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BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
ultrasonic flowmeter which measures the flow velocity and flow rate of fluid using ultrasonic
2. Description of the Related Art FIG. 3 shows a conventional example of this type. That is, the
ultrasonic flowmeter is mainly composed of the measuring pipe 1, the transducer 4 and the cable
5. The measuring pipe 1 is disposed between the cylindrical pipe 2 having flanges 2a at both
ends and the center line s of the pipe 2 and is opposed to the center line s with a predetermined
angle θ, A pair of ultrasonic transmitter-receivers (hereinafter also referred to as transmitterreceivers) 3a and 3b for transmitting and receiving ultrasonic signals are provided. The pair of
transducers 3a and 3b is connected via a cable 5 to a converter 4 for converting the signals of
the transducers 3a and 3b into fluid velocity and flow rate.
FIG. 4 is a diagram for explaining the operation, and (a) shows the transmitting operation to the
transducers 3a to 3b, and (b) shows the transmitting operation in the opposite direction. In the
ultrasonic flowmeter, for example, as shown in (a), an ultrasonic signal is transmitted from one of
the transducers 3a, and the other ultrasonic transducer (3b) receives the transmitted ultrasonic
signal. At this time, the propagation time t1 from pulse 1 at the start of transmission to pulse 2 at
the end of reception is measured, but in order to maintain stable measurement and maintain
measurement accuracy, receive waves with good S / N ratio and good rise You will need to get it.
The flow velocity of the fluid in the conduit can be measured by alternately switching the
transmission and reception of the ultrasonic signal and measuring the propagation time
difference between the upstream propagation time t2 and the downstream propagation time t1.
The flow rate can be measured by multiplying the flow rate by the cross-sectional area of the
pipe 2. The relational expression for obtaining the flow rate is as follows.
t1 = L / (C + V cos θ) (1) t2 = L / (C−V cos θ) (2) From the equations (1) and (2), V = L (1 / t1-1
/ t2) / 2 cos θ 3) Q = (π / 4) × D 2 × V × K (4) The meanings of the above symbols are as
follows. t1: Propagation time of ultrasonic wave transmitted from transducer 3a to 3b t2:
Propagation time of ultrasonic wave transmitted from transducer 3b to 3a C: Speed of sound of
fluid L: Between transducer 3a and 3b Distance V: measured flow velocity K: flow velocity
distribution coefficient D: inner diameter of pipe 2 Q: flow rate θ: angle between pipe axis and
installation axis of transducer
Heretofore, the thickness of the matching layer of the transducer has been considered to be good
in terms of sensitivity when it is expressed by the wavelength λ. There is also an example in
which 0.15 λ is used to improve the rising of the reception waveform. Here, the wavelength λ
of the matching layer is expressed by the following equation. = Wavelength of matching layer =
velocity of sound C of matching layer material C (m / s) / frequency f of ultrasonic wave (Hz = 1 /
Here, considering the relationship between the thickness of the matching layer and the
sensitivity and the ease of triggering, when the thickness of the matching layer is 0.24 λ, as
shown in FIG. Although the sensitivity is high at around 24λ and the sensitivity is high and the S
/ N ratio is good, as shown in FIG. 6A, the waveform has a bad rise (long rise time), and a trigger
for measuring the above propagation time It becomes difficult to apply. V1 to V6 and Vmax
shown in FIG. 5 respectively indicate the first to sixth waves and the maximum value of the
reception waveform shown in FIG.
On the other hand, when the thickness of the matching layer is 0.15λ, as shown in FIG. 5, the
maximum amplitude value Vmax is small and the S / N ratio is bad, but as shown in FIG.
Therefore, the trigger for measuring the propagation time becomes easy to put on. From the
above, it is not compatible with the problem of S / N ratio by the magnitude of the amplitude
value with respect to the thickness of the matching layer, the goodness of the rising waveform,
the easiness of triggering by badness, and the difficulty of putting it. Is also a problem. Therefore,
an object of the present invention is to provide an ultrasonic flowmeter that has a good S / N
ratio and is easy to trigger.
SUMMARY OF THE INVENTION In order to solve such problems, according to the invention of
claim 1, there is provided a pipe having at least two openings and passing a fluid from one open
end to the other open end, and An ultrasonic wave transmitter / receiver, transmission means for
exciting one ultrasonic wave transmitter / receiver to switch ultrasonic wave signals from the
upstream side or downstream side to the flow of fluid and transmitting the same, and
propagating in the fluid by the other ultrasonic wave transmitter / receiver A time measuring
means for detecting an ultrasonic signal, receiving processing at the receiving unit, detecting a
propagation time of the ultrasonic signal propagating in the fluid, and measuring a propagation
time of the ultrasonic signal in the downstream direction or the upstream direction; In an
ultrasonic flowmeter comprising a transducer having a flow rate calculating means for
calculating the flow velocity or flow rate of fluid from these propagation times, the thickness of
the matching layer of the ultrasonic transducer is the speed of sound C of the matching layer
material (M / s) and ultrasonic 0.22λ or more 0.18λ wavelength matching layer material
obtained from the wave number f (Hz) λ (m) or less, preferably characterized in that a 0.2?.
an embodiment of the present invention.
This shows the specific configuration of the transducer. A matching layer 11 is adhered to the
piezoelectric element 12 by an adhesive 18 and fixed to a case 16 by a lid 13. A piezoelectric
element 12 is held by the case 16 via an O-ring 14. The lead wire 15 soldered to the electrode
surface of the piezoelectric element 12 is connected to the electrode 17 by a screw 19 and a
voltage signal is transmitted. Although such a configuration itself is the same as a general one,
from the viewpoint as described above, the thickness of the matching layer 11 is optimized here.
Therefore, in the present invention, while changing the matching layer thickness, the ratio of the
first wave to the second wave (V2 / V1) and the ratio of Vmax to the third wave V3 (V3 / Vmax)
shown in FIG. Focusing and plotting, the relationship curves as shown in FIG. 2 were obtained.
Therefore, focusing on the intersection of these two curves, the ratio of V2 / V1 indicating
trigger stability and the ratio of V3 / Vmax indicating sensitivity are both relatively large at this
intersection point. The basic concept of the present invention is to adopt the matching layer
thickness 0.2λ at this time as the optimum value. It is also apparent from FIG. 2 that there is no
particular problem even if a margin of about 10% is taken around 0.2 λ and the range is 0.18 λ
to 0.22 λ. An example of the received waveform when the matching layer thickness is in this
range is shown in FIG. It can be seen that both the rising and the falling have good waveforms.
Thus, the thickness of the matching layer is at least 0.18 λ of the wavelength λ (m) of the
matching layer material, which is obtained from the speed of sound C (m / s) of the matching
layer material and the frequency f (Hz) of ultrasonic waves. By setting it as 0.22 λ or less, more
preferably 0.2 λ, it is possible to make the S / N ratio indicating sensitivity and the stability of
the trigger compatible, and as a result, the generation timing of the pulse generated from the
reception waveform is It becomes possible to stabilize and improve the measurement accuracy of
the propagation time.
According to the present invention, since the thickness of the matching layer of the ultrasonic
transmitter-receiver is optimized, the measurement accuracy of the propagation time is
improved, and the advantage of enabling the measurement with less error is brought about. .
Brief description of the drawings
1 is an explanatory diagram of an embodiment of the present invention.
2 is an explanatory view of the principle of the present invention.
3 is a block diagram showing a conventional ultrasonic flowmeter.
4 is an explanatory view of the measurement principle in FIG.
5 is a diagram for explaining the relationship between the matching layer thickness and the
amplitude value of the received waveform.
6 is a diagram of each received waveform when the matching layer thickness is specified.
7 is a waveform diagram showing an example of a reception waveform.
Explanation of sign
DESCRIPTION OF SYMBOLS 1 ... Measurement pipe ¦ tube 2. Piping 2a Flange 3a, 3b Ultrasonic
wave transmitter-receiver (transmitter / receiver) 4 Transducer 5 Cable 6 Sensor fixing part 11
Matching layer 12 Piezoelectric Elements, 13: Lids, 14: O-rings, 15: Leads, 16: Cases, 17:
Electrodes, 18: Adhesives, 19: Screws.
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