JP2009244215A - Flowmeter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a flowmeter capable of heightening switching accuracy of display state/non-display state of a flow pilot display. <P>SOLUTION: In this flowmeter equipped with a flow measuring part 3 for measuring intermittently an instantaneous flow rate of fluid passing through a channel, and an accumulated flow rate display part 51 for displaying an accumulated flow rate determined from the measured instantaneous flow rate and a measuring interval, a switching means is provided, for switching a display state for performing a visually-recognizable flow pilot display when a flow rate passing through the channel 1 exists, to/from a non-display state for not performing the flow pilot display when the flow rate does not exists, based on the first determination flow rate which is a representative flow rate of each flow rate passing through the channel during a prescribed period, and the first display flow rate threshold Qth1 which is a switching flow rate threshold from the non-display state to the display state is different from a non-display flow rate threshold Qth0 which is a switching flow rate threshold from the display state to the non-display state, and the first display flow rate threshold Qth1 is set larger than the non-display flow rate threshold Qth0. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a flow meter for measuring a flow rate of a fluid such as gas.

  In recent years, it has been considered to employ an ultrasonic flow meter to measure the amount of gas used in a gas meter. Such an ultrasonic flowmeter includes a flow rate measurement unit that intermittently measures an instantaneous flow rate of a fluid passing through a flow path, and an integrated flow rate that is obtained from the instantaneous flow rate measured by the flow rate measurement unit and a measurement interval. This is a representative example of a flow meter provided with a flow rate display unit.

  In the following description, the ultrasonic flowmeter will be mainly described. As is well known, an ultrasonic flow meter is arranged such that a pair of ultrasonic sensors are opposed to each other so as to cross a flow path so as to cross a flow path of a flow rate through which a fluid passes. The propagation time of ultrasonic waves propagating between the acoustic sensors is obtained, the instantaneous flow rate of the fluid passing through the flow path is obtained from the pair of propagation times, the integrated flow rate is calculated and displayed on the display unit (Patent Document 1).

  The ultrasonic flowmeter is provided with a liquid crystal display unit as shown in FIG. The flow rate display unit includes an integrated flow rate display unit 51 and a flow rate pilot display unit 52. The integrated flow rate display unit 51 is a display unit that displays the integrated flow rate, and the flow rate pilot display unit 52 is for displaying whether or not the flow rate is currently flowing.

  This flow pilot display is used to confirm that the gas meter is normally installed when the gas meter, which is a flow meter, is installed, and that there is no such thing as reverse installation. When a gas supply operator is in charge, the meter gas stopper on the upstream side of the gas meter is used to determine whether to manually close the meter gas stopper.

  Conventionally, the average value of instantaneous flow rate every 30 seconds (in the case of intermittent operation every 2 seconds, the average value of instantaneous flow rate for 15 times is updated every 30 seconds) is a constant threshold, for example, 4.5L ( If the flow rate pilot display unit 52 is turned on, the display state is set to be in the display state. If it does not exceed 4.5 L / h, the display is turned off to be in the non-display state.

Japanese Patent Laying-Open No. 2005-098794

By the way, when the supply pressure fluctuates temporarily due to the start of the use of a large water heater in the neighboring house or the governor control of the pipeline network, the gas is not contained in the meter even when the gas is not used on the downstream side of the flow meter. Flow may occur.
As described above, when switching the display state / non-display state of the flow rate pilot display based on the average value of the instantaneous flow rate for 30 seconds (that is, 15 times), the flow of 100 L / h even once out of 15 times. Even if all the rest are 0 L / h, the average value is 6.6 L / h, which exceeds the threshold value and switches to the display state. That is, even when the gas is not used, a display indicating that the gas is flowing is made, resulting in inconvenience such as causing misunderstanding of the customer.

  In addition, since the display / non-display state of the pilot display is switched every 30 seconds, there is also a problem that the response speed is slow as shown by a broken line in FIG. 4 described later. That is, when starting to use the gas equipment, there is a delay of up to 30 seconds until the pilot display is in the display state, and when the use of the gas equipment is finished, it is maximum until the pilot display is in the non-display state. A delay of 30 seconds occurs.

  The present invention has been made in view of the above reasons, and an object of the present invention is to provide a flow meter with improved switching accuracy between the display state / non-display state of the flow rate pilot display.

In order to achieve the above object, according to the present invention, a flow rate measurement unit that intermittently measures an instantaneous flow rate of a fluid passing through a flow path, and an integrated flow rate obtained from the instantaneous flow rate measured by the flow rate measurement unit and a measurement interval are obtained. The characteristic configuration of the flow meter with the integrated flow rate display to display is
Based on the first judgment flow rate that is a representative flow rate of the instantaneous flow rate measured during a predetermined period, a display state for performing flow rate pilot display that can be visually recognized when there is a flow rate that passes through the flow path, and when there is no flow rate, It has switching means to switch to the non-display state that does not perform flow rate pilot display,
The first display flow rate threshold that is the switching flow rate threshold value from the non-display state to the display state is different from the non-display flow rate threshold value that is the switching flow rate threshold value from the display state to the non-display state, One display flow rate threshold value is larger than the non-display flow rate threshold value, and is set to a value smaller than that associated with the lower limit value of the flow rate consumed when the fluid consuming device provided on the downstream side starts operation. It is in.

  This flow meter performs display / non-display of the flow rate pilot display by comparing the first determination flow rate with the first display flow rate threshold value and the non-display threshold value. Here, the relationship between the non-display threshold value and the first display flow rate threshold value is that the latter is larger than the former, and the first display flow rate threshold value is a flow rate that is consumed with the start of operation of the fluid consuming device provided on the downstream side. It is related to the lower limit value of and is determined as a smaller value. As a result, in the display change to the display side of the flow rate pilot display, it is possible to realize display switching in a state where the influence of disturbance from the upstream side is avoided, corresponding to the start of operation of the fluid consuming device provided on the downstream side.

  And a second display flow rate threshold value that is set on a smaller flow rate side than the first display flow rate threshold value, wherein the switching means is in a non-display state in which the first determination flow rate is smaller than the first display flow rate threshold value. It is preferable that the non-display state is switched to the display state when the representative value of the instantaneous flow rate measured during the period is larger than the second display flow rate threshold value.

  When a configuration that has a hysteresis that makes the first display flow rate threshold value different from the non-display threshold value is adopted, for example, a minute gas leak occurs after the flow rate is zero and the flow rate pilot display is in the non-display state. Then, the average of the instantaneous flow rate becomes a minute flow rate value from 0 at a certain point and continues as it is. At any timing, the condition of the first judgment flow rate> the first display flow rate threshold value is not satisfied, and there is a possibility that the flow rate pilot display is not displayed. There is sex. Therefore, a configuration is adopted in which a second display flow rate threshold is set and the flow rate pilot display is switched to the display state when the second determination flow rate (for example, the minimum value of the instantaneous flow rate during a predetermined period) exceeds this flow rate threshold value. Thus, the above problem can be solved.

  The non-display flow rate threshold may be a flow rate associated with a boundary value at which the flow meter determines that there is a leak downstream of the flow meter. By configuring in this way, the flow pilot display is maintained until the flow rate passing through the flow path falls below the flow rate associated with the boundary value that the flow meter determines to be leaking downstream from the flow meter. Can do.

  The second display flow rate threshold may be a flow rate associated with a boundary value at which the flow meter determines that there is a leak downstream from the flow meter. With this configuration, the flow rate passing through the flow path is smaller than the first display flow rate threshold value and exceeding the flow rate associated with the boundary value at which the flow meter determines that there is a leak downstream from the flow meter. The flow pilot display can be appropriately displayed.

  As said 1st determination flow volume, it can be set as the average value of the instantaneous flow volume calculated | required for every intermittent drive interval of instantaneous flow volume. In the case of this configuration, an average value can be obtained for each intermittent drive interval of the instantaneous flow rate, so that switching between the display state / non-display state of the flow rate pilot display can be made appropriate without delay.

  The second determination flow rate can be the minimum value of the instantaneous flow rate obtained at every intermittent drive interval of the instantaneous flow rate. In the case of this configuration, since the flow rate pilot display is in the non-display state, for example, even when a minute gas leak occurs, switching from the non-display state of the flow rate pilot display to the display state can be made appropriate.

  Regarding the measurement of instantaneous flow rate, a pair of ultrasonic sensors is provided in the flow rate measurement unit, and the instantaneous flow of the fluid passing through the flow path is determined based on the propagation time of the ultrasonic waves propagating between the pair of ultrasonic sensors. By using an ultrasonic flowmeter for determining the flow rate, highly accurate flow rate measurement can be realized with a relatively simple configuration.

  Embodiments of the present invention will be described below with reference to the drawings, taking an ultrasonic gas meter as an example. FIG. 1 is a schematic structural diagram showing a schematic structure of the ultrasonic gas meter 10 according to the present application, and FIG. 2 is a diagram showing a display state of a display unit 50 provided on the surface of the ultrasonic gas meter 10.

  The ultrasonic gas meter 10 is provided for each gas consumer to which the gas G is supplied, and serves primarily for measuring the total gas flow rate supplied to each gas consumer. Furthermore, when an abnormal use state of the gas G is confirmed on the supply side (for example, when a flow rate of a predetermined value or more is continuously used for a long time), an emergency shutoff can be performed to shut off the gas supply. It is configured.

  The ultrasonic gas meter 10 is provided with a shutoff valve 2 and a flow rate measuring unit 3 in a flow path 1 through which a gas G passes, and is disposed in a shutoff valve drive circuit 7 and a flow rate measuring unit 3 that drive the shutoff valve 2. An ultrasonic sensor drive circuit 8 is provided for driving the pair of ultrasonic sensors 3a and 3b according to a predetermined sequence. The shut-off valve drive circuit 7 and the ultrasonic drive circuit 8 are configured to work according to the drive control command from the control unit 40, and the ultrasonic drive circuit 8 controls the measurement results from the ultrasonic sensors 3a and 3b. It is configured to return to the unit 40. Further, the display unit 50 and the storage unit 60 are provided, and a configuration in which the flow rate information obtained by the control unit 40 is displayed on the display unit 50 is adopted, and the flow rate information is stored in the storage unit 60. The following predetermined processing can be executed. The control unit 40 includes a microcomputer (hereinafter, simply referred to as “microcomputer”) as a main component. This microcomputer is configured to execute an instantaneous flow rate detection process, an integrated flow rate calculation process, a pilot display information generation process, and a shutoff information generation process for the shutoff valve 2 as main processes.

In the illustrated example, the shut-off valve 2 is arranged on the upstream side of the measurement flow path section 3 in the flow path 1.
The shut-off valve 2 is shut off by the shut-off valve drive circuit 7 based on the shut-off information generated by the shut-off information generating means 45 provided in the control unit 40. The blocking information generating unit 45 generates blocking information when a flow rate of a predetermined value or more is continuously used for a long time.

  Specifically, the display unit 50 is configured as a liquid crystal display unit, and includes an integrated flow rate display unit 51 and a flow rate pilot display unit 52 as shown in FIG. The integrated flow rate display unit 51 displays the total gas flow rate integrated by the control unit 40. On the other hand, the flow rate pilot display unit 52 is turned on when there is a flow rate passing through the flow path 1 according to the pilot display information generated by the control unit 40 (display state), and is turned off when there is no flow rate (non-display state). ).

[Instantaneous flow measurement]
The flow rate measurement unit 3 is provided with ultrasonic sensors 3a and 3b that transmit and receive ultrasonic waves to the upstream portion and the downstream portion, respectively. The pair of ultrasonic sensors 3a and 3b are opposed to each other with the flow channel 1 interposed therebetween, and the traveling direction of ultrasonic waves transmitted and received between the ultrasonic sensors 3a and 3b and the direction in which fluid passes through the flow channel 1 Are arranged so as to intersect at an angle θ.
For the measurement of the instantaneous flow rate, the ultrasonic wave propagation time t1 when the ultrasonic wave is transmitted from the upstream ultrasonic sensor 3a to the downstream ultrasonic sensor 3b and the upstream ultrasonic sensor 3b upstream. The ultrasonic wave propagation time t2 when the ultrasonic wave is transmitted toward the ultrasonic sensor 3a on the side is used. Assuming that the distance between the ultrasonic sensors 3a and 3b is d, the fluid flow velocity is v, and the sound velocity is c, the following relationship is obtained.
(C + v · cos θ) t1 = (c−v · cos θ) t2 = d
Therefore, the flow velocity v can be expressed as follows:
v = (d / 2 cos θ) {(1 / t1) − (1 / t2)}
A value obtained by multiplying the flow velocity v thus obtained by the cross-sectional area S of the measurement flow path portion 1 is the instantaneous flow rate q.
That is, the instantaneous flow rate q is expressed by the following equation.
q = v · S

The control unit 40 is provided with an instantaneous flow rate detection means 41, which generates a drive control command at a predetermined timing and activates the ultrasonic sensor drive circuit 8. The ultrasonic sensor driving circuit 7 drives the ultrasonic sensors 3a and 3b as a set to transmit and receive ultrasonic waves as a set, and returns the obtained measurement result to the control unit 40 to detect instantaneous flow rate. The means 41 calculates and derives the instantaneous flow rate q for each set of operations according to the above formula.
The instantaneous flow rate measurement is performed intermittently. Usually, this intermittent drive interval is set to about 1 to 5 seconds.

(Integrated flow calculation)
The controller 40 is provided with an integrated flow rate calculation means 42, and the integrated flow rate Q is obtained by integrating the intermittent drive interval multiplied by the instantaneous flow rate q. The integrated flow rate Q thus obtained corresponds to the total amount of fluid that has passed through the flow path 1 after the meter is installed (the amount of gas G used). Information on the integrated flow rate Q is sent to the display unit 50 and displayed on the integrated flow rate display unit 51.

[Flow pilot display]
Hereinafter, ON / OFF of pilot display information will be described.
This pilot display information is obtained by moving and averaging the instantaneous flow rate q sent to the control unit 40 over time in the time domain, and comparing the obtained moving average value Qave of the instantaneous flow rate q or each instantaneous flow rate q with a threshold value. Generated by the pilot information generating means 43. As the threshold values, the first display flow rate threshold value Qth1, the second display flow rate threshold value Qth2, and the non-display flow rate threshold value Qth0 are stored and stored in the storage unit 60 in advance. Here, the first display flow rate threshold value Qth1 and the non-display flow rate threshold value Qth0 are compared with the moving average value Qmave30 of the instantaneous flow rate as described in detail in the processing flow shown in FIG. On the other hand, the second display flow rate threshold value Qth2 is compared with each instantaneous flow rate q (= Qbuf (i)). In the present application, a memory area 61 for individually storing the instantaneous flow rate q measured during a predetermined period is secured, and a register (not shown) provided therein is a first-in first-out method. The instantaneous flow rate Qbuf (i) is individually stored in the memory area 61. In the example described below, 15 areas are provided as the memory area 61 for storing the instantaneous flow rate Qbuf (i) in order to store the instantaneous flow rate q for 30 seconds measured every 2 seconds.

  In the control unit 40, when the average value Qmave30 of the instantaneous flow rate Qbuf (i) (an example of a first determination flow rate that is a representative flow rate of the flow rate that passes through the flow path in a predetermined period) is larger than the first display flow rate threshold value Qth1, Set pilot display information to ON. In the present application, a state in which the pilot display information is ON and the flow rate pilot display is performed is referred to as a “display state”. On the other hand, when the moving average value Qmave30 of the instantaneous flow rate is smaller than the non-display flow rate threshold value Qth0, the pilot display information is turned OFF. A state where the pilot display information is OFF and the flow rate pilot display is not performed is referred to as a “non-display state”.

  Therefore, based on the first determination flow rate Qmave 30, the control unit 40 displays the flow rate pilot display that can be visually recognized when there is a flow rate that passes through the flow path, and the flow rate pilot display when there is no flow rate. Switching means 44 is provided for switching to a non-display state that is not performed.

  The first display flow rate threshold value Qth1 that is the switching flow rate threshold value from the non-display state to the display state is different from the non-display flow rate threshold value Qth0 that is the switching flow rate threshold value from the display state to the non-display state. The flow rate threshold value Qth1 is set larger than the non-display flow rate threshold value Qth0. As an example, the first display flow rate threshold value Qth1 can be set to 50 L / h, and the non-display flow rate threshold value Qth0 can be set to 3 L / h. Here, the non-display flow rate threshold value Qth0 is a boundary value flow rate at which the gas meter 10 determines that there is a leak downstream from the flow meter. On the other hand, the first display flow rate threshold value Qth1 is larger than the non-display flow rate threshold value Qth0, and is related to the lower limit value (usually 80 to 100 L / h) of the flow rate that is consumed when the operation of the fluid consuming device provided downstream is started. (Set to a low flow rate of 30 to 50 L / h from this lower limit value), which is a smaller value.

  Next, the second display flow rate threshold value Qth2 will be described. This threshold value is a threshold value set on the smaller flow rate side than the first display flow rate threshold value Qth1, and the first determination flow rate Qmave is smaller than the first display flow rate threshold value Qth1. In the non-display state, this is a threshold value for switching the non-display state to the display state when an instantaneous flow rate larger than the second display flow rate threshold value Qth2 flows for a predetermined period, for example, 30 seconds. Therefore, the threshold value determination is also performed by the switching means 44 described above.

  This determination is necessary because, as described above, in the case of adopting a configuration having a hysteresis that makes the first display flow rate threshold value Qth1 and the non-display threshold value Qth0 different, the flow rate pilot display is not displayed. For example, if a minute gas leak occurs and continues, the moving average Qmave of the instantaneous flow rate changes from 0 to a minute flow rate value at a certain time and continues as it is, and the moving average of the instantaneous flow rate Qmave> first display flow rate threshold at any timing. This is because the condition of Qth1 is not satisfied and the flow rate pilot display does not switch to the display state. As an example, the second display flow rate threshold value Qth2 can be 3 L / h. The second display flow rate threshold value Qth2 is a flow rate associated with a lower limit boundary value for determining that there is a leak on the downstream side, since a minute flow rate is a problem. The second display flow rate threshold value Qth2 may be 3 L / h, or may be 1.5 L / h or 2.0 L / h with a margin. It can also be a value within the range of 3 L / h to 1.5 L / h.

  The reason why there is no problem even if such processing is added is that the flow of gas is generated in the meter even when the gas is not used when the supply pressure fluctuates temporarily due to the governor control of the upstream pipeline network, etc. However, in many cases, it is temporary for several seconds and does not flow continuously for 30 seconds (15 instantaneous flow rates).

Hereinafter, the generation / stop of the pilot display information will be specifically described with reference to FIG. As a premise of entering this flow, instantaneous flow rate measurement and moving averaging are executed at each instantaneous flow rate measurement timing, and the flow shown in FIG. 3 is entered.
In the description, the instantaneous flow rates stored in the memory area 61 that stores the instantaneous flow rate q separately are defined as Qbuf (1) to Qbuf (15). These instantaneous flow rates Qbuf (1) to Qbuf (15) are the instantaneous flow rates q from the present to the previous 15 stored in each area by the first-in first-out method as described above. Based on the 15 stored instantaneous flow rates Qbuf (1) to Qbuf (15), the moving average value Qmave30 of the latest 30 seconds at that time is obtained. The first display flow rate threshold value Qth1 is 50 L / h, the second display flow rate threshold value Qth2 is 3 L / h, and the non-display flow rate threshold value Qth0 is 3 L / h.

  When the flow shown in FIG. 3 is entered, the current flow pilot display ON / OFF state is determined (# 1). When the flow rate pilot display is not displayed (# 1: no), the first display flow rate threshold value Qth1 is used. When Qmave30> Qth1 (# 2: yes), the flow rate pilot display is displayed. (# 5). If Qmave30 ≦ Qth1 (# 2: no), the flow rate pilot display is continued in the non-display state.

  When the flow rate pilot display is in the display state (# 1: yes), the non-display flow rate threshold value Qth0 is used (# 3), and if Qmave30> Qth0 (# 3: yes), the flow rate pilot display is displayed. If Qmave30> Qth0 is not maintained (# 3: no), the flow rate pilot display is switched to the non-display state through the determination in step # 4 described below.

  When the flow rate pilot display is in a non-display state (# 1: no) and the first display flow rate threshold value Qth1 is used and Qmave30 ≦ Qth1 (# 2: no), the instantaneous flow rate for the latest 15 times If the minimum value of Qbuf (1) to Qbuf (15) is equal to or greater than the second display flow rate threshold value Qth2 (# 4: yes), the flow rate pilot display is switched to the display state (# 5). If any of the instantaneous flow rates Qbuf (1) to Qbuf (15) for the latest 15 times is less than the second display flow rate threshold value Qth2 (# 4: no), the flow rate pilot display is kept in the non-display state (# 6). ). In this way, a good display state can be ensured.

  FIG. 4 shows the determination result of the flow rate pilot display ON (display state) and OFF (non-display state) based on the moving average value Qmave30 obtained at each measurement point of the instantaneous flow rate (thin solid line) and the instantaneous flow rate (thick solid line). In addition, the flow pilot display ON / OFF determination result (dotted line) based on the average value obtained every 30 seconds, which is conventionally performed, is shown. The determination result shown in this figure is made by setting the threshold value between ON and OFF of the flow rate pilot display to the same threshold value (the display flow rate threshold value and the non-display threshold value are the same) 4.5 L / h. When the moving average value Qmave30 is used in this way, the instantaneous flow rate (thin solid line) is turned on immediately after the increase from 0 L / h, and after the instantaneous flow rate becomes 0 L / h, it is turned off earlier than the conventional one (dotted line). ing. Therefore, it can be seen that the ON / OFF display can be properly performed without delay only by using the moving average value Qmave30.

FIG. 5 shows instantaneous flow rate (thin solid line), ON / OFF judgment result of flow pilot display based on moving average value Qmave30 (thick solid line) when hysteresis is given to the threshold shown in FIG. The determination result (dotted line) of the ON / OFF of the flow rate pilot display when not having. In the case where no hysteresis is given (dotted line), it is erroneously turned on after 38000 to 39000 and after 39000, but in the above configuration, it is kept off. Accordingly, it can be seen that the flow rate pilot erroneous display can be suppressed by providing the threshold with hysteresis.
Although the example which was an ultrasonic flowmeter was shown as a flowmeter demonstrated so far, this application is applicable to the arbitrary flowmeters which can measure an instantaneous flow rate. In other words, even with a fluidic meter that detects fluid flow from the oscillation frequency of a gas flow by a target and a conventional membrane type flow meter, the pulse signal interval is narrower than the conventional one and less than 2 seconds. A pulse signal may be output. Alternatively, it may be a hot-wire flow meter that detects that the temperature distribution from the hot wire has changed according to the gas flow rate.

  It was possible to provide a flow meter with improved switching accuracy between the display state and non-display state of the flow rate pilot display.

Diagram showing the configuration of an ultrasonic gas meter The figure which shows the display state in a display part Diagram showing the flow of pilot display The figure which shows the state of the flow pilot display based on the moving average value of the instantaneous flow rate The figure which shows the state of the flow pilot display when hysteresis is set between the display flow rate threshold and the non-display flow rate threshold

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Flow path 3 Flow measurement part 40 Control part 41 Switching means 50 Display part 51 Integrated flow rate display part 52 Flow rate pilot display part

Claims (7)

A flow rate measurement unit that intermittently measures the instantaneous flow rate of the fluid passing through the flow path, and an integrated flow rate display unit that displays the integrated flow rate obtained from the instantaneous flow rate measured by the flow rate measurement unit and the intermittent drive interval A flow meter,
Based on the first judgment flow rate that is a representative flow rate of the instantaneous flow rate measured during a predetermined period, a display state for performing flow rate pilot display that can be visually recognized when there is a flow rate that passes through the flow path, and when there is no flow rate, It has switching means to switch to the non-display state that does not perform flow rate pilot display,
The first display flow rate threshold that is the switching flow rate threshold value from the non-display state to the display state is different from the non-display flow rate threshold value that is the switching flow rate threshold value from the display state to the non-display state, The one display flow rate threshold value is larger than the non-display flow rate threshold value, and is related to the lower limit value of the flow rate consumed at the start of operation of the fluid consuming device provided on the downstream side, and is set to a smaller value. Total.   A second display flow rate threshold value set on a smaller flow rate side than the first display flow rate threshold value, wherein the switching means is in a non-display state in which the first determination flow rate is smaller than the first display flow rate threshold value during a predetermined period. The flow meter according to claim 1, wherein when the second determination flow rate, which is a representative flow rate of the measured instantaneous flow rate, is larger than the second display flow rate threshold value, the non-display state is switched to the display state.   The flow meter according to claim 1 or 2, wherein the non-display flow rate threshold is a flow rate associated with a boundary value that is judged to be leaked downstream.   The flow meter according to claim 2, wherein the second display flow rate threshold value is a flow rate associated with a boundary value that is judged to be leaked downstream.   The flow meter according to any one of claims 1 to 4, wherein the first determination flow rate is an average value of instantaneous flow rates measured during a predetermined period, and is obtained at every intermittent drive interval.   The flowmeter according to claim 2, wherein the second determination flow rate is a minimum value of an instantaneous flow rate measured during a predetermined period, and is obtained at every intermittent drive interval.   An ultrasonic flowmeter comprising a pair of ultrasonic sensors in the flow rate measuring unit and obtaining an instantaneous flow rate of the fluid passing through the flow path based on propagation times of ultrasonic waves propagating between the pair of ultrasonic sensors. The flowmeter according to any one of claims 1 to 6.

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