Variable area flow meter

Abstract

A variable area flow meter for measuring the volumetric flow of a fluid flowing through the flow meter includes a body portion having a fluid inlet, a fluid outlet portion and a channel. A fixed boundary is defined between the channel and the fluid outlet portion. A fluid flow path extends between the fluid inlet and fluid outlet. A piston is arranged within the channel and longitudinally displaceable by the flow of fluid. The cross-sectional area of a portion the fluid flow path is variably determined by the structural shape of the piston and the instantaneous location of the piston relative to the fixed boundary. The longitudinal displacement of the piston in the direction of the fluid flow and consequently the cross-sectional area of the portion of the fluid flow path are dependent on the volumetric flow rate of the fluid.

Description

RELATED APPLICATIONS

The present application claims priority from United Kingdom Patent Application No. 0315497.8 filed Jul. 2, 2003, the disclosure of which is incorporated herein by reference in its entirety.

FIELD OF INVENTION

This invention relates to flow meters for measuring the flow rate of a fluid such as a liquid or gaseous medium. The invention particularly relates to variable area flow meters.

BACKGROUND OF THE INVENTION

Generally, flow meters comprise a flow channel and a dynamic barrier arranged to slide within the channel. As fluid flows through the channel the pressure of the fluid acts on the barrier so that it is displaced. The degree of displacement of the barrier is proportional to the rate of volumetric flow of the fluid.

The dynamic barrier is biased to return back to its original position. The biasing force may be provided by the weight due to gravity of the barrier, magnetic means or spring means.

A variable area flow meter is designed to enable the cross-sectional area of the flow path to vary as the flow rate of fluid varies. This feature enables variable area flow meters to measure a greater range of flow rates and to particularly measure low rates of volumetric flow. The cross-sectional area of the flow path generally increases as the flow rate increases. This is typically achieved by using a tapered cylindrical flow channel that widens as it extends towards the outlet of the flow channel. These types of variable area flow meters are very well known and commonly referred to as a “rotameter”. Alternatively, the flow channel may be formed with a tapered longitudinal recessed groove which widens in cross-sectional area as it extends from the inlet end to the outlet end of the channel. Thus, as the flow rate increases the displacement of the dynamic barrier relative to the flow channel increases and so the cross-sectional area of the flow path increases.

Patent document U.S. Pat. No. 4,466,293 (HUHTALA) describes a variable area flow meter comprising a cylindrical flow channel (3), an inlet point (10), an outlet point (12) and an axially movable, spring loaded indicator piston (4) which is shifted by the medium flowing through the channel to different positions depending on the flow quantities. An inclined outflow slot (27) is provided in the flow channel, the slot becoming deeper as it extends from the inlet point to the outlet point. The cross-sectional area of the flow path is formed between the indicator piston and outflow slot and it varies in accordance with the position of the indicator position.

A similar arrangement is disclosed in U.S. patent document U.S. Pat. No. 4,489,614 (deFASSELLE et al) which describes a variable flow meter comprising a body portion in which a flow path extends along a core tube between an inlet and an outlet. A first piston is vertically mounted within the core tube such that it may be displaced in accordance with the rate of flow of liquid along the flow path. A tapered recessed groove is formed within the inner surface of the core tube and the cross-sectional area of the fluid path is defined by the groove and first piston.

According to the an aspect of the invention, a variable area flow meter for measuring the volumetric flow of a fluid flowing through the flow meter comprises: a body portion having a fluid inlet, a fluid outlet portion and a channel and defining a fixed boundary between the channel and the fluid outlet portion; a fluid flow path extending between the fluid inlet and fluid outlet; a piston arranged within the channel and longitudinally displaceable by the flow of fluid; wherein the cross-sectional area of a portion the fluid flow path is variably determined by the structural shape of the piston and the instantaneous location of the piston relative to the fixed boundary whereby the longitudinal displacement of the piston in the direction of the fluid flow and consequently the cross-sectional area of the portion of the fluid flow path are dependent on the volumetric flow rate of the fluid.

Preferably, the cross-sectional area of the portion of the fluid flow path increases as the displacement of the piston increases in the direction of the flow of fluid.

Preferably, at least one longitudinally extending recess is formed in an outer surface of the piston and the said portion of the fluid path is defined by the fixed boundary and the at least one tapered recess. The at least one recess may extend longitudinally in a V-shape such that the width of the at least one recess decreases in the direction of the flow of fluid.

Furthermore, the depth of the at least one recess may vary in the direction of the flow of fluid. Optionally, the depth of the at least one recess increases in the direction of the flow of fluid or the depth of the at least one recess decreases in the direction of the flow of fluid.

Preferably, a lower portion of the at least one recess is flat. Alternatively, a lower portion of the at least one recess is curved.

Alternatively, the piston may comprise a hollow cylinder with a closed leading end and at least one aperture is formed in the cylinder wall, wherein the said portion of the fluid path is defined by the fixed boundary, the at least one aperture and the hollow cylinder.

The at least one aperture may be V-shaped and extends longitudinally along the cylinder wall such that the width of the at least one aperture decreases in the direction of the flow of the fluid. Or the at least one aperture is V-shaped and extends longitudinally along the cylinder wall such that the width of the at least one aperture increases in the direction of the flow of the fluid. Alternatively, the at least one aperture is rectangular in shape and extends longitudinally along the cylinder wall.

Preferably, the channel further comprises a pressure relief region arranged directly above the portion the fluid flow path and adjacent the fixed boundary of the channel.

The cross-sectional width of the piston may vary in the direction of the flow of fluid.

Preferably the flow meter further comprises cleaning means for cleaning the variable area flow meter. The cleaning means may comprise a cleaning membrane arranged to move longitudinally within the channel so as to clean an internal surface of the channel. The cleaning means may comprise a cleaning membrane arranged to clean the piston as the piston is longitudinally displaced. The variable area flow meter may optionally comprise biasing means operative to urge the piston in a direction opposite to the displacement of the piston resulting from the fluid flow. The biasing means may include at least one of a spring, repelling poles of a magnet and gravity.

Finally, the variable area flow meter may preferably comprise indicator means for indicating the volumetric flow rate of a fluid in response to the longitudinal displacement of the piston along the channel in the direction of the flow of fluid.

It has been found that it is significantly cheaper and easier to produce and maintain a flow meter if the piston is structurally shaped such that the cross-sectional area of the portion of the fluid flow path increases as the displacement of the piston increases in the direction of the flow of the fluid. For example, it is cheaper and easier to form a shaped piston than form recesses on an inner surface of the body.

The present invention advantageously lends itself to a modular design much more easily than the prior art. Flow meters having recesses formed in the body are often stored and sold as pre-assembled units, whereas flow meters with a shaped piston may be advantageously stored and sold as separate parts.

Pre-assembled flow meters of the prior art are limited in use since they are only able to determine the flow rate of fluids with similar parameters. In contrast, the modular design of the present invention is able to measure a range of different types of fluids and a range of different types of flow rates because the piston may be swapped to one that has the most appropriate type of shape in accordance with the fluid and flow rate to be tested. Furthermore, it is obviously cheaper and easier to exchange a piston for another piston with a different shape than to change the main body of the flow meter.

It is has been found that a flow meter can be much more compact in size if the piston is shaped rather than the body.

Finally, flow meters with at least one recess in the inner surface of the stationary body are very difficult to keep clean since the cleaning mechanisms are often unable to easily and efficiently access and remove dirt or debris from within the recess. In contrast, it has been found that dirt and debris that collects on or within the shaped piston may be removed much more easily because the piston moves axially within the flow channel relative to a cleaning mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how it may be carried into effect, reference shall now be made by way of example to the accompanying drawings, in which:

FIG. 1 depicts a sectional view of a first embodiment of a flow meter according to the present invention.

FIG. 2 depicts a sectional view of a second embodiment of a flow meter according to the present invention.

FIG. 3 depicts a sectional view of a third embodiment of a flow meter according to the present invention.

FIG. 4 depicts a piston of a flow meter, according to the present invention, with a tapered recess.

FIG. 5 depicts a piston of a flow meter, according to the present invention, with a plurality of tapered recesses.

FIG. 6 depicts the cross-sectional area of a flow path in a flow meter, according to the present invention, when measuring a low volumetric flow rate of fluid.

FIG. 7 depicts the cross-sectional area of a flow path in a flow meter, according to the present invention, when measuring a high volumetric flow rate of fluid.

FIG. 8 depicts a cleaning mechanism for a flow meter according to the present invention.

FIG. 9 depicts an overview of a piston of a flow meter, according to the present invention, with a wide-angled V-shaped recess.

FIG. 10 depicts an overview of a piston of a flow meter, according to the present invention, with a narrow-angled V-shaped recess.

FIG. 11 depicts a cross-sectional view of a piston of a flow meter, according to the present invention, with two shallow, flat-bottomed, tapered recesses.

FIG. 12 depicts a cross-sectional view of a piston of a flow meter, according to the present invention with two deep, flat-bottomed, tapered recesses.

FIG. 13 depicts a cross-sectional view of a piston of a flow meter, according to the present invention, with two curved-bottomed, tapered recesses.

FIG. 14 depicts a cross-sectional view of a piston of a flow meter, according to the present invention with four shallow, flat-bottomed, tapered recesses.

FIG. 15 depicts a sectional view of a piston of a flow meter, according to the present invention, with two sloping recesses.

FIG. 16 depicts a sectional view of a piston of a flow meter, according to the present invention, with two sloping recesses.

FIG. 17 depicts a sectional view of a hollow piston of a flow meter, according to the present invention, with a sloping recess.

FIG. 18 depicts a sectional view of a piston of a flow meter, according to the present invention, with a variable cross-sectional area, a curved tip and two tapered recesses.

FIG. 19 depicts a view of the piston of a flow meter, according to the present invention, as depicted in FIG. 18.

FIG. 20 depicts a cross-sectional profile a piston of a flow meter, according to the present invention, wherein the piston is a hollow cylinder with two rectangular shaped apertures formed in the cylinder wall.

FIG. 21 depicts an overview of a piston of a flow meter, according to the present invention, wherein the piston is a hollow cylinder with a V-shaped aperture formed in the cylinder wall.

FIG. 22 depicts a cross-sectional profile of a piston of a flow meter, according to the present invention, wherein the piston is a hollow cylinder with a V-shaped aperture extending from one end of the piston.

FIG. 23 depicts the cross-sectional area of a flow path in a flow meter, according to the present invention, when measuring a low volumetric flow rate of fluid using a piston as depicted in FIG. 22.

FIG. 24 depicts the cross-sectional area of a flow path in a flow meter, according to the present invention, when measuring a high volumetric flow rate of fluid using a piston as depicted in FIG. 22.

FIG. 25 depicts a cross-sectional area of a flow path in a flow meter, according to the present invention, when measuring a low volumetric flow using a piston with a V-shaped recess that tapers in the opposite direction to the flow of fluid.

FIG. 26 depicts a cross-sectional area of a flow path in a flow meter, according to the present invention, when measuring a high volumetric flow using a piston with a V-shaped recess that tapers in the opposite direction to the flow of fluid.

DETAILED DESCRIPTION OF THE INVENTION

The flow meter illustrated in the Figures consists of a body (1) through which fluid may flow. The body may be formed from a plastic material and/or a metallic material such as steel. The plastic material may be opaque or transparent.

The body comprises a fluid inlet (2), a fluid outlet portion (3) and a channel (4). The fluid inlet (2) and fluid outlet portion (3) are essentially connected by the channel (4).

The fluid inlet (2) and fluid outlet portion (3) may be arranged in parallel or perpendicular to the longitudinal axis of the channel. Furthermore, the fluid inlet (2) and fluid outlet portion (3) may be arranged at opposite ends of the body or part way along the body. FIGS. 1 and 3 depict a flow meter where the fluid inlet is mounted in a sidewall of the body in parallel to the longitudinal axis to the channel and the fluid outlet portion is arranged along an adjacent wall at the other end of the body perpendicular to the longitudinal axis of the channel. FIG. 2 depicts a flow meter where the fluid inlet and fluid outlet portion are arranged parallel to the longitudinal axis of the channel in sidewalls at opposite ends of the body.

The fluid outlet portion (3) may include at least one outlet guide channel (3a) to guide fluid out of the body (1).

The channel (4) is preferably, though not essentially cylindrical in shape.

A fixed boundary (10a) occurs between the channel (4) and fluid outlet portion (3). The boundary is a notional line defining the junction of the channel and the outlet portion (3). As discussed below, the fluid outlet portion (3) may be the outlet itself of may include a pressure relief region (11). As such, the boundary may be the junction of the channel with the outlet itself or the pressure relief zone.

The flow meter further comprises a piston (5) that is arranged to move longitudinally along the channel (4) relative to the fixed boundary (10a). The piston (5) is displaced in the direction of the flow of fluid if a sufficient force is exerted by the fluid as it flows through the flow meter. The displacement of the piston in the direction of the flow of fluid relates directly to the volumetric flow rate of the fluid.

The structural shape of the piston (5) varies along its longitudinal length. This may be achieved by providing at least one recess or aperture.

At least one recess (6) may be formed on the outer surface of the piston (5). FIGS. 1, 2, 3 and 4 depict a piston with only one recess, whereas FIGS. 5, 11 to 16 depict pistons with a plurality of recesses. The recesses are V-shaped and taper in the direction of the flow of fluid. The width of the V-shape recess preferably decreases in the direction of the flow of fluid such that the widest point of the recess is formed at the fluid inlet end of the piston and the narrowest point is formed at the fluid outlet end of the piston. The sidewalls of the V-shaped recess may taper linearly or in a curved manner. The lower portion of the recess may be flat or curved. The recess may alternatively be referred to as a “slot” or “groove” formed in the outer surface of the piston. The piston shaped to include a least one recess may be solid or a hollow cylinder with a closed trailing end.

Alternatively, the piston (5) may be a hollow cylinder with a closed leading end with at least one aperture (13) formed in the wall of the cylinder. The apertures may be V-shaped, rectangular or lozenge-shaped. If the apertures are V-shaped then they may arranged such that the width of the aperture either increases or decreases in the direction of the flow of fluid. FIG. 20 depicts a cross-sectional profile of a hollow piston with two rectangular apertures. FIG. 21 depicts an overview of a hollow piston with a single V-shaped aperture arranged to taper in the direction of the flow of fluid. FIG. 22 depicts an overview of a hollow piston with a single V-shaped aperture that extends from one end of the piston and tapers in the direction of the flow of fluid.

As the fluid flows through the flow meter it follows a fluid flow path. The fluid flow path extends between the fluid inlet and fluid outlet portion. A portion of the fluid flow path may be defined by the boundary of the channel and at least one recess. The cross-sectional area of this portion of the fluid flow path is determined by the shape of the recess and the position of the piston relative to the boundary. The cross-sectional area of this portion of the fluid path varies as the piston is displaced within the channel relative to the boundary. The recesses on the piston are arranged to taper in the direction of the flow of fluid such that the cross-sectional area of the portion of the fluid flow path increases as the piston is displaced by the force exerted by the fluid. Alternatively, a portion of the fluid flow path may be defined by the boundary, at least one aperture and the hollow cylinder. The cross-sectional area of this portion of the fluid flow path is determined by the shape of the aperture, the hollow cylinder and the position of the piston relative to the boundary. Again, the cross-sectional area of the portion varies as the piston is displaced within the channel relative to the boundary. The at least one aperture formed in the cylinder wall of the piston are arranged such that the cross-sectional area of the portion preferably increases as the piston is displaced by the force exerted by the fluid.

The body may include a bush (10) against which a portion of the piston slides as it is displaced within the channel. The piston is arranged such that it always extends longitudinally beyond the bush. The edge of the bush proximate the outlet end of the channel may act as the boundary of the channel.

The channel may further include a pressure relief region (11) defined by the boundary, the internal surface of the channel and the at least one recess. Alternatively, the pressure relief region may be defined by the boundary, the internal surface of the channel and the at least one aperture. The pressure relief region (11) may be formed in the fluid outlet portion (3).

FIGS. 6 and 7 show how the cross-sectional area of the fluid flow path, defined by the at least one V-shaped recess and boundary, varies as the piston is displaced relative to the boundary. FIG. 6 depicts the displacement of the piston when a fluid with a relatively low volumetric flow rate flows through the flow meter. Although the piston has moved to the left due to the fluid force, the width W1 across the V-shaped recess remains reasonably small and so the increase in the cross-sectional area of the fluid path is small. However, if the volumetric rate of flow is relatively high then the piston is pushed much further to the left (see FIG. 7) and so the width W2 of the recess significantly increases which leads to a significant increase in the cross-sectional area of the fluid path. Obviously, if the cross-sectional area increases then the volume of the fluid flow path defined by the recess, boundary and pressure relief region increases as the piston is displaced due to an increase in volumetric rate of flow.

FIGS. 23 to 26 show how the cross-sectional area of the fluid path, defined by the at least one aperture, hollow cylinder and boundary, varies as the piston is displaced relative to the boundary. FIG. 23 depicts the displacement of a piston, with a V-shaped aperture that decreases in width in the direction of the flow of fluid, when the fluid has a relatively low volumetric rate of flow and shows how the cross-sectional area Al of the fluid flow path is relatively small. However, if the flow rate increases, then the piston is pushed further to the left and so the cross-sectional area A2 of the portion of the fluid flow path increases. Likewise, FIGS. 25 and 26 show how the cross-sectional area of the fluid flow path increases when the rate of flow increases using a piston with a V-shaped aperture that increases in width in the direction of the flow of fluid.

Flow meters often become blocked due to impurities, dirt and debris carried by the fluid. For example, impurities in a liquid medium such as humus substances in water may form sediments on the internal surfaces of the channel. Also, solid particles may accumulate in the recesses. As dirt collects the fluid flow path becomes restricted and the flow meter becomes increasingly inaccurate. Therefore, the flow meter may include cleaning means to help remove impurities, dirt and debris. FIGS. 1, 2 and 3 depict a cleaning membrane (7) that is arranged within the channel of the flow meter between the fluid inlet and piston. FIG. (8) shows how the cleaning membrane includes holes (7a) so that the fluid can pass through freely and its flow it not hampered. The cleaning membrane is arranged such that it may be moved independently along the channel using a handle (7b). As the cleaning membrane slides along the channel the outer ring (7c) removes sediments and dirt that has collected on the internal surface of the channel. The removed debris is then carried by the fluid flowing through the flow meter and out through the fluid outlet. FIGS. 1, 2, and 3 also depict a cleaning membrane (7′) that is arranged within the channel between the piston and the fluid outlet. This particular cleaning membrane is permanently mounted with respect to the piston such that it can clean the outer surface of the piston, that at least one recess formed on the outer surface of the piston or may clean the inside of the piston if it is a hollow cylinder with at least one recess as the piston is displaced along the channel. Again, the unwanted debris is carried by the fluid towards the fluid outlet.

The flow meter may include biasing means (9) to provide a biasing force on the piston. The biasing force pushes the piston back to its original starting position if fluid stops flowing through the flow meter. The biasing means may include a coiled spring (91) as depicted in FIGS. 1 and 2, and/or may include the repelling poles of a magnet (92, 93) as depicted in FIG. 3. Furthermore, if the flow meter is arranged vertically then the weight of the piston due to the force of gravity acts as a biasing force.

Indicator means may work in conjunction with the piston in order to translate the displacement of the piston into a readable value of the volumetric flow of the fluid. The flow meter may include a linear measuring scale arranged relative to the piston such that a user may be able to measure the displacement of the piston and determine the subsequent volumetric flow rate of the fluid.

FIGS. 1 to 3, 6 & 7 show how a linear measuring scale (12) may be aligned on the body of the flow meter alongside the piston to indicate the rate of flow. In this particular example, a portion of the body is transparent so that the displacement of the piston within the channel may be detected and measured by the linear scale. As discussed above, the transparent portion of the body may be formed from transparent plastic. The flow meter may optionally or alternatively include a measuring dial that works in conjunction with the piston to indicate the total displacement of the indicator piston/volumetric flow rate of the fluid.

The operation of a variable area flow meter should now be readily apparent. Essentially, fluid flowing through the flow meter creates a fluid pressure that exerts a force on the piston. The piston is displaced in the direction of the flow of fluid until the cross-sectional area of a portion of the fluid flow path and consequently the volume of the fluid flow path, increases until it is sufficiently large enough to release the fluid pressure so that the flow meter reaches equilibrium. The volume of the portion of the fluid flow path equates to the volumetric flow of fluid when equilibrium occurs. Thus, the final displaced position of the piston indicates the volumetric flow rate of the fluid.

The number of recesses, angle of the V-shape, depth of the groove and cross-sectional profile of a recess has an effect on the pressure generated and released within the flow meter, the range of volumetric flow rates the flow meter can determine and the range of fluids a flow meter can measure. Thus, flow meters must be designed with the most appropriate type of recesses for measuring certain types of fluids in particular circumstances.

Also, the number of apertures, length of an aperture, angle of the V-shape or width of rectangle has an effect on the pressure generated within the flow meter, range of volumetric flow rates the flow meter can determine and the range of fluids a flow meter can measure. Accordingly, flow meters must be designed with the most appropriate type of aperture for measuring certain types of fluid in particular circumstances.

As discussed above, prior art variable area flow meters, having recesses or apertures formed in the body, are often stored and sold as pre-assembled units whereas the flow meters depicted in the Figures have a modular design such that they may be stored and sold as separate parts. The prior art models are limited to the testing fluids with similar parameters and are limited to a small range of flow rates. This is because the body cannot be swapped to another having more appropriate recesses in accordance with the fluid and flow rate to be tested. The present invention provides a flow meter that is able to determine the volumetric flow rate for a greater range of fluids under different types of conditions because the piston may be easily exchanged for a piston with more suitable recesses or apertures so that the flow meter can provide a more accurate reading.

The angle of the V-shaped recess or apertures may be varied so that different flow rates of different fluids may be more accurately measured. FIGS. 9 and 10 depict pistons with different sized V-shaped recesses. FIG. 9 depicts a piston with a wide angled V-shaped recess whilst FIG. 10 depicts a piston with a narrowed angled V-shaped recess. The angle of the V-shaped recess or aperture may range from 1 to 89°. However, it is preferable for the V-shaped recess or apertures to range from 5 to 70°. The size of angle is dependent on the length of the recess or aperture and the length and/or cross-sectional width of the piston.

FIGS. 11 and 12 depict the cross-sectional profile of a piston with two recesses. The recesses of the piston in FIG. 11 have a shallow depth whilst the recesses in the piston shown in FIG. 12 are much deeper.

Both sets of recesses in FIGS. 11 and 12 have a lower portion that is flat. Whilst the recesses of the piston depicted in FIG. 13 have a curved lower portion.

FIG. 14 depicts a cross-sectional profile of a piston that has four, shallow recesses with flat lower portions.

The recesses may also taper by sloping longitudinally in the direction of the flow of fluid. FIG. 15 depicts a piston in which the recesses incline at an angle in the opposite direction to the flow of fluid, whilst FIG. 16 depicts a piston in which the recesses incline at an angle in the direction of the flow of fluid. The angle of inclination may range from 0 to 90°. However, it is preferable for the angle of inclination to range from 5 to 70°. Again, the angle of inclination depends on the length of the recess and the length and/or cross-sectional width of the piston.

FIG. 17 depicts a hollow piston with a closed trailing end with only one recess. Obviously, a hollow piston is much lighter weight than a solid piston. Biasing means such as a spring and/or magnet may be placed within the hole of the piston.

The clearance gap between the bush and piston may need to be varied in accordance with the type of fluid being measured. This may be achieved by using pistons of different cross-sectional widths for different fluids and/or by tapering the cross-sectional width of a piston. FIGS. 18 and 19 depicts a piston where the width of the piston varies along its length and the tip of the piston at the fluid inlet end curves aerodynamically.

Claims

1. A variable area flow meter for measuring the volumetric flow of a fluid flowing through the flow meter comprising:

a body portion having a fluid inlet a fluid outlet portion and a channel and defining a fixed boundary between the channel and the fluid outlet portion;

a fluid flow path extending between the fluid inlet and fluid outlet;

a piston arranged within the channel and longitudinally displaceable by the flow of fluid;

wherein

the cross-sectional area of a portion the fluid flow path is variably determined by the structural shape of the piston and the instantaneous location of the piston relative to the fixed boundary whereby the longitudinal displacement of the piston in the direction of the fluid flow and consequently the cross-sectional area of the portion of the fluid flow path are dependent on the volumetric flow rate of the fluid.

2. The variable area flow meter according to claim 1 wherein the cross-sectional area of the portion of the fluid flow path increases as the displacement of the piston increases in the direction of the flow of fluid.

3. The variable area flow meter according to claim 1 wherein at least one longitudinally extending recess is formed in an outer surface of the piston and the said portion of the fluid path is defined by the fixed boundary and the at least one tapered recess.

4. The variable area flow meter according to claim 3 wherein the at least one recess extends longitudinally in a V-shape such that the width of the at least one recess decreases in the direction of the flow of fluid.

5. The variable area flow meter according to claim 3 wherein the depth of the at least one recess varies in the direction of the flow of fluid.

6. The variable area flow meter according to claim 5 wherein the depth of the at least one recess increases in the direction of the flow of fluid.

7. The variable area flow meter according to claim 5 wherein the depth of the at least one recess decreases in the direction of the flow of fluid.

8. The variable area flow meter according to claim 3 wherein a lower portion of the at least one recess is flat.

9. The variable area flow meter according to claim 3 wherein a lower portion of the at least one recess is curved.

10. The variable area flow meter according to claim 1 wherein the piston is a hollow cylinder with a closed leading end and at least one aperture is formed in the cylinder wall, wherein the said portion of the of the fluid path is defined by the fixed boundary, the at least one aperture and the hollow cylinder.

11. The variable area flow meter according to claim 10 wherein the at least one aperture is V-shaped and extends longitudinally along the cylinder wall such that the width of the at least one aperture decreases in the direction of the flow of the fluid.

12. The variable area flow meter according to claim 10 wherein the at least one aperture is V-shaped and extends longitudinally along the cylinder wall such that the width of the at least one aperture increases in the direction of the flow of the fluid.

13. The variable area flow meter according to claim 10 wherein the at least one aperture is rectangular in shape and extends longitudinally along the cylinder wall.

14. The variable area flow meter according to claim 1 wherein the channel further comprises a pressure relief region arranged directly above the portion the fluid flow path and adjacent the fixed boundary of the channel.

15. The variable area flow meter according to claim 1 wherein the cross-sectional width of the piston varies in the direction of the flow of fluid.

16. The variable area flow meter according to claim 1 further comprising cleaning means for cleaning the variable area flow meter.

17. The variable area flow meter according to claim 16 wherein the cleaning means comprise a cleaning membrane arranged to move longitudinally within the channel so as to clean an internal surface of the channel.

18. The variable area flow meter according to claim 16 wherein the cleaning means comprise a cleaning membrane arranged to clean the piston as the piston is longitudinally displaced.

19. The variable area flow meter according to claim 1 further comprising biasing means operative to urge the piston in a direction opposite to the displacement of the piston resulting from the fluid flow.

20. The variable area flow meter according to claim 19 wherein the biasing means comprise at least one member selected from the group comprising a spring, repelling poles of a magnet and gravity.

21. The variable area flow meter according to claim 1 further comprising indicator means for indicating the volumetric flow rate of a fluid in response to the longitudinal displacement of the piston along the channel in the direction of the flow of fluid.