Abstract:
A pitot-static device, comprising first and second pluralities of hollow spokes extending in a radial direction from a central hub, the hollow interiors of the spokes of the first plurality being connected so as to allow fluid communication therebetween, and at least all but one of the spokes of the first plurality having at least one aperture facing in a first axial direction that is transverse to the radial direction, and the hollow interiors of the spokes of the second plurality being connected so as to allow fluid communication therebetween, and at least one of the spokes of the second plurality having an aperture at an end portion thereof that faces in the radial direction.

Description:
This Application is a Section 371 National Stage Application of International Application No. PCT/GB2008/002879, filed Aug. 22, 2008 and published as WO 2009/024803 A2 on Feb. 26, 2009, the content of which is hereby incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a pitot-static device. 
     BACKGROUND ART 
     There are a range of airflow measurements for which an anemometer is the preferred solution. These include spot measurements of airflow are variable locations, such as beneath ceiling-mounted air-conditioning vents, for example. At such locations, it is aesthetically undesirable and technically unnecessary to provide a fixed or otherwise permanent airflow sensor, so a technician will take a handheld sensor to the location and hold it in the airflow of the vent 
     Such sensors typically comprise a rotateable vane that is mounted within a circular protective ring. The vane is typically of a metallic or plastics material, and a Hall effect or optical sensor in a handle portion extending from the ring can detect when one of the blades of the vane passes by. From this, the rotational speed of the vane can be determined, and knowledge of the aerodynamic properties of the vane will allow this to be converted to an airflow speed in the vicinity of the sensor. 
     Such anemometers suffer from distinct difficulties in practice. Principally, the rotating vane will have an inertia which must be overcome. This will impose a reaction time on the sensor output, and will make the sensor insensitive to small airflows. This can be reduced by reducing the mass of the vane, for example by using thin gauge sheet of a lightweight material such as aluminum, but such measures will reduce the rigidity of the vane and make it vulnerable to deformation on rough handling or shock, for example. Such deformation will change the aerodynamic properties of the vane and affect the accuracy of the sensor. 
     Further, the rotating vane is a moving part and hence in principle more vulnerable to wear, degradation, and the like. 
     Pitot-static devices are also used for measurement of airflow, as (for example) disclosed in GB-A-2,164,159. These are however bulky and have not been used for “on-the-spot” measurement via a handheld device. 
     A product known as the “Wilson Flow Grid” allows the measurement of airflow in a conduit such as a heating, ventilation or air-conditioning conduit or duct. It comprises a pair of square or circular grids of hollow conduits transverse to the airflow, one in front of the other. The frontmost grid has apertures in the sides of the conduits, facing into the airflow; these allow the dynamic pressure to be sampled. The rearmost grid has apertures on the two lateral sides of the conduit, at approximately 90° to the airflow, to sample the static pressure. In a rectangular grid (for a rectangular section conduit), the conduits form a gridiron pattern. In a circular grid (for a circular section conduit), the conduits are arranged as spokes from a central hub. 
     U.S. Pat. No. 4,453,419 shows a flow measurement device for use in a conduit, with two sets of radially-extending hollow spokes, one in front of the other. The spokes have apertures on their outer faces; thus one set has apertures facing forwards and one has apertures facing rearwardly. Each set of spokes emanates from one of two central hubs, from which dynamic and static pressure measurement are taken. 
     These devices are only suitable for use in fixed ducts or conduits, however. They are bulky and heavy, and not suited to portable use. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a handheld airflow measurement device that employs pitot-static principles instead of a rotating vane. A pitot-static device needs no moving parts and has little or no inertia. 
     In its first aspect, the present invention therefore provides a pitot-static device comprising a first plurality of hollow spokes and a second plurality of hollow spokes separated by an unimpeded flow path, the spokes of the first plurality being connected so as to allow fluid communication between their hollow interiors and each having at least one aperture facing in a first axial direction that is transverse to the spokes, the spokes of the second plurality being connected so as to allow fluid communication between their hollow interiors and each having at least one aperture facing in a second axial direction that is opposed to the first axial direction. 
     Such a device differs from the Wilson Flow Grid due to the location of the static ports. This change assists the accuracy of the device in measuring low rate airflows, and also allows the device to be made symmetrical. The latter advantage means that the device can be assembled from two identical half-mouldings and can be bi-directional; all these advantages assist in the creation of a handheld pitot-static device. 
     The spokes can extend radially from a central hub, with the hollow interiors of the spokes connected via one or more interior spaces within the hub. This allows the device to adopt a form and structure more closely resembling an anemometer, thereby clarifying it suitability as a direct replacement. 
     The second plurality of spokes can each have an aperture at the end thereof, which allows a more accurate determination of the static pressure. To maintain the symmetricality of the device and allow its manufacture as two half-mouldings, the spokes of the first plurality can also have an aperture at an end thereof, although that will need to be sealed against fluid communication at (perhaps) a later stage of manufacture. These end apertures can be additional to or as a replacement for the reverse-directed apertures of the second plurality. 
     The device can comprise a handle for manual support, to allow it to be carried and located as required. Alternatively, or in addition, it can comprise a socket for attaching the device to a pole or other support. 
     In another aspect, the present invention provides a pitot-static device comprising a first plurality of hollow spokes extending radially from a central hub and a second plurality of hollow spokes extending in a radial direction from a central hub, the spokes of the first plurality being connected so as to allow fluid communication between their hollow interiors, and at least all but one having at least one aperture facing in a first axial direction that is transverse to the radial direction, the spokes of the second plurality being connected so as to allow fluid communication between their hollow interiors, and at least all but one having at least one aperture, the device further comprising a handle for manual support. Having a handle, the device will be suited to portable uses; by combining this with a pitot-static measurement of airflow, the problems inherent in anemometer-based devices are avoided. 
     In this aspect, at least all but one of the spokes of the second plurality preferably has an aperture at an end portion thereof that faces in the radial direction. Likewise, for simplicity of manufacture, at least one of the spokes of the first plurality also preferably has an aperture at an end thereof which is sealed against fluid communication. The device of any of the above aspects can further comprise a ring around the central hub, the spokes extending from the hub to an inner face of the ring. Where some or all spokes have an aperture at the end thereof, the end apertures can extend through the ring. 
     A handle (where provided) can be conveniently attached to the ring. A conduit can usefully be provided between the central hub and the handle, to convey the pressure measurements to the handle and hence to an external measurement apparatus such as via a connector on the handle for external fluid conduits. The conduit preferably has no apertures between the central hub and the handle, as apertures on the conduit would contribute disproportionately to the pressure measurement. 
     The hub can be formed of a moulding integral with the spokes, covered by a suitable cap. This assists greatly in the manufacture of the device as simply as possible, and the present invention therefore also relates to such a device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; 
         FIG. 1  shows a perspective view of a device according to the invention; 
         FIG. 2  shows a perspective view of the top region of a half-moulding suitable for assembling into the device of  FIG. 1 ; 
         FIG. 3  shows a perspective view of the underside of the half-moulding of  FIG. 2 ; 
         FIG. 4  shows a longitudinal section through the device of  FIG. 1 ; 
         FIG. 5  shows an enlarged view of a region of  FIG. 4 ; 
         FIG. 6  shows a plot of actual air velocity (V t ) against measured air velocity (V m ) for a range of embodiments, between 0 and 25 ms −1 ; 
         FIG. 7  shows a plot of actual air velocity (V t ) against measured air velocity (V m ) for a range of embodiments, between 0 and 5 ms −1 ; 
         FIG. 8  shows a plot of actual air velocity (V t ) against measured air velocity (V m ) for a range of embodiments, between 0 and 2 ms −1 ; 
         FIG. 9  shows a plot of actual air velocity (V t ) against measured air velocity (V m ) for a range of embodiments, between 0 and 1 ms −1 ; 
         FIG. 10  shows an alternative constructional method for the device of the present invention; and 
         FIG. 11  shows a further alternative constructional method for the device of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows a device according to the present invention. An airflow meter  10  comprises a handle  12  attached to an outer surface of a ring  14 . The ring  14  comprises a short cylindrical section that serves to define a flow passage. The device measures the rate of flow of a fluid through that passage. 
     A central hub  16  is located substantially concentrically within the ring  14  and has two internal spaces, as will be described later. The central hub  16  is generally streamlined so as to cause relatively little disturbance to an airflow through the ring  14 . Each internal space is connected to a respective conduit  18 ,  20  that extends from the central hub  16  to the interior of the handle  12 . Within the handle  12 , connectors are provided to allow the conduits  18 ,  20  to be linked to flexible tubes leading to an external micromanometer for measuring pressure differences between the two conduits  18 ,  20 . The micromanometer may be as described in GB-A-2298281, for example. 
     Two arrays of spokes are located between the central hub  16  and the ring  14 , one in front of the other. The first array consists of five spokes  22  with hollow interiors communicating with one interior space of the hub  16  and thence conduit  18 . Together with the conduit  18 , these spokes are spaced at 60° intervals to form a symmetrical pattern. Each spoke  22  has a plurality of apertures  24  on the front face thereof, in this case three although there could be one, two or more than three apertures. As they are located on the front face of the spoke  22 , they face into an airflow that is flowing through the ring  14  and therefore sense a dynamic pressure created by that airflow. 
     The conduit  18  does not have any apertures. If an aperture on the conduit  18  did not face into the airflow then it would affect the dynamic pressure reading. If it did face into the airflow in the same orientation as the apertures  24  of the spokes  22 , then it would sense the same dynamic pressure but the apertures of the conduit  18  would be in a different topological location relative to the micromanometer and the hub  16  as compared to the apertures  24 , and this might distort the measured pressure. Accordingly, we prefer (as shown in  FIG. 1 ) to provide a sealed conduit  18 . 
     The second array of five spokes  26  are each located behind a spoke of the first array, and together with the conduit  20  are again spaced at 60° intervals. Each has three apertures (not visible in  FIG. 1 ) that are diametrically opposed to the apertures  24  of the first array of spokes  22 , i.e. point in an opposite direction. Again, there need not be three apertures although we find that this number is convenient. These apertures point in the lee direction of an airflow through the ring  14  and therefore sense a static pressure. That static pressure is fed through the conduit  20  and thence to the micromanometer. As a result, the micromanometer has access to a static and a dynamic pressure measurement and the airflow speed can be calculated using known techniques. 
     In addition to the apertures in the lee of the airflow, the spokes  26  of the second array each have an aperture  28  at an end thereof that extends through the ring  14  to the circumferential exterior face thereof. These allow a more balanced measurement of the static pressure. 
     The conduit  20  has no apertures, for the same reasons as set out above in relation to the conduit  18 . 
     Although five spokes in each array are shown, each forming (with its respective conduit) a symmetrical pattern with a rotational symmetry of 6, this number can be varied and strict symmetricality could be departed from. A balanced pattern with few spokes is likely to cause the least disturbance to the airflow being measured, although more spokes will provide for a greater number of sampling points in the airflow. We therefore prefer a symmetrical 6-spoke arrangement, but other arrangements are also likely to yield good results. 
     The spokes of the two arrays are shown as being aligned in the direction of the airflow, so that for each spoke of the first array there is a spoke of the second array directly behind it. Again, we prefer this arrangement as it is likely to cause the least disturbance to airflow, but other arrangements could be adopted, including arrangements in which the spokes of different arrays are not aligned and arrangements in which the arrays have different numbers of spokes. 
       FIG. 2  shows a half-moulding  30  from which the device  10  of  FIG. 1  can be produced. It is referred to as a half-moulding since the moulded item  30  provides approximately one half of the total device  10 ; two identical such half-mouldings  30  are assembled (together with other small parts) to form the device  10 . 
     Thus, the half-moulding  30  of  FIG. 2  is (by way of example) destined to form the front half of a device  10  and thus has a half handle  32 , a half ring  34 , a half hub  36  concentrically within the half ring  34 , a conduit  18  leading from the half hub  36  to the half handle  32 , and five spokes spaced at 60° intervals starting at the conduit  18  and leading from the half hub  36  to the half ring  34 . Each spoke  22  has three apertures  24  facing axially forward with respect to the central axis of symmetry of the half ring  34 . 
     An end aperture is also provided for each spoke  22 , extending from the hollow interior of the spoke  22  to the exterior face of the half ring  34 . This provision allows the half-moulding  30  to act as a rear half of a device  10  (in which case the spokes will be spokes  26  sensing static pressure). As part of the assembly process, these apertures  38  are sealed, for example by application of an adhesive tape to cover the aperture  38  or by insertion of a suitable plug into the end of the aperture  38 . As a alternative, 50% of the half mouldings  30  could be prepared without apertures  38 , but this would break the symmetry between the two items and hence double the tooling cost, increase inventory costs, etc. 
     A clip  40  extends from the half ring  34  towards the space that will be occupied by the companion half moulding that will make up the remainder of the device  10 . This is at a location on the half ring  34  offset from the conduit  18  by slightly more than 120° (to avoid the apertures  38 ), and is balanced by a recess  42  shown in  FIG. 3  at the mirror-image location on the half-ring  34 . Thus, when the half-moulding  30  and its companion are mated, the clip  40  of the half-ring  30  mates with the recess of the companion, and the clip of the companion mates with the recess  42 . Clip-locking elements in the clip  40  and the recess  42  of conventional design then ensure a snap fit between the half-moulding  30  and its companion. 
       FIG. 3  shows the reverse side of the half-hub  36 . It can be seen that this is a hemispherical shape (to provide the necessary streamlining) with the spokes  22  and conduit  18  communicating with the interior of the hemisphere. As a result, the pressures sensed by the spokes  22  can be averaged and sampled by the conduit  18 . During assembly, a cap is fitted to the half-hub  36  to close the hemisphere and provide a sealed interior space. An O-ring can be provided between the moulded half-hub  36  and the moulded cap to ensure a sufficient seal is obtained. 
     The conduit  18  leads into a hollow space within the half-handle  32 , and projects a short distance thereinto. This short projection acts as a connector for receiving a flexible hose that can convey the sensed pressure to a micromanometer. A circular hole  44  is provided in the half-handle opposite the conduit  18  to allow such a hose to leave the handle. Other forms of connector could be provided as desired or as required. 
     Pillars  46 ,  48  are provided within the half-handle  32  to mate with identical pillars on the companion half-moulding in a known fashion and secure the two parts together. 
       FIG. 4  shows a cross-section through the device  10 . Air or another fluid to be measured flows through the ring  14  in the direction of arrow  50  and impinges on the apertures  24  of the spoke  22  to establish a dynamic pressure within the hollow interior of the spoke  22 . This is conveyed to an interior space of the hub  16  defined by the half-hub hemisphere  36  and the cap  52 . This is averaged with the dynamic pressures from the other spokes not visible in  FIG. 4  and fed via the conduit  18  into a dynamic hose  54  connected to an end  56  of conduit  18 . The dynamic hose  54  departs the handle  12  via the hole  44  to a micromanometer (not shown). 
     Likewise, the apertures  58  on the spoke  26 , being directed in an opposite direction to the apertures  24 , are able to sense a static pressure. The end apertures  28  are also able to sense a static pressure outside the ring  14 . The static pressures sensed by the apertures  58  and end apertures  24  of the five spokes  26  are fed to a further interior space within the hub  16 , this time defined by the rearmost half-hub  36 ′ sealed by a further cap  52  and O-ring, where they are averaged and conveyed along the conduit  20  to a static hose  60  connected to an end  62  of the conduit  20 . This likewise exits the handle  12  via a further hole  44 ′. 
     As can be seen in  FIG. 4 , there is an unimpeded flow path past the two sets of spokes. In this example, there is an empty space between the two sets of spokes and therefore air (or the fluid concerned) can flow freely past the first set of spokes and then past the second. It is not strictly necessary for there to be a complete empty space; some support structures of other bracing could be provided between the two sets of spokes and if this did not extend beyond the cross-sections of the spokes in the direction of flow then this would not impede the fluid flow. However, this can be contrasted with the arrangement shown in, for example, U.S. Pat. No. 4,453,419 in which there is a transverse plate between the two sets of spokes which causes fluid flowing past the first set to divert outwardly, thereby affecting the flow pattern. 
       FIG. 4  also shows a seal  62  in the form of an adhesive layer over the apertures  38  at the ends of the spokes  22 . This adhesive layer can be in a number of short sections over each aperture  38 , or it can be a band around the relevant half of the ring  14 . 
       FIG. 5  shows an enlarged portion of the half-hub  36 , in section. Spokes  22  lead into the half-hub  36  and their hollow interiors  64  communicate with the interior of the half-hub  36  via openings  66 . A rear planar face of the half-hub  36  is initially open, but subsequently closed during assembly by way of a cap that seats opposite a shoulder  68  against which an O-ring can be compressed to provide a seal. 
       FIGS. 6 to 9  show graphs of the response of such a device, comparing various alternative embodiments. Data points are denoted as follows:
         ♦ a conventional vane anemometer   X an embodiment according to  FIGS. 1 to 5     * an embodiment according to  FIGS. 1 to 5  but without the apertures  58     ● an embodiment according to  FIGS. 1 to 5  but without the apertures  28         

       FIG. 6  shows the response at airflows between 0 and 25 ms −1 . Generally, all four show the same response at higher airflows. The absence of apertures  28  appears to give a proportionately slightly higher reading, but this could be corrected by suitable calibration. 
       FIG. 7  shows the response at airflow speeds up to 5 ms −1 , and shows a generally linear response for all four embodiments in the region above 1 ms −1 . That linear response can be corrected as required through calibration. 
       FIGS. 8 and 9  show the response at very low airspeeds of 1 ms −1  or less, and highlight a departure from linearity for the embodiment without the apertures  58  comparable, albeit opposite, to a departure from linearity of the conventional anemometer. It would seem that at low air speeds, the rotational inertia of the anemometer vane reduces the measured airflow as compared to the actual airflow. This difficulty is of course not faced by a pitot-static device. 
     It should be borne in mind that the graphs of  FIGS. 6 to 9  show a “best case” for the conventional anemometer. As the anemometer ages and is handled, the vanes and the rotating axle will inevitably degrade, creating additional resistance to rotation and uncertainties in the device calibration. No corresponding problems are applicable to a pitot-static device as described. 
       FIGS. 10 and 11  show alternative constructional methods for the hub region of the device. In  FIG. 10 , a single hub cap  100  sits between the two half-hubs  36 ,  36 ′. A pair of O-rings  102 ,  102 ′ are provided around a corresponding pair of snap-fit joins  104 ,  104 ′ which allow the cap  100  to fit to and seal with each of the two half-hubs  36 ,  36 ′. Assembly can be by fitting the cap to one half-hub  36 ′ first, then pressing the second half-hub  36  into place, or otherwise. 
     An internal dividing wall (not visible) within the cap  100  prevents flow between the two half-hubs and thus allows the pressures to be sensed independently. The pressure measurements can be obtained from the outer ends of the spokes  22 , or they can be extracted from the half-hubs.  FIG. 10  shows a single port  106  which leads into the cap  100  above the dividing wall (as illustrated). 
       FIG. 11  shows an alternative hub cap  110 , partially cut away to show the internal dividing wall  112 . As can be seen, this is stepped so that in part of the hub it is closer to the half-hub  36  (not shown in FIG.  11 , for clarity) and in another part it is closer to the half-hub  36 ′. This allows for two pressure ports  114 ,  166 , spaced circumferentially around the cap  110  and thus communicating with different sides of the dividing wall  112 . A flexible hose  118  is shown; the port  114  is oriented so as to lie between two spokes  22  and thus the hose  118  can fit between them for minimal obstruction to airflow. A half-aperture  120  is provided in each half-section of the ring  14  to allow the hose to pass through. An optional support  122  is provided on the adjacent spoke for the corresponding hose leading to the port  116 ; this could be replicated for the hose  118  if desired. 
       FIG. 11  also shows threaded inserts  124  moulded (or otherwise sealingly placed) into the ends of the spokes  22 . These allow for the connection of pressure sensing hoses (as required) or for the insertion of blanking plates where required. 
     The above embodiments are provided with a handle  12  for ease of use and positioning. As an alternative, or in addition, a socket such as a threaded insert could be provided on the device. Suitable locations include in one half-hub  36  (or both if symmetricality is required) or in the handle  12  itself. These could allow for the device to be mounted on a pole (or the like) to permit readings to be taken from difficult-to-reach locations. 
     Accordingly, the present invention provides a device that is lightweight, easily portable, and thus able to act as a direct like-for like replacement of a vane anemometer. At the same time, it provides an ab initio improvement in accuracy over an anemometer at low airflow rates and is more robust in long-term use with no moving parts and no fragile parts exposed to handling damage. 
     The device of the present invention is also more robust, in that it can be cleaned by simply directing a jet of high pressure air or other gas through the conduits  18 ,  20 . This will entrain any accumulated dust or grit and expel it via the apertures  24 ,  28 . Dust or grit that enters the bearings of a vane anemometer is difficult or impossible to remove and will necessitate replacement of the mechanism. 
     It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention. For example, the device could be incorporated into a larger apparatus for testing purposes or to remain there permanently for monitoring purposes. Other layouts of the spokes could be adopted, with (for example) different numbers of spokes or different dispositions such as parallel or grid layouts. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.