Patent Publication Number: US-9897475-B2

Title: Dispensing device with indicator member

Description:
The present invention relates to a dispensing device for use particularly, but not exclusively, to dispense metered doses of a chemical cleaning liquid. 
     When chemical cleaning products are provided for use on an industrial scale, for example for cleaning hospitals or hotels, they are generally packaged in concentrated form. It is therefore necessary to mix the concentrated product with water to achieve a desired dilution ratio for use. It is known to provide manual dispensing devices which dispense a metered dose of concentrated product for mixing with a particular quantity of water. These generally comprise a piston and cylinder arrangement which draws a particular quantity of the concentrated product from a reservoir in a priming stroke, and then dispenses that product in a dispensing stroke. The metered dose dispensed is the capacity of the cylinder. 
     A problem with this arrangement is that the dispensing device can be manually operated before the piston has returned to the top of the cylinder, which results in less than the intended dose of concentrated product being dispensed. If this occurs the resulting mixture has the incorrect dilution ratio. A further problem is that there is nothing to prevent users from repeatedly operating the dispensing device to dispense an excessive amount of the concentrated product. This is a particular problem if the concentrated product is corrosive or otherwise hazardous in its concentrated and less diluted forms. 
     In order to overcome these problems more advanced dispensing devices comprise an operating plunger which is automatically disengaged from the piston after a dispensing stroke thereof, and prevented from being reengaged until the piston has performed its priming stroke and the cylinder is fully primed. As such, the dispensing device cannot be operated to dispense anything other than the intended dose. In addition, it is also known to impede the performance of the priming stroke in order to introduce a delay time before the device can be operated once again. This serves to deter the repeated operation of the dispensing device to dispense an excessive amount. 
     An example of such a dispensing device is shown in WO 97/23156 to Unilever. Here the operating plunger can be manually depressed to move the piston up the cylinder and dispense a single shot of liquid therefrom, but the plunger is then moved by a spring back to a stand by position independently of the piston. It is then locked in that position so it cannot be depressed, and is automatically unlocked by the arrival of the piston back to its primed position. Detents lock the plunger in its stand by position, which are released by the piston when it achieves its primed position. 
     There are a number of drawbacks with the design shown in WO 97/23156 to Unilever. The first is that it is overly complex, and requires space to accommodate the movement of the resilient detents. These detents are also problematic because they can suffer from fatigue over time and lose their effectiveness. Secondly, there is the functional drawback that the operating plunger is locked in its stand-by position during the priming stroke of the piston, and a user unfamiliar with this locking functioning may attempt to force it in the belief that it is stuck. This can lead to damage. Finally, once the device is primed and the operating plunger unlocked, it can be manually operated merely by depression thereof. Therefore any inadvertent contact with the operating plunger can activate the dosing device. 
     The present invention is intended to overcome some of the above problems. 
     Therefore, according to a first aspect of the present invention, a dispensing device comprises a cylinder, a piston disposed in said cylinder and moveable therein between a top of the cylinder and a bottom of the cylinder in a priming direction and in a dispensing direction, a first spring device biasing said piston to move in said priming direction at a predetermined rate, and an operating plunger moveable between an engaged position in which it is operatively connected to said piston and movement of said operating plunger along a path is transmitted to said piston, and a disengaged position in which it is operatively disconnected from said piston, in which said operating plunger is movable between an outer point and an inner point, in which a second spring device biases said operating plunger to move from said inner point to said outer point independently of said piston, in which said piston comprises an indicator member, in which said operating plunger comprises an end wall comprising an aperture, and in which when said operating plunger is at said outer point and said piston is at the top of said cylinder, said indicator member is disposed in said aperture. 
     Therefore, the dispensing device has a visual indication to the user that the piston has returned independently of the operating plunger to its primed position, and the dispensing device can be operated once again to dispense the intended dosage. This is a much simpler solution to the problem than that presented by the prior art, as there is no requirement for any complex locking mechanism of the operating plunger. This arrangement also allows users to move the operating plunger before the piston has returned to the top of the cylinder, and therefore dispense a less than full dosage, if they wish to do so. Further, if the user is not familiar with the operation of the dispensing device, and in particular the fact that they are supposed to wait for the appearance of the indicator member before depressing the operating plunger once again, they will not damage it by depressing the operating plunger before the appearance of the indicator member. 
     Preferably the piston can comprises a face, the indicator member can extend outwardly from an underside of said face, and it can be so sized that it is disposed in said aperture when said operating plunger is at said outer point and said piston is at the top of said cylinder. 
     The indicator member can be any physical construction which can achieve the above technical results, but in a preferred construction it can comprise a third coil spring carrying an indicator body at an outer end thereof. 
     The first spring means can bias the piston to move in its priming direction at a predetermined rate in any known way. For example, it can simply comprise a relatively weak strength. However, in a preferred construction the first spring means can comprise a first coil spring biasing the piston to move in its priming direction, and an impedance mechanism adapted to impede movement of the piston in its priming direction to at or below the predetermined rate. The impedance mechanism can be any fluid flow restriction into the cylinder, which can be provided at an inlet thereof, or downstream in a supply hose. With this construction the first coil spring can be relatively strong, and it can drive the piston in the priming direction at the maximum rate permitted by the impedance mechanism. 
     In a preferred embodiment the piston can comprise a face, an annular side wall extending therefrom and comprising an outer end, a flange extending radially from said outer end and comprising an outer edge, and a collar extending substantially normally from said outer edge. When the piston is at a bottom of the cylinder after a dispensing stroke thereof the flange can overlie the top of the cylinder and the collar can be disposed radially outwardly of the cylinder. Further, the operating plunger can comprise a substantially tubular body disposed radially outwardly of the collar. 
     The first spring device can be mounted inside the cylinder, or in a spring chamber co-axial therewith. However, in a preferred embodiment the first spring device can be a first coil spring mounted around the cylinder between a first static surface of the dispensing device and an underside of the flange. This first static surface can be an area of the interior framework of the dispenser surrounding the bottom of the cylinder. 
     A first end stop surface can be provided which prevents the movement of the piston in the cylinder beyond the primed position. With this arrangement the first coil spring can be mounted under compression between the first static surface and the underside of the flange in all positions of the piston in the cylinder. Mounting the first coil spring in this position, and in this state is an expedient and compact arrangement. 
     The dispensing device can also comprise a second static surface disposed axially between the operating plunger and the piston. The second spring device can then comprise a second coil spring mounted inside the operating plunger between the second static surface and an underside of an end wall of the operating plunger. 
     Preferably, in versions of the invention in which the indicator member comprises a third coil spring carrying an indicator body at an outer end thereof, the indicator member can be disposed radially inside the second coil spring. This is also an expedient and compact arrangement. 
     The dispensing device can further comprise a tubular retention member which encloses the cylinder and piston. It can also comprise the end stop surface as referred to above in the form of an end stop ring which prevents movement of the piston beyond the top of said cylinder. In addition, the retention member can comprise an inwardly depending portion which can extend inwardly from said end stop ring, and a bottom of the inwardly depending portion can define the second static surface referred to above. 
     The dispensing device of the first aspect of the invention can comprise a simple linear operating plunger which only moves back and forth along the path, and which is axially aligned with the piston. As such, the operating plunger is in the engaged position when it is axial contact with the piston, and in the disengaged position when it is displaced from the piston by the second coil spring. With this arrangement, if the operating plunger is depressed at any time during which the piston is displaced from the top of the cylinder, then the operating plunger will be moved into the engaged position, and any further depression of the operating plunger will be transmitted to the piston. This means that it is possible for a user to drive the piston before it has completed a full priming stroke, resulting in the dispensing of a less than full dose. This may not be desired, but this arrangement does eliminate the possibility of a user damaging the dispensing device by attempting to force a movement which is prevented by some locking mechanism, as in the prior art. The indicator member feature is intended to address the issue of dispensing only a full dose, as the user is supposed to only depress the operating plunger when the indicator member is present in the aperture. 
     However, the first aspect of the present invention can still be used in a dispensing device which does comprises a mechanism to prevent the dispensing of a less than full dose. 
     Therefore, in one version of the invention the operating plunger can comprise engagement means, and it can be moveable between an engaged position in which said engagement means are operatively connected to said piston and movement of said operating plunger along said path is transmitted to said piston, and a disengaged position in which said engagement means are operatively disconnected from said piston and movement of said operating plunger along said path is independent of said piston. The engagement means can be arranged such that movement of the operating plunger between the engaged and disengaged positions is of a different kind to movement along said path. 
     Thus, in this version of the dispensing device it is necessary to first move the operating plunger into the engaged position before it can be moved along its path to drive the piston. As this movement is of a different kind to movement along the path, inadvertent activation of the device can be avoided. In addition, the operating plunger can be moved harmlessly along its path when it is not connected to the piston, which avoids the problems associated with locking the operating plunger in a stand by position. A dispenser can also be realised which achieves the various above described advantages of delayed single shot dispensers like that shown in WO 97/23156 to Unilever, without complex additional operating plunger locking features. In particular, as with such designs the piston is moved in the priming direction at a predetermined rate, which can be relatively slow in order to introduce a delay time before the device can be operated again, which deters repeated operation to dispense an excessive amount. 
     It will be appreciated that the engagement means can be arranged such that any movement of the operating plunger which is different to movement along its path could be provided. For example, in one version of the invention the engagement means can operatively connect with the piston through a lateral movement of the operating plunger which is substantially normal to movement along its path. If movement along the path comprises a depression of the operating plunger into a body of the dispenser, then this lateral movement could be from either side, or from above or below the depression position. The engagement means could be a tab which interacts laterally with a slot provided on the piston. 
     However, in a preferred construction the operating plunger can comprise a rotation axis, and it can be rotated on this axis to move it between the engaged and disengaged positions. This arrangement is advantageous because it is not necessary to provide for any lateral movement of the operating plunger, which would increase the size of the dispenser, and because there is less likelihood of inadvertent rotation of the operating plunger than of any inadvertent movement in a lateral direction. Furthermore, rotation requires a conscious manual effort of the user, which increases their interaction with the device and decreases the likelihood of accidental over dosing. 
     The operating plunger can be offset from the piston, and its path can be oblique to a stroke axis of the piston. For example, in one arrangement the operating plunger can be mounted to a hinge on the dispenser about which the part of the operating plunger which is connected to the piston rotates in a planetary fashion. As such, the path of the operating plunger can be actuate, and a joint between the operating plunger and the piston can transmit this motion into a linear motion of the piston along its stroke axis. This kind of arrangement is provided if the operating plunger is activated by, or is integrated with, an outer cover of the dispenser. 
     However, in a preferred construction the piston and the operating plunger can be arranged co-axially on a stroke axis, and the path of the operating plunger can comprise said stroke axis. As such, to drive the piston the operating plunger is simply depressed in a linear fashion by the user. 
     In a preferred construction the second spring device also biases the operating plunger to move from its engaged position to its disengaged position. As such, when the operating plunger is released by the user after a dispensing stroke, it not only moves back out to its original position, but it also rotates automatically back to the disengaged position. 
     In versions of the invention in which the second static surface is provided axially between the operating plunger and the piston, and the second spring device comprises a second coil spring mounted inside the operating plunger between the second static surface and an underside of an end wall of the operating plunger, the second spring device can be rotationally fixed at both ends to the second static surface and the end wall, such that rotation of the operating plunger into the engaged position places the second coil spring under torsional compression. As such, the second coil spring biases the operating plunger back into the disengaged position. 
     The engagement means can be any known mechanism which allows for two parts to be operatively connected for axial movement via a relative rotational movement. In one version of the invention the piston can comprise a radially outwardly extending slot, and the operating plunger can comprise a radially inwardly extending tab which moves into said slot when the operating plunger is rotated. However, in a preferred construction the piston can comprise a radially outwardly extending tab, the operating plunger can comprise a radially inwardly extending slot, and in the engaged position said tab can be disposed in said slot. 
     The tab and slot can be axially co-incident when the piston is at a top of the cylinder after a priming stroke thereof and the operating plunger is at its outer point. This is important because it means that rotation of the operating plunger to operatively connect it to the piston is only possible once the piston has completed its priming stroke, and not before. This prevents the dispensing of half doses of product by driving the piston before it has completed its priming stroke. 
     Following on from this, the operating plunger can comprise an axially extending wall, and when the operating plunger is at its outer point and in its disengaged position, and the piston is displaced from the top of said cylinder, said wall can engage the tab and prevent movement of the operating plunger from its disengaged position to its engaged position. This feature prevents the operating plunger from being rotated on its axis at all until the tab and the slot are axially co-incident. 
     In one version of the invention the piston can comprise a face, an annular side wall extending therefrom and comprising an outer end, a flange extending radially from said outer end and comprising an outer edge, and a collar extending substantially normally from said outer edge. When the piston is at the bottom of the cylinder after a dispensing stroke thereof said flange can overlie the top of the cylinder and said collar can be disposed radially outwardly of the cylinder. The operating plunger can then comprise a substantially tubular body disposed radially outwardly of said collar, and the tab can be provided on an outer surface of said collar, while said slot and wall can be provided on an inner surface of said tubular body. 
     The second static surface feature referred to above is a particularly expedient way to achieve the above described technical advantages, because it provides a compact and simple way to provide for the independent movement of the operating plunger. This arrangement can be used in dispensing devices in which there is no indicator feature as in the first aspect of the present invention. 
     Therefore, according to a second aspect of the present invention a dispensing device comprises a cylinder, a piston disposed in said cylinder and moveable therein between a top of the cylinder and a bottom of the cylinder in a priming direction and in a dispensing direction, a first spring device biasing said piston to move in said priming direction at a predetermined rate, and an operating plunger moveable between an engaged position in which it is operatively connected to said piston and movement of said operating plunger along a path is transmitted to said piston, and a disengaged position in which it is operatively disconnected from said piston, in which said operating plunger is movable between an outer point and an inner point, in which a second spring device biases said operating plunger to move from said inner point to said outer point independently of said piston, in which said dispensing device comprises a second static surface disposed axially between said operating plunger and said piston, and in which said second spring device comprises a second coil spring mounted inside said operating plunger between said second static surface and an underside of an end wall of said operating plunger. 
     The above described arrangement can find application with any of the features which are referred to above in relation to the first aspect of the invention. For example, the dispensing device can comprise a tubular retention member which encloses the cylinder and piston, and which comprises an end stop ring which prevents movement of the piston beyond the top of the cylinder. The retention member can then further comprise an inwardly depending portion which extends inwardly from the end stop ring, and a bottom of the inwardly depending portion can define the second static surface. 
     Further, the piston can comprise an indicator member extending outwardly from an underside of a face thereof, which can be so sized that it is disposed in an aperture provided in an end wall of the operating plunger when the operating plunger is at the outer point and the piston is at the top of the cylinder. 
     The first spring device can comprise a first coil spring biasing the piston to move in the priming direction, and an impedance mechanism adapted to impede movement of the piston in the priming direction to at or below the predetermined rate. 
     The piston can comprise an annular side wall extending from its face, a flange extending radially from an outer end of the side wall, and a collar extending substantially normally from and outer edge of the flange. When the piston is at the bottom of the cylinder after a dispensing stroke thereof the flange can overlie the top of the cylinder and the collar can be disposed radially outwardly of the cylinder. The operating plunger can comprise a substantially tubular body disposed radially outwardly of said collar. The first spring device can then comprise a first coil spring mounted around the cylinder between a first static surface of the dispensing device and an underside of the flange. 
     In addition, the invention of the second aspect can find application in dispensing devices which do not include any mechanism to prevent the user driving the piston before it has reached the end of its priming stroke, as well as those which do. 
     In terms of the versions of the second aspect of the invention which do include such a mechanism, it can be like that described above. In particular, the operating plunger can comprise engagement means and can be moveable between an engaged position in which the engagement means are operatively connected to the piston and movement of the operating plunger along the path is transmitted to the piston, and a disengaged position in which the engagement means are operatively disconnected from the piston and movement of the operating plunger along the path is independent of the piston. The engagement means can be arranged such that movement of the operating plunger between the engaged and disengaged positions is of a different kind to movement along the path. 
     Preferably the operating plunger can comprises a rotation axis, and can be rotated on said axis to move it between the engaged and disengaged positions. The piston and the operating plunger can be arranged co-axially on a stroke axis, and the path of the operating plunger can comprise said stroke axis. The second spring device can also bias the operating plunger to move from the engaged position to the disengaged position. 
     The piston can comprise a radially outwardly extending tab, the operating plunger can comprise a radially inwardly extending slot, and in the engaged position the tab can be disposed in the slot. The tab and slot can be axially co-incident when the piston is at a top of the cylinder after a priming stroke thereof and the operating plunger is at the outer point. The operating plunger can comprise an axially extending wall, and when the operating plunger is at the outer point and in the disengaged position and the piston is displaced from the top of the cylinder, said wall can engage the tab and prevent movement of the operating plunger from the disengaged position to the engaged position. 
     The piston can comprise a face, an annular side wall extending therefrom and comprising an outer end, a flange extending radially from said outer end and comprising an outer edge, and a collar extending substantially normally from said outer edge. When the piston is at the bottom of the cylinder after a dispensing stroke thereof said flange can overlie the top of the cylinder and said collar can be disposed radially outwardly of the cylinder. The operating plunger can then comprise a substantially tubular body disposed radially outwardly of said collar, and the tab can be provided on an outer surface of said collar, while said slot and wall can be provided on an inner surface of said tubular body. 
    
    
     
       The invention can be performed in various ways, but three embodiments will now be described by way of example, and with reference to the accompanying drawings, in which: 
         FIG. 1  is an oblique cross-sectional side view of the relevant parts of a first dispensing device according to the first aspect of the present invention in a first position; 
         FIG. 2  is a perspective view of the dispensing device as shown in  FIG. 1  in the first position; 
         FIG. 3  is a rear perspective view of an operating plunger part of the dispensing device as shown in  FIG. 1 ; 
         FIG. 4  is a front perspective view of a piston part of the dispensing device as shown in  FIG. 1 ; 
         FIG. 5  is an oblique cross-sectional side view of the relevant parts of the dispensing device as shown in  FIG. 1  in a second position; 
         FIG. 6  is a perspective view of the dispensing device as shown in  FIG. 1  in the second position; 
         FIG. 7  is an oblique cross-sectional side view of the relevant parts of the dispensing device as shown in  FIG. 1  in a third position; 
         FIG. 8  is a perspective view of the dispensing device as shown in  FIG. 1  in the third position; 
         FIG. 9  is an oblique cross-sectional side view of the relevant parts of the dispensing device as shown in  FIG. 1  in a fourth position; 
         FIG. 10  is an oblique cross-sectional side view of the relevant parts of the dispensing device as shown in  FIG. 1  in a fifth position; 
         FIG. 11  is an exploded perspective view of the piston part, a retention member part and the operating plunger part of the dispensing device as shown in  FIG. 1 ; 
         FIG. 12  is a cross-sectional top view of the relevant parts of the dispensing device as shown in  FIG. 1  in the first position; 
         FIG. 13  is a cross-sectional side view of the relevant parts of a second dispensing device according to the first aspect of the present invention; 
         FIG. 14  is a perspective view of the dispensing device as shown in  FIG. 13 ; 
         FIG. 15  is a rear perspective view of an operating plunger and piston part of the second dispensing device as shown in  FIG. 13 ; 
         FIG. 16  is a rear perspective view of the operating plunger part of the second dispensing device as shown in  FIG. 13 ; and 
         FIG. 17  is a cross-sectional side view of the relevant parts of a third dispensing device according to the second aspect of the present invention. 
     
    
    
       FIGS. 1 to 12  show a first dispensing device according to the first aspect of the present invention which includes a mechanism to prevent the user driving the piston before it has reached the end of its priming stroke.  FIGS. 13 to 16  show a second dispensing device according to the first aspect of the invention which does not include such a mechanism. In both cases the characterising feature of the first aspect of the invention is present, namely the indicator member.  FIG. 17  shows a third dispensing device which accords with the second aspect of the invention, but not the first, in order to illustrate how the second aspect of the invention can stand alone from the first. In all three embodiments of the invention the characterising feature of the second aspect of the invention is present, namely the second static surface. 
     As shown in  FIG. 1 , a dispensing device  1  comprises a cylinder  2 , and a piston  3  disposed in said cylinder  2 , and moveable therein in a priming direction, indicated by arrow A, and in a dispensing direction, indicated by arrow B. There is also a first spring means biasing said piston to move in said priming direction at a predetermined rate, in the form of first coil spring  4  and a fluid flow restrictor (not shown), and an operating plunger  5 , which as described further below is operatively connectable to said piston  3  and moveable along a path A-A to move said piston  3  in said dispensing direction, indicated by arrow B. The operating plunger  5  comprises engagement means, in the form of radially inwardly extending slot  6 , which is best seen in  FIG. 3 , and is moveable between an engaged position, as shown in  FIGS. 5-8 , in which said engagement means ( 6 ) are operatively connected to said piston  3  and movement of said operating plunger  5  along said path A-A is transmitted to said piston  3 , and a disengaged position, as shown in  FIGS. 1, 2, 9 and 10 , in which said engagement means ( 6 ) are operatively disconnected from said piston  3  and movement of said operating plunger  5  along said path A-A is independent of said piston  3 . As explained further below, the engagement means ( 6 ) are arranged such that movement of said operating plunger  5  between said engaged and disengaged positions is of a different kind to movement along said path A-A. 
     (For the sake of clarity  FIGS. 1, 5, 7, 9 and 10  show oblique cross-sectional side views which are at approximately 40 degrees to vertical. This angle has been chosen because it passes through the tabs  25  and  26  of the piston  3 , and therefore shows the operative connection between the tabs  25  and  26  and the slots  6  and  40  in the engaged position of the operating plunger  5 . This would not be visible in a standard cross-sectional side view because the tabs  25  and  26  do not extend through the vertical plane. The other parts of the dispensing device  1  shown in these Figures are annular, so appear much as they would in a standard cross-sectional side view.) 
     The dispensing device  1  is configured like known devices, in that it comprises a cylinder  2  and piston  3  pump arrangement which draws fluid to be dispensed from a reservoir (not shown), and then dispenses a metered dose through an outlet  7 . Referring to  FIG. 2 , a reservoir is attached to the cylinder  2  by a fluid line  8 , which passes through an inlet  9 , and travels up to the cylinder  2  above. The fluid product is drawn into the cylinder  2  automatically in use through an inlet (not visible) by negative pressure generated by movement of the piston  3  in the priming direction A, under pressure from the first coil spring  4 . A fluid flow restrictor (not shown) of a known kind is provided in the fluid line  8  (or adjacent the inlet), which restricts the rate at which the fluid product can pass through the fluid line  8 , and therefore the rate at which the piston  3  can move in the priming direction A, under pressure from the first coil spring  4 . As such, an in-built delay is incorporated into the automatic priming action of the piston  3 . The first coil spring  4  is of such a strength that it can drive the piston  3  in the priming direction A at the maximum rate permitted by the fluid flow restrictor. 
     A one-way valve (not shown) is provided at the fluid inlet which allows the fluid product to enter the cylinder  2 . Movement of the piston  3  in the dispensing direction B under pressure from the user acting on the operating plunger  5 , forces fluid product inside the cylinder  2  out of a fluid outlet  10  thereof. A one-way valve (not shown) is provided at the fluid outlet  10  which allows the fluid product to exit the cylinder  2 . It is then sent down a second fluid line  11  to the outlet  7  at the bottom  12  of the device  1 . A spout fixture or other fluid connection means can be provided at the outlet  7  to conveniently direct the fluid product for the user. 
     As shown in  FIG. 2 , the dispensing device comprises a back plate  13 , on which is provided a framework  14  which supports the cylinder  2  and piston  3  pump arrangement. The framework  14  also supports a number of other components which relate to adjusting the capacity of the cylinder  2  and piston  3  pump arrangement. These components are not relevant to the present invention so are not further described here. 
     Referring back to  FIG. 1 , the cylinder  2 , piston  3  and operating plunger  5  are all arranged co-axially on a stroke axis A-A. As such, the device  1  has a linear configuration in which the path of the operating plunger  5  comprises this stroke axis A-A, and to drive the piston  3  the operating plunger  5  is simply depressed by the user. 
     The piston  2  comprises a face  15 , an annular side wall  16  extending therefrom and comprising an outer end  17 , a flange  18  extending radially from said outer end  17  and comprising an outer edge  19 , and a collar  20  extending substantially normally from said outer edge  19 . As is clear from  FIG. 1  the annular side wall  16  fits snugly inside the cylinder  2 , and as is clear from  FIG. 7 , when the piston  3  is at a bottom  21  of the cylinder  2  after a dispensing stroke thereof the flange  18  overlies a top  22  of the cylinder  2 , and the collar  20  is disposed radially outwardly of the cylinder  2 . The first coil spring  4  is mounted around the cylinder  2 , between a first static surface (not shown) being an area of the framework  14  surrounding the bottom  21  of the cylinder  2 , and the flange  18 . 
     Mounted over the piston  2  is a retention member  23 , which can be seen in a perspective view in  FIG. 11 . It is a tubular part  34  which extends from the framework  14  and encloses the cylinder  2  and piston  3 . It also comprises an end stop ring  24  which prevents the movement of the piston  3  in the cylinder  1  beyond the primed position, as shown in  FIG. 1 . The flange  18  abuts against the end stop ring  24  in this position. In addition, the first coil spring  4  is held in compression in this position, so it is constantly held in some state of compression. 
     As is shown in  FIG. 4 , the collar  20  comprises a pair of opposite tabs  25  and  26  which extend radially outwardly therefrom. These tabs  25  and  26  are visible in  FIG. 1  because of the oblique cross-sectional view shown. The tubular part of the retention member  23  is slightly radially larger than the flange  18 , but it is radially smaller than the tabs  25  and  26 . It therefore comprises a pair of openings  27  and  28  on opposite sides thereof, through which the tabs  25  and  26  radially protrude. The collar  20 , and therefore the piston  3 , is held in its rotational position through interaction with the sides  29  of the openings  27  and  28 , only one of which is visible in  FIG. 1  because of the oblique cross-sectional view shown, but which can be seen clearly in  FIG. 11 . The collar  20  comprises a pair of opposite positioning fins  30  and  31 , and each of these abut against one side  29  of a corresponding opening  27  or  28 , and the far sides  32  and  33  of the tabs  25  and  26  respectively abut against the other side  29  of the corresponding opening  27  or  28 . The fins  30  and  31 , and the sides  32  and  33  ride against the sides  29  of the openings  27  and  28  when the piston  3  moves in the priming direction A or the dispensing direction B. 
       FIG. 12  shows a cross-sectional top view of the relevant parts of the dispenser  1 . In this view the shape and position of the retention member  23  can be better seen, because the cross-section passes through the tubular part  34  of the retention member  23 , as opposed to through the openings  27  and  28  as in the other Figures. It can also be seen from  FIG. 12  how the collar  20  fits inside the tubular part  34  of the retention member  23 . 
     The operating plunger  5  comprises a substantially tubular body  35  comprising an end wall  36 , an outer tubular portion  37  and an inner tubular portion  38  which comprises a greater radius from the stroke axis A-A than the outer tubular portion  37 . As is clear from the Figures the outer tubular portion  37  is slightly radially larger than the radial extent of the tabs  25  and  26 . Referring to  FIG. 3 , an interior surface  39  of the inner tubular portion  38  comprises a pair of radially inwardly extending slots  6  and  40 , only one of which  6  is visible in  FIG. 3 . Each slot  6 ,  40  comprises a circumferentially facing opening, only one of which  41  is visible. 
     The operating plunger  5  is rotatable on stroke axis A-A between a disengaged position, as shown in  FIGS. 1, 2, 9 and 10 , in which the slots  6  and  40  are displaced from the corresponding tabs  26  and  25  respectively, and an engaged position as shown in  FIGS. 5-8 , in which the tabs  26  and  25  are located in the slots  6  and  40  respectively. The extent of the possible rotation is defined by the interaction between the operating plunger  5 , the tabs  25  and  26 , and the retention member  23 . In particular, as is clear from  FIG. 3 , the slots  6  and  40  are formed in each case on a radially inwardly protruding section  42  and  43  respectively formed in the inner tubular portion  38 . The sections  42  and  43  are also provided in each case with an axially extending location tab  44  and  45  respectively. The sections  42  and  43  comprise a radius from the stroke axis A-A which is less than that of the tubular part  34  of the retention member  23 , and as such they protrude radially inwardly into the corresponding openings  28  and  27  respectively. This serves to define the limits of rotational movement of the operating plunger  5 , as the sections  42  and  43  cannot rotate in either direction beyond the sides  29  of the openings  28  and  27  respectively. (In the disengaged position of the operating plunger  5  the sections  42  and  43  actually come into contact with the positioning fins  31  and  30  respectively and not the sides  29 , because the positioning fins  31  and  30  are disposed against the sides  29  in a position which is axially co-incident with the sections  42  and  43 .) In addition, in the engaged position of the operating plunger  5  the tabs  26  and  25  are fully located in the slots  6  and  40  respectively, so further rotation of the operating plunger  5  is prevented. 
     Following on from this, the operating plunger  5  also comprises a pair of axially extending walls  46  and  47 , which extend from the inner ends, only one of which  48  is visible, of the slots  6  and  40 , to the ends of the location tabs  44  and  45 . When the operating plunger  5  is at an outer point thereof, and in the disengaged position, and the piston  3  is displaced from the top  22  of said cylinder  2 , as shown in  FIGS. 9 and 10 , any attempt to rotate the operating plunger  5  from the disengaged position to the engaged position results in the walls  46  and  47  rotationally engaging the tabs  26  and  25  respectively, which prevents the operating plunger  5  from moving into the engaged position. Therefore, in this particular in-use configuration rotational movement of the operating plunger  5  is restricted. The situation changes when the tabs  26  and  25  are once again axially co-incident with the slots  6  and  40  respectively, and therefore no longer axially co-incident with the walls  47  and  46  respectively. 
     The operating plunger  5  is connected to the retention member  23  by a second coil spring  49 . The retention member  23  comprises an inwardly depending portion  50 , which extends inwardly from the end stop ring  34 . At the bottom of the inwardly depending portion  50  is a base surface  51 , which defines the second static surface referred to above. It will be appreciated that this base surface  51  is disposed axially between the operating plunger  5  and the piston  3 . The second coil spring  49  is mounted inside the operating plunger  5  between the base surface  51  and an underside  52  of the end wall  36 . The second coil spring  49  is rotationally fixed in position at both ends. It is disposed such that movement of the operating plunger  5  from the disengaged position into the engaged position places it under torsional compression. Therefore, a first function of the second coil spring  49  is that it biases the operating plunger  5  to move from said engaged position to said disengaged position, by virtue of the way it is torsionally statically mounted at both ends. 
     Referring to  FIG. 11 , the retention member  23  comprises a pair of radially extending retention tabs  53  and  54 , which are disposed in the path of outer walls, only one of which  55  is visible in  FIG. 3 , of the slots  6  and  40  when the operating plunger  5  is disposed in its disengaged position, as shown in  FIGS. 1, 2, 9 and 10 , which prevents the operating plunger  5  from moving any further out from the position shown. Therefore, these retention tabs  53  and  54  define an outer point in the axial movement range of the operating plunger  5  in the disengaged position. The operating plunger  5  cannot be rotated to displace the walls  55  from the retention tabs  53  and  54  while the tabs  25  and  26  of the piston  3  are not axially co-incident with the slots  40  and  6  respectively, which would release the operating plunger  5 , because the axially extending walls  46  and  47  described above prevent any such rotation. The second coil spring  49  is held in axial compression in this position, so it is constantly held in some state of compression. Therefore, a second function of the second coil spring  49  is that it biases the operating plunger  5  to move to its outer position from any depressed position. As such, if the operating plunger  5  is released by the user after a dispensing stroke, as explained further below, the second coil spring  49  acts to return the operating plunger  5  to its disengaged position and to its outer point simultaneously, and independently of the piston  3 . 
     Referring to  FIGS. 1 and 5 , it will be appreciated that the tabs  25  and  26  and the slots  6  and  40  are axially co-incident when the piston  3  is at the top  22  of the cylinder  2  after a priming stroke thereof, and the operating plunger  5  is at its outer point. As such, rotation of the operating plunger  5  to operatively connect it to said piston  3  is only possible once the piston  3  has completed its priming stroke, and not before. 
     The piston  3  also comprises an indicator member  56 , which is mounted on a third coil spring  57 , which is itself mounted to an underside  58  of the face  15  of the piston  3 . The indicator member  56  comprises a body  59  and a face  60 . The end wall  36  of the operating plunger  5  also comprises an aperture  61 . An aperture  62  is also provided in the retention member  23  through which the third coil spring  57  passes. As is clear from  FIG. 1 , the indicator member  56  is so arranged that when the tabs  25  and  26  of the piston  3  and the slots  6  and  40  of the operating plunger  5  are axially co-incident and/or the operating plunger  5  is in the engaged position, as shown in  FIGS. 1, 2 and 5-8 , the face  60  is disposed in the aperture  61 . 
     The operating plunger  5  also comprises an indicator arrow  63 , which illustrates the manner of rotation required to move the operating plunger  5  into the engaged position. 
     The dispensing device  1  operates as follows. Firstly, an outer end of the fluid line  8  is placed into a reservoir of fluid to be dispensed (not shown). To prepare the dispensing device  1  for use the piston  3  must be driven back and forth in the cylinder  2  a few times to draw the fluid up the fluid line  8  to the inlet of the cylinder  2 , into the cylinder  2  itself and then down the second fluid line  11 . Once some fluid emerges from the end of the second fluid line  11 , or any spout fixture attached thereto (not shown), then the dispensing device  1  is ready to operate. (The manner in which the piston  3  is driven in this way is the same as the manner in which it is moved to dispense fluid in use, which is described below.) 
     A user places their receptacle at the outlet  7 , or any spout fixture attached thereto, to capture the dispensed dose of fluid. They then rotate the operating plunger  5  clockwise, as indicated by the arrow  63 , to move it from the disengaged position shown in  FIGS. 1 and 2  to the engaged position shown in  FIGS. 5 and 6 . As explained above, this movement places the second coil spring  49  under torsional compression, and as such the user will feel a degree of spring resistance. This rotational movement sees the slots  6  and  40  rotate into position over the tabs  26  and  25  respectively. The rotational movement ends when the sections  42  and  43  of the operating plunger  5  engaged the sides  29  of the openings  28  and  27  respectively of the retention member  23 , and the tabs  26  and  25  are fully located in the slots  6  and  40  respectively. The piston  3  is held axially in place during this action because the sides  32  and  33  of the tabs  25  and  26  respectively abut against the sides  29  of the openings  27  and  28  respectively. In the engaged position the operating plunger  5  and the piston  3  are operatively connected. 
     If a user does not rotate the operating plunger  5  in this way, and instead simply depresses it, then it will move from its outer point as shown in  FIG. 1 , to its inner point, as shown in  FIG. 7 , without engaging the piston  3 . As such, the dispensing device  1  will not be operated to dispense a dose of fluid. This will be immediately apparent to the user, who can then try again by first rotating the operating plunger  5  so it engages the piston  3 . In addition, it will be appreciated that if the operating plunger  5  is inadvertently depressed, this will not lead to any fluid being dispensed. 
     Before the user attempts to rotate the operating plunger  5  they can check the aperture  61  for the presence of the face  60  of the indicator member  56 . If the face  60  is present in the aperture  56 , then the piston  3  is located at the top  22  of the cylinder  2  in its primed position, and the dispensing device  1  is ready to operate. If the piston  3  is not located in its primed position, because it is still moving in the priming direction A after a dispensing stroke has been performed, as shown in  FIG. 10 , then the face  60  will not be located in the aperture  61 , and the user can know that they must wait a moment before attempting to rotate the operating plunger  5 . If the user attempts to rotate the operating plunger  5  before the piston  3  is located at the top  22  of the cylinder  2 , this will be prevented because the walls  46  and  47  of the operating plunger  5  will rotationally engaging the tabs  26  and  25  respectively, preventing the operating plunger  5  from moving into the engaged position. It will then be apparent to the user that the dispensing device  1  is not ready to be used. They can then wait until the face  60  appears in the aperture  61 , and the tabs  26  and  25  are once again axially co-incident with the slots  6  and  40  respectively. 
     Once the operating plunger  5  has been operatively connected to the piston  3  as shown in  FIGS. 5 and 6 , the user depresses the operating plunger  5  to drive the piston  3  in the dispensing direction B until it reaches the bottom  21  of the cylinder  2 , as shown in  FIGS. 7 and 8 . The slots  6  and  40  moveably engage the tabs  26  and  25  respectively, which moves the face  15  of the piston  3  down the cylinder  2 . This action is performed against the strength of the first coil spring  4  and the second coil spring  49 , as well as against the frictional resistance of the fluid exiting the cylinder  2  and travelling down the second fluid line  11 , and as such the user will feel a degree of spring resistance. It will be appreciated that the performance of a dispensing stroke will pump an amount of fluid from the second fluid line  11  equal to the capacity of the cylinder  2 . Therefore, the dose dispensed is precisely controlled. 
     The user then releases the operating plunger  5 , and it immediately rotates back into the disengaged position, under the torsional force of the second coil spring  49 . This rotates the slots  6  and  40  away from the tabs  26  and  25  respectively, and moves the operating plunger  5  until the sections  42  and  43  thereof engage the positioning fins  31  and  30  respectively, which are disposed against the corresponding sides  29  of the openings  28  and  27  respectively of the retention member  23 . However as soon as the tabs  26  and  25  are free of the slots  6  and  40  respectively the operating plunger  5  immediately moves axially along the stroke axis A-A back to its outer point, as shown in  FIG. 9 , under the axial force of the second coil spring  49 . It will be appreciated from  FIG. 9  that the sections  42  and  43  are of such a length thanks to the presence of the location tabs  44  and  45 , that even with the operating plunger  5  at its outer point, and the piston  3  in the fully dispensed position, the sections  42  and  43  are still axially co-incident with the positioning fins  31  and  32 . As such, the operating plunger  5  stays in the same rotational position in the disengaged position thereof, regardless of the axial position of the piston  3 . This axial movement of the operating plunger  5  is arrested when the outer walls  55  of the slots  6  and  40  come into contact with the retention tabs  53  and  54  of the retention member  23 . It will be appreciated that the return of the operating plunger  5  to its starting position is independent of the piston  3 . 
     At the same time that the user releases the operating plunger  5  the piston  3  begins its priming stroke, by moving in the priming direction B under the force of the first coil spring  4 . However, it takes longer for the piston  3  to complete its priming stroke than the operating plunger  5  takes to return to its outer point, because the fluid flow restrictor (not shown) provided in the fluid line  8  restricts the rate at which the fluid can pass therethrough, and therefore the rate at which the piston  3  can draw it into the cylinder  2 .  FIG. 10  illustrates a mid-point at which the piston  3  has travelled approximately half way back up the cylinder  2 . Therefore, an in-built delay is incorporated into the automatic priming action of the piston  3 . 
     If the user attempts to rotate the operating plunger  5  in order to engage the piston  3  for another stroke before it is once again located at the top  22  of the cylinder  2 , this is prevented because the walls  46  and  47  of the operating plunger  5  rotationally engaging the tabs  26  and  25  respectively. This serves to prevent the user from operating the dispensing device  1  while the cylinder  2  is only partially replenished with fluid, and less than a full dose would be dispensed. Only once the piston  3  has returned to the top of the cylinder  2  and the cylinder  2  is once again fully primed with a full dosage, can the user rotate the operating plunger  5  into the engaged position. It will be appreciated from  FIG. 9  that the walls  46  and  47  are of such a length thanks to the presence of the location tabs  44  and  45 , that even with the operating plunger  5  at its outer point, and the piston  3  in the fully dispensed position, the walls  46  and  47  are still axially co-incident with the tabs  26  and  25  respectively. Therefore, not only is there an in-built delay in the performance of the priming stroke of the piston  3 , but the user is prevented from operating the device  1  for the duration of the delay, and until a full dose can be dispensed again. This prevents repeated operations of the device  1  to dispense an excessive amount, and it also prevents the operating of the device  1  to dispense less than a full dose, which can lead to an incorrect dilution ratio in a mixed product to be used. 
     Before the user attempts to rotate the operating plunger  5  they can check the aperture  61  for the presence of the face  60  of the indicator member  56 . If the piston  3  is not yet located in its primed position, as shown in  FIG. 10 , then the face  60  is not located in the aperture  61 , and the user can know that they must wait a moment before attempting to rotate the operating plunger  5 . Once the face  60  is present in the aperture  61  again, as shown in  FIG. 1 , then the piston  3  is located at the top  22  of the cylinder  2  in its primed position, and the dispensing device  1  is ready to be operated again. 
     Referring now to  FIGS. 13 and 14 , these show a second dispensing device  100 , which is similar to dispensing device  1  described above, except that it does not feature a mechanism to prevent the driving of the piston  101  before it has completed its priming stroke. ( FIG. 13  shows a vertical cross section of the dispensing device  100 , not an oblique cross-section as shown in  FIGS. 1, 2, 7, 9, and 10 . There is no need to show an oblique cross-section because there is no need to illustrate features of a mechanism to prevent the driving of the piston  101  before it has completed its priming stroke. As a result of this vertical cross-section, retention tabs  122  and  123  of the retention member  104  are visible.) 
     The piston  101  is the same component as piston  3  described above, however some of its features are rendered redundant in dispenser  100 , as described further below. It moves axially up and down the cylinder  102  in the same way as piston  3  does, and it is biased in the priming direction A by first coil spring  103 . It is retained in the dispensing device  100  by retention member  104 , which is the same structure as retention member  23  described above. End stop ring  105  prevents the movement of the piston  101  beyond the primed position, as shown in  FIG. 13 . Flange  106  of the piston  101  abuts against the end stop ring  105  in this position. 
     The operating plunger  107  is similar to operating plunger  5  described above, except that it does not feature any slots on its interior surface  108 . Instead, the inner tubular portion  109  comprises a pair of opposed radially inwardly depending indented sections  110  and  111 , which are most clearly visible in  FIG. 16 . In  FIG. 13  the cross-sectional view passes through these indented sections  110  and  111 . Also visible in  FIG. 16  are a number of co-planar active surfaces  112 - 116  which are integrally formed on the interior surface  108  of the operating plunger  107 , and which collectively act on the flange  106  of the piston  101  to transmit a driving force thereto, as explained below. 
     As in dispenser  1  described above, the operating plunger  107  is connected to the retention member  104  by a second coil spring  117 . The retention member  104  comprises an inwardly depending portion  118 , which extends inwardly from the end stop ring  105 . At the bottom of the inwardly depending portion  118  is a base surface  119 , which defines the second static surface referred to above. It will be appreciated that as in dispenser  1  described above, this base surface  119  is disposed axially between the operating plunger  107  and the piston  101 . The second coil spring  117  is mounted inside the operating plunger  107  between the base surface  119  and an underside  120  of end wall  121 . 
     The retention member  104  comprises a pair of radially extending retention tabs  122  and  123 , which are disposed in the axial path of walls  124  and  125  which radially extend between the indented sections  110  and  111  respectively and an outer tubular portion  126  of the operating plunger  107 . Therefore, the retention tabs  122  and  123  prevent the operating plunger  107  from moving beyond an outer point as shown in  FIG. 13 . The second coil spring  117  is held in axial compression in this position, so it is constantly held in some state of compression. Therefore, the only function of the second coil spring  117  is to bias the operating plunger  107  to move to its outer point from any depressed position. Unlike in dispenser  1  described above, the operating plunger  107  is never rotated, so the second coil spring  117  is never placed under torsional compression, and it never acts to rotate the operating plunger  107 . 
     So, if the operating plunger  107  is released by the user after a dispensing stroke, as explained further below, the second coil spring  117  simply acts to return the operating plunger  107  to its outer point independently of the piston  101 . The piston  101  returns to its primed position as shown in  FIG. 13  at a rate which is controlled by a fluid flow restrictor (not shown) of a known kind which is provided in the fluid line  127 , or adjacent an inlet  128  of the cylinder  102 , as in dispensing device  1  described above. This restricts the rate at which the fluid product can pass through the fluid line  127 , and therefore the rate at which the piston  101  can move in the priming direction A, under pressure from the first coil spring  103 . The first coil spring  103  is of such a strength that it can drive the piston  101  in the priming direction A at the maximum rate permitted by the fluid flow restrictor. 
     The piston  101  also comprises indicator member  129 , which is mounted on a third coil spring  130 , which is itself mounted to an underside  131  of a face  132  of the piston  101 . The indicator member  129  comprises a body  133  and a face  134 . The end wall  121  of the operating plunger  107  also comprises an aperture  135 . An aperture  136  is also provided in the retention member  104  through which the third coil spring  130  passes. As is clear from  FIG. 13 , the indicator member  129  is so arranged that when the operating plunger  107  is at its outer point and the piston  101  is in its primed position, the face  134  is disposed in the aperture  135 . 
     Referring to  FIG. 15 , it can be seen that the piston  101  comprises tabs  137  and  138 , as well as positioning fins  139  and  140 . These are the same structures as tabs  25  and  26 , and fins  30  and  31  provided on piston  3  described above, but they have a different function in dispenser  100 . As is clear from  FIG. 15 , tab  137  and fin  139  are located on either side of indented section  110 , and tab  138  and fin  140  are located on either side of indented section  111 . As such, these features serve to maintain the piston  101  in one rotational position in relation to the operating plunger  107 . Furthermore, the active surfaces  112 - 116  of the operating plunger  107  act on the tabs  137  and  138  and fins  139  and  140  to transmit a driving force from the operating plunger  107  to the piston  101  in use, as described further below. The active surfaces  112  and  113  act on the tab  137 , active surface  114  acts on fin  140 , active surfaces  115  and  116  act on tab  138 , and another active surface which is not visible in  FIGS. 15 and 16 , acts on fin  139 . 
     As explained in greater detail below, the operating plunger  107  is movable between an engaged position and a disengaged position of the piston  101 . In the engaged position it is operatively connected to the piston  101 , by means of the active surfaces  112 - 116  being in contact with the piston  101 , and movement of the operating plunger  107  along a path in line with axis A-A is transmitted to the piston  101 . In the disengaged position the operating plunger  107  is operatively disconnected from the piston  101 , as the active surfaces  112 - 116  are axially displaced from the piston  101 . In  FIG. 13  the operating plunger  107  and the piston  101  are in the engaged position, as they are both at their outermost points, and depression of the operating plunger  107  will transmit a driving force to the piston  101 , moving it in a dispensing stroke in the direction of arrow B. 
     In use, the dispenser  100  operates as follows. Firstly, an outer end of the fluid line  127  is placed into a reservoir of fluid to be dispensed (not shown). To prepare the dispensing device  100  for use the piston  101  must be driven back and forth in the cylinder  102  a few times to draw the fluid up the fluid line  127  to the inlet  128  of the cylinder  102 , into the cylinder  102  itself and then down a second fluid line  141 . Once some fluid emerges from the end of the second fluid line  141 , or any spout fixture attached thereto (not shown), then the dispensing device  100  is ready to operate. (The manner in which the piston  101  is driven in this way is the same as the manner in which it is moved to dispense fluid in use, which is described below.) 
     Before the user attempts to depress the operating plunger  107  they can check the aperture  135  for the presence of the face  134  of the indicator member  129 . If the face  134  is present in the aperture  135 , then the piston  101  is located at the top of the cylinder  102  in its primed position, and the dispensing device  100  is ready to operate, as in  FIG. 13 . However, if the piston  101  is not located in its primed position, because it is still moving in the priming direction A after a dispensing stroke has been performed, then the face  134  will not be located in the aperture  135 , and the user can know that they must wait a moment before attempting to depress the operating plunger  107 . If the user attempts to depress the operating plunger  107  before the piston  101  is located at the top of the cylinder  102 , the operating plunger  107  will move along a path in line with axis A-A in the dispensing direction B until it comes into contact once again with the piston  101 , and the operating plunger  107  and the piston  101  will be in the engaged position. If the user continues to depress the operating plunger  107  at this point then a driving force will be transmitted to the piston  101  and it will perform an incomplete dispensing stroke. This will result in an unmeasured quantity of fluid being dispensed. This may or may not be desired. 
     Provided the user has waited for the piston  101  to complete a priming stroke and to have moved to the primed position as shown in  FIG. 13 , they can depress the operating plunger  107  to drive the piston  101  in the dispensing direction B until it reaches the bottom of the cylinder  102 . This action is performed against the strength of the first coil spring  103  and the second coil spring  117 , as well as against the frictional resistance of the fluid exiting the cylinder  102  and travelling down the second fluid line  141 , and as such the user will feel a degree of spring resistance. It will be appreciated that the performance of a full dispensing stroke will pump an amount of fluid from the second fluid line  141  equal to the capacity of the cylinder  102 . Therefore, the dose dispensed is precisely controlled. 
     The user then releases the operating plunger  107 , and it immediately moves axially along a path in line with axis A-A back to its outer point, under the force of the second coil spring  117 . This axial movement of the operating plunger  107  is arrested when the walls  124  and  125  come into contact with the retention tabs  122  and  123  respectively of the retention member  104 . It will be appreciated that the return of the operating plunger  107  to its starting position in this manner is independent of the piston  101 . It will also be appreciated that the indentations  110  and  111  are of such an axial length that even with the operating plunger  107  at its outer point, and the piston  101  in the fully dispensed position, the tabs  137  and  138  and the fins  139  and  140  are still axially co-incident therewith. As such, the operating plunger  107  and the piston  101  stay in the same rotational relationship regardless of their relative axial positions. 
     At the same time that the user releases the operating plunger  107  the piston  101  begins its priming stroke, by moving in the priming direction B under the force of the first coil spring  103 . However, it takes longer for the piston  101  to complete its priming stroke than the operating plunger  107  takes to return to its outer point, because the fluid flow restrictor (not shown) provided in the fluid line  127  restricts the rate at which the fluid can pass therethrough, and therefore the rate at which the piston  101  can draw it into the cylinder  102 . Therefore, an in-built delay is incorporated into the automatic priming action of the piston  101 . 
     As mentioned above, if the user depresses the operating plunger  107  before the piston  101  has returned to the top of the cylinder  102 , the operating plunger  107  will move down until it comes into contact with the piston  101 , and driving the piston  101  from there will result in an unmeasured quantity of fluid being dispensed. Therefore, in dispenser  101  the indicator member  129  is the sole mechanism which acts to prevent the dispensing of an incomplete dose. However, in contrast to dispenser  1  described above, depression of the operating plunger  107  will always result in the dispensing of some fluid, which avoids confusion. In addition, if it is desired to dispense an unmeasured quantity of fluid, this is possible with dispenser  100 . 
     Referring now to  FIG. 17 , this shows a third dispensing device  200  which is identical to dispensing device  100  described above, except that there is no indicator member feature. However, as is clear from  FIG. 17 , operating plunger  201  is connected to retention member  202  by a second coil spring  203 . The retention member  202  comprises an inwardly depending portion  204 , at the bottom of which is a base surface  205 . This constitutes the second static surface referred to above. It will be appreciated that as in dispensers  1  and  100  described above, this base surface  205  is disposed axially between the operating plunger  201  and piston  206 . The second coil spring  203  is mounted inside the operating plunger  201  between the base surface  205  and an underside  207  of end wall  208 . This arrangement is expedient and compact, and allows for the operating plunger  201  to move independently of the piston  206  in an efficient manner. Dispenser  200  therefore provides support for the second aspect of the present invention. 
     The dispensing devices described above can be altered without departing from the scope of claim  1 . For example, in alternative embodiments (not shown), the engagement means operatively connects with the piston through a lateral movement of the operating plunger which is substantially normal to its movement along the stroke axis. In different versions the lateral movement can be from either side, or from above or below the outer point of the operating plunger. In these alternative embodiments the engagement are tabs which interact laterally with slots provided on the pistons. 
     In another alternative embodiment (not shown), the operating plunger is offset from the piston, and its path is oblique to the stroke axis. The operating plunger is mounted to a hinge on the dispensing device about which the part of the operating plunger which is connected to the piston rotates in a planetary fashion. As such, the path is actuate, and a joint between the operating plunger and the piston transmits this motion into a linear motion of the piston along the stroke axis. 
     In another alternative embodiment (not shown), the piston comprises a radially outwardly extending slot, and the operating plunger comprises a radially inwardly extending tab which moves into the slot when the operating plunger is rotated. 
     Therefore, the first aspect of the present invention provides a dispensing device which has a visual indication to the user that the piston has returned independently of the operating plunger to its primed position, and the dispensing device can be operated once again to dispense the intended dosage. This is a much simpler solution to the problem than that presented by the prior art, as there is no requirement for any complex locking mechanism of the operating plunger. This invention finds application in a construction in which there is a mechanism to prevent the piston being driven until it has completed a full priming stroke, and in a construction in which no such mechanism is provided. The second aspect of the present invention provides a compact and expedient internal arrangement to facilitate the independent movement of the operating plunger in relation to the piston.