Patent Publication Number: US-2013243622-A1

Title: Pump

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of pending U.S. patent application Ser. No. 12/582,665, filed Oct. 20, 2009 entitled, “Pump”, which is a Continuation-in-Part of Application No. 10/593,174, filed Sep. 15, 2006, which is a national stage of International Application No. PCT/NZ2005/000046, filed Mar. 18, 2005. 
    
    
     BACKGROUND TO THE INVENTION 
     This invention relates to a pump. More particularly the present invention relates to a membrane pump. 
     Pumps, which incorporate a flexible element to achieve the pumping action, are known. For example, the flexible element can be in the form of a deformable tube or membrane. A deformable tube pump is described in our international patent specifications WO 99/01687 and WO 02/18790. 
     A membrane pump is disclosed in our PCT specification, WO 2005/088128. That pump uses an elastomeric membrane which is clamped between two pump halves. The membrane has outer dimensions greater than the size of the recess in which it is located, such that compressive forces are created in the elastomeric membrane. This pump provides an improved membrane life over prior pumps. However, the Applicant has found that still further improvements are possible in membrane pumps in order to improve the membrane life, accuracy and other operating parameters of the pump. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved membrane pump. 
     It is a further object of the invention to provide a membrane pump with a long membrane life. 
     It is a further object of the invention to provide a membrane pump with a reliable and accurate pump volume, and which remains accurate over a long life time. 
     It is a further object of the invention to provide improved efficiency over prior membrane pumps. 
     It is a further object of the present invention to provide improved methods of manufacturing membranes and membrane pumps. 
     Broadly according to a first aspect the invention provides a membrane pump including:
     an elongate cavity with opposing surfaces and having a ratio of width to depth in the range 8:1 to 16:1, where the depth is measured from one opposing surface to a mid-point of the cavity;   inlet and outlet passages communicating with the cavity;   a pressure port connected to the cavity; and   a pre-deformed non-elastomeric membrane located within the cavity;   wherein the pre-deformed non-elastomeric membrane:   has a first stable state in contact with one of the opposing surfaces, the first stable state corresponding to completion of an inlet stage of a pumping cycle;   has a second stable state in contact with the other opposing surface, the second stable state corresponding to completion of an exhaust stage of a pumping cycle; and   can be caused to invert from one stable state to the other stable state by application of positive or negative pressure to the cavity via the pressure port.   

     Preferably the ratio of width to depth is in the range 10:1 to 14:1. 
     Preferably the ratio of width to depth is around 12:1. 
     Preferably the non-elastomeric membrane is formed of a non-elastomeric sheet material. 
     Preferably the non-elastomeric membrane is resistant to corrosion by chemicals. 
     Preferably the non-elastomeric membrane is formed from a non-elastomeric fluoropolymer. 
     Preferably the non-elastomeric membrane is formed from one of: polytetrafluoroethylene, perfluoroalkoxy polymer resin or fluorinated ethylene-propylene. 
     Preferably the non-elastomeric membrane is has a thickness in the range 0.002 to 0.025 inches. Preferably the thickness is in the range 0.005 to 0.020 inches. Preferably the thickness is in the range 0.010 to 0.015 inches 
     Preferably the depth is less than 5 mm. Preferably the depth is less than 3 mm. Preferably the depth is in the range 1 to 3 mm. 
     Preferably the pressure port is situated adjacent one end of the cavity. 
     Preferably the outlet passage is situated adjacent the same end of the cavity as the pressure port. 
     Preferably the membrane is clamped between first and second housing sections, each section having a cavity section such that when the housing sections are assembled to form a housing, said cavity is formed. 
     Preferably each opposing surface has continuous curvature. 
     In a second aspect the invention provides a method of manufacturing a membrane pump, including:
     providing a first pump housing section and a second pump housing section, the first and second pump housing sections being shaped to form, when joined, a cavity with opposing surfaces;   positioning a non-elastomeric sheet material membrane between the first and second pump housing sections;   joining the first and second pump housing sections such that the non-elastomeric membrane extends through the cavity; and   permanently deforming the non-elastomeric membrane by applying a pressure to the cavity, thereby forcing the non-elastomeric membrane to conform to one of the opposing surfaces.   

     In a third aspect the invention provides a method of forming a membrane pump membrane, including:
     arranging a non-elastomeric material adjacent a concave surface;   securing the non-elastomeric material at two or more peripheral points; and   permanently deforming the non-elastomeric material by forcing it against the concave surface, such that the permanently deformed non-elastomeric material will conform to a pump surface.   

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following more detailed description of the invention according to one preferred embodiment, reference will be made to the accompanying drawings in which: 
         FIG. 1  is a longitudinal cross-section through the pump, 
         FIG. 2  is an exploded view in cross-section of the pump as shown in  FIG. 1 , 
         FIG. 3  is a transverse cross-sectional view taken between the inlet and outlet ports but showing only two sections of the pump body, 
         FIG. 4  is a perspective view of one housing section of the pump, 
         FIG. 5  is a schematic view of the pump on an exhaust cycle, 
         FIG. 6  is a view similar to  FIG. 5  but of the inlet cycle, 
         FIG. 7  is a cross-sectional view of a second embodiment which incorporates a different form of control mechanism, 
         FIG. 8  is a plan view of a first pump body half according to a further embodiment, 
         FIG. 8A  is an end view of the pump body half of  FIG. 8 , 
         FIG. 9  is a plan view of a second pump body half according to the embodiment of  FIG. 8 , 
         FIG. 9A  is an end view of the second pump body half of  FIG. 9 , 
         FIG. 10  is a plan view of a membrane for use in the pump of  FIGS. 8 to 9A , and 
         FIG. 11  is an end view showing the assembled pump of  FIGS. 8 to 10 . 
     
    
    
     DETAILS DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     Referring firstly to  FIGS. 1-3 , the pump  10  is, according to a preferred embodiment, formed of two housing sections  11  and  12 . When these are assembled together they define an internal pump cavity  13 . Clamped between the housing sections  11  and  12 , as will hereinafter be described, is a membrane  14  which is made from a suitable flexible material. 
     While prior membrane pumps have used flexible elastomeric materials, the Applicant has surprisingly found that the use of a flexible non-elastomeric material in a pump cavity designed specifically for reduced membrane stress provides much improved membrane life. 
     In the preferred form of the invention, the cavity  13  is elongate and, as shown in  FIG. 4 , each end  15  is complex curved. In cross-section as shown in  FIG. 1 , each end is also curved as indicated at  15 . Furthermore, in transverse cross-section as shown in  FIG. 3 , the cavity  13  is also of curved cross-section. The cavity curves gently towards its perimeter, in order to reduce the stresses on the membrane during use. The membrane therefore encounters a gentle continuous curved surface as it comes into contact with the cavity wall, rather than a sharp bend which would create stress in the membrane. 
     The Applicant&#39;s pump may use a small pump volume, defined by the volume of the cavity  13 . One cycle of the membrane pumps this volume of fluid from an inflow port to an outflow port, as will become clear below. Preferably the pump volume is less than 20 ml, more preferably less than 10 ml, ideally around 0.5 to 5 ml. Preferably the pump volume is in the range 0.5 to 20 ml, more preferably 0.5 to 10 ml, ideally around 0.5 to 5 ml. This low pump volume contributes both to the accuracy of the pump and the long life of the membrane. 
     The cavity  13  preferably has a small depth. This means that there is a large surface area of the membrane relative to the pump volume. The cavity depth, measured from one side of the cavity to the half way point of the cavity (this depth is marked “D” in  FIG. 1 ), may be less than 5 mm, preferably less than 3 mm, ideally around 1 to 3 mm. Again, this small depth contributes both to the accuracy of the pump and the long life of the membrane. 
     The cavity is preferably elongate. The cavity may have a length in the range 40 to 100 mm, preferably around 40 to 70 mm. The cavity may have a width in the range 10 to 40 mm, preferably 10 to 20 mm. 
     The pump volume and/or cavity dimensions result in only a small amount of movement of the membrane from one side of the cavity to the other. This reduces stress on the membrane and therefore contributes to long life of the membrane. 
     Preferably the ratio of width of the cavity to depth (as defined above) of the cavity is preferably in the range 8:1 to 16:1, more preferably 10:1 to 14:1, ideally around 12:1. The Applicant has found that these ratios, with appropriate shaping of the chamber walls, determine an arc which significantly reduces the stress on the membrane, leading to long membrane life. Lower ratios place excess stress on the membrane, while higher ratios interfere with the efficient working of the bi-stable membrane. 
     Housing section  11  incorporates a rebate  16 , which effectively results in an upstand or projecting portion  17 . Thus, the cavity section  13   a  is effectively located, at least in part, in the resultant upstanding portion  17 . 
     The other housing section  12  has a recessed portion  18  with cavity section  13   b  extending away from the floor of the recess  18 . Thus, when the two housing sections  11  and  12  are brought together the projecting portion  17  engages snugly within recess  18 . However, the arrangement is such that surface  20  of projecting portion  17 , terminates a distance from the floor  19  of recess  18 . In the preferred form of the invention, this distance D (see  FIG. 1 ) is less than the thickness of the membrane  14 . The reason for this gap D will hereinafter become apparent. 
     The membrane  14  is, in the preferred form of the invention, cut from sheet material. The material is of a type which is compatible with the fluid that is intended to be pumped through the pump  10 . For example, if the fluid to be pumped through the pump  10  is corrosive, then the membrane material is selected such as to be able to withstand the corrosive nature of the fluid. By way of further example, the membrane is selected from a food grade material in the event that the pump is to handle a liquid foodstuff. 
     The various types of materials and applications to which a pump of this type can be put are well known to those skilled in the art. Therefore further description herein is not necessary for the purposes of describing the construction and operation of the pump according to the invention. 
     According to the invention, the membrane  14  is cut in a shape and to a size, which enables it to be snugly fitted into the recess  18 . 
     When the housing section  11  is combined with housing section  12  (the membrane  14  being in place in recess  18 ) the fact that distance D is less than the thickness of the membrane  14  causes the peripheral edge margin portion of the membrane  14  to be sandwiched and securely clamped between opposing surfaces  19  and  20 . This clamping force provides a secure seal between the two sides of the membrane, preventing fluid from flowing between the two sides. One or more sealing elements, such as O-rings, may be provided to assist with this seal. 
     A port  22  is formed in the housing section  12  and opens into the cavity section  13   b.  This port  22  can be offset toward one end of the cavity  13 , as shown in the drawings, or else it can be located midway in the length of the cavity  13 . 
     In one form of the invention, a recessed flow path in the form of a narrow groove  22   a  can be formed in the wall surface of the cavity section  13   b  and extend along the length of the cavity  13  either side of from the port  22 . Also a similar recessed flow path in the form of a narrow groove (not shown) can be formed in cavity  13   b.  The effect of the recessed flow path is to prevent the pump from “choking” when the membrane approaches contact with the surface of the cavity. Such contact could prevent fluid flow from occurring and thereby result in the cavity not fully filling or exhausting. The recessed flow path ensures that flow occurs right down to when the membrane comes into full overall contact with the cavity surface. As an alternative to a single groove, the recessed flow path could be a series of grooves, or lowpoints in a profiled surface (e.g. a ribbed surface, or a roughened surface, or even a surface with projecting pins). 
     In addition to preventing “choking”, the recessed flow paths are believed to contribute to efficient flow of fluid into the cavity, particularly into the cavity from the pressure port. 
     At each end of the cavity section  13   a  is a port, which opens from the cavity  13  to the outer surface  23  of housing section  11 . Port  24  functions as an inflow or inlet port while port  25  functions as an outflow, outlet or exhaust port. Each of inlet ports  24  and exhaust port  25  can, as shown, be made up by a plurality of separate passages  24   a  and  25   a  respectively. A recess  26  is formed in the surface  23  of housing section  11  and into this is engaged a disk of flexible material which forms valve element  27 . Likewise, a valve element  28  in the form of a disk of flexible material is provided in the exhaust valve  25  but it locates in a recess  29  in cover  30 . 
     Cover  30  has connecting pieces  31  and  32  (e.g. in the form of annular walls or turrets) which respectively provide connections for an inlet line (not shown) to inlet valve  24  and an outlet or exhaust line (also not shown) from exhaust valve  25 . 
     As mentioned above, the membrane is formed from a non-elastomeric material. Preferably the membrane is formed from a non-elastomeric sheet material, such as a non-elastomeric sheet polymer material. Preferably the membrane material is chemically inert and/or resistant to corrosion by chemicals. The membrane may be formed from a non-elastomeric fluoropolymer. The membrane may be formed from PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxy polymer resin) or FEP (fluorinated ethylene-propylene). 
     The use of a non-elastomeric fluoropolymer such as PTFE (Teflon) provides a cheap, chemical resistant membrane which will be suitable for almost all uses of the pump. Thus a standard pump can be produced without the need for different membrane materials for different applications. 
     The membrane is permanently deformed such that the deformed shape of the material conforms to the shape of the opposing surfaces of the pump cavity  13 . The membrane will then have a first stable state, in which the membrane lies without further deformation (e.g. extension) against one of the opposing surfaces, and a second stable state, in which the membrane lies without further deformation (e.g. extension) against the other of the opposing surfaces. 
     Permanent deformation of the membrane may be achieved by forcing the non-elastic membrane against a shaped surface. In one embodiment the Applicant&#39;s pump may be assembled. A pressure is then applied to the cavity  13 , to force the membrane against one of the cavity&#39;s opposing surfaces. This pressure must be sufficiently high to cause the membrane to conform completely to the surface and to permanently deform to this shape, so will generally be significantly greater than an operating pressure of the pump. The pressure can be applied via one or more of the flow ports communicating with the cavity  13 . In one embodiment the deformation pressure is around 40 to 50 psi, significantly higher than an operating pressure around 10 to 20 psi. 
     This method has the advantage that the permanent deformation can be achieved as part of the assembly process. The membrane need be formed only as a section of planar sheet material, with three dimensional permanent deformation occurring in situ after assembly of the pump. 
     Alternatively, permanent deformation of the membrane may be achieved by forcing the membrane against a shaped surface before fitting the membrane to the rest of the pump. This shaped surface would be shaped such that the resulting permanently deformed membrane conforms to the shape of the opposing surfaces of the pump cavity  13 . 
     The force used in deforming the membrane can be applied by any suitable mechanism. However, pressure is most easily applied by a pressurised fluid, preferably a pressurised gas. 
     The membrane is non-elastic but still flexible. The membrane may be formed from a sheet material with a thickness in the range 0.002 to 0.025 inches, preferably in the range 0.005 to 0.020 inches, ideally around 0.010 to 0.015 inches. This provides the necessary flexibility to allow the membrane to travel between the two stable states, sufficient stability to cause the membrane to naturally conform to the stable states, allows satisfactory permanent deformation of the membrane as discussed above and provides a durable membrane for long life. Thinner materials tend to lack sufficient stability, while thicker materials are placed under greater stress. 
     The permanent deformation of the membrane may be plastic deformation. The deformation process may be carried out at low temperature (e.g. room temperature). 
     Furthermore, the permanent deformation of the membrane can be contrasted with other techniques such as injection moulding, which would result in a membrane which sits naturally in only one of the stable states. 
     The permanent deformation of the membrane  14  as described above, results in the membrane  14  being bi-stable. One stable position of the membrane  14  is shown in full detail in  FIG. 1  while the other stable position is shown in dotted detail. Thus, in the first stable position the membrane  14  is in the cavity section  13   b  and when in the second stable position the membrane  14  is located in the cavity section  13   a.  In effect therefore, the membrane  14  adopts a stable position in either a position which conforms with completion of intake of fluid through inlet valve  24  (i.e. the position shown in the drawings) and a full or completed exhaust position. 
     A stable position is a position adopted by the membrane in the absence of applied pressure. In the Applicant&#39;s pump there are two such positions as described above. 
     The membrane  14  is moved between its two stable positions by application of negative P 1  and positive P 2  pressures applied to the cavity  13   b  through port  22 . Consequently with the pump in the configuration shown in  FIG. 1  and inlet and outlet conduits or lines attached to connectors  31  and  32  a positive pressure P 2  (see  FIG. 5 ) applied through port  22  will force the membrane  14  into an opposite stable position. In this “stroke” of the membrane  14 , the inlet valve  24  is forced closed while the outlet valve  25  is forced open and any fluid within the cavity  13  i.e. to that side of the membrane opposite to that which faces port  22 , is exhausted through the outlet valve  25 . 
     Upon this “stroke” having been completed a negative pressure P 1  applied via port  22  (see  FIG. 6 ) causes the membrane  14  to return to the position shown in  FIG. 1  which also causes the exhaust valve  25  to close but the inlet valve  24  to open and enable fluid in the inlet line to be drawn into cavity  13 . The cavity  13  thus fills with the fluid ready to be exhausted through the outlet valve  25  upon the next cycle occurring when membrane  14  moves back into cavity section  13   a  under positive pressure P 2 . 
     The means for applying negative and positive pressures can take on many forms as will be apparent to the person skilled in the art. The means could comprise, for example, sources of positive and negative pressure, which via suitable valves can be coupled to the port  22 . 
     Examples of mechanisms we have developed for applying the positive and negative pressures via port  22  are shown in  FIGS. 1 and 7 . 
     As shown in  FIG. 1 , there is a pneumatic operator  33  that has a body  34  which defines a chamber  35  in which a piston  36  is reciprocally mounted. A piston rod  37  is pivotally connected via pivot  38  to the piston  36 . This piston rod  37  is pivotally connected by pivot  39  at its other end to a rotating drive member  40 . The drive member  40  is connected to a drive means (not shown) which can be in the form of an electric motor or some other form of motive power. 
     A port  41  in the end wall  42  of the body  34  is in communication with port  22 . As shown in  FIG. 1  the body  34  is in close proximity to the pump  10  but it will be appreciated by those skilled in the art that the pneumatic operator  33  could be located quite some distance away from the pump  10  and connected by a conduit extending between ports  22  and  41 . 
     A recess  43  is formed in the inside surface of the side wall  34   a  of body  34 . The recess is located adjacent the end of wall  42 . 
     At a position in the length of the side wall  34   a  of the body  34  there is a port  43   a  which opens to atmosphere. As illustrated, the port  43   a  is shown in one preferred position where it is adjacent the inner end of the piston  36  when the piston is at its full stroke away from end wall  42  of body  34 . Thus, once the piston has moved past the port  43   a  (i.e. into the position of  FIG. 1 ) the chamber  35  is fully vented to atmosphere. The position of port  43   a  can be varied dependent on use requirements that may require venting before the full stroke of piston  36  has been completed. 
     Consequently, when the piston  36  advances toward end wall  42  the air in chamber  35  becomes compressed and the resultant positive pressure P 2  works on the membrane  14  to force it into cavity section  13   a.  However, when the piston  36  has completed its stroke toward wall  42  the piston sealing ring  36   a  is positioned within the area of the recess  43  whereby air can flow past the sealing ring  36   a  and exhaust through the clearance between the piston  36  and surface of wall  36   a.    
     Upon its reverse stroke commencing the piston  36  moves so that sealing ring  36   a  moves away from recess  43  and once again seals against the entire peripheral surface of wall  36   a.  Consequently, the movement of the piston creates negative pressure P 1  until the port  43   a  opens to vent the chamber  35  to atmosphere and hence complete the pumping cycle. 
     An alternative arrangement is shown in  FIG. 7 . 
     A port  43 ′ in the wall  34   a  is connected to a conduit  44  which is, in turn, connected to a vent housing  45 . One wall of the vent housing  45  has a vent opening  49  which opens into a chamber  50  in which a pin  51  is moveably located. The pin  51  is therefore moveable between the position where conduit  44  is isolated from vent  49  to a position where the vent  49  is connected to conduit  44 . 
     Mounted with a periphery of the driving member  40  and projecting there from is a pair of curved or shaped (e.g. ramped) projections  52  and  53 . Consequently, as the rotating member  40  rotates, a projection  52  or  53  comes into contact pin  51  which forces the pin  51  inwardly (relative to the housing) thereby connecting or disconnecting the vent  49  from the conduit  44 . 
     This action causes the chamber  35  to vent to atmosphere (via vent  49 ) for the period of time that the pin  51  fails to seal closed the conduit  44 . In the preferred form of the invention the pin  51  is biased by suitable biasing means (not shown) such as a spring or the like into a position where the vent  49  is closed i.e. isolated from conduit  44 . 
     As a consequence, continued movement of the piston  36  creates a positive pressure build up which via port  22  forces the membrane  14  from the position shown in  FIG. 7  to its other stable position in cavity section  13   a.  Material resident in the cavity  13  is thus forced out through the exhaust port  25 . 
     As the piston  36  moves back along the chamber  35  from the second position the vent port  49  will still be closed. This will continue to be the situation until the engagement projection  52  comes into contact with pin  51  to effectively open the vent port  49 . As a result, the vent port  49  once again vents the chamber  35  to atmosphere. After the vent  49  is closed from conduit  44  by movement of the pin  51  and as a result of the pin clearing the projection  52 , the continued movement of the piston  36  back to its first position will create a negative pressure. 
     This negative pressure build up will cause the membrane  14  to move back to the position shown in  FIG. 7  thereby creating a negative pressure within the chamber  13  which draws pumpable medium on the inlet  24  to be drawn through the inlet valve  24  and into the cavity  13 . This inflow will continue until the membrane  14  is fully back into its position shown in  FIG. 7 . 
     Preferably the point and the movement of the piston  36  where contact between the pin  51  and projections  53  respectively occurs is adjustable. According to the preferred form of the invention, projections  52  and  53  can be adjustable in position on the periphery of the driving member or rotor  40  so that, for example, the period during which the piston creates a positive pressure could be less. This would result in the time that the membrane is under negative pressure to be greater than the period that it is under positive pressure. 
     The bi-stable flexible membrane  14  effectively has a small amount of travel between its two states. It is not mechanically connected to any drive thereby giving the membrane free movement in the cavity  13 . The cavity shape is round rectangular and its contoured to fit the bi-stable shape of the membrane. Consequently, the cavity supports the diaphragm over its full surface when the diaphragm is in a so-called stable state. The membrane is therefore subject to uniform pressure not only when in the stable states but during the transition between the states as it is supported on both surfaces by the incoming or outgoing pumpable medium and the positive or negative pressure applied across the whole membrane surface via port  22 . 
     It is believed that the bi-stable nature of the membrane, the cavity shape and contour, as well as the uniform pressure to which the membrane is subjected will lead to a significant reduction in mechanical stress on the membrane. This will therefore equate to longer membrane life. Furthermore, during operation of the pump there will be full removal of fluid on the exhaust stroke and full uptake on the inlet stroke as the membrane  14  moves fully from contact and support within the two sections of the chamber. 
     The pump therefore provides maximum efficiency and good linear flow characteristics, the latter being more critical as viscosity of the pumpable medium increases. The outlet pressure will be governed by the drive pressure therefore no need for pressure limiting. Suction (lift) is governed by the negative pressure. There is thus consistent through put over a wide range of drive pressures. 
     The valves  24  and  25  are located at the half round extremities of the cavity and in close proximity to the cavity. This proximity of the valves to the cavity thus minimises voids thereby giving optimum dry prime and compression ratio. 
     The pump arrangement is such that only low inertia needs to be overcome in order to drive the membrane. The valves are progressively closed and finally close before full exhaust or intake. This means that the last thing to occur as the membrane  14  reaches its stable position is movement of the valves into a closed position or opening is the first thing to occur upon the membrane  14  moving from a stable position. 
       FIG. 8  shows the pressure port side of a pump according to a further embodiment. The pump body half  80  includes a generally flat surface  81  with a shallow depression  82  which forms one half of the pump cavity in the assembled pump. The flat surface  81  may have one or more grooves formed therein for receiving one or more O-ring seals to form a sealed connection with the other pump body half  90 . The depression  82  preferably is dimensioned and shaped as described above and includes a surface feature  84  defining a recessed flow path communicating with the pressure port  85 . 
     A number of holes  86  may be formed on the flat surface  81  and as will become clear below these aid with correct assembly and alignment of the pump body halves and membrane. 
     Note that the pressure port  85  is preferably positioned at the top of the chamber, at the same end as the output port. Counter-intuitively, the Applicant has found that the positioning of the pressure port at the same end as the output port actually improves the performance of the pump. 
       FIG. 8A  is an end view of the pump body half  80 , looking down from the top. This shows that the pump body half is formed essentially as a half cylinder. A connection port  87  communicates with the pressure port  85  to allow connection of a positive/negative pressure source to the pump. 
       FIG. 9  shows the second pump body half  90 . This pump body half includes a flat surface  91  which will rest against the flat surface  81  of the first pump body half in an assembled pump. A depression  92  is formed in the flat surface  91  and has a shape matching the shape of the depression  82  in the first pump body half. 
     An inflow port  93  and an outflow port  94  are formed in the depression, and a recessed flow path is also provided to avoid the “choking” problem described above. Note that the inflow port  93  is preferably positioned at the bottom of the pump chamber, with the outflow port  94  at the top of the chamber. This helps to ensure that air is not trapped within the chamber, since it will naturally flow towards the outflow port and be removed from the chamber as part of the natural operation of the pump. 
     In contrast, prior pumps suffer from decreased accuracy resulting from trapped air in the chamber. Essentially trapped air occupies space in the pump volume and/or limits movement of the membrane and therefore reduces the pump volume in an uncontrolled and unpredictable manner, resulting in inaccuracy and lowered efficiency. Air may be introduced to the pump during priming, and the Applicant&#39;s configuration naturally purges air from the pump. 
     A number of pins  96  extend from the flat surface  91  and cooperate with the holes  86  to ensure correct alignment of the two pump body halves  80 ,  90 . 
       FIG. 9A  is an end view of the top of the second pump body half  90 . An outflow connection port  97  communicates with the outflow port  94  for connection of an outflow conduit to the pump. A similar inflow connection port is provided in the bottom of the second pump body half for connection of an inflow conduit. 
       FIG. 10  is a plan view of the membrane  100  used in this embodiment, before permanent deformation of the membrane. The membrane  100  is a Out sheet material with a number of apertures  101  which cooperate with the pins to ensure correct positioning and alignment of the membrane during assembly. The membrane will be permanently deformed as described above to match the inner surfaces of the depressions  82 ,  92 . 
     One or more sealing elements (e.g. the O-rings described above) create seals between the two flat surfaces  81 ,  91  and the membrane so as to close the pump chamber. 
     The pump body halves may be formed from any suitable material. However, preferably a plastics material is used for ease of manufacture. In addition the material should be resistant to the fluid used to apply pressure and the fluid being pumped. Polypropylene may be suitable for many applications. 
     The pump body halves may be held together by a cover which slides over the assembled cylinder. Alternatively the cover could clamp around the pump halves, or any suitable fasteners could be used. 
     The embodiment of  FIGS. 8 to 11  may otherwise operate in similar manner to the embodiments of  FIGS. 1 to 7 , with valve arrangements, sources of positive and negative pressure etc as described above. 
     The improvements and advantages of the Applicant&#39;s pump are such that for many applications the membrane need no longer be regarded as a part which will require replacement or maintenance during the life of the pump. This is in complete contrast to prior devices where membranes require regular replacement. This alone represents a significant saving in ongoing operational expenditure. Furthermore, because the membrane is a reliable and long-lived component, complex and costly backup systems for preventing contamination in the event of membrane failure will generally not be required. 
     The Applicant&#39;s pump will continue to deliver reliable, accurate pumping throughout the long life of the pump. The pre-deformation of the membrane, small cavity depth and recessed flow paths all contribute to reliable and complete travel of the membrane from one stable state in contact with one opposing surface of the cavity to the other stable state in contact with the other opposing surface of the cavity. This means that the pump volume is reliably pumped from the inflow port to the outflow port with each and every cycle of the membrane. This accuracy is expected to be retained throughout the long life of the pump, with less than 5% change in accuracy over the life of the device. This is a significant improvement over prior pumps. 
     Furthermore the design of the Applicant&#39;s pump housing and membrane means that only a very low level of power is required to cause motion of the membrane. The membrane is pre-deformed, so that input power is efficiently converted into movement of fluid through the pump, not expended in deformation of the membrane. Once motion of the membrane passes a certain point, the pre-deformed membrane tends to move of its own accord into one of its stable states, which is very efficient (despite the fact that this motion is of course resisted by the fluid being pumped). The small chamber depth also means that the distance travelled by the membrane is small. The Applicant&#39;s pump therefore operates at around 95% efficiency, which is around 2 to 2.5 times better than most prior devices. This represents a significant saving in ongoing energy consumption and operating cost. In fact the Applicant&#39;s pump can be adequately powered of a small number of conventional 1.5V battery cells and has twice the battery life of some prior pumps. 
     While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of the Applicant&#39;s general inventive concept.