JPH09257539A - Ultrasonic flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure the resolution in a practical flow speed range, and to reduce the power consumption. SOLUTION: Switches 7, 8 are set in the condition expressed with a figure, ultrasonic pulse is sent from a transmitter 2 of the upstream to a transmitter 3 of the downstream in the normal direction. This transmission is repeated at (n) times with a sing-around method so as to measure the total transmission time nt1 with a second counter 11. The switchers 7, 8 are switched for the measurement in the opposite direction, and the total transmission time nt2 is measured. Measured values nt1 and nt2 of the second counter 11 is read out by a microcomputer 4 so as to compute the flow speed. At the time except for the during the measurement, a power source of a received wave detecting unit 5 of an analog circuit is turned off so as to reduce the current. In the case of a large flow speed, (n) is set small, and in the case of small flow speed, (n) is set large so as to widen an interval.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a sing-around type ultrasonic flowmeter.

[0002]

2. Description of the Related Art In FIG. 8, the speed of sound in a stationary fluid is C,
Assuming that the velocity of the fluid flow is V, if the propagation direction of the sound wave coincides with the direction along the flow (hereinafter referred to as the forward direction), the propagation velocity becomes (C + V), and the direction opposite to the flow (hereinafter the reverse direction) (CV).

A pair of ultrasonic transducers 2 separated by a distance L 2.
3 is arranged on the upstream side and the downstream side of the flow tube 1 separately, and when an ultrasonic pulse is transmitted in the forward direction from the ultrasonic transducer 2 on the upstream side, the ultrasonic wave is transmitted to the ultrasonic transducer 3 on the downstream side. The time required for the sound wave to reach t ₁ is required for the ultrasonic wave to reach the upstream ultrasonic wave transmitter / receiver 2 when an ultrasonic wave pulse is transmitted in the reverse direction from the downstream ultrasonic wave transmitter / receiver 3. If time is t ₂ , then t ₁ = L / (C + V) (1) t ₂ = L / (C−V) (2)

The above-mentioned propagation time t of ultrasonic waves in the forward and reverse directions
₁ , ₁ and t ₂ were measured, the flow velocity V of the fluid was calculated from them, and the flow rate and the integrated flow rate (volume) were obtained. The flow velocity V of the fluid is calculated from the above equations (1) and (2) as V = (L / 2) · {(1 / t ₁ ) − (1 / t ₂ )} (3) regardless of the sound velocity C. .

Actually, the resolution of the propagation time measurement is
To measure forward and backward propagation times to increase
At the same time as receiving ultrasonic waves,
The next transmission is performed and transmission / reception in the same direction is repeated multiple times (n times).
Iteratively repeats from the first transmission to the last nth transmission.
The time until the second reception is calculated for the forward and backward directions.
Find each of them, and their value nt₁, Nt_TwoMore than one transmission
Propagation time t from reception to reception ₁, T_TwoTrying to ask
You.

[0006]

Recently, there has been a demand for an ultrasonic flowmeter which consumes less power and has high measurement accuracy.

However, in the former technique of the related art, in order to improve the measurement accuracy of the flow velocity and the flow rate, it is necessary to first increase the resolution of the propagation time measurement, and the frequency of the reference clock for the propagation time measurement is increased. It was

However, if the frequency of the reference clock is increased, the current consumption, and thus the power consumption and power consumption also increase. In addition, there is a limit to high frequency with low power consumption and low price.

Therefore, by repeatedly transmitting and receiving a plurality of times (n times) in the forward direction and the reverse direction, respectively,
There is a tendency to improve the resolution by measuring nt ₁ and nt ₂ of the propagation time (the latter of the above-mentioned prior art).

However, if the frequency of the reference clock is lowered in order to reduce the power consumption, the measurement accuracy will drop, so
In order to cover this, it is necessary to increase the number of times n of transmission and reception is repeated. Then, by doing so, it takes a long time for the measurement, and the average power consumption, that is, the power consumption increases.

Therefore, the effect of another measure as a measure for reducing the power consumption, for example, oscillating the reference clock only during the measurement or limiting the power supply to the analog portion which easily consumes a large current only during the measurement There was a dilemma that was not fully realized.

The inventor of the present invention has conducted extensive studies to solve such a dilemma. As a result, the resolution required to improve the measurement accuracy of the flowmeter depends on the flow velocity V of the fluid, and when the flow velocity is large, Realized that the resolution could be low. That is, if the ratio is the same, it can be said that the required resolution is inversely proportional to the flow velocity V if the flow velocity V is much smaller than the sound velocity C.

However, in the latter of the above-mentioned prior arts, since the number of repetitions n is fixed, the measurement is unnecessarily high resolution when the flow velocity V is high. Since there is an error due to the frequency deviation of the reference clock, the flow velocity distribution, etc., it is meaningless to increase the resolution of only the flow velocity measurement.

In other words, this consumes unnecessary power. From this point of view, therefore, the present invention improves the above-mentioned conventional technique so that the practically required resolution can be exerted in a wide range from a place where the flow velocity V of the fluid is small to a place where the flow velocity V of the fluid is large, and low power consumption. An object is to provide an ultrasonic flowmeter that can realize quantification.

[0015]

In order to achieve the above-mentioned object, the invention of claim 1 is an ultrasonic flowmeter characterized by having the following requirements 1) to 9).

1). A pair of ultrasonic wave transmitters / receivers (2) (3) that act on both the transmitting side and the receiving side for transmitting / receiving ultrasonic waves in the same or oblique direction as the flow in a fluid flow. 2). When the transmission wave switching signal from the control section (4) indicates the forward direction, the downstream side transducer (3) is connected, and when the transmission direction signal indicates the reverse direction, the upstream side transducer (2) is connected to receive the reception wave. Received wave detection unit (5) for detecting.

3). When the transmission / reception switching signal from the control section (4) indicates the forward direction, the upstream / downstream transducer (2)
Is configured to drive the downstream side wave transmitter / receiver (3) when indicating the reverse direction, performs the first drive when the first transmission command signal is input, and thereafter receives the reception wave detection unit (5). A wave drive unit (6) that drives the transmitter / receiver (2 or 3) on the transmitting side every time the received wave detection signal is input, until the nth received wave detection signal is input.

4). The reception wave detection signal from the reception wave detection unit (5) is received, and the nth reception wave detection signal is detected to detect the nth reception wave.
A first counter (10) that outputs a received wave detection signal. 5). A second counter (11) for measuring the time from the first transmission command signal to the nth received wave detection signal.

6). The transmission / reception switching signal is alternately switched between the forward direction and the reverse direction at a constant measurement interval (T), and the
A control unit (4) that outputs a transmission command signal and, when receiving the nth received wave detection signal, reads the measured value of the second counter (11) and calculates the flow velocity, the flow rate, and the like.

7). The second counter (11) has a reference clock oscillator, and measures the time from the first transmission command signal to the nth received wave detection signal by counting the reference clock that is the output of the oscillator, and at the same time It is configured to stop the oscillation of the reference clock except when measuring 8). The received wave detector (5) is powered off except during measurement
9). The control unit (4) is configured so that the n of the first counter can be set, and when the detected flow velocity is relatively high, next, n is set relatively low,
When the flow velocity is relatively small, n is set to be relatively large next, and the control unit (4) is configured to output the first transmission command signal to start the measurement.

According to the present invention, during forward measurement and reverse measurement, transmission and reception are repeated n times each,
By collectively measuring nt ₁ and nt ₂ of the propagation time, the resolution is n times as high as the former case of the prior art, like the latter case of the prior art.

When the flow velocity is high, the number of repetitions n
, The oscillation time of the reference clock is shortened and the power consumption is reduced. Furthermore, since the power of the received wave detection unit (5) composed of an analog circuit that consumes a large amount of current is turned off except during measurement, the power ON time is shortened,
From this aspect as well, the power consumption is low.

When the flow velocity is low, n is increased in order to improve the resolution. Therefore, this tends to increase the measurement time nt ₁ and nt ₂ and increase the power consumption.

The invention of claim 2 is characterized in that, in the ultrasonic flowmeter of claim 1, when the flow rate is relatively small, the measurement interval (T) is set relatively large next time. It is a thing.

In the invention of claim 1, when n is increased in order to increase the resolution when the flow velocity is low, nt ₁ or nt ₂ of the measurement time becomes long, which tends to increase the power consumption. .

Generally, in the case of an integrated flow meter having an integrated flow rate display, when the flow velocity is small, the integration is slowed down, so the speed of the integration is also slowed down. Then, when the flow velocity is so small that the lowest digit of the integrated value increases by 1 only after several times of measurement, it is unreasonable to perform the measurement frequently.

Therefore, in the invention of claim 2, the resolution is increased by increasing n, but the measurement interval (T) is increased,
Prevents the average current consumption from increasing. Claim 3
In the ultrasonic flowmeter according to claim 1, when the flow velocity is relatively low, the measurement interval (T) is made relatively large next time to perform measurement, and the flow velocity and flow rate are measured at each measurement interval (T). Etc., the interval (T) is further shortened, and the measurement is performed at a relatively small n, and the difference between the reading values of the second counter (11) in the forward direction and the backward direction is equal to or more than a certain value Then, the two readings are used to calculate the flow velocity, the flow rate, etc., and from then on, the measurement interval (T) is set to a relatively small value, and n
Is set to a comparatively small value commensurate with the flow rate and is measured.

If the measurement interval (T) is increased, there is a possibility that it will not be possible to follow a sudden change in the flow velocity, and a problem remains. Therefore, in the present invention, in order to monitor only the change in the flow velocity, the measurement with a small n that can be detected is performed at a short interval (from T).

According to the invention of claim 4, in the ultrasonic flowmeter of claim 1, when it is determined that the flow velocity is zero or a constant value close to zero, n is set to be relatively small and measurement is performed. It is a feature.

In the present invention, the measurement is performed with a small n when the fluid is not flowing. Depending on how the flow meter is used, it is often the longest time without flow. Therefore, the power consumption can be reduced by doing so. It is absurd to consume power when the fluid is not flowing, as when it is flowing.

Further, when the flow rate is near zero, it is usual to operate a low level cut, which is regarded as zero and discards the measured value, and it is useless to perform measurement with a large n. When the flow rate and flow rate exceeds a certain level beyond the low level cut region, increase n. Flow velocity in the low level cut area
This is because the flow rate does not require much resolution and it is sufficient to be able to determine whether or not it is within the low level cut region.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION Next, a preferred embodiment of the present invention will be described with reference to FIGS. Reference numerals 2 and 3 denote a pair of ultrasonic wave transmitters / receivers, which transmit and receive ultrasonic waves in the same direction as the flow of the fluid or in the oblique direction, as in the prior art.

4 is a control section, 5 is a received wave detection section,
6 is a transmitter driving unit, 7 is a first switching unit, 8 is a second switching unit, and both switching units 7 and 8 constitute a switching unit 9. The control unit 4 is composed of a microcomputer, and controls the entire flow meter and performs arithmetic processing such as flow velocity / flow rate and integrated flow rate.

The transmission / reception switching signal output from the control unit 4 is operated by operating the first switching unit 7 and the second switching unit 8 forming the switching unit 9 to alternately perform forward measurement and reverse measurement. To instruct.

When the transmission / reception switching signal indicates the forward direction, the upstream side transmitter / receiver 2 is connected to the transmitter drive unit 6 by the first switch 7 as shown in FIG. 1, and the downstream side transmitter / receiver 3 is connected. Is connected to the received wave detector 5 by the second switch 8 as shown in FIG.

When the transmission / reception switching signal indicates the opposite direction, the first switching device 7 and the second switching device 8 are switched from the state shown in FIG. 1, and the downstream transmission / reception device 3 is switched to the first switching device. The transmitter / receiver 2 on the upstream side is connected to the received wave detector 5 via the second switching unit 8.

The control unit 4 alternately switches the transmission / reception switching signal between the forward direction and the reverse direction at a constant measurement interval T, and outputs the first transmission command signal to the transmitter driving unit 6 each time. When the first transmission command signal from the control unit 4 is input, the wave transmitter driving unit 6 drives and excites the transmitter / receiver unit 2 or 3 on the transmission side connected via the first switching unit 7, After that, each time the received wave detection signal from the received wave detection unit 5 is input, the transmitter / receiver 2 or 3 on the transmission side is driven until the nth received wave detection signal described later is input.

Reference numeral 10 is a first counter, which is a received wave detector 5
Receives the received wave detection signal from, detects the nth received wave detection signal, and outputs the nth received wave detection signal. The first transmission command signal from the control unit 4 is input to the reset terminal R of the first counter 10, and the count value is reset to zero by the first transmission command signal.

The first counter 10 is constructed so that the value of n can be changed by an n selection signal from the control section 4. FIG. 2 shows a specific example of the electric circuit of the first counter 10, and 10a is a counter that counts the received wave detection signals from the received wave detection unit 5 and outputs the count values of 10, 20, 40 and 80, which are controlled by the counter. Each time the first transmission command signal from the unit 4 is received, its content is reset to zero.

Reference numerals 10b, 10c and 10d denote n selection switches which are operated by an n selection signal from the control section 4. For example, when the n selection switches 10b, 10c, 10d operated by the n selection signal are all in the "0" position as shown in FIG. 2, the output terminal 10 of the counter 10a is
Is selected, the first counter 10 detects the tenth received wave detection signal and outputs the tenth received wave detection signal.

By appropriately switching the n-selection switches 10b, 10c, 10d by the n-selection signal from the control section, one of the four outputs 10, 20, 40 or 80 of the counter 10a is sent to the n-th received wave detection signal. It is supposed to be selected as.

The n selection signals are two signals, one for operating the n selection switch 10b and the other for the n selection switch 1
It consists of operating 0c and 10d. Note that n
The selection switches 10b, 10c and 10d can be easily realized by combining AND gates.

As described above, in the specific example of FIG. 2, any one of 10, 20, 40 and 80 can be selected as n by the n selection signal. Reference numeral 11 denotes a second counter, which measures the time from the first transmission command signal to the nth received wave detection signal every time the forward measurement and the backward measurement are performed. The first transmission command signal from the control unit 4 is input to the reset terminal R of the second counter 11, and the count value is reset to zero by the first transmission command signal.

The second counter 11 has a built-in reference clock oscillator (not shown), and counts the reference clock from the reference clock oscillator to count from the first transmission command signal to the nth received wave detection signal. Is measured as the count value.

The reference clock oscillator (not shown) of the second counter 11 is controlled by an oscillation ON / OFF signal from the control section 4 to generate an oscillation ON / O signal.
It is configured to oscillate when the FF signal is ON, and stop the oscillation when the FF signal is OFF.

FIG. 3 shows a specific example of the electric circuit of the second counter 11, and 11a is a counter which receives the first transmission command signal from the control section 4 and resets its content to zero and starts counting.

Reference numeral 11b is a reference clock oscillator, which receives an oscillation ON / OFF signal from the control unit 4, transmits the signal at "H" level, and stops oscillation at "L" level. The reference clock of the reference clock oscillator 11b is input to the counter 11a via the gate 11c and counted. When the nth received wave detection signal is input, the gate 11c is closed and the counter 11a stops counting.

Thus, the counter 11a measures the time from the first transmission command signal to the nth received wave detection signal as the count value of the reference clock of the reference clock oscillator 11b for each of the forward measurement and the backward measurement, and thereafter, The control unit 4 reads the count value as a time measurement value.

The reference clock oscillator 11b is configured to stop the oscillation under the control of the oscillation ON / OFF signal from the control unit 4 except when this time is being measured.

The control section 4 alternately switches the transmission / reception switching signal in the forward direction and the reverse direction at a constant measurement interval T, outputs the first transmission command signal each time, and receives the nth received wave detection signal. The measured value (count value) of the second counter 11 is read and the flow velocity, flow rate, integrated flow rate, etc. are calculated.

The received wave detecting section 5 is composed of an analog circuit, and the power supply of its operating power is controlled by the control section 4 to supply power during measurement and stop power supply during non-measurement. This control by the control unit 4 is performed by inputting an oscillation ON / OFF signal to the received wave detection unit 5 as shown in FIG. 1 and supplying the operating power of the received wave detection unit 5 when the signal is at “H” level. However, it can be realized by stopping the power supply at the “L” level.

When the detected flow velocity is relatively high, the control unit 4 next sets n relatively low, and when the flow velocity is relatively low, next n is relatively high set. , And outputs the first transmission command signal to start the measurement.

Next, the operation of the control unit in the above-mentioned embodiment will be mainly described in more detail with reference to FIGS. As an example, a case where the measurement is performed in the forward direction and then in the reverse direction every one second will be described.

The control section 4 is composed of a microcomputer. An interval timer is set to occur every second by a timer inside the microcomputer 4 (Fig. 4).

When an interval interrupt every 1 second is applied,
First, the microcomputer 4 sets the transmission / reception switching signal in the forward direction, turns on the oscillation of the reference clock oscillator 11b, and turns on the power of the received wave detection unit 5 formed of an analog circuit.
In this example, the reference clock oscillation is turned on / off.
The control and the power ON / OFF of the received wave detector 5 are both configured to be performed at the output port of the microcomputer 4.

Then, the microcomputer 4 outputs the first transmission command signal. n has been set to either. With this, the measurement in the forward direction is started, and the microcomputer 4 waits for the completion signal of the nth received wave detection signal as an interrupt.

When the transmission / reception of ultrasonic waves for n consecutive times is completed (FIG. 5), an interruption is applied by the nth received wave detection signal (FIG. 6). Here, the microcomputer 4 reads the measurement value (count value) of the second counter 11.

This value is first stored as the measurement value nt ₁ of the forward measurement, and this time, the transmission / reception switching signal is set to the reverse direction, the first transmission command signal is output again, and the measurement in the reverse direction is started.

Then, the microcomputer 4 waits for the nth received wave detection signal as an interrupt. When the measurement in the reverse direction is completed, the n-th received wave detection signal is input from the first counter 10 to the microcomputer 4 as an interrupt, and the interrupt occurs.

The microcomputer 4 reads the count value of the second counter 11, oscillates the reference clock 11b, and turns off the power of the received wave detector 5. If the low level is not being cut, the calculation of Δf = {1 / (nt ₁ −τ)} − {1 / (nt ₂ −τ)} (4) is performed. Here, nt ₂ is the count value of the second counter 11 as a measurement value in the opposite direction, and τ is the sum of delays such as the time required for detection of the received wave (for example, detection of the zero cross point). It is a value that has been determined and determined by, etc., and is calculated in consideration of n.

It can be said that the value Δf obtained by the equation (4) is almost proportional to the flow velocity. Where α ₁ ,
By comparing α ₂ , α ₃ , α ₄ and Δf, it is possible to determine whether the current flow velocity is relatively high or low. However, α ₁ <α ₂ <α ₃ <α ₄ is satisfied.

Based on the comparison result, n is set in the second counter 11. The next measurement is the newly set n
Done in When Δf <α ₁ , the flow velocity is almost zero,
The flow rate is regarded as zero as a low level cut region, and the flow rate is not integrated.

When the low level is being cut originally, it is first checked whether or not nt ₂ −nt ₁ <β with respect to the constant value β, and if nt ₂ −nt ₁ ≧ β, the flow velocity changes from the low level cut region. When it is judged that it has come out, it goes to the calculation of Δf of the equation (4), but when nt ₂ −nt ₁ <β, it is judged that it is still within the low level cut region, and the calculation of the equation (4) of Δf is performed. Without doing so, the flow rate is not integrated by imitating that the flow velocity is zero.

In the example of FIG. 5, n is set during the low level cut.
= 20 is set (set) in the second counter 11, but it is effective to set it to a value as small as possible so that it can be reliably detected when the flow velocity exits the low level cut region determined by the specifications. Is.

The constant value β is similarly determined. According to this method, the value of n is relatively small when the flow velocity that does not require a high resolution is large, and conversely, when the flow velocity that requires a high resolution is small, the measurement is performed with a large value of n. The current can be reduced.

Further, when the low level cut works and the flow velocity is near zero, the measurement is performed with a small value n which can be used to judge whether or not the flow velocity region of the low level cut has been exited. .

[0067]

FIG. 7 is a diagram for explaining the operation of an embodiment corresponding to claim 3. In this embodiment, the block diagram of the entire flow meter is the same as in FIGS. The only difference is the microcomputer software.

In the second aspect of the present invention, when the flow velocity is low, n is increased to increase the resolution and the measurement interval T is increased to prevent the average current consumption from increasing. If the measurement interval T is set to be large, it becomes impossible to cope with a sudden change in the flow velocity.

Therefore, in the embodiment, in order to monitor only the change in the flow velocity, the measurement with a small n that can be detected is performed at short intervals. In FIG. 7, the horizontal axis represents the elapsed time, and the mark ◯ represents the time when the measurement and calculation are performed with a large n, and the interval T. △
The mark indicates the measurement time with a small n, and when it is judged that the flow velocity has increased, it is calculated and the interval is set to T / 4 of 1/4.

When it is detected that the flow velocity has become large in the measurement with a small n, the measurement value is used for the calculation at that time, and the subsequent measurement is performed at the interval T and n corresponding to the flow velocity.

[0071]

EFFECT OF THE INVENTION Since the ultrasonic flowmeter of the present invention is constructed as described above, it can exhibit the practically required resolution over a wide range from the small flow velocity to the high flow velocity of the fluid, and Low power consumption can be realized.

According to the second aspect of the present invention, further, the measurement interval (T) is lengthened and the current consumption is reduced on average, avoiding the irrationality of frequent measurement when the flow velocity is small. Even lower power consumption can be achieved.

According to the third aspect of the invention, it is possible to follow and respond to a sudden change in the flow velocity. Further, according to the invention of claim 4, it is possible to further reduce the power consumption in the low level cut region.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is an electric circuit diagram of a specific example of a first counter in the embodiment of FIG.

FIG. 3 is an electric circuit diagram of a specific example of a second counter in the embodiment of FIG.

FIG. 4 is a flowchart illustrating an operation of the control unit according to the exemplary embodiment of FIG.

5 is a flowchart mainly of a control unit in the embodiment of FIG.

FIG. 6 is a flowchart mainly of a control unit in the embodiment of FIG.

FIG. 7 is a diagram illustrating a measurement interval according to the embodiment of this invention.

FIG. 8 is a schematic diagram illustrating the principle of the prior art.

[Explanation of symbols]

 2, 3 ... Ultrasonic wave transmitter / receiver 4 ... Control unit (microcomputer) 5 ... Received wave detection unit 6 ... Wave transmitter drive unit 10 ... First counter 10a ... Counters 10b, 10c, 10d ... N selection switch 11 ... 2 counter 11a ... Counter 11b ... Reference clock oscillator 11c ... Gate T ... Interval

Claims (4)

[Claims] 1. An ultrasonic flowmeter having the following requirements 1) to 9). 1). A pair of ultrasonic wave transmitters / receivers (2) (3) that act on both the transmitting side and the receiving side for transmitting / receiving ultrasonic waves in the same or oblique direction as the flow in a fluid flow. 2). When the transmission wave switching signal from the control section (4) indicates the forward direction, the downstream side transducer (3) is connected, and when the transmission direction signal indicates the reverse direction, the upstream side transducer (2) is connected to receive the reception wave. Received wave detection unit (5) for detecting. 3). When the transmission / reception switching signal from the control unit (4) indicates the forward direction, it drives the upstream side transducer (2), and when it indicates the reverse direction, it drives the downstream side transducer (3). , The first drive is performed when the first transmission command signal is input, and thereafter, every time the reception wave detection signal from the reception wave detection unit (5) is input, transmission is performed until the nth reception wave detection signal is input. A wave transmitter drive unit (6) for driving the side wave transmitter / receiver (2 or 3). 4). A first counter (10) that receives a received wave detection signal from the received wave detection unit (5), detects the nth received wave detection signal, and outputs the nth received wave detection signal. 5). A second counter (11) for measuring the time from the first transmission command signal to the nth received wave detection signal. 6). The transmission / reception switching signal is alternately switched to the forward direction and the reverse direction at a constant measurement interval (T), and each time the first transmission command signal is output and the n-th received wave detection signal is received, the second counter (11 ) A control unit (4) that reads the measured value and calculates the flow velocity and flow rate. 7). The second counter (11) has a reference clock oscillator, and measures the time from the first transmission command signal to the nth received wave detection signal by counting the reference clock which is the output of the oscillator, and at the same time It is configured to stop the oscillation of the reference clock except when measuring 8). The received wave detector (5) is powered off except during measurement
9). The control unit (4) is configured to be able to set the n of the first counter, and when the detected flow velocity is relatively high, next set n to be relatively small,
When the flow velocity is relatively low, then n is set to be relatively high and the control unit (4) is configured to output the first transmission command signal to start the measurement. 2. The ultrasonic flowmeter according to claim 1, wherein when the flow rate is relatively small, the measurement interval (T) is set relatively large next time. 3. When the flow velocity is relatively small, next, the measurement interval (T) is made relatively large to perform measurement, and the flow velocity, the flow rate, etc. are calculated for each measurement interval (T), and the interval (T ) Is shortened, and measurement is performed at a relatively small n, and the second counter (1
When the difference between the readings in 1) becomes a certain value or more, the two readings are used to calculate the flow velocity, the flow rate, etc. From then on, the measurement interval (T) is set to a relatively small value, and n is The ultrasonic flowmeter according to claim 1, wherein the ultrasonic flowmeter is set to a relatively small value corresponding to the flow rate for measurement. 4. The ultrasonic flowmeter according to claim 1, wherein when the flow velocity is determined to be zero or a fixed value close to zero or less, n is set to be relatively small and measurement is performed.