Abstract:
Pressure sensing devices and fluid assemblies may sense fluid pressure, including, for example, a difference in pressure, or differential pressure, between a fluid at a first pressure and a fluid at a second pressure. The fluid may be a gas, a liquid, or a mixture of gases, liquids, and/or solids.

Description:
[0001]    This application claims priority based on U.S. Provisional Application No. 60/608,876, which was filed on Sep. 13, 2004, and which is incorporated by reference in its entirety for any and all purposes. 
     
    
     DISCLOSURE OF THE INVENTION 
       [0002]    The invention relates to pressure sensing devices and fluid assemblies. For example, devices and assemblies embodying the invention may be used to sense the difference in pressure, i.e., the pressure differential, between a fluid at a first pressure and a fluid at a second pressure. The fluid may be a gas, a liquid, or a mixture of gases, liquids, and/or solids. 
         [0003]    In accordance with one aspect of the invention, a pressure sensing device may comprise a housing, a deflectable element, a deflection sensing circuit, and a layer of solid, insulative material. First and second fluid passages are associated with the housing, and the deflectable element has first and second opposite sides. The deflectable element is mounted to the housing with the first side coupled to the first fluid passage and the second side coupled to the second fluid passage. When a fluid at a first pressure is directed along the first fluid passage, the first pressure is applied against the first side of the deflectable member. When a fluid at a second pressure is directed along the second fluid passage, the second pressure is applied against the second side of the deflectable member. The deflectable member may then deflect toward the first fluid passage if the second pressure is greater than the first pressure or toward the second fluid passage if the first pressure is greater than the second pressure. The amount of deflection will depend on the pressure differential. The deflection sensing circuit and the solid, inorganic, insulative layer are supported by the first side of the deflectable element. The deflection sensing circuit senses the deflection of the deflectable element and, therefore, the differential pressure. The solid, insulative layer overlies the deflection sensing circuit to electrically insulate the deflection sensing circuit from the first fluid passage and any fluid in the first fluid passage. 
         [0004]    In accordance with another aspect of the invention, a pressure sensing device may comprise a housing, a deflectable metal diaphragm, a mechanical stop, a deflection sensing circuit, a ring seal, and an electrical connector. The housing has a metal portion, and first and second fluid passages are associated with the housing. The deflectable metal diaphragm has first and second opposite sides. The deflectable metal diaphragm is mounted to the metal portion of the housing with the first side of the diaphragm coupled to the first fluid passage and the second side coupled to the second fluid passage. Again, when a fluid at a first pressure and a fluid at a second pressure are directed along the first and second fluid passages, the first and second pressures are respectively applied against the first and second opposite sides of the deflectable metal diaphragm, deflecting the diaphragm in accordance with the differential pressure. The mechanical stop is associated with the first side of the diaphragm and is arranged to limit the deflection of the diaphragm toward the first fluid passage a predetermined value. The deflection sensing circuit, which is supported by the first side of the deflectable metal diaphragm, includes first and second thin insulative layers and a thin-film strain gauge circuit positioned between the first and second insulative layers. The deflection sensing circuit, including the strain gauge circuit, senses the amount of deflection of the deflectable metal diaphragm and, therefore, the differential pressure. The seal is positioned between the housing and the first side of the deflectable metal diaphragm, defining an inner region and an outer region of the first side of the diaphragm. The inner region includes at least a portion of the strain gauge circuit and is coupled to the first fluid passage, while the outer region is isolated from the first fluid passage by the seal. Thus, the fluid in the first fluid passage exerts the first pressure against the first side of the deflectable metal diaphragm, and the insulative layers of the deflection sensing circuit electrically insulate the strain gauge circuit from the fluid in the first passage and from the deflectable metal diaphragm. The electrical connector is associated with the strain gauge circuit and extends between the inner and outer regions of the first side of the deflectable metal diaphragm. 
         [0005]    In accordance with another aspect of the invention, a pressure sensing assembly may comprise a fitting, deflectable element, a deflection sensing circuit, an insulative layer, and a receptacle. A first fluid passage and a second fluid passage are associated with the fitting, and the deflectable element has a first and second opposite sides. The deflectable element is mounted to the fitting with the first side coupled to the first fluid passage and the second side coupled to the second fluid passage. Again, when a fluid at a first pressure and a fluid at a second pressure are directed along the first and second fluid passages, the first and second pressures are respectively applied against the first and second opposite sides of the deflectable element, deflecting the deflectable element in accordance with the differential pressure. The deflection sensing circuit, which is supported on the first side of the deflectable element and includes a thin-film strain gauge circuit, senses the deflection of the deflectable element and, therefore, the differential pressure. The insulative layer is also supported by the first side of the deflectable element and overlies the deflection sensing circuit to electrically insulate the deflection sensing circuit from the first fluid passage and any fluid in the first fluid passage. The receptacle includes a pressure sensing port having a well. The well of the pressure sensing port receives the fitting with the deflectable element positioned within the well. The receptacle fluidly connects the first fluid passage of the fitting to a source of the fluid at the first pressure and the second fluid passage of the fitting to a source of the fluid at the second pressure. 
         [0006]    Pressure sensing devices and fluid assemblies embodying one or more aspects of the invention have many advantages. For example, with an insulative layer overlying the deflection sensing circuit and supported by the deflectable element, the deflection sensing circuit is electrically insulated from any fluid which might damage the deflection sensing circuit. Fluids can be coupled to both sides of the deflectable element or diaphragm, and the first and second pressures can be directly applied to both sides of the deflectable element or diaphragm, without the use of additional protective features, such as isolating diaphragms, intermediate dielectric liquids, and complex manifold arrangements. This not only substantially reduces the size, weight, and complexity of the devices and assemblies embodying the invention, it also significantly enhances their reliability and responsiveness. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1A  is a cross sectional elevation view of a pressure sensing device and 
           [0008]      FIG. 1B  is a cross sectional elevation view of the pressure sensing device rotated by 90°. 
           [0009]      FIG. 2  is a plan view of a deflectable element, a deflection sensing circuit, and an overlying insulative layer. 
           [0010]      FIG. 3  is a cross sectional elevation view of a portion of a deflectable element and a deflection sensing circuit with the thicknesses exaggerated for clarity. 
           [0011]      FIG. 4  is a cross sectional elevation view of a fluid assembly. 
           [0012]      FIG. 5  is a cross sectional elevation view of a fluid assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0013]    Pressure sensing devices and assemblies embodying one or more aspects of the invention may be structured in a wide variety of ways. One of many examples of a pressure sensing device  10  is shown in  FIGS. 1A-3 . The illustrated pressure sensing device  10  includes a housing  11  and a deflectable element  12  mounted to the housing  11 . The housing  11  includes a first fluid passage  13  which is coupled to one side, e.g., the outboard side  14 , of the deflectable element  12  and a second fluid passage  15  coupled to the opposite side, e.g., the inboard side  16 , of the deflectable element  12 . A deflection sensing circuit  20  is supported by the outboard side  14  of the deflectable element  12 . An electrically insulative layer  21  overlies the deflection sensing circuit  20  and is also supported by the outboard side  14  of the deflectable element  12 . 
         [0014]    A fluid at a first pressure may be directed along the first fluid passage  13 , applying the first pressure directly against the outboard side  14  of the deflectable element  12 . A fluid at a second pressure may be directed along the second fluid passage  15 , applying the second pressure directly against the inboard side  16  of the deflectable element  12 . The deflectable element  12  deflects in proportion to the pressure differential. The deflection sensing circuit  20  senses the deflection and, therefore, the differential pressure. The insulative layer  21  protects the deflection sensing circuit  20 , for example, by electrically insulating the deflection sensing circuit  20  from the fluid in the first fluid passage  13 . 
         [0015]    The housing of the pressure sensing device may be fashioned from any material which has sufficient structural integrity and is sufficiently impervious, such as a metallic material or a polymeric material, and may be configured in numerous ways. The housing may be a single piece structure or a multipiece structure. As shown in  FIGS. 1A and 1B , the housing  11  may comprise several pieces, including mating inboard and outboard pieces  22 , 23  between which the deflectable element  12  may be located. The housing  11  may define a chamber  24  in which various electronic components  25 , such as printed circuit boards, interfaces, signal conditioners and amplifiers, may be located. The electronic components may serve a variety of purposes. For example, they may be used to calibrate the pressure sensing device or for signal conversion, e.g., to convert a strain gauge signal to other formats such as voltage or current. One or more temperature sensors  26  may also be located in the chamber  24  preferably in the vicinity of the first fluid passage  13 . The temperature sensors may also serve a variety of purposes. For example, they may be used to compensate the pressure sensing device for change in output due to temperature changes or to provide an independent indication of temperature as an output. The chamber  24  may be filled with a thermally conductive, electrically insulative material such as silicone. 
         [0016]    The first and second fluid passages may be variously configured and may be associated with the housing in several ways. As shown in  FIGS. 1A and 1B , for example, the first and second fluid passages may extend through the housing. For instance, the first fluid passage  13  may comprise a radial channel  30  which extends radially between the outer surface and the center of the outboard housing piece  23 , a gap  31  between the outboard side  14  of the deflectable element  12  and the outboard housing piece  23 , and an axial channel  32  which extends between the gap  31  and the inner end of the radial channel  30 . The second fluid channel  15  may comprise a bore  33  which extends from the inboard end of the inboard housing piece  22  to the deflectable element  12 . To prevent foulants from collecting in the fluid passages, one or both passages, e.g., the first fluid passage  13 , may contain a porous material  34  to filter the foulants from fluid flowing along the passages. While the first and second fluid passages may comprise the channels, gap and bore shown in  FIGS. 1A and 1B , the first and second fluid passages may alternatively comprise any other channels, grooves, crevices, chambers, spaces or openings which allow fluid to fluidly communicate with, for example, contact, the opposite sides of the deflectable element. For example, the deflectable element may be located at one end of the housing with the outboard side of the deflectable element facing the housing and the inboard side facing away from the housing. The first fluid passage may be a passage which extends within the housing and fluidly communicates with the outboard side of the deflectable element, and the second fluid passage may be a space at the end of the housing which faces the inboard side of the deflectable element. 
         [0017]    The deflectable element may be variously configured. For example, the deflectable element may comprise a thin, regularly or irregularly shaped diaphragm which can deflect upon application of a pressure differential on both sides of the diaphragm. The thickness may be uniform or non-uniform. In the embodiment illustrated in  FIGS. 1A-3 , the deflectable element  12  may comprise a thin, circular diaphragm having a thickened center section  35  and a thinner annular section  36  surrounding the thickened section  35 . The thickened section  35  concentrates the deflection in the thinner annular section  36 , which can enhance the sensitivity of the pressure sensing device  10 . The thickened section  35  may have a generally uniform thickness in the range from about 0.035 inch or less to about 0.050 inch or more, and the thinner annular section  36  may have a generally uniform thickness in the range from about 0.007 inch or less to about 0.014 inch or more. While the thickened section may protrude on both sides of the diaphragm, it preferably protrudes on only one side. For example, the thickened section  35  may protrude on only the inboard side  16  of the diaphragm  12 , leaving the outboard side  14  generally flat. 
         [0018]    The deflectable element  5  may be fashioned from any suitably deflectable material, including, for example, silicon, sapphire, a metal, an elastomer, or a polymer, and may be a single or multi piece structure distinct from the housing. For many embodiments, the deflectable element and the housing are fashioned from materials having similar coefficients of thermal expansion to reduce thermal stress. The deflectable element may be mounted in a variety of ways and in a variety of locations to the housing with the outboard and inboard sides coupled to the first and second fluid passages. For example, the deflectable element may be welded, bonded or mechanically connected to the housing at either end of the housing or intermediate the ends of the housing. Further, the deflectable element may be mounted generally perpendicular to a longitudinal axis of the housing, generally parallel to a longitudinal axis o the housing, or at any angle between perpendicular and parallel. 
         [0019]    In the embodiment as shown in  FIG. 1A-3 , the deflectable element  12  may be a unitary portion of the housing  11 , including, for example, a unitary portion of the outboard piece  23  or the inboard piece  22 , e.g., the inboard piece  22 . The bore  33  may be drilled from the inboard end of the inboard piece  22 , terminating short of the outboard end. The diaphragm  12  may then be machined at the outboard end of the inboard piece  22 . The diaphragm  12  may thus be unitarily mounted integrally to the inboard piece  22  of the housing  11 , which matches the thermal coefficients of expansion and eliminates the need for a seal between the inboard piece  22  and the diaphragm  12 . The outboard housing piece  23  may also be fashioned from a metal having a coefficient of thermal expansion similar to, e.g., generally equal to, the metal of the inboard housing piece  22 . Matching the thermal coefficients of expansion reduces thermal stress and enhances performance. The inboard housing piece  22  may then be attached to the outboard housing piece  23  with the diaphragm  12  between them. The inboard and outboard pieces may be threaded to one another. Alternatively, the two pieces  22 ,  23  may be welded to one another with the outboard side  14  of the diaphragm  12  facing the gap  31  of the first fluid passage  13  and the inboard side  16  facing the bore  33  of the second fluid passage  15 . The inboard and outboard pieces  22 ,  23  may be fusion welded by a variety of techniques, including laser, election beam, or TIG welding. Welding the pieces  22 ,  23  eliminates a seal and simplifies the construction. 
         [0020]    The deflection sensing circuit  20  may be configured in any manner which enables the deflection of the deflectable element  12  to be sensed. For example, the deflection sensing circuit  20  may have an electrical parameter which changes in response to deflection of the deflectable element  12 . For many embodiments, the deflection sensing circuit  20  may comprise a strain gauge circuit  40 , such as a thin-film strain gauge circuit, which includes a resistance network that changes resistance in proportion to the deflection of the deflectable element. 
         [0021]    The deflection sensing circuit may be supported by one or both sides of the deflectable element. For many embodiments, the deflection sensing circuit  20  may be supported by only one side of the deflectable element  12 , e.g., the side closest to the electronic components  25 . In the embodiment shown in  FIGS. 1A-3 , the thin-film strain gauge circuit  40  may be supported by the outboard side  14  of the diaphragm  12  and at least a portion of the strain gauge circuit  40  may be supported in the thin annular section  36  of the diaphragm  12 . Further, the deflection sensing circuit may be supported by the deflectable element in a variety of ways. For example, the deflection sensing circuit may be mounted directly on the deflectable element, especially if the deflectable element is fashioned from an insulative material. As another example, the deflection sensing circuit may be mounted directly on one or more intermediate layers which, in turn, are mounted on the deflectable element. In the embodiment shown in  FIGS. 1A-3 , the thin-film strain gauge circuit  40  may be mounted directly on an underlying insulative layer  41  which, in turn, may be mounted directly on the outboard side  14  of the metal diaphragm  12 . The underlying insulative layer  41  may be formed from any sufficiently insulative material, including a solid inorganic material, such as glass. The underlying insulative layer  41 , and the thin-film strain gauge circuit  40 , may be deposited in any suitable manner, including, for example, by sputtering or chemical vapor deposition. 
         [0022]    The deflection sensing circuit  20  may be supported by one or both sides of the deflectable element  12  facing one or both adjacent fluid passages  13 ,  15 . To electrically insulate the deflection sensing circuit  20  from the fluid passages  13 ,  15 , and any fluid in the fluid passages  13 ,  15 , an insulative layer  21  which is supported by the deflectable element  12  overlies the deflection sensing element  20 . The overlying insulative layer  21  may also be fashioned from any suitably insulative material, including a solid, inorganic material, such as glass. In the embodiment shown in  FIGS. 1A-3 , the overlying insulative layer  21  may be formed as a component of the deflection sensing circuit  20 . For example, the underlying insulative layer  41 , the thin-film strain gauge circuit  40 , and the overlying insulative layer  21  may be deposited sequentially on the diaphragm  12 . In other embodiments, the overlying insulative layer may be a component distinct from the deflection sensing circuit. The overlying insulative layer, as well as the underlying insulative layer, may extend over an entire side of the deflectable element or only a portion of the deflectable element, e.g., the portion occupied by the deflection sensing circuit. 
         [0023]    Electrical signals may be supplied to or from the deflection sensing circuit in any of numerous ways. For example, the deflection sensing circuit may be electrically coupled via a wireless connection to other electrical components within the pressure sensing device or elsewhere. Alternatively, the deflection sensing circuit may be electrically coupled to other electrical components by one or more electrical connectors. The electrical connectors may be variously configured and may be routed to and/or from the deflection sensing circuit in a variety of ways. 
         [0024]    For many embodiments, the deflection sensing circuit may be electrically coupled to other electrical components via one or more electrical connectors positioned on the same side of the deflectable element as the deflection sensing circuit and in a region isolated from the fluid passages. For example, in the embodiment shown in  FIGS. 1A-3 , a seal  43  may be sealingly positioned between the housing  11  and the deflectable element  12 , e.g., between the outboard housing piece  23  and the generally flat outboard side  14  of the diaphragm  12 . The seal may have any of a variety of geometries. For some embodiments, the seal may comprise a ring seal, such as an O-ring seal or a ring gasket. The ring seal  43  may seal directly against the diaphragm  12  and/or the deflection sensing circuit  20 , which form a seal seat  47 . The ring seal  43  defines an inner region  44  and an outer region  45  of the deflectable element  12 . The inner region  44  may include at least a portion of the deflection sensing circuit  20 , e.g., the thin-film strain gauge circuit  40 , and at least a portion of the thinner annular section  36  of the diaphragm  12 . The inner region  44  may be coupled to the first fluid passage  13 . The outer region  45  may be isolated from the first fluid passage  13  and any fluid in the first fluid passage  13  by the seal ring  43 . One or more electrical connectors  46  may extend between the inner region  44  and the outer region  45 , for example, along the deflectable element  12  under the ring seal  43 . The electrical connector  46  may be a portion of the deflection sensing circuit  20 , e.g., a portion of the strain gauge circuit  40 , which extends under the ring seal  43 . Alternatively, the electrical connector  46  may be a separate conductor which is coupled to the deflection sensing circuit  20 , e.g., the strain gauge circuit  40 , and extends under the ring seal  43 . For many embodiments, the electrical connector  46  may be deposited on the deflectable element  12  with the thin-film strain gauge circuit  40  and sandwiched between the underlying and overlying insulative layers  41 ,  21 . In the embodiment shown in  FIGS. 1A-3 , the electrical connector  46  may be coupled to one or more contact pads  50  in the isolated outer region  45 . Additional electrical connectors  51 , such pins, wires, or ribbons, may extend from the contact pads  50  through the outboard housing piece  23  to the electronic components  25 . In the embodiment illustrated in  FIGS. 1A-3 , the additional electrical connectors  51  may comprise insulated, spring-loaded electrical probes. Isolating fluid in the fluid passages from the electrical connectors  46 ,  50 ,  51  and their interconnections significantly enhances the reliability of the pressure sensing device. 
         [0025]    Pressure sensing devices embodying the invention may be operated in numerous ways. In one mode of operation, fluid at a lower pressure may be directed along the first fluid passage  13  and fluid at a higher pressure may be directed along the second fluid passage  15 . The lower pressure fluid is thus coupled to the outboard side  14  of the deflectable element  12  while the higher pressure fluid is coupled to the inboard side  16  of the deflectable element  12 . One advantage of this mode of operation is that the weld between the inboard and outboard pieces  22 ,  23  is maintained in compression by the differential pressure, which enhances the durability and reliability of the pressure sensing device. 
         [0026]    With the low pressure fluid and the high pressure fluid respectively coupled to the outboard and inboard sides  14 ,  16  of the deflectable element  12 , the deflectable element  12  deflects toward the gap  31  of the first fluid passage  13  and the outboard piece  23  of the housing  11 . For many embodiments, the amount of deflection may be limited by a stop  52 . The stop  52  may be arranged to allow the deflectable element  12  to deflect freely over a normal operating range but to contact the deflectable element  12  and limit further deflection beyond the normal operating range. For example, the stop  52  may be arranged to limit the deflection of the deflectable element to a predetermined value. The predetermined value may vary for different diaphragm configurations and may depend on several factors, including, for example, the diameter of the diaphragm, the diameter of the thickened section, the thickness of the annular section, and the diaphragm material. For some embodiments, the predetermined value may be about 0.010 inch or less, or about 0.005 inch or less, or about 0.003 inch or less, e.g., about 0.002 inch or less. The stop thus protects the deflectable element from over-pressure or line pressure in second fluid passage. For example, the pressure sensing device may be used to measure differential pressures on the order of 100 psid for fluids which may have a line pressure on the order of 5000 psi. If the first fluid passage were to leak to atmosphere, the differential pressure across the deflectable element would be about 5000 psid and the stop would prevent undue deflection of the deflectable element. The stop and the small gap width allow the pressure sensing device to be used in environments where the ratio of over-pressure or line pressure to nominal differential pressure is up to about 50:1 or even greater than about 50:1. 
         [0027]    The stop may be configured in a variety of ways and may be located in a variety of positions. For example, the stop may be an additional mechanical structure mounted to the housing or the deflectable element. For many embodiments, the stop may comprise an existing portion of the housing or the deflectable element, eliminating the need for additional structure and, thereby, reducing both the size and weight of the pressure sensing device. For example, in the embodiment shown in  FIGS. 1A-3 , the stop  52  may comprise the inboard end of the outboard piece  23  of the housing  11  on the opposite side of the gap  31  from the outboard side  14  of the diaphragm  12 . The depth of the gap  31 , e.g., about 0.002 inch, thus limits the amount of deflection of the diaphragm  12 . 
         [0028]    The deflectable element  12  deflects in proportion to the differential pressure, and the deflection sensing circuit  20  senses the deflection and provides an electrical signal indicative of the differential pressure. For example, the thin-film strain gauge circuit  40  may have a resistance network which changes resistance in proportion to the amount of deflection and provides a corresponding electrical signal indicative of the differential pressure. The electrical signal may be sent to an electrical system which monitors the differential pressure via the electrical connectors  46 ,  51  and electronic components  25 . 
         [0029]    Pressure sensing devices embodying the invention allow the low pressure fluid and the high pressure fluid to be directly coupled to both sides of the deflectable element, i.e., the fluids contact both sides of the deflectable element or contact a structure, such as the thin-film strain gauge circuit and the insulative layers, which is supported by one or both sides of the deflectable element. The pressures of the fluids are applied directly against both sides of a deflectable element without the use of additional protective features, such as isolating diaphragms, intermediate dielectric liquids, and complex manifold arrangements which direct the low pressure fluid and the high pressure fluid to separate sensing diaphragms. Pressure sensing devices embodying the invention thus provide a rapidly responsive, highly accurate indication of differential pressure in a small, light-weight package. 
         [0030]    Pressure sensing devices embodying the invention may be used in a wide variety of fluid assemblies. For example, many fluid assemblies include filter elements to filter impurities from fluids flowing through the fluid assemblies. The filter element may be used to remove impurities, for example, from hydraulic liquids or lubricant liquids. The pressure sensing device may be used in such a fluid assembly to monitor the differential pressure across the filter element and determine if the filter element should be replaced by a clean filter element. For many embodiments, the pressure sensing device may be small enough to fit the active portion of the device, e.g., the deflectable element and the deflection sensing circuit, into the pressure sensing port of the fluid assembly. 
         [0031]    For example, the fluid assembly  100  shown in  FIG. 4  includes a filter element  101  and an inlet line  102  and an outlet line  103  connected to the filter element  101 . The fluid assembly  100  also includes a receptacle  104  which receives a pressure sensing device  10 . The receptacle  104  may be part of any structure of the fluid assembly  100  and may include pressure sensing port  105  which comprises a well  106 . The well  106  may be configured in a variety of ways. For example, the well  106  may have one or more cylindrical walls  110  and may terminate at a base  111 . A first channel  112  may extend from one line, e.g., the lower pressure outlet line  102 , to the cylindrical wall  110  of the well  106 , while a second channel  113  may extend from another line, e.g., the higher pressure inlet line  103 , to the base  11  of the well  106 . The outboard end of the well  106  opposite the base  111  may be surrounded by a ledge  114 . 
         [0032]    The pressure sensing device, for example, a pressure sensing device  10  similar to that shown in  FIGS. 1A and 1B , may be mounted to the receptacle  104  in any convenient manner, e.g., by a threaded connection (not shown). All or a portion of the housing  11  of the pressure sensing device  10  may be configured as a fitting  53  which may be inserted in the well  106  of the pressure sensing port  105  with the first channel  112  fluidly coupled to the first fluid passage  13  and the second channel  113  fluidly coupled to the second fluid passage  15 . 
         [0033]    The fitting  53  may be variously configured. In the embodiment shown in  FIG. 4 , the fitting  53  may have a generally cylindrical outer surface which extends from a flange  54  to the inboard end of the fitting  53 . The first fluid passage  13  may intersect the cylindrical outer surface of the fitting  53  and the second fluid passage  15  may open onto the inboard end of the fitting  53 . The outer diameter of the fitting  53  may be about equal to the inner diameter of the cylindrical wall  110  of the well  106  of the receptacle  104 . The fitting  53  may be inserted into the well  105  with the flange  54  abutting the ledge  114 , with the first channel  112  fluidly communicating with the first fluid passage  13 , and with the second channel  113  fluidly communicating with the second fluid passage  15 . The first channel  112  and the first fluid passage  13  may be aligned with one another or they may fluidly communicate via a clearance between the cylindrical wall  110  of the well  106  and the outer cylindrical surface of the fitting  53 . 
         [0034]    A seal  55 , for example, an O-ring seal, may be mounted between the fitting  53  and the receptacle  104  to seal any fluid in the first channel  112  and the first fluid passage  13  from the ambient environment. The seal  55  may be mounted, for example, between the flange  54  and the ledge  114  or between the outer cylindrical surface of the fitting  53  and the cylindrical wall  110  of the well  106 . The seal  55  is axially positioned outboard of the intersection of the first fluid passage  13  with the outer cylindrical surface of the fitting  53  and outboard of the intersection of the first channel  112  with the cylindrical wall  110  of the well  106 . 
         [0035]    Another seal  56 , for example, another O-ring seal, may be mounted between the fitting  53  and the receptacle  104  to seal any fluid in the first channel  112  or the first fluid passage  13  from any fluid in the second channel  113  or the second fluid passage  15 . The seal  56  may be mounted between the outer cylindrical surface of the fitting  53  and the cylindrical wall  110  of the well  106  and may be axially positioned inboard of the intersection of the first fluid passage  13  with the outer cylindrical surface of the fitting  53  and inboard of the intersection of the first channel  112  with the cylindrical wall  110  of the well  106 . Alternatively, the seal may be mounted in the bore of the inboard housing piece between an inner wall of the inboard housing piece and a hollow cylindrical boss (not shown) which extends into the bore from the base of the well. 
         [0036]    In one mode of operation, lower pressure fluid from the filter outlet line  103  may be coupled to the outboard side  14  of the deflectable element  12  via the first channel  112  and the first fluid passage  13 , while higher pressure fluid from the filter inlet line  102  may be coupled to the inboard side  16  of the deflectable element  12  via the second channel  113  and the second fluid passage  15 . The deflection sensing circuit  20  responds to the deflection of the deflectable element  12  and provides a signal indicative of the differential pressure, which, for the fluid assembly  100  shown in  FIG. 4 , corresponds to the pressure drop through the filter element  101 . With the deflectable element  12  and the deflection sensing circuit  20  located in the fitting  53  of the pressure sensing device  10  and in the well  106  of the pressure sensing port  105 , the differential pressure is sensed with a minimum of manifolding outside the receptacle  104 . Consequently, the pressure sensing device  10  provides a highly responsive and reliable indication of the differential pressure. 
         [0037]    While pressure sensing devices and fluid assemblies embodying one or more aspects of the invention have been previously described and/or illustrated in the Figures, the invention is not limited to these embodiments. For instance, one or more of the features of these embodiments may be eliminated without departing from the scope of the invention. For example, one or more of the electronic components  25  and/or temperature sensors  26  may be eliminated from the chamber  24  of the housing  11 . This may further reduce the size and weight of the pressure sensing device. 
         [0038]    Further, one or more features of one embodiment may be combined with one or more features of other embodiments and/or one or more features of the embodiments may be modified without departing from the scope of the invention. For example, as shown in  FIG. 5 , a pressure sensing device  10  may include a temperature sensor  27  that extends through the housing  11  and into a reservoir or fluid line of the fluid assembly  100 . This temperature sensor  27  may be configured as a probe having an encased end  28  sealed to the housing  11 . 
         [0039]    The pressure sensing device  10  shown in  FIG. 5  may also include a second stop  57  arranged to limit deflection of the deflectable element  12  toward the second fluid passage  15  to a predetermined value. The second stop  57  may be useful if the fluid assembly  100  is subject to occasional reverse pressures where the pressure in the first fluid passage  13  may exceed the pressure in the second fluid passage  15  for a brief or extended period of time. Further, a pressure sensing device with stops on both sides of the deflectable element may be more versatile, allowing the higher pressure fluid to be coupled to either of the first fluid passage or the second fluid passage. 
         [0040]    The second stop  57  may be configured in many different ways. In the embodiment shown in  FIG. 5 , the second stop  57  may comprise a hollow, cylindrical sleeve which may be inserted in the bore  33  of the inboard housing piece  22  and attached to the inboard piece  22  in any convenient manner. For example, the second stop  57  may be bonded, threaded or welded to the inboard piece  22 . The second stop  57  may have an annular end face  58  which faces the inboard side  16  of the deflectable element  12  with a gap  59  between them. For example, the end face  58  of the second stop may face the inboard side of the thin annular section  36  of the diaphragm  12  with a gap  59  between them, allowing the thickened center section  35  to extend into the hollow interior of the sleeve  57 . The thickness of the gap  59  facing the inboard side  16  of the diaphragm  12  may then limit the deflection of the diaphragm  12  toward the gap  59  in the same manner, and to a similar or different extent, as the thickness of the gap  31  facing the outboard side  14  limits the deflection of the diaphragm  12  toward that gap  31 . To prevent foulants from fouling the second fluid passage  15 , especially a thin gap  59 , a porous filter material  60  may be mounted in the center of the sleeve  57 . 
         [0041]    Further, a pressure sensing device embodying the invention may be combined with other components, such as other sensing elements, to create a more multipurpose device. For example, the multipurpose device may also include additional temperature sensors, a gauge pressure sensor, a water content sensor, and/or a flow sensor, e.g., a device that senses differential pressure across an orifice.