Abstract:
Optical sensors used in harsh environments require a sealed pressure tight passage of an optical waveguide into an interior of the sensor. In one embodiment, a pressure sensor assembly for determining the pressure of a fluid in a harsh environment includes a sensing element suspended within a fluid filled housing. An optical waveguide that provides communication with the sensing element couples to a feedthrough assembly, which includes a cane-based optical waveguide forming a glass plug sealingly disposed in the housing. The glass plug provides optical communication between the optical waveguide and the sensing element. A pressure transmitting device can transmit the pressure of the fluid to the fluid within the housing. The assembly can maintain the sensing element in a near zero base strain condition and can protect the sensing element from shock/vibration.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is related to application Ser. No. ______, (Attorney Docket Number WEAT/0588) filed herewith, entitled “Optical Waveguide Feedthrough Assembly,” which is herein incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to optical sensors sealed within a housing to protect the sensors from a surrounding environment. More particularly, embodiments of the invention relate to an optical pressure sensor assembly that has an optical waveguide feedthrough assembly and is for use in a harsh environment.  
         [0004]     2. Description of the Related Art  
         [0005]     An exemplary pressure sensor is described in U.S. Pat. No. 6,439,055, issued Aug. 27, 2002, which is hereby incorporated by reference. The pressure sensor generally includes a fiber optic sensing element suspended within a fluid filled housing. Small diameter optical waveguides penetrate the housing at a pressure seal or feedthrough member where a relatively high fluid or gas differential pressure may exist. One or both sides of the feedthrough member may be subjected to relatively high temperatures and other harsh environmental conditions, such as corrosive or volatile gas, fluids and other materials. Thus, there exists a need for an improved optical sensor assembly capable of operating in relative high temperature and high pressure environments.  
       SUMMARY OF THE INVENTION  
       [0006]     Optical sensors used in harsh environments require a sealed pressure tight passage of an optical waveguide into an interior of the sensor. In one embodiment, a pressure sensor assembly for determining the pressure of a fluid in a harsh environment includes a sensing element suspended within a fluid filled housing. An optical waveguide that provides communication with the sensing element couples to a feedthrough assembly, which includes a cane-based optical waveguide forming a glass plug sealingly disposed in the housing. The glass plug provides optical communication between the optical waveguide and the sensing element. A pressure transmitting device can transmit the pressure of the fluid to the fluid within the housing. The assembly can maintain the sensing element in a near zero base strain condition and can protect the sensing element from shock/vibration. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.  
         [0008]     It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0009]      FIG. 1  illustrates a cross section view of an optical waveguide feedthrough assembly.  
         [0010]      FIG. 2  illustrates a cross section view of an optical waveguide feedthrough assembly having diagnostic sensors disposed therein.  
         [0011]      FIGS. 3-5  illustrate graphs of signals received from the diagnostic sensors where the feedthrough assembly is at a fixed temperature and different pressure for each graph.  
         [0012]      FIGS. 6-8  illustrate graphs of signals received from the diagnostic sensors where the feedthrough assembly is at a fixed pressure and different temperature for each graph.  
         [0013]      FIG. 9  illustrates a cross section view of an optical waveguide feedthrough assembly that provides bi-directional seal performance.  
         [0014]      FIG. 10  illustrates a cross sectional view of an optical waveguide feedthrough assembly that includes a compression seal element.  
         [0015]      FIG. 11  illustrates the optical waveguide feedthrough assembly shown in  FIG. 10  after compression of the compression seal element.  
         [0016]      FIG. 12  illustrates a cross section view of another optical waveguide feedthrough assembly.  
         [0017]      FIG. 13  illustrates a side cross section view of a pressure sensor assembly.  
         [0018]      FIG. 14  illustrates a side cross section view of a first alternative embodiment of a pressure transmitting device.  
         [0019]      FIG. 15  illustrates a side view of a second alternative embodiment of a pressure transmitting device.  
         [0020]      FIG. 16  illustrates a side cross section view of a first alternative embodiment of a pressure sensor.  
         [0021]      FIG. 17  illustrates a side cross section view of a second alternative embodiment of a pressure sensor assembly.  
         [0022]      FIG. 18  illustrates a cross sectional view of the pressure sensor assembly of  FIG. 17  taken substantially along line  5 - 5 .  
         [0023]      FIG. 19  illustrates a side cross section view of a third alternative embodiment of a pressure sensor assembly.  
         [0024]      FIG. 20  illustrates a side cross section view of a fourth alternative embodiment of a pressure sensor assembly. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]     Epoxy-free optical fiber feedthrough assemblies applicable for use in high temperature, high pressure environments are provided. In one embodiment, a feedthrough assembly includes a glass plug disposed in a recess of a feedthrough housing. The glass plug is preferably a large-diameter, cane-based, waveguide adapted to seal the recess in the housing and provide optical communication through the housing. All embodiments described herein provide for sealing with respect to the housing at or around the glass plug of an optical waveguide element passing through the housing.  
         [0026]     As used herein, “optical fiber,” “glass plug” and the more general term “optical waveguide” refer to any of a number of different devices that are currently known or later become known for transmitting optical signals along a desired pathway. For example, each of these terms can refer to single mode, multi-mode, birefringent, polarization maintaining, polarizing, multi-core or multi-cladding optical waveguides, or flat or planar waveguides. The optical waveguides may be made of any glass, e.g., silica, phosphate glass, or other glasses, or made of glass and plastic, or solely plastic. For high temperature applications, optical waveguides made of a glass material is desirable. Furthermore, any of the optical waveguides can be partially or completely coated with a gettering agent and/or a blocking agent (such as gold) to provide a hydrogen barrier that protects the waveguide. In addition, the feedthrough assemblies can include a single such optical waveguide or may include a plurality of such optical waveguides.  
         [0000]     An Exemplary Feedthrough Assembly  
         [0027]      FIG. 1  shows a cross section view of an optical fiber feedthrough assembly  100  that includes a front housing  10  coupled to a back housing  12 . An optical waveguide element  14  passes through a passageway  16  common to both housings  10 ,  12 . The passageway  16  is defined by bores extending across the housings  10 ,  12 . The optical waveguide element  14  includes a glass plug  18  defining a large-diameter, cane-based, optical waveguide preferably having an outer diameter of about 3 millimeters (mm) or greater. The glass plug  18  can have appropriate core and cladding dimensions and ratios to provide the desired outer large-diameter.  
         [0028]     For some embodiments, first and second fiber pigtails  19 ,  20  extend from each end of the glass plug  18 . Each of the pigtails  19 ,  20  preferably include an optical waveguide such as an optical fiber  26  encased or embedded in a carrier  28  or larger diameter glass structure allowing the fiber  26  to be optically coupled to the glass plug  18 . U.S. patent application Ser. No. 10/755,722, entitled “Low-Loss Large-Diameter Pigtail” and hereby incorporated by reference in its entirety, describes exemplary pigtails that can facilitate subsequent optical connection of the fiber  26  to other fibers, connectors, or other optical components by suitable splicing techniques known in the art. Further, U.S. application Ser. No. 10/755,708, entitled “Large Diameter Optical Waveguide Splice,” which is herein incorporated by reference in its entirety, describes a large-diameter splice suitable for splicing the fiber pigtails  19 ,  20  to the glass plug  18 . For some embodiments, the glass plug  18  can be spliced to or otherwise optically coupled with fibers in optical communication with each end of the glass plug  18  by other techniques and methods.  
         [0029]     Sealing of the optical waveguide element  14  with respect to the front housing  10  occurs at and/or around the glass plug  18  to enable isolation of fluid pressure in communication with a first end  22  of the passageway  16  from fluid pressure in communication with a second end  24  of the passageway  16 . This sealing of the glass plug  18  with respect to the front housing  10  provides the feedthrough capabilities of the feedthrough assembly  100 . In the embodiment shown in  FIG. 1 , the glass plug  18  has a cone shaped tapered surface  50  for seating against a complimentary tapered seat  51  of the front housing  10 . Engagement between the tapered surface  50  and the complimentary tapered seat  51  that is located along the passageway  16  forms a seal that seals off fluid communication through the passageway  16 . The glass plug  18  can be machined to provide the cone shaped tapered surface  50 . Additionally, the glass plug  18  is preferably biased against the tapered seat  51  using a mechanical preload.  
         [0030]     A recess  30  formed in one end of the front housing  10  aligns with a corresponding recess  31  in one end of the back housing  12  where the housings  10 ,  12  are coupled together. Preferably, the front housing  10  is welded to the back housing  12  along mated features thereof. The housings  10 ,  12  preferably enclose the glass plug  18 , a biasing member such as a first stack of Belleville washers  34 , and a plunger  32 , which are all disposed within the recesses  30 ,  31 .  
         [0031]     The first stack of Belleville washers  34  supply the mechanical preload by pressing the plunger  32  onto an opposite end of the glass plug  18  from the tapered surface  50 . Since the plunger  32  is moveable with the glass plug  18 , this pressing of the plunger  32  develops a force to bias the glass plug  18  onto the tapered seat  51  of the front housing  10  located along the passageway  16  that passes through the front housing  10 . Transfer of force from the plunger  32  to the glass plug  18  can occur directly via an interface  54  between the two, which can include mating conical surfaces. The first stack of Belleville washers  34  compress between a base shoulder  44  of the recess  31  in the back housing  12  and an outward shoulder  46  of the plunger  32  upon make-up of the front housing  10  to the back housing  12 . Once the back housing  12  is welded or otherwise attached to the front housing  10  in order to keep the front and back housings  10 ,  12  connected, the first stack of Belleville washers  34  maintains the compression that supplies force acting against the plunger  32 .  
         [0032]     In some embodiments, the feed through assembly  100  further includes a gasket member  52  disposed between the tapered seat  51  and the tapered surface  50  of the glass plug  18 . As shown in  FIG. 1 , the gasket member  52  comprises an annular gasket. The gasket member  52  may be a gold foil that is shaped to complement the tapered surface  50  and the tapered seat  51 . The gasket member  52  deforms sufficiently to accommodate imperfections on the tapered surface  50  and/or the tapered seat  51 , thereby completing the seal and reducing stress between contacting surfaces due to any imperfections on the surfaces. Gold is preferred because of its ability to withstand high temperature, its ductility and its inert, non-reactive, non-corrosive nature. However, other materials possessing these characteristics may also be suitable, including aluminum, lead, indium, polyetheretherketone (“PEEK™”), polyimide, other suitable polymers, and combinations thereof.  
         [0033]     An additional gasket member (not shown) may be disposed between the interface  54  of the glass plug  18  and the plunger  32  for some embodiments to reduce the surface stress that may occur between these two components. In further embodiments, a layer of gold or other suitable material is deposited on the contact surfaces as an alternative to using the gasket member  52 . For example, the gold may be deposited using chemical vapor deposition, physical vapor deposition, plating, or combinations thereof to reduce surface stress and maximize the seal performance. Other embodiments utilize the gasket member  52  punched from sheets of a gasket material.  
         [0034]     For some embodiments, the housings  10 ,  12  additionally enclose a cup-shaped backstop sleeve  36 , a second stack of Belleville washers  38 , a perforated washer  40 , and a centering element  42  that are all disposed within the recesses  30 ,  31 . An outward shoulder  56  of the backstop sleeve  36  is trapped by the end of the front housing  10  and an inward shoulder  57  along the recess  31  in the back housing  12 . Contact upon sandwiching of the shoulder  56  of the backstop sleeve  36  provides the point at which the housings  10 ,  12  are fully mated and can be secured together. Clearance is provided such that the end of the back housing  12  does not bottom out prior to the housings  10 ,  12  being fully mated.  
         [0035]     The centering element  42  includes an elastomeric sealing component disposed between the glass plug  18  and the front housing  10  that can act as a back-up seal in addition to facilitating alignment of the glass plug  18  with respect to the seat  51 . Although the centering element  42  is described as providing a back up seal to the tapered surface  50  of the glass plug  18  seated with the gasket member  52  on the complimentary tapered seat  51 , the centering element  42  can be omitted or used independently to seal off the passageway  16  through the housings  10 ,  12  in other embodiments.  
         [0036]     In some applications, the pressure in the recesses  30 ,  31  entering from the second end  24  of the passageway  16  is higher than the pressure entering from the first end  22  of the passageway  16 . This pressure differential advantageously causes the centering element  42  to deform and press against the wall of the recess  30  and the wall of the glass plug  18 , thereby creating a pressure energized seal. In some embodiments, one or more holes or annular channels  43  are formed on the outer surface of the high pressure side of the centering element  42 . These holes or channels  43  facilitate the deformation of the centering element  42  and the formation of the seal between the centering element  42  and the walls of the recess  30  and the glass plug  18 . Additionally, the perforated washer  40  enables pressurized fluid to fill the centering element  42  for providing the energized seal.  
         [0037]     Preferably, force transferred through the perforated washer  40  biases the centering element  42  into the recess  30 . The second stack of Belleville washers  38  pressed by the backstop sleeve  36  supplies the preloading force to the perforated washer  40 . The second stack of Belleville washers  38  allow a maximum pressure force to act on the centering element  42  such that pressure of the centering element  42  against the wall of the glass plug  18  does not override force being put on the glass plug  18  to press the tapered surface  50  against the seat  51 .  
         [0038]     Embodiments of the feedthrough assembly  100  are capable of performing in temperature environments of between −50° C. and 300° C. Additionally, the feedthrough assembly  100  is capable of withstanding pressure up to about 30 kpsi.  
         [0000]     Embedding Diagnostic Sensors  
         [0039]      FIG. 2  illustrates a cross section view of an optical waveguide feedthrough assembly  200  that operates similar to the feedthrough assembly  100  shown in  FIG. 1 . However, the feedthrough assembly  200  includes first and second diagnostic sensors  201 ,  202  disposed within a glass plug  218 . The diagnostic sensors  201 ,  202  can include any optical sensing element, such as fiber Bragg gratings, capable of reflecting or transmitting an optical signal in response to a parameter being measured. The first diagnostic sensor  201  is disposed within the glass plug  218  proximate an interface  254  where a plunger  232  pushes on the glass plug  218 . The second diagnostic sensor  202  is disposed within the glass plug  218  proximate where a tapered surface  250  of the glass plug  218  mates with a seat  251 . Preferably, each of the diagnostic sensors  201 ,  202  span a length of the glass plug  218  across the respective feature that the sensor is proximate.  
         [0040]     Interpreting the signals generated by the sensors  201 ,  202 , such as by use of a suitable algorithm or comparison to a calibration, enables monitoring of temperature and/or pressure. This detection ability allows real-time monitoring of the state of the feedthrough assembly  200 . Information derived from the sensors  201 ,  202  can be beneficial both during fabrication of the feedthrough assembly  200  and during use thereof. For diagnostic purposes, signals received from the second sensor  202  can be monitored to identify when and/or if proper contact of the tapered surface  250  with the seat  251  occurs to ensure that sealing is established or maintained. Further, monitoring one or both the sensors  201 ,  202  can ensure that excess force that might break the glass plug  18  is not applied to the glass plug  18  in embodiments where the amount of force can be controlled. Monitoring signals received from the first sensor  201  can detect the presence and condition of hydrostatic loads from surrounding fluid since these hydrostatic loads dominate the response of the first sensor  201 . When the feedthrough assembly  200  is part of a wellhead outlet of an oil/gas well, the sensors  201 ,  202  can be used to detect pressure increases and set an alarm indicating that seals have been breached in the well.  
         [0041]      FIGS. 3-5  illustrate graphs of signals received from the diagnostic sensors  201 ,  202  where the feedthrough assembly  200  is at a fixed temperature but has different pressures introduced at end  224  for each graph. In all of the graphs herein, first sensor responses  301  correspond to signals received from the first sensor  201  while second sensor responses  302  correspond to signals received from the second sensor  202 . In  FIG. 3 , an initial distortion or spreading of the second sensor response  302  visible specifically as a spectral chirp  303 , providing positive feedback that preload of the glass plug  18  at the tapered surface  250  against the seat  251  has been established.  
         [0042]     As visible in  FIGS. 4 and 5 , this distortion in the second sensor responses  302  grows relative to pressure due to non-uniform seal loads. However, the first sensor responses  301  show little change as pressure increases since uniform hydrostatic pressure dominates the first sensor  201 . Additionally, the first sensor responses  301  provide an indication of a thermo-mechanical state of the housing of the feedthrough assembly  200  and a small pressure driven change in the preload of the plug  232 .  
         [0043]      FIGS. 6-8  show graphs of signals received from the diagnostic sensors  201 ,  202  where the feedthrough assembly  200  is at a fixed pressure but is at a different temperature for each graph. The graphs show that as temperature increases both of the responses  301 ,  302  shift in wavelength relative to the temperature increase in the same direction. For example, the peak at approximately 1534.5 nanometers (nm) in the first responses  301  at 25° C. shifts to approximately 1536.5 nm at 194° C. Other than small changes from temperature driven changes in the preloads, shapes of the responses  301 ,  302  do not change with temperature changes.  
         [0044]     With reference to  FIG. 1 , pressure entering the first end  22  of the passageway  16  may be significantly higher than the pressure entering the second end  24  of the passageway  16  in some applications. In this instance, if the higher pressure from the first end  22  exceeds a threshold value, then the seals formed by the seated tapered surface  50  of the glass plug  18  and/or the centering element  42  may be unseated. Accordingly, non-epoxy feedthrough assemblies in some embodiments can be adapted to seal against pressure from either side of a glass plug.  
         [0000]     A Bi-Directional Seal Assembly  
         [0045]      FIG. 9  shows an exemplary feedthrough assembly  900  having a bi-directional pressurized seal assembly  930 . A cone shaped glass plug  920  is disposed in a recess  925  of a feedthrough housing  910  formed by two body sections  911 ,  912 . The body sections  911 ,  912  can be coupled together using a weld or various other coupling configurations. A bore  915  sized to accommodate portions of an optical waveguide element  922  on either side of the glass plug  920  extends through the feedthrough housing  910 . A tapered seat  913  can be formed on each body section  911 ,  912  for receiving the glass plug  920 . Similar to the embodiment shown in  FIG. 1 , a gasket member  945  such as an annular gold foil can be disposed between the glass plug  920  and the tapered seats  913  of the body sections  911 ,  912 . The symmetrical configuration of tapered seats  913  in sections  911 ,  912  creates the primary bidirectional seal design.  
         [0046]     In one embodiment, a back-up bidirectional seal assembly  930  is disposed in the recess  925  to provide an additional seal against any leakage from either body section  911 ,  912 . The seal assembly  930  includes two cup-shaped, annular sealing elements  931 ,  932  and a positioning device  940  to maintain the sealing elements  931 ,  932  in their respective seal seats  941 ,  942 . The sealing elements  931 ,  932  are positioned such that their interior portions are opposed to each other and the positioning device  940  may be disposed in the interior portions of the sealing elements  931 ,  932 . The positioning device  940  may comprise a preloaded spring to bias the sealing elements  931 ,  932  against their respective seal seats  941 ,  942 , or against the body sections  911 ,  912 . In one embodiment, the sealing elements  931 ,  932  are made of an elastomeric material. The sealing elements  931 ,  932  can also comprise other suitable flexible materials capable of withstanding high temperature and high pressure.  
         [0047]     In operation, if fluid leaks through the tapered surfaces between the glass plug  920  and the first body section  911 , then the fluid pressure forces the glass plug  920  against the tapered seat in the body section  912  to activate the reverse direction seal. The fluid pressure will also act against the second sealing element  932 , which is biased against the second body section  912 . Particularly, the fluid pressure acts on the interior portion of the second sealing element  932  and urges sealing lips  934  of the second sealing element  932  outward, thereby sealing off any fluid path between the second sealing element  932  and the glass plug  920  and between the second sealing element  932  and the body section  911 . In this manner, the leaked fluid is prevented from entering the bore of the second body section  912  because of redundant seals.  
         [0048]     Similarly, if fluid leaks through the tapered surfaces between the glass plug  920  and the second body section  912 , then the fluid pressure forces the glass plug  920  against the tapered seat  913  in body section  911 . The fluid pressure will also act against the first sealing element  931  biased against the first body section  911 . In this respect, the fluid pressure causes sealing lips  933  of the first sealing element  931  to sealingly engage the glass plug  920  and the body section  911 . Thus, the leaked fluid is prevented from entering the of bore of the first body section  911  because of redundant seals.  
         [0000]     Feedthrough Assembly with Compression Bushing  
         [0049]      FIG. 10  illustrates a cross sectional view of an optical waveguide feedthrough assembly  500  that includes a housing  110 , an externally threaded bushing  102 , a compression driver bushing  104 , a compression seal element  106 , and a glass plug  118  portion of an optical waveguide element that sealingly passes through the housing  110 . The bushings  102 ,  104  and the seal element  106  are disposed adjacent to one another in a recess  130  in the housing  110  and encircle a portion of the glass plug  118 . Specifically, the externally threaded bushing  102  threads into a portion of the recess  130  in the housing  110  defining mating internal threads. The seal element  106  is located next to the driver bushing  104  and proximate an inward tapering cone  131  along the recess  130  in the housing  110 .  
         [0050]     A seal can be established with the glass plug  118  with respect to the housing  110  by driving the seal element  106  down the cone  131 . To establish this seal, rotation of the threaded bushing  102  with respect to the housing  110  displaces the threaded bushing  102  further into the recess  130  due to the threaded engagement between the threaded bushing  102  and the housing  110 . The driver bushing  104  in turn moves further into the recess and pushes the sealing element  106  toward the cone  131 . One function of the driver bushing  104  includes reducing torque transferred to the seal element  106  from the threaded bushing  102 .  
         [0051]     Preferably, the glass plug  118  has a cone shaped tapered surface  150  for seating against a complimentary tapered seat  151  of the housing  110 . The engagement between the tapered surface  150  and the complimentary tapered seat  151  can also or alternatively seal off fluid communication through the housing  110  around the glass plug  118  in a redundant manner. A gasket member  152  such as an annular gold foil can be disposed between the tapered surface  150  of the glass plug  118  and the tapered seat  151  of the housing  110  to reduce stress risers.  
         [0052]      FIG. 11  illustrates the optical waveguide feedthrough assembly  500  after compressing the seal element  106 . The seal element  106  packs within an annulus between an exterior of the glass plug  118  and an interior of the housing  110  after being driven down the cone  131 . Once packed in the annulus, the seal element  106  provides sealing contact against both the glass plug  118  and the housing  110 . Examples of suitable materials for the seal element  106  include TEFLON™, VESPEL™, polyimide, PEEK™, ARLON™, gold or other ductile metals for high temperature applications. During lower temperature usage, element  106  can be nylon, DELRIN™ or metal such as tin or lead. The driving of the seal element  106  can additionally move the glass plug  118  to force the tapered surface  150  to mate with the seat  151 . The glass plug  118  is of sufficient diameter and structural integrity that the compression of the seal element  106  around the glass plug does not disturb the optical qualities thereof. The feedthrough assembly  500  is capable of sealing the glass plug  118  with respect to the housing  110  regardless of which side of the housing  110  is exposed to a higher pressure.  
         [0000]     An Additional Exemplary Feedthrough Assembly  
         [0053]      FIG. 12  shows a cross-section view of a feedthrough assembly  400  that includes a feedthrough housing  410  for retaining a glass plug  418 . A recess  425  is formed in one end of the housing  410  to receive the glass plug  418 . Preferably, the recess  425  has a corresponding tapered seat  451  for receiving a cone shaped tapered surface  450  of the glass plug  418 . The glass plug  418  is preferably biased against the tapered seat  451  that is located along a bore  416  that connects to the recess  425  and provides a passageway through the housing  410 .  
         [0054]     In one embodiment, a fitting  436  having an axial bore  437  extending therethrough is disposed between the glass plug  418  and a washer cap  412 . One end of the fitting  436  has a surface that mates with the glass plug  418  and an outer diameter that is about the same size as the inner diameter of the recess  425 . In this respect, the fitting  436  assists with supporting the glass plug  418  in the recess  425 . The other end of the fitting  436  has a neck  435  that connects to the washer cap  412 . Particularly, a portion of the neck  435  fits in a hole of the washer cap  412 . The washer cap  412  may be attached to the feedthrough housing  410  by any manner known to a person of ordinary skill in the art, such as one or more screws or bolts. For example, bolts  438  (two of three are visible in  FIG. 12 ) may be used to attach the washer cap  412  to the feedthrough housing  410  via three screw holes  440  (only one is visible in  FIG. 12 ) formed through the washer cap  412  and into the feedthrough housing  410 .  
         [0055]     The inner portion of the washer cap  412  facing the feedthrough housing  410  has a cavity  431  for retaining a preload member such as a spring. In one example, the preload member is a Belleville washer stack  434 . The washer stack  434  may be disposed on the neck  435  of the fitting  436  and between the washer cap  412  and an outward shoulder  446  formed by a reduced diameter of the neck  435  of the fitting  436 . In this manner, the washer stack  434  may exert a preloading force on the glass plug  418  to maintain a seal between the glass plug  418  and the tapered seat  451  of the feedthrough housing  410 . Similar to the embodiments described above, a gasket member such as an annular gold foil (not shown) can be disposed between the glass plug  418  and the tapered seats  451  and/or the glass plug  418  and the fitting  436 .  
         [0056]     The feedthrough assembly  400  may further include a centering element  442  to act as a back-up seal. The centering element  442  comprises an elastomeric sealing component that is disposed between the glass plug  418  and the feedthrough housing  410 . A pressure differential across the glass plug  418  advantageously causes the centering element  442  to deform and press against the wall of the recess  425  and the wall of the glass plug  418 , thereby creating a pressure energized seal. Although the centering element  442  is described as providing a back up seal, the centering element  442  may be used independently to seal off the bore  416  of the feedthrough housing  410 .  
         [0000]     A First Exemplary Sensor Assembly  
         [0057]      FIG. 13  shows a sensing assembly  1318  that includes a pressure sensor element  1310  disposed within a volume  1312  partially defined by a sensor housing  1314  that is filled with a first viscous fluid  1316  to essentially “float” the sensor element  1310  within the sensor housing  1314 . The first viscous fluid  1316  “floats” the sensor element  1310  within the sensor housing  1314  providing fluid dampening to the sensor and allowing for uniform pressure distribution about the sensor element  1310 . Sensing assembly  1318  further includes a pressure transmission device  1320 , such as a bellows, disposed within a pressure housing  1324  and in fluid communication with the volume  1312 . The pressure transmission device  1320  is exposed to a second viscous fluid  1325 , which may be the same or different than the first viscous fluid  1316 , having a first pressure P 1  entering the pressure housing  1324  through an inlet  1326  from a source (not shown), such as an oil production tube. The pressure transmission device  1320  reacts to the first pressure P 1  in the direction indicated by arrow  1321  and produces a corresponding second pressure P 2  within the volume  1312 . Further, the pressure transmission device  1320  may be configured to maintain the first viscous fluid  1316  in a relatively void free condition.  
         [0058]     In some embodiments, the second fluid  1325  comprises those fluids typically encountered within an oil production well including oil, gas, water and air among others. The sensor housing  1314  is filled with the first viscous fluid  1316  such as a viscous fluid, grease, silicone oil, or other fluids that provide shock and/or vibration isolation and prevent the sensor element  1310  from violently contacting the inner walls of the housing when subject to shock or vibration. For some embodiments, the first viscous fluid  1316  is comprised of a silicone oil, such as Dow Corning 200 Fluid, having a nominal viscosity of 200 centistokes.  
         [0059]     The pressure transmission device  1320  is shown in  FIG. 13  as a bellows by way of example and may include any transmission or accumulator (or similar) device that effectively transmits the first pressure P 1  to the volume  1312  while maintaining the volume  1312  in a fluid filled, void free, condition at the second pressure P 2 . A change in the first pressure P 1  causes bellows  1320  to react in the direction of arrow  1321  changing the internal volume of the bellows and the pressure P 2  within volume  1312  thereby. The bellows can have a maximum extension volume that maintains the second pressure P 2  of the first viscous fluid  1316  at a predictable minimum quasi-hydrostatic pressure suspending the sensor element  1310  within the volume  1312  with an average gap  1328  between the sensor element  1310  and the sensor housing  1314 .  
         [0060]     The pressure sensor element  1310  is exposed to the second pressure P 2  and transmits a signal corresponding to the level of pressure of the first viscous fluid  1316  via transmission cable  1330 . In order to insure that the sensor element  1310  is free to float within the housing  1314 , the transmission cable  1330  may be provided with a strain relief, or flexure portion  1331  which creates a low stiffness attachment between the sensor element  1310  and its base structure, the sensor housing  1314 . Although shown as a loop, the flexure portion  1331  may comprise any configuration that relieves attachment strain to the sensor element  1310  such as a coil, serpentine arrangement, helix, or other similar flexible configuration.  
         [0061]     The transmission cable  1330  passes through the sensor housing  1314  via a feedthrough assembly  1332  and is routed to other sensors or to an instrumentation or interrogation system (not shown). The feedthrough assembly  1332  can be any feedthrough assembly embodiment described above. The pressure sensor element  1310  may be any type of known optical pressure sensor benefiting from shock and vibration protection. When the sensor element  1310  is fiber optic based, the transmission cable  1330  may comprise one or more fiber optic cables.  
         [0000]     Alternative Pressure Transmission Devices  
         [0062]      FIGS. 14 and 15  show alternative embodiments of the pressure transmission device  1320 . In  FIG. 14 , the pressure transmission device  1320  comprises a diaphragm that transmits the first pressure P 1  to the volume  1312  while maintaining a fluid filled, void free, chamber. Referring to  FIG. 15 , the pressure transmission device  1320  comprises a pressure biased valve that transmits the first pressure P 1  to the volume  1312 . The valve is shown in an open position and is biased in a closed position (not shown) with a biasing force provided by a spring hinge  1337  that is overcome once the second pressure P 2  reaches a predetermined minimum pressure. The valve is shown as pivoting in the direction of arrow  1338  between the open and closed positions but may comprise any known type of pressure biased valve such as a check valve, slide valve, duck&#39;s bill, or other similar type valve.  
         [0000]     A Second Exemplary Sensor Assembly  
         [0063]      FIG. 16  shows an alternative embodiment of a pressure sensing assembly  1318  including an inlet tube  1327  having an inside diameter  1329  exposed to a first pressure P 1  and transmitting that pressure to a sensor housing  1314 . In the embodiment shown, fluids  1325  and  1316  may be the same fluid and expose a sensor element  1310  to a second pressure P 2  that is equal to the first pressure P 1 . For a given first fluid  1316  and a predetermined diameter  1329 , a sufficient capillary force is provided within the inlet tube  1327  to preclude fluid flow between a pressure housing  1324  and the sensor housing  1314  below some minimum pressure threshold. Once a volume  1312  of the sensor housing  1314  is filled with the first fluid  1316  the capillary force provided by the inlet tube  1327  essentially prevents the flow of fluids between the sensor housing  1314  and the pressure housing  1324 . In some embodiments, the inlet tube  1327  may be exposed directly to a source without the pressure housing  1324  being intermediate. It is advantageous to minimize flow in and out of the sensor housing  1314  in the directions indicated by arrows  1334 ,  1336 . For this, as well as for other reasons, a buffer tube  1333  is coupled to an inlet  1326 .  
         [0064]     The sensor housing  1314  includes a pass through arrangement. A transmission cable  1330  enters the sensor housing  1314  on one end and is coupled to the sensor element  1310 . An exit transmission cable  1360  is similarly coupled to the sensor element  1310  and exits the sensor housing  1314  via a feedthrough assembly  1332 . The pass through arrangement allows multiplexing of a plurality of the sensor assemblies  1318  wherein the transmission cables  1330 ,  1360  are similarly connected to other sensors. Alternatively, one or both, of the transmission cables  1330 ,  1360  may be connected to a signal processing system (not shown).  
         [0000]     A Third Exemplary Sensor Assembly  
         [0065]      FIGS. 17 and 18  show a sensing assembly  1318  including bumper elements  1340 ,  1342  that are attached to and extend beyond the dimensions of a sensor element  1310  to prevent the sensor element  1310  from directly contacting a sensor housing  1314 . The bumper elements  1340 ,  1342  may be comprised of a suitable material, such as polyamide, epoxy, polymers, elastomers, TEFLON™, VITON™, for example, and are sized to provide a predetermined clearance  1344  between the bumpers and the sensor housing  1314  allowing the sensor element  1310  to float radially in the direction indicated by arrow  1346  within the housing and within the clearance dimension.  
         [0066]     Features further limit the motion of the sensor element  1310  in a rotational and translational direction. The bumper elements  1340 ,  1342  include slots  1348 ,  1350  that cooperate with housing mounted tangs  1352 ,  1354  to limit the translational movement of the sensor element  1310  in the direction indicated by arrow  1356  and further limits rotational movement of the sensor element  1310  in the direction indicated by arrow  1358 . The bumpers  1340 ,  1342 , and slots  1348 ,  1350  allow the sensor element  1310  to float within a volume  1312  within a limited envelope determined by the gaps between the bumpers and the sensor housing  1314  and the gaps between the tangs  1348 ,  1350  and the slots.  
         [0067]     Limiting the radial motion of the sensor element  1310  prevents the sensor element from contacting the sensor housing  1314  directly. Limiting the translational movement of the sensor element  1310  reduces the amount of strain relief  1331  needed to allow for float and further prevents the sensor element  1310  from directly contacting the ends of the sensor housing  1314 . Further, limiting the rotational envelope of the sensor element  1310  prevents the sensor element from spinning within the volume  1312  and further reduces problems spinning would create with the transmission cable  1330  and its attachment to the sensor element.  
         [0068]     For some embodiments, bumpers are mounted to the housing  1314  to limit the movement of the sensor element  1310  within the volume  1312  similar to that described herein with reference to sensor mounted bumpers. The bumpers can include a pair of grooves to cooperate with a pair of tangs in any radial arrangement about the sensor housing. Further, the bumpers  1340 ,  1342  fill the volume  1312  to advantageously reduce the amount of a first fluid  1316  therein.  
         [0000]     A Fourth Exemplary Sensor Assembly  
         [0069]      FIG. 19  illustrates another embodiment of a pressure transducer subassembly  1500  having feedthrough assemblies  1510 ,  1511  that utilize glass plugs  1504 ,  1505  as described above. The subassembly  1500  includes the feedthrough assemblies  1510 ,  1511  coupled to splits  1522  of a collar  1520  to form a sealed fluid chamber  1530 . A housing  1525  is attached to the feedthrough assemblies  1510 ,  1511 . A sensor element  1535  can be connected directly to the glass plug  1505  of the feedthrough assemblies  1510 . The other end of the sensor element  1535  is cantilever mounted to the collar  1520 . In this respect, the sensor element  1535  is isolated from the housing  1525 .  
         [0070]     In one embodiment, the sensor element  1535  is mounted to the collar  1520  using a ball joint  1550 . The ball joint  1550  comprises a ball  1551  coupled to a tracking device, such as a washer  1552 . The ball  1551  can have a hole for receiving the sensor element  1535 . The ball  1551  is supported by the washer  1552  that is coupled to the collar  1520  and is locked in a neutral position. The ball joint  1550  advantageously limits deflection of the sensor element  1535  that is cantilevered. To reduce the strain on the sensor element  1535 , a service loop  1540 , a flexible carrier  1545 , or combinations thereof may be utilized for connection of the sensor element  1535  to an optical fiber  1503 .  
         [0000]     A Fifth Exemplary Sensor Assembly  
         [0071]      FIG. 20  shows a pressure sensor assembly  1418  having an optical pressure sensor element  1410  disposed within a sensor housing  1414  that is filled with a fluid. A bellows  1420  is disposed within a pressure housing  1424  and has an inner volume in fluid communication with an interior of the sensor housing  1414 . The bellows  1420  is exposed to a fluid pressure entering the pressure housing  1424  through an inlet  1426  that is coupled to a source (not shown). Since the pressure sensor assembly  1418  operates similar to other embodiments described above, a detailed discussion of the operational particulars of the pressure sensor assembly  1418  is omitted.  
         [0072]     An optical transmission cable  1430  is coupled to a feedthrough assembly  1432 , which is illustrated as the feedthrough assembly that is shown in detail in  FIG. 1 . For some embodiments, the feedthrough assembly  1432  can be any other feedthrough assembly embodiment described above. Accordingly, the feedthrough assembly  1432  provides optical communication between the transmission cable  1430  and the sensor element  1410 .  
         [0073]     Clam shell members  1440  hold the sensor element  1410  in position within the sensor housing  1414  and aid in filling an interior volume of the sensor housing. Annular passages between the clam shell members  1440  and the sensor housing  1414 , transverse apertures in the clam shell members  1440  and/or internal longitudinal passages defined by the claim shell members  1440  provide fluid communication between an interior of the bellows  1420  and the sensor element  1410  that detects pressure changes caused by expansion/compression of the bellows  1420 .  
         [0074]     The pressure housing  1424  includes a base  1422  that the bellows  1420  are coupled to. Preferably, the bellows  1420  are welded to the base  1422  around a reduced diameter protrusion  1421  of the base  1422  that aids in alignment of the bellows  1420  with the base  1422 . A longitudinal bore  1425  through the base  1422  opens into the interior of the bellows  1420 . A male end  1423  of the base  1422  of the pressure housing  1424  is threaded into a female end  1415  of the sensor housing  1414 . The male end  1423  can include a conical tapered surface for mating with a corresponding conical tapered surface of the female end  1415 . The tapered surfaces facilitate alignment of the bore  1425  in the base  1422  of the pressure housing  1424  with a channel  1426  extending into the interior of the sensor housing  1414 . A weld (not shown) circumscribing the male end  1423  at a junction between the male and female ends  1423 ,  1415  can seal the interior of the bellows  1420  and the interior of the sensor housing  1414  from a surrounding fluid environment. Alternatively, the male and female ends  1423 ,  1415  can be reversed for some embodiments.  
         [0075]     As described herein, pressure transducer assemblies having a non-epoxy feedthrough assembly are provided for operation in high temperature and high pressure environments. The invention heretofore can be used and has specific utility in applications within the oil and gas industry. Further, it is within the scope of the invention that other commercial embodiments/uses exist with one such universal sealed sensor arrangement shown in the figures and adaptable for use in (by way of example and not limitation) industrial, chemical, energy, nuclear, structural, etc. Although the sensors described heretofore detect pressure, other environmental conditions may be detected by optical elements, such as Bragg grating based sensors, disposed and arranged within a housing for detection of seismic disturbances, chemicals, etc., as is well known in the art.  
         [0076]     It should be understood that, unless otherwise stated herein, any of the features, characteristics, alternatives or modifications described regarding a particular embodiment herein may also be applied, used, or incorporated with any other embodiment described herein. While the foregoing is directed to preferred embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.