Patent Publication Number: US-2022236127-A1

Title: Pressure sensing device and blood purification apparatus using same

Description:
TECHNICAL FIELD 
     The present invention relates to a pressure sensing device and a blood purification apparatus using the same. 
     BACKGROUND 
     In blood purification apparatuses, pressure sensing devices are used to measure liquid pressure of, e.g., blood or dialysate, etc. It is not preferable that blood and dialysate, etc., whose pressure is detected in the blood purification apparatuses, come into contact with air. For this reason, a pressure sensing device, which is configured to use a case (a pressure detection container) with a diaphragm provided so as to partition between a first space, through which a liquid whose pressure is to be detected, such as blood or dialysate, flows and a second space through which the liquid does not flow, and to measure pressure of a gas (air) filling the second space by a pressure sensor, is used (see, e.g., Patent Literature 1). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Translation of PCT International Application Publication No. JP-T 2017/504389. 
     SUMMARY 
     Technical Problem 
     In case of the pressure sensing device described above, a detection value or pressure detection range of the pressure sensor changes depending on an initial position of the diaphragm. Thus, to accurately detect pressure and to make the pressure detection range a desired range, the initial position of the diaphragm is desired to be accurately adjusted to a given position. 
     Therefore, it is an object of the invention to provide a pressure sensing device capable of accurately adjusting an initial position of a diaphragm to a given position, and a blood purification apparatus using the same. 
     Solution to Problem 
     To solve the problem mentioned above, the invention provides a pressure sensing device, comprising: 
     a case provided on a liquid flow path through which a liquid subject to pressure measurement flows; 
     a diaphragm that is provided so as to divide a space in the case into a first space through which the liquid flows and a second space through which the liquid does not flow, and can be displaced according to pressure of the liquid in the first space; 
     a pressure sensor to measure pressure of a gas filling the second space; and 
     a diaphragm initial position adjustment mechanism capable of adjusting an initial position of the diaphragm to a desired position by adjusting a filling amount of the gas filling the second space, 
     wherein the diaphragm initial position adjustment mechanism comprises a reciprocating pump that comprises a cylinder in communication with the second space, a plunger provided so as to be able to advance and retract within the cylinder, and a plunger driving part to advance and retract the plunger, and adjusts the filling amount of the gas filling the second space by advancing and retreating the plunger in the cylinder by the plunger driving part. 
     To solve the problem mentioned above, the invention also provides a blood purification apparatus, comprising: 
     the pressure sensing device described above as at least one of pressure sensing devices provided on a blood circuit to extracorporeally circulate blood of a patient, a liquid supply flow path to supply a supply liquid to the blood circuit or to a blood purifier provided on the blood circuit, and a waste liquid flow path to discharge a waste liquid from the blood purifier. 
     Advantageous Effects of Invention 
     According to the invention, it is possible to provide a pressure sensing device capable of accurately adjusting an initial position of a diaphragm to a given position, and a blood purification apparatus using the same. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram illustrating a blood purification apparatus in an embodiment of the present invention. 
         FIG. 2A  is a schematic configuration diagram illustrating a pressure sensing device in the embodiment of the invention. 
         FIG. 2B  is a cross-sectional view showing a case. 
         FIG. 2C  is a schematic configuration diagram illustrating a reciprocating pump. 
         FIG. 3  is a flowchart showing a control flow executed by a control unit. 
         FIG. 4A  is a flowchart showing a modification of the control flow executed by the control unit. 
         FIG. 4B  is a flowchart showing a modification of the control flow executed by the control unit. 
         FIG. 5  is a flowchart showing a modification of the control flow executed by the control unit. 
         FIG. 6A  is a schematic configuration diagram illustrating the pressure sensing device in another embodiment of the invention. 
         FIG. 6B  is a flowchart showing the control flow executed by the control unit of the pressure sensing device of  FIG. 6A . 
         FIG. 7A  is a schematic configuration diagram illustrating the pressure sensing device in another embodiment of the invention. 
         FIG. 7B  is a flowchart showing the control flow executed by the control unit of the pressure sensing device of  FIG. 7A . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 
     An embodiment of the invention will be described below in conjunction with the appended drawings. 
     Blood Purification Apparatus 
     Firstly, a blood purification apparatus in which a pressure sensing device of the present embodiment is used will be described.  FIG. 1  is a schematic configuration diagram illustrating a blood purification apparatus in the present embodiment. 
     As shown in  FIG. 1 , a blood purification apparatus  10  includes a liquid supply flow path  13  to supply a supply liquid to a blood circuit  11  extracorporeally circulating blood of a patient or to a blood purifier  12  provided on the blood circuit  11 , and a waste liquid flow path  14  to discharge a waste liquid from the blood purifier  12 .  FIG. 1  shows an example in which the liquid supply flow path  13  is a dialysate flow path  13   a  to supply a dialysate to the blood purifier  12 . However, it is not limited thereto, and the liquid supply flow path  13  may be a replenishing liquid flow path to supply a replenishing liquid directly to the blood circuit  11 , or may include both the dialysate flow path  13   a  and the replenishing liquid flow path. 
     The blood circuit  11  is composed of, e.g., a flexible tube, etc. A blood pump  111 , the blood purifier  12 , a second blood pump  113  and an air trap chamber  112  are sequentially provided on the blood circuit  11  from the upstream to the downstream of a blood flow. The blood pump  111  is a liquid feed pump to send blood. The air trap chamber  112  is a device to remove air bubbles from the blood. 
     A first pressure sensing device  181  to measure pressure of blood on the upstream side of the blood pump  111  is provided on the upstream side of the blood pump  111  (at upstream of the blood flow). A second pressure sensing device  182  to measure pressure of the blood on the upstream side of the blood purifier is provided between the blood pump  111  and the blood purifier  12 . A third pressure sensing device  183  to measure pressure of the blood on the downstream side of the blood purifier is provided between the blood purifier  12  and the second blood pump  113 . A fourth pressure sensing device  184  to measure pressure inside the air trap chamber  112  is provided on the air trap chamber  112 . The third and fourth pressure sensing devices  183  and  184  are essential when the second blood pump  113  is provided, but the third pressure sensing device  183  can be omitted when the second blood pump  113  is omitted. 
     From a RO (Reverse Osmosis) device (not shown) which produces clean dialysis water using a reverse osmosis membrane (RO membrane), dialysis water is supplied to the dialysate flow path  13   a . Two types of undiluted dialysate fluids, an undiluted fluid A and an undiluted fluid B, are also supplied to the dialysate flow path  13   a . The two undiluted fluids are respectively stored in undiluted fluid storage tanks  151 , and the undiluted fluid A and the undiluted fluid B are supplied to the dialysate flow path  13   a  respectively from the undiluted fluid storage tanks  151  via undiluted fluid flow paths  152 . Undiluted fluid injection pumps  153 , which are liquid feed pumps pumping out the undiluted fluid A or the undiluted fluid B, are respectively provided on the two undiluted fluid flow paths  152 . The undiluted fluid A and the undiluted fluid B are mixed with the dialysis water in the dialysate flow path  13   a  and the dialysate is thereby prepared. The prepared dialysate is introduced into the blood purifier  12  via a duplex pump  16 . 
     A fifth pressure sensing device  185  to measure pressure of supplying the dialysis water is provided on the dialysate flow path  13   a  on the upstream side relative to portions connected to the undiluted fluid flow paths  152 . A sixth pressure sensing device  186  to measure pressure of the dialysate to be introduced into the blood purifier  12  is provided on the dialysate flow path  13   a  between the duplex pump  16  and the blood purifier  12 . In addition, a seventh pressure sensing device  187  to measure pressure of the waste liquid discharged from the blood purifier  12  is provided on the waste liquid flow path  14  between the blood purifier  12  and the duplex pump  16 . 
     The waste liquid from the blood purifier  12  is discharged through the waste liquid flow path  14 . The duplex pump  16  is provided over the dialysate flow path  13   a  and the waste liquid flow path  14  and performs pump operation so that an amount of the dialysate introduced into the blood purifier  12  is equal to an amount of the waste liquid discharged from the blood purifier  12 . In addition, a water removal flow path  14   a  is provided on the waste liquid flow path  14  so as to bypass the dual pump  16 , and a water removal pump  17  is provided on the water removal flow path  14   a . When the water removal pump  17  is driven, the amount of the waste liquid discharged from the blood purifier  12  becomes larger than the amount of the dialysate introduced into the blood purifier  12  and water is removed from the blood. It is possible to adjust the amount of water removed from the blood by adjusting the amount of liquid sent by the water removal pump  17 . 
     In the blood purification apparatus  10  of the present embodiment, at least one of the pressure sensing devices  181 - 187  provided on the blood circuit  11 , the liquid supply flow path  13  (the dialysate flow path  13   a  in this example) and the waste liquid flow path  14  is a pressure sensing device  1  in the present embodiment. In the blood purification apparatus  10 , the pressure sensing device  1  in the present embodiment may be used as at least one of the first to seventh pressure sensing devices  181 - 187 , or the pressure sensing device  1  in the present embodiment may be used as a pressure sensing device provided at another position. 
     The configuration of  FIG. 1  is only an example and a specific configuration of the blood purification apparatus  10  can be changed appropriately. 
     Pressure Sensing Device  1   
       FIG. 2A  is a schematic configuration diagram illustrating the pressure sensing device  1  in the present embodiment,  FIG. 2B  is a cross-sectional view showing a case, and  FIG. 2C  is a schematic configuration diagram illustrating a reciprocating pump. 
     As shown in  FIGS. 2A to 2C , the pressure sensing device  1  includes a case  2  provided on a liquid flow path  4  (the blood circuit  11  in this example) through which a liquid subject to pressure measurement (blood in this example) flows, a diaphragm  3  dividing a space in the case  2  into a first space  2   a  with the liquid (blood) flowing therethrough and a second space  2   b  filled with a gas (air in this example), a pressure sensor  6  to measure pressure of the gas filling the second space  2   b , a diaphragm initial position adjustment mechanism  5  capable of adjusting an initial position of the diaphragm  3  to a desired position by adjusting a filling amount of the gas filling the second space  2   b , and a control unit  7  that controls the diaphragm initial position adjustment mechanism  5 . Although the liquid subject to pressure measurement is blood and the gas filling the second space  2   b  is air in the following description, the liquid subject to measurement and the gas filling the second space  2   b  are not limited thereto. In addition, in the following description, the term simply referred as “air” means the air filling the second space  2   b , etc. 
     The liquid flow path  4  has an inflow path  4   a  to let the blood flow into the first space  2   a  and an outflow path  4   b  to let the blood flow out of the first space  2   a . When the pressure sensing device  1  is used as, e.g., the second pressure sensing device  182  in  FIG. 1 , the inflow path  4   a  serves as the blood circuit  11  extending from the blood pump  111  and the outflow path  4   b  serves as the blood circuit  11  extending to the blood purifier  12 . 
     The case  2  is composed of a hard resin molded article, etc., and is provided on the liquid flow path  4 . The case  2  integrally includes a first connection part  21  being in communication with the first space  2   a  and connected to the inflow path  4   a  and a second connection part  22  being in communication with the first space  2   a  and connected to the outflow path  4   b . The case  2  also integrally includes a protruding part  23  that is in communication with the second space  2   b  and protrudes outward. The pressure sensing device  1  also includes a socket part  24  which is provided separately from the case  2  and into which the protruding part  23  is inserted and connected. Although it is not shown, a measurement flow path  61  (described later) is connected to the socket part  24 , and the measurement flow path  61  is communicated with the second space  2   b  by inserting and connecting the protruding part  23  to the socket part  24 . The case  2  is configured to be removable from the socket part  24  by detaching the protruding part  23  from the socket part  24 , which allows the case  2  to be disposable. Thus, adjustment of the initial position of the diaphragm  3  (described later) is performed every time a new case  2  is attached. In this regard, the case  2  does not need to be entirely disposable and may be configured to be splittable on, e.g., the second space  2   b  side relative to the diaphragm  3  so that only a portion of the case  2  including the first space  2   a  can be disposable. It is not necessary to separately form the case  2  and the socket part  24 , and the case  2  may be integrally formed with the socket part  24 . 
     The diaphragm  3  is a flexible membrane and is provided in the case  2  so as to divide an internal space of the case  2  into two spaces, the first space  2   a  and the second space  2   b . Materials of the case  2  and the diaphragm  3  are not specifically limited. The diaphragm  3  is configured to be displaceable according to pressure of the blood in the first space  2   a  and serves to transmit the pressure of the blood in the first space  2   a  to the air in the second space  2   b.    
     The pressure sensor  6  is connected to the second space  2   b  of the case  2  via the measurement flow path  61  and measures pressure of the air filling the second space  2   b  and the measurement flow path  61 . A detection signal of the pressure sensor  6  is output to the control unit  7 . 
     The diaphragm initial position adjustment mechanism  5  has a reciprocating pump  51 , a pump flow path  53  connecting the reciprocating pump  51  to the second space  2   b , and a pump flow path opening/closing valve  52  that is provided on the pump flow path  53  and serves as a pump flow path opening/closing mechanism to open/close the pump flow path  53 . Although the pump flow path  53  in the present embodiment is connected to the second space  2   b  via the measurement flow path  61 , the pump flow path  53  may be directly connected to the second space  2   b  of the case  2 . As the pump flow path opening/closing valve  52 , it is desirable to use a solenoid valve so that opening/closing can be controlled automatically, but, e.g., a clamp mechanism, etc., may be used. In the present embodiment, a solenoid valve is used as the pump flow path opening/closing valve  52 . 
     The reciprocating pump  51  is also called a plunger pump or a piston pump, and has a cylinder  511  in communication with the second space  2   b  via the pump flow path  53  and the measurement flow path  61 , a plunger  512  (or a piston) provided so as to be able to advance and retract within the cylinder  511 , and a plunger driving part  513  to advance and retract the plunger  512 . The cylinder  511  and the pump flow path  53  are filled with the air in the same manner as the second space  2   b  and the measurement flow path  61 , and by advancing and retracting the plunger  512  in the cylinder  511  by the plunger driving part  513 , it is possible to adjust the filling amount of the air filling the second space  2   b  and thereby adjust the position of the diaphragm  3 . As the plunger driving part  513 , it is possible to use, e.g., a stepping motor. 
     The control unit  7  adjusts the initial position of the diaphragm  3  by controlling the reciprocating pump  51  and the pump flow path opening/closing valve  52 . The control unit  7  is realized by appropriately combining an arithmetic element such as CPU, a storage device such as memory, a software, and an interface, etc. 
     In the present embodiment, the filling amount of the air filling the second space  2   b , the measurement flow path  61 , the cylinder  511  and the pump flow path  53  is known. In this case, since it is known where to position the plunger  512  to cause the diaphragm  3  to be located at a desired position, the control unit  7  controls to move the pusher  512  to a predetermined position (referred to as an adjusted position) so that the diaphragm  3  is located at a desired position. 
     In particular, as shown in  FIG. 3 , the control unit  7  firstly opens the pump flow path opening/closing valve  52  in Step S 11 , moves the plunger  512  from its initial position (e.g., retracts toward the decompression side from a position of the plunger  512  when fully pushed, or advances toward the compression side from a position of the plunger  512  when fully withdrawn) in Step S 12 , and determines whether a predetermined time has elapsed in Step S 13 . Alternatively, whether a travel amount of the plunger  512  has reached a predetermined travel amount may be determined in Step S 13 . When the determination made in Step S 13  is NO, the process returns to Step S 12 . When the determination made in Step S 13  is YES, the plunger  512  is stopped in Step S 14 , the pump flow path opening/closing valve  52  is then closed in Step S 15  and the process ends. The time taken for the determination in Step S 13  (the predetermined time mentioned above) is set to the time required to move the plunger  512  from the initial position to the adjusted position. It is desirable that the control unit  7  adjust the initial position of the diaphragm  3  by executing the control flow of  FIG. 3  prior to blood purification treatment, e.g., during priming. 
     In the present embodiment, to adjust the filling amount of the air in the second space  2   b  by moving the plunger  512 , it is possible to linearly change pressure of the air. When, e.g., a peristaltic pump is used as a means to compress and decompress the air, the pressure of the air changes stepwise and it is thus difficult to precisely adjust the initial position of the diaphragm  3 . That is, by using the reciprocating pump  51  as in the present embodiment, it is possible to adjust the initial position of the diaphragm  3  more precisely. In addition, the peristaltic pump is configured to squeeze a flexible tube and variation in the discharge amount occurs due to production tolerance on the tube or deterioration of the tube over time, but the reciprocating pump  51  does not use any tube and such a problem can be suppressed. In addition, the tube of the peristaltic pump needs to be periodically replaced, but the reciprocating pump  51  does not need such periodic replacement of component and thus has high durability, and also contributes to cost reduction by reducing the number of components. 
     Furthermore, by configuring such that the pump flow path  53  can be closed by the pump flow path opening/closing valve  52  after adjusting the initial position of the diaphragm  3  by the reciprocating pump  51 , it is possible to reduce the filling amount of the air whose pressure is transmitted from the second space  2   b  to the pressure sensor  6 . Since the size of the pressure sensing device  1  is determined by the filling amount of the air whose pressure is transmitted from the second space  2   b  to the pressure sensor  6 , the size of the pressure sensing device  1  can be further reduced by providing the pump flow path opening/closing valve  52 . However, the pump flow path opening/closing valve  52  is not essential and can be omitted. 
     If the diaphragm  3  is damaged, the air leaks into the blood circuit  11  when performing compression by the reciprocating pump  51  and thus cannot be compressed. Thus, it is also possible to detect the failure of the pressure sensing device  1  based on an output value of the pressure sensor  6  when compression is performed by the reciprocating pump  51 . 
     Modifications 
     The filling amount of the air filling the second space  2   b , the measurement flow path  61 , the cylinder  511  and the pump flow path  53  is known in the present embodiment, but a means to estimate the filling amount of the air is required when the filling amount of the air is unknown. The filling amount of the air can be estimated from, e.g., a change in pressure of the air when the plunger  512  is moved. In this case, the control unit  7  estimates the filling amount of the air based on a travelled distance of the plunger  512  and a change in pressure of the air at that time, calculates the adjusted position, which is a position of the plunger  512  causing the diaphragm  3  to be located at a desired position, based on the estimated filling amount of the air, and then moves the plunger  512  to the adjusted position. When estimating the filling amount of the air, it may be estimated using, e.g., Boyle-Charles&#39;s law. 
       FIG. 4A  shows a control flow when estimating the filling amount of the air based on the travelled distance of the plunger  512 . As shown in  FIG. 4A , the control unit  7  firstly opens the pump flow path opening/closing valve  52  in Step S 21 , retracts the plunger  512  from the initial position (e.g., retracts toward the decompression side from a position of the plunger  512  when fully pushed) in Step S 22 , and determines whether the plunger  512  has reached a predetermined decompression position (whether the plunger  512  has been moved by a predetermined distance) in Step S 23 . When the determination made in Step S 23  is NO, the process returns to Step S 22 . When the determination made in Step S 23  is YES, the plunger  512  is stopped in Step S 24 , and then, the filling amount of the air is estimated based on the travelled distance of the plunger  512  and a pressure change of the air detected by the pressure sensor  6  and the adjusted position of the plunger  512  causing the diaphragm  3  to be located at a desired position is calculated in Step S 25 . After that, the plunger  512  is advanced in Step S 26 , and whether the plunger  512  has reached the adjusted position is determined in Step S 27 . When the determination made in Step S 27  is NO, the process returns to Step S 26 . When the determination made in Step S 27  is YES, the plunger  512  is stopped in Step S 28 , the pump flow path opening/closing valve  52  is then closed in Step S 29  and the process ends. 
       FIG. 4B  shows a control flow when executed based on the pressure change of the air. The control flow of  FIG. 4B  is a modification of the control flow of  FIG. 4A  in which Step S 23  is replaced with Step S 23   a  of determining whether the pressure of the air is a predetermined pressure. That is, the pressure change of the air when moving the plunger  512  by a predetermined distance is detected in  FIG. 4A , while the travelled distance of the plunger  512  causing the pressure change of the air to be a predetermined value is detected in  FIG. 4B . With either of these methods, it is possible to estimate the filling amount of the air and calculate the adjusted position of the plunger  512 . 
     In the examples of  FIGS. 4A and 4B , a plunger position detection unit to detect the position of the plunger  512  is required. As the plunger position detection unit, it is possible to use, e.g., an encoder that detects a rotational speed of a motor used for the plunger driving part  513 , etc., or a linear potentiometer that directly detects the position of the plunger  512 , etc. In addition, when using a stepping motor (pulse motor) as the plunger driving part  513 , it is possible to detect the position of the plunger  512  also based on a driving amount (a number of output pulses) of the stepping motor. When operating the plunger  512  at a constant speed, it is also possible to detect the plunger  512  from the operating time. Furthermore, as the plunger position detection unit, it also possible to use a contact type sensor such as limit switch, strain gauge or piezoelectric element sensor and it is also possible to use an indirect sensor such as photoelectric sensor or pressure sensor. 
     A diaphragm position detection unit capable of detecting that the diaphragm is located at a predetermined position (e.g., a position at which the volume of the first space  2   a  reaches the maximum or the minimum) may be further provided. The diaphragm position detection unit may be configured to detect the position of the diaphragm  3  using, e.g., a sensor or the like such as photoelectric sensor. It is also possible to estimate the position of the diaphragm  3  by using the detection result of the encoder or linear potentiometer mentioned above, or the driving amount of the stepping motor, or an output of the pressure sensor  6 . 
     When the diaphragm position detection unit is provided, the control unit  7  preferably estimate the filling amount of the air based on the travel amount of the plunger  512  until the diaphragm position detection unit detects that the diaphragm  3  is located at a predetermined position since the plunger  512  is moved from the initial position. In more particular, the filling amount of the air can be estimated from a retraction distance of the plunger  512  which is required for the diaphragm  3  to reach the predetermined position (e.g., the position at which the volume of the first space  2   a  reaches the maximum) from the no-load position. The control flow in this case is shown in  FIG. 5 . The control flow of  FIG. 5  is a modification of the control flow of  FIG. 4A  in which Step S 23  is replaced with Step S 23   b  of determining whether the diaphragm  3  is located at a predetermined position and Step S 25  is replaced with Step S 25   b  of estimating the filling amount of the air based on the travelled distance of the plunger  512  and calculating the adjusted position. 
     Functions and Effects of the Embodiment 
     As described above, the pressure sensing device  1  in the present embodiment includes the diaphragm initial position adjustment mechanism  5  capable of adjusting the initial position of the diaphragm  3  to a desired position by adjusting the filling amount of the air filling the second space  2   b , and the diaphragm initial position adjustment mechanism  5  has the reciprocating pump  51  that adjusts the filling amount of the gas filling the second space  2   b  by advancing and retreating the plunger  512  in the cylinder  511 . 
     By adjusting the initial position of the diaphragm  3  using the reciprocating pump  51 , the initial position of the diaphragm  3  can be accurately adjusted to a given position and variation in the initial position of the diaphragm  3  can be suppressed, as compared to when a peristaltic pump, etc., is used. By suppressing the variation in the initial position of the diaphragm  3 , it is possible to improve repeat accuracy of the initial position adjustment of the diaphragm  3  which is performed every time the case  2  is changed, it is possible to improve pressure detection accuracy and also possible to accurately adjust the pressure detection range. In addition, when the variation in the initial position of the diaphragm  3  is large, the filling amount of the air filling the second space  2   b  and the measurement flow path  61  needs to be increased to absorb the variation. However, in the present embodiment, since the variation in the initial position of the diaphragm  3  can be suppressed, the filling amount of the air can be reduced and this contributes to size reduction of the pressure sensing device  1 . 
     Other Embodiments 
     A pressure sensing device  1   a  shown in  FIG. 6A  is a device in which the pressure sensing device  1  of  FIG. 2A  includes a pressure release mechanism  8  capable of releasing the air filling the second space  2   b , etc., to the atmosphere. The pressure release mechanism  8  has a pressure release flow path  81  with one end connected to the pump flow path  53  between the reciprocating pump  51  and the pump flow path opening/closing valve  52  and the other end opened to the atmosphere, and a pressure release valve  82  that is provided on the pressure release flow path  81  and is capable of opening/closing the pressure release flow path  81 . Opening/closing of the pressure release valve  82  is controlled by the control unit  7 . A solenoid valve is used as the pressure release valve  82  in the present embodiment, but, e.g., a clamp mechanism may be used. 
     The control unit  7  of the pressure sensing device  1   a  retracts the plunger  512  from the initial position and moves to a predetermined decompression position (a pressure application position), releases the air to the atmosphere by opening the pressure release valve  82  and then closes the pressure release valve  82 , and after that, moves the plunger  512  to the adjusted position which is a position of the plunger  512  causing the diaphragm  3  to be located at a desired position. In this regard, after the plunger  512  is advanced from the initial position and moved to a predetermined compression position, it may be opened to the atmosphere. 
     When a new case  2  is attached to the socket part  24 , the position of the diaphragm  3  in the case  2  in the unadjusted state can be various positions. In this case, by controlling to open to the atmosphere after moving the diaphragm  3  in one direction by the reciprocating pump  51 , the diaphragm  3  can be positioned at a fixed position (the no-load position) and also the air filling the second space  2   b , etc., can be brought to atmospheric pressure, thereby making the filling amount of the air a known amount. Thereafter, by moving the plunger  512  to a predetermined adjusted position, the diaphragm  3  can be accurately adjusted to a desired position. 
       FIG. 6B  shows the control flow executed by the control unit  7  of the pressure sensing device  1   a . The control flow of  FIG. 6B  is a modification of the control flow of  FIG. 4A  in which Step S 25  is replaced with Step S 30  of opening the pressure release valve  82  and Step S 31  of closing the pressure release valve  82  after Step S 30 . 
     The pressure sensing device  1   a  also includes an air filter  62  that is provided in the measurement flow path  61  to prevent foreign matter from entering the piping. The air filter  62  is a so-called hydrophobic filter, and is configured to allow gases to pass therethrough but to not allow liquids to pass therethrough (very high resistance to the passage of liquids). 
     A pressure sensing device  1   b  shown in  FIG. 7A  is a device in which the pressure sensing device  1   a  of  FIG. 6A  further includes a diaphragm position fixing mechanism  9  capable of fixing the position of the diaphragm  3  by closing the liquid flow path  4 . The diaphragm position fixing mechanism  9  is configured to be capable of simultaneously closing both the inflow path  4   a  and the outflow path  4   b . In this embodiment, the diaphragm position fixing mechanism  9  has a first diaphragm position fixing valve  9   a  which is provided on the inflow path  4   a  and is capable of opening/closing the inflow path  4   a , and a second diaphragm position fixing valve  9   b  which is provided on the outflow path  4   b  and is capable of opening/closing the outflow path  4   b . By closing both the diaphragm position fixing valves  9   a  and  9   b , the liquid flow path  4  between the two diaphragm position fixing valves  9   a  and  9   b , including the first space  2   a , is closed and the position of the diaphragm  3  is fixed. Although solenoid valves are used as both diaphragm position fixing valves  9   a  and  9   b  in this embodiment, e.g., a clamp mechanism may be used. Opening/closing of both diaphragm position fixing valves  9   a  and  9   b  is controlled by the control unit  7 . In this embodiment, the pump flow path opening/closing valve  52  also serves as the diaphragm position fixing mechanism  9 . 
     The control unit  7  of the pressure sensing device  1   b  opens the pressure release valve  82  after the position of the diaphragm  3  is fixed by the diaphragm position fixing mechanism  9 . In particular, as shown in  FIG. 7B , Step S 32  of fixing the diaphragm  3  by closing both diaphragm position fixing valves  9   a  and  9   b  is inserted between Step S 24  and Step S 30  in the control flow of  FIG. 6B , and Step S 33  of releasing the diaphragm  3  from the fixed position by opening both diaphragm position fixing valves  9   a  and  9   b  is inserted between Step S 31  and Step S 26 . In this regard, Step S 33  (opening both diaphragm position fixing valves  9   a  and  9   b ) may be performed after returning the plunger  512  to the adjusted position, i.e., after Step S 28 . 
     As a result, the diaphragm  3  does not return to the no-load position when opening the pressure release valve  82  in Step S 30  and is held at a position at which, e.g., the volume of the first space  2   a  is the largest. Therefore, as compared to  FIG. 6A  and  FIG. 6B  in which the diaphragm  3  returns to the no-load position, the position of the diaphragm  3  when opened to the atmosphere can be kept more fixed, and accuracy of position adjustment of the diaphragm  3  is further improved. 
     Summary of the Embodiment 
     Technical ideas understood from the embodiments will be described below citing the reference numerals, etc., used for the embodiments. However, each reference numeral, etc., described below is not intended to limit the constituent elements in the claims to the members, etc., specifically described in the embodiments. 
     [1] A pressure sensing device ( 1 ), comprising: a case ( 2 ) provided on a liquid flow path through which a liquid subject to pressure measurement flows; a diaphragm ( 3 ) that is provided so as to divide a space in the case ( 2 ) into a first space ( 2   a ) with the liquid flowing therethrough and a second space ( 2   b ) filled with a gas and can be displaced according to pressure of the liquid in the first space ( 2   b ); a pressure sensor ( 6 ) to measure pressure of the gas filling the second space ( 2   b ); and a diaphragm initial position adjustment mechanism ( 5 ) capable of adjusting an initial position of the diaphragm ( 3 ) to a desired position by adjusting a filling amount of the gas filling the second space ( 2   b ), wherein the diaphragm initial position adjustment mechanism ( 5 ) comprises a reciprocating pump ( 51 ) that comprises a cylinder ( 511 ) in communication with the second space ( 2   b ), a plunger ( 512 ) provided so as to be able to advance and retract within the cylinder ( 511 ), and a plunger driving part ( 53 ) to advance and retract the plunger ( 512 ), and adjusts a filling amount of the gas filling the second space ( 2   b ) by advancing and retreating the plunger ( 512 ) in the cylinder ( 511 ) by the plunger driving part ( 513 ). 
     [2] The pressure sensing device ( 1 ) described in [1], wherein the diaphragm initial position adjustment mechanism ( 5 ) comprises a pump flow path ( 52 ) connecting the reciprocating pump ( 51 ) to the second space ( 2   b ), and a pump flow path opening/closing mechanism ( 53 ) provided on the pump flow path ( 52 ) to open/close the pump flow path ( 52 ). 
     [3] The pressure sensing device ( 1 ) described in [1] or [2], comprising: a control unit ( 7 ) that adjusts the initial position of the diaphragm ( 3 ) by controlling the reciprocating pump ( 51 ), wherein the control unit ( 7 ) estimates the filling amount of the gas based on a travelled distance of the plunger ( 512 ) and a change in pressure of the gas at that time, calculates an adjusted position, which is a position of the plunger at which the diaphragm ( 3 ) is located at a desired position, based on the estimated filling amount of the gas, and moves the plunger ( 512 ) to the adjusted position. 
     [4] The pressure sensing device ( 1 ) described in [1] or [2], comprising: a control unit ( 7 ) that adjusts the initial position of the diaphragm ( 3 ) by controlling the reciprocating pump ( 51 ); and a diaphragm position detection unit capable of detecting that the diaphragm ( 3 ) is located at a predetermined position, wherein the control unit ( 7 ) estimates the filling amount of the gas based on a travel amount of the plunger ( 512 ) until the diaphragm position detection unit detects that the diaphragm ( 3 ) is located at the predetermined position since the plunger ( 512 ) is moved from the initial position, calculates an adjusted position, which is a position of the plunger ( 512 ) at which the diaphragm ( 3 ) is located at a desired position, based on the estimated filling amount of the gas, and moves the plunger ( 512 ) to the adjusted position. 
     [5] The pressure sensing device (la) described in [1] or [2], comprising: a pressure release mechanism ( 8 ) comprising a pressure release valve ( 82 ) capable of releasing the gas to the atmosphere; and a control unit ( 7 ) that adjusts the initial position of the diaphragm ( 3 ) by controlling the reciprocating pump ( 51 ) and the pressure release mechanism ( 8 ), wherein the control unit ( 7 ) advances or retracts the plunger ( 512 ) to a predetermined pressure application position, closes the pressure release valve ( 82 ) after opening the pressure release valve ( 82 ) and releasing the gas to the atmosphere, and then moves the plunger ( 512 ) to an adjusted position which is a position of the plunger ( 512 ) at which the diaphragm ( 3 ) is located at a desired position. 
     [6] The pressure sensing device ( 1   b ) described in [5], comprising: a diaphragm position fixing mechanism ( 9 ) capable of fixing the position of the diaphragm ( 3 ) by closing the liquid flow path ( 4 ), wherein the control unit ( 7 ) opens the pressure release valve ( 82 ) after the position of the diaphragm ( 3 ) is fixed by the diaphragm position fixing mechanism ( 9 ). 
     [7] The pressure sensing device ( 1 ) described in any one of [1] to [6], comprising: a socket part ( 24 ) to which the pressure sensor ( 6 ) is connected, wherein the case ( 2 ) and the diaphragm ( 3 ) are provided so as to be removable from the socket part ( 24 ). 
     [8] A blood purification apparatus ( 10 ), comprising: a blood circuit ( 11 ) to extracorporeally circulate blood of a patient; a liquid supply flow path ( 13 ) to supply a supply liquid to the blood circuit ( 11 ) or to a blood purifier ( 12 ) provided on the blood circuit ( 11 ); and a waste liquid flow path ( 14 ) to discharge a waste liquid from the blood purifier ( 12 ), wherein at least one of pressure sensing devices ( 181 - 187 ) provided on the blood circuit ( 11 ), the liquid supply flow path ( 13 ) and the waste liquid flow path ( 14 ) comprises the pressure sensing device ( 1 ,  1   a ,  1   b ) described any one of [1] to [7]. 
     Although the embodiments of the invention have been described, the invention according to claims is not to be limited to the embodiments described above. In addition, not all combinations of the features described in the embodiments are necessary to solve the problem of the invention. In addition, the invention can be appropriately modified and implemented without departing from the gist thereof. 
     REFERENCE SIGNS LIST 
     
         
           1  pressure sensing device 
           2  case 
           2   a  first space 
           2   b  second space 
           3  diaphragm 
           4  liquid flow path 
           5  diaphragm initial position adjustment mechanism 
           51  reciprocating pump 
           511  cylinder 
           512  plunger 
           513  plunger driving part 
           52  pump flow path opening/closing valve 
           53  pump flow path 
           6  pressure sensor 
           7  control unit 
           8  pressure release mechanism 
           82  pressure release valve 
           9  diaphragm position fixing mechanism 
           10  blood purification apparatus 
           11  blood circuit 
           12  blood purifier 
           13  liquid supply flow path 
           14  waste liquid flow path