Pump

1. A membrane pump including:

i. an elongate cavity with opposing surfaces and having a length greater than its width and a ratio of width to depth in the range 10:1 to 16:1, where the depth is measured from one opposing surface to a mid-point of the cavity;

ii. inlet and outlet passages communicating with the cavity;

iii. a pressure port connected to the cavity wherein the pressure port is offset along the length of the cavity; and wherein an exhaust passage is directly connected to the cavity to feed fluid directly from the cavity to the outlet passage, wherein the exhaust passage is formed at the same end of the cavity as the pressure port

iv. a pre-deformed non-elastomeric membrane located within the cavity, and substantially coextensive with the cavity;

wherein the pre-deformed non-elastomeric membrane:

a) 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;

b) has a second stable state in contact with the other opposing surface, the second stable state corresponding to completion of an exhaust stage of the pumping cycle; and

c) 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.

2. A pump as claimed in claim 1 wherein the ratio of width to depth is in the range 10:1 to 14:1.

3. A pump as claimed in claim 1 wherein the ratio of width to depth is around 12:1.

4. A pump as claimed in claim 1 wherein the non-elastomeric membrane is formed of a non-elastomeric sheet material.

5. A pump as claimed in claim 1 wherein the non-elastomeric membrane is resistant to corrosion by chemicals.

6. A pump as claimed in claim 1 wherein the non-elastomeric membrane is formed from a non-elastomeric fluoropolymer.

7. A pump as claimed in claim 1 wherein the non-elastomeric membrane is formed from one of: polytetrafluoroethylene, perfluoroalkoxy polymer resin or fluorinated ethylene-propylene.

8. A pump as claimed in claim 1 wherein the non-elastomeric member is has a thickness in the range 0.002 to 0.025 inches.

9. A pump as claimed in claim 8 wherein the thickness is in the range 0.005 to 0.020 inches.

10. A pump as claimed in claim 8 wherein the thickness is in the range 0.010 to 0.015 inches.

11. A pump as claimed in claim 1 wherein the depth is less than 5 mm.

12. A pump as claimed in claim 1 wherein the depth is less than 3 mm.

13. A pump as claimed in claim 1 wherein the depth is in the range 1 to 3 mm.

14. A pump as claimed in claim 1 wherein the pressure port is offset along the length of the cavity so as to be adjacent one end of the cavity and wherein the nonelastomeric membrane is substantially coextensive with the cavity.

15. A pump as claimed in claim 14 wherein the outlet passage is situated adjacent the same end of the cavity as the pressure port.

16. A pump as claimed in claim 1 wherein 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.

17. A pump as claimed in claim 15 or 16 wherein each opposing surface has continuous curvature.

18. 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.

19. A pump as claimed in claim 1 , wherein the elongate cavity has a length in the range 40 to 70 mm and a width in the range 10 to 20 mm.

20. A pump as claimed in claim 1 wherein recessed flow paths are formed in the opposing surfaces such that fluid can flow along each surface even when the membrane is in contact with that surface.

21. A membrane pump including:

i. an elongate cavity with opposing surfaces and having a length greater than its width and a ratio of width to depth in the range 10:1 to 16:1, where the depth is measured from one opposing surface to a mid-point of the cavity;

ii inlet and outlet passages communicating with the cavity;

iii a pressure port connected to the cavity and offset towards one end thereof, and wherein an exhaust passage is directly connected to the cavity to feed fluid directly from the cavity to the outlet passage wherein the exhaust passage is formed at the same end of the cavity as the pressure port; and

iv. a pre-deformed non-elastomeric membrane located within the cavity, and substantially coextensive with the cavity;

wherein the pre-deformed non-elastomeric membrane:

a) has a first stable state in contact with one of the opposing surfaces, the first stable sate corresponding to completion of an inlet stage of a pumping cycle;

b) has a second stable state in contact with the other opposing surface, the second stable state corresponding to completion of an exhaust stage of the pumping cycle; and

c) 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.

22. The method of claim 18 wherein the non elastomeric sheet material membrane is substantially planar when inserted.

23. The membrane pump of claim 1 where in the pre-deformed non-elastomeric membrane is substantially coextensive with the elongated cavity.