WO2011144641A1

WO2011144641A1 - Coaxial means for measuring a flow and a method of measuring a flow

Info

Publication number: WO2011144641A1
Application number: PCT/EP2011/058008
Authority: WO
Inventors: Göran Cewers
Original assignee: Mindray Medical Sweden AB
Current assignee: Mindray Medical Sweden AB
Priority date: 2010-05-17
Filing date: 2011-05-17
Publication date: 2011-11-24
Anticipated expiration: 2012-11-17
Also published as: EP2572170A1

Abstract

A method is described for measuring a flow of gas passing a flow meter device. The device causes a pressure drop when a gas flows through it. The pressure drop across the device is a measure of the gas flow. The device is designed as a gas permeable tube (81) which has a flow channel on the upstream side along the tube designed so as to decrease the cross-section of the channel downstream of the gas permeable tube. This provides for a minimum volume in the channel upstream of the device's pressure-drop- generating component. By combining a gas permeable element with a flow meter element having a variable restriction, an advantageous flow meter with fast response times, and a high dynamic flow range is provided.

Description

TITLE: COAXIAL MEANS FOR MEASURING A FLOW AND A METHOD OF MEASURING A FLOW

Related applications

The present application relates to the following applications of the same inventor as the present

application with the following titles: "COAXIAL MEANS FOR MEASURING A FLOW AND A METHOD OF MEASURING A FLOW"

US61/345, 771; and "FLOW RESTRICTOR AND A METHOD TO REDUCE RESISTANCE OF A FLOW" US61/345, 788 ; which are incorporated herein by reference in their entirety for all purposes.

Background of the invention

Technical Field

The disclosure pertains to flow meter elements and methods for measuring flows. More exactly, the disclosure refers to a device and a method which are based on

measuring a pressure drop across a flow restrictor being passed by a flow, giving a measurement of the flow of a medium through the channel where the flow restrictor is positioned. Even more precisely, the disclosure refers to use in medical ventilators, anesthesia machines or similar breathing apparatuses. Some embodiments include a variable flow restrictor and a method for variably altering flow resistance in a flow restrictor. More exactly, the

embodiment refer to reducing the flow resistance in large flows in order to increase the dynamic flow range.

Related Art

Measuring flow by means of a flow restrictor is common and the method works both for liquids and gases, e.g. in flow regulators for ventilators. Examples of ventilator applications are disclosed in US patent

5,265,594 and SE patent 529,989. A gas delivery sensor is disclosed in US 6,164,141.

In US patent 5,265,594 it is disclosed how a flow restrictor is used for measuring gas flow through a gas channel upstream of a control valve.

In patent SE 529,989 it is disclosed how a flow restrictor is used for measuring gas flow through a gas channel downstream of a control valve.

One important difference between gases and liquids when measuring flow is that gases are compressible and liquids are virtually uncompressible . This results in a number of undesirable effects such as the pressure in the gas channel affecting the flow measurement since a change in pressure results in a pressure-generated flow flowing directly into the volume between the flow restrictor and a control valve which generates or controls a flow based on the flow signal from the flow meter.

Should the flow restrictor be located upstream of the control valve, false flows will be generated by pressure variations in the outlet to the device. In a ventilator application this means that valves comprising a plurality of such aggregates with a common outlet will interfere with each other.

One way of minimizing said undesired gas compression effects is to make the volume between the flow restrictor and the control valve as small as possible, which can be done by using a small flow restrictor. However, this is then at the expense of the linearity and dynamics of the flow pressure drop vs. the flow due to increased turbulence at high flows. Flow restrictors can be provided in a variety of ways, e.g. by fluid being led through one or more narrow tubes, various types of nets, sintered fluid permeable tubes or slits.

Flow restrictors are for instance disclosed in

US2007/283962 or US 4,006,634. However, US2007/283962 relates to nasal devices and is not suitable for flow restrictors in flow measurement applications.

In US patent number 4,006,634 a flow meter is disclosed consisting of a differential pressure gauge and variable flow restrictor which diminishes as flow

increases. In this invention the flow restrictor consists of a film with radial slits forming a number of circular shaped flaps. At high flows the flaps bend and the flow resistance decreases. Since it is difficult to cut extremely thin slits in a film, the flow resistance is relatively small in this design and thus it is difficult measure low flows.

One object of the disclosure is to provide a device with flow restriction with a relatively large surface and small volume between the device's flow restrictor and the control valve interface.

One object is to provide a larger initial flow resistance than is present in a design which uses flaps bending under large flows as described in US patent

4,006, 634.

Yet another object is to provide a device with flow restriction with reduced flow resistance in large flows. This is important in removing increased flow resistance caused by turbulence and in increasing the dynamic flow range. By designing the geometry of the device according to the invention for the abovementioned geometries it is possible to make flow restrictors in accordance with the invention .

Summary of the Invention

At least some of these objects are met by means of the devices and the methods in accordance with the appended independent claims, while particular embodiments are dealt with in the dependent claims.

Thus, the present invention seeks primarily to mitigate, alleviate or eliminate one or more of the above- identified deficiencies or disadvantages in the art, singly or in any combination, and solves at least partly the abovementioned issues by providing equipment and methods according to the appended patent claims.

A method is described for measuring a flow of gas passing a flow meter device. The device causes a pressure drop when a gas flows through it. The pressure drop across the device is a measure of the gas flow. The device is designed as a gas permeable tube which has a flow channel on the upstream side along the tube designed so as to decrease the cross-sectional area of the channel downstream of the gas permeable tube. A small volume is produced by adapting the device flow channel and flow restriction to the geometry of the flow profile of the gas flowing out from the flow valve. This provides for a minimum volume in the channel upstream of the device's pressure-drop- generating component.

In some embodiments the outlet from a gas valve is centered, and the outlet flow profile has the appearance of a jointed cone. In other cases the outlet is coaxial with a flow profile which may be described as a tube.

By designing the geometry of the device according to the embodiments of the invention for the abovementioned flow profiles, it is possible to make a flow restrictor for each of the mentioned valve types.

According to a first aspect of the invention, a flow meter element is provided comprising a tube element and a flow restrictor arranged inside the latter.

The tube element may comprise a connection interface to a flow valve. The flow restrictor comprises a closed fluid permeable body so as to form a fluid permeable tube which on the upstream side along the tube has a flow channel designed to decrease the cross-section of the flow channel downstream of the tube.

In an embodiment, the tube element is an outer tube. The outer tube and the fluid permeable body are arranged relative to each other at a distance which decreases along the direction of the flow meter element between the inner side of the outer tube and the fluid permeable body.

By adapting the flow meter element's flow channel and the flow restrictor to the geometry of the flow profile from a fluid flowing out from a flow valve it is possible to obtain a relatively large surface in the flow restrictor and at the same time a small volume between the flow restrictor and the interface (outer tube) to a valve. An increased pressure is generated upstream before the flow restrictor, e.g. by compressing a gas when flow resistance increases, and a pressure drop downstream of the flow restrictor. The pressure differential is proportional to the flow in the channel, allowing the flow to be measured. In a particular embodiment of the flow meter element, the closed fluid permeable body is a cone.

A way of creating the abovementioned design is for the fluid permeable body to be conical in shape turned either upstream or downstream.

In a particular embodiment of the flow meter element the closed fluid permeable body is a partial cone.

A way of creating the abovementioned design is for the fluid permeable body to be partly conical in shape turned either upstream or downstream.

The cross-section of the tube geometry of the outer tube and the fluid permeable tube may either be of circular shape or polygonal shape or ellipsoid shape.

In some embodiments of the invention the flow meter element comprises a differential pressure meter which is connected to each side of the fluid permeable body. The differential pressure meter measures the differential pressure on both sides of the fluid permeable body, which is then used to calculate the flow

Some embodiments of the invention comprise the flow meter element having a flow valve which is connected to a first connection interface to the flow meter element.

Some embodiments of the invention comprise the flow meter element having a second connection interface

identical to the flow valve's connection interface.

By the flow meter element having a mechanical

connection interface adapted after a closed flow valve and a flow meter element having a second connection interface identical to that of the flow valve, the device can be coupled into a link with an already existing design. Another aspect of the invention comprises a flow measurement method comprising a flow restrictor with a relatively large surface and a relatively small volume between the fluid permeable body placed in the flow channel of the flow restrictor and the outer tube of the connection interface; wherein the relatively small volume is provided by the outer tube and the fluid permeable body being arranged at a relative distance from one another which decreases along the longitudinal direction of the flow meter element between the inside of the outer tube and the fluid permeable body thus boosting the pressure upstream (PI) and producing a pressure drop downstream (P2) of the fluid permeable body, the difference between which is proportional to the flow.

Yet another aspect of the invention provides a method for measuring a flow which comprises measuring a flow of a fluid passing a flow meter device, wherein the device causes a pressure drop in the fluid flowing through it, and where the pressure drop across the device is a measure of the fluid flow. The method comprises providing a device which is designed as a fluid permeable tube which on the upstream side along the tube has a flow channel which is designed so that the cross-sectional area of the channel decreases downstream of the fluid permeable tube, whereby this method comprises providing a minimal volume in the channel upstream of the device's pressure-drop-generating part .

The advantages of this method are, as for the above described equipment, that the flow in a flow channel or out of a valve may be measured easily without the occurrence of false flows which might affect flow measurement. According to yet another aspect of the invention a variable flow restrictor is provided. The variable flow restrictor comprises a flexible foil with movable parts such as flaps, and a fluid permeable body. The flexible foil is arranged immediately next to the fluid permeable body where the movable parts are pre-tensioned against the fluid permeable body resulting in a variable fluid flow through the flow restrictor as by increased displacement of the flexible parts with increased flow through the fluid permeable body and the flexible foil.

In order to reduce the flow resistance during

increased flows, a thin flexible foil with flaps is placed close to the fluid permeable body. Thanks to the flaps being in tension, the flow resistance is high in small flows, which is then reduced when the flow increases. The flexible foil should preferably be placed adjoining the fluid permeable body by bending it and placing it with its convex side against the fluid permeable body. The flexible foil may be shaped as e.g. a cone, partial cone or as a cylinder. The tube geometry of the entire design may have a cross-section that may be circular and/or ellipsoid or and/or polygonal.

This may be obtained according to an embodiment where a foil with flaps downstream is placed next to a fixed flow restrictor in the form of a net or some other fluid

permeable material.

One advantageous embodiment of the invention is to place the abovementioned flaps pre-tensioned against the fixed flow restrictor. This method ensures that the flow resistance is maximized at low flows. An advantageous way of pre-tensioning the flaps is to bend the foil with flaps and allow the convex side to lie against the flow

restrictor, which has a matching shape. The flaps will in this case lie pre-tensioned against the flow restrictor. The curvature of the foil may be arbitrary in shape, such as a cone, sphere or cylinder.

Yet another advantageous embodiment of the invention is to provide the flow restrictor with holes or thin material where it is covered by the abovementioned flaps.

In some embodiments of the variable flow restrictor, the fluid permeable body has openings which allow the fluid to flow under the flaps.

Adding these openings or holes, which may also be thinner parts in the material, increases the effect of the variable flow restrictor by reducing the flow resistance more during flows.

In some embodiments of the variable flow restrictor, a differential pressure gauge is connected upstream of the fluid permeable body and downstream of the flexible foil.

Another aspect of the invention comprises a method for reducing the flow resistance in a flow restrictor at an elevated flow rate. The method comprises pre-tensioning moveable parts, such as flexible flaps, on a flexible foil by placing the flexible foil next to a fluid permeable body, thus creating a variable flow restrictor in that the moveable parts are moved more and more as the flow

increases, thus providing a variable fluid flow through the flow restrictor.

The advantages of this method are the same as for the equipment described above, i.e. providing a flow restrictor which reduces flow resistance as flow increases. This is important in removing increased flow resistance caused by turbulence and in increasing the dynamic flow range.

The term flow resistance refers to actual flow resistance as distinct from pressure drop.

A gas permeable element is an element through which gas can flow. For instance, gas can flow through an outer to an inner side of a cylindrical or conical gas permeable element. Its flow resistance will differ for different gas flows there through (across it) . An example for such an element is for instance a sintered metal having a defined pore size. Such sintered elements are commercially

available as "Sica Fil". A gas permeable element has a defined permeability for both fluids and gases. For

example, a fixed gas permeable element may be provided in the form of a net, sintered metal or ceramics of a

particular pore size or cell size, or some other fluid permeable material. For example, gas permeable elements in the form of flow restrictors are used in flow meters to create pressure drops with a desired profile. Differential pressure across a gas permeable element may be measured and provides a measure of the

Brief Description of the Drawings

These and other aspects, features and advantages of which the invention at least is capable of will be apparent and elucidated from the following description of

embodiments of the present invention, reference being made to the accompanying drawings, in which

Figure 1 shows in a schematic view an exemplary embodiment suitable for a flow valve with peripheral circular outlet upstream; Figure 2 shows in a schematic view yet another exemplary embodiment suitable for a flow valve with peripheral circular outlet upstream;

Figure 3 shows in a schematic view an exemplary embodiment suitable for a flow valve downstream with peripheral circular inlet;

Figure 4 shows in a schematic view yet another exemplary embodiment suitable for a flow valve downstream with peripheral circular inlet;

Figure 5 shows in a schematic view an exemplary embodiment suitable for a flow valve with centered outlet upstream;

Figure 6 shows in a schematic view an exemplary embodiment suitable for a flow valve downstream with centered inlet;

Figure 7 shows in a schematic view an exemplary embodiment suitable for a flow valve upstream with peripheral circular outlet;

Figure 8 shows in a schematic view an exemplary embodiment having a flow valve upstream with peripheral circular outlet;

Figure 9 shows in a schematic view an exemplary embodiment using a foil with cut out flap structures 10;

Figures 10-12 show in schematic views exemplary embodiments with a bent foil 21 and a flow restrictor 20; and

Figure 13 shows in a schematic view an exemplary embodiment where the flow restrictor 50 is provided with holes in the area below flaps 52. Description of Embodiments

Specific embodiments of the invention will now be described with reference to the accompanying drawings.

This invention may, however, be embodied in many different forms and should not be construed as limited to the

embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The terminology used in the detailed description of the embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, like numbers refer to like elements .

An exemplary embodiment of a device according to the invention may be obtained as shown in Fig. 1 by a gas flow from a flow valve flowing in the inside of a tube 10 and passing downstream through a gas permeable partial cone 11. Thanks to the shape of cone 11 the cross-sectional area of the flow channel gradually decreases downstream of cone 11. In this manner turbulence minimizes as well as the volume between cone 11 and inlet upstream of tube 10. A pressure PI builds up upstream the cone depending on the flow.

Pressure P2 is measured downstream the cone. The difference between P2 and PI (P2-P1) is a measure of the flow.

Advantageous flow measurement is possible thanks to minimum turbulence and the minimal volume between cone 11 and the inlet upstream of tube 10. Thus flow measurement is largely independent of pressure variations in the flow channel. The signal from the differential pressure P2-P1 provides rapid and reliable gas flow control with a flow valve . The device shown in Figure 1 is suitable for a flow valve with upstream peripheral circular outlet. For example, the flow valve can be fastened to the flow meter using a suitable flange (not shown) or using grooves which close on seals, see, for example, Fig. 8.

In some embodiments of the flow meter system, besides the differential pressure P2-P1, other parameters such as gas temperature, outlet pressure P2, gas viscosity and gas density are measured. These parameters may be linearized. This allows flow to be calculated with extreme accuracy.

Figure 2 shows a schematic view of yet another exemplary embodiment suitable for a flow valve with

peripheral circular outlet upstream. In this embodiment, gas permeable element 21 is cylindrical in shape with a closed end upstream. The cross-section of the flow channel gradually decreases downstream thanks to insert 22

gradually decreasing the cross-sectional area of the flow channel up to the fastening point of the cylindrical gas permeable element 21.

In other embodiments, a conical gas permeable element may be combined with a conical insert, see e.g. Figs. 5 and 6.

The tube and the insert may be designed as an

integrated component.

Figure 3 shows a schematic view of an exemplary embodiment suitable for a flow valve downstream with peripheral circular inlet. A conical gas permeable element 31 is arranged in a tube 30. Here too, the distance in longitudinal direction of the flow meter element between the inside of the outer tube and the fluid permeable body is changed. Figure 4 shows a schematic view of yet another exemplary embodiment suitable for a flow valve downstream with peripheral circular inlet. In this embodiment, a gas permeable element 41 is cylindrical in shape with a closed end downstream. Here too, the distance in longitudinal direction of the flow meter element between the inside of the outer tube and the fluid permeable body is changed.

Figure 5 shows a schematic view of an exemplary embodiment suitable for a flow valve with centered outlet upstream. A conical gas permeable element 51 is combined with a conical insert 52, but the pitch of the two elements differs. The conical gas permeable element 51 has a larger pitch than conical insert 52. In this manner an extremely small volume is provided between the device's flow

restrictor and the interface to a control valve.

Figure 6 shows a schematic view of an exemplary embodiment suitable for a flow valve downstream with centered inlet. A conical gas permeable element 61 is combined with a conical insert 62, but the pitch of the two elements differ. The conical gas permeable element 61 has a lower pitch than the conical insert 62. In this manner an extremely small volume is provided between the device's flow restrictor and the interface to the control valve.

Figure 7 shows in a schematic view an exemplary embodiment suitable for a flow valve with peripheral circular outlet upstream. A conical gas permeable element 71 is combined with a cylindrical insert. Here too the distance in longitudinal direction of the flow meter element between the inside of the outer tube and the fluid permeable body is changed so there is an extremely small volume between the device's flow restrictor and the

interface with the control valve.

In Figure 8 an exemplary embodiment is shown where a valve 200 with central inlet and coaxial outlet is

connected to the device. The device shown in this figure has a mechanical interface to the valve which fits together with the valve's interface. The device interface shown at the bottom of the figure is identical with the valve's interface. This allows the device to be connected as a link in an existing design. A first connection interface is adapted to receive valve 200. A second connection interface at the opposed end of the tubes 80, 82 is provided as a corresponding interface as that of the valve 200. Thus, the unit 80, 81 and other parts 81, 82, 201 may easily be inserted and operated at existing equipment hitherto operated by valves 200 without integrated flow and/or pressure measurement.

An inner wall 82 has a receiving interface for the inner, for instance protruding, first channel (here inlet) of the valve. An outer wall 80 forms an outer wall of the coax arrangement. The outer wall coaxially surrounds at least partly along its length the inner wall 82. Outer wall has a receiving interface for the outer valve interface (upper black ring in Fig. 8) . The differential pressure sensor 201 has ports on each side of a cone 81 of gas permeable material. Cone 81 is a gas permeable element. The gas flow may be measured with the help of the signal emitted by this differential pressure sensor 201. A control loop for the flow and pressure delivered by the valve 200 may thus be provided based on the pressure sensor signals. Some specific gas permeable elements are now

described in more detail with reference to Figs. 9 to 13.

Figure 9 shows a schematic view of foil 101 with cut out flap structures 110. The foil may be attached to a gas permeable elements in form of a flow restrictor, which is described below.

A gas permeable elements in form of a flow restrictor has a defined permeability for both fluids and gases. For example, a fixed flow restrictor may be provided in the form of a net, sintered metal or ceramics of a particular pore size or cell size, or some other fluid permeable material. For example, flow restrictors are used in flow meters to create pressure drops with a desired profile. Differential pressure across a flow restrictor may be measured and provides a measure of the flow.

Figure 10 shows a schematic view of an exemplary embodiment with part of a bent foil 21 and a part of a flow restrictor 120 with a shape matching the contour of the foil 121. In this case the flow restrictor and the foil are cylindrical in shape. Here it is shown how the radially bent out flaps 122 in none tensioned state look before the flow restrictor 20 is assembled together with foil 121.

Figure 11 shows the bent foil 121 and flow restrictor 120 with a shape matching the contour of foil 121

assembled. Here the flaps 132 are shown in contact to flow restrictor 120 in the tensioned condition.

Figure 12 shows the bent foil 121 and flow restrictor 120 with a shape matching the contour of foil 121 when a flow illustrated by the arrows in Fig. 12 passes through flow restrictor 120 and foil 121 installed downstream. Here it is shown how the now radially inwardly bent flaps 142 are bent by a larger flow, shown by the arrows.

Figure 13 shows a schematic view of an exemplary embodiment where flow restrictor 150 is provided with holes 153 in the area below flaps 152 according to a principle of the invention. Here the flaps 152 are shown being bent by a major flow, passing the holes in the flow restrictor 150, shown by the arrows .

An example of a device in Figure 13 according to the invention may be obtained by a fluid flow illustrated by arrows streaming through a flow restrictor 150 and further downstream through foil 121 whose flaps 152, which are an integrated part of foil 150, in major flows are bent downstream and thus open up the area of flow restrictor 150 with the consequent reduction in flow resistance.

As an alternative to or in combination with holes 153, the flow restrictor may be designed of thinned down material or large pore sizes or cell sizes or filter density where it is covered by the abovementioned flaps.

By combining a gas permeable element (flow

restrictor) as described in the embodiments of figs. 9 to 13 with a flow meter element as described above with reference to Figs. 1-8, an advantageous synergy is

obtained. A flow meter with fast response times, and a high flow range is provided.

In the present exemplary embodiment the parts used are circular. However, the geometry of the device is not restricted to these shapes, but the circular shape can be replaced by polygons, ellipses or combinations thereof.

The present invention has been described above with reference to specific embodiments. However, other

embodiments than the above described are equally possible within the scope of the invention. Different method steps than those described above, performing the method by hardware or software, may be provided within the scope of the invention. The different features and steps of the invention may be combined in other combinations than those described. The scope of the invention is only limited by the appended patent claims.

Claims

1. A flow meter element device comprising a tube element (10) and a flow restrictor arranged inside the latter, wherein said tube element may comprise a connection interface to a flow valve, and wherein the flow restrictor comprises a closed fluid permeable body(ll, 21, 31, 41, 51, 61, 71) so as to form a fluid permeable tube which on the upstream side along the tube (10) has a flow channel designed to decrease the cross-section of the flow channel downstream of said tube (10), wherein a differential pressure meter is connected to each side of the fluid permeable body (11, 21, 31, 41, 51, 61, 71), and is

arranged to measure a differential pressure upon a flow across the fluid permeable body (11, 21, 31, 41, 51, 61, 71) .

2. The flow meter element of claim 1, wherein said tube element (10) is an outer tube; wherein said outer tube and said fluid permeable body (11, 21, 31, 41, 51, 61, 71) are arranged to each other at a distance which decreases along the longitudinal direction of the flow meter element between the inner side of the outer tube and the fluid permeable body (11, 21, 31, 41, 51, 61, 71) .

3. The flow meter element of claim 1 or 2, wherein the flow restrictor is a fluid permeable body (11, 21, 31, 41, 51, 61, 71) is shaped as a tube with one closed end.

4. The flow meter element of claim 3, wherein said flow channel downstream of said closed tube has a gradually decreasing cross-sectional area.

5. The flow meter element of claim 3, characterized in that said flow channel upstream of said closed tube has a gradually decreasing cross-sectional area.

6. The flow meter element of claim 3, wherein said closed fluid permeable body (51, 61) is a cone.

7. The flow meter element according to any of claims 1-6, wherein said tube element (10) comprises an insert element (22) that conically changes the flow channel.

8. The flow meter element of claim 3, wherein the closed fluid permeable body is a partial cone (11, 31) .

9. The flow meter element according to any of claims 1-8, wherein the cross-section of the tube geometries is of circular shape, or polygonal shape, or ellipsoid shape.

10. The flow meter element of any of claims 1-8, comprising the cross-section of the tube geometries is a combination of circular shape, and/or polygonal shape, and/or ellipsoid shape

11. The flow meter element of any of claims 1-10, comprising a flow valve which is connected to a first connection interface to said flow meter element.

12. The flow meter element of claim 11, wherein said flow meter element having a second connection interface identical to said flow valve's connection interface.

13. The flow meter element according to claim 1, comprising a flow restrictor with a surface and a volume between the fluid permeable body (11, 21, 31, 41, 51, 61, 71) placed in the flow channel of the flow restrictor and the outer tube (10) of the connection interface; wherein the volume is provided by the outer tube and the fluid permeable body (11, 21, 31, 41, 51, 61, 71) being arranged at a distance from one another which decreases along the longitudinal direction of the flow meter element between the inside of the outer tube (10) and the fluid permeable body (11, 21, 31, 41, 51, 61, 71) thus boosting the pressure upstream (PI) and producing a pressure drop downstream (P2) of the fluid permeable body (11, 21, 31, 41, 51, 61, 71), the difference between which is

proportional to the flow.

14. The flow meter element according to any of the preceding claims, wherein said flow restrictor is a variable flow restrictor for a flow meter element, comprising

a flexible foil (21) with movable parts, such as flaps (22, 32, 42, 52); and

a fluid permeable body (20, 50);

wherein the flexible foil (21) is arranged adjoining the fluid permeable body (20, 50) .

15. The flow meter element according to claim 14, wherein said flexible foil is bent and has a convex side, and wherein the flexible foil (21) is with its convex side positioned against said fluid permeable body (20, 50), wherein the movable parts are tensioned against the fluid permeable body (20, 50) resulting in a variable fluid flow through the flow restrictor by increased displacement of the movable parts with increased flow through the fluid permeable body (20, 50) and the flexible foil (21) .

16. The flow meter element according to claim 14 or 15, wherein the foil (21) is positioned downstream the fluid permeable body.

17. The flow meter element according to any of claims 14-16, wherein the flexible foil (21) is cone shaped.

18. The flow meter element according to any of claims 14-17, wherein the flexible foil (21) is shaped as a partial cone.

19. The flow meter element according to any of claims 14-16, wherein the flexible foil (21) is shaped as a cylinder .

20. The flow meter element according to any of claims 14-19, wherein the fluid permeable body (20, 50) comprises openings (53) adjacent said movable parts which allow fluid to flow there through.

21. The flow meter element according to any of claims 14-19, wherein the fluid permeable body (20, 50) comprises thinner parts in the material or larger pore size or cell size or filter density adjacent the movable parts.

22. The flow meter element according to any of claims 14-21, wherein the tube geometry of design has a cross- section that is shaped circular, polygonal, or elliptical.

23. The flow meter element according to any of claims

14-21, wherein the tube geometry of design has a cross- section that is shaped circular and/or elliptical and/or or polygonal .

24. The flow meter element according to any of claims

14-23, wherein a differential pressure gauge is connected upstream of the fluid permeable body (20, 50) and

downstream of the flexible foil (21) .

25. A method of measuring a flow, which comprises measuring a flow of a fluid passing a flow meter device, wherein the device causes a pressure drop in the fluid flowing through it, and where the pressure drop across the device is a measure of the fluid flow, and wherein the method comprises providing a device which is designed as a fluid permeable tube (11, 21, 31, 41, 51, 61, 71) which on the upstream side along a tube (10) has a flow channel which is designed so that the cross-sectional area of the channel decreases downstream of the tube (10), whereby said method comprises providing a minimal volume in the channel along the tube (10) and upstream of the device's pressure- drop-generating part, said method further comprising measuring a differential pressure on each side of the fluid permeable body (11, 21, 31, 41, 51, 61, 71) .

26. The method of claim 25, comprising measuring the gas temperature, outlet pressure, gas viscosity and gas density, for determining the flow.

27. The method of any of claims 24-26, further comprising reducing a flow resistance in a flow restrictor at an elevated flow rate, said method comprising tensioning moveable parts, such as flexible flaps (22, 32, 42, 52), of a flexible foil (21); this is achieved by bending said flexible foil (21) creating a convex side and placing the flexible foil (21) with its convex side next to a fluid permeable body (20, 50), thus creating a variable flow restrictor in that the moveable parts are increasingly moved as the flow increases, thus providing a variable fluid flow through the flow restrictor.

28. A method for reducing a flow resistance in a flow restrictor at an elevated flow rate, said method comprising tensioning moveable parts, such as flexible flaps (22, 32, 42, 52), of a flexible foil (21); this is achieved by bending said flexible foil (21) creating a convex side and placing the flexible foil (21) with its convex side next to a fluid permeable body (20, 50), thus creating a variable flow restrictor in that the moveable parts are increasingly moved as the flow increases, thus providing a variable fluid flow through the flow restrictor.