Patent Publication Number: US-2023145490-A1

Title: Ultrasonic transceiver, ultrasonic flowmeter, ultrasonic flow velocimeter, ultrasonic densitometer, and manufacturing method

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an ultrasonic transceiver and measuring instruments configured to measure the flow rate, flow velocity, and concentration of a fluid, respectively, by using the ultrasonic transceiver. 
     2. Description of the Related Art 
     Patent Literature 1 discloses ultrasonic transceiver  51  including an acoustic matching body having high sensitivity to transmit and receive ultrasonic waves, high mechanical strength, and high heat-resistance.  FIG.  15 A  is a plan view of conventional ultrasonic transceiver  51 .  FIG.  15 B  is a cross-sectional view of ultrasonic transceiver  51  taken along line VA-VA illustrated in  FIG.  15 A . As illustrated in  FIGS.  15 A and  15 B , ultrasonic transceiver  51  includes a sound matching layer. The sound matching layer includes a plate-like base member having a predetermined thickness, dense portion  52 , and recessed portion  53 . The base member includes: joint face  55  formed on one side of the base member and joined to ultrasonic wave source  54 ; and oscillating face  56  formed on the other side of the base member and configured to emit an ultrasonic wave. Dense portion  52  and recessed portion  53  are partly provided in at least oscillating face  56  toward joint face  55 . 
       FIG.  16    is a diagram illustrating a conventional ultrasonic transceiver. As illustrated in  FIG.  16   , Patent Literature 2 discloses an ultrasonic transceiver in which edge portion  62  of one main face  61  of sound matching layer  60  is fixed to the upper end face of tubular case  63 , the other main face  64  of sound matching layer  60  is covered with first water-proof member  65 , side face  66  of sound matching layer  60  is covered with second water-proof member  67 , second water-proof member  67  is joined to first water-proof member  65  without a gap in the vicinity of edge portion  68  of the other main face  64  of sound matching layer  60  and is also joined to case  63  without a gap in side face  69  of case  63 . 
       FIG.  17    is a diagram illustrating a conventional ultrasonic transceiver. As illustrated in  FIG.  17   , Patent Literature 3 discloses an ultrasonic transceiver including: a matching member including dense layer  72  laminated on a face of porous body  70  and formed of a thermosetting resin and flow-preventing particles; and side wall member  75  adhering to ultrasonic radiation face  73  and the outer wall face of porous body  70 , wherein porous body  74  is sealed by dense layer  72  and side wall member  75 , and the radial thickness of side wall member  75  is approximately uniform in the direction of ultrasonic radiation. 
     Citation List 
     
         
         Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2019-12921 
         Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2002-135894 
         Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2010-268262 
       
    
     SUMMARY 
     The present disclosure provides an ultrasonic transceiver capable of stably measuring a measurement target fluid with high accuracy for a long period even when the measurement target fluid is a fluid of high temperature and high humidity, and provides an ultrasonic flowmeter, an ultrasonic flow velocimeter, and an ultrasonic densitometer, each including the ultrasonic transceiver. 
     The ultrasonic transceiver according to the present disclosure is an ultrasonic transceiver including a piezoelectric body and an acoustic matching body disposed in one face of the piezoelectric body. The acoustic matching body includes: a top plate, a bottom plate, and a side wall that define a closed space; and a perpendicular partition wall adhering to the top plate and the bottom plate and formed substantially perpendicularly to the bottom plate so as to divide the closed space. 
     The ultrasonic transceiver according to the present disclosure includes a piezoelectric body and an acoustic matching body disposed in one face of the piezoelectric body. The acoustic matching body includes a top plate, a bottom plate, and a side wall that define a closed space, and is formed so that the closed space is divided. Accordingly, even when corrosion deterioration occurs in the outer circumference of the acoustic matching body and a fluid of high humidity enters the acoustic matching body from a gap formed due to the corrosion deterioration, the spread of moisture entry in the whole of the acoustic matching body can be substantially prevented because of a plurality of the partitions. Thus, it is less prone to cause an apparent change in the density of the acoustic matching body due to the moisture entry, and therefore, a decrease in the measurement performance of a measuring instrument including the ultrasonic transceiver can be substantially prevented. Thus, an ultrasonic flowmeter including the ultrasonic transceiver is capable of stably measuring the flow rate of a fluid of high temperature and high humidity with high accuracy for a long period. Furthermore, an ultrasonic flow velocimeter including the ultrasonic transceiver is capable of stably measuring the velocity of a fluid of high temperature and high humidity with high accuracy for a long period. Furthermore, an ultrasonic densitometer including the ultrasonic transceiver is capable of stably measuring the concentration of a fluid of high temperature and high humidity with high accuracy for a long period. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-sectional view of a configuration example of an ultrasonic transceiver according to a first embodiment. 
         FIG.  2    is a cross-sectional view of a configuration example of an acoustic matching body according to the first embodiment. 
         FIG.  3    is a diagram illustrating a procedure for manufacturing the acoustic matching body according to the first embodiment by using perspective views. 
         FIG.  4    is a schematic block diagram illustrating a configuration example of an ultrasonic flowmeter according to a second embodiment. 
         FIG.  5    is a schematic block diagram illustrating a configuration example of an ultrasonic densitometer according to a third embodiment. 
         FIG.  6    is a cross-sectional view of a configuration example of an acoustic matching body according to a fourth embodiment. 
         FIG.  7    is a diagram illustrating a procedure for manufacturing the acoustic matching body according to the fourth embodiment by using perspective views. 
         FIG.  8    is a cross-sectional view of a configuration example of an acoustic matching body according to a fifth embodiment. 
         FIG.  9    is a cross-sectional view of another configuration example of the acoustic matching body according to the fifth embodiment. 
         FIG.  10    is a cross-sectional view of a configuration example of an ultrasonic transceiver according to a sixth embodiment. 
         FIG.  11    is a diagram illustrating a procedure for manufacturing the ultrasonic transceiver according to the sixth embodiment by using cross-sectional views. 
         FIG.  12    is a cross-sectional view and a plan view of a configuration example of the ultrasonic transceiver according to the sixth embodiment. 
         FIG.  13 A  is a cross-sectional view of a configuration example of an ultrasonic transceiver according to a seventh embodiment. 
         FIG.  13 B  is a cross-sectional view of another configuration example of the ultrasonic transceiver according to the seventh embodiment. 
         FIG.  13 C  is a diagram illustrating a procedure for manufacturing the ultrasonic transceiver of the another configuration example according to the seventh embodiment by using cross-sectional views. 
         FIG.  14 A  is a cross-sectional view of a configuration example of an acoustic matching body according to an eighth embodiment. 
         FIG.  14 B  is a cross-sectional view of another configuration example of the acoustic matching body according to the eighth embodiment. 
         FIG.  15 A  is a plan view of a conventional ultrasonic transceiver. 
         FIG.  15 B  is a cross-sectional view of the ultrasonic transceiver taken along line VA-VA illustrated in  FIG.  15 A . 
         FIG.  16    is a diagram illustrating a conventional ultrasonic transceiver. 
         FIG.  17    is a diagram illustrating a conventional ultrasonic transceiver. 
     
    
    
     DETAILED DESCRIPTIONS 
     Underlying Knowledge Forming Basis of the Present Disclosure 
     At the time when the inventors came up with the present disclosure, efficient propagation of ultrasonic waves through a measurement target fluid was needed in order to measure the flow velocity, flow rate, or concentration of a combustible gas or a dry gas, such as air, as the measurement target fluid. For that purpose, it was necessary to control the physical properties of an acoustic matching body interposed between the measurement target fluid and a piezoelectric body. 
     A physical interpretation on the above-mentioned acoustic matching body will be described below. 
     The product of density and acoustic velocity, that is, the definition of an acoustic impedance in a certain substance, indicates the momentum of a substance constituting a microscopic unit element of the certain substance. In other words, letting the momentum of the substance constituting the microscopic unit element be ΔP, letting the mass of the substance be ΔM, and letting the speed of the substance be V, the following formula (1) is derived based on the theorem of momentum.  
     
       
         
           
             Δ 
             P 
             
               
                 momentum 
               
             
             = 
               
             Δ 
             M 
             × 
             V 
             
               
                 acoustic 
                   
                 impedance 
               
             
           
         
       
     
      The formula (1) indicates that an acoustic impedance is the momentum of a substance constituting a microscopic unit element. 
     Hence, in order to efficiently perform energy propagation of ultrasonic waves from a certain substance serving as an ultrasonic wave source into another substance adjacent to the certain substance, the acoustic impedances of these two substances are desirably close to each other. 
     Based on the above, a phenomenon occurring in the above-mentioned acoustic matching body will be described. 
     Generally, the acoustic velocity of a substance is expressed by the following formula (2).  
     
       
         
           
             V 
             = 
             
               
                 
                   κ 
                   / 
                   ρ 
                 
               
             
             
               1 
               / 
               2 
             
           
         
       
     
      In the formula (2), κ represents a bulk modulus, and ρ represents a density. In other words, the acoustic velocity of the substance is uniquely determined by a bulk modulus and a density. This indicates that intentionally controlling the acoustic velocity is difficult. 
     Therefore, in order to reduce the acoustic impedance of a certain substance, the density of the substance is effectively reduced. 
     An acoustic matching body according to the present disclosure includes a top plate, a bottom plate, and a side wall that define a closed space, and includes perpendicular partition walls formed substantially perpendicularly to the top plate and the bottom plate inside the closed space. The perpendicular partition walls are formed to adhere to the top plate and the bottom plate, thereby dividing the closed space. Thus, a method of reducing an apparent density is applied to the acoustic matching body of the present disclosure. 
     In the case of a prior art, when a gas of high temperature and high humidity is measured as a measurement target fluid, moisture mixes in a through portion (for example, recessed portion  53  of  FIGS.  15 A and  15 B ), and as a result, the density of an acoustic matching body sometimes becomes large seemingly. In this case, the acoustic impedance of the acoustic matching body becomes larger and thereby the propagation efficiency of ultrasonic waves from the acoustic matching body into the measurement target fluid falls, and as a result, there is a possibility that the performance of a measuring instrument including the acoustic matching body, for example, the flow rate measurement performance of an ultrasonic flowmeter decreases, or measurement becomes impossible. 
     The inventors found the above-mentioned problem in the prior art, and to solve the problem, the inventors accomplished configurations for the subject matters of the present disclosure. 
     The present disclosure provides an ultrasonic flowmeter, an ultrasonic flow velocimeter, and an ultrasonic densitometer, each being capable of stably measuring a measurement target fluid with high accuracy for a long period even when the measurement target fluid is a fluid of high temperature and high humidity. 
     Hereinafter, embodiments will be described in detail with reference to the drawings. Descriptions that are more detailed than necessary may, however, be omitted. For example, detailed descriptions on already well-known matters and overlapping descriptions on substantially identical configurations may be omitted. This is intended to avoid redundancy in the descriptions below and to aid those skilled in the art in the understanding of the descriptions. 
     The accompanying drawings and the following descriptions are provided to help those skilled in the art fully understand the present disclosure and are not intended to limit the subject matters recited in the claims. 
     In the following embodiments, as a manner of convenience, three axes, namely, the X-axis, the Y-axis, and the Z-axis are provided in the drawings illustrating the shapes of constituents of the present disclosure, and descriptions are given using the X-axis, the Y-axis, and the Z-axis, as needed. Furthermore, in the following embodiments, as a manner of convenience, when an ultrasonic transceiver is disposed in the orientation illustrated in  FIG.  1   , a direction from the left toward the right in  FIG.  1    is taken as an X-axis positive direction, a direction from the bottom toward the top in  FIG.  1    is taken as a Z-axis positive direction, and a direction from the front side to the back side in  FIG.  1    is taken as a Y-axis positive direction. Furthermore, a size of a constituent in a direction parallel to the Z-axis is sometimes called “thickness”, the Z-axis positive direction is sometimes called “upper” or “upward”, and the Z-axis negative direction is sometimes called “lower” or “downward”. Note that a description using the term, “the X-axis”, “the Y-axis”, “the Z-axis”, “upper”, or “lower” is merely used for convenience to facilitate the understanding of the present disclosure, and the terms “upper” and “lower” are relative terms that change with the orientation of installation of the ultrasonic transceiver according to the present disclosure. Therefore, the present disclosure is not limited by descriptions using the above-mentioned terms in the following embodiments. 
     First Embodiment 
     Hereinafter, an ultrasonic transceiver according to a first embodiment will be described using  FIG.  1    to  FIG.  3   . 
     1-1. Configuration 
       FIG.  1    is a schematic cross-sectional view of a configuration example of ultrasonic transceiver  1  in the first embodiment.  FIG.  1    is a cross-sectional view (a cross-sectional view in the X-Z plane) taken along the thickness direction (parallel to the Z-axis) of ultrasonic transceiver  1 . 
     As illustrated in  FIG.  1   , ultrasonic transceiver  1  includes acoustic matching body  2 , piezoelectric body  3 , lead wire  6  connected to electrode  4  of piezoelectric body  3 , and lead wire  7  connected to electrode  5  of piezoelectric body  3 . Electrode  4  of piezoelectric body  3  and acoustic matching body  2  are joined using a joining material. For example, using a common adhesive, such as an epoxy adhesive, a phenol adhesive, or a cyanoacrylate adhesive, electrode  4  and acoustic matching body  2  can be joined. 
     Next, an internal structure of acoustic matching body  2  will be described using  FIG.  2   .  FIG.  2    is a cross-sectional view of a configuration example of acoustic matching body  2  according to the first embodiment. Note that (a) of  FIG.  2    is a cross-sectional view (a cross-sectional view in the X-Z plane) taken along the thickness direction (parallel to the Z-axis) of acoustic matching body  2 . Furthermore, (b) of  FIG.  2    is a cross-sectional view taken along line II-II illustrated in (a) of  FIG.  2   , specifically, a cross-sectional view (a cross-sectional view in the X-Y plane) taken along a direction (parallel to the X-Y plane) perpendicular to the thickness direction of acoustic matching body  2 . 
     As illustrated in  FIG.  2   , acoustic matching body  2  includes top plate  8 , bottom plate  9 , side wall  10 , and perpendicular partition walls  12 . In acoustic matching body  2 , top plate  8  and bottom plate  9  are joined to side wall  10  to define closed space  11 , and perpendicular partition walls  12  are formed to divide closed space  11  into a plurality of closed spaces. Perpendicular partition walls  12  are disposed substantially perpendicularly to top plate  8  and bottom plate  9  (extend in substantially parallel to the Z-axis), and are integrally joined to top plate  8  and bottom plate  9 . 
     Thus, as illustrated in  FIG.  2   , closed space  11  is partitioned into the closed spaces by perpendicular partition walls  12 . In the example illustrated in  FIG.  2   , acoustic matching body  2  includes two perpendicular partition walls  12  disposed concentrically and eight perpendicular partition walls  12  disposed to linearly extend in the radial direction, when the acoustic matching body  2  according to the first embodiment is viewed from above (in parallel to the Z-axis). Thus, closed space  11  is divided into one circular closed space and  16  sector-shaped closed spaces. Note that the shape and number of perpendicular partition walls  12  disposed in closed space  11  are not limited to the shape and number illustrated in  FIG.  2   . Other examples of the shape of the perpendicular partition walls will be described later. 
     1-2. Procedure for Manufacturing Acoustic Matching Body 
     Next, a procedure for manufacturing acoustic matching body  2  will be described using  FIG.  3   . 
       FIG.  3    is a diagram illustrating a procedure for manufacturing acoustic matching body  2  in the first embodiment by using perspective views. The steps for manufacturing acoustic matching body  2  are performed in the order of (a), (b), (c), and (d) illustrated in  FIG.  3   . 
     As illustrated in (a) of  FIG.  3   , first, one or a plurality of metal plates  13  large enough to cut out a plurality of metal plates  14   a  and  14   b  is prepared. (a) of  FIG.  3    illustrates one metal plate  13 . Next, as illustrated in (b) of  FIG.  3   , metal plate  13  is circularly patterned to produce a plurality of metal plates  14   a  serving as top plate  8  and bottom plate  9 , and a plurality of metal plates  14   b  patterned with side wall  10  and perpendicular partition walls  12  of acoustic matching body  2  is produced from metal plate  13 . For the patterning of metal plate  13 , for example, punching of metal plate  13  with a press, etching by photolithography, laser processing, or processing using an electric discharge wire can be applied. Note that, in the present disclosure, there is illustrated an example in which metal plates  14   a  and  14   b  are formed to have a circular (disc-like) external shape when viewed from above (viewed in parallel to the Z-axis). However, this is merely an example, and the external shape of metal plates  14   a  and  14   b  according to the present disclosure is not limited to a circular shape (a disc-shape), and may be an elliptical shape or a polygonal shape. 
     Next, as illustrated in (c) of  FIG.  3   , metal plates  14   a  and metal plates  14   b  are positioned and laminated. Specifically, first, a predetermined number of metal plates  14   b  are laminated. Next, metal plate  14   a  serving as top plate  8  is laminated on the top face of laminated metal plates  14   b  (a face on the Z-axis positive direction side of metal plate  14   b  disposed at an end in the Z-axis positive direction). Next, metal plate  14   a  serving as bottom plate  9  is laminated on the bottom face of laminated metal plates  14   b  (a face on the Z-axis negative direction side of metal plate  14   b  disposed at an end in the Z-axis negative direction). The patterned metal plates are joined by heating and pressurization to become an integrated member by diffusion joining. As for the heating temperature, in the case of stainless having a melting point of approximately 1500° C., temperature in the diffusion joining is approximately 1000° C., and therefore, in the case where laminated metal plates  14   a  and  14   b  are made of stainless steel, laminated metal plates  14   a  and  14   b  are heated to the above-mentioned temperature and pressurized to perform diffusion joining. In the diffusion joining, flatness is required, and therefore, depending on the way of processing illustrated in (c) of  FIG.  3   , post-processing is needed to eliminate burrs or deformations of metal plates  14   a  and  14   b  after the step illustrated in (b) of  FIG.  3   . 
     By the above-described manufacturing procedure, acoustic matching body  2  of ultrasonic transceiver  1  according to the first embodiment in which the patterned metals are joined by diffusion joining can be produced as illustrated in (d) of  FIG.  3   . Note that the following embodiments including the present embodiment describe an example in which an acoustic matching body is formed to have a cylindrical external shape. However, this is merely one example, and the shape of the acoustic matching body described in the present disclosure is not limited to a cylindrical shape, and may be an elliptic cylinder shape or a polygonal column shape. 
     1-3. Effect 
     As described above, ultrasonic transceiver  1  in the present embodiment includes: piezoelectric body  3 ; and acoustic matching body  2  disposed in one face of piezoelectric body  3 . In acoustic matching body  2 , closed space  11  is defined by top plate  8 , bottom plate  9 , and side wall  10 . Inside closed space  11 , perpendicular partition walls  12  are provided to be substantially perpendicular to top plate  8  and bottom plate  9 . Perpendicular partition walls  12  is formed to adhere to top plate  8  and bottom plate  9 , thereby dividing closed space  11 . 
     Thus, in the case where ultrasonic transceiver  1  according to the present disclosure is used in a fluid of high temperature and high humidity or in a high-temperature and high-humidity environment, even when corrosion deterioration occurs in the outer circumference of acoustic matching body  2  and moisture enters closed space  11  from a gap caused by the corrosion deterioration in the outer circumferential portion of acoustic matching body  2 , the spread of moisture entry in the whole of acoustic matching body  2  can be substantially prevented because closed space  11  is divided into a plurality of closed spaces by perpendicular partition walls  12 . Thus, it is less likely to cause an apparent change in the density of acoustic matching body  2  due to the moisture entry, and therefore degradation in the measurement performance of a measuring instrument including ultrasonic transceiver  1  can be substantially prevented. Therefore, even when ultrasonic transceiver  1  is used in a fluid of high temperature and high humidity or in a high-temperature and high-humidity environment, ultrasonic transceiver  1  can stably operate for a long period. 
     In the present embodiment, a method for manufacturing acoustic matching body  2  is such that the step of forming a pattern in metal plate  13 , the step of laminating patterned metal plates  14   a  and  14   b , and the step of joining metal plates  14   a  and  14   b  by applying a load at a high temperature performed in this order. 
     Thus, acoustic matching body  2  can be patterned with high accuracy and the metal plates can be firmly joined without a gap, whereby acoustic matching body  2  can be stably manufactured with high accuracy. As a result, ultrasonic transceiver  1  can be manufactured with less variation in quality in mass production. 
     Second Embodiment 
     Next, ultrasonic flowmeter  80  according to the present embodiment will be described using  FIG.  4   . Note that ultrasonic flowmeter  80  described hereinafter can be replaced with ultrasonic flow velocimeter  81 . In this case, the term “flow rate” in the following description is beneficially replaced with the term “flow velocity”. Alternatively, a measuring instrument illustrated in  FIG.  4    may be capable of measuring both the flow rate and the flow velocity. 
     2-1. Configuration 
       FIG.  4    is a schematic block diagram illustrating a configuration example of ultrasonic flowmeter  80  in the second embodiment. 
     As illustrated in  FIG.  4   , ultrasonic flowmeter  80  in the present embodiment is configured such that a pair of ultrasonic transceivers  16  and  17  having the configuration of ultrasonic transceiver  1  described in the first embodiment are disposed on the upstream side and the downstream side of flow path  15 , respectively, to face each other. An arrow in flow path  15  indicates the direction of the flow of a fluid. In  FIG.  4   , the left-hand side is the upstream side of flow path  15 , while the right-hand side is the downstream side of flow path  15 . A dashed-line arrow L1 in  FIG.  4    indicates a propagation path of an ultrasonic wave propagating from ultrasonic transceiver  16  disposed on the upstream side into ultrasonic transceiver  17 . Another dashed-line arrow L2 in  FIG.  4    indicates a propagation path of an ultrasonic wave propagating from ultrasonic transceiver  17  disposed on the downstream side into ultrasonic transceiver  16 . Ultrasonic flowmeter  80  in the present embodiment includes: clocking device  18  connected to ultrasonic transceivers  16  and  17  and configured to clock the amount of time elapsed before the arrival of an ultrasonic wave from one of ultrasonic transceivers  16  and  17  at the other; and calculator  19  connected to clocking device  35  and configured to calculate the flow rate of a fluid flowing through flow path  15  by using the amount of ultrasonic arrival time determined by clocking device  18 . 
     Note that, in the case where the measuring instrument illustrated in  FIG.  4    is ultrasonic flow velocimeter  81 , ultrasonic flow velocimeter  81  is configured in the same manner as ultrasonic flowmeter  80 , except that calculator  19  is configured to calculate the flow velocity of a fluid flowing through flow path  15 , from the amount of ultrasonic arrival time determined by clocking device  18 . Note that calculator  19  may be configured to calculate both the flow velocity and flow rate of a fluid flowing through flow path  15 . 
     2-2. Measurement Operation of Flow Velocimeter or Flowmeter 
     V represents the flow velocity of a fluid flowing through flow path  15 , C (not illustrated) represents the velocity of an ultrasonic wave in the fluid, and θ represents an angle between the direction of the flow of the fluid and the direction of propagation of the ultrasonic wave. When ultrasonic transceiver  16  is used as an ultrasonic transmitter and ultrasonic transceiver  17  is used as an ultrasonic receiver, propagation time t1 elapsed until an ultrasonic wave emitted from ultrasonic transceiver  16  reaches ultrasonic transceiver  17  is expressed by the following formula (3). 
     
       
         
           
             t1 
             = 
             
               L 
               / 
               
                 
                   
                     C+Vcos 
                     θ 
                   
                 
               
             
           
         
       
     
      Next, propagation time t2 elapsed until an ultrasonic pulse emitted from ultrasonic transceiver  17  reaches ultrasonic transceiver  16  is expressed by the following formula (4).  
     
       
         
           
             t2 
             = 
             
               L 
               / 
               
                 
                   
                     C 
                     − 
                     Vcos 
                     θ 
                   
                 
               
             
           
         
       
     
      Then, when the acoustic velocity C of the fluid is eliminated from both the formula (3) and the formula (4), the following formula (5) is obtained.  
     
       
         
           
             V 
               
             = 
               
             
               L 
               / 
               
                 2cos 
                 θ 
                 
                   
                     
                       1 
                       / 
                       
                         t1 
                       
                     
                     − 
                     
                       1 
                       / 
                       
                         t2 
                       
                     
                   
                 
               
             
           
         
       
     
      When L and θ are known, the flow velocity V can be determined by measuring t1 and t2 by using clocking device  18 . In addition, by multiplying the flow velocity V by a cross section S and a correction factor K by using calculator  19 , a flow rate Q can be determined. Calculator  19  of ultrasonic flowmeter  80  is configured to perform an operation of the above-mentioned Q = KSV. 
     2-3. Effect 
     As described above, in the present embodiment, ultrasonic flowmeter  80  includes: flow path  15  allowing a measurement target fluid to flow therethrough; a pair of ultrasonic transceivers  16  and  17  disposed on the upstream side and the downstream side of flow path  15 , respectively, to face each other; clocking device  18  configured to clock the amount of arrival time elapsed from transmission of an ultrasonic signal from one of ultrasonic transceivers  16  and  17  to reception of the ultrasonic signal by the other; and calculator  19  configured to calculate the flow rate of the measurement target fluid flowing through flow path  15  from the amount of ultrasonic arrival time determined by clocking device  18 . Note that, when the measuring instrument illustrated in  FIG.  4    is ultrasonic flow velocimeter  81 , ultrasonic flow velocimeter  81  is configured in the same manner as ultrasonic flowmeter  80 , except that calculator  19  is configured to calculate the flow velocity of the measurement target fluid flowing through flow path  15  from the amount of ultrasonic arrival time determined by clocking device  18 . 
     Thus, in the case where ultrasonic flowmeter  80  or ultrasonic flow velocimeter  81  according to the present disclosure is used in a fluid of high temperature and high humidity or in a high-temperature and high-humidity environment, even when corrosion deterioration occurs in the outer circumference of acoustic matching body  2  and moisture enters closed space  11  from a gap caused by the corrosion deterioration in the outer circumferential portion of acoustic matching body  2 , moisture entry can be substantially prevented from spreading in the whole of acoustic matching body  2 , because closed space  11  is divided into a plurality of closed spaces by perpendicular partition walls  12 . Thus, it is less likely to cause an apparent change in the density of acoustic matching body  2  due to moisture entry, and therefore degradation in the measurement performance of ultrasonic flowmeter  80  or ultrasonic flow velocimeter  81  can be substantially prevented. Therefore, even when a fluid of high temperature and high humidity is used for a long period, ultrasonic flowmeter  80  can stably measure the flow rate of the measurement target fluid with high accuracy. In addition, even when a fluid of high temperature and high humidity is used for a long period, ultrasonic flow velocimeter  81  can stably measure the flow velocity of the measurement target fluid with high accuracy. 
     Third Embodiment 
     Next, a fluid densitometer using ultrasonic waves according to the present embodiment will be described using  FIG.  5   . 
     3-1. Configuration 
       FIG.  5    is a schematic block diagram illustrating a configuration example of ultrasonic densitometer  90  in a third embodiment. Ultrasonic densitometer  90  according to the present disclosure includes casing  30  including concentration measurement space  37  for measuring a fluid concentration. Casing  30  includes vent  31  configured to allow a measurement target fluid to pass out of or into casing  30  through vent  31 . Concentration measurement space  37  in casing  30  has, for example, a rectangular parallelepiped shape or a cylindrical shape. Concentration measurement space  37  is not necessarily entirely surrounded by a wall of casing  30 , and is beneficially a space at least capable of transmitting and receiving an ultrasonic wave between the pair of ultrasonic transceivers  32  and  33 . For example, casing  30  is made partially defective, and, in the defect portion, concentration measurement space  37  may be opened to the outside of casing  30 . 
     Inside concentration measurement space  37 , the pair of ultrasonic transceivers  32  and  33  each having the configuration of ultrasonic transceiver  1  described in the first embodiment are disposed to face each other. Furthermore, temperature sensor  34  is accommodated in concentration measurement space  37 . Ultrasonic transceivers  32  and  33  are connected to clocking device  35 . Clocking device  35  and temperature sensor  34  are connected to calculator  36 . 
     3-2. Operation of Concentration Measurement 
     When ultrasonic transceiver  32  is used as an ultrasonic transmitter, ultrasonic transceiver  32  transmits an ultrasonic wave, based on the operation of clocking device  35 . In this case, ultrasonic transceiver  33  functions as an ultrasonic receiver. The ultrasonic wave transmitted from ultrasonic transceiver  32  propagates through the measurement target fluid filled in concentration measurement space  37 . Ultrasonic transceiver  33  used as an ultrasonic receiver receives the ultrasonic wave. Clocking device  35  measures a propagation time elapsed from the transmission of an ultrasonic wave from ultrasonic transceiver  32  to the reception of the ultrasonic wave by ultrasonic transceiver  33 , and determines the propagation velocity Vs of the ultrasonic wave, based on a predetermined ultrasonic propagation distance L. 
     The propagation velocity Vs of an ultrasonic wave propagating through a mixed gas as the measurement target fluid is determined by the average molecular weight M, the specific heat ratio γ, the gas constant R, and the gas temperature T (K) of the mixed gas, as expressed by the following formula (6). By measuring the acoustic velocity and the temperature, the average molecular weight is determined. [0055] 
     
       
         
           
             Vs 
               
             = 
               
             γ 
             ⋅ 
             R 
             ⋅ 
             
               T 
               / 
               M 
             
           
         
       
     
      When gas components in the mixed gas are known, the gas temperature T and the propagation velocity Vs are measured to determine the average molecular weight M, whereby a gas concentration can be calculated from the average molecular weight M. In the case of an ideal gas mixture including two types of gases, namely, a and b, a concentration equation is expressed as the following formula (7).  
     
       
         
           
             Concentration 
               
             of 
               
             gas 
               
             a 
               
             
               % 
             
             = 
               
             M 
             − 
             
               
                 mb 
               
               / 
               
                 ma 
               
             
             − 
             mb 
               
             × 
               
             100 
           
         
       
     
      In the formula, ma represents the molecular weight of gas a, and mb represents the molecular weight of gas b. 
     3-3. Effect 
     As described above, in the present embodiment, ultrasonic densitometer  90  includes: casing  30  including a vent allowing a measurement target fluid to pass out of or into casing  30 ; a pair of ultrasonic transceivers  32  and  33  disposed at a predetermined distance from each other and facing each other in casing  30 ; temperature sensor  34  disposed inside casing  30 ; clocking device  3  configured to clock the amount of arrival time elapsed from transmission of an ultrasonic signal from one of the pair of ultrasonic transceivers  32  and  33  to the reception of the ultrasonic signal by the other; and calculator  36  configured to calculate the propagation velocity of the ultrasonic wave propagating through the measurement target fluid, the average molecular weight of a mixed gas, and the gas concentration of the mixed gas, from the amount of arrival time determined by clocking device  35 . 
     Thus, in the case where ultrasonic densitometer  90  including ultrasonic transceivers  32  and  33  according to the present disclosure is used in a fluid of high temperature and high humidity or in a high-temperature and high-humidity environment, even when corrosion deterioration occurs in the outer circumference of acoustic matching body  2  and moisture enters closed space  11  from a gap caused by the corrosion deterioration in the circumferential portion of acoustic matching body  2 , the spread of moisture entry in the whole of acoustic matching body  2  can be substantially prevented, because closed space  11  is divided into a plurality of closed spaces by perpendicular partition walls  12 . Thus, it is less likely to cause an apparent change in the density of acoustic matching body  2  due to moisture entry, and therefore degradation in the measurement performance of ultrasonic densitometer  90  can be substantially prevented. Therefore, even when a fluid of high temperature and high humidity is used for a long period, ultrasonic densitometer  90  can stably measure the gas concentration of the measurement target fluid with high accuracy. 
     Fourth Embodiment 
     Hereinafter, an ultrasonic transceiver according to a fourth embodiment will be described using  FIGS.  6  to  9   . 
     4-1. Configuration 
     The ultrasonic transceiver in the present embodiment is different only in the internal structure of the acoustic matching body from that in the first embodiment, and the configuration of the ultrasonic transceiver is the same as that in the first embodiment, and therefore, descriptions thereof will be omitted. Here, the internal structure of the acoustic matching body will be described using  FIG.  6   .  FIG.  6    is a cross-sectional view of a configuration example of acoustic matching body  20  in the fourth embodiment. Note that (a) of  FIG.  6    is a cross-sectional view (a cross-sectional view in the X-Z plane) taken along the thickness direction (parallel to the Z-axis) of acoustic matching body  20 . Furthermore, (b) of  FIG.  6    is a cross-sectional view taken along line VI-VI illustrated in (a) of  FIG.  6   , in other words, a cross-sectional view (a cross-sectional view in the X-Y plane) taken along a direction (parallel to the X-Y plane) perpendicular to the thickness direction of acoustic matching body  20 . 
     As illustrated in  FIG.  6   , acoustic matching body  20  according to the present disclosure includes top plate  8 , bottom plate  9 , side wall  10 , perpendicular partition walls  12 , and horizontal partition walls  39 . In acoustic matching body  20 , closed space  11  is defined by top plate  8 , bottom plate  9 , and side wall  10 . Furthermore, in acoustic matching body  20 , perpendicular partition walls  12  are formed substantially perpendicularly to top plate  8  and bottom plate  9  (to extend in substantially parallel to the Z-axis), meanwhile horizontal partition walls  39  are formed to be substantially horizontal to top plate  8  and bottom plate  9  (extends substantially horizontally to the X-Y plane) inside closed space  11 . Perpendicular partition walls  12  are formed to adhere to top plate  8  and bottom plate  9  so that perpendicular partition walls  12  divide closed space  11  into a plurality of closed space when acoustic matching body  20  is viewed from above (viewed in parallel to the Z-axis). Horizontal partition walls  39  are formed to adhere to side wall  10  and perpendicular partition walls  12  so that horizontal partition walls  39  divide closed space  11  into upper and lower parts (along the Z-axis) when acoustic matching body  20  is viewed horizontally (viewed in parallel to the X-axis and the Y-axis). 
     Thus, as illustrated in  FIG.  6   , closed space  11  is partitioned into a plurality of spaces by perpendicular partition walls  12  and horizontal partition walls  39 . In the example illustrated in  FIG.  6   , acoustic matching body  20  in the fourth embodiment includes: two perpendicular partition walls  12  disposed concentrically and eight perpendicular partition walls  12  disposed to extend linearly and radially, when acoustic matching body  20  is viewed from above (in parallel to the Z-axis); and two disc-shaped horizontal partition walls  39 . Thus, closed space  11  is partitioned into three circular closed spaces and  48  sector-shaped closed spaces. Note that the shape and number of perpendicular partition walls  12  and horizontal partition walls  39  disposed in closed space  11  are not limited to the shape and number illustrated in  FIG.  6   . Other examples of the shape of the perpendicular partition walls will be described later. 
     4-2. Procedure for Manufacturing Acoustic Matching Body 
     Next, a procedure for manufacturing acoustic matching body  20  will be described using  FIG.  7   .  FIG.  7    is a diagram illustrating the procedure for manufacturing acoustic matching body  20  in the fourth embodiment by using perspective views. The steps for manufacturing acoustic matching body  20  are performed in the order of (a), (b), (c), and (d) illustrated in  FIG.  7   . 
     As illustrated in (a) of  FIG.  7   , first, one or a plurality of metal plates  13  large enough to cut out a plurality of metal plates  14   a  and  14   b  is prepared. (a) of  FIG.  7    illustrates one metal plate  13 . Next, as illustrated in (b) of  FIG.  7   , metal plate  13  is circularly patterned to produce a plurality of metal plates  14   a  serving as top plate  8 , bottom plate  9 , and horizontal partition walls  39 , and furthermore, a plurality of metal plates  14   b  patterned with perpendicular partition walls  12  and side wall  10  that are formed substantially perpendicularly to top plate  8  and bottom plate  9  of acoustic matching body  2  is produced from metal plate  13 . For the patterning of metal plate  13 , for example, punching of metal plate  13  with a press, etching by photolithography, laser processing, or processing using an electric discharge wire can be applied. The above-described steps are the same as the steps described using (a) and (b) of  FIG.  3    in the first embodiment, and the shapes of the metal plates  14   a  and  14   b  are also the same as those described in the first embodiment. 
     Next, as illustrated in (c) of  FIG.  7   , metal plates  14   a  and metal plates  14   b  are positioned and alternately laminated. Specifically, metal plates  14   a  are laminated as horizontal partition walls  39  meanwhile metal plates  14   b  are laminated as perpendicular partition walls  12 . Then, metal plate  14   a  serving as top plate  8  is laminated on the top face of alternately laminated metal plates  14   a  and  14   b  (a face on the Z-axis positive direction side of metal plate  14   b  disposed at an end in the Z-axis positive direction). Next, metal plate  14   a  serving as bottom plate  9  is laminated on the bottom face of alternately laminated metal plates  14   a  and  14   b  (a face on the Z-axis negative direction side of metal plate  14   b  disposed at an end in the Z-axis negative direction). The patterned metal plates are joined by heating and pressurization to become an integrated member by diffusion joining. As for the heating temperature, in the case of stainless having a melting point of approximately 1500° C., the temperature in the diffusion joining is approximately 1000° C., and therefore, when alternately laminated metal plates  14   a  and  14   b  are made of stainless steel, laminated metal plates  14   a  and  14   b  are heated to the above-mentioned temperature and pressurized to perform diffusion joining. In the diffusion joining, flatness is required, and therefore, depending on the way of processing illustrated in (c) of  FIG.  7   , post-processing is needed to eliminate burrs or deformations of metal plates  14   a  and  14   b  after the step illustrated in (b) of  FIG.  7   . 
     By the above-described manufacturing procedure, acoustic matching body  20  in the fourth embodiment in which patterned metals are joined by diffusion joining can be produced as illustrated in (d) of  FIG.  7   . 
     4-3. Effect 
     As described above, acoustic matching body  20  of the ultrasonic transceiver in the present embodiment includes top plate  8 , bottom plate  9 , and side wall  10  that define closed space  11 , and further includes perpendicular partition walls  12  formed substantially perpendicularly to top plate  8  and bottom plate  9  inside closed space  11 , and horizontal partition walls  39  formed substantially horizontally to top plate  8  and bottom plate  9  inside closed space  11 . Perpendicular partition walls  12  adhere to top plate  8  and bottom plate  9 , thereby dividing closed space  11 , meanwhile horizontal partition walls  39  adhere to side wall  10  and perpendicular partition walls  12  to divide closed space  11  into upper and lower parts (along the Z-axis). 
     Thus, in the case where the ultrasonic transceiver including acoustic matching body  20  according to the present disclosure is used in a fluid of high temperature and high humidity or in a high-temperature and high-humidity environment, even when corrosion deterioration occurs in the outer circumference of acoustic matching body  20  and moisture enters closed space  11  from a gap caused by the corrosion deterioration in the outer circumferential portion of acoustic matching body  20 , the spread of moisture entry in the whole of acoustic matching body  20  can be substantially prevented, because closed space  11  is divided into a plurality of closed spaces by perpendicular partition walls  12  and horizontal partition walls  39 . Thus, it is less likely to cause an apparent change in the density of acoustic matching body  20  due to moisture entry, and therefore degradation in the measurement performance of a measuring instrument including the ultrasonic transceiver including acoustic matching body  20  can be substantially prevented. Furthermore, in acoustic matching body  20  used in the ultrasonic transceiver according to the present disclosure, closed space  11  is partitioned by perpendicular partition walls  12  and horizontal partitions  39 , and hence, closed space  11  is divided into more closed spaces than that in acoustic matching body  2  in the first embodiment. Therefore, the ultrasonic transceiver including acoustic matching body  20  is capable of stably operating for a still longer period. 
     In the present embodiment, the method for manufacturing acoustic matching body  20  is such that the step of forming a pattern in metal plate  13 , the step of alternately laminating patterned metal plates  14   a  and  14   b , and the step of joining metal plates  14   a  and  14   b  by applying a load at a high temperature are performed in this order. 
     Thus, acoustic matching body  20  can be patterned with high accuracy and the metal plates can be firmly joined without a gap, whereby the acoustic matching body can be stably manufactured with high accuracy. As a result, the ultrasonic transceiver can be manufactured with less variation in quality in mass production. 
     The ultrasonic transceiver according to the present embodiment can be used as an ultrasonic transceiver used of ultrasonic flowmeter  80  or ultrasonic flow velocimeter  81  described in the second embodiment or ultrasonic densitometer  90  described in the third embodiment. 
     Fifth Embodiment 
     Next, another shape of perpendicular partition wall  12  of an acoustic matching body used in an ultrasonic transceiver will be described in a fifth embodiment. 
     5-1. Pattern of Perpendicular Partition Wall 
     In the fifth embodiment, a different shape of pattern of perpendicular partition wall  12  from the shapes illustrated in  FIGS.  2 ,  3 ,  6 , and  7    will be illustrated. Note that the present disclosure is not intended to limit a pattern of perpendicular partition wall  12  to the patterns illustrated in the embodiments. The acoustic matching body described in the present embodiment is the same as acoustic matching bodies  2  and  20  respectively described in the first embodiment and the fourth embodiment, except the pattern of perpendicular partition wall  12 , and therefore, descriptions on the configuration, except on the pattern of perpendicular partition wall  12  will be omitted. 
       FIG.  8    is a cross-sectional view of a configuration example of the acoustic matching body in the fifth embodiment.  FIG.  9    is a cross-sectional view of another configuration example of the acoustic matching body in the fifth embodiment.  FIGS.  8  and  9    are cross-sectional views (cross-sectional views in the X-Y plane) taken along a direction (parallel to the X-Y plane) perpendicular to the thickness direction of the acoustic matching body. 
     In each of  FIGS.  8  and  9   , side wall  10 , and perpendicular partition walls  12  formed substantially perpendicularly to top plate  8  and bottom plate  9  of the acoustic matching body are illustrated. The pattern of perpendicular partition wall  12  can be arbitrarily selected according to an usage environment or a required strength, and examples of the pattern include a lattice shape illustrated in  FIG.  8    and a honeycomb shape illustrated in  FIG.  9   . Besides the patterns illustrated in  FIGS.  7 ,  8 , and  9   , for example, a pattern in which circles are spread around can be selected for perpendicular partition wall  12 . 
     5-2. Thickness of Partition Wall 
     The thickness of perpendicular partition wall  12  defined inside closed space  11  of the acoustic matching body is preferably thinner than the thickness of side wall  10 . As the acoustic matching body is lighter in weight, the acoustic matching body can more efficiently transmit an ultrasonic wave to a measurement target fluid. Therefore, perpendicular partition wall  12  is preferably thinner than top plate  8  and bottom plate  9 , and the number of perpendicular partition walls  12  is preferably smaller. However, when used in a high-temperature and high-humidity environment, corrosion deterioration begins in side wall  10 . Therefore, by making side wall  10  larger in thickness, corrosion resistance is enhanced. 
     In view of the above, perpendicular partition walls  12  defined inside closed space  11  of the acoustic matching body is made thinner than side wall  10 , whereby, while substantially preventing a decrease in the propagation efficiency of an ultrasonic wave, the resistance of the acoustic matching body to a high-temperature and high-humidity environment in which the acoustic matching body easily corrodes can be enhanced. 
     5-3. Ultrasonic Propagation Efficiency Owing to Partition Walls 
     Perpendicular partition wall  12  has the function of partitioning closed space  11 , and also functions as a frame that resonates with ultrasonic vibration generated in piezoelectric body  3 . Perpendicular partition wall  12  and top plate  8  are firmly joined by diffusion joining. However, when the area of each region obtained by the partition by perpendicular partition walls  12  is larger, top plate  8  is bent, and accordingly, a vibration different from a targeted vibration occurs, whereby the efficiency of propagation of an ultrasonic wave to the measurement target fluid falls as a result. 
     Table  1  illustrates a relation among the area (mm 2 ) of a region resulting from partition by perpendicular partition walls  12  in the acoustic matching body, the projected area ratio (%) of perpendicular partition walls  12 , and the efficiency of ultrasonic propagation. Note that the projected area ratio (%) of perpendicular partition walls  12  means the ratio of the total area of perpendicular partition walls  12  to the area of the acoustic matching body except side wall  10 , when the acoustic matching body is viewed from above (viewed in parallel to the Z-axis). As the projected area ratio (%) of perpendicular partition walls  12  is larger, perpendicular partition wall  12  is larger in thickness or the number of perpendicular partition walls  12  is larger. Furthermore, a larger value of the efficiency of ultrasonic propagation of waves from a higher propagation efficiency. 
     From Table  1 , it is understood that the followings are preferable in order to enhance the efficiency of ultrasonic propagation into a measurement target fluid. An area (an area when the acoustic matching body is viewed from above (viewed in parallel to the Z-axis)) of a region resulting from perpendicular partition walls  12  is preferably 0.2 mm 2  or larger, and more preferably in a range of 0.30 mm 2  to 1.0 mm 2 . The projected area ratio of perpendicular partition walls  12  is preferably 15% or lower, and more preferably in a range of 8% to 13%. 
     [Table 1] Relation among region resulting frame partition of perpendicular partition walls of acoustic matching body, projected area ratio of perpendicular partition walls, and ultrasonic efficiency of propagation 
     
       
         
           
               
               
               
               
               
               
               
               
               
            
               
                 Region resulting from partition by perpendicular partition walls (mm2) 
                 0.08 
                 0.14 
                 0.22 
                 0.31 
                 0.42 
                 0.87 
                 1.25 
                 1.95 
               
               
                 Projected area ratio of perpendicular partition walls (%) 
                 23.7 
                 18.4 
                 15.0 
                 13.0 
                 11.4 
                 8.0 
                 6.7 
                 5.4 
               
               
                 Efficiency of ultrasonic propagation into measurement target fluid 
                 0.20 
                 0.26 
                 0.36 
                 0.55 
                 1.00 
                 0.82 
                 0.45 
                 0.20 
               
            
           
         
       
     
     When the thickness of top plate  8  of the acoustic matching body is thinner than the thickness of one patterned metal plate (for example, metal plate  13 ), the efficiency of ultrasonic propagation into a measurement target fluid can be enhanced. 
     5-4. Effect 
     In the present embodiment, perpendicular partition walls  12  defined formed inside closed space of the acoustic matching body are thinner than side wall  10 . 
     Thus, while substantially preventing a decrease in the efficiency of propagation of an ultrasonic wave from the ultrasonic transceiver including the acoustic matching body into a measurement target fluid, the resistance of the ultrasonic transceiver to a high-temperature and high-humidity environment in which the acoustic matching body easily corrodes can be enhanced. 
     In the acoustic matching body in the present embodiment, the area of each region resulting from partition by perpendicular partition walls  12  (the area of the region when the acoustic matching body is viewed from above (viewed in parallel to the Z-axis)) is 1 mm 2  or smaller, and the projected area of perpendicular partition walls  12  (the total area of perpendicular partition walls  12  when the acoustic matching body is viewed from above (viewed in parallel to the Z-axis)) is 10% or less of the projected area of the acoustic matching body except side wall  10  (the area of the acoustic matching body except side wall  10  when the acoustic matching body is viewed from above (viewed in parallel to the Z-axis)). 
     Thus, the efficiency of propagation of an ultrasonic wave from the ultrasonic transceiver including the acoustic matching body into a measurement target fluid can be further enhanced. 
     In the present embodiment, the acoustic matching body is formed by laminating a plurality of patterned metal plates. 
     Thus, perpendicular partition wall  12  having a more complicated shape can be produced with high definition. Thus, in the mass production of the acoustic matching body, variations in characteristics can be reduced. Thus, when the ultrasonic transceiver including the acoustic matching body is used in a flowmeter, a flow velocimeter, or a densitometer, measurement with high accuracy can be achieved. 
     In the present embodiment, the acoustic matching body is formed so that top plate  8  is thinner than one patterned metal plate (for example, metal plate  13 ). 
     Thus, the efficiency of ultrasonic propagation from the ultrasonic transceiver including the acoustic matching body into a measurement target fluid can be further enhanced. 
     The ultrasonic transceiver according to the present embodiment can be used as an ultrasonic transceiver of ultrasonic flowmeter  80  or ultrasonic flow velocimeter  81   described in the second embodiment or ultrasonic densitometer  90  described in the third embodiment. 
     Sixth Embodiment 
     Hereinafter, a sixth embodiment will be described using  FIGS.  10  to  12   . 
     6-1. Configuration 
       FIG.  10    is a cross-sectional view of a configuration example of ultrasonic transceiver  21  in the sixth embodiment.  FIG.  10    is a cross-sectional view (a cross-sectional view in the X-Z plane) taken along the thickness direction (parallel to the Z-axis) of ultrasonic transceiver  21 . 
     As illustrated in  FIG.  10   , ultrasonic transceiver  21  includes: closed-top tubular metal case  42 ; piezoelectric body  3  disposed in top inner wall  42   a  of closed-top tubular metal case  42 ; and acoustic matching body  2  described in the first embodiment or acoustic matching body  20  described in the fourth embodiment, which is disposed in top outer wall  42   b  of closed-top tubular metal case  42 . Top inner wall  42   a  is a top face on the inner side (a face on the Z-axis negative direction side) of closed-top tubular metal case  42 , meanwhile top outer wall  42   b  is a top face on the outer side (a face on the Z-axis positive direction side) of closed-top tubular metal case  42 . Terminal  44  is the equivalent of lead wire  6  illustrated in  FIG.  1    and is joined to terminal plate  43  configured to allow the passage of electric current between terminal  44  and closed-top tubular metal case  42 , and is electrically connected to electrode  4  of piezoelectric body  3  via terminal plate  43  and conductive closed-top tubular metal case  42 . Terminal  45  is the equivalent of lead wire  7  illustrated in  FIG.  1    and is electrically connected to electrode  5  of piezoelectric body  3  via conductive rubber  47 . Through-hole  46  provided in terminal plate  43  is a hole for allowing terminal  45  to penetrate, and terminal  45  penetrates through-hole  46  upward from the bottom (in substantially parallel to the Z-axis), and comes into contact with a conductive center of conductive rubber  47 . The diameter of conductive rubber  47  is larger than the diameter of through-hole  46 , an outer circumferential portion of conductive rubber  47  has insulation properties, and the outer circumferential portion of conductive rubber  47  is pressurized upward (in the Z-axis positive direction) by the circumferential edge portion of through-hole  46 . Note that, in the following descriptions, acoustic matching body  2  is assumed to be joined to top outer wall  42   b  of closed-top tubular metal case  42 , but, acoustic matching body  2  may be replaced with acoustic matching body  20 . In that case, descriptions may be the same as the following descriptions, and therefore will be omitted. For the joining of closed-top tubular metal case  42  to acoustic matching body  2  and piezoelectric body  3 , for example, an organic adhesive, low melting glass, soldering, or brazing can be used. 
     6-2. Procedure for Manufacturing Ultrasonic Transceiver 
     Next, a procedure for manufacturing ultrasonic transceiver  21  will be described using  FIG.  11   . 
       FIG.  11    is a diagram illustrating the procedure for manufacturing ultrasonic transceiver  21  in the sixth embodiment by using cross-sectional views. 
     As illustrated in (a) of  FIG.  11   , first, acoustic matching body  2  described in the first embodiment is prepared. At the same time, as illustrated in (b) of  FIG.  11   , a thermosetting adhesive to be used as joining material  40  is applied to the upper face (a face on the Z-axis positive direction side) of piezoelectric body  3 , and the same joining material  41  is applied to top outer wall  42   b  of closed-top tubular metal case  42 . Next, as illustrated in (c) of  FIG.  6   , closed-top tubular metal case  42  is laminated on piezoelectric body  3 , and joining material  40  is interposed between the upper face (a face in the Z-axis positive direction side) of piezoelectric body  3  and top inner wall  42   a  of closed-top tubular metal case  42  to paste the upper face and top inner wall  42   a  together. Furthermore, acoustic matching body  2  is laminated on closed-top tubular metal case  42 , and joining material  41  is interposed between top outer wall  42   b  of closed-top tubular metal case  42  and the lower face (a face in the Z-axis negative direction side) of acoustic matching body  2  to paste the upper face and top inner wall  42   a  together. Here, piezoelectric body  3 , closed-top tubular metal case  42 , and acoustic matching body  2  are heated while being pressurized at approximately 2 kg/cm 2  to 10 kg/cm 2 , whereby the thermosetting adhesive is cured. Thus, acoustic matching body  2  and piezoelectric body  3  are stuck fast to closed-top tubular metal case  42 . 
     Next, as illustrated in (d) of  FIG.  6   , terminal plate  43  in which conductive rubber  47  is inserted into a recessed portion provided above through-hole  46  is superimposed on a joined member from below, the joined member including acoustic matching body  2 , closed-top tubular metal case  42 , and piezoelectric body  3  and being obtained by heat-curing and laminating through the above-described steps. Then, a flange of closed-top tubular metal case  42  and a circumferential edge portion of terminal plate  43  are welded. During the welding, an inert gas, such as argon gas, nitrogen gas, or helium gas, is sealed in a closed space surrounded by terminal plate  43  and closed-top tubular metal case  42 . Thus, deterioration of an electrode of piezoelectric body  3  and deterioration of a joint between piezoelectric body  3  and closed-top tubular metal case  42  can be reduced. 
     Then, terminal  44  is joined to terminal plate  43 , and terminal  45  is brought into contact with a center portion of conductive rubber  47 . 
     A material for forming closed-top tubular metal case  42  is beneficially iron, brass, copper, aluminum, stainless steel, or an alloy thereof, or a conductive material such as a metal obtained by plating a surface of the above-mentioned metals. 
     The thermosetting adhesive used as joining materials  40  and  41  is beneficially a thermosetting resin, such as an epoxy resin, a phenolic resin, a polyester resin or, a melamine resin, and is not particularly limited. In some cases, as the adhesive, there may be used a thermoplastic resin having a glass-transition temperature that is equal to or higher than a high-temperature use temperature (for example, 70° C. or higher), the high-temperature use temperature being a temperature defined as the upper limit of an operating temperature of ultrasonic transceiver  21 . 
     Thus, as illustrated in (e) of  FIG.  6   , ultrasonic transceiver  21  is in a finished state. 
     6-3. Relation Between Projected Plane of Joint in Piezoelectric Body and Projected Plane of Joint in Side Wall 
     Next, a relation of area of a joint between acoustic matching body  2  and piezoelectric body  3  will be described using  FIG.  12   . 
       FIG.  12    is a cross-sectional view and a plan view of a configuration example of ultrasonic transceiver  21  in the sixth embodiment. Note that (a) of  FIG.  12    is a cross-sectional view (a cross-sectional view in the X-Z plane) taken along the thickness direction (parallel to the Z-axis) of ultrasonic transceiver  21 . Furthermore, (b) of  FIG.  12    is a plan view obtained when ultrasonic transceiver  21  is viewed from above (viewed in parallel to the Z-axis). In (b) of  FIG.  12   , a relation between a projected plane of j oint in piezoelectric body  3  and a projected plane of joint in side wall  10  in the sixth embodiment is illustrated. The projected plane of joint in piezoelectric body  3  is a joint face between piezoelectric body  3  and top inner wall  42   a  when ultrasonic transceiver  21  is viewed from above (viewed in parallel to the Z-axis). The projected plane of j oint in side wall  10  is a joint face between side wall  10  and top outer wall  42   b  when ultrasonic transceiver  21  is viewed from above (viewed in parallel to the Z-axis). Hereinafter, the projected plane of joint in piezoelectric body  3  is referred to as piezoelectric body joint projected plane  48 , and the projected plane of joint of side wall  10  of acoustic matching body  2  is referred to as side wall j oint projected plane  49 . 
     Piezoelectric body  3  vibrates at a predetermined frequency by an ultrasonic signal, and acoustic matching body  2  resonates to this vibration, whereby an ultrasonic signal having a higher amplitude is produced. Thus, an ultrasonic wave propagates from ultrasonic transceiver  21  into a measurement target fluid. In the present disclosure, the measurement target fluid is assumed to be a fluid of high temperature and high humidity. In acoustic matching body  2 , closed space  11  is defined by top plate  8 , bottom plate  9 , and side wall  10 , and perpendicular partition walls  12  formed substantially perpendicularly to top plate  8  and bottom plate  9  are provided inside closed space  11 . Perpendicular partition walls  12  are formed to adhere to top plate  8  and bottom plate  9  so that perpendicular partition wall  12  and side wall  10  divide closed space  11 . The thickness of side wall  10  is preferably 0.3 mm or more in order to further enhance the moisture resistance of acoustic matching body  2 . However, such specification causes acoustic matching body  2  to be larger in weight, whereby there is a risk of a decrease in the efficiency of propagation of ultrasonic waves into the measurement target fluid. 
     Therefore, as illustrated in (b) of  FIG.  12   , piezoelectric body  3  and acoustic matching body  2  are formed so that the projected plane of joint of piezoelectric body  3  to closed-top tubular metal case  42 , namely, piezoelectric body joint projected plane  48  is included in side wall joint projected plane  49  of acoustic matching body  2 . It was confirmed that, with this configuration, a decrease in the efficiency of ultrasonic propagation from ultrasonic transceiver  21  into the measurement target fluid can be substantially prevented. Thus, with this configuration, while a decrease in the efficiency of propagation of ultrasonic waves is substantially prevented, the thickness of side wall  10  can be 0.3 mm or more, whereby the moisture resistance of acoustic matching body  2  can be further enhanced. 
     Descriptions about an operation of ultrasonic flowmeter  80 , an operation of ultrasonic flow velocimeter  81 , and an operation of ultrasonic densitometer  90 , in which ultrasonic transceiver  21  in the present embodiment is used, will be omitted because these operations are the same as those described in the second and third embodiments. 
     6-4. Effect 
     As described above, in the present embodiment, ultrasonic transceiver  21  is configured to include: closed-top tubular metal case  42 ; piezoelectric body  3  disposed in top inner wall  42   a  of closed-top tubular metal case  42 ; and acoustic matching body  2  described in the first embodiment and disposed in top outer wall  42   b  of closed-top tubular metal case  42 . Note that ultrasonic transceiver  21  may be configured to include acoustic matching body  20  described in the fourth embodiment, in place of acoustic matching body  2 . 
     With this configuration, in the case where ultrasonic transceiver  21  according to the present disclosure is used in a fluid of high temperature and high humidity or in a high-temperature and high-humidity environment, even when corrosion deterioration occurs in the outer circumference of acoustic matching body  2  (or acoustic matching body  20 ) and moisture enters closed space  11  from a gap caused by the corrosion deterioration in the outer circumferential portion of acoustic matching body  2  (or acoustic matching body  20 ), the spread of moisture entry in the whole of acoustic matching body  2  (or acoustic matching body  20 ) can be substantially prevented, because closed space  11  is partitioned into a plurality of closed spaces by perpendicular partition walls  12  (or perpendicular partition walls  12  and horizontal partition walls  39 ). Thus, it is less likely to cause an apparent change in the density of acoustic matching body  2  (or acoustic matching body  20 ) due to moisture entry, and therefore, degradation in the measurement performance of a measuring instrument including ultrasonic transceiver  21  including acoustic matching body  2  (or acoustic matching body  20 ) can be substantially prevented. Therefore, even when ultrasonic transceiver  21  is used in a fluid of high temperature and high humidity or in a high-temperature and high-humidity environment, ultrasonic transceiver  21  can stably operate for a long period. Furthermore, in ultrasonic transceiver  21 , piezoelectric body  3  is sealed by closed-top tubular metal case  42  and terminal plate  43 , so that corrosion of electrodes  4  and  5  of piezoelectric body  3  and deterioration of joining material  40  are inhibited. Thus, the reliability of the measuring instrument including ultrasonic transceiver  21  is secured for a long period. 
     In the present embodiment, piezoelectric body  3  and acoustic matching body  2  (or acoustic matching body  20 ) are formed so that piezoelectric body joint projected plane  48  is included in side wall joint projected plane  49  of acoustic matching body  2  (or acoustic matching body  20 ). Thus, while a decrease in the efficiency of propagation of ultrasonic waves from ultrasonic transceiver  21  into a measurement target fluid is substantially prevented, the moisture resistance of ultrasonic transceiver  21  can be further enhanced. 
     Ultrasonic transceiver  21  according to the present embodiment can be used as an ultrasonic transceiver of ultrasonic flowmeter  80  or ultrasonic flow velocimeter  81  described in the second embodiment or ultrasonic densitometer  90  described in the third embodiment. 
     Seventh Embodiment 
     Hereinafter, a seventh embodiment will be described using  FIG.  13 A .  FIG.  13 A  is a cross-sectional view illustrating a configuration example of ultrasonic transceiver  23  in the seventh embodiment. 
     7-1. Configuration 
     As illustrated in  FIG.  13 A , ultrasonic transceiver  23  includes: piezoelectric body  3 ; closed-top tubular metal case  42  disposed in one face of piezoelectric body  3 ; and acoustic matching body  22  disposed in top outer wall  42   b  of closed-top tubular metal case  42 . Note that, unlike acoustic matching bodies  2  and  20  respectively described in the first and fourth embodiments, acoustic matching body  22  in the present embodiment does not include bottom plate  9 , and top outer wall  42   b  of closed-top tubular metal case  42  is used in place of bottom plate  9 . Specifically, in acoustic matching body  22 , closed space  11  is defined by top plate  8 , side wall  10 , and top outer wall  42   b  of closed-top tubular metal case  42 . Furthermore, inside closed space  11 , there is provided perpendicular partition walls  12  formed substantially perpendicularly to top plate  8  of acoustic matching body  22  and top outer wall  42   b  of closed-top tubular metal case  42 . Perpendicular partition walls  12  adhere to top plate  8  of acoustic matching body  22  and top outer wall  42   b  of closed-top tubular metal case  42  viajoining material  41  described in the sixth embodiment so as to divide closed space  11 . 
     Acoustic matching body  22  in the present embodiment is configured by eliminating bottom plate  9  from acoustic matching bodies  2  and  20  respectively illustrated in the first and fourth embodiments, and this configuration allows acoustic matching body  22  to be lighter in weight than acoustic matching bodies  2  and  20 . Thus, the efficiency of propagation of ultrasonic waves from ultrasonic transceiver  23  including acoustic matching body  22  into a measurement target fluid can be further enhanced. 
     Note that acoustic matching body  22  is configured in substantially the same manner as acoustic matching bodies  2  and  20 , except that acoustic matching body  22  does not include bottom plate  9 . In other words, in acoustic matching body  22 , an inner space is defined by top plate  8  and side wall  10 , and an edge of side wall  10 , the edge being more distant from top plate  8  (an end on the Z-axis negative direction side), adheres to top outer wall  42   b  of closed-top tubular metal case  42  to define closed space  11 . Except the above, acoustic matching body  22  is configured in substantially the same manner as acoustic matching bodies  2  and  20 , and therefore detailed descriptions thereof will be omitted. Furthermore, a procedure for manufacturing ultrasonic transceiver  23  in the present embodiment is the same as the procedure for manufacturing ultrasonic transceiver  21  illustrated in  FIG.  11    in the sixth embodiment, except that acoustic matching body  22  does not include bottom plate  9 , and therefore descriptions about the procedure will be omitted. Furthermore, an operation of ultrasonic flowmeter  80 , an operation of ultrasonic flow velocimeter  81 , and an operation of ultrasonic densitometer  90 , each including ultrasonic transceiver  23  in the present embodiment, are the same as the operations described in the second and third embodiments, and therefore, descriptions about the operations will be omitted. 
     7-2. Effect 
     As described above, in the present embodiment, ultrasonic transceiver  23  includes: piezoelectric body  3 ; closed-top tubular metal case  42  disposed in one face of piezoelectric body  3 ; and acoustic matching body  2  disposed in top outer wall  42   b  of closed-top tubular metal case  42 . In acoustic matching body  2 , closed space  11  is defined by top plate  8 , side wall  10 , top outer wall  42   b  of closed-top tubular metal case  42 , and perpendicular partition walls  12  formed substantially perpendicularly to top plate  8  of acoustic matching body  22  and top outer wall  42   b  of closed-top tubular metal case  42  are provided inside closed space  11 . Perpendicular partition walls  12  adhere to top plate  8  of acoustic matching body  22  and top outer wall  42   b  of closed-top tubular metal case  42 , thereby dividing closed space  11 . 
     This configuration allows ultrasonic transceiver  23  to be lighter in weight by the weight of eliminated bottom plate  9  than ultrasonic transceiver  21  described in the sixth embodiment. Thus, the efficiency of propagation of ultrasonic waves from ultrasonic transceiver  23  into a measurement target fluid can be further enhanced. 
       FIG.  13 B  is a cross-sectional view of another configuration example of the ultrasonic transceiver in the seventh embodiment. In the present embodiment, an example in which, as described above, ultrasonic transceiver  23  is configured by joining acoustic matching body  22  to top outer wall  42   b  of closed-top tubular metal case  42  by using joining material  41  is illustrated in  FIG.  13 A . However, ultrasonic transceiver  25  similar to ultrasonic transceiver  23  can be configured without using joining material  41 . For example, as illustrated in  FIG.  13 B , ultrasonic transceiver  25  may be produced in a manner that acoustic matching body  24  similar to acoustic matching body  22  is used and closed-top tubular metal case  42  and acoustic matching body  24  are integrated so that the top face of closed-top tubular metal case  42  also serves as bottom plate  9  of acoustic matching body  24 . 
       FIG.  13 C  is a diagram illustrating a procedure for manufacturing ultrasonic transceiver  25  in the seventh embodiment by using cross-sectional views. 
     First, by using the procedure described using  FIG.  3   , metal plate  14   a  serving as top plate  8  is laminated on a plurality of metal plates  14   b  patterned with side wall  10  and perpendicular partition walls  12 . Furthermore, as illustrated in (a) of  FIG.  13 C , a metal plate produced to have a size corresponding to the shape of closed-top tubular metal case  42  is laminated as bottom plate  9 . Then, the laminated metal plates are integrated by diffusion joining. Next, illustrated in (a′) of  FIG.  13 C , bottom plate  9  is formed in the shape of closed-top tubular metal case  42  by pressing. At the same time, as illustrated in (b) of  FIG.  13 C , piezoelectric body  3  is prepared. Next, ultrasonic transceiver  25  is produced by using the procedure illustrated in (c), (d), and (e) of  FIG.  13 C . Note that (c), (d), and (e) of  FIG.  13 C  are the same as (c), (d), and (e) of  FIG.  11   , and therefore descriptions thereof will be omitted. 
     Eighth Embodiment 
     Hereinafter, an eighth embodiment will be described using  FIG.  14 A  and  FIG.  14 B . 
     8-1. Configuration 
     Acoustic matching body  26  described in the present embodiment is different only in the internal structure from acoustic matching body  2  described in the first embodiment, and has substantially the same configuration as that of acoustic matching body  2 , except the internal structure. Furthermore, the configuration of the ultrasonic transceiver in the present embodiment is also the same as that in the first, sixth, and seventh embodiments, and therefore descriptions thereof will be omitted. 
     Next, the internal structure of acoustic matching body  26  will be described using  FIG.  14 A . 
       FIG.  14 A  is a cross-sectional view of a configuration example of acoustic matching body  26  in the eighth embodiment. Note that (a) of  FIG.  14    is a cross-sectional view (a cross-sectional view in the X-Z plane) taken along the thickness direction (parallel to the Z-axis) of acoustic matching body  26 . Furthermore, (b) of  FIG.  14    is a cross-sectional view taken along line XA-XA illustrated in (a) of  FIG.  14   (A), in other words, a cross-sectional view (a cross-sectional view in the X-Y plane) taken along a direction (parallel to the X-Y plane) perpendicular to the thickness direction of acoustic matching body  26 . 
     As illustrated in  FIG.  14 A , acoustic matching body  26  according to the present disclosure includes top plate  8 , bottom plate  9 , side wall  10 , perpendicular partition walls  12 , and horizontal partition walls  39 . In acoustic matching body  26 , closed space  11  is defined by top plate  8 , bottom plate  9 , and side wall  10 . Furthermore, in acoustic matching body  26 , inside closed space  11 , perpendicular partition walls  12  are formed substantially perpendicularly to top plate  8  and bottom plate  9  (extends in substantially parallel to the Z-axis), meanwhile horizontal partition walls  39  are formed substantially horizontally to top plate  8  and bottom plate  9  (extends in substantially parallel to the X-Y plane). Perpendicular partition walls  12  are formed to adhere to top plate  8  and bottom plate  9 , thereby dividing closed space  11  into a plurality of closed spaces when acoustic matching body  26  is viewed from above (in parallel to the Z-axis). Horizontal partition walls  39  are formed to adhere to side wall  10  and perpendicular partition walls  12 , thereby dividing closed space  11  into upper and lower parts (along the Z-axis) when acoustic matching body  26  is viewed horizontally (in parallel to the X-axis and the Y-axis). Furthermore, when perpendicular partition walls  12  are viewed from above (in parallel to the Z-axis), the thickness of the partition walls (hereinafter, simply referred to as “the thickness”) is thinner in top portion  28  of acoustic matching body  26  than in bottom portion  29  of acoustic matching body  26 . In other words, the thickness of perpendicular partition wall  12  is gradually thinner from the lower part (the bottom plate  9  side) to the upper part (the top plate  8  side). The closed spaces resulting from the division by perpendicular partition walls  12  are gradually larger from the lower portion (the bottom plate  9  side) to the upper portion (the top plate  8  side). 
     A procedure for manufacturing acoustic matching body  26  in the present embodiment is the same as the procedure for manufacturing acoustic matching body  20  described in the fourth embodiment by using  FIG.  7   , and therefore descriptions of the procedure will be omitted. Furthermore, a procedure for manufacturing the ultrasonic transceiver in the present embodiment is the same as the procedure for manufacturing ultrasonic transceiver  21  described in the sixth embodiment by using  FIG.  11   , and therefore descriptions thereof will be omitted. Furthermore, an operation of ultrasonic flowmeter  80 , an operation of ultrasonic flow velocimeter  81 , and an operation of ultrasonic densitometer  90  in the present embodiment are the same as the operations described in the second and third embodiments, and therefore, descriptions thereof will be omitted. 
     8-2. Effect 
     As described above, in the present embodiment, acoustic matching body  26  includes top plate  8 , bottom plate  9 , and side wall  10  that define closed space  11 , and further includes: perpendicular partition walls  12  formed substantially perpendicularly to top plate  8  and bottom plate  9  inside closed space  11 ; and horizontal partition walls  39   formed horizontally to top plate  8  and bottom plate  9  inside closed space  11 . Perpendicular partition walls  12  are formed to adhere to top plate  8  and bottom plate  9 , thereby dividing closed space  11 , meanwhile horizontal partition walls  39  are formed to adhere to side wall  10  and perpendicular partition walls  12 , thereby dividing closed space  11  into upper and lower parts (along the Z-axis). In addition, perpendicular partition walls  12  are formed to be gradually thinner in top portion  28  of acoustic matching body  2  than in bottom portion  29  of acoustic matching body  2 . 
     As described in “Underlying Knowledge Forming Basis of the Present Disclosure”, for the purpose of the efficient propagation of ultrasonic waves through a measurement target fluid, it is most efficient that an acoustic impedance expressed by multiply a density of the acoustic matching body by an acoustic velocity is continuously reduced toward an ultrasonic propagation direction. In the present embodiment, the manufacturing method in which metal plates are freely patterned and laminated as described in the first and seventh embodiments is selected, whereby the thickness of perpendicular partition walls  12  formed substantially perpendicularly to top plate  8  and bottom plate  9  can be arbitrarily controlled, depending on a perpendicular position (a position on the Z-axis). Thus, the apparent density of acoustic matching body  26  can be successively reduced in the ultrasonic propagation direction. Hence, this allows a designed acoustic impedance of acoustic matching body  26  to be closer to a theoretical value. As a result, the efficiency of propagation of ultrasonic waves from the ultrasonic transceiver using acoustic matching body  26  into a measurement target fluid can be enhanced. Thus, while a decrease in the efficiency of propagation of ultrasonic waves from the ultrasonic transceiver into the measurement target fluid is substantially prevented, the resistance of the ultrasonic transceiver to a high-temperature and high-humidity environment in which the acoustic matching body easily corrodes can be enhanced. 
       FIG.  14 B  is a cross-sectional view of another configuration example of the acoustic matching body in the eighth embodiment. (a) of  FIG.  14 B  is a cross-sectional view of acoustic matching body  27  taken along the thickness direction (parallel to the Z-axis), and (b) of  FIG.  14 B  is a cross-sectional view taken along line XB-XB in (a) of  FIG.  14 B . For example, as illustrated in  FIG.  14 B , acoustic matching body  27  may be configured such that horizontal partition walls  39  are eliminated from acoustic matching body  26  illustrated in  FIG.  14 A , and closed space  11  is divided only by perpendicular partition walls  12 . Also in this case, the same effects as those achieved by acoustic matching body  26  illustrated in  FIG.  14 A  can be obtained. 
     Note that the above-described embodiments are merely for exemplifying the technology of the present disclosure, and therefore, the embodiments may be subjected to various modifications, substitutions, additions, omissions, and the likes within the scope of the claims and their equivalents. 
     Industrial Applicability 
     The present disclosure is applicable to an ultrasonic flowmeter, an ultrasonic flow velocimeter, and an ultrasonic densitometer that are respectively configured to measure the flow rate, the flow velocity, and the concentration of gas. Specifically, the present disclosure is applicable to, for example, a home flowmeter, a medical anesthetic gas densitometer, and a hydrogen densitometer for fuel cells. 
     Reference Marks in the Drawings 
       1 ,  16 ,  17 ,  21 ,  23 ,  25 ,  32 ,  33 ,  51 ...ultrasonic transceiver  2 ,  20 ,  22 ,  24 ,  26 ,  27 ...acoustic matching body
       3 ...piezoelectric body     8 ...top plate     9 ...bottom plate     10 ...side wall     11 ...closed space     12 ...perpendicular partition wall     13 ,  14   a ,  14   b ...metal plate     15 ...flow path     18 ...clocking device     19 ...calculator     39 ...horizontal partition wall     30 ...casing     31 ... vent     34 ...temperature sensor     35 ...clocking device     36 ...calculator     40 ,  41 ...joining material     42 ...closed-top tubular metal case     42   a ...top inner wall     42   b ...top outer wall     43 ...terminal plate     44 ,  45 ...terminal     46 ...through-hole     47 ...conductive rubber     48 ...piezoelectric body joint projected plane     49 ...side wall joint projected plane     52 ...dense portion     53 ...recessed portion     54 ...ultrasonic wave source     55 ...joint face     56 ...oscillating face     60 ..sound matching layer     61 ...one main face     62 ...edge portion     63 ... case     64 ...another main face     65 ... first water-proof member     66 ,  69 ...side face     67 ... second water-proof member     68 ...vicinity of edge portion     70 ,  74 ...porous body     72 ...dense layer     73 ...ultrasonic radiation face     75 ...side wall member     80 ...ultrasonic flowmeter     81 ...ultrasonic flow velocimeter     90 ...ultrasonic densitometer