Patent Publication Number: US-2023152175-A1

Title: Pressure transmitter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage filing under 35 U.S.C. 371 of International Application No. PCT/KR2021/004248, filed on Apr. 6, 2021, which claims the benefit of earlier filing date of and rights of priority to Korean Application 10-2020-0046064 filed on Apr. 16, 2021, the contents of which are all hereby incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a pressure transmitter, and more particularly, to a pressure transmitter having a structure capable of correcting a measurement error caused by thermal expansion of a heat transfer fluid. 
     BACKGROUND ART 
     Facilities such as nuclear power systems are operated in a range of several hundred degrees from 200° C. to 700° C., and in such facilities, substances in a liquid state in a wide range of temperature under normal pressure are used as a working fluid. For example, liquid sodium having a melting point of 98° C. and a boiling point of 890° C. may be used as a working fluid. 
     In order to recognize states of such systems, pressures such as differential pressure and static pressure of the working fluid need to be measured. However, as a sensor of a pressure gauge is not capable of withstanding high temperature of the working fluid, the pressure of the working fluid is measured by a pressure transmitter. 
     In addition, the pressure transmitter is configured to transmit the pressure of the working fluid to the pressure gauge through a separate medium fluid, in case the working fluid solidifies at room temperature. 
     The medium fluid of such a pressure transmitter is pressurized by the working fluid, and the pressure of the working fluid is sensed by a pressure sensor provided in the medium fluid. 
     Meanwhile, in order for facilities to operate stably, the pressure of the working fluid needs to be measured with a high level of precision. However, a temperature of the working fluid may change in a range of several hundred degrees, and a temperature of the medium fluid adjacent to the working fluid also changes in a range of several hundred degrees. When the temperature of the medium fluid changes in such a wide range, a volume of the medium fluid itself changes according to the change of the temperature. 
     A flexible member for transmitting pressure such as a diaphragm is provided between the medium fluid and the working fluid, and the flexible member may be elastically deformed to be curved as the medium fluid is thermally expanded by the change of the temperature. 
     However, a force to be restored to its original state is generated in the flexible member that is elastically deformed to be curved, and the medium fluid is pressurized by the flexible member as much as the generated restoring force. 
     In other words, since the restoring force is applied to the medium fluid in addition to the pressure of the working fluid, a measurement error is generated. 
     When a size of the flexible member is increased, the above-described measurement error can be reduced, but in this case, the size of the flexible member may be excessively increased. 
     Prior art document (U.S. Patent Publication No. U.S. Pat. No. 7,624,642 B2) discloses a differential pressure transmitter. 
     However, there is a limitation in that the prior art document does not disclose a solution for correcting an error that may be generated by an elastic force of the flexible member. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         (Patent Document 1) Prior Art Document: U.S. Patent Publication No. U.S. Pat. No. 7,624,642 B2 (Dec. 1, 2009) 
       
    
     DISCLOSURE 
     Technical Problem 
     The present disclosure is directed to providing a pressure transmitter having a structure capable of solving the above problems. 
     First, an aspect of the present disclosure is to provide a pressure transmitter having a structure capable of correcting a measurement error caused by thermal expansion of a medium fluid. 
     Another aspect of the present disclosure is to provide a pressure transmitter having a structure capable of correcting a measurement error caused by thermal expansion of a medium fluid while not excessively increasing a size of a flexible member. 
     In addition, an aspect of the present disclosure is to provide a pressure transmitter having a structure capable of correcting a measurement error caused by thermal expansion of a medium fluid even when a variable range of a temperature of a working fluid is large. 
     Technical Solution 
     In order to achieve the above aspects, a pressure transmitter according to an embodiment of the present disclosure includes a pressure measuring chamber filled with a medium fluid for pressure transmission and pressurized by a target fluid that is a target of a pressure measurement. 
     One side of the pressure measuring chamber is opened toward the target fluid, and the medium fluid in the pressure measuring chamber and the target fluid pressurize each other with a flexible member interposed therebetween. 
     The flexible member is made of a material having thermal conductivity and elasticity, and the medium fluid is thermally expanded by heat transferred from the target fluid. 
     In addition, the pressure transmitter is provided with a reference chamber. 
     The reference chamber is configured same as the pressure measuring chamber, but one side of the reference chamber is not opened toward the target fluid flowing through a pipe, but is opened toward an external fluid outside the pipe. 
     In addition, the reference chamber is configured to receive heat from the target fluid. Accordingly, the medium fluid in the reference chamber is thermally expanded by the heat transferred from the target fluid. 
     A degree in which a thermal energy is distributed in the medium fluid in the reference chamber and a degree in which a thermal energy is distributed in a medium fluid in the pressure measuring chamber are preferably similar to each other. 
     Specifically, a temperature of the medium fluid in the reference chamber and a temperature of the medium fluid in the pressure measuring chamber are preferably similar at a portion adjacent to a portion at which the flexible member is pressurized. 
     The pressure measuring chamber and the reference chamber are each provided with a pressure measuring sensor, and pressure values sensed by each of the pressure measuring sensors are transmitted to a controller so as to be used to derive a pressure of the target fluid. 
     A measurement error that may be generated by thermal expansion can be corrected by subtracting a pressure value measured in the reference chamber from a pressure value measured in the pressure measuring chamber. 
     In addition, a pressure transmitter according to an embodiment of the present disclosure includes: a first pressure measuring space with opened one side and filled with a pressure transmitting fluid therein; a first flexible member covering the opened one side of the first pressure measuring space; a second pressure measuring space with opened one side and filled with a pressure transmitting fluid therein; a second flexible member covering the opened one side of the second pressure measuring space; a first pressure sensor provided in the first pressure measuring space to measure a pressure of the pressure transmitting fluid in the first pressure measuring space; a second pressure sensor provided in the second pressure measuring space to measure a pressure of the pressure transmitting fluid in the second pressure measuring space; and a controller electrically connected to the first pressure sensor and the second pressure sensor, and configured to calculate a pressure of a target fluid that is a target of a pressure measurement by using a pressure value sensed by the first pressure sensor and a pressure value sensed by the second pressure sensor. 
     Further, the first flexible member is disposed to be pressurized by the target fluid, and the second flexible member is disposed to be pressurized by an external fluid. 
     In addition, the first flexible member is configured to be curved toward the target fluid as the pressure transmitting fluid in the first pressure measuring space is thermally expanded, and the second flexible member is configured to be curved toward the external fluid as the pressure transmitting fluid in the second pressure measuring space is thermally expanded. 
     In addition, the opened one side of the second pressure measuring space and the opened one side of the first pressure measuring space are in opposite directions. 
     In addition, the opened one side of the first pressure measuring space and the opened one side of the second pressure measuring space are both provided in one member having thermal conductivity. 
     In addition, a pressure transmitter according to an embodiment of the present disclosure includes: a coupling portion inserted into a pipe through which a target fluid, which is a target of a pressure measurement, flows, and disposed to be partially contactable with the target fluid; a first pressure chamber connected to a portion other than the portion partially contactable with the target fluid in the coupling portion, extending in a direction away from the target fluid, and provided therein with a first measuring space in which one side thereof is opened toward the coupling portion; and a second pressure chamber connected to a portion other than the portion partially contactable with the target fluid in the coupling portion, extending in a direction away from the target fluid, and provided therein with a second measuring space in which one side thereof is opened toward the coupling portion. 
     In addition, the coupling portion is provided with a first connecting space, in which one end thereof is opened toward the target fluid and another end thereof communicates with the opened one side of the first measuring space, formed therethrough, and a second connecting space, in which one end thereof is opened toward an external fluid that flows outside the pipe and another end thereof communicates with the opened one side of the second measuring space, formed therethrough. 
     In addition, the opened one end of the first connecting space is covered by a first flexible member, and the opened one end of the second connecting space is covered by a second flexible member. The first measuring space and the second measuring space, and the first connecting space and the second connecting space are filled with pressure transmitting fluids. 
     In addition, the opened one end of the second connecting space and the opened one end of the first connecting space are in opposite directions. In addition, the pressure transmitter includes: a first pressure sensor provided in the first measuring space to measure a pressure of the pressure transmitting fluid in the first measuring space; a second pressure sensor provided in the second measuring space to measure a pressure of the pressure transmitting fluid in the second measuring space; and a controller electrically connected to the first pressure sensor and the second pressure sensor, and configured to calculate a pressure of the target fluid by using a pressure value sensed by the first pressure sensor and a pressure value sensed by the second pressure sensor. 
     Advantageous Effects 
     According to an embodiment of the present disclosure, the following effects can be achieved. 
     First, a pressure measuring portion pressurized by a target fluid that is a target of a pressure measurement and a reference pressure measuring portion pressurized by an external fluid outside a pipe are provided. 
     In addition, a part of the pressure measuring portion that is pressurized by the target fluid and a part of the reference pressure measuring portion that is pressurized by the external fluid are provided inside one thermally conductive member. 
     Accordingly, at the portion pressurized by the target fluid and at the portion pressurized by the external fluid, a difference in temperature between the pressure measuring portion and the reference pressure measuring portion may be insignificantly small. 
     Therefore, a difference between a pressure in which a medium fluid in the pressure measuring portion pressurizes the flexible member by thermal expansion and a pressure in which a medium fluid in the reference pressure measuring portion pressurizes the flexible member by thermal expansion may be insignificantly small. 
     As a result, an accuracy of an operation of correcting an error that may be generated by thermal expansion of the medium fluid in the pressure measuring portion can be improved. 
     In addition, since the portion of the pressure measuring portion that is pressurized by the target fluid and the portion of the reference pressure measuring portion that is pressurized by the external fluid are both disposed close to the target fluid, a difference between a temperature at the pressure measuring portion and a temperature at the reference pressure measuring portion may be extremely small even when the temperature of the target fluid changes. 
     Therefore, even when the temperature of the target fluid changes, a degree of thermal expansion of the flexible member and the medium fluid in the pressure measuring portion and a degree of thermal expansion of the flexible member and the medium fluid in the reference pressure measuring portion are similar to each other. 
     Therefore, even when the temperature of the target fluid is changes, a pressure applied to the flexible member of the pressure measuring portion and a pressure applied to the flexible member of the reference pressure measuring portion are similar to each other. 
     As a result, even when the temperature of the target fluid changes, an error caused by thermal expansion can be accurately corrected corresponding to the change of the temperature. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG.  1    is a conceptual view of a pressure transmitter according to an embodiment of the present disclosure. 
         FIG.  2    is a perspective view of a pressure measuring portion according to an embodiment of the present disclosure. 
         FIG.  3    is a planar view of the pressure measuring portion according to  FIG.  2   . 
         FIG.  4    is a cross-sectional view illustrating the pressure measuring portion according to  FIG.  3    taken along line IV-IV. 
         FIG.  5    is an enlarged partial cross-sectional view illustrating an area A of  FIG.  4   . 
         FIG.  6    is a conceptual view illustrating a process of a flexible member being curved in a coupling portion according to  FIG.  4   . 
     
    
    
     BEST MODE 
     Hereinafter, description will be given in detail of a pressure transmitter in accordance with an embodiment disclosed herein, with reference to the accompanying drawings. 
     In the following description, description of several components will be omitted in order to clarify the technical features of the present disclosure. 
     1. Definition of Terms 
     The term “energization” used in the following description means that one component is electrically connected to another component or is connected to enable information communication. The energization may be implemented by conducting wires, communication cables, or the like. 
     The term “forward” used below refers to a direction toward a target fluid TF flowing through a pipe  1000 . 
     The term “rearward” used below refers to a direction away from the target fluid TF flowing through the pipe  1000 . 
     2. Description of a Pressure Transmitter  1  in Accordance with an Embodiment of the Present Disclosure 
     Referring to  FIG.  1   , a pressure transmitter  1  according to an embodiment of the present disclosure is coupled to the pipe  1000  through which the target fluid TF flows to measure a pressure of the target fluid TF. 
     In the illustrated embodiment, the target fluid TF flows in a hollow pipe  1000 , but the embodiment is not limited thereto. 
     In an embodiment not illustrated, the target fluid TF may flow in various accommodating spaces that may be used in a nuclear power plant instead of the pipe  1000 . 
     In addition, in an embodiment not illustrated, the pipe  1000  through which the target fluid TF flows may be formed such that one side thereof is closed. Accordingly, a dynamic pressure of the target fluid TF flowing through the pipe  1000  can be removed. 
     The pressure transmitter  1  includes a pressure measuring portion  10 , a controller  20 , and a display  30 . 
     (1) Description of the Pressure Measuring Portion  10   
     The pressure measuring portion  10  is inserted into an opened portion of the pipe  1000 . A part of the pressure measuring portion  10  inserted into the pipe  1000  is pressurized by the target fluid TF flowing through the pipe  1000 . 
     The pressure measuring portion  10  includes a first pressure measuring space V1 filled with a first pressure transmitting fluid F1 and a second pressure measuring space V2 filled with a second pressure transmitting fluid F2. 
     In an embodiment, the first pressure transmitting fluid F1 and the second pressure transmitting fluid F2 may be metals capable of maintaining a liquid state within a variable temperature range of a system. 
     In an embodiment, the first pressure transmitting fluid F1 and the second pressure transmitting fluid F2 may be a molten metal of at least one selected from alkali metal containing sodium (Na), sodium-potassium (Na—K) alloy, lead-bismuth (Pb—Bi) alloy, and lead (Pb). 
     In an embodiment, for accuracy of error correction, the first pressure transmitting fluid F1 and the second pressure transmitting fluid F2 may be metals identical to each other. In addition, for accuracy of error correction, a volume of the first pressure transmitting fluid F1 and a volume of the second pressure transmitting fluid F2 may be same as each other. 
     One side of the first pressure measuring space V1 is opened toward an inner side of the pipe  1000  so as to be in contact with the target fluid TF. The first pressure measuring space V1 extends in a direction away from the target fluid TF. 
     The opened one side of the first pressure measuring space V1 is covered by a first flexible member  191 . The first pressure measuring space V1 is sealed by the first flexible member  191 . 
     The first flexible member  191  may be formed in a shape and a material that can be elastically deformed by being pressurized by the first pressure transmitting fluid F1 or the target fluid TF. 
     One side of the second pressure measuring space V2 is opened toward an outer side of the pipe  1000  so as to be in contact with a fluid outside the pipe  1000 . The opened one side of the second pressure measuring space V2 and the opened one side of the first pressure measuring space V1 are in opposite directions. 
     In the illustrated embodiment, the second pressure measuring space V2 is opened toward the outer side of the pipe  1000 , but the embodiment is not limited thereto. 
     In an embodiment not illustrated, the opened one side of the second pressure measuring space V2 may be formed in various positions that may be opened toward the fluid outside the pipe  1000 . 
     The opened one side of the second pressure measuring space V2 is covered by a second flexible member  192 . The second pressure measuring space V2 is sealed by the second flexible member  192 . 
     The second flexible member  192  may be formed in a shape and a material that can be elastically deformed by being pressurized by the second pressure transmitting fluid F2 or the fluid outside the pipe  1000 . 
     In an embodiment, the first flexible member  191  and the second flexible member  192  may be a flexible film such as a membrane and a diaphragm. 
     In an embodiment, the shape and material of the first flexible member  191  and the shape and material of the second flexible member  192  may be same as each other. 
     The first pressure measuring space V1 extends in a rear-to-front direction, and a front end portion of the first pressure measuring space V1 is opened toward the target fluid TF. 
     The second pressure measuring space V2 extends by a predetermined distance in the rear-to-front direction and then bent rearward. One end portion of the bent second pressure measuring space V2 is opened rearward. 
     In other words, the opened one end of the first pressure measuring space V1 and the opened one end of the second pressure measuring space V2 are in opposite directions. 
     In addition, the first flexible member  191  and the second flexible member  192  are disposed to overlap each other in a direction in which the pressure measuring portion  10  is inserted. 
     In addition, a value of a shortest distance between the first flexible member  191  and the second flexible member  192  may be equal to or smaller than a value of a thickness of the pipe  1000 . 
     The target fluid TF pressurizes the first pressure transmitting fluid F1 through the first flexible member  191 . In other words, the pressure of the target fluid TF may be transmitted to the first pressure transmitting fluid F1. 
     In addition, the first flexible member  191  is a thermally conductive member, whereby heat may be transferred from the target fluid TF to the first pressure transmitting fluid F1. 
     Accordingly, the first pressure transmitting fluid F1 is expanded by the heat, and the first flexible member  191  is elastically deformed by the expansion of the first pressure transmitting fluid F1 and is curved toward the target fluid TF. 
     Since the elastically deformed first flexible member  191  generates a restoring force to return to its original state, the first pressure transmitting fluid F1 is pressurized by the restoring force of the first flexible member  191 . 
     In other words, the first pressure transmitting fluid F1 is pressurized by the target fluid TF and the first flexible member  191 . 
     The second pressure transmitting fluid F2 is also expanded by the heat transferred from the target fluid TF. 
     Accordingly, the second flexible member  192  is elastically deformed by the expansion of the second pressure transmitting fluid F2 and is curved toward the fluid outside the pipe  1000 . 
     Since the elastically deformed second flexible member  192  generates a restoring force to return to its original state, the second pressure transmitting fluid F2 is pressurized by the restoring force of the second flexible member  192 . 
     In other words, the second pressure transmitting fluid F2 is pressurized by the second flexible member  192 . 
     Since the restoring force due to thermal expansion is applied to the first pressure transmitting fluid F1 and the second pressure transmitting fluid F2, a measurement error due to the thermal expansion may be corrected by subtracting a pressure value measured in the second pressure transmitting fluid from a pressure value measured in the first pressure transmitting fluid. 
     However, for accurate error correction, it is preferable that the restoring force generated in the first flexible member  191  and the restoring force generated in the second flexible member  192  are the same. 
     However, when a degree of the thermal expansion of the first pressure transmitting fluid F1 at a portion where the first flexible member  191  is disposed and a degree of the thermal expansion of the second pressure transmitting fluid F2 at a portion where the second flexible member  192  is disposed are different, a magnitude of the restoring force generated by the elastic deformation of the first flexible member  191  and a magnitude of the restoring force generated by the elastic deformation of the second flexible member  192  may be different. 
     In addition, when the size and material of the first flexible member  191  and the size and material of the second flexible member  192  are different, the magnitude of the restoring force generated by the elastic deformation of the first flexible member  191  and the magnitude of the restoring force generated by the elastic deformation of the second flexible member  192  may be different. 
     Therefore, the size and material of the first flexible member  191  and the size and material of the second flexible member  192  are preferably same as each other. 
     In addition, both the first flexible member  191  and the second flexible member  192  are preferably disposed adjacent to the target fluid TF. 
     As described above, the value of the shortest distance between the first flexible member  191  and the second flexible member  192  may be equal to or smaller than a value of the thickness of the pipe  1000 . 
     Accordingly, a difference between a temperature at a portion where the first flexible member  191  is disposed and a temperature at a portion where the second flexible member  192  is disposed may be insignificantly small. 
     The first pressure measuring space V1 is provided therein with a first pressure sensor  181 , and the second pressure measuring space V2 is provided therein with a second pressure sensor  182 . 
     The first pressure sensor  181  is configured to measure a pressure of the first pressure transmitting fluid F1. And, the second pressure sensor  182  is configured to measure a pressure of the second pressure transmitting fluid F2. 
     The first pressure sensor  181  senses information about a pressure of the target fluid TF and a pressure generated by the elastic deformation of the first flexible member  191 , and the second pressure sensor  182  senses information about a pressure generated by the elastic deformation of the second flexible member  192 . 
     The first pressure sensor  181  and the second pressure sensor  182  are each electrically connected to the controller  20 . The information sensed by the first pressure sensor  181  and the information sensed by the second pressure sensor  182  are transmitted to the controller  20 . 
     (2) Description of the Controller  20  and the Display  30   
     The controller  20  may be implemented in any form capable of inputting, outputting, and calculating information. In an embodiment, the controller  20  may be implemented in a form of a microprocessor, a central processing unit (CPU), a printed circuit board (PCB), or the like. 
     The controller  20  calculates information about the pressure of the target fluid TF in a predetermined method using each of the received information. The controller  20  may include a calculating portion that calculates information about the pressure. 
     In an embodiment, the predetermined method may be a method for subtracting a pressure value received by the second pressure sensor  182  from a pressure value received by the first pressure sensor  181 . 
     However, the embodiment is not limited thereto, and a calculation for correcting an additional error that may be generated according to a design condition of the pressure transmitter  1  may be added. 
     Information about the pressure of the target fluid TF calculated by the controller  20  is transmitted to the display  30 . 
     The display  30  converts the received information into information in a visual form and outputs the converted information. 
     In an embodiment, the display  30  may be an interface (not illustrated) having a display provided in the pressure transmitter  1 . A user recognizes information about the pressure value of the target fluid TF through the information outputted on the display  30 . 
     2. Description of the Pressure Measuring Portion  10  According to an Embodiment of the Present Disclosure 
       FIGS.  2  to  5    illustrate the pressure measuring portion  10  according to an embodiment. 
     The pressure measuring portion  10  according to this embodiment measures the pressure of the target fluid TF through error correction as described above. 
     The pressure measuring portion  10  includes a coupling portion  110  inserted into a portion formed through the pipe  1000 , a first pressure chamber  140  and a second pressure chamber  150  coupled to the coupling portion  110 , a hollow main body  160  surrounding the first pressure chamber  140  and the second pressure chamber  150 , and a rear body  170  coupled to the hollow main body  160 . 
     (1) Description of the Coupling Portion  110  and the Flexible Members  191  and  192   
     The coupling portion  110  is partially inserted into the pipe  1000  through which the target fluid TF flows, and a part of the coupling portion  110  is brought into contact with the target fluid TF. A front surface of the coupling portion  110  is brought into contact with the target fluid TF. 
     The coupling portion  110  includes a first connecting space filled with the first pressure transmitting fluid F1 and a second connecting space filled with the second pressure transmitting fluid F2. 
     The first connecting space includes a first curved passage  121  and a first straight passage  131  to be described later, and the second connecting space includes a second curved passage  122  and a second straight passage  132  to be described later. 
     The coupling portion  110  includes a first coupling portion  120  and a second coupling portion  130  that are sequentially arranged in a direction away from the target fluid TF. 
     The first coupling portion  120  is entirely inserted into the pipe  1000 , and the second coupling portion  130  is partially inserted into the pipe  1000 . 
     The first curved passage  121  and the second curved passage  122  are formed in the first coupling portion  120 . 
     One end of the first curved passage  121  is disposed at a front surface of the first coupling portion  120  and another end of the first curved passage  121  is disposed at a rear surface of the first coupling portion  120 . The first curved passage  121  is formed such that the opened one end and the opened another end communicate with each other. In an embodiment, the one end of the first curved passage  121  may be disposed at a central portion of the front surface, and the another end of the first curved passage  121  may be disposed adjacent to a circumference of the rear surface. 
     A first groove  123  is formed to be recessed at a portion where the one end of the first curved passage  121  is disposed. In other words, the one end of the first curved passage  121  is disposed at a central portion of the first groove  123 . 
     The first flexible member  191  is coupled to the first groove  123  to cover the opened one end of the first curved passage  121 . The first flexible member  191  has been described above, and will not be repeated. 
     A circumference of a rear surface of the first flexible member  191  is coupled to the first groove  123 . Accordingly, an inner portion of a circumference of the first flexible member  191  may be elastically deformed to be spaced apart from the first groove  123 . 
     One end and another end of the second curved passage  122  are both disposed at the rear surface of the first coupling portion  120 . The second curved passage  122  is formed such that the opened one end and the opened another end communicate with each other. In other words, the second curved passage  122  extends in the rear-to-front direction by a predetermined distance from the rear surface of the first coupling portion  120 , and then is bent to extend rearward. 
     In an embodiment, the one end of the second curved passage  122  may be disposed at a central portion of the rear surface of the first coupling portion  120 , and the another end of the second curved passage  122  may be disposed adjacent to the circumference of the rear surface. 
     In an embodiment, the one end of the second curved passage  122  may be disposed between the another end of the second curved passage  122  and the another end of the first curved passage  121 . 
     A second groove  124  is formed to be recessed at a portion where the one end of the second curved passage  122  is disposed. In other words, the one end of the second curved passage  122  is disposed at a central portion of the second groove  124 . 
     The second flexible member  192  is coupled to the second groove  124  to cover the opened one end of the second curved passage  122 . The second flexible member  192  has been described above, and will not be repeated. 
     A circumference of a front surface of the second flexible member  192  is coupled to the second groove  124 . Accordingly, an inner portion of a circumference of the second flexible member  192  may be elastically deformed to be spaced apart from the second groove  124 . 
     In an embodiment, the coupling portion  120  may be defined in a cylindrical shape. 
     The second coupling portion  130  is coupled to the rear surface of the first coupling portion  120 . 
     The second coupling portion  130  is provided with the first straight passage  131 , the second straight passage  132 , and an external fluid inflow passage  134  formed therethrough in a front-to-rear direction. 
     Opposite ends of the first straight passage  131  are opened to communicate with each other and opposite ends of the second straight passage  132  are opened to communicate with each other. One end of the first straight passage  131  is disposed at a front surface of the second coupling portion  130  and another end of the first straight passage  131  is disposed at a rear surface of the second coupling portion  130 . And, one end of the second straight passage  132  is disposed at the front surface of the second coupling portion  130  and another end of the second straight passage  132  is disposed at the rear surface of the second coupling portion  130 . 
     The one end of the first straight passage  131  is disposed at a position communicating with the another end of the first curved passage  121  when the first coupling portion  120  and the second coupling portion  130  are coupled. Accordingly, the first curved passage  121  and the first straight passage  131  communicate with each other. 
     The one end of the second straight passage  132  is disposed at a position communicating with the another end of the second curved passage  122  when the first coupling portion  120  and the second coupling portion  130  are coupled. Accordingly, the second curved passage  122  and the second straight passage  132  communicate with each other. 
     Opposite ends of the external fluid inflow passage  134  are opened to communicate with each other, wherein one end of the external fluid inflow passage  134  is disposed at the front surface of the second coupling portion  130  and another end of the external fluid inflow passage  134  is disposed at the rear surface of the second coupling portion  130 . 
     In addition, an external fluid inflow groove  133  is formed to be recessed at a portion where the one end of the external fluid inflow passage  134  is disposed. 
     The external fluid inflow groove  133  is covered by a part of the first coupling portion  120  where the second flexible member  192  is installed. An external fluid inflow space V3 is formed between the external fluid inflow groove  133  and the rear surface of the first coupling portion  120 . 
     The external fluid inflow space V3 is communicated with the outside of the pipe  1000  through the external fluid inflow passage  134 , and the fluid outside the pipe  1000  is introduced into the external fluid inflow space V3 through the external fluid inflow passage  134 . The external fluid pressurizes the second flexible member  192  with a predetermined pressure. The predetermined pressure may be atmospheric pressure. 
     The second flexible member  192  is disposed to overlap the external fluid inflow space V3 in a direction away from the target fluid TF. Accordingly, an entire rear surface of the second flexible member  192  may be pressurized by the external fluid. 
     The second flexible member  192  is disposed between the first coupling portion  120  and the second coupling portion  130 . In other words, the second flexible member  192  is disposed to be spaced apart in a forward direction by a predetermined distance from the rear surface of the second coupling portion  130 . 
     Accordingly, the second flexible member  192  can be disposed closer to the target fluid TF. In other words, a distance between the second flexible member  192  and the first flexible member  191  is reduced. 
     In an embodiment, a value of a distance between the first flexible member  191  and the second flexible member  192  may be smaller than the value of the thickness of the pipe  1000 . 
     As the distance between the first flexible member  191  and the second flexible member  192  is small, a difference between a temperature at a portion where the first flexible member  191  is disposed and a temperature at a portion where the second flexible member  192  is disposed may be reduced. 
     Specifically, as the value of the distance between the first flexible member  191  and the second flexible member  192  is smaller than the value of the thickness of the pipe  1000 , the difference between the temperature at the portion where the first flexible member  191  is disposed and the temperature at the portion where the second flexible member  192  is disposed may be insignificantly small. 
     As a result, the restoring force generated in the first flexible member  191  and the restoring force generated in the second flexible member  192  due to thermal expansion may be similar to each other. In other words, the accuracy in correcting errors caused by thermal expansion may be improved. 
     When the target fluid TF is constantly provided, errors can be corrected by continuously correcting a specified error value. However, when the temperature of the target fluid TF continuously changes, an error value to be corrected is changed, and this may cause a problem in applying a method for continuously correcting a specific value. 
     However, in the pressure measuring portion  10  according to this embodiment, even when the temperature of the target fluid TF changes, the temperatures at the portions where the first flexible member  191  and the second flexible member  192  are disposed also change similarly thereto. Accordingly, even when the temperature of the target fluid TF changes, an error caused by thermal expansion can be accurately corrected. 
     The second coupling portion  130  has a cylindrical shape, and an outer circumferential surface of the second coupling portion  130  has a flange portion  135  protruding therefrom in a circumferential direction. 
     A fastening member (not illustrated) passes through the flange portion  135  to be inserted into the pipe  1000 , and the coupling portion  110  is coupled to the pipe  1000  by a fastening force between the fastening member and the pipe  1000 . 
     The first pressure chamber  140  and the second pressure chamber  150  are coupled to the rear surface of the second coupling portion  130 . 
     (2) Description of the First Pressure Chamber  140  and the Second Pressure Chamber  150   
     A first measuring space  141  opened forward is formed in the first pressure chamber  140 , and a second measuring space  151  opened forward is formed in the second pressure chamber  150 . 
     The first measuring space  141  communicates with the first connecting space  121  and  131  to form the first pressure measuring space V1, and the second measuring space  151  communicates with the second connecting space  122  and  132  to form the second pressure measuring space V2. 
     The first pressure transmitting fluid F1 is filled in the first pressure measuring space V1, and the second pressure transmitting fluid F2 is filled in the second pressure measuring space V2. 
     The first pressure chamber  140  and the second pressure chamber  150  each may be formed in a hollow shape extending in the front-to-rear direction. 
     Specifically, an opened front end of the first pressure chamber  140  is coupled to the another end of the first straight passage  131  disposed at the rear surface of the second coupling portion  130 . A sealing structure may be provided at a portion where the front end of the first pressure chamber  140  and the another end of the first straight passage  131  are coupled to prevent a leakage of the first pressure transmitting fluid F1. 
     An opened front end of the second pressure chamber  150  is coupled to the another end of the second straight passage  132  disposed at the rear surface of the second coupling portion  130 . A sealing structure may be provided at a portion where the front end of the second pressure chamber  150  and the another end of the second straight passage  132  are coupled to prevent a leakage of the second pressure transmitting fluid F2. 
     The first pressure chamber  140  and the second pressure chamber  150  are surrounded by the hollow main body  160 . 
     (3) Description of the Hollow Main Body  160   
     The hollow main body  160  protects the first pressure chamber  140  and the second pressure chamber  150  from external impacts and dissipates heat from the first pressure chamber  140  and the second pressure chamber  150 . 
     The hollow main body  160  is formed in a hollow shape extending in the front-to-rear direction, and provided with a connecting portion  161  coupled to the rear surface of the second coupling portion  130 , and a heat dissipating portion  162  coupled to a rear end of the connecting portion  161 . 
     The connecting portion  161  is defined in a cylindrical shape with a rear edge thereof is chamfered, and has a hollow shape with opposite ends thereof are opened. However, the shape is not limited thereto, and may be formed in various shapes. 
     The heat dissipating portion  162  is defined in a hollow cylindrical shape extending in the front-to-rear direction. A plurality of heat dissipating holes  162   a  are formed through an outer circumferential surface of the heat dissipating portion  162 . As the heat dissipating holes  162   a  are formed, heat discharged from the first pressure chamber  140  and the second pressure chamber  150  can be smoothly dissipated outwardly of the heat dissipating portion  162 . 
     The rear body  170  is coupled to a rear end of the heat dissipating portion  162 . 
     (4) Description of the Rear Body  170   
     The rear body  170  may be provided to improve a reliability of the pressure transmitter  1 . 
     For accuracy of measurement, the first pressure chamber  140  and the second pressure chamber  150  are each defined in a tube shape having a small diameter. In this case, there exists a danger in which the first pressure chamber  140  and the second pressure chamber  150  are damaged by an external impact. In addition, since inner areas of the first pressure chamber  140  and the second pressure chamber  150  are small, cost of directly inserting the pressure sensors  181  and  182 , to be described later, into the first pressure chamber  140  and the second pressure chamber  150  may be increased. 
     With this reason, a first pressure sensor accommodating space  171  and a second pressure sensor accommodating space  172  are formed in the separate rear body  170  and are each connected to the first pressure chamber  140  and the second pressure chamber  150 . 
     The first pressure sensor accommodating space  171  and the second pressure sensor accommodating space  172  are formed at opposite sides of the rear body  170  in a direction intersecting the front-rear direction. 
     In addition, a first coupling hole  170   a  is formed through from an outer surface of the rear body  170  to the first pressure sensor accommodating space  171 , and a second coupling hole  170   b  is formed through from the outer surface of the rear body  170  to the second pressure sensor accommodating space  172 . 
     A rear side of the first pressure chamber  140  and a rear side of the second pressure chamber  150  exit outwardly of the hollow main body  160  through the heat dissipating holes  162   a , and then are bent to be inserted in the first coupling hole  170   a  and the second coupling hole  170   b , respectively. 
     A first coupling member  173  having a structure capable of being screwed to the first coupling hole  170   a  is coupled to a rear end of the first pressure chamber  140 . As the first coupling member  173  is coupled to the first coupling hole  170   a , the first measuring space  141  of the first pressure chamber  140  communicates with the first pressure sensor accommodating space  171 . The first pressure sensor accommodating space  171  is provided therein with the first pressure sensor  181 . 
     The first connecting space  111 , the first measuring space  141 , and the first pressure sensor accommodating space  171  communicate among others to form the first pressure measuring space V1. 
     The pressure of the target fluid TF applied to the first flexible member  191  provided in the first connecting space  111  and the pressure due to the elastic deformation of the first flexible member  191  are applied to the first pressure transmitting fluid F1. 
     The first pressure sensor  181  disposed in the first pressure sensor accommodating space  171  is pressurized by the first pressure transmitting fluid F1 to measure the pressure of the target fluid TF applied to the first flexible member  191  and the pressure applied by the elastic deformation of the first flexible member  191 . 
     A second coupling member  174  having a structure capable of being screwed to the second coupling hole  170   b  is coupled to a rear end of the second pressure chamber  150 . As the second coupling member  174  is coupled to the second coupling hole  170   b , the second measuring space  151  of the second pressure chamber  150  communicates with the second pressure sensor accommodating space  172 . The second pressure sensor accommodating space  172  is provided therein with the second pressure sensor  182 . 
     The second connecting space  112 , the second measuring space  151 , and the second pressure sensor accommodating space  172  communicate among others to form the second pressure measuring space V2. 
     The second pressure transmitting fluid F2 is pressurized by the elastic deformation of the second flexible member  192 . 
     The second pressure sensor  182  disposed in the second pressure sensor accommodating space  172  is pressurized by the second pressure transmitting fluid F2 to measure the pressure applied by the elastic deformation of the second flexible member  192 . 
     The first pressure sensor  181  and the second pressure sensor  182  are each preferably disposed at a rear side of the first pressure measuring space V1 and a rear side of the second pressure measuring space V2 in a direction away from the target fluid TF. 
     When the first pressure sensor  181  and the second pressure sensor  182  are disposed to be close to the target fluid TF, the first pressure sensor  181  and the second pressure sensor  182  may be damaged by excessive heat. 
     Damage on the first pressure sensor  181  and the second pressure sensor  182  due to excessive heat can be suppressed by disposing the first pressure sensor  181  and the second pressure sensor  182  in the rear body  170  and separating them from the target fluid TF. 
     The first pressure sensor  181  and the second pressure sensor  182  are electrically connected to the controller  20  (see  FIG.  1   ) disposed outside the rear body  170 . 
     The controller  20  calculates information about the pressure of the target fluid TF in a predetermined method by using information detected by the first pressure sensor  181  and the second pressure sensor  182 . 
     A method for calculating pressure information about the target fluid TF by the controller  20  has been described above, and will not be repeated. 
     The pressure information about the target fluid TF calculated by the controller  20  is converted into a visual form so as to be outputted on the display  30 . 
     3. Description of an Effect of a Measurement Structure According to an Embodiment of the Present Disclosure 
     Referring to  FIG.  6   , the first flexible member  191  is elastically deformed to be curved due to thermal expansion of the first pressure transmitting fluid F1, and the second flexible member  192  is elastically deformed to be curved due to thermal expansion of the second pressure transmitting fluid F2. 
     Since a force of the elastically deformed first flexible member  191  to be restored to its original state is applied to the first pressure transmitting fluid F1, an error may be generated when measuring the pressure of the target fluid TF. 
     The error may be corrected by measuring the force of the elastically deformed second flexible member  192  to be restored to its original state, and then subtracting the measured force value from a pressure value measured by the first pressure sensor  181 . 
     For accuracy of correction, it is preferable that a difference between the restoring force of the first flexible member  191  and the restoring force of the second flexible member  192  is as small as possible. 
     The structure of the pressure transmitter  1  according to this embodiment has an effect of forming as small as possible the difference between the restoring force of the first flexible member  191  and the restoring force of the second flexible member  192 . 
     Specifically, the effect is as follows. 
     First, the first pressure measuring space V1 pressurized by the target fluid TF that is a target of a pressure measurement and the second pressure measuring space V2 pressurized by the fluid outside the pipe  1000  are provided. 
     In addition, a portion of the first pressure measuring space V1 that is pressurized by the target fluid TF and a portion of the second pressure measuring space V2 that is pressurized by the external fluid are both formed inside the first coupling portion  120 . 
     Accordingly, at the portion pressurized by the target fluid TF and at the portion pressurized by the external fluid, a difference in temperature between the first pressure measuring space V1 and the second pressure measuring space V2 may be insignificantly small. 
     Therefore, a difference between a pressure in which the first pressure transmitting fluid F1 of the first pressure measuring space V1 pressurizes the first flexible member  191  by thermal expansion and a pressure in which the second pressure transmitting fluid F2 of the second pressure measuring space V2 pressurizes the second flexible member  192  by thermal expansion may be insignificantly small. 
     As a result, an accuracy of an operation of correcting an error that may be generated by thermal expansion of the first pressure transmitting fluid F1 in the first pressure measuring space V1 can be improved. 
     In addition, since a portion of the first pressure measuring space V1 that is pressurized by the target fluid TF and a portion of the second pressure measuring space V2 that is pressurized by the external fluid are both disposed close to the target fluid TF, a difference between a temperature at the first pressure measuring space V1 and a temperature at the second pressure measuring space V2 may be extremely small even when the temperature of the target fluid TF changes. 
     Therefore, even when the temperature of the target fluid TF changes, a degree of thermal expansion of the first flexible member  191  and the first pressure transmitting fluid F1 in the first pressure measuring space V1 and a degree of thermal expansion of the second flexible member  192  and the second pressure transmitting fluid F2 in the second pressure measuring space V2 are similar to each other. 
     Therefore, even when the temperature of the target fluid TF changes, a pressure applied to the first flexible member  191  of the first pressure measuring space V1 and a pressure applied to the second flexible member  192  of the second pressure measuring space V2 are similar to each other. 
     As a result, even when the temperature of the target fluid TF changes, an error caused by thermal expansion can be accurately corrected corresponding to the change of the temperature. 
     Although the foregoing description has been given with reference to the preferred embodiment, it will be understood that those skilled in the art will be able to variously modify and change the present disclosure without departing from the scope of the disclosure described in the claims below.