Patent Publication Number: US-10768069-B2

Title: Pressure measuring device for protection of pressure sensor from thermomechanical stress

Description:
TECHNICAL FIELD 
     The present invention relates to a pressure measuring device having a carrier, a base connected to the carrier, and a pressure sensor mounted on the base whose base surface is larger than a base surface of the base. 
     BACKGROUND DISCUSSION 
     Pressure measuring devices are used for the measurement of pressures—especially, absolute pressures, relative pressures, and differential pressures—and are used in industrial measurement technology. 
     In pressure measurement technology, so-called semiconductor pressure sensors are frequently used. Semiconductor sensors are today regularly produced that are based upon silicon, e.g., using silicon-on-insulator (SOI) technology. They are fashioned, for example, as pressure sensor chips that typically have a membrane carrier and a measurement membrane arranged on the membrane carrier. 
     These pressure sensors are very sensitive and are therefore placed in a housing that as a rule is metallic, and are subjected to the pressure to be measured via a diaphragm seal that is connected upstream and that is filled with a liquid that transmits pressure. The housing and the pressure sensor are thus made of different materials that have very different coefficients of thermal expansion. Therefore, as a result of the mechanical connection between the housing and the pressure sensor required for the sensor assembly, mechanical stresses can occur that affect the transmission characteristic of the measurement membrane, thus impairing the achievable measurement precision and its reproducibility. This holds especially for a temperature-dependent hysteresis of the measurement results. 
     In order to reduce temperature-dependent stresses acting on the pressure sensor, German Patent, DE 10 2007 052 364 A1 describes the arrangement of the pressure sensor chip on a ceramic intermediate carrier that has a coefficient of thermal expansion that matches the coefficient of thermal expansion of the semiconductor material. The intermediate carrier is glued directly onto a metallic carrier of the housing via an adhesive bond realized with an elastic adhesive. Kovar is indicated as the material for the carrier. Kovar has a coefficient of thermal expansion of 6 ppm/K, and is therefore significantly closer to the coefficient of thermal expansion of silicon—used for pressure sensors—of 2.6 ppm/K than more standard housing materials, which are also significantly lower in cost, such as stainless steel, which has a coefficient of thermal expansion of 16 ppm/K. 
     In the described pressure measurement devices, the ceramic intermediate carrier has a base surface that is larger than the base surface of the pressure sensor arranged thereon. This has the consequence that, despite the comparatively well-matched coefficient of thermal expansion of the intermediate layer, remaining thermal mechanical stresses can have effects on the pressure sensor over the entire base surface. 
     As an alternative to this, complementary solution approaches are known from the prior art, in which a reduction in temperature-dependent stresses acting on the pressure sensor is achieved by arranging the pressure sensor on a base whose base surface is significantly smaller than the base surface of the pressure sensor mounted thereon. For this purpose, two different specific embodiments are known, which are described, for example, in German Patent, DE 34 36 440 A1. In one specific embodiment, the base is an integral component of the metallic carrier of the housing, and is made of the material of the housing. In the second specific embodiment, the base is fashioned as a separate component that is placed into a bore in the carrier by means of a glazing. The glazing offers the advantage that it brings about an electrical insulation of the pressure sensor against the housing. However, the material combinations of the base material and the carrier material, between which hermetically-sealed glazings can be produced, are limited. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to indicate a pressure measuring device of the type cited above in which the pressure sensor is protected from thermomechanical stresses. 
     For this purpose, the present invention comprises a pressure measuring device having
         a carrier,   a base connected to the carrier, and   a pressure sensor mounted on the base, whose base surface is larger than a base surface of the base, distinguished in that an end of the base oriented away from the pressure sensor is glued into a recess in the carrier by means of an adhesive bond.       

     A development is distinguished in that the pressure sensor is mounted by means of a joint—especially, an adhesive bond; especially, an adhesive bond realized with an epoxy resin-based adhesive, a thermoplastic adhesive, or a silicon adhesive; especially, a silicon rubber, on an end of the base protruding from the recess; especially, a protruding end having a length on the order of a few tenths of a millimeter. 
     A first variant is distinguished in that the adhesive bond between the carrier and the base extends over an external jacket surface of the end of the base running in the recess. 
     A development of the first variant is characterized in that
         the base has an overall length of greater than or equal to 3 mm—especially, an overall length of from 3 mm to 7.5 mm, and   the end of the base running in the recess has a length greater than or equal to 2.5 mm,   the end of the base protruding from the recess and bearing the pressure sensor has a length of especially a few tenths of a millimeter—especially, on the order of 0.5 mm.       

     A second variant is distinguished in that the adhesive bond between the carrier and the base extends over an end surface of the base oriented towards a floor surface of the recess. 
     A third variant is distinguished in that the adhesive bond between the carrier and the base extends over an external jacket surface of the end of the base running in the recess and over an end face of the base oriented towards a floor surface of the recess. 
     A development of the second or third variant is distinguished in that base  3  has an overall length of less than or equal to 3 mm—especially, an overall length on the order of 0.5 mm to 3 mm. 
     A development is distinguished in that, in the recess, there is provided a centering device for centering the base—especially, a centering device formed by a base surface of the recess that tapers towards the base surface of the base—tapering in the direction of a floor surface of the recess onto which the base is placed. 
     A further development is distinguished in that the base is made of metal—especially, of stainless steel, Invar, or Kovar—of silicon carbide, or of an insulator—especially, of ceramic—especially, of aluminum oxide, silicon nitride, or aluminum nitride—or of glass—especially, borosilicate glass. 
     A further development is distinguished in that the adhesive bond between the base and the carrier is an adhesive bond realized with an adhesive based upon epoxy resin, a thermoplastic adhesive, or a silicon adhesive—especially, a silicon rubber. 
     A further development is distinguished in that the adhesive bond between the base and the carrier is an adhesive selected as a function of an adhesive surface available for the bonding of the base and the carrier, a more elastic adhesive being used, the larger the available adhesive surface is. 
     A further development is characterized in that
         the pressure measuring device is a differential pressure measuring device or an absolute or relative pressure measuring device for measuring higher pressures—especially, pressures greater than or equal to 4 MPa (40 bar)—and   the base is made of a material—especially, of stainless steel, Invar, Kovar, or ceramic—especially, of aluminum oxide, silicon nitride, silicon carbide, or aluminum nitride—that has a modulus of elasticity greater than or equal to 200,000 MPa.       

     A further development is distinguished in that the base is made of a material that has a coefficient of thermal expansion that is matched to the coefficient of thermal expansion of the pressure sensor. 
     A further development is distinguished in that the base has a length in the range of 0.5 mm to 7.5 mm, an outer diameter in the range of 0.5 mm to 7 mm, and is bar-shaped or has an inner diameter in the range of 0.25 mm to 4 mm. 
     According to a preferred embodiment, the carrier is made of a metal—especially, of a stainless steel. 
     The pressure measuring device according to the invention has the advantage that, due to the base being set into a recess in the carrier via an adhesive bond that dissipates thermomechanical stresses, the pressure sensor is protected from temperature-dependent stresses acting thereon. Here, an adhesive bond surrounding the base externally has the advantage that this provides a large adhesive surface, so that, even with an adhesive that is very elastic and is therefore particularly well-suited for dissipating thermomechanical stresses, a mechanically solid and hermetically-sealed bond can be achieved. In connection with the base having a smaller base surface than the pressure sensor, in this way, a significant reduction is achieved in the temperature dependence of the measurement results. Moreover, there is a significantly greater flexibility with regard to the choice of material for the components used for the mounting of the pressure sensor, so that, even in connection with carriers made of a material that is more advantageous with regard to cost, such as a stainless steel, an improved measurement precision is achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and its advantages will now be explained in detail using the figures in the drawing, which show three examples of embodiments. The same elements are indicated by the same reference numbers in the figures. 
         FIG. 1  shows: a pressure measuring device having an externally glued-in base; 
         FIG. 2  shows: a pressure measuring device having a base glued in at its end face; 
         FIG. 3  shows: a pressure measuring device having a base glued in externally and at its end face; and 
         FIG. 4  shows: a pressure measuring device having a stainless steel carrier with a stainless steel base. 
     
    
    
     DETAILED DISCUSSION IN CONJUNCTION WITH THE DRAWINGS 
       FIG. 1  shows a pressure measuring device according to the present invention. It comprises a carrier  1 , a base  3  connected to carrier  1 , and a pressure sensor  5  mounted on base  3 . Pressure sensor  5  is a so-called semiconductor pressure sensor, e.g., a silicon-based pressure sensor chip, and has, for example, a membrane carrier  7  and a measurement membrane  9  arranged thereon under which a pressure chamber  11  is enclosed. 
     The pressure measuring device can be fashioned as a differential pressure measuring device, as a relative pressure measuring device, or as an absolute pressure measuring device. 
     For the detection of differential pressures, a first side of measurement membrane  9  is subjected to a first pressure p 1  and its second side is subjected to a second pressure p 2  via a pressure transmission line  13 —shown as a dashed line in  FIG. 1 —that runs through carrier  1 , base  3 , and membrane carrier  7  and that opens into pressure chamber  11 . In this specific embodiment, the pressure difference acting on measurement membrane  9  between first and second pressures p 1 , p 2  causes a deflection of measurement membrane  9  that is a function of differential pressure p that is to be measured. 
     For the detection of relative pressures, the first side of measurement membrane  9  is subjected to the pressure p that is to be measured, and the second side of measurement membrane  9  is supplied with, instead of second pressure p 2 , a reference pressure p ref , e.g., an ambient pressure, related to which the pressure p acting on the first side is to be measured. In this specific embodiment, the pressure difference acting on measurement membrane  9  between pressure p and reference pressure p ref  causes a deflection of measurement membrane  9  that is a function of the relative pressure to be measured. 
     For the detection of absolute pressures, pressure chamber  11  enclosed under measurement membrane  9  is evacuated, and is completely sealed from the external environment by measurement membrane  9  and membrane carrier  7 . In this case, pressure transmission line  13 , which runs through carrier  1 , base  3 , and membrane carrier  7 , leading to pressure chamber  11 , is omitted, and pressure p acting on the first side of measurement membrane  9  causes a deflection of measurement membrane  9  that is a function of the absolute pressure to be measured. 
     In all three cases, the resulting deflection of measurement membrane  9  is registered via, for example, sensor elements  15  arranged on or in measurement membrane  9 , e.g., piezoresistive elements connected together to form a resistance measuring bridge, and is converted into an electrical output signal that is then available for further processing and/or evaluation. 
     In differential pressure measuring devices and relative pressure measuring devices, base  3  is tubular in shape, and, in absolute pressure measuring devices, bar-shaped, and has a base surface that is smaller than the base surface of pressure sensor  5 . As a result of this geometry, a decoupling is already brought about of measurement membrane  9  and carrier  1  that protects pressure sensor  5  from thermomechanical stresses. 
     Semiconductor pressure sensors usually have a base surface that is as a rule square in shape and whose size is on the order of 1 mm 2  to 100 mm 2 , depending upon the measurement range and measurement sensitivity. In contrast, base  3  preferably has a circular or circular-annular base surface. Depending upon the size of the base surface of pressure sensor  5 , the outer diameter of base  3  is preferably in the range of 0.5 mm to 7 mm. In the case of a bar-shaped base  3 , this corresponds to a base surface on the order of approximately 0.2 mm 2  to 40 mm 2 . In the case of bases  3  having a circular-annular base surface, the joint surface via which base  3  and pressure sensor  5  are connected to one another is still smaller, wherein base  3  with a circular-annular base surface preferably has an inner diameter in the range of 0.25 mm to 4 mm. Base  3  preferably has an overall length in the range of 0.5 mm to 7.5 mm. According to the present invention, this decoupling is reinforced in that carrier  1  has a recess  17 , oriented towards pressure sensor  5 , into which an end of base  3  is placed by means of an adhesive bond  19 . To produce adhesive bond  19 , adhesives based upon epoxy resin, thermoplastic adhesives, or silicon adhesives, such as silicon rubber, are especially suitable. 
     Adhesives are significantly more elastic than are rigid connections such as glazings, bonded connections, soldered connections, or welded connections, and are thus able to compensate for stresses resulting from the different coefficients of thermal expansion of carrier  1  and base  3 . Adhesive bond  19  thus brings about a reduction in thermomechanical stresses that would otherwise act on or be transmitted to pressure sensor  5 —especially, its measurement membrane  9 . The reduction in thermomechanical stresses brought about by adhesive  19  is greater, the more elastic adhesive bond  19  is. 
     In order to achieve an adhesive surface that is as large as possible, adhesive bond  19  between base  3  and carrier  1  preferably extends over an external jacket surface of the end of base  3  running in recess  17 . Here, the available adhesive surface increases linearly as length L of the end of base  3  placed into recess  17  increases, which length in turn corresponds to the depth of recess  17 . 
     Adhesive bond  19 , extending over the entire external jacket surface of base  3  situated in recess  17 , offers the advantage of a comparatively large adhesive surface that makes it possible to achieve a comparatively high degree of tightness and strength, even given the use of a very elastic adhesive such as a silicon rubber. 
     Moreover, this specific embodiment offers the advantage that adhesive bond  19  is essentially exposed only to shearing loads distributed over the entire adhesive surface, which, in comparison with pressure-loaded or tension-loaded adhesive bonds, has the advantage of reduced deformation hysteresis that may occur under high pressure loading. 
     In the specific embodiment shown in  FIG. 1 , base  3  preferably has an overall length of greater than or equal to 3 mm—especially, an overall length of 3 mm to 7.5 mm—and the end of base  3  running in recess  17  preferably has a length L of greater than or equal to 2.5 mm, length L being determined as a function of the outer diameter of base  3 , the pressure measurement range in which the pressure measuring device is to be used, and the adhesive used. In order to achieve an adhesive surface that is as large as possible, base  3  is preferably placed into recess  17  in such a way that the end of base  3 , protruding from recess  17  and bearing pressure sensor  5 , has a significantly smaller length than does the end running in recess  17 . Here, it is sufficient if the end protruding from recess  17  has a minimum length of a few tenths of a millimeter—especially, on the order of 0.5 mm. 
     Preferably, in recess  17 , there is provided a centering device  21  for centering base  3  in recess  17 , by which a tipping of base  3  when being placed into recess  17 , and during the subsequent adhesive process, is prevented. This is advantageous especially in connection with bases  3  having a comparatively large overall length—especially, an overall length greater than or equal to 3 mm—and/or with adhesive bonds  19  using an adhesive that has low viscosity during the carrying out of the adhesive process, because, here, the danger of tipping base  3  is particularly great. Centering device  21  is preferably formed by recess  17  having a base surface that tapers in the direction of its floor surface, onto which base  3  is placed, towards the base surface of base  3 . This centering device  21  is preferably produced directly during the manufacture of recess  17 , and in this case forms an integral component of carrier  1 . Alternatively, a correspondingly-shaped insert can be placed into recess  17  of carrier  1 . 
     Alternatively, instead of external adhesive bond  19  shown in  FIG. 1 , an adhesive bond  23  can be provided that extends over an end face, oriented towards a floor surface of recess  17 , of the end of base  3  placed into recess  17 . An exemplary embodiment of this is shown in  FIG. 2 . 
     A further alternative is formed by adhesive bonds  25 , shown in  FIG. 3 , between base  3  and carrier  1 , extending over the external jacket surface of base  3  and the end face, oriented towards the floor surface of carrier  1 , of base  3 . 
     The specific embodiments shown in  FIGS. 2 and 3  are suitable especially for pressure measuring devices having shorter bases  3 —especially, bases  3  having an overall length on the order of 0.5 mm to 3 mm—and/or for pressure measuring devices in which—for example, for reasons of space—a depth of recess  17  cannot be provided that is adequately large for an exclusively external adhesive bond  19 . 
     Both specific embodiments differ from the exemplary embodiment shown in  FIG. 1  in the smaller depth of recess  17  and the region over which the respective adhesive bond  23  or  25  extends. In other respects, reference is made to the description of  FIG. 1 . In these specific embodiments as well—especially, in connection with bases  3  having a larger overall length—a centering device (not shown in  FIGS. 2 and 3 ) for centering base  3  can be provided. 
     In all described adhesive bonds  19 ,  23 ,  25 , the strength and tightness of the joint brought about by adhesive bond  19 ,  23 ,  25  is a function, on the one hand, of the choice of adhesive and, on the other hand, of the adhesive surface available for the bonding of base  3  and carrier  1 . Correspondingly, the adhesive used to produce adhesive bond  19  is preferably selected as a function of the available adhesive surface and the demands on strength, which are a function of the pressure measurement range in which the pressure measuring device is to be used, a more elastic adhesive being used, the larger the available adhesive surface is and the lower the demands on strength are. 
     Here, it is to be kept in mind that the strength of adhesives is, as a rule, a function of temperature and, in some circumstances, the type of load on the adhesive bonds  19 ,  23 ,  25 . While adhesive bond  19  surrounding base  3  externally is essentially exposed to shear forces, end-face adhesive bond  23  is essentially exposed to pressure stresses, and also to tensile stresses in the case of difference pressure measurements and relative pressure measurements. In both cases, however, loads on adhesive bonds  19 ,  23 ,  25  oriented from carrier  1  in the direction of pressure sensor  5  require a higher strength of the adhesive than do loads acting in the reverse direction, from pressure sensor  5  in the direction of carrier  1 , because the latter are absorbed not only by the adhesive, but also by carrier  1 . 
     Therefore, the modulus of elasticity of the adhesive that is used for the respective direction of load is a measure of the strength. Thus, for example as a function of the dimension of the components, the choice of material, and the size of the adhesive surface, in connection with a differential pressure measuring device according to the invention or absolute or relative pressure measuring devices according to the invention for measuring higher pressures—especially, pressures greater than or equal to 4 MPa (40 bar)—it can be advantageous to use an adhesive having a modulus of elasticity greater than or equal to 2,000 MPa—especially, a modulus of elasticity in the range of 2,000 MPa to 10,000 MPa. In contrast, for example as a function of the dimensions of the components, the choice of material, and the size of the adhesive surface, in connection with absolute or relative pressure measuring devices according to the present invention for measuring smaller pressures—especially, pressures less than or equal to 40 bar—depending upon the size of the adhesive surface, it can be advantageous to use an adhesive having a modulus of elasticity less than or equal to 2,000 MPa—especially, a modulus of elasticity in the range of 10 MPa to 1,000 MPa—and, in connection with absolute or relative pressure measuring devices for measuring smaller pressures—especially, pressures less than or equal to 4 MPa (40 bar)—depending upon the size of the adhesive surface, even adhesives having a modulus of elasticity less than or equal to 10 MPa can be advantageous. 
     All of the described adhesive bonds  19 ,  23 ,  25  offer the advantage that, through them, not only are thermomechanical stresses dissipated, but, moreover, a large spectrum of different materials of base  3  and carrier  1  can be bonded to one another. 
     Here, for carrier  1 , with regard to its pressure strength, preferably a material is used having a high modulus of elasticity, and, with regard to its comparatively large volume, preferably a low-cost material, such as stainless steel, is used. 
     The material of base  3  can then be selected from a large number of different materials with regard to the specific demands made on the pressure measuring device. These materials include metals—especially, stainless steel, Kovar, or Invar—ceramics—especially, aluminum oxide (Al 2 O 3 ), silicon carbide (SiC), silicon nitride (Si 3 N 4 ), or aluminum nitride (AlN)—and glasses—especially, borosilicate glass. 
     Here, for the measurement of differential pressures or high absolute or relative pressures, such as pressures greater than or equal to 4 MPa (40 bar), preferably bases  3  are used made of a material having a high modulus of elasticity—especially, a modulus of elasticity greater than or equal to 200,000 MPa. Due to the comparatively high modulus of elasticity, influences, which are a function of the pressure acting on base  3 , on the achievable measurement precision are largely prevented. For this purpose, especially suitable are bases  3  made of stainless steel, Kovar, Invar, or of ceramic, e.g., of aluminum oxide (Al 2 O 3 ), silicon carbide (SiC), silicon nitride (Si 3 N 4 ), or aluminum nitride (AlN). For measuring lower absolute or relative pressures, such as pressures in the range of up to 4 MPa (40 bar), materials having a lower modulus of elasticity, such as glass, can also be used. 
     Moreover, an electrical insulation of pressure sensor  5  from carrier  1  can be brought about via the choice of an insulator as material for base  3 . An electrical insulation can be brought about, for example, by a base  3  made of glass, e.g., of borosilicate glass, or ceramic, e.g., aluminum oxide (Al 2 O 3 ), silicon nitride (Si 3 N 4 ), or aluminum nitride (AlN), wherein ceramic is preferably used for the measurement of differential pressures or high absolute or relative pressures, such as pressures greater than or equal to 4 MPa (40 bar), because ceramic has a significantly higher modulus of elasticity than do glasses. 
     In pressure measuring devices whose pressure sensors already have a base plate to be connected to base  3  or a membrane carrier made of an insulator, e.g., glass or ceramic, such an insulation is not required. 
     The second end of base  3 , situated opposite the first end of base  3 , bears pressure sensor  5  and is connected thereto via a joint  27 . Joint  27  is preferably also an adhesive bond, having the advantages described above of a further reduction in thermomechanical stresses and great flexibility with regard to the materials that can be used as material for base  3 . This adhesive bond can also be realized by, for example, an adhesive based upon epoxy resin, a thermoplastic adhesive, or a silicon adhesive—especially, a silicon rubber. 
     Adhesive bonds  19 ,  23 , or  25  and  27 , at both ends of base  3 , offer the advantage that, through their interaction, a significant reduction in thermomechanical stresses is still brought about, even if both adhesive bonds  19 ,  23  or  25  and  27  are realized using an adhesive having a comparatively high modulus of elasticity. 
     Alternatively, pressure sensor  5  and base  3  can be connected using a different joining method, e.g., soldering. 
     For the further reduction in temperature-dependent stresses acting on pressure sensor  5 —especially, its measurement membrane  9 —base  3  is preferably made of a material that has a coefficient of thermal expansion that matches the coefficient of thermal expansion of pressure sensor  5 . Related to the coefficient of thermal expansion of silicon of 2.6 ppm/K, for this purpose, borosilicate glasses are especially suitable, which have a coefficient of thermal expansion on the order of 3.2 ppm/K. In pressure measuring devices for measuring differential pressures or higher absolute or relative pressures, however, preferably a material is selected having a higher modulus of elasticity, such as Kovar, having a coefficient of thermal expansion on the order of 5 ppm/K, Invar, having a coefficient of thermal expansion on the order of 0.55 ppm/K, or ceramic, e.g., aluminum oxide (Al 2 O 3 ), having a coefficient of thermal expansion on the order of 8.5 ppm/K, silicon carbide (SiC), having a coefficient of thermal expansion on the order of 3.5 ppm/K to 4.5 ppm/K, aluminum nitride (AlN), having a coefficient of thermal expansion on the order of 4 ppm/K to 5 ppm/K, or silicon nitride (Si 3 N 4 ), having a coefficient of thermal expansion on the order of 2.8 ppm/K to 2.9 ppm/K. 
     In order to illustrate the influence of the various materials of base  3  on the temperature-dependent measurement errors of a pressure measuring device according to the present invention, temperature-dependent hystereses of differential pressure measurements carried out with the pressure measuring device shown in  FIG. 1 , with unloaded measurement membrane  9 , were measured. In each case, the measurements took place by exposing both sides of measurement membrane  9  to the same pressure p=p 1 =p 2 , and the hystereses H were determined as the difference, related to the differential pressure measurement range of 50 kPa (500 mbar), between a differential pressure Δp (25° C.) measured at an ambient temperature of 25° C., and a differential pressure Δp (135° C.) measured at an ambient temperature of 135° C., according to: 
     
       
         
           
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                   50 
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                       500 
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     For all the measurements, bases  3  were used that were identical in design, glued into recess  17  with an epoxy resin adhesive having, at 25° C., a modulus of elasticity of 4500 MPa, the bases having an overall length of 5 mm, a length L running in recess  17  and glued in over the external jacket surface of 4.5 mm, and an outer diameter of 2 mm. For a base  3  made of stainless steel, the measurements showed a hysteresis H of 0.025%, and, for bases  3  made of aluminum oxide, Kovar, and borosilicate glass, in each case, hystereses H resulted of less than 0.005%. 
     In comparison to this, with the pressure measuring device shown in  FIG. 4 , with an identically-fashioned pressure sensor  5  arranged on a stainless steel base  29 , 0.5 mm in height, of a stainless steel carrier  31 , by a joint realized identically to joint  27 , under identical measurement conditions, a temperature-dependent hysteresis H of 0.125% was measured. 
       1  support 
       3  base 
       5  pressure sensor 
       7  membrane carrier 
       9  measurement membrane 
       11  pressure chamber 
       13  pressure transmission line 
       15  sensor elements 
       17  recess 
       19  adhesive bond 
       21  centering device 
       23  adhesive bond 
       25  adhesive bond 
       27  joint 
       29  stainless steel base 
       31  stainless steel carrier