Abstract:
The present disclosure relates generally to flow sensors, and more particularly, to methods and devices for reducing variations in fluid flow across the flow sensor for increased accuracy and/or reliability. In one illustrative embodiment, a flow sensor assembly includes a housing with an inlet flow port and an outlet flow port. The housing defines a fluid channel extending between the inlet flow port and the outlet flow port, with a flow sensor positioned in the housing and exposed to the fluid channel. The flow sensor is configured to sense a measure related to the flow rate of a fluid flowing through the fluid channel. A porous insert is situated in the fluid channel, sometimes upstream of the flow sensor. When so configured, and during operation of the flow sensor assembly, a fluid may pass through the inlet flow port, through the porous insert, across the flow sensor, and through the outlet flow port. The porous insert may include pores that are configured to reduce the turbulence in the fluid passing the flow sensor.

Description:
RELATED APPLICATION 
       [0001]    This application is related to U.S. application Ser. No. ______ (Attorney Docket No. 1326.1145101), entitled “SENSOR ASSEMBLY WITH HYDROPHOBIC FILTER”, filed on the even date herewith, which is hereby incorporated by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates generally to flow sensors, and more particularly, to methods and devices for reducing variations in fluid flow across a flow sensor for increased accuracy and/or reliability. 
       BACKGROUND 
       [0003]    Flow sensors are often used to sense the flow rate of a fluid (e.g. gas or liquid) traveling through a fluid channel. Such flow sensors are often used in a wide variety of applications including, for example, medical applications, flight control applications, industrial process applications, combustion control applications, weather monitoring applications, as well as many others. In some instances, the fluid flow entering the flow sensor may be turbulent, which can result in increased noise in the flow sensor output signal. This noise can affect the accuracy, repeatability and/or reproducibility of the measurement of the flow sensor. 
       SUMMARY 
       [0004]    The present disclosure relates generally to flow sensors, and more particularly, to methods and devices for reducing variations in fluid flow across the flow sensor for increased accuracy and/or reliability. In one illustrative embodiment, a flow sensor assembly includes a housing with an inlet flow port and an outlet flow port. The housing may define a fluid channel extending between the inlet flow port and the outlet flow port, with a flow sensor positioned in the housing exposed to the fluid channel. The flow sensor may sense a measure related to the flow rate of a fluid flowing through the fluid channel. A porous insert may be situated in the fluid channel, sometimes upstream of the flow sensor. When so configured, and during operation of the flow sensor assembly, a fluid may pass through the inlet flow port, through the porous insert, across the flow sensor, and through the outlet flow port. The porous insert may include pores that are configured to reduce the turbulence in the fluid passing the flow sensor. In some instances, the pores of the porous insert(s) may be configured o help laminarize the fluid flow past the flow sensor. In some cases, the one or more porous inserts may be configured to provide a predetermined pressure drop of the fluid flowing through the fluid channel of the housing at a given flow rate. 
         [0005]    The preceding summary is provided to facilitate an understanding of some of the innovative features unique to the present disclosure and is not intended to be a full description. A full appreciation of the disclosure can be gained by taking the entire specification, claims, drawings, and abstract as a whole. 
     
    
     
       BRIEF DESCRIPTION 
         [0006]    The disclosure may be more completely understood in consideration of the following detailed description of various illustrative embodiments of the disclosure in connection with the accompanying drawings, in which: 
           [0007]      FIG. 1  is a schematic diagram of an illustrative flow sensor for measuring a fluid flow rate of a fluid passing through a fluid channel; 
           [0008]      FIG. 2  is a schematic diagram of an illustrative thermal flow sensor assembly useful for measuring the flow rate of a fluid passing through a fluid channel; 
           [0009]      FIG. 3  is a partially exploded perspective view of an illustrative flow sensor assembly that includes one or more porous inserts; 
           [0010]      FIG. 4  is a cross-sectional view of the illustrative flow sensor assembly of  FIG. 3  including porous inserts in both inlet and outlet flow ports; 
           [0011]      FIGS. 5 and 6  are cross-sectional views of the illustrative flow sensor assembly of  FIG. 3  including porous inserts in only one of the inlet and outlet flow ports; 
           [0012]      FIG. 7  is a graph showing output variations of a flow sensor with porous inserts; and 
           [0013]      FIG. 8  is a graph showing output variations of a flow sensor without porous inserts. 
       
    
    
     DESCRIPTION 
       [0014]    The following description should be read with reference to the drawings wherein like reference numerals indicate like elements throughout the several views. The description and drawings show several embodiments which are meant to be illustrative of the claimed disclosure. 
         [0015]      FIG. 1  is a schematic diagram of an illustrative flow sensor  10  for measuring a fluid flow rate of a fluid flow  14  passing through a fluid channel  12 . The term “fluid” as used herein can refer to a gas flow or a liquid flow, depending on the application. In the illustrative embodiment, the flow sensor  10  may be exposed to and/or disposed in the fluid channel  12  to measure one or more properties of the fluid flow  14 . For example, the flow sensor  10  may measure the mass flow and/or velocity of the fluid flow  14  using one or more thermal sensors (e.g. see  FIG. 2 ), pressure sensors, acoustical sensors, optical sensors, pitot tubes, and/or any other suitable sensor or sensor combination, as desired. In some cases, the flow sensor  10  may be a microbridge or a Microbrick™ sensor assembly available from the assignee of the present application, but this is not required. Some illustrative methods and sensor configurations that are considered suitable for measuring the mass flow and/or velocity of the fluid flow  14  are disclosed in, for example, U.S. Pat. Nos. 4,478,076; 4,478,077; 4,501,144; 4,581,928; 4,651,564; 4,683,159; 5,050,429; 6,169,965; 6,223,593; 6,234,016; 6,502,459; 7,278,309; 7,513,149; and 7,647,842. It is contemplated that flow sensor  10  may include any of these flow sensor configurations and methods, as desired. It must be recognized, however, that flow sensor  10  may be any suitable flow sensor, as desired. 
         [0016]    In the illustrative example, the fluid channel  12  may experience a range of flow rates of fluid flow  14 . For example, the fluid channel  12  may include a high-volume fluid flow, a mid-volume fluid flow, or a low-volume fluid flow. Example fluid flow applications can include, but are not limited to, respirometers, flow meters, velocimeters, flight control, industrial process stream, combustion control, weather monitoring, as well as any other suitable fluid flow applications, as desired. 
         [0017]    Turning now to  FIG. 2 , which is a schematic diagram of an illustrative thermal flow sensor assembly for measuring the flow rate of a fluid flow  14  passing through a fluid channel  12 . In the illustrative embodiment, the flow sensor assembly may include one or more heater elements, such as heater element  16 , and one or more sensor elements  18  and  20 , for sensing a flow rate of a fluid  28  in the fluid channel  12 . 
         [0018]    As illustrated in  FIG. 2 , the flow sensor assembly may also include one or more porous inserts  22  and  24  positioned in the fluid channel  12  upstream and/or downstream of the heater element  16  and one or more sensor elements  18  and  20 . The porous insert(s)  22  and/or  24  may include a plurality of pores that are configured to reduce the turbulence in the fluid passing the flow sensor. In some instances, the pores of the porous insert(s)  22  and/or  24  may be configured to help laminarize the fluid flow in the fluid channel  12  past the flow sensor. In some cases, the one or more porous insert(s)  22  and/or  24  may be configured to provide a predetermined pressure drop of the fluid flowing through the fluid channel  12  of the housing at a given flow rate. 
         [0019]    In some instances the porous insert(s)  22  and/or  24  may cause a more consistent flow of fluid past the flow sensor. The consistent flow of fluid through the plurality of pores can cause the flow to be laminar and mitigate turbulent effects of flow as the fluid passes the flow sensor. In some cases, the laminar flow can reduce the noise seen by the flow sensor assembly, providing a more consistent, reliable, repeatable, and stable output of the flow sensor assembly. 
         [0020]    As illustrated in  FIG. 2 , the flow sensor assembly may include a heater element  16 , a first sensor element  18  positioned upstream of the heater element  16 , and a second sensor element  20  positioned downstream of the heater element  16 . While the first sensor element  18  is shown as upstream of the heater element  16 , and the second sensor element  20  is shown as downstream of the heater element  16 , this is not meant to be limiting. It is contemplated that, in some embodiments, the fluid channel  12  may be a bi-directional fluid channel such that, in some cases, the first sensor element  18  is downstream of the heater element  16  and the second sensor element  20  is upstream of the heater element  16 . In some instances only one sensor element may be provided, and in other embodiments, three or more sensor elements are provided. In some instances, both sensor elements  18  and  20  may be positioned upstream (or downstream) of the heater element  16 . 
         [0021]    In some cases, the first sensor element  18  and the second sensor element  20  may be thermally sensitive resistors that have a relatively large positive or negative temperature coefficient, such that the resistance varies with temperature. In some cases, the first and second sensing elements  18  and  20  may be thermistors. In some instances, the first sensor element  18 , the second sensor element  20 , and any additional sensor elements may be arranged in a Wheatstone bridge configuration, but this is not required in all embodiments. 
         [0022]    In the example shown, when no fluid flow is present in the fluid channel  12  and the heater element  16  is heated to a temperature higher than the ambient temperature of the fluid in the fluid flow  28 , a temperature distribution may be created and transmitted in a generally symmetrical distribution about the heater element  16  to upstream sensor element  18  and downstream sensor element  20 . In this example, upstream sensor element  18  and downstream sensor element  20  may sense the same or similar temperature (e.g. within 25 percent, 10 percent, 5 percent, 1 percent, 0.001 percent, etc.). In some cases, this may produce the same or similar output voltage in the first sensor element  18  and the second sensor element  20 . 
         [0023]    When a fluid flow  28  is present in the fluid channel  12  and the heater element  16  is heated to a temperature higher than the ambient temperature of the fluid in the fluid flow  28 , the symmetrical temperature distribution may be disturbed and the amount of disturbance may be related to the flow rate of the fluid flow  28  in the fluid channel  12 . The flow rate of the fluid flow  28  may cause the upstream sensor element  18  to sense a relatively cooler temperature than the downstream sensor element  20 . In other words, the flow rate of the fluid flow  28  may cause a temperature differential between the upstream sensor element  18  and the downstream sensor element  20  that is related to the flow rate of the fluid flow  28  in the fluid channel  12 . The temperature differential between the upstream sensor element  18  and the downstream sensor element  20  may result in an output voltage differential between the upstream sensor element  18  and the downstream sensor element  20 . 
         [0024]    In another illustrative embodiment, the mass flow and/or velocity of the fluid flow  28  may be determined by providing a transient elevated temperature condition in the heater element  16 , which in turn, causes a transient elevated temperature condition (e.g. heat pulse) in the fluid flow  28 . When there is a non-zero flow rate in the fluid flow  28 , the upstream sensor element  18  may receive a transient response later than the downstream sensor element  20 . The flow rate of the fluid flow  28  can then be computed using the time lag between the upstream sensor element  18  and downstream sensor element  20 , or between the time the heater is energized and when the corresponding elevated temperature condition (e.g. heat pulse) is sensed by one of the sensors, such as the downstream sensor  20 . 
         [0025]    In the illustrative embodiment, the one or more porous inserts  22  and  24  may provide a laminar fluid flow  28  across the heater element  16  and sensing elements  18  and  20 . For example, in some cases, a relatively unstable or turbulent fluid flow  26  may enter the fluid channel  12  and the one or more porous inserts  22  and  24  may help laminarize fluid flow  28  across heater element  16  and sensing elements  18  and  20 . The pressure drop due to porous inserts  22  and  24  may be dependant upon the density and length of the porous inserts  22  and  24 . In some embodiments, the porous inserts  22  and  24  can have a porous structure with pore sizes in the range of microns to millimeters depending on the desired pressure drop and other factors, as desired. In some embodiments, the porous inserts  22  and  24  can have lengths of less than one inch, one inch, or greater than one inch, depending on the desired pressure drop, pore size, and other factors. In some cases, the porous inserts  22  and  24  can have the same pore size and length or, in other cases, can have different pore sizes and lengths, as desired. 
         [0026]    As illustrated, porous insert  22  is positioned in the fluid channel  12  upstream of the heater element  16  and one or more sensor elements  18  and  20 , and porous insert  24  is positioned in the fluid channel  12  downstream of the heater element  16  and one or more sensor elements  18  and  20 . In some embodiments, however, it is contemplated that only one porous insert  22  or  24  may be provide in the fluid channel  12 . For example, only porous insert  22  or only porous insert  24  may be provided in the fluid channel  12 . It is contemplated that only an upstream porous insert may be used, only a downstream porous insert may be used, or that multiple upstream and/or downstream porous inserts may be used, as desired. For example, in a uni-directional sensor, upstream porous insert  22  may serve to laminarize the fluid flow, however, in a bi-directional flow sensor, either upstream porous insert  22  or downstream porous insert  24  may serve to laminarize the fluid flow, depending on the direction of the flow. 
         [0027]    In some embodiments, the porous inserts  22  and  24  may include suitable porous materials, such as, for example, porous polymer and/or porous fiber material (e.g. sintered polymer particulates), foams (e.g. reticulated foams, open-cell foams), woven fibers (e.g. precision woven mesh), non-woven fibers (e.g. felt), polyurethane, polytetraflouride (PTFE), polyethylene (PE), nylon, polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polypropylene (PP), and/or any other material that, for example, helps laminarize and/or causes a desired pressure drop in a fluid flowing through the flow channel  12 . The porous polymer material may include, for example, thermoset polymers, thermoplastic polymers, elastomer materials, organic or synthetic materials, and any other suitable polymer material, as desired. Example porous materials include POREX porous polymer materials and POREX fiber media available from POREX Technologies. Other porous materials are UHMW Polyethylene or PE copolymers available from GenPore. An example of a precision woven mesh is Sefar Tetex® DLW available from Sefar Filtration Incorporated. An example of non woven fiber material is Gore Acoustic filter GAW102 available from W.L Gore &amp; Associates. 
         [0028]    It is to be understood that the illustrative heater element  16  and sensing elements  18  and  20  are merely illustrative and, in some embodiments, may not be present, as desired. For example, it is contemplated that the porous inserts  22  and  24  may be incorporated into one or more pressure sensors, acoustical sensors, optical sensors, pitot tubes, and/or any other suitable sensor or sensor combination that may be used to sense a measure related to a fluid flow in fluid channel  12 , as desired. 
         [0029]      FIG. 3  is a partially exploded perspective view of an illustrative flow sensor assembly  30  that includes one or more porous inserts  22  and/or  24 . In the illustrative embodiment, the flow sensor assembly  30  includes an outer protective housing including a top protective cover  37  and a bottom protective cover  36 . As illustrated, the top protective cover  37  may be inserted into a recess of the bottom protective cover  36 . With such a configuration, the top and bottom protective covers  37  and  36  may protect the flow sensing element (shown as  42  in  FIG. 4 ) and any signal conditioning circuitry that may be provided in the housing. In some cases, the top protective cover  37  and the bottom protective cover  36  may be formed from, for example, plastic. However, it is contemplated that any other suitable material may be used, as desired. 
         [0030]    In the illustrative embodiment, the outer protective housing including the top protective cover  37  and the bottom protective cover  36  are formed as a composite. However, it is contemplated that the outer protective housing can be molded in a single piece from a plastic or other suitable material according to design considerations. For example, it is contemplated that the outer protective housing may be formed by injection molding or made by other suitable methods and materials, as desired. 
         [0031]    As illustrated in  FIG. 3 , the top protective cover  37  of the housing includes a first flow port  32  and a second flow port  34 , which a flow channel extending therebetween. The flow sensing element is exposed to the fluid in the flow channel. In some cases, flow port  32  may be an inlet flow port, and flow port  34  may be an outlet flow port, but this is not required. In some cases, it is contemplated that the flow sensor assembly  30  may be a bi-directional flow sensor assembly and, in this case, either flow port  32  or flow port  34  may serve as the inlet flow port or the outlet flow port, depending on the current direction of the fluid flow through the flow channel. 
         [0032]    Although not shown in  FIG. 3 , the flow sensor assembly  30  may include one or more electrical leads (shown as  44  in  FIG. 4 ) electrically connected to the flow sensing element  42  and extending external of the outer protective housing. In some cases, the one or more electrical leads  44  may include a metal, however, any suitable conducting material may be used, as desired. 
         [0033]    In some embodiments, the outer protective housing may also include one or more mounting holes  38 . As illustrated, bottom protective housing  36  includes two mounting holes  38 , but any suitable number of mounting holes may be used, as desired. The mounting holes  38  may be configured to receive a fastener, such as a screw, bolt, or nail, to mount the bottom protective cover  36  to a desired surface to accommodate the particular equipment for which the flow sensor assembly  30  may be used. It is contemplated that bottom protective cover  36  or the top protective cover  37  may include additional mounting holes  38  or no mounting holes  38 , as desired. 
         [0034]    In the illustrative embodiment, porous insert  22  may be inserted, pressed, or otherwise positioned in or adjacent to flow port  32 . Porous insert  24  may be inserted, pressed, or otherwise positioned in or adjacent to flow port  34 . In some embodiments, the porous inserts  22  and  24  may be generally cylindrical in shape. However, it is contemplated that any suitable shape may be used, depending on the shape of the port that the insert is to be inserted. In other cases, it is contemplated that the porous inserts  22  and  24  may be any shape and, when inserted in the flow ports  32  and  34 , the porous inserts  22  and  24  may be deformable to accommodate the shape of the flow ports  32  and  34 . 
         [0035]    The porous inserts  22  and  24  can be configured to have a length and/or density that will produce a desired or predetermined pressure drop along the fluid channel at a given flow rate. For example, increasing the length and/or increasing density of the porous inserts  22  and  24  (e.g. reducing the pore size) may increase the pressure drop through the flow channel, whereas decreasing the length and/or decreasing the density of the porous inserts  22  and  24  may decrease the pressure drop. In some cases, increasing the density of upstream porous insert  22  (e.g. reducing the pore size) and/or providing relatively uniform pore sizes may help to provide a more laminar fluid flow. It is contemplated that any suitable length and/or density may be used for the porous inserts  22  and  24 , depending on the desired pressure drop and/or laminarization of the fluid flow in the flow channel. 
         [0036]      FIGS. 4-6  are cross-sectional views of the flow sensor assembly  30  of  FIG. 3 . In the illustrative embodiment of  FIG. 4 , the flow sensor assembly  30  may include a flow sensing element  42  mounted on a package substrate  40 . The flow sensing element  42  may be configured to sense a measure related to flow rate of a fluid flowing through in flow channel  46 . The package substrate  40  may include a ceramic material, however, other suitable types of material may be used, as desired. 
         [0037]    In the illustrative embodiment, the housing of the flow sensor assembly  30  may include a top housing cover  37  and a bottom housing cover  36 . As shown in  FIGS. 4-6 , the top housing cover  37  and bottom housing cover  36  may define a cavity for receiving package substrate  40  with the flow sensing element  42  mounted thereon. In the illustrative embodiment, an upper surface of the package substrate  40 , which includes the flow sensing element  42 , and an inner surface of the top housing cover  37  may define flow channel  46  of the flow sensor assembly  30 . The flow channel  46  may extend from flow port  32  of the top housing cover  37 , along the flow sensing element  42 , and to flow port  34  of the top housing cover  37 . The flow channel  46  may expose the flow sensing element  42  to a fluid flow. 
         [0038]    As illustrated in  FIG. 4 , the flow sensor assembly  30  may include porous insert  22  disposed in flow port  32  and/or porous insert  24  disposed in flow port  34 . The porous inserts  22  and  24  may help laminarize the fluid flow, and/or control the pressure drop, across flow sensing element  42 . As illustrated in  FIGS. 5 and 6 , only one porous insert  22  and  24  is provided. As shown in  FIG. 5 , porous insert  22  is provided in flow port  32  without any porous insert provided in flow port  34 . As shown in  FIG. 6 , porous insert is provided in flow port  34  without any porous insert in flow port  32 . While only one porous insert  22  or  24  is shown in the embodiments of  FIGS. 5 and 6 , the flow sensor assembly may still provide a laminar flow and/or a controlled pressure drop across the flow sensing element  42 . 
         [0039]    While porous inserts  22  and  24  are shown inserted into their respective flow ports  32  and  34 , this is not meant to be limiting. It is contemplated that porous inserts  22  and  24  may be mounted over or provided adjacent to their respective flow ports  32  and  34 . Further, it is contemplated that the porous inserts  22  and  24  can be provided in any suitable position to, for example, help laminarize the fluid flow and/or control the pressure drop in the fluid flow, as desired. For example, porous inserts  22  and  24  may be provided in the flow channel  46  between the package substrate  40  and inner surface of the top housing cover  37 , if desired. 
         [0040]    In the illustrative embodiment, flow sensor assembly  30  may include one or more electrical leads  44  mounted to the package substrate  40 . The one or more electrical leads  44  may be configured to receive a signal transmitted from the flow sensing element  42  corresponding to the sensed flow rate of a fluid flowing through flow channel  42 , via one or more traces provided on the package substrate  40 . In some cases, the one or more electrical leads  44  may include a metal, however, any suitable conductive material may be used, as desired. 
         [0041]      FIG. 7  is a graph showing output variations of an illustrative flow sensor having porous inserts in the flow ports. In the illustrative example, the porous inserts may include a polyethylene material having a pore size of 45 pores per inch (PPI) and a length of about 6.35 millimeters (0.25 inches). As illustrated, the graph shows forty consecutive data points or readings from a digital output of the flow sensor having a fluid flow of 1000 standard cubic centimeters per minute (sccm). In the illustrative example, the digital output is measure in “counts”, where there are 6.55 counts per sccm. The data points or readings from the flow sensor can be obtained at 1 millisecond intervals. To help illustrate the variations in the digital output of the flow sensor, the digital output is normalized to the first reading. As shown in  FIG. 7 , with the porous inserts in the flow ports of the flow sensor, the variations in the digital output ranges from 0 to 4 counts. 
         [0042]      FIG. 8  shows a graph of the output variations of the illustrative flow sensor used in  FIG. 7  without porous inserts in the flow ports. Similar to the graph shown in  FIG. 7 , the graph shows forty consecutive data points or readings from a digital output of the flow sensor having a fluid flow of 1000 sccm with the data points or readings taken at 1 millisecond intervals. Without the porous inserts, the variation in the digital output (normalized to the first reading) ranges from about −68 counts to about 12 counts. Comparing the graphs shown in  FIGS. 7 and 8 , the digital output of the graph shown in  FIG. 7 , with porous inserts, shows fewer variations and less noise in the digital output. The reduced number of variations and noise in the digital output may be due, at least in part, to the porous inserts providing a more laminar and less turbulent fluid flow through the flow sensor used for the graph of  FIG. 7  as compared to the flow sensor without porous inserts used for the graph of  FIG. 8 . 
         [0043]    While the foregoing description has been described with reference to a porous inserts  22  and  24 , it is contemplated that any suitable insert, porous or not, that allows a fluid to flow therethrough and that decreases the instability in the fluid flow may be used, as desired. 
         [0044]    Having thus described the preferred embodiments of the present disclosure, those of skill in the art will readily appreciate that yet other embodiments may be made and used within the scope of the claims hereto attached. Numerous advantages of the disclosure covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respect, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the disclosure. The disclosure&#39;s scope is, of course, defined in the language in which the appended claims are expressed.