Abstract:
A pressure sensor has a housing with an opening exposing a cavity, a barrier diaphragm across the opening, a pressure sensing element within the cavity, and a fill fluid within the cavity. The fill fluid comprises a fluid and a filler, the fluid has a TCE associated therewith, the housing has a TCE associated therewith, and the filler lowers the TCE of the fluid to a level matching the TCE of the housing. Accordingly, the housing and the fill fluid do not exhibit temperature induced dimensional changes relative to one another that cause plastic deformation of the barrier diaphragm.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to a pressure sensor having a barrier diaphragm that retains a fill fluid in a cavity of the pressure sensor. 
   BACKGROUND OF THE INVENTION 
   Pressure sensors are used in a wide variety of applications in order to sense pressure. Such pressure sensors typically include a transducer that converts the pressure input to an electrical output signal. In some pressure sensors, the transducer is located in a cavity that is filled with a fluid and that is sealed by a diaphragm. Thus, the diaphragm acts as a barrier between the input pressure and the fill fluid. The diaphragm responds to the pressure input and transmits the pressure input through the fill fluid to the transducer. The transducer converts this pressure to the electrical output signal. 
   The fill fluid used for barrier diaphragm pressure sensors has a very high temperature coefficient of expansion (TCE) compared to the materials, typically stainless steel, forming the enclosure that confines the fill fluid. Therefore, as the pressure sensor is exposed to extreme temperatures, the fill fluid tends to expand more quickly than the enclosure resulting in a plastic deformation of the barrier diaphragm. 
   The present invention solves this and/or other problems. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the present invention, a pressure sensor comprises a housing having an opening exposing a cavity, a barrier diaphragm across the opening, a pressure sensing element supported within the cavity, and a fill fluid within the cavity. The fill fluid comprises a fluid mixed with a filler, the fluid has a TCE, and the filler is selected to lower the TCE of the fill fluid. 
   In accordance with another aspect of the present invention, a pressure sensor comprises a housing having a cavity, a barrier diaphragm in communication with the cavity, a pressure sensing element within the cavity, and a fill fluid within the cavity. The fill fluid comprises a fluid and a filler, the fluid has a TCE associated therewith, the housing has a TCE associated therewith, and the filler lowers the TCE of the fill fluid to a level more closely matching the TCE of the housing. 
   In accordance with still another aspect of the present invention, a pressure sensor comprises a housing, a pressure sensing element, a diaphragm stop, a barrier diaphragm, electrical conductors, and a fill fluid. The housing has an opening exposing a cavity. The pressure sensing element is supported by the housing within the cavity. The diaphragm stop is within the cavity, and the pressure sensing element is between the diaphragm stop and the housing. The barrier diaphragm is across the opening, and the diaphragm stop is between the pressure sensing element and the barrier diaphragm. The electrical conductors extends through the housing and electrically engages the pressure sensing element. The fill fluid is within the cavity, the fill fluid has a TCE, the housing has a TCE, and the TCE of the fill fluid has a value relative to the TCE of the housing so as to prevent plastic deformation of the barrier diaphragm. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which: 
       FIG. 1  illustrates an exploded view of a pressure sensor in accordance with one embodiment of the present invention; 
       FIG. 2  is a cross sectional view of the sensor assembly of  FIG. 1 ; and, 
       FIG. 3  is a cross sectional view of the sensor assembly and barrier diaphragm of the pressure sensor of FIG.  1 . 
   

   DETAILED DESCRIPTION 
   As shown in  FIGS. 1 ,  2 , and  3 , a pressure sensor  10  includes a sensor assembly  12  having a housing body  14 . The housing body  14  has an opening  16  that receives a diaphragm stop  18 . When the diaphragm stop  18  is inserted through the opening  16  and fully engages the housing body  14  as shown in  FIG. 3 , a retainer ring  20  is snapped into a recess  22  formed in the housing body  14  so as to retain the diaphragm stop  18  in the housing body  14 . Alternatively, the diaphragm stop  18  may be retained in the housing body  14  by use of a weld instead of the retainer ring  20 . 
   One or more sintered plugs  23  may be used in combination with a hole through the housing body  14  in order to provide a path from the outside of the pressure sensor  10  to the back side of a sensor die  36  (discussed below). This path implements the pressure sensor  10  as a gauge pressure sensor. The one or more plugs  23  are not present when the pressure sensor  10  is to be used to measure absolute pressure. 
   A barrier diaphragm  24  covers the opening  16 . The barrier diaphragm  24  has an outer circumference  26  that engages an annular lip  28  of the housing body  14  at the opening  16 . The barrier diaphragm  24  may be attached to the housing body  14  by a weld around the annular lip  28 . A pressure adapter  30  may be arranged to thread over the annular lip  28  or be welded to housing body  14  so as to couple a pressure to be sensed to the housing body  14 . The barrier diaphragm  24  thus forms a seal for the housing body  14 . 
   A connector plug  32  fills and seals the end of the housing body  14  that is opposite the opening  16 . The connector plug  32 , for example, may be an elastomeric plug. The connector plug  32  has electrical conductors  34  therethrough. The electrical conductors  34  are coupled to the sensor die  36  that is supported by a die support  38  located within a recess  40  of the housing body  14 . The die support  38  supports the sensor die  36  so that the sensor die  36  is positioned within a cavity  42  between the barrier diaphragm  24  and the housing body  14 . The purpose of the connector plug  32  is to provide mechanical strain relief to the electrical conductors  34 . Although the connector plug  32  is shown as a plug, the connector plug  32  could instead be formed in place by, for example, filling the volume with epoxy and curing the epoxy. 
   A fill hole  48  extends through the housing body  14  and communicates with the cavity  42  through and/or around the diaphragm stop  18 . A fill plug  50  is inserted into a fill hole  48 . A fill fluid  52  is supplied through the fill plug  50  and fills the cavity  42 . The fill fluid  52  transmits pressure on the barrier diaphragm  24  to the sensor die  36 . The sensor die  36  converts this pressure to an electrical signal and communicates this electrical signal through one or more of the electrical conductors  34  to the exterior of the housing body  14 . The sensor die  36  is provided by Honeywell International under parts number 22025995-002. 
   The housing body  14 , for example, may be a stainless steel body. Similarly, the barrier diaphragm  24  may be stainless steel. The diaphragm stop  18  may be alloy steel, stainless steel, or ceramic, and the retainer ring  20  may be stainless steel or spring steel. 
   The fill fluid  52  is chosen so that it has a TCE more closely matching the TCE of the housing body  14 . Accordingly, the housing body  14  and the fill fluid  52  do not exhibit temperature induced dimensional changes relative to one another to such an extent that the barrier diaphragm  24  undergoes a plastic deformation. 
   The fill fluid  52  may be a low TCE fill fluid. For example, the fill fluid  52  may be a combination of a fluid, such as a silicon fluid, and a filler, such as particles having a low TCE. As a more particular example, the fluid of the fill fluid  52  may be silicon polydimethylsiloxane, and the filler may be ceramic oxide particles. As a still more particular example, the fluid may be DC200 supplied by Dow Corning, or DC210H, which is a high temperature fill fluid also supplied by Dow Corning, and the filler may be particles of silicon dioxide. 
   The filler is added to the fluid to lower the TCE of the fluid. The filler should be mixed with the fluid so that the filler is uniformly distributed throughout the fluid. The filler, for example, may be a fine monodispersed powder having a particle size of around 1 micron. The particles of the filler are preferably, although not necessarily, spherical. The fill fluid  52  may comprise 50% fluid and 50% filler by volume. 
   The linear thermal expansion coefficient for  316  stainless steel, which can be used for the stainless steel parts described above, may be on the order of 16.5×10 −6  m/m/deg C. or a volume thermal expansion coefficient of 50×10 −6  m 3 /m 3 /deg C. The volume thermal expansion coefficient of the fill fluid, DC200H, with no filler is about 950×10 −6  m 3 /m 3 /deg C. The volume thermal expansion coefficient of the fill fluid with filler is about 476×10 −6  m 3 /m 3 /deg C. 
   Accordingly, the fill fluid  52  has a lower temperature coefficient of expansion (TCE) because of the filler than it would have had without the filler. The fill fluid  52  may have a temperature coefficient of expansion (TCE) that is higher than the TCE of the housing body  14  (as well as relevant other parts, such as stainless steel parts, of the pressure sensor  10 ). Therefore, as the pressure sensor  10  is exposed to extreme temperatures, the fill fluid  52  does not expand so quickly that plastic deformation of the barrier diaphragm  24  results. 
   An alternative to the present invention is to build a sensor with a housing body and a barrier diaphragm having a diameter that is large enough to avoid plastic deformation of the barrier diaphragm as a result of extreme temperature excursions. However, such a pressure sensor will be much larger than the pressure sensor  24  described herein. Small size is an advantage in most pressure sensors. 
   Certain modifications of the present invention have been described above. Other modifications will occur to those practicing in the art of the present invention. For example, as described above, the housing body  14 , the barrier diaphragm  24 , the diaphragm stop  18 , and the retainer ring  20  may be stainless steel. However, these elements may be other materials or different combinations of other materials that can benefit from the present invention. 
   The fill fluid  52  may be either conducting or non-conducting depending on application. 
   Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.