Abstract:
A construction for mounting a pressure detector prevents the detector diaphragm from being strained by stress applied to the pressure detector as the detector is mounted in a fixture main body provided in a pipe line or the like, thereby keeping the output characteristics and temperature characteristics of the detector from greatly differing before and after the mounting. The pressure detector is constructed by combining and fastening together a diaphragm base having a diaphragm and a sensor base having a sensor element therein that is activated by displacement of the diaphragm base. The pressure detector, with a gasket placed thereunder, is disposed in a mounting hole of a fixture main body that is mounted in a pipe line. The pressure detector is air-tightly pressed and fastened by a presser member inserted from above in the mounting hole. The presser member is brought in contact with a block upper surface of the diaphragm base, and the gasket is also brought in contact with the block lower surface of the diaphragm base. A shallow groove is defined in the form of a ring on the lower surface of the block at a place inward of the portion contacting the metal gasket so that the shallow groove absorbs strain caused by the presser member.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation of PCT Application No. PCT/JP00/05235 filed Aug. 3, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to improvements in and relating to a mounting structure or construction for mounting a pressure detector utilizing mainly a sensor chip as a pressure sensitive element.  
         BACKGROUND OF THE INVENTION  
         [0003]    Diaphragm type pressure detectors utilizing a sensor chip as the pressure sensitive element or strain gauge have been widely used for the detection of fluid pressure in pipes.  
           [0004]    [0004]FIGS. 10 and 11 illustrate diaphragm type pressure detectors as disclosed in the inventors&#39; Japanese patent applications No. 10-82707 and 10-008841. The pressure detectors each comprise a sensor base  1  for supporting a pressure sensitive element in the form of a sensor chip  2 , a diaphragm  3 , a diaphragm base  4 , a pressure transfer medium (silicone oil)  5 , a seal ball  6 , output lead pins  7 , and a weld  8 . If a fluid pressure  10  is applied to sensor chip  2  through diaphragm  3  and pressure transfer medium  5 , voltage signals proportional to the pressure from a semiconductor pressure transducer forming the sensor chip  2  are produced at output lead pins  7 .  
           [0005]    [0005]FIGS. 12 and 13 show the pressure detectors of FIGS. 10 and 11, respectively, mounted to measure pressure in a pipeline or the like. FIG. 14 is an enlarged sectional view of a portion A of FIG. 13.  
           [0006]    In FIG. 12, a fixture main body  11  has a fluid channel  11   b  therein, the channel extending from one side of the main body to an opposite side thereof. The main body  11  is mounted between pipe line end sections  52  so that fluid may flow between the end sections via channel  11   b . A fluid passage  22  connects with channel  11   b  and permits pressure in the channel to be applied to diaphragm  3 .  
           [0007]    A presser member  12  rests on diaphragm base  4  and a bearing  14  rests on presser  12 . A threaded clamping element such as a clamping bolt  15  is inserted into a threaded opening in the fixture main body  11  and, as the clamp is tightened, a force acting through bearing  14  and presser  12  pushes diaphragm  4  downward against a metal gasket  17 .  
           [0008]    In FIG. 13, clamping element  16  presses down on presser member  13  which in turn presses down on the sensor base  1  so that diaphragm  4  is pressed against metal gasket  17 .  
           [0009]    In both FIGS. 12 and 13, the pressure applied by the presser member  12 ,  13  creates an air-tight seal, via the metal gasket  17 , between the diaphragm  4  and the fixture main body  11 . The metal gasket  17  is made of a material that has a high resistance to corrosion and wear, and does not generate dust.  
           [0010]    Diaphragm type pressure detectors constructed as shown in FIGS. 10 and 11 can minimize the so-called dead space when mounted on a pipe line or the like. This offers practical advantages in that the gas exchangeability is high and a desired passive state film without spots, and with a uniform thickness, can be formed with relative ease on the gas-contact surface of the diaphragm  3 .  
           [0011]    The metal gasket  17 , being made of a material having a high resistance to corrosion and not prone to generating dust, is almost free from O-ring corrosion-caused problems, unlike the mountings of diaphragm type pressure detectors using an O-ring. However, other problems exist, the most serious problem being fluctuations in measurements attributable to stress, strain or the like on the diaphragm  3 .  
           [0012]    Diaphragm  3  is very thin, on the order of 0.05 to 0.06 mm. The diaphragm thickness is reduced to raise the pressure detection sensitivity. In such a state as shown in FIG. 12, therefore, stress or strain on the diaphragm  3  inevitably results at the time of tightening clamping bolt  15  when the contact surface of metal gasket  17  is brought into contact with the block lower surface  4   f  (FIG. 10) of the diaphragm base  4 . This changes greatly the stress applied to the sensor chip  2  through silicone oil  5 .  
           [0013]    Experiments were conducted using a pressure detector with a diaphragm 0.05-0.06 mm thick, about 10 mm in inside diameter, and having a detection pressure range from several Torr to 7 kgf/cm 2  abs. The pressure P acting on the diaphragm  3  was high at some Ps=7 kgf/cm 2  abs, and the pressure detector was mounted on the fixture main body  11 . The output Vs (mv) and temperature characteristics ZTC (% FS/° C.) were not much different from those observed when the pressure detector was in a free state, that is, not mounted on the fixture main body  11 .  
           [0014]    However, in the case where the pressure P applied to the diaphragm  3  was low, for example, Po=0 kfg/cm 2  abs, the output Vo changed by more than 5.2 mv when the pressure detector was mounted. (The output was 16.66 mv before the mounting of the pressure detector and 21.86 mv after the mounting.) The temperature characteristics ZTC (% FS/° C.), too, greatly fluctuated from 0.162 to 0.719. That is, as far as the output is concerned, differences in measurements are too large. In respect of temperature characteristics, too, fluctuations are too large to compensate. Thus, this pressure detector presents problems when used in practice.  
           [0015]    On the other hand, if the peripheral portion of the diaphragm base  4  is formed as shown in FIG. 11, and if the pressure detector is tightened and clamped with the outer circumferential surface  4   d  of the block of the diaphragm base  4 , and the inner circumferential surface  17   d  of the gasket  17  not in contact with each other (FIG. 14), the magnitude of change ΔVo in output before and after the mounting under pressure Po=0 kgf/cm 2  abs can be reduced to less than ± about 3.5 mv. Likewise, the temperature characteristics ZTC (% FS/° C.) will come within a range between 0.052 and 0.259. If the pressure detector is mounted on a pipe line etc., the characteristics will be well applicable in practice through a specific correction procedure.  
           [0016]    In the mounting construction shown in FIGS. 13 and 14, the magnitude of the change ΔVo in output before and after mounting of the sensor will be small. This is because the gasket  17  is placed between the collar lower surface  4   c  of a collar  4   a  provided on the diaphragm base  4  and the outer circumferential surface  4   d  of the thick (about 2 mm) block  4   b  of the diaphragm base  4 , and further because the inner circumferential surface  17   d  of the metal gasket  17  and the outer circumferential surface  4   d  of the block  4   b  are not in contact with each other. Therefore, even if the presser member  13  applies downward pressure on the metal gasket  17  through the sensor base  1  and diaphragm base  4 , the upward and downward reaction forces of the metal gasket  17  are all received by the collar  4   a  of the diaphragm base  4 . In other words, almost no strain or stress, resulting from tightening, acts on the diaphragm  3  formed integrally in the block  4   b  of the diaphragm base  4 .  
           [0017]    However, it is desirable that the change ΔVo in output before and after the mounting of the pressure detector and the temperature characteristic ZTC (%FS/° C.), be as small as possible. With the prior art construction or mounting structure shown in FIG. 14, the trouble is that the magnitude of ΔVo is still too large.  
         SUMMARY OF THE INVENTION  
         [0018]    The main object of the present invention is to solve the above-mentioned problem encountered when the diaphragm type pressure detector having the constitution shown in FIGS.  10  to  14  is actually applied to the pipe line or the like. That is, an object of the invention is to solve the problem of lowered measurement precision, resulting from large changes in output and temperature characteristics caused by differences in stress or strain on the diaphragm, that arise when the pressure detector is mounted in the fixture main body. This object is achieved by improvements in the structure or construction for mounting the pressure detector on the pressure detector fixture main body so that (1) when the pressure detector is fixed in the fixture main body the output and temperature characteristics will be hardly different from those observed when the pressure detector is not mounted, and (2) the pressure detector can be applied to the pipe line etc. without increasing the dead space in the fluid passage.  
           [0019]    An object of the invention is to provide a structure or construction for mounting a pressure detector, the pressure detector comprising a diaphragm base provided with a diaphragm and a sensor base fixed to the diaphragm base and having a built-in sensor element that is activated with displacement of the diaphragm base, the pressure detector being inserted in a mounting hole of a fixture main body mounted in a pipe line with a gasket placed under the pressure detector, the pressure detector being pressed and fixed in an air-tight manner in the mounting hole by a presser member inserted in the mounting hole from above, wherein the presser member is brought in contact with a block upper surface  4   e  of the diaphragm base  4  and the gasket  17  is brought in contact with a block lower surface  4   f  of the diaphragm base, and a shallow groove  18   b  in the form of a ring is defined on the block lower surface  4   f  at a place inward of the portion contacting the metal gasket  17  so that the strain arising from pressing by the presser member  12  is absorbed by the shallow groove  18   b.    
           [0020]    Another object of the invention is to provide a structure or construction for mounting a pressure detector, the pressure detector comprising a diaphragm base provided with a diaphragm and a sensor base fixed to the diaphragm base and having a built-in sensor element that is activated with displacement of the diaphragm base, the pressure detector being inserted in a mounting hole of a fixture main body mounted in a pipe line with a gasket placed under the pressure detector, the pressure detector being pressed and fixed in an air-tight manner in the mounting hole by a presser member inserted in the mounting hole from above, wherein the presser member is brought in contact with a block upper surface  4   e  of the diaphragm base  4  and the gasket  17  is brought in contact with a block lower surface  4   f  of the diaphragm base, a shallow groove  18   a  in the form of a ring is defined in the block upper surface  4   e  at a place inward of a portion contacting the presser member  12 , and a shallow groove  18   b  in the form of a ring is defined on the block lower surface  4   f  at a place inward of the portion contacting the metal gasket  17  so that the strain arising from pressing by the presser member  12  is absorbed by the shallow groove  18   b    
           [0021]    A further object of the invention is to provide a structure or construction for mounting a pressure detector, the pressure detector comprising a diaphragm base provided with a diaphragm and a sensor base fixed to the diaphragm base and having a built-in sensor element that is activated with displacement of the diaphragm base, the pressure detector being inserted in a mounting hole of a fixture main body mounted in a pipe line with a gasket placed under the pressure detector, the pressure detector being pressed and fixed in an air-tight manner in the mounting hole by a presser member inserted in the mounting hole from above, wherein a first step portion  19  and a second step portion  20  are provided in a lower portion of the mounting hole  11   a  of the fixture main body  11 , with the area between a horizontal plane  20   b  of the second step portion  20  and the lower contacting surface  17   b  of the gasket  17  serving as a seal portion, a collar  1   a  being provided on the sensor base  1  of the pressure detector and a collar  4   a  provided in an upper portion of the diaphragm base  4 , the two collars being placed opposite to each other and fixed, in addition, a seal portion between the collar lower surface  4   c  of the collar  4   a  of the diaphragm base  4  and the upper contacting surface  17   a  of the metal gasket  17 , and a shallow groove  18   c  being defined at an inward portion of the collar upper surface  1   b  of the sensor base  1  and a shallow groove  18   d  being defined at an inward portion of the collar lower surface  4   c  of the diaphragm base  4 , both in a ring form, the metal gasket  17  having an almost rectangular section and an upper contact face  17   a  and a low contact surface  17   b  so strain arising from pressing the collar upper surface  1   b  of the sensor base  1  by the presser member  13  is absorbed by the shallow grooves  18   c ,  18   b.    
           [0022]    A further object of the invention is to provide a structure or construction for mounting a pressure detector, the pressure detector comprising a diaphragm base provided with a diaphragm and a sensor base fixed to the diaphragm base and having a built-in sensor element that is activated with displacement of the diaphragm base, the pressure detector being inserted in a mounting hole of a fixture main body mounted in a pipe line with a gasket placed under the pressure detector, the pressure detector being pressed and fixed in an air-tight manner in the mounting hole by a presser member inserted in the mounting hole from above, wherein a first step portion  19  and a second step portion  20  are provided in a lower portion of the mounting hole  11   a  of the fixture main body  11 , with the area between a horizontal plane  20   b  of the second step portion  20  and the lower contacting surface  17   b  of the gasket  17  serving as a seal portion, a collar  1   a  being provided on the sensor base  1  of the pressure detector, the collar  1   a  and an upper surface  4   e  of the diaphragm base  4  being positioned opposite of each other and fastened, the diaphragm base  4  protruding downward from a diaphragm base block lower surface  4   f  to form a seal surface  4   g  with an area between the seal surface  4   g  and the upper contact face  17   a  of the metal gasket  17  serving as a seal portion, a shallow groove  18   e  being defined at an inward portion of the collar upper surface  1   b  of the collar  1   a  of the sensor base  1 , a shallow groove  18   f  being defined at an inward portion of the block lower surface  4   f  of the diaphragm base  4 , and shallow grooves  18   g ,  18   h  defined opposite to each other at a position upward of the seal surface  4   g  protruding downward, each in the form of a ring, the metal gasket  17  having an almost rectangular section with an upper contact face  17   a  and a low contact surface  17   b  so strain arising from pressing on the collar upper surface  1   b  of the sensor base  1  by the presser member  13  is absorbed by the shallow grooves  18   e ,  18   f ,  18   g ,  18   h.    
           [0023]    Another object of the invention is to provide a pressure sensor mounting construction as described above wherein an outer circumferential portion  24  of the collar  1   a  of the sensor base  1  and an outer circumferential portion  25  of the block  4   b  are made of a material with a high hardness. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a schematic section view of a pressure detector according to a first embodiment of the present invention.  
         [0025]    [0025]FIG. 2 is a schematic section view of a pressure detector according to a second embodiment of the present invention.  
         [0026]    [0026]FIG. 3 is a partially enlarged sectional view showing the mounting construction of the pressure detector according to the second embodiment.  
         [0027]    [0027]FIG. 4 is a schematic section view of a pressure detector according to a third embodiment of the invention.  
         [0028]    [0028]FIG. 5 is a partially enlarged sectional view showing the mounting construction of the pressure detector according to the third embodiment.  
         [0029]    [0029]FIG. 6 is a schematic section view of a pressure detector according to a fourth embodiment of the invention.  
         [0030]    [0030]FIG. 7 is a partially enlarged sectional view showing the mounting construction of the pressure detector according to the fourth embodiment.  
         [0031]    [0031]FIG. 8 is a schematic section view of a pressure detector according to a fifth embodiment.  
         [0032]    [0032]FIG. 9 is a partially enlarged sectional view showing the mounting construction of the pressure detector according to the fifth embodiment  
         [0033]    [0033]FIG. 10 is a vertical, sectional view showing an example of construction of a prior art pressure detector.  
         [0034]    [0034]FIG. 11 is a vertical, sectional view showing an example of construction of another prior art pressure detector.  
         [0035]    [0035]FIG. 12 is a vertical, sectional view showing the construction of mounting the prior art pressure detector in FIG. 10.  
         [0036]    [0036]FIG. 13 is a vertical, sectional view showing the construction of mounting the prior art pressure detector in FIG. 11.  
         [0037]    [0037]FIG. 14 is an enlarged sectional view of the area indicated by A in FIG. 13. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    In the following description, the same reference numbers are used to refer to the same elements as in FIGS.  10  to  14 .  
       Embodiment 1  
       [0039]    In FIG. 1, a pressure detector according to the first embodiment of the present invention comprises a sensor base  1 , a sensor chip  2 , a diaphragm  3 , a diaphragm base  4 , a pressure transfer medium  5 , the sensor lead pins  7  and a weld  8 .  
         [0040]    The sensor base  1  is made of stainless steel in the form of a thick disk, with a chip housing  1   c  formed in the center of the underside and an oil inlet hole  1   d  and lead pin through holes (not shown) provided therein.  
         [0041]    The sensor chip or pressure sensitive element is a known diffusion-type semiconductor pressure transducer. That is, the sensor chip  2  has a diaphragm construction that deforms when pressure is received. The chip includes four resistors formed by the same manufacturing method as integrated circuits, the resistors being connected in the form of a bridge. The resistance across the bridge varies as the resistors are subjected pressure changes so that a voltage output signal proportional to the degree of applied pressure is produced at the output terminals of the bridge.  
         [0042]    The diaphragm  3  and diaphragm base  4  are formed integrally in one piece of stainless steel about 50 μm in thickness and about 10 mm in inside diameter. The thickness of the diaphragm  3  can be changed depending on the detection pressure range of the detector. For a pressure detector for measurement of absolute pressure values from several torr to 7 kgf/cm 2  according to the present embodiment, the diaphragm may be 10 mm in diameter and preferably about 50 μm thick.  
         [0043]    The diaphragm  3  and the diaphragm base  4  may be formed separately and welded together.  
         [0044]    It is also noted that the diaphragm  3  has a so-called passive state film formed on its gas-contact surface by a known technique. Formed on the outside surface layer of the gas contact surface is a passive state film about 200 Å thick of about 100% of chromium oxide or a fluoride passive state film some 1,000 to 3,000 Å thick or an oxide passive state film some 200 Å thick of a mixture mainly of aluminum oxide and chromium oxide.  
         [0045]    The pressure transfer medium  5  transfers to the sensor chip  2  the pressure  10  applied to the diaphragm  3 . Silicone oil is used as the pressure transfer medium because it has a low temperature expansion coefficient and compressibility coefficient and is chemically stable.  
         [0046]    A seal ball  6 , made of ball-bearing steel, is used to seal an oil inlet hole  1   d  through which the silicone oil  5  is entered into the pressure detector.  
         [0047]    The construction of the diaphragm type pressure detector itself is known and will not be described in detail.  
         [0048]    In the diaphragm type pressure detector according to the first embodiment, a shallow groove  18   b  to release strain is formed in the shape of a ring on the lower surface  4   f  of the peripheral flange or block  4   b  of the diaphragm base  4  as shown in FIG. 1. Shallow groove  18   b  is provided in the form of a ring at a place radially inward of the portion contacting the metal gasket  17 . See FIG. 3. The groove is V-shaped (or reverse U-shaped) or U-shaped (reverse V-shaped). The depth of the groove may be about 0.3 to 0.5 mm for a diaphragm base  4  having a thickness of 1.5 to 2.5 mm.  
         [0049]    The shape and effects of the shallow groove  18   b  are the same as in the second embodiment described below.  
       Embodiment 2  
       [0050]    [0050]FIG. 2 is a schematic section view of a pressure detector according to the second embodiment of the present invention, and FIG. 3 is a partially enlarged sectional view showing the mounting construction of the pressure detector according to the second embodiment. The pressure detector according to the second embodiment is essentially identical with the pressure detector according to the first embodiment except that another shallow groove  18   a  is provided. In other words, the pressure detector according to the second embodiment has so-called strain-releasing shallow grooves  18   a  and  18   b  formed in the shape of rings, the former being in the upper surface  4   e  of the block  4   b  of the diaphragm base  4  and the latter on the lower surface  4   f . The embodiment shown in FIG. 3 thus differs from the pressure detector according to the first embodiment only in that it includes the shallow groove  18   a.    
         [0051]    [0051]FIG. 3 shows the pressure detector of FIG. 2 mounted in a fixture main body  11 , the main body having a fluid flow channel  11   b . It will be understood that fixture main body  11  is mounted in or on a pipe line or machine part  52  so that pressure of a fluid in the pipe or machine part is communicated through channel  52  and a fluid passage  22  to the diaphragm  3 .  
         [0052]    In FIG. 3, as the clamp  15  is tightened into fixture main body  11 , upward and downward compressive forces (upward and downward reaction forces) are applied to the block  4   b  outside the grooves  18   a  and  18   b  via the presser member  12  and the metal gasket  17 . If, at the time of tightening the clamp  15 , a strain force is applied to the diaphragm  3  by upward and downward compressive forces acting on the block  4   b  (if, for example, component forces of the upward and downward compressive forces arise and act on the diaphragm  3 ), deformation by strain force is absorbed near the thin part P between the shallow grooves  18   a ,  18   b  (second embodiment) or  18   b  (first embodiment) provided opposite to each other on the block upper surface  4   e  and block lower surface  4   f , respectively, of the diaphragm base  4 . As a result, the strain force is kept from reaching the diaphragm  3  directly. Thus mounting of the pressure detector in the fixture main body  11  causes no strain on diaphragm  3 .  
       Embodiment 3  
       [0053]    [0053]FIG. 4 is a schematic section view of a pressure detector according to a third embodiment of the invention, and FIG. 5 is a partially enlarged sectional view showing the mounting construction of the pressure detector.  
         [0054]    In the third embodiment, a collar  1   a  and a collar  4   a  formed in the sensor base  1  and the diaphragm base  4 , respectively. The two collars  1   a ,  4   a  are positioned opposite each other and united by a weld  8  on their outer circumferential portion.  
         [0055]    The diaphragm base  4  is formed of a ring-shaped block  4   b  and collar or flange  4   a . The lower surface  4   c  of the collar  4   a  is a seal surface that comes in contact with the upper contact face  17   a  of the gasket  17  as shown in FIG. 5. Therefore, the lower surface  4   c  of the collar  4   a  is finished to a high-precision smooth surface.  
         [0056]    In the third embodiment the diaphragm base  4  may have a diameter of 13 mm with the diameter of the diaphragm pressure-receiving surface being 11 mm and the thickness of the diaphragm 3 about 0.06 mm. The passive state film is about 200 Å of chromium oxide, the total thickness of the diaphragm base  4  is 4 mm and lead pins  7  are metal pieces of which one is an earth electrode. To an input circuit (not shown), a 1.5 mA d.c. current is applied. As the pressure applied on the sensor element of sensor chip  2  changes, four resistors formed of sensor chips change in resistance, so that an output voltage V is developed between the output terminals.  
         [0057]    In the pressure detector according to the third embodiment, a shallow groove  18   c  is formed on the collar upper surface  1   b  of the sensor base  1  at an inward portion thereof, and a shallow groove  18   d  is formed on the collar lower surface  4   c  of the diaphragm base  4  at an inward portion thereof. The shallow groove  18   c  is dish-shaped in section, while the shallow groove  18   d  is reverse U-shaped (or reverse V-shaped).  
         [0058]    In the third embodiment, a metal gasket  17  and the pressure detector are inserted in a cylindrical mounting hole  11   a  formed in the upper center of fixture main body  11  as shown in FIG. 5. Fixture main body  11  is made of stainless steel. With the collar upper surface  1   b  of the sensor base  1  pressed by a clamp (not shown) via the presser member  13 , the pressure detector is mounted airtight via the metal gasket  17 . The mounting hole  11   a  is reduced in size at its bottom portion to define a first step portion  19  and a second step portion  20 . The circumferential wall surface  19   a  of the first step portion  19  is a guide surface for the presser member  13 . The circumferential wall surface  20   a  of the second step portion  20  is in contact with the outer circumferential surface  17   c  of the gasket  17 , and the horizontal surface  20   b  is in contact with the lower face  17   b  of the gasket  17 . The circumferential wall surface  20   a  and the horizontal plane  20   b  define the fitting portion receiving the gasket  17 .  
         [0059]    The inside portion of the horizontal plane  20   b  of the second step portion  20  is formed in a taper  21 . In the center of the bottom of the mounting hole  11   a , a fluid passage  22  is provided.  
         [0060]    The gasket  17  is ring-shaped and the section of the gasket is rectangular with the width being longer than the height and with the four corners of the rectangle chamfered.  
         [0061]    The inner circumferential surface  17   d  of the gasket  17  and the outer circumferential surface  4   d  of the block  4   b  of the diaphragm base  4  are not in contact with each other. The upper contact face  17   a  of the gasket  17  is in contact with the lower surface  4   c  of the collar  4   a  of the diaphragm base  4 . Furthermore, the outer circumferential surface  17   c  of the gasket  17  is in contact with the circumferential wall surface  20   a  of the second step portion  20 . That is, the collar lower surface  4   c  of the diaphragm base  4 , the circumferential wall surface  20   a  of the second step portion  20  and the horizontal plane  20   b  form a fitting portion receiving the gasket  17 . The distance between the circumferential wall surface  20   a  and the outer circumferential surface  4   d  of the collar main body is set at about the same as or a little longer than the width of the gasket  17 .  
         [0062]    In FIG. 5, the gasket  17  is 14.7 mm in outside diameter, 13.0 mm in inside diameter, 1.5 mm in width of the seat, 0.9 mm in thickness of the seat (height), and 0.8 mm in width of the contact faces  17   a ,  17   b  of the seat. The gasket is made of stainless steel under the JIS designation SUS 316L-P (w melt).  
         [0063]    The effects of the aforesaid shallow grooves  18   c ,  18   d  are the same as in the second embodiment shown in FIG. 3. That is, the strain force caused by upward and downward compressive forces (reaction forces) applied through the presser member  13  is absorbed by the grooves  18   c ,  18   d  and the thin thickness part P between the grooves, whereby the strain force is kept from directly reaching the diaphragm  3 .  
       Embodiment 4  
       [0064]    [0064]FIG. 6 is a schematic section view of a pressure detector according to the fourth embodiment of the present invention, and FIG. 7 is a partially enlarged sectional view showing the mounting construction of the pressure detector.  
         [0065]    In the fourth embodiment, the diaphragm  3  and the diaphragm base  4  are formed separately and united to each other by providing a weld  23 .  
         [0066]    A seal surface  4   g  is formed on the lower surface side of the diaphragm base  4  in such a way that the seal surface protrudes from the diaphragm  3  to a downward position. That is, the seal surface  4   g  extends below the lower surface  4   f  of the diaphragm base  4 . Shallow grooves  18   g ,  18   h  with a U-shaped (or V-shaped) section are formed symmetrically above the surface  4   g  at a position almost as high as the diaphragm.  
         [0067]    Shallow grooves  18   e ,  18   f  are formed at an inward position of the collar upper surface  1   b  of the sensor base  1  and in the central position of the block lower surface  4   f  of the diaphragm base  4 , respectively.  
         [0068]    The grooves  18   e ,  18   f  absorb the strain force caused by the upward and downward pressure forces applied to the diaphragm base  4  via the presser member  13 , thereby reducing the effects of the strain force that directly acts on the diaphragm  3  when a clamp  16  (see FIG. 13) is tightened up.  
       Embodiment 5  
       [0069]    [0069]FIG. 8 is a schematic section view of a pressure detector according to the fifth embodiment and FIG. 9 is a partially enlarged sectional view showing the mounting construction for the pressure detector.  
         [0070]    The structure and construction for mounting the pressure detector according to the fifth embodiment are basically identical with those of the third embodiment shown in FIGS. 6 and 7. However, in the fifth embodiment the outer circumferential portion  24  of the collar  1   a  of the sensor base  1  and the outer circumferential portion  25  of the diaphragm base  4 , that is, the outer circumferential portions outward of the dotted lines in FIGS. 8 and 9, are hardened.  
         [0071]    The outer circumferential portions of the sensor base  1  and the diaphragm base  4  are hardened to reduce the strain force caused in the radial direction by compressive forces applied upward and downward when the presser member  13  is tightened up. The shallow grooves  18   e  to  18   f  absorb the strain force, thereby further keeping down the strain on the diaphragm  3 .  
         [0072]    Needless to say, the formation of hardened portions  24 ,  25  is applicable to the pressure detector shown in FIGS. 4 and 5.  
       Test Results  
       [0073]    Experiments were conducted with the pressure detectors and mounting constructions described above. In the first and second embodiments, the magnitude of change ΔVo in output before and after the mounting under a pressure Po=0 kgf/cm 2 . abs was less than about ±2.0 mv, a decrease of some 30 to 35% from that of the prior art.  
         [0074]    In the third embodiment, the magnitude of change ΔVo in output was less than about ±1.0 mv, a decrease of 60 to 70% from that of the prior art.  
         [0075]    Similarly, in the fourth embodiment, the magnitude of change ΔVo in output was less than about ±1.0 mv.  
         [0076]    With respect to the fifth embodiment, it was confirmed that the magnitude of change ΔVo output is further reduced from that of the fourth embodiment.  
       Effects of the Invention  
       [0077]    In the first and second embodiments described above, the shallow groove  18   b  is formed at the inside portion of the lower surface of the block of the diaphragm base  4  (or shallow grooves  18   a ,  18   b  are formed symmetrically at the inside portions of the upper surface and the lower surface of the block of the diaphragm base  4 ) so that the strain force arising when the pressure detector is tightened and clamped by the presser member  12  may be absorbed by the shallow groove  18   b  (or shallow grooves  18   a ,  18   b ). Therefore, the stress or strain acting on the diaphragm  3  is further reduced, thereby substantially decreasing the magnitude of change in measurements before and after the tightening.  
         [0078]    In the third embodiment, a plurality of step portions are defined in the lower portion of the mounting hole  11   a  of the mounting fixture main body of the pressure detector, and the diaphragm base of the pressure detector is formed of a collar and a thick block. The gasket  17  with an almost rectangular section is placed in a fitting portion formed with the circumferential wall surface and the flat surface of the second step portion and the collar lower surface of the diaphragm base. As a result, even if the sensor presser is inserted in the detector mounting hole, with the upper part of the collar of the sensor base pressed downward, the reaction force applied on the diaphragm base  4  through the presser member  13  is absorbed by the collar  4   a  of the diaphragm base  4  and the thick block  4   b , so that little strain is caused by the reaction force on the diaphragm  3  formed integrally with the block  4   b.    
         [0079]    Such an advantage, together with the fact that the shallow grooves  18   c - 18   d  absorb the stress or strain as seen in the second embodiment, minimizes the fluctuations in output before and after the mounting of the pressure detector on the fixture main body and temperature characteristics even in the area where the fluid pressure is low, thus solving problems in practical use. This makes it possible to apply this kind of diaphragm type pressure detector to a pipe line and the like.  
         [0080]    The fourth embodiment is so constituted that shallow grooves  18   g ,  18   h  are formed at places higher than the seal surface  4   g  of the diaphragm base  4 . To the effects of the second embodiment, this adds the effects of absorbing strain or stress by the shallow grooves  18   g ,  18   h , thereby further reducing fluctuations in output before and after mounting.  
         [0081]    In the fifth embodiment, the portion on which the tightening force of the presser member  13  is applied is made of a hardened material, which reduces the material deformation by stress in tightening. This greatly reduces the strain on the diaphragm, which further keeps down the fluctuations in output.