BACK-FLOW PREVENTION VALVE FOR A FLUID DISPENSER STATEMENT OF CORRESPONDING APPLICATIONS The present invention is based on the complete specification filed in relation to New Zealand Patent No. 600489, the entire contents of which are incorporated herein. TECHNICAL FIELD The present invention relates generally to a back-flow prevention valve for a fluid dispenser. Particularly, although not exclusively, the present invention relates to a back-flow prevention valve for dispensing a measured volume of liquid of a fixed size from a reservoir such as for dispensing measures of alcoholic spirits in a bar or restaurant. BACKGROUND ART The increasingly stringent regulatory standards (e~g. Australian liquor dispensing measurement authority) for dispensing of fluids such as alcoholic spirits for consumption in bars or restaurants have given rise to the need for reliable means to dispense full measures of fluids without significant loss in volume. The standard volume of spirit is usually defined as a "shot" which can range from 15 to 30 millilitres in volume in New Zealand and up to 60 millilitres in the United States. Shot dispensers have been developed so that bar staff can be sure they are providing a set amount of spirits to the person who requested the drink. Customers at public bars are highly conscious of receiving value for money from the drinks they purchase. If bar staff do not have accurate shot dispensers and dispense small measures or "short shots" for the same price as a standard measure, customers may avoid the bar in the future.
In addition, accurate shot dispensers are required by the public so that a person may gauge exactly how much alcohol they have consumed and what effect this will have on their behaviour. This is especially important if a customer is worried that they may start acting irresponsibly, or whether they will be "over the limit' and will be guilty of drunk driving if they drive themselves home. Accurate shot dispensers are also invaluable to the managers of bars who have to account for the amount of spirits dispensed and tally this with the amount of money received. An accurate shot dispenser is required to ensure that a bar is not either cheating its customers, or being too generous with its measures. Numerous known dispensing apparatus exist for the repeated dispensing of a predetermined volume of liquid from an associated reservoir. However a disadvantage of these apparatus is that when the volume of fluid in the reservoir is only large enough to dispense one more full measure, the measured volume that is dispensed can fall below the average calibrated shot size resulting in back-flow of a proportion of the measured fluid volume back into the liquid reservoir before dispensing frorn the dispensing apparatus. For example the last measured shot can vary by up to 0.7 millilitres for a 15 millilitre shot size. Consequently, it is desirable to provide a dispensing apparatus capable of accurate dispensing of the last full measure of fluid from an associated fluid reservoir by being configured to prevent back-flow of liquid from the dispensing apparatus back into the reservoir. It is an object of the present invention to address the foregoing problem or at least to provide the public with a useful choice. All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference 2 constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinence of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country. It is acknowledged that the term 'comprise' may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term 'comprise' shall have an inclusive meaning - i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term 'comprised' or 'comprising' is used in relation to one or more steps in a method or process. Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only. DISCLOSURE OF INVENTION According to one aspect of the present invention there is provided a back-flow prevention valve for a fluid dispenser, the back-flow prevention valve comprising: * a dispensing chamber in fluid communication with a fluid reservoir; * a dispensing chamber inlet for fluid communication with the fluid reservoir; * a dispensing chamber outlet; and * an actuation member configured to positively displace a measured volume of fluid from the dispensing chamber through the dispensing chamber outlet 3 wherein the back-flow prevention valve also comprises a sealing means mounted in the dispensing chamber and adjacent to the dispensing chamber inlet and configured to seal the dispensing chamber inlet to substantially prevent fluid back-flow from the dispensing chamber back to the fluid reservoir when a positive pressure is created between the dispensing chamber inlet and the actuation member by the positive displacement of fluid by the actuation member. Such a pressure differential is created when there is only one full measured dispensing volume remaining in the fluid reservoir and dispensing chamber, this represents a lower 'head pressure' of fluid resisting the back-flow of fluid from the dispensing chamber. For the purposes of the specification the term "positive displacement" means a piston type device acting within a dispensing chamber to displace the fluid within the dispensing chamber to exit through the dispensing chamber outlet. This positive displacement principle allows the amount of fluid dispensed to be controlled by installing different sizes of 'spacer' in the piston assembly to displace more or less fluid within the dispensing chamber as required. This is in contrast to gravity fed fluid dispensers which rely on the controlled timing of inlet and outlet valves being opened and closed to determine how much fluid is dispensed. Preferably, the fluid reservoir used to contain the fluid may be a substantially airtight container such as a bottle. The fluid dispensing chamber and associated piston may be of complementary shapes and sizes so that the piston fits snugly inside the walls of the dispensing chamber and forms a base to the fluid reservoir. Preferably, the actuation member may be a piston. 4 Preferably, the actuation member may be configured to move through the dispensing chamber via motive force provided by a solenoid in order to cycle the dispensing chamber outlet and dispensing chamber inlet and displace a measured volume of fluid from the reservoir. In a preferred embodiment the actuation member may be any type of substantially rigid protrusion that fits within the dispensing chamber with the ability to seal either the valve inlet or dispensing chamber outlet alternately but not both at the same time. In a further preferred embodiment, the actuation member includes a piston shaft on which a motive force is enacted preferably by way of an electronic solenoid reacting with a ferrous slug which is attached to the piston shaft. When the actuation member is operated by the solenoid the component is able to move upward, breaking seal contact with the chamber outlet and sealing the chamber inlet sequentially. By allowing the fluid within the chamber to escape through the outlet and sealing the chamber inlet in the same instance, the fluid reservoir is conversely isolated so no further fluid can enter the chamber while the measured volume is dispensed. Alternative embodiments of the present invention may not ernploy a solenoid as a source of motive effort. For example, in alternative embodiments any form of spring or substantially elastic material which may be compressed and then allowed to expand, transferring kinetic energy to adjacent elements may be used as the energy storage device. For example in alternative embodiments some forms of foam rubber or other resilient materials may be used as the energy storage device. In a preferred embodiment, where the actuation member is configured with a substantially complementary shape to that of the interior of the dispensing chamber, the actuation member is moved through the dispensing chamber as the dispenser spring is allowed to expand. In a further preferred embodiment the expansion of the 5 dispenser spring is controlled so that every time the spring expands the actuation member is moved a set distance within the body of the fluid reservoir. As the actuation member head is configured to contact the inner sides of the dispensing chamber, displacement of the actuation member a set distance also displaces a set volume of spirits from the dispenser. The measured volume of fluid dispensed by the actuation member will be equal to the free volume present within the dispensing chamber minus the volume occupied by the actuation member and the calibrated 'spacer' component. It should readily be appreciated by those skilled in the art that the measured volume of fluid displaced by the dispenser may be readily calculated and also modified dependant on the size of the spacer component used as part of the actuation member assembly. Preferably, the actuation member also comprises at least one upper seal and at least one lower seal. In a closed configuration, the actuation member seals the dispensing chamber outlet on the bottom of the dispensing chamber via the lower seal, which in turn means that the dispensing chamber inlet at the top of the dispensing chamber is open to allow fluid to enter from the fluid reservoir. When the actuation member cycles upward, the lower dispensing chamber outlet is opened and sequentially the upper dispensing chamber inlet is sealed by the actuation member. Preferably, the measured volume of fluid is in the range 15 millilitres to 30 millilitres. Preferably, the sealing means comprises a valve diaphragm. More preferably, the valve diaphragm may be a flat disc More preferably, the sealing means may also comprise a valve diaphragm seat complimentary in shape to the valve diaphragm. 6 BRIEF DESCRIPTION OF THE FIGURES The invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows a section side view of a preferred embodiment of the present invention in the form of a back-flow prevention valve for a fluid dispenser; Figure 2 shows a section perspective close-up view of the preferred embodiment shown in Figure 1; Figure 3 shows section side close-up view of the preferred embodiment shown in Figure 1; and Figure 4a&b shows close-up photographic views of the preferred embodiment shown in Figure 1 with the back-flow diaphragm in an open and closed position. BEST MODES FOR CARRYING OUT THE INVENTION The present invention has use in the accurate dispensing of a volume of fluid from an associated fluid reservoir and specifically in providing accurate dispensing of the last measured volume of liquid of between 15 milliliters and 30 milliliters without significant loss of volume. Reference throughout this specification is made to the use of the present invention in conjunction with dispensers used to dispense alcoholic spirits. However it should be appreciated that the present invention can be used to dispense other fluids and other amounts, for example in a laboratory situation. In a preferred form of the invention, a back-flow prevention valve for a fluid dispenser is indicated by generally indicated by arrow 1. The valve 1 comprises a dispensing chamber 2, a dispensing chamber inlet 3 positioned in the dispensing chamber 2, the 7 dispensing chamber inlet 3 in fluid communication with a fluid reservoir 4 in the form of a substantially airtight alcoholic spirits or liquor bottle (not shown). A chamber 5 receives fluid from the fluid reservoir 4 via the dispensing chamber inlet 3. An dispensing chamber outlet 6 of the chamber 5 enables dispensing a measured volume of fluid from the fluid dispenser 1 when an actuation member in the form of a rigid piston 7 is activated to move via motive force supplied by a solenoid (not shown) interacting with a slug of magnetic ferrous material attached to the piston 7 within the dispensing chamber outlet 6. The solenoid is charged by way of an electronic circuit initiated by a button (not shown) pressed by the user. The set volume of alcoholic spirits dispensed by the actuation member 7 will be equal to the free volume present within the dispensing chamber 2 minus the volume occupied by the actuation member 7 and a calibrated 'spacer' component 13. The fluid dispenser 1 also comprises a sealing means comprising a valve diaphragm 8 in the form of an 18.5 mm diameter flat plastic disc and a corresponding valve seat 9 positioned in the chamber 5 and adjacent the dispensing chamber inlet 3. The valve diaphragm 8 is configured to move on locating pin 10 against the valve seat 9 to seal fluid flow in the dispensing chamber inlet 3 from the chamber 5 and the fluid reservoir 4 when a positive pressure situation occurs on dispensing the last measured volume of alcoholic spirit from the fluid reservoir. The valve diaphragm 8 is of sufficient low profile so as to not impede travel of the piston 7 during dispensing of alcoholic spirit from the fluid dispenser 1. A lower seal 11 and upper seal 12 prevent fluid flow from the dispensing chamber outlet 6 when the piston 7 is in a closed position with respect to the valve body 2 (as shown in Figure 4a). 8 It was found that the drop in volume of the final dispensed measure volume is caused by the remaining liquid contained in the liquor bottle and chamber 5 reacting to the upward movement of the piston 7. The piston 7 is triggered by a large solenoid (not shown); Referring to Figure 3, as the piston 7 begins to travel in the direction of arrow A, the volume of liquid above the upper seal 12 is displaced in the direction of arrow B. As a column of fluid is displaced above the upper seal 12 before it comes in contact with the top of the chamber 5, a corresponding volume of air enters through the breather tube 14 in the direction of arrow C by way of vacuum generated by the initial fluid displacement. As fluid is displaced from the measurement chamber by air, this accounts for the drop in volume of the last shot' dispenser The drop in volume of the final dispensed measured volume only occurs on the 'last shot' as the pressure generated by the reduced 'head' of fluid within the bottle resisting the back-flow from the chamber is not sufficient to prevent fluid from leaving the dispensing chamber. To overcome this anomaly, the 'back flow' valve diaphragm 8 is positioned above the upper seal 12 in order to arrest the movement of fluid caused by the main valve piston cycle. The 'back flow' valve diaphragm 8 is in an open state due to gravity (as shown in Figure 4b) when the fluid in which it is suspended is static, or when the fluid is travelling down through the valve to fill the chamber 5. Upon upward movement of the fluid the valve diaphragm 8 shuts (as shown in Figure 4a), preventing the fluid from discharging back into the reservoir. With the 'back flow' valve diaphragm 8 in place, there is no perceivable drop in dispensed volume on the final shot. Example 1: Measurement of dispensing volume accuracy 9 An alcoholic spirits (liquor) bottle was weighed, the weight zeroed on a balance and 96.4 grams of alcoholic spirits added to the bottle. This weight equated to 7 shots of spirits due to the specific gravity of the spirits used. The bottle containing the spirits was connected to the back-flow prevention valve of the present invention and the first dispensed shot (measured volume) discarded to prime the system. Subsequent shots were then measured in a calibrated measuring cylinder and the volume of each shot measured until the bottle was empty. This process was repeated seven times The results of this trial are illustrated in the below table and show that there was no significant drop in dispensing volume on the last measured shot dispense before the spirits bottle is emptied- All dispensed shot volumes must fit inside a strict volume range whilst also allowing a processing window above the minimal volume allowance and below the maximum volume allowance. 18.5mm flat OD Centre disc, 15.1mm ID sealing disc Test 1.1 1.2 1.3 1.4 1.5 1.6 1.7 96.4 98.15 100.66 103,1 106.11 108.6 111.01 15.5 15.4 15.2 15,5 15.4 15.4 15.4 15.2 15,4 15.4 15.4 15.5 15.2 15.2 15,2 15.4 15.2 15.2 15.3 15.4 15.4 15.5 153 15.4 15.2 15.1 15.3 15.4 10~ Totalprd 0.3 0.2 0 2 0.3 0.4 012 0.2 Average first 4 15.4 15A4 15.3 15.3 15.3 '15.3 15.3 Difrence between Average ln ast ...... dISpense -0.2 -0 01 -4 01 0.0 The present invention offers the notable advantage over the prior art of improved accuracy in dispensing the last full measure of fluid from an associated reservoir. 10 This in turn provides the advantage of ensuring regulatory standards for dispensing volumes of fluids such as alcoholic spirits are met. Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof as defined in the appended claims. 11