Abstract:
A precompression system for a liquid dispensing device is presented, that prevents liquid from being discharged until a predetermined pressure has been built up. The system comprises a pump for drawing liquid through an inlet and discharging it through an outlet, and a precompression valve disposed between the pump and the outlet. The valve allows liquid in the pump to reach the outlet once the predetermined pressure is established, and stops liquid from reaching the outlet once the pressure falls below it. The precompression valve comprises an elastic diaphragm normally closing the valve opening, with a concave surface facing the valve opening and in fluid communication with the pump and a convex surface in fluid communication with atmospheric pressure. The diaphragm may be stretched around a valve seat. Methods of assembling such a precompression system in a dispensing device are also presented.

Description:
This application is a continuation of U.S. Ser. No. 11/743,554, filed May 2, 2007, which claims priority to European Patent Application No. 07006107.2 filed Mar. 24, 2007, the contents of each of which are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to liquid dispensers and more specifically relates to a precompression system for a liquid dispenser whereby liquid in a container is not discharged from the dispenser until a predetermined pressure level is reached. The invention also relates to a method of assembling such a precompression system in a liquid dispenser. 
     BACKGROUND OF THE INVENTION 
     Containers having liquid dispenser assemblies secured thereto are well known. U.S. Pat. No. 5,730,335 discloses a liquid dispenser including a precompression system. This liquid dispenser is a trigger sprayer having a sprayer housing that may be fixed onto the neck of a container. The sprayer housing contains a manually operated pump. An operating element in the shape of a trigger as pivotally connected to the housing for operating the pumb. A dip tube may extend from the pump and into the container so that the liquid in the container may be drawn through the dip tube and into the pump during operation thereof. The trigger sprayer also includes an outlet in fluid communication with the pump for discharging the fluid. The trigger sprayer further includes a spring located in the pump for biasing the piston of the pump to return to a charged position at the end of a discharging pump stroke. 
     The precompression system of this prior art trigger sprayer serves to prevent liquid from leaving the outlet at too low a pressure, which would result in insufficient atomization of the liquid with large drops of fluid or liquid being formed in the spray pattern. The precompression system includes a precompression valve moveable between a position that closes off communication between the pump and the outlet and an open position in which it is spaced from a valve seat for opening communication between the pump and the outlet. The precompression valve is a shallow dome made of a spring material, such as stainless spring steel or a stiff but resilient plastic material. It is biased toward a closed position, in which its convex side engages the valve seat, by its inherent spring characteristics. The precompression valve is flexed to its open position only when a predetermined pressure is attained within the pump. 
     Among the problems associated with this prior art liquid dispenser and its precompression system are the large number of separate parts, which moreover are made from different materials, and the sometimes irregular dispensing pressures achieved by the precompression system. 
     The high number of parts results in a product that is both difficult to manufacture and assemble. As a result, both the manufacturing and the assembly of the dispenser parts are expensive and time consuming. In addition, the different materials pose problems in handling and recycling the trigger sprayer and the container when the items are ready to be discarded. For example, the metal spring used for returning the piston and the stainless steel spring valve must both be removed from the trigger sprayer before the plastic portion of the item may be recycled. 
     The variations in the pressure that is built up in the prior art precompression system is due to the fact that the convex side of the dome shaped spring valve is moved away from the valve seat by flexing the valve such that it assumes a somewhat “wavy” shape in cross section. This is an unstable situation, which may lead to the same amount of pump pressure resulting in varying deformation and consequently varying degrees of opening of the coring valve. Moreover, there is a risk that the spring valve may abruptly snap to an inverted position, thus leaving an open connection between the pump and the outlet. 
     In response to the above problems, commonly assigned U.S. Pat. No. 6,378,739 discloses another liquid dispenser which includes a precompression system. In this prior art liquid dispenser, which has generally the same functionality and structure as the dispenser of the &#39;335 patent discussed above, both the number of separate parts and the use of different materials is reduced in comparison to the liquid dispenser of the &#39;335 patent. To this end the springs for returning the piston at the end of a pump stroke are made from a plastics material and are integrally molded with the neck of the container. Moreover, the precompression system of this brier art liquid dispenser includes a precompression valve that is made of a plastics material as well and that is integrally molded with a sleeve which mounts the valve in a valve chamber. This extensive, use of integrally molded plastic structures limits the number of separate parts, resulting in a liquid dispenser that is easy to manufacture and assemble. Moreover, handling and recycling of the liquid dispenser when it is discarded after use is facilitated. 
     The precompression valve of the liquid dispenser disclosed in the &#39;739 patent includes a dome shaped elastic diaphragm that engages the precompression valve seat with its convex side. Therefore, this elastic diaphragm is still prone to inversion when subjected to pump pressure. In order to limit the amount of deflection of the diaphragm and prevent it from being moved to an inverted position, a stop member protrudes from the concave side of the diaphragm towards a fixed part of the dispenser housing. Nevertheless, the degree to which the diaphragm deflects when the pressure in the pump increases and consequently also the valve opening may vary. 
     SUMMARY OF THE INVENTION 
     The present invention relates to various types of precompression systems for liquid dispensers and assembly methods for making such precompression systems that overcome the problems described above. 
     In accordance with a first aspect of the present invention, a precompression system for a liquid dispensing device that has an inlet and an outlet comprises a pump chamber and a valve chamber. The pump chamber includes a piston that is movable in the pump chamber for drawing liquid through the inlet and discharging the liquid through the outlet. The valve chamber includes a valve member that is disposed between the pump chamber and the outlet and that is operable to allow liquid in the pump chamber to reach the outlet only after a predetermined pressure is established in said pump chamber and to stop liquid from reaching the outlet when the pressure in the pump chamber falls below said predetermined pressure. The valve chamber has an inlet end in fluid communication with said pump chamber, an outlet end in fluid communication with the outlet and a valve seat that is arranged between the inlet end and the outlet end and that has an opening extending there through. The valve member comprises an elastic diaphragm that normally closes the valve seat opening and that includes a concave surface facing the valve seat opening and in fluid communication with the pump chamber and a convex surface in fluid communication with atmospheric pressure. By arranging the elastic diaphragm such that its concave surface faces and engages the valve seat, the pressure at which the precompression valve opens may be controlled more accurately. This is due to the fact that the valve is opened by stretching of the elastic diaphragm, rather than flexing. Moreover, this configuration of the valve member avoids any risk of inversion of the diaphragm. 
     In a preferred embodiment, she elastic diaphragm is stretched around the valve seat. By stretching the diaphragm it is prestressed, which results in improved sealing and better control of the opening pressure. 
     In a further preferred embodiment the elastic diaphragm has an outer periphery and the valve member includes a sleeve surrounding and holding the outer periphery of the diaphragm and extending substantially perpendicular to the plane of the diaphragm, the sleeve being sealingly arranged in the valve chamber. In this way the elastic diaphragm may be easily mounted in the valve chamber. 
     In order to reduce the number of separate parts and to facilitate manufacture and assembly of the precompression system, it is preferred that the elastic diaphragm and the sleeve be integrally molded from a plastics material. Since the diaphragm is arranged with its concave side against the valve seat and the valve is operated by stretching, rather than by deflection of the diaphragm, the plastics material may be more flexible than in the case of a convex valve as described in the prior art. Suitable plastic materials are e.g. polypropylene or polyethylene. 
     The elastic diaphragm may advantageously be molded in an unstretched shape that is substantially less concave than its shape when stretched over the valve seat. In this manner a suitable degree of prestress may be obtained. Preferably, the elastic diaphragm is molded in a convex shape and is stretched to a concave shape when the sleeve is arranged in the valve chamber. 
     In order to ensure that deformation of the valve member will be limited to the elastic diaphragm only, the sleeve preferably includes a plurality of ribs extending along an inner wall thereof substantially perpendicular to the plane of the diaphragm. In this way movement of the diaphragm is well defined, while the sleeve will continue to seal the valve chamber. 
     In a further preferred embodiment of the precompression system of the invention, the sleeve has a lengthwise dimension substantially perpendicular to the plane of the diaphragm and a diametral dimension substantially parallel to the plane of the diaphragm, wherein the lengthwise dimension is greater than a corresponding dimension of the valve chamber. This ensures that the sleeve is clamped tightly in the valve chamber when the precompression system is assembled. 
     Where the sleeve has a lengthwise dimension substantially perpendicular to the plane of the diaphragm and a diametral dimension substantially parallel to the plane of the diaphragm, this diametral dimension may further advantageously be greater than the lengthwise dimension. This results in a relatively short and sturdy sleeve, which is less prone to deformation when the valve member is subjected to the pressure generated by the pump. 
     A precompression system which is relatively easy to assemble is obtained when the dispensing device comprises a shroud including an end wall, and the end wall of the shroud is in alignment with the valve chamber and in contact with she sleeve for securing the valve member within the valve chamber. 
     The invention further provides a liquid dispensing device having an inlet and an outlet and a precompression system arranged between the inlet and outlet, wherein the precompression system comprises a pump chamber including a movable piston, and a valve chamber including a valve member disposed between the pump chamber and the outlet. The valve chamber has an inlet end, an outlet end and a valve seat arranged between the inlet end and the outlet end, with an opening extending through the valve seat. The valve member comprises an elastic diaphragm normally closing the valve seat opening and including a concave and a convex surface. The concave surface of the elastic diaphragm faces the valve seat opening and is in fluid communication with the pump chamber, while its convex surface is in fluid communication with atmospheric pressure. 
     In accordance with yet another aspect of the invention a method is provided for assembling a precompression system for a liquid dispensing device having an inlet and an outlet. This inventive method comprises providing a pump chamber including a piston movable therein and providing a valve chamber disposed between the pump chamber and the outlet. This valve chamber has an inlet end in fluid communication with said pump chamber, an outlet end in fluid communication with the outlet and a valve seat arranged between the inlet end and the outlet end and having an opening extending therethrough. The method further includes arranging a valve member in the valve chamber such that it normally closes the valve seat opening. In this method the valve member comprises an elastic diaphragm including a concave surface facing the valve seat opening and in fluid communication with the pump chamber and a convex surface in fluid communication with atmospheric pressure. 
     In another embodiment the invention provides a precompression system for a liquid dispensing device having an inlet and an outlet. This precompression system comprises a pump chamber including a piston movable in the pump chamber for drawing liquid through the inlet, and discharging the liquid through the outlet; and a valve chamber including a valve member disposed between the pump chamber and the outlet and being operable to allow liquid in the pump chamber to reach the outlet only after a predetermined pressure is established in said pump chamber and to stop liquid from reaching the outlet when the pressure in the pump chamber falls below said predetermined pressure. The valve chamber has an inlet end in fluid communication with the pump chamber, an outlet end in fluid communication with the outlet and a valve seat arranged between the inlet end and the outlet end and having an opening that extends through the valve seat. The valve member comprises an elastic diaphragm that is stretched around the valve seat and that normally closes the valve seat opening. 
     In accordance with yet another aspect of the invention a method of assembling such a precompression system is provided. The inventive method comprising the steps of providing a pump chamber, providing a valve chamber and arranging a valve member in the valve chamber. The pump chamber that is provided includes a movable piston, while the valve chamber is disposed between the pump chamber and the outlet. The valve chamber that the method provides has an inlet end in fluid communication with the pump chamber, an outlet end in fluid communication with the outlet and a valve seat arranged between the inlet end and the outlet end and having an opening extending therethrough. The valve member that is arranged in the valve chamber comprises an elastic diaphragm and is arranged such that the elastic diaphragm is stretched around the valve seat and normally closes the valve seat opening. 
     Finally, the invention provides a valve member for use in a valve chamber of a precompression system for a liquid dispensing device. The valve member of the invention comprises an elastic diaphragm engaging a valve seat in the valve chamber. This elastic diaphragm includes a concave surface engaging the valve seat and a convex surface facing away from the valve seat. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a longitudinal sectional view of a liquid dispenser subassembly having a housing, a piston, a trigger, an outlet nozzle and a precompression system in accordance with a first embodiment of the present invention. 
         FIG. 2  shows a longitudinal sectional view of the precompression valve used in the liquid dispenser of  FIG. 1 . 
         FIG. 3  shows a bottom perspective view of the precompression valve of  FIG. 2 . 
         FIG. 4  shows a first step for assembling the precompression system of the liquid dispenser in accordance with the first embodiment of the present invention. 
         FIG. 5  shows the dispenser subassembly with the precompression valve loosely arranged in a valve chamber. 
         FIG. 6  shows a fragmentary longitudinal sectional view of the liquid dispenser after a shroud of the housing has been mounted so as to secure and prestress the precompression valve. 
         FIG. 7  shows a longitudinal sectional view of the liquid dispenser of  FIG. 1  during a pump stroke, when the precompression valve is opened. 
         FIG. 8  is a view corresponding with  FIG. 7  and showing the liquid dispenser at the end of the pump stroke, when the precompression valve is closed again. 
         FIG. 9  is a view corresponding with  FIG. 2  and showing a precompression valve used in a second embodiment of the present invention. 
         FIG. 10  is a view corresponding with  FIG. 5  and showing the second embodiment of the precompression valve loosely arranged in a valve chamber. 
         FIG. 11  is a view corresponding with  FIG. 1  and showing the second embodiment of the liquid dispenser after assembly. 
         FIG. 12  is an exploded view of a liquid dispenser including a housing, a pushbutton type operating element, a precompression valve, a dip tube, a locking element and a container, in accordance with a third embodiment of the present invention. 
         FIG. 13  shows a fragmentary cross-sectional view of the liquid dispenser of  FIG. 23  after final assembly thereof. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a fragmentary longitudinal sectional view of a liquid dispenser  1  in accordance with a first embodiment of the present invention. The liquid dispenser  1  comprises a housing  2 , a pump  3 , an operating mechanism  4 , an inlet  5 , an outlet  6  and a precompression system  7 . A discharge nozzle  49  is arranged on the outlet  6  for atomizing the liquid that is dispensed. The liquid dispenser  1  is connected to a container  9  having an opening  10  bordered by a neck  11 . In the illustrated embodiment this connection is a snap connection, which is effected by snapping lugs  12  arranged on an inner surface of the housing  2  into recesses  13  formed in the outer surface of the neck  11 . A dip tube  14  extends from the inlet  5  of the liquid dispenser  1  into the container  9  for drawing liquid from the container  9  into the liquid dispenser  1 . 
     The pump  3  includes a pump chamber  15  and a piston  16  that is arranged in the pump chamber  15  for reciprocating movement. Pump chamber  15  has an inlet opening  17  communicating with the liquid dispenser inlet  5  and an outlet opening  18  communicating with a discharge conduit  19  that leads to the liquid dispenser outlet  6 . Pump chamber  15  further has an aerating opening  20  communicating with the interior of the container  9 . This aerating opening  20  is selectively opened and closed by two peripheral flaps  21 ,  22  arranged on the piston  16 . 
     The operating mechanism  4  includes a trigger  23 , the top of which is pivotally connected to the housing  2  by means of a hinge (not shown here). Trigger  23  is also pivotally connected to piston  16  by means of a pin  24  received in an opening  25 . The trigger  23  is biased to its extended position as shown in  FIG. 1  by a pair of flexion springs (not shown here), which are arranged in the housing  2  outside the pump chamber  15 . 
     The precompression system  7  is arranged between the pump chamber  15  and the outlet  6 . It includes a valve chamber  26  in which a precompression valve member  27  is arranged. The valve chamber  26  has an inlet end  28  communicating with the pump chamber outlet opening  18  and an outlet end  29  communicating with the discharge conduit  19  and hence the liquid dispenser outlet  6 . Arranged between the inlet and outlet ends  28 ,  29  is an annular valve seat  31 , which surrounds a valve opening  30  that constitutes the outlet end  29  of the valve chamber. Precompression valve member  27  includes an elastic diaphragm  32  which normally closes the valve opening  30 . This elastic diaphragm  32  is dome shaped and includes a concave surface  32 A facing the valve seat  31  and its opening  30 , as well as a convex surface  32 B facing away from the valve seat opening  30  towards the interior of valve chamber  26 . A stabilizing member  45  is attached to the center of the convex surface  32 B. 
     Precompression valve member  27  further includes a sleeve  33  surrounding and holding an outer periphery  34  of the elastic diaphragm  32 . This sleeve  33  is arranged in the valve chamber  26  and seals against an inner wall  35  thereof by means of a peripheral flap  36  and an annular ridge  37  arranged on an outer surface  38  of the sleeve  33 . Sleeve  33  further includes a second peripheral flap  39  which serves as a flap valve between liquid dispenser inlet  5  and inlet opening  17  of pump chamber  15 . Finally, as shown more clearly in  FIGS. 2 and 3 , sleeve  33  includes a plurality of ribs  40  evenly distributed in peripheral direction and extending along an inner surface  41  of the sleeve  33 . In the illustrated embodiment there are four ribs  40  each spaced 90 degrees from the adjacent ribs  40 . 
     Sleeve  33  has a stepped contour which corresponds with the stepped configuration of the inner wall  35  of the valve chamber  26 . Sleeve  33  extends beyond the plane of the elastic diaphragm and has an inner ridge  42 —when considered in the direction of valve chamber  26 —which engages a bottom surface  46  of the valve chamber  26 . The inner ridge  42  includes a plurality of openings  43  allowing liquid to flow from the pump chamber  15  towards the discharge conduit  19 . The length of the sleeve  33  measured from the inner ridge  42  to an outer ridge  44  is slightly greater than the corresponding depth of the valve chamber  26 . This ensures that the valve member  27  is tightly clamped in the valve chamber  26  when the liquid dispenser  1  is assembled. The force required for pressing the valve member  27  tightly into the valve chamber  26  is provided by an end wall  47  that forms part of a shroud  48  of the dispenser housing  2   
     Valve member  27  including the sleeve  33  and elastic diaphragm  32  is integrally molded front a plastics material, like e.g. polypropylene. When molded, the elastic diaphragm  32  has a shape which is substantially less concave—considered in the direction facing the valve seat  31 —than it has when the valve member  27  is arranged in the valve chamber  26 . In the illustrated embodiment the elastic diaphragm  32  is actually molded in a convex shape, which is inverted when the valve member  27  is pressed into the valve chamber  26  by the end wall  47 . In this way the elastic diaphragm  32  is prestressed against or stretched over the valve seat  31 , which is an important feature with a view to obtaining excellent sealing until the liquid in the pump chamber  15  reaches the predetermined pressure at which the precompression valve should open. 
     Referring to  FIG. 4 , the precompression system  7  is assembled by first inserting the valve member  27  in the valve chamber  26 , which is integrally formed as part of the housing  2  of the liquid dispenser  1 . The valve member  27  is first pressed into the valve chamber  26  until the elastic diaphragm  32  engages the valve seat  31 . In this position, which is shown in  FIG. 5 , the inner ridge  42  does not yet engage the bottom  46  of valve chamber  26 . Since the distance between the elastic diaphragm  32 —when unstressed—and the outer ridge sleeve  33  is greater than the distance between the valve seat  31  and the end of valve chamber  26 , sleeve  33  of valve member  27  still protrudes somewhat from valve chamber  26 . 
     In a final assembly step the shroud  48  is connected to the rest of the housing  2 . During this step the end wall  47  engages the protruding outer ridge  44  of sleeve  33  and presses valve member  27  tightly into valve chamber  26  until the inner ridge  42  abuts the valve chamber bottom  46 . Since the valve seat  31  protrudes further from the valve chamber bottom  46  than the distance between the sleeve inner ridge  42  and the elastic diaphragm  32 , the latter is stretched over the valve seat  31  and the face  32 A of the diaphragm  32  assumes its concave shape, as shown in  FIG. 6 . The liquid dispenser  1  is now ready for operation. 
     When the trigger  23  is first operated, the piston  16  will move inwards, reducing the volume of the pump chamber  15  and thereby compressing the air inside—assuming the pump  3  has not been primed. The resulting air pressure is not enough to force the precompression valve away from the valve seat  31 . When the trigger  23  is released, it will be returned to its original position by the springs. During this return or suction stroke, the pressure in she pump chamber  16  will be lowered, thus drawing liquid from the container  9  through the dip tube  14  and the dispenser inlet  5 , past the flap valve  39 , through the inlet opening  17  into the pump chamber  16 . 
     When the trigger  23  is operated again, movement of the piston  16  will result in a sharp increase in the pressure within the pump chamber  16 , since the liquid is not compressible. This pressure acts on all parts of the pump chamber  16  and is also present in the outlet opening  18 , which is closed by the elastic diaphragm  32  of the precompression valve  27 . Once the pressure exceeds a predetermined value, for instance in the order of three bar, the elastic diaphragm  32  will stretch and be lifted from the valve seat  31 , as shown in  FIG. 7 . This pressure is determined by the elasticity of the diaphragm  32  and the ambient pressure, which acts on the convex surface  320  of the diaphragm  32 . Once the diaphragm  32  is lifted from the valve seat  32  pressurized liquid from the pump chamber  16  may flow through the outlet opening  18 , between the valve seat  31  and the elastic diaphragm  32 , into the valve opening  30 . From there the liquid will flow through the discharge conduit  19  to the outlet  6  of the liquid dispenser  1 . Since the liquid is dispensed only after reaching the predetermined pressure, it will be properly atomized upon leaving the outlet  6  and the spraying pattern will be evenly distributed, without any large drops being dispensed. 
     Referring now to  FIG. 8 , when the pressure in the pump chamber  16  drops below the predetermined level at the end of the pump stroke, the elasticity of the diaphragm  32  will overcome the liquid pressure. Consequently the diaphragm  32  will contract again until it comes to rest against the valve seat  31 . This closes the valve opening  30  and instantly interrupts the flow of liquid from the pump  3  to the outlet  6 . In this way the liquid dispenser  1  will not “drip” at the end of the pump stroke. 
       FIG. 9  shows a valve member  127  for use in a second embodiment of the precompression system  107 . This valve member  127  has a square, rather than elongated shape, since its length—the distance between the inner and outer edges  142  and  144 , respectively—is no larger than its diameter. This configuration results in a sturdy sleeve  133 , which has even less tendency to deform when pressure is applied to the diaphragm  132 . Although the length of this alternative valve member  127  is smaller than that of the valve member  27  of the first embodiment, it is still longer than the depth of the valve chamber  126 . Consequently, the outer ridge  144  still protrudes from the valve chamber  126  when the valve member  127  has been inserted up to the point where the diaphragm  132  contacts the valve seat  131 , as shown in  FIG. 10 . Therefore, also in this embodiment the elastic diaphragm  132  is stretched and prestressed when the valve member  127  is finally clamped tight in the valve chamber  126  by connecting the shroud  148  including she end wall  147  to the rest of the liquid dispenser  101 , as illustrated in  FIG. 11 . 
       FIG. 12  shows a liquid dispenser  201  in accordance with a third embodiment of the present invention. Like the first and second embodiments, this liquid dispenser  201  comprises a housing  202 , a pump  203 , an operating mechanism  204 , an inlet  205 , an outlet  206  and a precompression system  207 . The liquid dispenser  201  is again connected to a container  209  having an opening  210  bordered by a neck  211 . A dip tube  214  again extends from the inlet  205  of the liquid dispenser  201  into the container  209  for drawing liquid from the container  209  into the liquid dispenser  201 . 
     This liquid dispenser  201  is not a trigger sprayer, but is intended for dispensing more viscous liquids like e.g. hand soap. Consequently, the discharge nozzle  249  at the outlet  206  is not arranged for atomizing the liquid, but merely for deflecting the flow of liquid downward. The dispenser further has a different mechanism for operating the pump  203 , using a pushbutton  223  that is slidable within the housing  202 , rather than a hinged trigger. The pushbutton  223  is biased to a position of rest by two substantially S-shaped combined torsion/flexion springs  250 , only one of which is shown. In this embodiment of the liquid dispenser  201  the piston  216  is integrated in the pushbutton  223 . This embodiment of the liquid dispenser  201  further includes a vent chamber  251  arranged next to the pump chamber  215 . The pushbutton  223  also includes a second piston (not shown here) that is arranged for reciprocating movement in the vent chamber  251 . 
     The valve member  227  of this third embodiment is somewhat different from that of the first two embodiments in that the elastic diaphragm  232  is arranged substantially halfway the sleeve  233 , rather than near its inner ridge  242 . Like in the first two embodiments, the diaphragm  232  is stretched over the valve seat  231 , as shown in  FIG. 13 . Its concave side  232 A again faces both the valve opening  230  and the outlet opening  218  of the pump chamber  215  and is exposed to the pressure generated by the pump  203 . The convex side  232 B of the elastic diaphragm  232  faces the rear of the valve chamber  226  and is exposed to atmospheric pressure. 
     Again, the elastic diaphragm  232  is originally molded in a shape that is substantially less concave than the shape it has to assume by being stretched over the valve seat  231  when valve member  227  is inserted into valve chamber  226 . This deformation of the elastic diaphragm  232  leads to a certain degree of prestress that results in an excellent seal between the diaphragm  232  and the valve seat  231 . Depending on the degree of prestress that is required to obtain the required sealing action and a specific precompression of the liquid, the elastic diaphragm  232  may also be molded in a straight or even a convex shape. 
     The sleeve  233  includes an opening  243  in its side wall  235  for allowing liquid to pass from the outlet opening  218  of the pump chamber  215  to the valve opening  230 . Since in this embodiment the pump  203  and the inlet  205  are arranged on opposite sides of the valve chamber  226 , the sleeve  233  further includes a groove  252  allowing liquid to pass along the outside of the sleeve  233 . In this embodiment, the outer ridge  244  of the sleeve  233  has a somewhat greater diameter than the outer end of the valve chamber  226  so that it is held thereby. The valve member  227  is locked in position by a plurality of ribs  253  protruding from end wall  247  of shroud  248 . 
     Reciprocating movement, of the pushbutton  223  between its two positions also reciprocates the pump piston  216  and the vent piston in the pump chamber  215  and vent chamber  251 , respectively. During a suction stroke, the pump piston  216  moves in an onward direction to create a vacuum in the pump chamber  215 , thereby drawing liquid from the container  209  through dip tube  214  and inlet  205 , past the sleeve  233  and into the pump chamber  215 . During a discharge stroke, the pump piston  216  moves in a downward direction to reduce the volume of the pump chamber  215 . Once the pressure within the pump chamber  215  is greater than the combined elastic force of the diaphragm  232  and the ambient pressure on the convex face  232 B of the diaphragm, the diaphragm  232  stretches and moves away from the valve seat  231  and the liquid is free to pass through the valve opening  230  and into the discharge conduit  219  towards the outlet  206 . 
     Although the invention has been illustrated by means of a number of examples, it should be apparent that it is not limited thereto. For example, the precompression system might be used in other types of liquid dispensers. Moreover, the flexible diaphragm and sleeve of the valve member could be formed separately. In addition, both the configuration of the elastic diaphragm and sleeve and the choice of materials might be varied as well. Accordingly, the scope of the invention is defined solely by the appended claims.