concave wall defining the cavity, having the cavity formed therein a plurality of passages communicating with the fluid flow passage to inhibit, during the fluid measurement step, the formation of occluded regions of fluid between the wall of the cavity and the first flexible membrane and securing so that substantially all of the fluid leaving its reservoir is pumped during the fluid filling step during the fluid measurement step. Each of the plurality of passages is preferably a slot. The combination of the disposable pump unit of the invention and a reusable pump actuator can constitute a beverage dispenser as described more particularly in our co-pending PCT application of the same date, the pump unit serving to measure a predetermined amount of a beverage concentrate, for example an orange concentrate, which is then mixed with water, preferably in a predetermined ratio, supplied by the distributor. For that particular purpose, the body of the disposable pump unit preferably incorporates a diluent inlet, eg, water, which communicates with an outlet passage formed in the body connected to the outlet port, whereby, since the Fluid flows from the outlet port through the outlet passage, mixes with the diluent and is then distributed in a receptacle such as a cup or glass. Preferably downstream of the outlet port of the pump cavity and immediately upstream of the diluent inlet, there are a number of flow path obstructions adapted to moderate the pumped viscous fluid to help mix with the diluent. The exit passage preferably includes means, for example a static turbulator, to assist in the mixing of the fluid and the diluent. In addition, means for adjusting the flow rate of the diluent and the feedback means can be provided in order to ensure substantially constant logometric mixing at a target distribution rate. The pump actuator includes a complementary surface that is connected to the aforementioned surface of the disposable pump unit. Both surfaces are substantially flat. An essential function of the pump actuator surface is to control the degree to which the first flexible membrane can exit from the disposable pump body and consequently in part, to define the predetermined measured fluid volume. Preferably, the pump actuator surface also has at least one opening (the number corresponding to the number of cavities in the body of the disposable pump) defined therein to receive the first flexible membrane during the fluid filling stage, serving the opening wall to limit the movement of the membrane. The cavity (s) of the disposable pump unit and the opening (s) (if any) of the pump actuator are preferably concave in shape. Preferably, each pump actuator has associated with each opening, a means for detecting whether the or each opening is full of fluid or is empty. Preferably, the detection means comprises ultrasonic transducers, whose variation in signals indicates the volume of fluid within each opening. Another function of the pump actuator is to provide drive means for the closing of the inlet and outlet ports. The drive means for the inlet preferably comprises a solenoid operated armature which, by means of a compression spring, drives part of the respective flexible membrane in sealing contact with the inlet port in order to close it, but which assumes, when the solenoid is energized, a position spaced from the membrane when the port is required to be open. It will be recognized by those skilled in the art that other conventional driving means, for example tires, could be used to activate the armature in place from the solenoid described above. In a preferred arrangement, the valve actuation means associated with the closing of the outlet port is capable of affecting a variable, pre-selected degree of opening of the outlet port. This can be achieved by using, for example, a stepper motor or a variable end stop solenoid associated with an armature or other actuator acting on the second flexible membrane where it is superimposed on the output port. This feature allows the disposable pump unit to control the output flow rate as desired, depending on the viscosity of the fluid being measured. For example, with a fluid of relatively higher viscosity, it may be desirable to have the outlet fully open in order to achieve the desired output flow ratio and thus the mixing ratio control required with a diluent such as water, while that with a relatively lower viscosity fluid, it may be desirable to have the outlet only partially open for that purpose. For example, alternatively, the armatures can be operated pneumatically. In order to improve the fluid seal between the ports and the flexible membranes in the closed port position, each port is preferably surrounded by a raised edge. Preferably, the drive means are provided with soft edges, for example of a silicone rubber, which do not damage the membrane and provide uniform pressure at the raised edge. The disposable pump unit is permanently connected to, or integral with, a reservoir containing the fluid so that, once the reservoir is emptied or otherwise needs to be changed, the combination of reservoir and pump unit is disconnected from the reservoir. pump actuator and can be discarded. Then a tank / pump unit replacement can be connected to the pump actuator. Preferably, a closure is provided between the pump unit and the reservoir so that the reservoir and the disposable pump unit can be transported together while preventing the migration of fluid in the disposable pump unit. Once in place and connected to the pump actuator, the closure moves from a closed position in which the flow between the reservoir and the disposable pump unit is blocked, to an open position in which fluid can flow from the reservoir. deposit to the disposable pump unit. Especially in the context of distributing beverages, the tank and the disposable pump unit are preferably cooled by a cooling system comprised in the reusable part of a beverage dispensing machine and which can also serve to cool said diluent. In a preferred arrangement, the action of loading and unloading the disposable pump unit of the pump actuator automatically opens and closes the closure, respectively. It will be appreciated that the measured fluid comes into contact only with the components of the disposable pump unit, and consequently, that the pump actuator can be reused continuously without the need to wash it regularly. In this way, the pump actuator will be, for example, part of a fixed beverage machine installed in a bar, restaurant or the like, it being possible for a given machine to distribute different beverages depending on the nature of the fluid concentrate contained in a tank / disposable pump unit selected. Because different concentrates will commonly require different degrees of dilution, the disposable pump unit preferably includes an identification means and the reusable pump actuator includes reading means for automatically reading the identification means, whereby the pump / Combined pump actuator, for example, a beverage dispensing machine, can adapt its distribution mode, eg, diluent flow ratio, and / or provide the user with audio and / or visual information depending on the identified characteristics. Such characteristics may include, for example, one or more of the viscosities of the fluid to be measured in a particular case, its type (eg, orange juice or other), its storage / expiration date and the desired dilution ratio. . The identification means and the reading means can be based, for example, on radio frequency identification (RDIF) technology, integrated circuits Electro-Programmable-Read Only Memory (EEPROM) Erasable-Programmable Electro-only Reading Memory), bar code or color detection technology, whose general nature is known. Preferably, the reusable pump actuator has associated therewith a read / write device capable of both reading information from an identification means associated with the disposable pump unit and of writing information to said identification means. According to the third aspect of the present invention, there is provided a disposable pump unit as defined above adapted to mix two or more fluids, especially viscous fluids, the body defining two or more of said cavities and an inlet port associated with each cavity and with reservoirs for the respective fluids, and a common outlet associated with the cavities, whereby the fluids, in association with a reusable pump actuator, can be distributed and mixed simultaneously. Such a unit has a number of different applications, and mention is made by way of example of the mixing of the two epoxy resin precursor materials (e.g., "Araldite" - Registered Trademark). The embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings in which: Figure 1 is a perspective view of a disposable pump unit of the invention; Figure 2 is a longitudinal cross section of the disposable pump unit of Figure 1; Figure 3 is a perspective view of a pump actuator for mounting with the pump unit shown in Figures 1 and 2; Figure 4 is a cross section of the assembled pump and pump drive unit; Figure 5 is a perspective view of the pump unit shown in Figure 1 which additionally has a diluent inlet; Figure 6 is a view similar to Figure 5, but in which the pump outlet has a convoluted integral path mixing section; Figure 7 is a perspective view of a disposable pump unit of the invention showing the channels provided for prevention of occluded volumes of fluid in the pump; Figure 8 is a perspective view of a disposable pump unit of the invention showing the closure between the pump unit and the reservoir; and Figure 9 is a perspective view of a pre-formed membrane for use with the disposable pump unit. With reference to Figures 1 and 2, a disposable dual chamber pump unit 100 is shown. A fluid inlet 14 is separated to feed each of the two pump cells la, Ib comprised of a rigid body 2 having, on a substantially flat surface thereof, an area 3 containing a chamber inlet port 4, the inlet port 4 being surrounded by a raised edge 5, and a concave cavity 6 defining one side of a pump chamber 7. The second side of the chamber 7 comprises a membrane 8 made of a flexible sheet material, eg, low density polyethylene (LDPE), secured around its periphery to the aforementioned body surface 2 in order to contain each entrance area of flow 3 and their respective concave cavities 6 so that the fluid can pass from the inlet port 4, when opened, to the respective concave cavities 6. Located in each concave cavity 6 of each pump chamber the, lb, is enclosed There is an arrangement of chamber outputs 9. Each chamber outlet 9 is in fluid communication with an exit port 10 that can be closed, surrounded by an elevated edge 11. The flow paths of the two exit ports 10 that can be closed , converge together in a single outlet 12. The two exit ports 10 that can be closed and the outlet 12 are together sealedly enclosed by a membrane 13 comprising a flexible sheet material, shown integral with the membrane 8, secured around from its periphery to the surface of body 2 mentioned above. With reference to Figure 3, there is shown a non-disposable pump drive unit 200 for the dual chamber pump unit 100. The drive unit 200 comprises a rigid body 15 containing two concave cavities 16, each surrounded by a sealing seal 17. The concave cavities 16 and the seal seal 17 are configured in such a way that they fit into the configuration of the pump cells la, Ib so that when they are placed in contact with them, they form a seal around them. the circumference of the pump cells the, lb. located inside each cavity 16 is a compressed air inlet / outlet port 18 defined in part by transverse channels extending over a substantial base area of the cavity 16. It is also located within each cavity 16 a armature operated by solenoid 19 extending through the body 15 and into the cavity 16. A pair of armatures 20 also extend through the body 15 adjacent the cavities 16. With reference to Figures 1, 2 and 4, the pump drive unit 200 is shown in Figure 4 releasably connected to the disposable pump unit 100 to form a complete pump. The cavity 16 in the unit 200 together with the membrane 8 forms a drive chamber 21 which can alternatively be connected to negative and positive pressure air supplies through a passage 22. Each cavity 16 in the pump drive unit 200 and its opposite cavity 6 in the disposable pump unit 100 jointly define a fixed volume of fluid that will be displaced in each cycle of the pump. The operating sequence of the pump is that each armature 20 extends in order to drive the membrane 13 locally on the respective raised edges 11 of the outlet ports 10 thus closing the outlet of the pump chamber, and the armature 19 is located separated from the membrane 8 so that the flow path between the inlet port 4 and the concave cavity 6 is opened. Armatures 19 and 20 have associated seals 19a, 20a that prevent the entry of any substance beyond the induced ones. A first source of pump drive fluid at a negative pressure, ie, below ambient pressure, is connected to the drive fluid port 18 through passage 22, the application of negative pressure causing the flexible membrane 8 leaves towards and inside the cavity 16 whereby the fluid in the latter is released from a reservoir (not shown) through the inlet 14 and the inlet port 4, the inlet port 4 being open by the negative pressure tending to raise the membrane 8 locally away from the inlet port 4. The channels in the transverse configuration of the port 18 ensure that the membrane 8 can completely exit into the cavity 16 and prevent the membrane 8 from blocking the port 18 before that the membrane 8 is substantially completely contained in the cavity 16. When the membrane 8 comes out completely in the cavity 16 and the volume defined by the cavity 16 and the cavity 6 is lena or substantially filled with the fluid to be dispensed, the armatures 19 and 20 are activated so that the armature 19 moves to the pump cell, pressing locally the membrane 8 against the raised edge 5 of the inlet port 4 for closing the flow path between the inlet 14 and the pump chamber 7, and the armature 20 moves away from the inlet port 10 allowing the membrane 13 to move away from the outlet port 10 of the outlet of the pump cell (12 , Figure 1) . Substantially at the same time, positive air pressure is applied to the membrane 8 through the port 18 which drives the membrane 8 towards, and substantially completely into the cavity 6 whereby the fluid is pumped through the outlet 12 through from the output port 10. Then the pump filling / distribution cycle can be repeated. The output armatures 20 are connected to stepper motors 20b which can vary the position of each of the armatures 20 in relation to the raised edges 11 of their respective output port 10 thus allowing the opening of the outlet valve. it is controlled to vary the output flow of the pump. In operation, the two pump cells can be operated in opposite phase so that when one distributes the other one is filled, preferably the filling cycle is faster than the distribution cycle so that there can be an overlap of the distribution cycles for Ensure a constant exit. If there are more than two pump cells then it is not necessary for the filling cycle to be faster than the distribution cycle. With reference to Figure 5, a pump unit similar to that shown in Figure 1 is shown and operates in the same manner, but has the additional feature of a diluent inlet 23 through which a diluent enters the cell of pump and mixed with the pumped fluid to pass through it through the outlet of the pump cell 12 through which the diluted fluid is distributed. The flow of diluent is controlled by means of an external control valve (not shown) that can be variable and controlled to give a constant ratio measurement mixture of pumped fluid to diluent. With reference to Figure 6, a pump unit similar to that shown in Figure 5 is shown and operates in the same way. However, additionally, it comprises a mixing section 24 downstream of the point at which the diluent is added. When the pumped fluid is of high viscosity (e.g., above 10,000 centipoise) the difficulty to obtain a homogeneous diluted fluid increases; the convoluted path 25 of the mixing section 24 is designed to cut the viscous fluid and create turbulence to ensure that the two components mix thoroughly. Referring to Figure 7 there is shown a plastic pump unit comprising a fluid inlet 14 which leads to two inlet ports of two chambers 4 from which a flow path to the concave cavity 6 and its chamber outlet 9 exits. associated Provided on the surface of the concave cavity 6 and in the flat area 3 are open slots 26 which, if the flexible film (not shown) traps an occluded area of the remote pumped fluid at the outlet of the chamber 9, there will always be a channel so that the fluid is pushed out ensuring that the chamber is completely empty, thus giving an amplifiable volumetric output. To the pump unit shown in this figure all excess plastic has been removed and designed for production by injection molding techniques. With reference to Figure 8, the rigid plastic pump unit of Figure 7 is shown further comprising an integrated static mixer 27 which is formed as a characteristic of the plastic mold enclosed by the flexible films that are melted by heat thereon. Additionally, an arrangement of obstructions 28 is provided between the inlet ports 10t the static mixer 27 so that the fluid is cut prior to mixing with the diluent entering through the diluent inlet 23. Once mixed with the diluent , the fluid passes through the static mixer 27 and is distributed therefrom as a homogeneous fluid. At the fluid inlet, (14, Figure 7) there is a closure 29 that can be rotated by means of the lever 30 to open or close the fluid from the reservoir (not shown) to the inlet ports 4. With reference to the Figure 9 shows a pre-formed flexible membrane suitable for melting by heat to a pump zone of the invention.

Claims (30)

1. CLAIMS 1. A disposable pump unit for receiving and measuring a predetermined volume of fluid, the pump comprising a body having a surface on which the inlet of a cavity formed in the body is opened, an inlet port for the fluid that it opens to the surface adjacent to the entrance of the cavity, whereby, when the inlet port is opened, the fluid can flow from the inlet port into the cavity through the entrance thereof, a first flexible membrane secured in a sealed manner at its periphery to the surface and superimposed on the cavity and the inlet port, an outlet port for the fluid, there being a passage of fluid flow that extends through the body that connects the cavity to the port of exit, and a second flexible membrane secured in a sealed manner on its periphery and superimposed on the outlet port, serving the portions of the first and second flexible membranes, where they overlap the ports s input and output respectively, as closures for the ports. A disposable pump unit according to Claim 1 wherein the outlet port is also opened on the same surface, in which the opening of the cavity is opened and the first and second flexible membranes are integral with each other. 3. A disposable pump unit according to Claim 1 or Claim 2 wherein the flexible membrane that overlaps the cavity and the inlet port is substantially non-stretchable and is pre-formed to a configuration substantially similar to the shape of the surface of the pump cavity so that, during the fluid measurement step, it can be driven by the driving fluid in contact with substantially the entire surface of the wall of the cavity whereby substantially all of the fluid extracted from the reservoir during the fluid filling stage, it is pumped out during the fluid measurement step. 4. A disposable pump unit according to Claim 3 wherein during storage and transport, the pre-formed configuration of the membrane remains level with the concavely curved surface of the pump cavity thereby reducing the susceptibility of the membrane to damage during transit. A disposable pump unit according to any of Claims 1 to 4 wherein the flexible membranes comprise a flexible laminated film comprising a substantially non-stretchable polymer layer, and a layer of a heat-weldable polymer. 6. A disposable pump unit according to Claim 5 wherein the flexible membranes comprise a polyamide layer and a polyurethane layer. A disposable pump unit according to any of Claims 1 to 6 wherein downstream of the cavity is a variable flow reducer. A disposable pump unit according to any of Claims 1 to 7 wherein a plurality of cavities are provided each having respective inlet ports, outlet ports and flexible membranes whereby, when coupled to a pump actuator , the fluid can be introduced into at least one of the cavities while simultaneously being pumped out of at least one other. 9. A pump unit according to Claim 8 comprising a pair of cavities. A disposable pump unit according to any of Claims 1 to 9 wherein on the surface of the cavity a plurality of passages communicating with the passage of the fluid flow to inhibit are formed during the measurement step of fluid, the formation of occluded regions of fluid between the wall of the cavity and the first flexible membrane and thus ensuring that substantially all of the fluid withdrawn from its reservoir is pumped during the fluid filling stage during the fluid measurement step thus ensuring distribute a substantially repeatable volume of fluid each time. A disposable pump unit according to Claim 10 wherein each of the plurality of passages is a slot. 12. A disposable pump unit according to any of Claims 1 to 11 further comprising, downstream of the outlet port (s), a chamber having a diluent inlet and a diluted fluid outlet whereby the pumped fluid is mixed with a diluent before leaving the disposable pump unit. A disposable pump unit according to Claim 12 wherein between the outlet port (s) and the diluent inlet a number of flow obstructions are provided to moderate the pumped fluid to assist the diluent in the mixed A disposable pump unit according to Claims 12 or 13 wherein the outlet passage includes a mixing means. 15. A disposable pump unit according to claim 14 wherein the mixing means is a static mixer through which the mixture flows. 16. A disposable pump unit according to any of Claims 1 to 15 wherein the disposable pump unit is connected, or is integral with, a disposable reservoir containing the fluid so that, in use, once the tank is emptied or otherwise needs to be changed, the combined tank and pump unit can be discarded. 17. A disposable pump unit according to Claim 16 wherein a lockable closure is provided between the disposable pump unit and the reservoir such that the reservoir and the disposable pump unit can be transported together while avoiding the migration of the fluid to the disposable pump unit. 18. A disposable pump unit according to any one of Claims 1 to 17 including means of identification such that, in use, the reusable pump actuator coupled to the disposable pump unit automatically reads the information included in the means of identification, whereby the combined pump / actuator of pump can adapt its mode of operation depending on the identified information. 19. A disposable pump unit according to Claim 18 wherein the identification means of the disposable pump unit is capable of receiving and storing information from the reusable pump actuator. A disposable pump unit according to Claim 18 or Claim 19 wherein the identification means is a radio frequency identification mark (RDIF) or an integrated circuit of Electro-Erasable-Programmable-Read Only Memory (EEPROM) ) (Erasable-Programmable Electro Reading Only Memory),. 21. A disposable pump unit according to Claim 8 or any claim appended thereto wherein the cavities of the disposable pump unit are connected through their respective input ports to respective sources of different fluids, and the exit ports joined together to form a common outlet associated with the cavities, whereby the respective fluids, in association with a reusable pump actuator, can be distributed and mixed simultaneously with each other. 22. The combination of a disposable pump unit according to any of Claims 1 to 21 and, removably coupled thereto, a reusable pump actuator, sealingly sealing said surface of the disposable pump unit. to the pump actuator, the pump actuator comprising: a source of positive and negative pressure driving fluid, and first and second valve actuating means, each having an associated actuator, associated respectively with the closing of the inlet port and the closure of the outlet port, the arrangement being such that, when the outer surface of the first flexible membrane is exposed to the source of negative pressure fluid, with the inlet port open and the outlet port closed, it is removed from the body of the disposable pump whereby the fluid is extracted from a reservoir thereof through the port of entry to substantially all of the defi nest through the cavity and the first flexible membrane and, with the inlet port closed, and the outlet port open, when the positive pressure fluid is applied to the outer surface of the first flexible membrane, the membrane is driven back into and into the cavity and pumps the fluid from the cavity through said passage to the exit port. 23. The combination according to claim 22, wherein the first and second valve actuation means are axially movable. The combination according to claim 22 or Claim 23 wherein the actuator associated with the output port closing armature is activated by a stepper motor so that the opening of the outlet port can be varied to provide a variable flow restriction. The combination according to claim 23 or claim 24 wherein seals are provided around the armatures associated with at least the inlet port preventing any substance from passing the armature towards the actuator. 26. The combination according to any of Claims 22 to 25 wherein the reusable pump actuator has cavities therein so that when coupled to the disposable pump unit, the pumped volume is defined on the one hand by the wall of the pump cavity, and on the other hand by the wall of the cavity in the pump actuator. 27. The combination according to claim 26 wherein the armature associated with the inlet port extends toward the volume between the cavity and the flexible membrane of the disposable pump unit. 28. The combination according to any of Claims 22 to 27 wherein the means for detecting whether the, or each of the cavities is full or empty, is provided in or on the pump actuator. 29. The combination according to claim 28 wherein the detection means is an ultrasonic detection means. 30. A reusable pump actuator as defined in any of Claims 22 to 29 for use in combination with a disposable pump unit as defined in any of Claims 1 to 21. SUMMARY A disposable pump unit (100) for receiving and measuring a predetermined volume of fluid has a body with a surface on which it opens the mouth of a cavity (7) formed in the body. An inlet port (4) for the fluid opens in the surface adjacent to the mouth of the cavity (7) and a plurality of outlets (9) are provided in the cavity (7) that opens to a flow passage of fluid that extends through the body and connects the cavity (7) to an outlet port (10). A flexible membrane (8) overlaps the cavity (7), the inlet port (4) and the outlet port (10) and seals in a sealed manner on its periphery to the surface. A reusable pump drive unit (200) cooperates with the pump unit (100) to position the membrane (8) between the cavity (7) in the pump unit (100) and a coupling unit (21) in the drive unit (200). The membrane (8) is located towards and away from the cavity (7) by connecting the cavity (21) to a positive and negative fluid pressure source through a port (18). The drive unit (200) has an armature (19) operable to move the membrane (8) to close the input port (4) when the output port (10) opens and an armature (20) operable to move the membrane (8) to close the outlet port (10) when the inlet port (4) is opened. In use, the pump unit (100) draws fluid into the cavity (7) when the inlet port (4) opens and a negative fluid pressure is applied to the chamber (21) and pumps the fluid out of the cavity (7), when the inlet port (4) is closed and a positive fluid pressure is applied to the chamber (21).