Abstract:
The present disclosure relates generally to sensors, and more particularly, to methods and devices for reducing moisture, dust, particulate matter, and/or other contaminates entering a sensor. In one illustrative embodiment, a 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 sensor positioned in the housing and exposed to the fluid channel. The illustrative sensor is configured to sense a measure related to the flow rate of a fluid flowing through the fluid channel. A hydrophobic filter may be situated in the fluid channel, sometimes upstream of the sensor. When so configured, and during operation of the sensor assembly, a fluid may pass through the inlet flow port, through the hydrophobic filter, across the sensor, and through the outlet flow port. The hydrophobic filter may be configured to reduce the moisture entering the fluid channel of the sensor.

Description:
RELATED APPLICATION 
       [0001]    This application is related to U.S. application Ser. No. ______ (Attorney Docket No. 1326.1143101), entitled “FLOW SENSOR ASSEMBLY WITH POROUS INSERT”, filed on the even date herewith, which is hereby incorporated by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates generally to sensors, and more particularly, to methods and devices for reducing moisture, dust, particulate matter and/or other contaminants in a sensor. 
       BACKGROUND 
       [0003]    Sensors, such as pressure and flow sensors, are often used to sense the pressure and/or flow of a fluid (e.g. gas or liquid) in a fluid channel. Such 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, moisture, dust, particulate matter, and/or other contaminants can enter the sensor during use. Over time, such contaminants can impact the accuracy, repeatability, functionality and/or other aspects of the sensor. For example, moisture in a sensor can increase corrosion or electromigration in the flow sensor itself, which may impact the accuracy, repeatability, functionality and/or other aspects of the sensor. Also, dust, particulate matter, or other contaminants can build-up and possibly obstruct the sensor. Therefore, there is a need for new and improved systems and methods for reducing moisture, dust, particulate matter, and/or other contaminants from entering a sensor. 
       SUMMARY 
       [0004]    The present disclosure relates generally to sensors, and more particularly, to methods and devices for reducing moisture, dust, particulate matter, and/or other contaminants in a sensor. 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 and exposed to the fluid channel. The flow sensor may sense a measure related to the flow rate of the fluid flowing through the fluid channel. A hydrophobic filter may be situated in or adjacent to the fluid channel, sometimes upstream of the flow sensor. When so configured, and during operation of the flow sensor assembly, the fluid may pass through the hydrophobic filter and across the flow sensor. The hydrophobic filter may be configured to reduce moisture ingress into the flow sensor, while still allowing fluid flow through the flow channel and past the flow sensor. While this example includes a flow sensor, it is contemplated that a hydrophobic filter may be used in conjunction with many other types of sensors including pressure sensors, humidity sensors, temperature sensors, or any other type of sensor that is exposed to a fluid (e.g. gas or liquid). 
         [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 filters; 
           [0010]      FIG. 4  is a cross-sectional view of the illustrative flow sensor assembly of  FIG. 3  including filters adjacent to both the inlet and the outlet flow ports; 
           [0011]      FIGS. 5 and 6  are cross-sectional views of the illustrative flow sensor assembly of  FIG. 3  including a filter in only one of the inlet and outlet flow ports; 
           [0012]      FIGS. 7-9  are cross-sectional views of other illustrative flow sensor assemblies that include one or more filter structures. 
       
    
    
     DESCRIPTION 
       [0013]    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 illustrative embodiments and are not meant to be limiting in any way. 
         [0014]    While the illustrative embodiments described below includes a flow sensor, it is contemplated that a filter may be used in conjunction with many other types of sensors including pressure sensors, humidity sensors, temperature sensors, or any other type of sensor that is exposed to a fluid (e.g. gas or liquid). 
         [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 or a liquid, 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 filters  22  and/or  24  positioned in the fluid channel  12  upstream and/or downstream of the heater element  16  and the one or more sensor elements  18  and  20 . The filter(s)  22  and/or  24  may be configured to reduce moisture, dust, and/or other contaminants from the fluid flow  28  passing through the flow sensor housing and/or produce a desired or predetermined pressure drop along the fluid channel at a given flow rate. In some instances, the reduction of moisture, dust, and/or other contaminants in the fluid flow may provide a more consistent, reliable, accurate, repeatable, and stable output of the flow sensor for a longer period of time due to the reduction of corrosion, electromigration, and/or contaminant build-up obstructing fluid flow in the flow sensor. 
         [0019]    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 may be provided. In some instances, both sensor elements  18  and  20  may be positioned upstream (or downstream) of the heater element  16 . 
         [0020]    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. 
         [0021]    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 . 
         [0022]    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 . 
         [0023]    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 . 
         [0024]    In the illustrative embodiment, the one or more filters  22  and  24  may reduce the amount of moisture, dust, and/or other contaminants in fluid flow  28  across the heater element  16  and sensing elements  18  and  20 . In some cases, the filters  22  and  24  may include a plurality of pores sized to filter out and/or reduce the presence of moisture, dust, particulate matter, and/or other contaminants in the fluid flow  28  across the heater element  16  and sensing elements  18  and  20 . In some embodiments, the filters  22  and  24  can have a porous structure with pore sizes in the range of microns to millimeter depending on the desired filtration rate and filtration application. In some embodiments, the filters  22  and  24  can have lengths in the flow direction of less than one inch, one inch, or greater than one inch, depending on the desired filtration, as well as other factors. In some cases, the filters  22  and  24  can have the same pore size and length or, in other cases, can have different pore sizes and lengths, as desired. 
         [0025]    As illustrated, filter  22  is positioned in the fluid channel  12  upstream of the heater element  16  and one or more sensor elements  18  and  20 , and filter  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 filter  22  or  24  may be provide in the fluid channel  12 , such as for example, only the upstream filter. It is also contemplated that multiple upstream and/or downstream filters may be used, as desired, sometimes with different filter characteristics. 
         [0026]    In one example, to reduce the introduction of dust, particulate matter, and/or other contaminants in the flow sensor, a filter  22  or  24  may be provided upstream of the heater element  16  and one or more sensor elements  18  and  20 . In another example, to reduce the introduction of moisture into the flow sensor, both filters  22  and  24  may be provided upstream and downstream of the heater element  16  and one or more sensor elements  18  and  20 . In bi-directional flow sensor applications, for example, both filter  22  and  24  may be provided. 
         [0027]    In some embodiments, the filters  22  and  24  may include suitable filter materials to reduce moisture ingress into the flow sensor, such as, hydrophobic or hydrophobic treated material. For example, the filter material may include woven fibers, such as, for example, a precision woven mesh, having hydrophobic treatments, non-woven fibers (e.g. felt) with hydrophobic treatment, polytetraflouride (PTFE), expanded polytetraflouride (ePTFE), porous polymer and/or porous fiber material with hydrophobic treatment (e.g. sintered polymer particulates), and/or any other material that, for example, helps reduce moisture ingress in a fluid flowing through the flow channel  12 . Examples ePTFE materials include Teflon® available from DuPont, and Gore-Tex® available from W.L. Gore &amp; Associates and Versapore membrane available from PALL Life Sciences. Examples of hydrophobic porous materials are UHMW Polyethylene or PE copolymers available from GenPore. An example of a precision woven mesh with a hydrophobic treatment is Acoustex available from SAATItech and hydrophobically treated acoustic filters available from Sefar Filtration Incorporated. An example of non woven fiber material with hydrophobic treatment is Gore Acoustic filter GAW102 available from W.L Gore &amp; Associates. Other materials that can be used include, for example, foams (e.g. reticulated foams, open-cell foams), polyurethane, polyethylene (PE), nylon, polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polypropylene (PP), pressed metal, and/or any other suitable material, as desired. Further, in some embodiments, the filter may include materials that are untreated (non-hydrophobic). In these embodiments, the filter materials may include, for example, woven fibers, such as, for example, a precision woven mesh, non-woven fibers (e.g. felt), and/or any other material that, for example, helps reduce moisture ingress in a fluid flowing through the flow channel  12 . The foregoing materials are merely illustrative and it is to be understood that any suitable hydrophobic material or hydrophobic treated material may be used to reduce moisture ingress in the flow channel  12  of the flow sensor. 
         [0028]    In some embodiments, the filters  22  and  24  may include a backing for the filter materials, an adhesive applied to the materials for adhering to the material to the flow channel  12 , and/or other insert (e.g. plastic ring, screen, etc.) for mounting the filter material to. In one example, the filter material may be mounted on a plastic ring, other insert, or backer, and pressed or otherwise inserted into or otherwise positioned to filter fluid flow entering the flow channel  12  of the flow sensor. In another example, an adhesive may be applied to a surface of the filter material for adhering the filter to the inside of the flow channel  12  or over a flow channel  12  port, as desired. An example backer for the filter material may be non-woven PET. Furthermore, it is contemplated that any other suitable technique or material may be used to mount the filters  22  and  24  to the flow sensor or flow sensor housing. 
         [0029]    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 filters  22  and  24  may be incorporated into one or more flow sensors (such as the flow sensors described and/or referred to previously), 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 (e.g. gas of liquid) in fluid channel  12 , as desired. 
         [0030]      FIG. 3  is a partially exploded perspective view of an illustrative flow sensor assembly  30  that includes one or more filters  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 . In 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. 
         [0031]    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. 
         [0032]    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 , and helps define 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. 
         [0033]    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 pin or trace, however, any suitable conducting material or configuration may be used, as desired. 
         [0034]    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. 
         [0035]    In the illustrative embodiment, filter  22  may be inserted, pressed, or otherwise positioned in, on, or adjacent to flow port  32 . Filter  24  may be inserted, pressed, or otherwise positioned in, on, or adjacent to flow port  34 . In some embodiments, the filters  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 filter is to be inserted. In other cases, it is contemplated that the filters  22  and  24  may be any shape and, when inserted in the flow ports  32  and  34 , the filters  22  and  24  may be deformable to accommodate the shape of the flow ports  32  and  34 . In some instances, the filters  22  and  24  may be positioned in, on, or adjacent to flow ports  32  and  34 , respectively, using an adhesive or other backing. In other instances, the filters  22  and  24  may be mounted on or formed on a backer or insert and pressed into flow ports  32  and  34 , respectively. In yet other instances, the hydrophobic material of filters  22  and  24  can be inserted or pressed into flow ports  32  and  34 , respectively, without any backer or insert. 
         [0036]    The filters  22  and  24  can be configured to have a length in the flow direction and/or pore density that will produce a desired or predetermined pressure drop along the fluid channel at a given flow rate. For example, increasing the length, increasing the pore size and/or decreasing the pore density of the filters  22  and  24  may increase the pressure drop through the flow channel, whereas decreasing the length, increasing the pore size and/or increasing the pore density of the filters  22  and  24  may decrease the pressure drop. It is contemplated that any suitable length, pore size and/or pore density may be used for the filters  22  and  24 , depending on the desired pressure drop and other considerations. 
         [0037]      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. 
         [0038]    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. 
         [0039]    As illustrated in  FIG. 4 , the flow sensor assembly  30  may include filter  22  disposed in flow port  32  and/or filter  24  disposed in flow port  34 . The filters  22  and  24  may help reduce the introduction of moisture, dust, particulate matter, and/or other contaminants in the fluid flow and/or control the pressure drop across flow sensing element  42 . As illustrated in  FIGS. 5 and 6 , only one filter  22  and  24  may be provided. As shown in  FIG. 5 , filter  22  is provided in flow port  32  without any filter provided in flow port  34 . As shown in  FIG. 6 , filter  24  is provided in flow port  34  without any filter in flow port  32 . While only one filter  22  or  24  is shown in the embodiments of  FIGS. 5 and 6 , the flow sensor assembly may still reduce moisture, dust, particulate matter, and/or other contaminants from entering the flow sensor and/or provide a controlled pressure drop across the flow sensing element  42 . 
         [0040]    While filters  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 filters  22  and  24  may be mounted over or provided adjacent to their respective flow ports  32  and  34 . Further, it is contemplated that the filters  22  and  24  can be provided in any suitable position to, for example, help reduce moisture, dust, particulate matter, and/or other contaminants in the fluid flow passing across the flow sensor and/or control the pressure drop in the fluid flow, as desired. For example, filters  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. 
         [0041]    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 one or more signals from the flow sensing element  42  corresponding to the sensed flow rate (and/or other parameter) of a fluid flowing through flow channel  42 , via one or more traces (not shown) provided on the package substrate  40 . In some cases, the one or more electrical leads  44  may be metal, however, it is contemplated that they may be any suitable conductive material, as desired. 
         [0042]    In some embodiments, water ingress into flow sensors employing filters using hydrophobic 3 micrometer ePTFE on non-woven PET backer and hydrophobic non-woven material (i.e. Gore Acoustic filter) may be reduced. For example, the weight of the flow sensors may change by less than 0.01% when immersed in water, indicating little or no ingress of water. In one example, flow sensors including filters using hydrophobic 3 micrometer ePTFE membrane on non-woven PET backer was installed in a fixture including 80 milliliters of water. The apparatus was shaken with ten cycles for one orientation and then rotated 90 degrees, in which ten more cycles were then performed. This was repeated for four different orientations of the assembly. The pre-test weight and the post-test weight of the flow sensors were then compared. For the hydrophobic 3 micrometer ePTFE membrane on non-woven PET backer, two examples sensors weighed about 11.43 and 11.41 ounces. After the test, the two example sensors weighed about 11.44 and 11.42 ounces, respectively, or both had about a 0.01 ounce change. For hydrophobic non-woven material (e.g. Gore—GAW1020308), the three example flow sensors had a pre-test weight of about 11.20, 11.33, and 11.48 ounces. The post-test weight of the three example flow sensors was about 11.22, 11.34, and 11.49 ounces, respectively, or a change of about 0.02, 0.01, and 0.01 ounces, respectively. 
         [0043]    In addition, the pressure drop was also measured for dry and wet hydrophobic non-woven material. For the three example flow sensor when dry, the pressure drop was about 20.81, 22.45, and 24.18 mmH 2 O for a 1000 standard cubic centimeters per minute (sccm) flow rate, about 16.06, 17.35, and 18.52 mmH2O for a 800 sccm flow rate, about 11.61, 12.57, and 13.31 mmH2O for a 600 sccm flow rate, about 7.481, 8.113, and 8.514 sccm for a flow rate of 400 sccm, about 3.638, 3.936, and 4.084 mmH2O at a flow rate of 200 sccm, about 1.664, 1.805, and 1.873 mmH2O for a flow rate of 100 sccm, about 1.238, 1.348, and 1.377 mmH2O for a flow rate of 70 sccm, about 1.059, 1.136, and 1.166 mmH2O for a flow rate of 60 sccm, and about 0.8639, 0.9360, and 0.9660 mmH2O for a flow rate of 50 sccm. 
         [0044]    For a wet hydrophobic non-woven material, the three example filters for the flow sensor soaked in water for about 1 hour. The pressure drop for the wet hydrophobic non-woven material was about 20.61, 22.29, and 23.83 mmH2O for a 1000 sccm flow rate, about 15.90, 17.21, and 18.31 mmH2O for a 800 sccm flow rate, about 11.48, 12.43, and 13.19 mmH2O for a 600 sccm flow rate, about 7.405, 8.106, and 8.471 sccm for a flow rate of 400 sccm, about 3.586, 3.958, and 4.111 mmH2O at a flow rate of 200 sccm, and about 1.631, 1.861, and 1.927 mmH2O for a flow rate of 100 sccm. 
         [0045]    For the hydrophobic 3 micrometer ePTFE membrane on non-woven PET backer, two examples sensors had pressure drops of about 133.0 and 136.3 mmH2O for a flow rate of 70 sccm, about 113.4 and 117.1 mmH2O for a flow rate of 60 sccm, and about 94.89 and 95.71 mmH2O for a flow rate of 50 sccm. As can be seen, the pressure drop of the hydrophobic non-woven filter (e.g. Gore—GAW1020308) was less than the hydrophobic 3 micrometer ePTFE membrane on non-woven PET backer. For some applications, a maximum pressure drop of about 5 mmH2O at 200 sccm is desired. For these applications, the hydrophobic non-woven material (e.g. Gore—GAW1020308) had acceptable pressure drops. However, the desired maximum pressure drop may vary depending on the application, and for some applications, a higher pressure drop may be acceptable or desirable. 
         [0046]      FIGS. 7-9  are cross-sectional views of other illustrative flow sensor assemblies  50 ,  60 , and  70  similar to flow sensor  30 , but has one or more alternative filters  52 ,  54 ,  62 ,  64 ,  72 , and/or  74 . While  FIGS. 7-9  are shown having filters in the flow ports  32  and  34 , it is to be understood that only one filter may be provided in either the upstream or downstream flow port, and/or in another location within the channel  46 , as desired. 
         [0047]    As shown in  FIG. 7 , filters  52  and  54  may be inserts including an end having a plurality of orifices  58  configured to reduce moisture, dust, particulate matter, and/or other contaminants in the fluid flow of flow sensor  50 . The size of the orifices  58  may be small enough to help reduce water penetration because of surface tension. In the illustrative embodiment, filter  52  and  54  may also include one or more tabs  56  extending from the end of filters  52  and  54 . The tabs  56  may be configured to be compressed by the flow ports  32  and  34  to maintain the filters  52  and  54  in flow ports  32  and  34 , respectively. Filters  52  and  54  may also provide a desired pressure drop across the flow sensing element  42  depending on the size, number and/or density of orifices  58 . In the illustrative example, filters  52  and  54  have seven orifices  58 . However, it is contemplated that filters  52  and  54  may have two, three, four, five, six, seven, eight, nine, ten or any other number of orifices  58 , as desired. In some cases, the size of orifices  58  may be on the order of hundredths of inches. For example, the diameter (or other dimension) of orifices  58  may be about 0.010 inches, 0.012 inches, 0.015 inches, 0.018 inches, 0.020 inches, 0.030 inches, or any other size, as desired. 
         [0048]    As shown in  FIG. 8 , filters  62  and  64  may include a first end  65 , a second end  67 , and a tortuous path  66  extending between the two ends  65  and  67 . For example, an upper end  65  of filters  62  and  64  may include an opening  61  and the bottom end  67  of filters  62  and  64  may include an opening  63 . The tortuous path may extend from the opening  61  to opening  63 , and may extend in a helical pattern in the filters  62  and  64  to help reduce moisture, dust, particulate matter, and/or other contaminants which may be present in the fluid flow from entering the flow channel  46  of the flow sensor  60 . 
         [0049]    As shown in  FIG. 9 , flow sensor  70  may include flow ports  32  and  34  having integral flow restrictors  72  and  74 , respectively, to reduce moisture, dust, particulate matter, and/or other contaminants in the fluid flow of flow sensor  70 . The flow restrictors  72  and  74  may include one or more orifices  76  utilizing surface tension to reduce moisture, dust, particulate matter, and/or other contaminants. The diameter of orifices  78  may be about 0.010 inches, 0.012 inches, 0.015 inches, 0.018 inches, 0.020 inches, 0.030 inches, or any other size, as desired. In the illustrative example, the flow restrictors  72  and  74  include four orifices  76  having a 0.015 inch diameter, but this is just one example. 
         [0050]    A number of illustrative implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of this disclosure. Accordingly, other implementations are with the scope of the following claims.