Abstract:
An apparatus (10) for varying the flow area of a conduit (12). The apparatus comprises a flap (14) mounted in said conduit (12) which is off-center pivotally mounted to pivot in response to flow of fluid (F) through said conduit (12). The apparatus (10) also includes biasing means associated with the flap to rotationally urge the flap against the action of the flow (F).

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus for varying the flow area of a conduit. 
     The invention has been developed primarily for use as a variable size opening for flow measurement in conjunction with upstream and downstream pressure tappings and will be described hereinafter with reference to this application. 
     BACKGROUND OF THE INVENTION 
     A known method of measuring the flow rate of a fluid (liquid or gas) through a conduit involves measuring the pressure either side of a restriction of constant area in the conduit. If the pressure drop across the restriction is recorded for a variety of known flow rates then the function of the flow rate with respect to the pressure drop can be determined. The flow rate is basically related to the pressure drop by a quadratic function (ie. the pressure drop is proportional to the square of the flow rate). When the function is known then, by measuring the pressure drop, it is possible to calculate the corresponding flow rate. Devices capable of measuring the pressure drop include, for example, electronic differential pressure transducers and fluid manometers. 
     Due to the quadratic relationship, small variations or errors in the measuring of the pressure drop at low flow rates produce large variations or errors in the calculated flow rate. Restrictions of constant area also produce a large pressure drop at high flow rates. Accordingly, a restriction of constant area is unsuitable when a large range of flow rates is required to be measured, especially where accuracy is required at the lower end of the range. 
     An example of an application where the above properties are undesirable is the flow rate measurement of air (or other breathable gas) supplied to a patient undergoing continuous positive airway pressure (CPAP) treatment for obstructive sleep apnea. In particular, when the pressure of the gas supplied to the patient is bi-level (in synchronism with patient inspiration and expiration) or autosetting in level, then accurate flow rate readings of down to zero flow are required for triggering purposes by a control system. Also, the flow rate must also be able to be measured at peak flows of up to about 200 liters per minute without a large pressure drop being caused. 
     A restriction of variable area can ameliorate some of the above problems. A prior art variable area restriction includes a resilient plastic flap that, in an unstressed state, almost occludes a restriction in the conduit. The flap deflects to enlarge the permitted flow area of the conduit under the influence of the fluid flowing through the restriction. The higher the air flow, the more the flap deflects, and the larger the restriction area becomes. The resilient flap can be configured to provide an almost linear relationship between pressure drop and flow rate over a useful range of flows. In this way, the resilient flap provides the desired level of accuracy at both relatively low and high flows. Further, as the area of the restriction is increased at high flows, the resilient flap does not cause a large pressure drop at these high flows. 
     However, the resilient flap suffers from the disadvantage that, after continuous use, it can take on a permanently deflected set and therefore provide erroneous or inaccurate readings at low flow rates. 
     It is an object of the present invention to substantially overcome or at least ameliorate these prior art deficiencies, 
     SUMMARY OF THE INVENTION 
     Accordingly, in a first aspect, the present invention discloses an apparatus for varying the flow area of a conduit, said apparatus including: a flap mounted in said conduit, said flap being off-centre pivotally mounted to pivot in response to flow of fluid through said conduit; and biasing means associated with said flap to rotationally urge said flap against the action of said flow. 
     The biasing means preferably urges said flap to a rest position when no fluid is flowing through the conduit. The flap preferably substantially occludes the conduit in the rest position. The flap is also preferably substantially perpendicular to the general direction of fluid flow through said conduit when in the rest position. 
     Desirably, the flap progressively pivots to progressively increase the flow area of the conduit in response to increasing fluid flow therethrough. 
     In a preferred embodiment, the biasing means takes the form of a magnet mounted on the flap which, in the rest position, is positioned between one or more, preferably two, magnets mounted on the conduit remote from the flap magnet. In another preferred embodiment, the flap pivots about a shaft and the biasing means includes a flap magnet eccentrically mounted on the shaft, the flap magnet, in the rest position, being positioned between at least two magnets mounted on the conduit remote the flap magnet. In one form, the shaft passes through a wall of the conduit and the flap magnet is mounted exterior the conduit. The conduit magnets are preferably symmetrically mounted either side of the flap magnet. The flap magnet and the conduit magnets are preferably adapted to provide a repelling or an attracting force therebetween to urge the flap to the rest position. 
     In another preferred embodiment, an attracting magnet is placed on the conduit adjacent the flap magnet, when the flap is in the rest position, to therefore attract the flap magnet and urge the flap into the rest position. In a variation of this embodiment, a ferro-magnetic metal, such as steel, is placed on the flap and is attracted by the conduit magnet to bias the flap to the rest position. Alternatively, a ferro-magnetic metal can be placed on the conduit and a flap magnet used to attract the flap into the rest position. 
     In a still further embodiment, a spring, for example, a clock spring, has one end attached to the flap and the other end attached to the conduit to urge the flap to the rest position. 
     In yet another embodiment, gravity can be used to urge the flap to the rest position. In one form of this embodiment, a weight is preferably placed on the portion of the flap below the hinge axis. In another form of this embodiment, the portion of the flap below the hinge axis is preferably configured to be larger and thereby heavier than that above. These embodiments are particularly suitable when the orientation of the apparatus is constant. 
     In a second aspect, the invention discloses a device for measuring flow rate as a function of differential pressure, the device including an apparatus for varying the flow area of a conduit according to the first aspect, and a means to measure the differential pressure in the conduit upstream and downstream of said flap. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 is a cross-sectional side view of a first embodiment; 
     FIG. 2 is a cross-sectional end view along line 2--2 of the embodiment shown in FIG. 1; 
     FIG. 3 is a side view of the embodiment shown in FIG. 1 under the influence of a relatively low flow rate; 
     FIG. 4 is a side view of the embodiment shown in FIG. 1 under the influence of a relatively high flow rate; 
     FIG. 5 is a side view of a second embodiment; 
     FIG. 6 is a cross sectional end view along line 8--8 of the embodiment shown in FIG. 7; 
     FIG. 7 is a diagranunatical end view of the embodiment shown in FIG. 1; 
     FIG. 8 is a diagrammatical end view of a third embodiment; 
     FIG. 9 is a diagrammatical end view of a fourth embodiment; 
     FIG. 10 is a side view of a fifth embodiment; 
     FIG. 11 is a cross sectional end view of the embodiment shown in FIG. 10; and 
     FIG. 12 is a side view of a sixth embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring firstly to FIGS. 1 and 2, there is shown a first embodiment of an apparatus 10 for varying the flow area of a conduit 12. The apparatus takes the form of a substantially rigid flap 14 which is mounted for off-centre pivotal rotation about axis 16 in response to a flow of fluid F through the conduit 12. The flap 14 has two pivot shafts 15 that each terminate in a relatively sharp cone 19. The pivot shafts 15 are received within two pivot bores 17 which each terminate in a relatively wide cone 21 to provide point contact of low friction between each of the pivot shafts 15 and the associated pivot bores 17. 
     The apparatus 10 also includes biasing means in the form of flap magnet 18 and conduit magnets 20. The magnets 18 and 20 are preferably fabricated from Neodynium Iron Boron type material. The conduit magnets 20 are oriented so that they repel the flap magnet 18 and thereby urge the flap to the position shown in FIG. 1, being the position of the flap when there is no fluid flow through the conduit 12 (hereinafter referred to as the &#34;rest&#34; position). 
     The flap 14 has a relatively large region 22 and a relative small region 24 either side of the pivot axis 16. Accordingly, when fluid flow F is occurring in the conduit 12 the flap 14 is forced to pivot in the direction of arrow 26 (see FIGS. 5 and 6) about axis 16 as the force of the fluid flow F on the relatively large area 22 is higher than that on the relatively smaller area 24. The rotation of the flap 14 in the direction of arrow 26 is opposed and balanced by the repelling force between magnets 18 and 20. 
     FIG. 3 shows the flap 14 rotated to a position θ L  in response to the fluid flow F L . In this position, the force on the flap 14 in the direction of arrow 26 is balanced by the repelling force provided by the magnets 18 and 20 in the direction of arrow 30. The flow area A L  of the conduit 14 is this position is (assuming conduit width to be W) A L  =W×(X L  +Y L ). 
     FIG. 4 shows a flap under the influence of higher force F H  where the flap 14 is rotated to the position θ H . In this position the flow area A H  Of the conduit is=W (X H  +Y H ) which is larger than A L . Accordingly, the higher the flow of fluid rate F through the conduit 12 the larger the flow area of the restriction provided by the flap 14. 
     The apparatus 10 also includes an upstream pressure tapping 32 and a downstream pressure tapping 34. By measuring the differential pressure in the conduit 14 between the upstream and downstream pressure tappings 32 and 34, the function of the flow rate through the conduit with respect to the differential pressure can be determined. The function can be configured to be close to linear over a useful range of flows. 
     FIGS. 5 and 6 show a second embodiment of an apparatus 50 for varying the flow area of the conduit 14. The apparatus 50 has a shaft 52 which protrudes from the conduit 14. The flap magnet 18 is eccentrically mounted on the shaft 52 external the conduit 14. The conduit magnets 20 bias the flap 14 to the position shown in FIG. 7 in a similar manner to that of the first embodiment. 
     FIG. 7 is a diagrammatical representation of the flap 14 within the conduit 12 of the first embodiment. 
     FIG. 8 is a diagrammatical representation of a third embodiment having an irregular shaped flap 60 provided within a correspondingly shaped conduit 62 to exemplify that the flap 60 and the conduit 62 can be of many different shapes. 
     FIG. 9 is a diagrammatical representation of a fourth embodiment having a flap 70 which pivots about the axis 16 near the upper edge 72 of the flap 70 to exemplify that the off-centre pivot axis can be placed at different positions in the flap 70. Further, in this embodiment, gravity alone, or with the assistance of a weight 74, can bias the flap 70 to the rest position. 
     FIGS. 10 and 11 show a fifth embodiment 80 in which a flap magnet 82 is used to attract a conduit magnet 84 to bias the flap 14 to the rest position shown. In a variation, one of the flap magnet or the conduit magnet can be replaced by ferromagnetic metal. 
     FIG. 12 shows a sixth embodiment 90 which uses a spring 92 to bias the flap 14 to the rest position shown. One end of the spring 92 is attached to the conduit 12 and the other to the flap 14. 
     In the first embodiment, the flap 14 substantially occludes the cross-sectional area of the conduit 12 in the rest position. This is a preferred configuration only and in other embodiments (not shown) the flap 14 only partially occludes the conduit when in the rest position. Similarly, in the embodiment of FIG. 1 the flap 14, in the rest position, is substantially perpendicular to the direction of fluid flow F through the conduit 12. This is also a preferred configuration and in other embodiments (not shown) the flap 14 is angled with respect to the flow direction at the rest position. 
     The apparatus&#39; described above possesses the previously described advantages over the fixed area restriction as they function in the manner of a variable area restriction. Further, the apparatus&#39; are also advantageous over the flexible flap restriction previously described as the rigid pivotable flap cannot take on a permanent deflection or set through constant use thereby increasing the accuracy of the apparatus when used for flow measurement. Also, the function between flow rate and differential pressure can be altered to provide other desired relationships by varying the flap size, shape, pivot axis or the force of the biasing means. 
     Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art, that the invention can be embodied in many other forms.