Patent Publication Number: US-11035714-B2

Title: Flow meter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims foreign priority based on Japanese Patent Application No. 2017-131551, filed Jul. 4, 2017, the contents of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a flow meter for calculating a flow rate of a fluid flowing in a pipe. 
     2. Description of Related Art 
     A flow meter (flow velocity meter) is used to measure a value of a flow rate of a fluid flowing in a pipe. For example, Japanese Patent Laid-open Publication No. 2007-147631 describes a monitoring system for monitoring a flow velocity of tap water flowing in a water distribution pipe. In this monitoring system, the flow velocity of the tap water is detected by a flow velocity sensor disposed in the water distribution pipe. A flow velocity meter conversion part calculates a measured value corresponding to the flow velocity detected by the flow velocity sensor. The flow velocity meter conversion part has a built-in data logger that stores data such as the measured value of the flow velocity for a predetermined period. 
     In the monitoring system of Japanese Patent Laid-open Publication No. 2007-147631, the flow velocity sensor and the flow velocity meter conversion part (data logger) are provided independently. Thus, in order to store (log) data, such as the flow velocity detected by the flow velocity sensor, into the data logger, a user needs to give the data logger an operation to instruct execution of logging. 
     Hence, it is not guaranteed that the data logger always logs data. Suppose the user forgets to perform the operation, even when the flow velocity sensor detects a flow velocity, the data cannot be logged. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a flow meter capable of logging data more reliably. 
     (1) A flow meter according to the present invention is a flow meter that operates by receiving power from a power supply, the flow meter including: a detection element that repeatedly detects a physical quantity related to a flow rate of a fluid flowing in a pipe in a predetermined sampling cycle; a flow rate calculation part that sequentially calculates the flow rate of the fluid in the pipe based on the physical quantity detected by the detection element; a control storage part that stores a logging target to be logged, the logging target being a part or whole of parameters including the flow rate calculated by the flow rate calculation part, a logging cycle for the logging target, and logging start definition information for defining start of logging of the logging target; a log storage part that stores a numerical value of the logging target; a time measurement part that measures time; and a control part that automatically starts logging of the logging target in accordance with startup of the flow meter based on the logging start definition information stored in the control storage part, and causes the log storage part to store the time measured by the time measurement part and the numerical value of the logging target in association with each other. 
     This flow meter operates by receiving power from a power supply. In this operation, a detection element repeatedly detects a physical quantity related to a flow rate of a fluid flowing in a pipe in a predetermined sampling cycle. Based on physical quantities detected by the detection element, a flow rate calculation part sequentially calculates the flow rate of the fluid in the pipe. Further, a time measurement part calculates the time. A control storage part is caused to store a logging target, a logging cycle, and logging start definition information. Based on the logging start definition information stored in the control storage part, a control part automatically starts logging of the logging target in response to startup of the flow meter, and causes a log storage part to store the time measured by the time measurement part and a numerical value of the logging target in association with each other in every logging cycle. 
     With this configuration, the control part automatically performs logging based on the logging target, the logging cycle, and the logging start definition information. Hence, there is no need for the user to perform a separate setting (operation) in order to store the logging target into the log storage part (logging). It is thus guaranteed that the logging target is always stored in the log storage part together with the time measured by the time measurement part after starting the flow meter. As a result, it is possible to log data more reliably. 
     (2) Each of the logging target, the logging cycle, and the logging start definition information stored in the control storage part may be made up of a parameter not settable by a user, a default parameter decided before setting, or an execution program of the control part. In this case, the user can execute the logging without setting the logging target, the logging cycle, or the logging start definition information. 
     (3) The flow meter may include a display that displays a current instantaneous flow rate calculated by the flow rate calculation part. The control storage part may further store display format information for identifiably displaying the logging target on the display. The display may simultaneously or switchably display the instantaneous flow rate and the logging target, based on the display format information stored in the control storage part. 
     In this case, the user can easily visually recognize the logging target together with the current instantaneous flow rate calculated by the flow rate calculation part. Hence, the user can perform simple management and determine the occurrence or non-occurrence of abnormality by using only the flow meter without outputting the logging target to a management device such as a personal computer outside the flow meter. When abnormality occurs, the user can simply analyze the cause thereof by using the flow meter and can also analyze the cause in detail by using the management device outside the flow meter. 
     (4) The flow meter may include: an operation part that accepts an input based on a user&#39;s operation; a setting part that sets a flow rate threshold serving as a reference for comparison with the flow rate calculated by the flow rate calculation part based on the input received by the operation part; a first signal output part that outputs an on/off signal related to the flow rate based on the flow rate calculated by the flow rate calculation part and the flow rate threshold set by the setting part; and a display for displaying the current instantaneous flow rate calculated by the flow rate calculation part. The display may simultaneously or switchably display the instantaneous flow rate, the flow rate threshold, and the logging target. 
     In this case, the user can easily set the flow rate threshold by operating the operation part. In addition to the current instantaneous flow rate calculated by the flow rate calculation part, the user can easily confirm the threshold set by the setting part by visually recognizing the display. 
     (5) The display may be capable of selectively displaying the logging target stored in the log storage part in every period corresponding to the logging cycle. In this case, the user can easily visually recognize the logging target in each period. Thus, when abnormality occurs, the user can easily manage the flow meter in each period and analyze the cause of the abnormality. 
     (6) The flow meter may include a display that displays a current instantaneous flow rate calculated by the flow rate calculation part. The flow rate calculation part may further decide a maximum flow rate and a minimum flow rate within a period corresponding to the logging cycle in every logging cycle from the sequentially calculated flow rate of the fluid in the pipe. The control storage part may further store the maximum flow rate and the minimum flow rate as the logging targets. The control part may cause the log storage part to store the time measured by the time measurement part, the maximum flow rate and the minimum flow rate in association with each other in every logging cycle. The display may simultaneously or switchably display the instantaneous flow rate, the maximum flow rate, and the minimum flow rate. 
     In this case, the user can easily visually recognize the maximum flow rate and the minimum flow rate in each period together with the current instantaneous flow rate calculated by the flow rate calculation part. By visually recognizing the maximum flow rate and the minimum flow rate, the user can simply analyze a cause when trouble occurs in the pipe, based on the maximum flow rate and the minimum flow rate. Further, the volume of the logging target data to be stored into the log storage part is reduced, so that the logging target can be stored in the log storage part for a longer period. 
     (7) The flow meter may include a display that displays a current instantaneous flow rate calculated by the flow rate calculation part. The control storage part may store the instantaneous flow rate as the logging target. The control part may cause the log storage part to store the time measured by the time measurement part and the instantaneous flow rate in association with each other in every logging cycle. The flow rate calculation part may further decide a maximum flow rate and a minimum flow rate within a period corresponding to the logging cycle in every logging cycle from the instantaneous flow rate stored in the log storage part. The display may simultaneously or switchably display the instantaneous flow rate, the maximum flow rate, and the minimum flow rate. 
     In this case, the user can easily visually recognize the maximum flow rate and the minimum flow rate in each period, together with the current instantaneous flow rate calculated by the flow rate calculation part without logging the maximum flow rate and the minimum flow rate as the logging targets. By visually recognizing the maximum flow rate and the minimum flow rate, the user can simply analyze a cause when trouble occurs in the pipe. 
     (8) The flow meter may include a display that displays a current instantaneous flow rate calculated by the flow rate calculation part. The flow rate calculation part may further calculate an integrated flow rate from the sequentially calculated flow rate of the fluid in the pipe. The control storage part may further store the integrated flow rate as the logging target. The control part may cause the log storage part to store the time measured by the time measurement part and the integrated flow rate in association with each other in every logging cycle. The display may simultaneously or switchably display the instantaneous flow rate and the integrated flow rate. 
     In this case, the user can easily manage the flow rate of the fluid based on the logged integrated flow rate. Further, the user can easily visually recognize the current instantaneous flow rate calculated by the flow rate calculation part and the integrated flow rate in each period. By visually recognizing the integrated flow rate, the user can simply analyze a cause when trouble occurs in the pipe. 
     (9) The flow meter may include a display that displays a current instantaneous flow rate calculated by the flow rate calculation part. The control storage part may store the instantaneous flow rate as the logging target. The control part may cause the log storage part to store the time measured by the time measurement part and the instantaneous flow rate in association with each other in every logging cycle. The flow rate calculation part may further calculate an integrated flow rate from the instantaneous flow rate stored in the log storage part. The display may simultaneously or switchably display the instantaneous flow rate and the integrated flow rate. 
     In this case, the user can easily manage the flow rate of the fluid based on the integrated flow rate without logging the integrated flow rate as the logging target. Further, the user can easily visually recognize the current instantaneous flow rate calculated by the flow rate calculation part and the integrated flow rate in each period. By visually recognizing the integrated flow rate, the user can simply analyze a cause when trouble occurs in the pipe. 
     (10) The flow meter may further include a temperature measurement part that measures a temperature of a fluid. The control storage part may further store a representative temperature within a period corresponding to the logging cycle based on the temperature measured by the temperature measurement part as the logging target. The control part may cause the log storage part to store the time measured by the time measurement part and the representative temperature in association with each other in every logging cycle. In this case, the user can easily manage the temperature of the fluid based on the logged representative temperature. When trouble occurs in the pipe due to the temperature, the user can easily and simply analyze the cause of the trouble. 
     (11) The flow meter may further include a second signal output part that outputs a binary signal based on the flow rate calculated by the flow rate calculation part. The control storage part may further store a history of the change when the state of the binary signal output by the second signal output part changes within a period corresponding to the logging cycle. The control part may cause the log storage part to store the time measured by the time measurement part and the change history in association with each other in every logging cycle. In this case, the user can easily manage the history of the change in state of the logged binary signal. Further, the user can easily and simply determine whether or not trouble has occurred in the pipe by visually recognizing the change in state of the binary signal. 
     (12) The flow meter may further include a parameter selection part that accepts selection of a parameter to be the logging target. In this case, the user can select a desired parameter as the logging target while causing automatic execution of the logging. Thus, the usefulness of the flow meter can be improved. 
     (13) The flow meter may further include a data output part provided capable of outputting the logging target stored in the log storage part. In this case, it is possible to output the logging target to the outside of the flow meter. As a result, it is possible for the external management device to manage the flow rate of the fluid in detail based on the logging target. When trouble occurs in the pipe, the user can easily analyze the cause of the trouble in detail by using the management device. 
     (14) The control part may initialize an operation state of the detection element in response to receiving power from the power supply. The flow rate calculation part may operate after initialization by the control part. In this case, after the operation state of the detection element has been initialized, the logging target is stored into the log storage part. Thus, inappropriate information before the initialization of the operation state of the detection element is prevented from being stored into the log storage part as the logging target. 
     (15) The log storage part may include a ring buffer. When a logging target is stored into all storage areas of the ring buffer, the control part may overwrite the logging target stored last and stores the latest logging target. In this case, it is possible to prevent logging targets from being not stored in the log storage part due to insufficient capacity of the ring buffer. 
     (16) The time measurement part may include a real-time clock that operates independently of power supply from the power supply. The control part may cause the log storage part to store an absolute time based on the real-time clock and the logging target in association with each other. In this case, the period corresponding to the logging cycle can be associated with the absolute time. This makes it easier to manage the logging target. 
     (17) The flow meter may further include a secondary battery that is charged with power from the power supply and supplies power to the real-time clock. In this case, there is no need to frequently replace the secondary battery. Thus, the maintainability of the flow meter can be improved. 
     (18) The flow meter may further include a mode selection part that accepts selection of an operation mode to be executed by the control part out of a first operation mode in which the logging target is stored into the log storage part and a second operation mode in which the logging target is not stored into the log storage part. In this case, the user can stop the logging depending on the use. 
     (19) The detection element may detect at least one of transmission of ultrasonic waves to the fluid flowing in the pipe and reception of ultrasonic waves from the fluid flowing in the pipe to detect the ultrasonic waves. The flow rate calculation part may sequentially calculate the flow rate of the fluid in the pipe based on the ultrasonic waves detected by the detection element. In this case, the flow rate of the fluid flowing in the pipe by can be detected from the outside of the pipe by using ultrasonic waves. 
     According to the present invention, it is possible to log data more reliably. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an external perspective view of a flow meter according to an embodiment of the present invention; 
         FIG. 2  is a schematic sectional view showing an internal configuration of the flow meter of  FIG. 1 ; 
         FIG. 3  is a perspective view of a casing; 
         FIG. 4  is a block diagram showing a configuration of a control board; 
         FIG. 5  is a diagram schematically showing various pieces of data stored in a control storage part; 
         FIG. 6  is a diagram for explaining execution timing of logging based on logging start definition information; 
         FIG. 7  is a diagram schematically showing various pieces of data stored in a log storage part; 
         FIGS. 8A to 8D  are each a diagram for explaining an operation of a control part; 
         FIGS. 9A to 9D  are views each showing a display screen of a display; 
         FIGS. 10A to 10D  are views each showing a data display screen for displaying other piece of information included in log data; 
         FIGS. 11A to 11H  are views showing a data display screens corresponding to respective periods in which the log data is stored; 
         FIG. 12  is a plan view showing a configuration of a terminal block in a casing; 
         FIG. 13  is a plan view showing a terminal block in a state where a closing member has been removed; 
         FIG. 14  is a flowchart showing an algorithm of control processing of a flow meter executed by a control program; 
         FIG. 15  is a flowchart showing an algorithm of sensing processing in Step S 3  of  FIG. 14 ; 
         FIG. 16  is a flowchart showing an algorithm of logging processing in Step S 4  of  FIG. 14 ; and 
         FIG. 17  is a view for explaining execution timing of logging based on logging start definition information in another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     (1) Schematic Configuration of Flow Meter 
     Hereinafter, a flow meter according to an embodiment of the present invention will be described with reference to the drawings.  FIG. 1  is an external perspective view of a flow meter according to an embodiment of the present invention.  FIG. 2  is a schematic sectional view showing an internal configuration of a flow meter  100  of  FIG. 1 . As shown in  FIG. 1 , in the present embodiment, clamp members  1 ,  2  are mounted on the outer peripheral surface of a relatively large pipe P by a mounting tool in a state where the clamping members face each other across the pipe P. The diameter of the pipe P is, for example, 40 mm to 200 mm. Fluid flows in the pipe P. 
     As shown in  FIGS. 1 and 2 , the flow meter  100  includes casings  10 ,  20 , main units  30 ,  40 , a control board  50 , a power supply board  60 , a display board  70 , and a terminal block  80 . In the casing  10 , the main unit  30 , the control board  50 , the power supply board  60 , the display board  70 , and the terminal block  80  are accommodated. The main unit  40  is accommodated in the casing  20 . The casings  10 ,  20  are fixed to the clamp members  1 ,  2  by a plurality of fixing screws  101 . As a result, the flow meter  100  is mounted on the outer peripheral surface of the pipe P. 
     The casing  10  has an elongated shape extending along the direction in which the fluid flows (the axial direction of the pipe P). Hereinafter, in the casing  10 , the direction toward the pipe P is defined as a downward direction, and the opposite direction is defined as an upward direction. The vertical direction of the casing  10  is orthogonal to the longitudinal direction and the width direction of the casing  10 . 
     The casing  10  is mounted on the pipe P by the clamp member  1 , with a part of the lower part of the casing  10  in contact with the pipe P. The surface of the casing  10  mounted on the pipe P is referred to as a mounting surface  10   a , and the surface of the casing  10  opposite from the mounting surface  10   a  is referred to as a main surface  10   b . On the main surface  10   b  of the casing  10 , a translucent window member  17  is provided. 
     A vertically penetrating opening h 1  is formed on the mounting surface  10   a  of the casing  10 . A pair of projections  10   p  projecting downward is formed on the mounting surface  10   a . A temperature measurement part  102  is, for example, a thermistor. The pair of projections  10   p  is arranged so as to longitudinally face each other across the opening h 1 . A temperature measurement part  102  is provided on one projection  10   p . In a state where the casing  10  is mounted on the pipe P, the pair of projections  10   p  is in contact with the pipe P, and the mounting surface  10   a  is not in contact with the pipe P. The temperature of the fluid is measured by the temperature measurement part  102  through the pipe P by one projection  10   p  coming into contact with the pipe P. 
     The casing  20  is mounted on the pipe P by the clamp member  2  in a state where a part thereof is in contact with the pipe P. A vertically penetrating opening h 2  is formed on a mounting surface  20   a  of the casing  20  mounted on the pipe P. A pair of projections  20   p  similar to the projections  10   p  is formed on the mounting surface  20   a.    
     The main unit  30  includes a detection element  31 , a path member  32 , and an acoustic couplant  33  having a solid shape. In the present embodiment, the detection element  31  is an ultrasonic element, is disposed to transmit and receive ultrasonic waves to and from the pipe P at a predetermined angle, and is joined to the upper portion of the path member  32  with an acoustic cement. 
     The path member  32  is formed of a material that is non-metallic and has high rigidity and high sound permeability. Further, the path member  32  is preferably formed of a material having high environmental resistance. In the present example, the path member  32  is formed of polyphenylene sulfide (PPS) resin or ULTEM (registered trademark) resin. The path member  32  is mounted on the casing  10  with a seal member, not shown, interposed so as to close the opening h 1 . The lower surface of the path member  32  protrudes slightly downward from the opening h 1 . The amount of projection of the lower surface of the path member  32  from the mounting surface  10   a  is smaller than the amount of projection of each projection  10   p  from the mounting surface  10   a.    
     The acoustic couplant  33  is mounted on the lower surface of the path member  32 . By mounting the casing  10  on the pipe P, the acoustic couplant  33  is disposed between the lower surface of the path member  32  and the outer peripheral surface of the pipe P in a slightly crushed state. Note that the pair of projections  10   p  regulates the crushed amount of the acoustic couplant  33 . 
     The main unit  40  includes a detection element  41 , a path member  42 , and an acoustic couplant  43  respectively having the same configurations as those of the detection element  31 , the path member  32 , and the acoustic couplant  33 . The detection element  41  is disposed to transmit and receive ultrasonic waves to and from the pipe P at a predetermined angle, and is joined to the path member  42  with an acoustic cement. The path member  42  is mounted on the casing  20  with a seal member, not shown, interposed so as to close the opening h 2 . The acoustic couplant  43  is disposed between the path member  42  and the pipe P. 
     In the above arrangement (so-called Z-type arrangement), the ultrasonic waves transmitted by the detection element  31  are incident on the fluid in the pipe P at an incident angle θ through the path member  32  and the acoustic couplant  33 . The ultrasonic waves having passed through the fluid pass through the pipe P at an exit angle θ, and are received by the detection element  41  through the acoustic couplant  43  and the path member  42 . The ultrasonic waves transmitted by the detection element  41  are incident on the fluid in the pipe P at an incident angle θ through the path member  42  and the acoustic couplant  43 . The ultrasonic waves having passed through the fluid are transmitted through the pipe P at an exit angle θ, and are received by the detection element  31  through the acoustic couplant  33  and the path member  32 . 
     The control board  50  is connected to the main unit  30  in the casing  10  and connected to the main unit  40  in the casing  20  through a cable  5 . The control board  50  calculates the velocity and the flow rate of the fluid flowing in the pipe P based on an output signal indicating the result of the transmission and reception of the ultrasonic waves output from the main units  30 ,  40 , and also controls operations of various mechanisms in the flow meter  100 . 
     The power supply board  60  converts a high voltage (e.g., AC voltage of 100 V to 240 V) input from a plurality of terminals provided on the terminal block  80  to a low voltage (e.g., DC voltage of smaller than 36 V), and outputs the converted voltage to the control board  50  and the display board  70 . Note that the power supply board  60  may include a capacitor for power storage. As a result, even when a voltage input from the terminal block  80  is momentarily interrupted, the power supply board  60  can continue to output the voltage to the control board  50  and the display board  70 . 
     The display board  70  is provided with a display  71  including a reflecting member and a light shielding member. In the present embodiment, the display  71  is a segment display device, but may be a dot matrix display device or a liquid crystal display device. Various pieces of information such as the velocity and flow rate of the fluid, calculated by the control board  50 , are displayed on the display  71 . The user can visually recognize the display  71  from above the casing  10  through the window member  17 . 
     The terminal block  80  is provided with a plurality of terminals to be connected to devices outside the casing  10 . Detailed structures of the casing  10 , the control board  50 , and the terminal block  80  will be described later. 
     (2) Casing 
       FIG. 3  is a perspective view of the casing  10 . As shown in  FIG. 3 , one end face in the longitudinal direction of the casing  10  is referred to as an end face  10   c , and the other end face is referred to as an end face  10   d . On the end face  10   c  of the casing  10 , ports  13 ,  14  for connecting cables  3 ,  4  are formed so as to be aligned widthwise. The port  13  holds the cable  3  while passing the cable  3  from the outside to the inside of the casing  10 . The port  14  holds the cable  4  while passing the cable  4  from the outside to the inside of the casing  10 . 
     The cable  3  is provided with a plurality of electric wires for inputting a voltage from an external power supply  200  ( FIG. 4  described later) of the casing  10  to the power supply board  60  of  FIG. 2 . The external power supply  200  is a commercial power supply that supplies an AC voltage of 100 V to 240 V, for example. The cable  4  is provided with a plurality of electric wires for transmitting signals between an external device  300  ( FIG. 4  described later) and the control board  50  of  FIG. 2 . The external device  300  is mainly a programmable logic controller (PLC), but may be a personal computer (PC). A plurality of electric wires of the cables  3 ,  4  are connected to a plurality of terminals of the terminal block  80 . In  FIG. 3 , illustration of the electric wires of the cables  3 ,  4  is omitted. 
     On the end face  10   d  of the casing  10 , a connection part  15  connected to the control board  50  of  FIG. 2  inside the casing  10  is formed. One end of the cable  5  is connected to the connection part  15  of the casing  10  and the other end of the cable  5  is connected to the main unit  40  in the casing  20  of  FIG. 2 . A control signal (an excitation signal of the detection element  41 ) for control is given from the control board  50  to the main unit  40  through the cable  5 , and an ultrasonic output signal is given from the main unit  40  to the control board  50 . 
     On the main surface  10   b  of the casing  10 , vertically penetrating openings h 3 , h 4  in a substantially rectangular shape are formed so as to be aligned in this order longitudinally from the end face  10   c  toward the end face  10   d . In the casing  10 , the terminal block  80  is disposed at a position vertically overlapping the opening h 3  and close to the opening h 3 . As a result, the terminal block  80  in the casing  10  is exposed from the opening h 3 . Hence the user can easily perform the operation of connecting the plurality of electric wires in the cables  3 ,  4  to the plurality of terminals of the terminal block  80  and the operation of outputting the log data described later from above the casing  10 . 
     When the connection work and the log data output work are not performed, the cover member  16  for covering the opening h 3  is mounted on the main surface  10   b  of the casing  10  with a seal member, not shown, interposed. As a result, the terminal block  80  is protected, and the user is prevented from coming into contact with the terminal of the terminal block  80 . 
     The window member  17  is fitted into the opening h 4  with a seal member, not shown, interposed. The window member  17  is formed of glass, for example. In the casing  10 , the display  71  vertically overlaps the window member  17  and is disposed at a position close to the window member  17 . As a result, the user can visually recognize the display  71  in the casing  10  from above the casing  10  through the window member  17 . 
     On the main surface  10   b  of the casing  10 , a plurality of (three in the present example) operation parts  18  are provided so as to be aligned widthwise close to one side of the opening portion h 4 . The operation parts  18  are connected to the control board  50  through the display board  70  of  FIG. 2 . The operation parts  18  are used to input various pieces of information to the flow meter  100 . The various pieces of information include information (e.g., the inner diameter of the pipe P) necessary for calculating the velocity of the fluid flowing in the pipe P and a threshold concerning the flow rate of the fluid flowing in the pipe P. 
     An indicator lamp  19  is provided in the vicinity of the end face  10   d  on the main surface  10   b  of the casing  10 . The indicator lamp  19  includes, for example, a plurality of light emitting diodes for emitting light of different colors, and is connected to the control board  50  through the display board  70 . As will be described later, the flow meter  100  also operates as a flow rate switch. The indicator lamp  19  lights up in different display states (e.g., color) according to the operation state of the flow rate switch. 
     (3) Control Board 
       FIG. 4  is a block diagram showing a configuration of the control board  50 . As shown in  FIG. 4 , a microcomputer  51 , a control storage part  52 , a log storage part  53 , and a communication part  54  are mounted on the control board  50 . In  FIG. 4 , the control board  50  is illustrated by a dotted line, and the microcomputer  51  is illustrated by a one-dot chain line. The microcomputer  51  is achieved by, for example, a central processing unit (CPU), a built-in analog and digital (A/D) converter, and a built-in memory (cache memory, etc.). The microcomputer  51  includes a control part  51 A, a measurement part  51 B, a setting part  51 C, a calculation part  51 D, a signal output part  51 E, a lamp control part  51 F, a temperature acquisition part  51 G, a time acquisition part  51 H, and a display control part  51 I. 
     In the present embodiment, the measurement part  51 B and the temperature acquisition part  51 G are achieved by the built-in A/D converter of the microcomputer  51 , but the present invention is not limited thereto. For example, the measurement part  51 B and the temperature acquisition part  51 G may be achieved by an external A/D converter provided outside the microcomputer  51 . That is, a part or the whole of the functional blocks in the control board  50  of  FIG. 4  may be made up of hardware such as an electronic circuit. 
     The control storage part  52  is made up of, for example, a nonvolatile memory, a hard disk or a flash read only memory (ROM) and stores (holds) various pieces of data and control programs (system programs) for operating the flow meter  100 . By the control part  51 A reading and executing the system program of the control storage part  52 , the functions of the measurement part  51 B, the setting part  51 C, the calculation part  51 D, the signal output part  51 E, the lamp control part  51 F, the temperature acquisition part  51 G, the time acquisition part  51 H, and the display control part  51 I are achieved. 
     The log storage part  53  is made up of a nonvolatile memory such as an electrically erasable and programmable (EEP) ROM, for example, and stores various pieces of data that can be set by the user (rewritable by the user) in order to operate the flow meter  100 . 
     When executing the control program, the control part  51 A refers to data stored in the control storage part  52  or the log storage part  53  as necessary. In the present embodiment, the control storage part  52  and the log storage part  53  are achieved by separate memories, but the present invention is not limited thereto. For example, by allocating a storage area, the control storage part  52  and the log storage part  53  may be achieved by a common memory. 
     The flow meter  100  is brought into a startup state when the power supply is turned on. In the present embodiment, when the power supply is turned on, power (voltage) is supplied from the external power supply  200  to the microcomputer  51  via the plurality of terminals  81   a  to  81   c  provided on the terminal block  80  of  FIG. 2  and the power supply board  60  of  FIG. 2 . 
     In a startup state, the control part  51 A reads and executes the control program stored in the control storage part  52 , and executes startup processing (preparation) including sensing processing for calculating a normal flow rate and logging processing in every fixed period. In the present embodiment, the logging target, the logging cycle, and the logging start definition information which are necessary for the logging processing are previously incorporated into the control program. 
     Here, “the logging target, the logging cycle, and the logging start definition information are previously incorporated into the control program” means that the logging target, the logging cycle, and the logging start definition information are included in the control program. In other words, the logging target, the logging cycle, and the logging start definition information correspond to a part of the execution program of the control part  51 A. 
     Further, the control part  51 A reads the thresholds and the like which are stored in the log storage part  53  and used for on/off determination, and refers to and sets various parameters necessary for the sensing processing. Here, if an operation mode (a second operation mode described later) in which the logging processing is not executed has been selected by the user, the logging processing is not executed. Details of the pieces of data stored in the control storage part  52  and the log storage part  53  will be described later. 
     Next, the control part  51 A initializes the operation state of the flow meter  100 . In the initialization of the operation state, based on an instruction (command) of the control part  51 A, for example, optimization of the intensity of ultrasonic waves emitted from the detection elements  31 ,  41  via the measurement part  51 B and subtraction of an initial value (e.g., 0) for a variable to be used in the calculation part  51 D, or the like is performed. In the present embodiment, initialization processing is performed in response to an instruction from the control part  51 A, but the present invention is not limited thereto, and the initialization processing may be performed by a processing module (e.g., initialization part) other than the control part  51 A. 
     After ending of initialization, the startup of the measurement part  51 B, the calculation part  51 D, the signal output part  51 E, the lamp control part  51 F, the temperature acquisition part  51 G, the time acquisition part  51 H, and the display control part  51 I is completed. As a result, the startup state ends and the flow meter  100  is in a steady state. 
     Hereinafter, the operation of the flow meter  100  in a steady state will be described. The measurement part  51 B causes the detection elements  31 ,  41  to transmit and receive ultrasonic waves and acquires output signals from the detection elements  31 ,  41 . Based on the acquired output signals, the measurement part  51 B measures a difference (hereinafter referred to as a time difference) between the time until the detection element  41  receives the ultrasonic waves transmitted by the detection element  31  and the time until the detection element  31  receives the ultrasonic waves transmitted by the detection element  41 . The measurement result from the measurement part  51 B is given to the calculation part  51 D. 
     The setting part  51 C sets various pieces of information input by the user from the operation parts  18 . As described above, the information input from the operation parts  18  includes thresholds concerning the inner diameter of the pipe P and the flow rate of the fluid flowing in the pipe P. The inner diameter of the pipe P is used when the calculation part  51 D calculates the flow rate. The threshold is used when the signal output part  51 E generates a switching signal. The information input from the operation parts  18  includes an operation mode of the control part  51 A and information on a display screen to be displayed on the display  71 . 
     The operation mode of the control part  51 A includes first and second operation modes. Details will be described later. The information on the display screen is, for example, switching information used for switching the display on the display  71 , and is different from display format information (display position, display size, display color, display font, etc.) indicating how to display the logging target on the display  71 . For example, the display format information is information used for identifiably displaying the logging target on the display  71 . The display format information includes display layout information specifying that, for example, the maximum flow rate is displayed in the upper stage (upper stage display area  71   a  described later) of the display  71 , while the minimum flow rate is displayed in the lower stage (lower stage display area  71   b  described later). The display format information is stored in the control storage part  52 . A part of the setting of information by the setting part  51 C may be performed as an initial setting in the startup state. 
     The calculation part  51 D calculates an instantaneous value (instantaneous flow rate) of a flow rate Q of the fluid flowing in the pipe P based on Expression (1) below. Here, Δt is a time difference measured by the measurement part  51 B, and d is an inner diameter of the pipe P, set by the setting part  51 C. θ is an incident angle of ultrasonic waves, V s  is a velocity of the ultrasonic waves, and K is a flow rate correction coefficient for converting the velocity of the fluid having a predetermined distribution in the cross section of the pipe P into an average velocity. The incident angle θ, the velocity V s , and the flow rate correction coefficient K are known quantities. Further, the calculation part  51 D calculates an integrated value (integrated flow rate) of the fluid in the pipe P by integrating the calculated instantaneous flow rates. Moreover, the calculation part  51 D can also calculate a velocity V f  of the fluid flowing in the pipe P based on Expression (2) below. 
     
       
         
           
             
               
                 
                   
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     The signal output part  51 E generates a switching signal (ON/OFF signal) based on the comparison result between the threshold set by the setting part  51 C and the instantaneous flow rate calculated by the calculation part  51 D, and outputs the generated switching signal (ON/OFF signal) to the external device  300  through the terminals  82   a  to  82   d  and the cable  4  of  FIG. 3 . The switching signal is a binary signal for switching between the on-state and the off-state of the external device  300 . In this manner, the flow meter  100  can operate as a flow rate switch that changes the state of the switching signal to be given to the external device  300  based on whether or not the fluid is flowing in the pipe P at a flow rate not smaller than the threshold. 
     The terminals  82   a  to  82   d  are terminals that interface the external device  300  such as a PLC and are provided on the terminal block  80  of  FIG. 2  similarly to the terminals  81   a  to  81   c . When the external device  300  is a general-purpose PC or the like instead of the PLC, a communication interface that meets a predetermined standard similar to that the communication part  54  (described later) meets may be provided in the control board  50  instead of the terminals  82   a  to  82   d.    
     The lamp control part  51 F causes the indicator lamp  19  to light up so as to distinguish between the on-state and the off-state of the external device  300 . In the present embodiment, the indicator lamp  19  is lit in green when the external device  300  is in the on-state, and lit in red when the external device  300  is in the off-state. Thus, the user can easily distinguish between the on-state and the off-state of the external device  300 . 
     The temperature acquisition part  51 G acquires an instantaneous value (instantaneous temperature) of the temperature of the fluid, measured by the temperature measurement part  102 . Further, the flow meter  100  is provided with a time measurement part  103 . The time measurement part  103  includes, for example, a real-time clock that is operated by a secondary battery  104  independently of turning-on of the power supply, and measures time regardless of whether or not the flow meter  100  has been started up. In the present embodiment, the secondary battery  104  is a lithium battery that can be charged by turning-on of the power supply. The time acquisition part  51 H acquires the time measured by the time measurement part  103 . 
     Data in which the time measured by the time measurement part  103  and various pieces of information (logging target) acquired at the time are associated with each other is called log data. In the present embodiment, based on the setting by the setting part  51 C, a part or the whole of logging targets, such as the maximum flow rate, the minimum flow rate, the integrated flow rate, the maximum temperature, the minimum temperature, and the history of the occurrence of change in state of the switching signal (event, described later) can be included in the log data. In the present embodiment, since the period of the log data can be associated with the absolute time based on the real-time clock, it is possible to facilitate the management of the log data. 
     The control part  51 A selectively operates in the first operation mode and the second operation mode. Specifically, in the first operation mode, the control part  51 A stores the log data into the log storage part  53  in a predetermined format in every predetermined period (five minutes in the present example). In the present embodiment, the period cannot be set as a default value to the user, but the present invention is not limited to thereto. The period may be appropriately settable by the operation of the operation parts  18  or the like. On the other hand, the control part  51 A does not cause the log storage part  53  to store the logging target in the second operation mode. Therefore, the user can stop storing the log data by selecting the second operation mode in accordance with the intended use. In the following description, the control part  51 A operates in the first operation mode. 
     The log data is stored in a format (data format) made up of character strings (YEAR, MONTH, DAY, HOUR, MINUTE, FLOW_PEAK, FLOW_BOTTOM, TOTAL_PER_DAY, TEMP_PEAK, TEMP_BOTTOM, EVENT, line feed), for example. Specifically, the data is stored with each piece of data delimited by a comma and each record delimited by a line feed. Thus, the year, month, day, hour and minute when the log data was created, the maximum flow rate, minimum flow rate, integrated flow rate (in units of one day), maximum temperature, minimum temperature, and event history are associated with each other. There are three types of time resolution of the integrated flow rate, which are one day unit, one week unit, and one month unit, but in the above example, the time resolution is set as one day unit for the convenience of description. The time resolution may be one week unit or one month unit, such as TOTAL_PER_WEEK or TOTAL_PER_MONTH. 
     The data format may be various formats besides the above example. For example, each record may be represented by a comma and a tab, or a special character may be interposed between a record and a record. As described above, the logged logging target is stored in a predetermined data format assuming that the logging target is output by communication to a general-purpose PC or the like. The predetermined data is previously stored into the control storage part  52  and is read and referred to by the control part  51 A. 
     The communication part  54  includes, for example, a communication interface based on a predetermined standard (e.g., RS 232C standard). When the communication cable is connected to an output terminal  86  of  FIG. 13  described later, the control part  51 A gives the log data stored in the log storage part  53  to a management device  400  through the communication part  54  and the communication cable. The management device  400  is, for example, a PC, and can perform detailed management of the flow rate of the fluid based on the log data, analysis of a detailed cause when a trouble occurs in the pipe P, and the like. 
     Here, the transmission format at the time of transmitting the logging target may be the same as or different from the data format stored in the control storage part  52 . Specifically, in the present embodiment, the transmission format may be the same as a data format at the time of storage into the control storage part  52 , namely, a format (data format) made up of a character string (YEAR, MONTH, DAY, HOUR, MINUTE, FLOW_PEAK, FLOW_BOTTOM, TOTAL_PER_DAY, TEMP_PEAK, TEMP_BOTTOM, EVENT, line feed), or in addition to these, CHECKSUM may be added when an output is made to an external general-purpose PC by communication. CHECKSUM is data for error detection, and by adding this last, even if garbled characters occur during communication, the management device  400  can recognize the error. 
     Further, header information indicating the order of character strings may be included in the transmission format when the log data is transmitted. The header information is information such as a character string (YEAR, MONTH, DAY, HOUR, MINUTE, FLOW_PEAK, FLOW_BOTTOM, TOTAL_PER_DAY, TEMP_PEAK, TEMP_BOTTOM, EVENT). As a result, the management device  400  analyzes the header information to recognize that the character strings (numeral strings) to be received are in the order of YEAR, MONTH, DAY, HOUR, MINUTE, FLOW_PEAK, FLOW_BOTTOM, TOTAL_PER_DAY, TEMP_PEAK, TEMP_BOTTOM, and EVENT. 
     The display control part  51 I causes the display  71  to display a normal display screen and a data display screen in a switchable manner based on the setting of the setting part  51 C. The normal display screen is a screen for displaying the threshold set by the setting part  51 C, the instantaneous flow rate, the integrated flow rate or the velocity V f  calculated by the calculation part  51 D, the instantaneous temperature acquired by the temperature acquisition part  51 G, or the like. The data display screen is a screen for displaying information included in the log data in every period of the log data under the control of the control part  51 A. Detailed operations of the control part  51 A and the display control part  51 I will be described later. 
     (4) Control Storage Part and Log Storage Part 
       FIG. 5  is a diagram schematically showing various pieces of data stored in the control storage part  52 . As shown in  FIG. 5 , the control storage part  52  stores data that cannot be set by the user. Specifically, in the control storage part  52 , in addition to the control program, the logging target, the logging cycle, and the logging start definition information are stored. The logging target indicates an object to be logged among various parameters. The logging cycle indicates a constant cycle when logging targets are logged. The logging start definition information indicates information for defining the start of logging the logging target. 
     In the present embodiment, the logging start definition information is information specifying that the logging is started when the specific time is reached every five minutes after turning-on of the power supply for the flow meter  100  and after ending of the startup processing of the flow meter  100 .  FIG. 6  is a diagram for explaining execution timing of logging based on the logging start definition information. 
     In the example of  FIG. 6 , when the power supply is turned on for the flow meter  100  at 14:36, startup processing is started at the same time. At 14:39, the startup processing ends, and the sensing processing is started at the same time. After that, logging is executed at 14:40 as a specific time. Also, logging is repeated every five minutes such as 14:45, 14:50, 14:55, and 15:00 As described above, the logging start definition information includes whether or not a specific time, which comes in every predetermined time, has reached since ending of the startup processing. 
     In the present embodiment, as indicated by a set  1  in  FIG. 5 , six pieces of information, which are the maximum flow rate, the minimum flow rate, the maximum temperature, the minimum temperature, the integrated flow rate, and the event, are stored as the logging targets. For each of the maximum flow rate, the minimum flow rate, the maximum temperature, and the minimum temperature, “five minutes” is stored as the logging cycle. For the integrated flow rate, one day, one week, and one month are stored as the logging cycle. For the event, the event occurrence time is stored as the logging cycle. 
     “Five minutes” means that the logging is performed every five minutes at an absolute time, and for example, the logging is performed every five minutes at absolute times such as 14:05, 14:10, and 14:15. “One day” means that the logging is performed at an absolute time, such as 12 o&#39;clock. “One week” means that logging is performed, for example, at 12 o&#39;clock on Sunday, and “one month” means that the logging is performed at 12 o&#39;clock on the first day every month. “Event occurrence time” means that the logging is performed at the timing when an event such as an alarm occurs. 
     As described above, for the integrated flow rate, a plurality of logging cycles are associated. Hence, at the time of displaying the integrated flow rate in one week unit or one month unit, it is unnecessary to convert the integrated flow rate in one week unit to these flow rates one by one. On the other hand, when a configuration is provided to convert the integrated flow rate in one day unit to the integrated flow rate in one week unit or one month, the logging cycle of one week unit or one month unit does not need to be associated with the integrated flow rate. 
     As described above, in the present embodiment, the control storage part  52  stores, in addition to the control program, a control program (execution program) including the logging target, the logging cycle, and the logging start definition information, and when the control program is executed by the control part  51 A, the logging target is logged automatically. Also, as described above, the user can set none of the logging target, the logging cycle, and the logging start definition information described in the control program. 
     In this case, the control part  51 A reads and executes the control program in response to the turning-on of the external power supply  200 , so that the logging target is automatically stored into the log storage part  53  in every logging cycle without the user inputting a trigger for starting the logging into the flow meter  100 . This eliminates the need for the user to make a setting required for the logging, such as the logging target or the logging cycle. 
     In the present embodiment, the logging targets indicated by the set  1  in  FIG. 5 , the logging cycles, and the logging start definition information are previously incorporated into the control program, but the present invention is not limited thereto. Apart from the control program, each parameter of the logging target, the logging cycle, and the logging start definition information may be stored in the control storage part  52  as a default parameter that cannot be set by the user. 
     In this case, in response to the turning-on of the external power supply  200 , the control part  51 A reads and executes the control program and refers to default parameters of the logging target, the logging cycle, and the logging start definition information from the control storage part  52 . Also in this case, the logging target is automatically stored into the log storage part  53  in every logging cycle without the user inputting a trigger for starting the logging into the flow meter. This eliminates the need for the user to make a setting required for the logging, such as the logging target or the logging cycle. 
     Further, in the case where the logging targets indicated by the set  1  in  FIG. 5  according to the present embodiment are default parameters which are decided before setting by the user, when the user makes a setting related to the logging, the parameter may be changed from the default value to the value set by the user in accordance with the set content. For example, it may be configured such that the user can select a set of parameters different from those of the set  1  like a set  2  or a set  3  in  FIG. 5 . 
     In this case, the selection information may be stored into the log storage part  53 . For example, when the user selects the set  2 , the control part  51 A reads the selection information stored in the log storage part  53  and takes as the logging targets the four parameters which are the maximum flow rate, the minimum flow rate, the integrated flow rate, and the event. When the user selects the set  3 , the control part  51 A reads the selection information stored in the log storage part  53 , and takes as the logging targets the four parameters which are the maximum flow rate, the minimum flow rate, the maximum temperature and the minimum temperature. 
     In this manner, the user can automatically start the logging without making a setting related to the logging, whereas the user can reflect the setting content in the logging processing when making the setting related to the logging. 
       FIG. 7  is a diagram schematically showing various pieces of data stored in the log storage part  53 . As shown in  FIG. 7 , the data settable by the user is stored into the log storage part  53 . Specifically, setting parameters related to the logging processing and setting parameters related to the sensing processing are stored into the log storage part  53 . 
     The setting parameters related to the sensing processing include a threshold, an on/off operation setting, an output setting, and the like. The threshold is used for on/off determination in the flow rate switch. The on/off operation setting is to set whether to bring the switch into the on-state when the instantaneous value of the flow rate exceeds the threshold or to bring the switch into the off-state when the instantaneous value falls below the threshold. The output setting is to set whether to output a switching signal from the signal output part  51 E or output an analog signal indicating the calculated instantaneous flow rate value. 
     The setting parameters related to the logging processing include a mode parameter, a parameter set indicating a logging target at the time of outputting log data by communication, a baud rate (transmission velocity), an amount of data output at one time, and the like. The mode parameter indicates whether or not to enable the mode (second operation mode) for not executing the logging processing, and no (first operation mode) is set as a default value. Also, as described above, when the user makes a setting related to the logging, the setting information (e.g., selection information of the set  2  or the set  3  in  FIG. 5 ) is also stored in the log storage part  53 . 
     (4) Control Part 
       FIGS. 8A to 8D  are diagrams for explaining the operation of the control part  51 A. A vertical axis in  FIG. 8A  represents the instantaneous flow rate calculated by the calculation part  51 D. A vertical axis in  FIG. 8B  represents the integrated flow rate calculated by the calculation part  51 D. A vertical axis in  FIG. 8C  represents the instantaneous temperature acquired by the temperature acquisition part  51 G. A vertical axis in  FIG. 8D  represents the value (state) of the switching signal output by the signal output part  51 E. 
     Each of horizontal axes in  FIGS. 8A to 8D  represent the time from the time 14:40 to the time 15:00. In the description of  FIGS. 8A to 8D , a period from the time 14:40 to the time 14:45 is referred to as a period T 1 , and a period from the time 14:45 to the time 14:50 is referred to as a period T 2 . A period from the time 14:50 to the time 14:55 is referred to as a period T 3 , and a period from the time 14:55 to the time 15:00 is referred to as a period T 4 . 
     In the example of  FIG. 8A , a set of data including the maximum flow rate and the minimum flow rate is set as the information included in the log data. In this case, the control part  51 A causes the log storage part  53  to store the maximum and minimum instantaneous flow rates within each of the periods T 1  to T 4  among the instantaneous flow rates calculated by the calculation part  51 D as the maximum flow rate and the minimum flow rate, respectively. 
     Specifically, the control part  51 A sequentially holds the instantaneous flow rates continuously calculated by the calculation part  51 D in the period T 1 , and determines whether the newly held instantaneous flow rate is larger than the maximum instantaneous flow rate previously held within the period T 1  or smaller than the minimum instantaneous flow rate previously held within the period T 1 . When the newly held instantaneous flow rate is larger than the previously stored maximum instantaneous flow rate, the control part  51 A decides the newly held instantaneous flow rate as the maximum flow rate within the period T 1  and stores the newly stored instantaneous flow rate into the log storage part  53 . Similarly, when the newly held instantaneous flow rate is smaller than the previously stored minimum instantaneous flow rate, the control part  51 A decides the newly held instantaneous flow rate as the minimum flow rate within the period T 1 , and stores the newly held instantaneous flow rate into the log storage part  53 . 
     Also in each of the periods T 2  to T 4  after the period T 1 , the control part  51 A operates similarly. As a result, the periods T 1  to T 4  and the maximum flow rates and the minimum flow rates in the respective periods T 1  to T 4  are sequentially stored into the log storage part  53  as the log data of the periods T 1  to T 4 . In  FIG. 8A , “∘” and “□” are indicated for the maximum flow rate and the minimum flow rate in each of the periods T 1  to T 4 . 
     In the example of  FIG. 8B , a set of data including the integrated flow rate is set as the information included in the log data. In this case, the control part  51 A acquires the integrated flow rate calculated by the calculation part  51 D at a specific time (e.g., the end time of each of the periods T 1  to T 4 ) in each of the periods T 1  to T 4  and stores the acquired integrated flow rate into the log storage part  53 . As a result, the periods T 1  to T 4  and the integrated flow rates in the respective periods T 1  to T 4  are sequentially stored into the log storage part  53  as log data of the periods T 1  to T 4 . 
     In the example of  FIG. 8C , a set of data including the maximum temperature and the minimum temperature is set as the information included in the log data. In this case, the control part  51 A causes the log storage part  53  to store the highest and lowest instantaneous temperatures within each of the periods T 1  to T 4  among the instantaneous temperatures acquired by the temperature acquisition part  51 G as the maximum temperature and the minimum temperature, respectively. 
     Specifically, the control part  51 A sequentially holds the instantaneous temperatures continuously acquired by the temperature acquisition part  51 G in the period T 1 , and determines whether or not the newly held instantaneous temperature is higher than the highest instantaneous temperature previously held within the time period T 1  or lower than the lowest instantaneous temperature previously held within the time period T 1 . When the newly held instantaneous temperature is higher than the previously held highest instantaneous temperature, the control part  51 A decides the newly held instantaneous temperature as the maximum temperature within the period T 1 , and stores the newly held instantaneous temperature into the log storage part  53 . Similarly, when the newly held instantaneous temperature is lower than the previously held lowest instantaneous temperature, the control part  51 A decides the newly held instantaneous temperature as the minimum temperature within the period T 1 , and stores the newly held instantaneous temperature into the log storage part  53 . 
     Also in each of the periods T 2  to T 4  after the period T 1 , the control part  51 A operates similarly. As a result, the periods T 1  to T 4  and the maximum temperatures and the minimum temperatures within the respective periods T 1  to T 4  are sequentially stored into the log storage part  53  as the log data of the periods T 1  to T 4 . In  FIG. 8C , “∘” and “□” are indicated for the maximum temperature and the minimum temperature in each of the periods T 1  to T 4 . 
     Hereinafter, a change in state of the switching signal output by the signal output part  51 E is referred to as an event. In the example of  FIG. 8D , a history of events is set as information included in the log data. In this case, when an event occurs, the control part  51 A causes the log storage part  53  to store event information indicating the content of the event. 
     More specifically, in the periods T 1 , T 4 , the state of the switching signal does not change from the on-state. In this case, the control part  51 A does not cause the log storage part  53  to store the event information. On the other hand, in the period T 2 , the switching signal changes from the on-state to the off-state at time t 1 . Therefore, the control part  51 A causes the log storage part  53  to store the event information in the period T 2  and its occurrence time t 1 . Similarly, in the period T 3 , the switching signal changes from the off-state to the on-state at the time t 2 , so that the control part  51 A causes the log storage part  53  to store the event information in the period T 3  and its occurrence time t 2 . As a result, the periods T 2 , T 3  and the event information and the occurrence times t 1 , t 2  within the periods T 2 , T 3  are sequentially stored in the log storage part  53  as the log data of the periods T 2 , T 3 . 
     Here, in the present embodiment, the log storage part  53  includes a ring buffer, and the above-described log data (logging target) is sequentially stored into this ring buffer. When the log data is stored into all the storage areas of the ring buffer, the control part  51 A first deletes the log data stored last into the log storage part  53 , and then stores the latest log data into the storage area made available by the deletion. As a result, the log data stored last is overwritten with the latest log data. In this manner, a part of the log storage part  53  functions as the ring buffer, and the log data once stored into the ring buffer is held for a certain period until overwritten by the latest log data. 
     With this configuration, it is possible to prevent the log data from being not stored into the log storage part  53  due to insufficient capacity of the ring buffer. In the present embodiment, the log data in each period includes the maximum flow rate and the minimum flow rate, and does not include the instantaneous flow rate at other points in time. Similarly, the log data in each period includes the maximum and minimum temperatures and does not include instantaneous temperatures at other points in time. Therefore, the volume of the log data in each period is reduced, so that the log data in each period continues to be stored in the log storage part  53  for a long time before being overwritten. 
     (5) Display Control Part 
       FIGS. 9A to 9D  are views each showing a display screen of the display  71 . As shown in  FIGS. 9A to 9D , the display  71  has an upper stage display area  71   a  and a lower stage display area  71   b . Hereinafter, the three operation parts  18  are referred to as an up button  18   a , a down button  18   b  and a mode button  18   c , respectively. Further, continuously pressing the up button  18   a , the down button  18   b , or the mode button  18   c  for a certain period of time or longer is referred to as “long press”, and pressing the up button  18   a , the down button  18   b , or the mode button  18   c  shorter than the certain period of time is simply referred to as “push”. 
     By long-pressing the up button  18   a , the down button  18   b  and the mode button  18   c  at the same time, the user can give an instruction for switching between the normal display screen and the data display screen to the display control part  51 I. Every time the instruction is given, the display control part  51 I switches between the normal display screen of  FIG. 9A  and the data display screen of  FIG. 9B  displayed on the display  71 . 
     In the example of the normal display screen of  FIG. 9A , an instantaneous flow rate “35.6 (L/min)” calculated by the calculation part  51 D is displayed in the upper stage display area  71   a . An instantaneous temperature “25.4° C.” acquired by the temperature acquisition part  51 G is displayed in the lower stage display area  71   b . In the example of the data display screen of  FIG. 9B , “September 16” which is a month and a date when the storage of the log data in the period T 4  of  FIGS. 8A to 8D  was started is displayed in the upper stage display area  71   a . Also, “14:55” which is the time when the storage of the log data was started is displayed in the lower stage display area  71   b.    
     Here, as described above, display format information indicating how the log data is displayed on the display  71  is stored in the control storage part  52 . By reading the display format information from the control storage part  52  and referring to the read information, the control part  51 A causes the start month and date of logging to be displayed in the upper stage display area  71   a  and the start hour and minute of logging to be displayed in the lower stage display area  71   b , for example as shown in  FIG. 9B . Further, as will be described later with reference to  FIG. 10 , various types of log data are displayed so as to be distinguishable from each other. 
     By pressing the mode button  18   c  on the normal display screen shown in  FIG. 9A , the display is switched to the normal display screen shown in  FIG. 9C . On the normal display screen shown in  FIG. 9C , the current instantaneous flow rate similar to that in  FIG. 9A  is displayed in the upper stage display area  71   a , and the threshold as a reference for comparison with the instantaneous flow rate is displayed in the lower stage display area  71   b    
     On the normal display screen shown in  FIG. 9C , the threshold can be changed by pressing the up button  18   a  or the down button  18   b . In the example of  FIG. 9D , the threshold is changed from 30.0 to 32.0 by pressing the up button  18   a . In the present embodiment, the setting of the threshold is also changed simultaneously with the change of the display of the threshold. Specifically, the threshold stored in the log storage part  53  is overwritten with a new numerical value. 
     In order to prevent an unintentional setting change of the threshold, it is also possible to set a key lock state in which the change of display of threshold is not accepted. In the present embodiment, it is possible to set the key lock state by long-pressing the down button  18   b  and the mode button  18   c . In the key lock state, by long-pressing the down button  18   b  and the mode button  18   c , the key lock state can be canceled. 
     As described above, in the flow meter  100 , the user can freely set the threshold. Therefore, as will be described later with reference to  FIGS. 10A to 11H , when the user visually recognizes the log data and finds an abnormal value, the user can adjust the threshold trying to cancel the abnormal state, instead of immediately outputting the log data to the general-purpose PC. For example, when there are many abnormal values logged because the threshold has been too severe (the tolerance has been too narrow), the user may first consider loosening the threshold (widening the tolerance). 
       FIG. 10A to 10D  are views each showing a data display screen for displaying other information included in the log data. By pressing the mode button  18   c  in a state where the data display screen is displayed on the display  71 , the user can give an instruction for displaying other information included in the log data to the display control part  51 I. Every time the instruction is given, the display control part  51 I sequentially switches the data display screens of  FIG. 9B  and  FIGS. 10A to 10D  displayed on the display  71 . 
     In the example of the data display screen of  FIG. 10A , a maximum flow rate “1252 (L/min)” and a minimum flow rate “1053 (L/min)” in a specific period are respectively displayed in the upper stage display area  71   a  and the lower stage display area  71   b . In the example of the data display screen of  FIG. 10B , a maximum temperature “27.6° C.” and a minimum temperature “19.3° C.” in the specific period are respectively displayed in the upper stage display area  71   a  and the lower stage display area  71   b.    
     In the example of the data display screen of  FIG. 10C , an integrated flow rate “576258 (L/min)” in the specific period is displayed on the display  71 . As in the example of  FIG. 10C , when the digit of the integrated flow rate value exceeds a predetermined number, the value of the upper digit is displayed left-aligned in the upper stage display area  71   a  and the lower digit is displayed right-aligned in the lower stage display area  71   b.    
     As will be described later, the signal output part  51 E of  FIG. 4  is capable of outputting a switching signal from each of the first and second control channels. In the example of the data display screen of  FIG. 10D , “the first control channel” which is the control channel where the event occurred in a specific period and “OFF” which is the content of the event are displayed in the upper stage display area  71   a  and the lower stage display area  71   b , respectively. The display of  FIG. 10D  indicates that the switching signal output from the first control channel has changed from the on-state to the off-state in a specific period. 
       FIGS. 11A to 11H  are diagrams showing data display screens corresponding to respective periods in which log data is stored. As described above, the display format information indicating how the log data (logging target) stored in the log storage part  53  is displayed on the display  71  is previously stored in the control storage part  52  at the time of factory shipment. Therefore, it is not necessary for the user to separately set the setting for visually recognizing log data on the display  71 . 
     The data display screens of  FIGS. 11A to 11D  show the start times of periods T 4  to T 1  of  FIGS. 8A to 8D , respectively. The data display screens of  FIGS. 11E to 11H  show sets of the maximum flow rate and the minimum flow rate during the periods T 4  to T 1 , respectively. When an instruction to display the data display screen is given for the first time, the display control part  51 I causes the display  71  to display the data display screen of  FIG. 11A  showing the start time of the latest log data. 
     By pressing the up button  18   a  or the down button  18   b , the user can give an instruction for switching the data display screen of  FIGS. 11A to 11D  to the display control part  51 I. Every time the down button  18   b  is pressed, the display control part  51 I sequentially switches the displayed data display screen in the order from the state of  FIG. 11A  to the state of  FIG. 11D . Further, each time the up button  18   a  is pressed, the display control part  51 I sequentially switches the displayed data display screen in the order from the state of  FIG. 11D  to the state of  FIG. 11A . 
     When the mode button  18   c  is pressed with the data display screens of  FIGS. 11A to 11D  being displayed, the display control part  51 I respectively switches the data display screens to the displayed data display screen as shown in  FIGS. 11E to 11H . Thereafter, every time the down button  18   b  is pressed, the display control part  51 I sequentially switches the displayed data display screen in the order from the state of  FIG. 11E  to the state of  FIG. 11H . Further, every time the up button  18   a  is pressed, the display control part  51 I sequentially switches the displayed data display screen in the order from the state of  FIG. 11H  to the state of  FIG. 11E . 
     When the mode button  18   c  is pressed with the data display screen of  FIGS. 11E to 11H  being displayed, the display control part  51 I switches the displayed data display screen to data display screen displaying the other data included in the log data in the same period. When the user finds abnormality in the flow rate, temperature, event or the like in a specific period, the user can easily confirm the other information of the log data in the period by switching the data display screen. As a result, it is possible to simply analyze the cause of the abnormality occurrence. 
     In this manner, the user can perform simple management and determine the occurrence or non-occurrence of abnormality by using only the flow meter  100  without outputting the log data to the management device  400  outside the flow meter  100 . Further, when abnormality occurs, the user can simply analyze the cause by the flow meter, and can analyze the cause in detail by using the management device  400  outside the flow meter  100 . 
     (6) Terminal Block 
       FIG. 12  is a plan view showing the configuration of the terminal block  80  in the casing  10 . As shown in  FIG. 12 , the terminal block  80  is formed of an insulating material such as resin and has terminal mounting areas  81 ,  82  aligned widthwise. The terminal mounting areas  81 ,  82  are disposed so as to be respectively aligned with the ports  13 ,  14  along the longitudinal direction. 
     In the terminal mounting area  81 , terminals  81   a ,  81   b ,  81   c  formed of a metallic material such as copper are provided. The terminals  81   a ,  81   b  are connected to the voltage input part on the power supply board  60  of  FIG. 2 . The terminal  81   c  is connected to the reference potential of the power supply board  60 . 
     The cable  3  of  FIG. 3  includes a plurality of electric wires connected to a live terminal, a neutral terminal and a ground terminal of the external power supply  200  of  FIG. 4 , respectively. The tips of these electric wires are inserted through the port  13  from the end face  10   c  of the casing  10  to the inside and are electrically connected to the terminals  81   a  to  81   c , respectively. 
     In the terminal mounting area  82 , terminals  82   a ,  82   b ,  82   c ,  82   d  formed of a metallic material such as copper are provided. The terminals  82   a  to  82   d  are connected to the signal output part  51 E of  FIG. 4 . The terminal  82   a  constitutes a first control channel, and the terminal  82   c  constitutes a second control channel. A switching signal is output from each control channel. 
     The cable  4  of  FIG. 3  includes a pair of electric wires each connected to the first and second input/output channels of the external device  300  of  FIG. 4 , and includes the other pair of electric wires connected to a positive potential and a reference potential of an external control power supply. The tips of the pair of electric wires are inserted through the port  14  from the end face  10   c  of the casing  10  to the inside, and are electrically connected to the terminals  82   a ,  82   b , respectively. Similarly, the other pair of electric wires are inserted through the port  14  from the end face  10   c  of the casing  10  to the inside and electrically connected to the terminals  82   c ,  82   d , respectively. 
     In this embodiment, it is possible to select the output method for the switching signal by an NPN method and a PNP method. In the NPN method, the reference potential of the external device  300  is maintained so as to be equal to the positive potential of the control power supply (the potential of the terminal  82   b ). In the PNP method, the reference potential of the external device  300  is maintained so as to be equal to the reference potential of the control power supply (the potential of the terminal  82   d ). 
     According to this wiring connection, for example, an AC voltage of 100 V to 240 V supplied from the external power supply  200  is input into the power supply board  60  of  FIG. 2  through the cable  3  and the terminals  81   a  to  81   c  of the terminal block  80 . In the power supply board  60 , the input AC voltage is converted into a DC voltage of, for example, less than 36 V. The DC voltage converted by the power supply board  60  is output to the control board  50  and the display board  70  of  FIG. 2 . As a result, the flow meter  100  operates. 
     The switching signal output from the signal output part  51 E of  FIG. 4  is input into the first input/output channel of the external device  300  through the terminal  82   a  (first control channel) of the terminal block  80  and the cable  4 . Similarly, the switching signal output from the signal output part  51 E is input into the second input/output channel of the external device  300  through the terminal  82   c  (second control channel) of the terminal block  80  and the cable  4 . Hence it is possible to switch between the on-state and the off-state of the external device  300  corresponding to each input/output channel based on the flow rate of the fluid flowing in the pipe P. 
     Further, the external device  300  can output an instruction signal for giving various instructions to the flow meter  100  from the second input/output channel. In this case, the instruction signal output from the second input/output channel of the external device  300  is given to the control board  50  through the cable  4  and the terminal  82   c  (second control channel) of the terminal block  80 . 
     Between the terminal mounting area  81  and the terminal mounting area  82  and between each of the terminals  81   a ,  81   b , and  82   a  to  82   d  are electrically isolated from each other by the partition member  84  so as to be non-short-circuited. In the present embodiment, the terminal  82   a  and the terminal  82   c  are aligned so as to be aligned widthwise. Further, the terminal  82   b  and the terminal  82   d  are aligned widthwise at positions slightly lower than those of the terminals  82   a ,  82   c  at the positions displaced in the longitudinal direction from the terminals  82   a ,  82   c . It is thus possible to easily prevent the interference of the electric wires in the wiring connection. 
     A part of the partition member  84  positioned between the terminal mounting area  81  and the terminal mounting area  82  includes closing members  84   a ,  84   b . The user can expose the structure formed in the terminal block  80  by holding the upwardly projecting partition wall portions of each of the closing members  84   a ,  84   b  and pulling those portions upward so as to remove the closing members  84   a ,  84   b  from the terminal block  80 . 
       FIG. 13  is a plan view showing the terminal block  80  in a state in which the closing members  84   a ,  84   b  are removed. As shown in  FIG. 13 , as the closing member  84   a  of  FIG. 12  is detached from the terminal block  80 , the battery accommodating portion  85  for housing the secondary battery  104  of  FIG. 4  formed on the terminal block  80  is exposed. 
     In the present embodiment, the secondary battery  104  can be mounted on the closing member  84   a . When the secondary battery  104  is mounted, the closing member  84   a  is fitted into the battery accommodating portion  85 , whereby the secondary battery  104  is accommodated in the battery accommodating portion  85  and the battery accommodating portion  85  is closed. The secondary battery  104  is charged by the external power supply  200  of  FIG. 4  in the battery accommodating portion  85  and supplies power to the time measurement part  103  of  FIG. 4 . In the present embodiment, it is unnecessary to frequently replace the secondary battery  104 , so that the maintainability of the flow meter can be improved. 
     By removing the closing member  84   b  of  FIG. 12  from the terminal block  80 , the output terminal  86  formed on the terminal block  80  is exposed. By inserting a predetermined communication cable (RS 232C cable in the present example) connected to the management device  400  of  FIG. 4  into the output terminal  86 , the user can output the log data recorded in the log storage part  53  of  FIG. 4  to the management device  400  and save the output data. The user can easily perform the connection work and the log data output work from above the casing  10  through the opening h 3 . 
     (7) Control Processing of Flow Meter 
       FIG. 14  is a flowchart showing the algorithm of the control processing of the flow meter  100  executed by the control program. As shown in  FIG. 14 , the control processing includes the startup processing executed in a startup state, and the sensing processing and the logging processing executed in a steady state. Hereinafter, the control processing of the flow meter  100  will be described mainly with reference to  FIG. 4 . 
     In the startup processing in the startup state, the microcomputer  51  is started up in response to turning on the power supply (Step S 1 ). Next, the control part  51 A (or the microcomputer  51 ) initializes the operation state of the flow meter  100  (Step S 2 ). As a result, the inner diameter of the pipe P, the thresholds and the like stored in the log storage part  53  are read. These inner diameters and thresholds are information set by the setting part  51 C and stored in the log storage part  53  during the previous operation of the flow meter  100 . Further, in the present example, the first operation mode is set as the operation mode of the control part  51 A. Moreover, the control part  51 A reads the control program stored in the control storage part  52 . The logging targets such as the maximum flow rate, the minimum flow rate and the integrated flow rate are previously incorporated into this control program. Thereafter, the startup state ends, and the flow meter  100  comes into the steady state. In the steady state, the sensing processing and the logging processing are executed in parallel and independently (Steps S 3  and S 4 ). 
       FIG. 15  is a flowchart showing the algorithm of the sensing processing in Step S 3  of  FIG. 14 . In the sensing processing, first, the measurement part  51 B causes the detection elements  31 ,  41  to transmit and receive ultrasonic waves (Step S 11 ). Next, the measurement part  51 B measures the time difference based on the output signals from the detection elements  31 ,  41  (Step S 12 ). 
     The calculation part  51 D calculates the instantaneous flow rate based on the time difference measured in Step S 12  and the inner diameter of the pipe P set in Step S 2  (Step S 13 ). Among the instantaneous flow rates calculated in Step S 13 , the control part  51 A holds the instantaneous flow rate larger than the previous maximum instantaneous flow rate and the instantaneous flow rate smaller than the previous minimum instantaneous flow rate as the maximum flow rate and the minimum flow rate, respectively (Step S 14 ). 
     The calculation part  51 D calculates the integrated flow rate by integrating the instantaneous flow rate calculated in Step S 13  (Step S 15 ). The signal output part  51 E compares the instantaneous flow rate calculated in Step S 13  with the threshold set in Step S 2  (Step S 16 ). Thereafter, the signal output part  51 E outputs a switching signal based on the comparison result of Step S 16  (Step S 17 ), and the processing is returned to Step S 11 . In the sensing processing, Steps S 11  to S 17  described above are repeated. 
       FIG. 16  is a flowchart showing the algorithm of the logging processing in Step S 4  of  FIG. 14 . In the present embodiment, as described above, the control part  51 A reads from the control storage part  52  the control program with the logging target, the logging cycle, and the logging start definition information incorporated thereinto and executes the read program, so that the logging processing is automatically started in parallel with and independently of the sensing processing. 
     In the logging processing, first, the display control part  51 I updates the display content of the display  71  (Step S 21 ). As a result, the display screen of  FIG. 9  to  FIG. 11  or the like is displayed on the display  71  based on the setting of the setting part  51 C. Further, the setting part  51 C accepts the operation of the operation parts  18  (Step S 22 ). Thereafter, the setting part  51 C updates the setting based on the operation accepted in Step S 22  (Step S 23 ). 
     Here, the control part  51 A determines whether or not the first operation mode is selected in the setting after updating in Step S 23  (Step S 24 ). When the first operation mode is not selected, namely, when the second operation mode is selected, the control part  51 A returns to Step S 21 . When the first operation mode is selected, the control part  51 A confirms the time based on the time acquired by the time acquisition part  51 H (Step S 25 ). 
     Next, the control part  51 A determines whether or not a predetermined period has elapsed (Step S 26 ). When the period has not elapsed, the control part  51 A returns the processing to Step S 21 . When the period has elapsed, the control part  51 A acquires the set of the maximum flow rate and the minimum flow rate held in Step S 14  and the integrated flow rate calculated in Step S 15  as log data (Step S 27 ). 
     Thereafter, the control part  51 A initializes by updating the maximum instantaneous flow rate and the minimum instantaneous flow rate in Step S 14  to the latest instantaneous flow rate (Step S 28 ). Further, the control part  51 A causes the log storage part  53  to store the log data acquired in Step S 27  (Step S 29 ) and returns the processing to Step S 21 . In the logging processing, Steps S 21  to S 29  described above are repeated. 
     In the above control processing, some of the processing may be executed at other points in time. For example, in the sensing processing of  FIG. 15 , either Step S 14 , Step S 15  or Steps S 16 , S 17  may be executed first or simultaneously. In the logging processing of  FIG. 16 , either Step S 21  and Steps S 22 , S 23  may be executed first. Any one of Step S 28  and Step S 29  may be executed first or Steps S 28 , S 29  may be executed at the same time. 
     In the above control processing, when a combination of the maximum temperature and the minimum temperature is set as the log data, the temperature acquisition part  51 G acquires the instantaneous temperature from the temperature measurement part  102  in parallel with Steps S 11  to S 13 . In Step S 14 , among the acquired instantaneous temperatures, the control part  51 A holds the instantaneous temperature higher than the previous maximum instantaneous temperature as the maximum temperature, and holds the instantaneous temperature lower than the previous minimum instantaneous temperature as the minimum instantaneous temperature. 
     Thereafter, in Step S 27 , the control part  51 A acquires a set of the held maximum temperature and minimum temperature as log data. In Step S 28 , the control part  51 A initializes the maximum instantaneous temperature and the minimum instantaneous temperature by updating those temperatures to the latest instantaneous temperatures. 
     In the above control processing, when the event history is set as the log data, the control part  51 A holds the state of the switching signal output in Step S 17 . Thereafter, when a change in state of the held switching signal occurs in Step S 27 , the control part  51 A acquires the content of the event as the log data. 
     (8) Effect 
     In the flow meter  100  according to the present embodiment, in response to reception of power from the external power supply  200 , the control part  51 A initializes the operation states of the detection elements  31 ,  41 . After the initialization, ultrasonic waves are transmitted and received to and from the fluid flowing in the pipe P by the detection elements  31 ,  41 , and the calculation part  51 D calculates the instantaneous flow rate and integrated flow rate of the fluid in the pipe P. Further, the temperature acquisition part  51 G acquires the temperature measured by the temperature measurement part  102 , and the time acquisition part  51 H acquires the time measured by the time measurement part  103 . Then, the calculated instantaneous flow rate is displayed on the display  71  in real time. In this manner, the sensing processing is performed as the steady state of the flow meter  100 . 
     On the other hand, the logging processing for logging a predetermined logging target is performed in parallel with and independently of this sensing processing. Specifically, the control part  51 A reads and executes the control program stored in the control storage part  52 . The control program includes a logging target, a logging cycle, and logging start definition information. 
     Therefore, the control part  51 A causes the log storage part  53  to automatically store the time measured by the time measurement part  103  and the maximum flow rate and the minimum flow rate calculated by the calculation part  51 D as log data in every predetermined logging cycle. The log data may include a part or the whole of the integrated flow rate calculated by the calculation part  51 D, the maximum temperature and the minimum temperature acquired by the temperature acquisition part  51 G, and the event history in the signal output part  51 E. 
     With this configuration, in order to store the log data in the log storage part  53 , there is no need for the user to perform a separate setting (operation). In particular, the logging start definition information for starting the logging at a specific time every five minutes after ending of the start-up processing of the flow meter  100  is included in the control program, so that the logging processing is started automatically. Therefore, it is guaranteed that the log data is always stored in the log storage part  53  after starting the flow meter  100 . It is thus possible to log data more reliably. As a result, the user can easily manage the usage of the fluid based on the log data. 
     Further, in the above configuration, inappropriate information before initialization of the operation states of the detection elements  31 ,  41  is prevented from being stored into the log storage part  53  as log data. Moreover, when the user finds abnormality in the flow rate, temperature, event, or the like in a specific period, the user can easily confirm other information in the period. As a result, it is possible to easily and simply analyze a cause when abnormality occurs. 
     (9) Other Embodiments 
     (a) In the above embodiment, the calculation part  51 D calculates the flow rate of the fluid flowing in the pipe P by Expression (1) based on the propagation time difference method, but the present invention is not limited thereto. The calculation part  51 D may calculate the flow rate of the fluid flowing in the pipe P based on the Doppler method. In this case, one of the detection elements  31 ,  41  may be made up of an ultrasonic transmission element, and the other one of the detection elements  31 ,  41  is made up of an ultrasonic reception element. 
     (b) In the above embodiment, the main units  30 ,  40  are provided in a so-called Z-type arrangement, but the present invention is not limited thereto. In a case where the pipe P is relatively small, the main units  30 ,  40  may be provided in an arrangement (so-called V-type arrangement) aligned in the direction in which the pipe P extends. In this configuration, the main unit  40  may be provided not in the casing  20  but in the casing  10 . 
     In the V-type arrangement, the ultrasonic waves transmitted by the detection element  31  are incident on the fluid in the pipe P at the incident angle θ through the path member  32  and the acoustic couplant  33 . The ultrasonic waves having passed through the fluid are reflected by the inner surface of the pipe P at a reflection angle θ and received by the detection element  41  through the acoustic couplant  43  and the path member  42 . Similarly, the ultrasonic waves transmitted by the detection element  41  are incident on the fluid in the pipe P at an incident angle θ through the path member  42  and the acoustic couplant  43 . The ultrasonic waves having passed through the fluid are reflected by the inner surface of the pipe P at the reflection angle θ and received by the detection element  31  through the acoustic couplant  33  and the path member  32 . 
     (c) In the above embodiment, the flow meter  100  includes the casing  20 , but the present invention is not limited thereto. When the main units  30 ,  40  are provided in the V-shaped arrangement described above, the flow meter  100  does not need to include the casing  20 . Further, when a main unit having the same function as that of the main unit  40  is separately provided, the flow meter  100  does not need to include the casing  20  or the main unit  40 . 
     (d) In the above embodiment, the flow meter  100  is an ultrasonic flow meter that measures the ultrasonic waves and calculates the flow rate of the fluid, but the present invention is not limited thereto. The flow meter  100  may be an electromagnetic flow meter that measures the electromotive force of the electrically conductive fluid to calculate the flow rate of the fluid or may be a vortex flow meter that measures the flow rate of the fluid by measuring the Karman vortex generated by the fluid flow. 
     Alternatively, the flow meter  100  may be an impeller type flow meter that detects the magnetism generated by the fluid flow with an element provided at the tip of the impeller to calculate the flow rate of the fluid, or may be a thermal type flow meter that heats the fluid and detects a temperature distribution of the heated fluid to calculate the flow rate of the fluid. 
     (e) In the above embodiment, the maximum flow rate and the minimum flow rate within the predetermined period are stored in the log storage part  53  as log data, but the present invention is not limited to this. The instantaneous flow rate at each time within a predetermined period may be stored into the log storage part  53  as log data. 
     Similarly, the maximum temperature and the minimum temperature within a predetermined period are stored into the log storage part  53  as log data, but the present invention is not limited to this. Instantaneous temperatures at respective times within a predetermined period may be stored into the log storage part  53  as log data. Alternatively, a representative temperature (e.g., average temperature) within a predetermined period may be stored into the log storage part  53  as log data. 
     (f) In the above embodiment, the time measurement part  103  is a real-time clock that measures time independently of turning-on of the power supply, but the present invention is not limited thereto. The time measurement part  103  may be a count-up timer for measuring the time from the operation start time of the flow meter  100 . In this case, the time measurement part  103  may operate by turning on the power supply, and the secondary battery  104  does not need to be provided in the flow meter  100 . 
     (g) In the above embodiment, the flow meter  100  includes the display  71 , but the present invention is not limited thereto. When the flow rate calculated by the flow meter  100  or the information based on the log data or the like can be displayed on the display device outside the flow meter  100 , the flow meter  100  does not need to include the display  71 . 
     (h) In the present embodiment, logging is started when the specific time comes every five minutes after ending of the start-up processing of the flow meter  100 , by incorporating the logging start definition information into the control program, but the present invention is not limited thereto.  FIG. 17  is a diagram for explaining execution timing of logging based on logging start definition information in another embodiment. 
     In the example of the pattern  1  of  FIG. 17 , a predetermined delay time T 1  of five minutes to ten minutes both inclusive is provided after ending of the startup processing at 14:39. Thereafter, the logging is executed at 14:50 as a specific time. Also, the logging is repeated every five minutes, such as 14:55 and 15:00. In this case, the logging start definition information includes whether or not a specific time coming at regular time intervals has elapsed since ending of the startup processing and elapse of the predetermined delay time T 1 . This is determined by the control part  51 A. 
     In the example of the pattern  2 , after turning-on of the power supply at 14:36, a predetermined delay time T 2  of five minutes and ten minutes both inclusive is provided. After that, the logging is executed at 14:45 as a specific time. The logging is repeated every five minutes, such as 14:50, 14:55 and 15:00. Pattern  2  may be used when the fluctuation of the time required for the startup processing is short. In this case, the logging start definition information includes whether or not a specific time coming at regular time intervals has elapsed since turning-on of the power supply and elapse of a predetermined delay time T 2 . This is determined by the control part  51 A. 
     In the example of pattern  3 , the startup processing ends at 14:39 and the logging is executed at the same time. Also, logging is repeated every five minutes such as 14:44, 14:49, 14:54, and 14:59. In this case, the logging start definition information includes whether or not the startup processing has ended. This is determined by the control part  51 A. 
     In the example of pattern  4 , the power supply is turned on at 14:36, and the logging is executed at the same time. The logging is repeated every five minutes, such as 14:41, 14:46, 14:51, and 14:56. In this case, the logging start definition information includes whether or not the power supply has been turned on. This is determined by the control part  51 A. In Patterns  3  and  4 , there is no need to measure the absolute time, so the time measurement part  103  does not need to include the real-time clock. 
     Here, as described above, in the pattern  3  shown in  FIG. 17 , whether or not the startup processing has ended is determined by the control part  51 A, but the present invention is not limited thereto. For example, in the control program, the processing procedure of the logging processing may be associated with the processing procedure of the starting processing so that the logging processing is automatically started when the starting processing ends. In this case, the control part  51 A does not determine whether or not the startup processing has ended, and the control program is merely executed. That is, the logging start definition information corresponds to (a part of) the execution program executed by the control part  51 A. 
     Similarly, in the pattern  4  shown in  FIG. 17 , the control part  51 A determines whether or not the power supply has been turned on, but the present invention is not limited thereto. For example, in the control program, the processing procedure of the logging processing may be associated with the processing procedure of power-on or the processing procedure to be executed first so that the logging processing is automatically started when power supply is turned on. In this case, the control part  51 A does not determine whether or not the power supply has been turned on, and the control program is merely executed. That is, the logging start definition information corresponds to (a part of) the execution program executed by the control part  51 A. 
     In this manner, the control part  51 A automatically starts the logging target logging, based on the logging start definition information stored in the control storage part  52 , regardless of whether or not to determine the start timing of the logging processing, and stores the measurement time and the numerical value of the logging target in association with each logging cycle in the log storage part  53 . 
     (i) In the above embodiment, the instantaneous flow rate may be logged. Specifically, the control part  51 A stores the instantaneous flow rate calculated by the calculation part  51 D in, for example, a random access memory (RAM) every one second. Further, the control part  51 A causes the log storage part  53  to store the average value (mL/min) per part time (e.g., one minute) of the instantaneous flow rate stored in the RAM in association with the time measured by the time measurement part  103 . In this example, since the logging cycle is five minutes, five instantaneous flow rates are logged in each period. 
     With this configuration, the calculation part  51 D can decide the maximum flow rate and the minimum flow rate in each period based on the instantaneous flow rate stored in the log storage part  53  and can calculate the integrated flow rate in each period. Further, the display  71  can display such information on the data display screen. Therefore, it is not necessary for the maximum flow rate and the minimum flow rate to be logged as logging targets, or the integrated flow rate does not need to be logged. 
     (10) Correspondence Relation Between Each Constituent Element of Claim and Each Part of Embodiment 
     Hereinafter, an example of correspondence between each constituent element of the claims and each part of the embodiment will be described, but the present invention is not limited to the following example. As each constituent element of the claims, various other elements having the constitution or function described in the claims can be used. 
     In the above embodiment, the external power supply  200  is an example of a power supply, the flow meter  100  is an example of a flow meter, the pipe P is an example of a pipe, the detection elements  31 ,  41  are examples of a detection element, the calculation part  51 D is an example of a flow rate calculation part. The control storage part  52  is an example of a control storage part, the log storage part  53  is an example of a log storage part and a ring buffer, and the time measurement part  103  is an example of a time measurement part and a real-time clock. 
     The control part  51 A is an example of a control part, the display  71  is an example of a display, the operation parts  18  are an example of an operation part, a parameter selection part or a mode selection part, and the setting part  51 C is an example of a setting part. The signal output part  51 E is an example of the first and second signal output parts, the temperature measurement part  102  is an example of the temperature measurement part, the output terminal  86  is an example of the data output part, and the secondary battery  104  is an example of a secondary battery.