JP3738891B2 - Ultrasonic flow meter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an ultrasonic flowmeter that measures the flow velocity and flow rate of a fluid using ultrasonic waves.
[0002]
[Prior art]
FIG. 3 shows this type of conventional example.
That is, the ultrasonic flowmeter is mainly composed of a measuring tube 1, a converter 4, and a cable 5. The measurement tube 1 is arranged so as to sandwich a cylindrical pipe 2 having flanges 2a at both ends and a center line s of the pipe 2 and to face the center line s with a predetermined angle θ. It has a pair of ultrasonic transceivers (hereinafter also referred to as transducers) 3a and 3b for transmitting and receiving ultrasonic signals. The pair of transducers 3a and 3b is connected via a cable 5 to a converter 4 that converts the signals of the transducers 3a and 3b into fluid velocity and flow rate.
[0003]
FIG. 4 is a diagram for explaining the operation. FIG. 4A shows the transmission operation from the transducers 3a to 3b, and FIG. 4B shows the transmission operation in the opposite direction.
In the ultrasonic flowmeter, as in (a), for example, an ultrasonic signal is transmitted from one transducer 3a, and the transmitted ultrasonic signal is received by the other transducer (3b). 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. In order to maintain a measurement accuracy by applying a stable trigger, a received wave with a good S / N ratio and a good rise is obtained. It is necessary to get. By alternately switching the transmission / reception of this ultrasonic signal and measuring the propagation time difference between the propagation time t2 to the upstream side and the propagation time t1 to the downstream side, the flow velocity of the fluid in the pipeline can be measured. The flow rate can be measured by multiplying this flow velocity by the cross-sectional area of the pipe 2. The relational expression for obtaining the flow rate is as follows.
[0004]
t1 = L / (C + V cos θ) (1)
t2 = L / (C−Vcos θ) (2)
From equations (1) and (2)
V = L (1 / t1-1 / t2) / 2 cos θ (3)
Q = (π / 4) × D ² × V × K (4)
In addition, the meaning of each said symbol is as follows.
t1: Propagation time of ultrasonic waves transmitted from the transducers 3a to 3b t2: Propagation time of ultrasonic waves transmitted from the transducers 3b to 3a C: Sound velocity of fluid L: Between the transducers 3a and 3b Distance V: Measurement flow velocity K: Flow velocity distribution coefficient D: Inner diameter of pipe 2 Q: Flow rate θ: Angle formed between the pipe shaft and the installation shaft of the transducer [0005]
[Problems to be solved by the invention]
Conventionally, the thickness of the matching layer of the transmitter / receiver has been considered to be 0.24λ in terms of sensitivity when described in terms of wavelength λ. There is also an example in which 0.15λ is used to improve the rising of the received waveform. Here, the wavelength λ of the matching layer is expressed by the following equation.
Matching layer wavelength λ
= Sonic velocity of matching layer material C (m / s) / frequency of ultrasonic wave f (Hz = 1 / s)
[0006]
Here, considering the relationship between the thickness of the matching layer, the sensitivity, and the ease of triggering, when the thickness of the matching layer is 0.24λ, as shown in FIG. Trigger for measuring the propagation time as shown in FIG. 6 (a), which has a maximum sensitivity in the vicinity of 24λ and a high S / N ratio, but has a poor rise (long rise time) as shown in FIG. Is difficult to apply. V1 to V6 and Vmax shown in FIG. 5 indicate the first to sixth waves and the maximum value of the received waveform shown in FIG. 7, respectively.
[0007]
On the other hand, when the thickness of the matching layer is 0.15λ, the maximum amplitude value Vmax is small as shown in FIG. 5 and the S / N ratio is bad, but the waveform has a good rise as shown in FIG. Therefore, it is easy to apply a trigger for measuring the propagation time.
From the above, the problem of S / N ratio due to the magnitude of the amplitude value with respect to the thickness of the matching layer is not compatible with the goodness of the rising waveform, the ease of triggering due to the badness, and the difficulty of applying, and the measurement accuracy of the propagation time It is also a problem.
Accordingly, an object of the present invention is to provide an ultrasonic flowmeter having a good S / N ratio and easy to trigger.
[0008]
[Means for Solving the Problems]
In order to solve such a problem, in the invention of claim 1, a pipe having at least two openings and allowing fluid to pass from one opening end to the other opening end, an ultrasonic transceiver, and one ultrasonic wave Transmitting means that excites the transmitter / receiver to switch the ultrasonic signal from the upstream side or the downstream side to the flow of the fluid and transmits it, and the other ultrasonic transmitter / receiver detects the ultrasonic signal propagating in the fluid, and the receiving unit And a time measuring means for detecting the propagation time of the ultrasonic signal that is received and processed in the fluid and measuring the propagation time of the ultrasonic signal in the downstream direction or the upstream direction. In an ultrasonic flowmeter comprising a converter having a flow rate calculation means for calculating a flow rate,
The thickness of the matching layer of the ultrasonic transmitter / receiver is set to 0.18λ of the wavelength λ (m) of the matching layer material obtained from the sound velocity C (m / s) of the matching layer material and the frequency f (Hz) of the ultrasonic wave. The above is 0.22λ or less, preferably 0.2λ.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is an explanatory diagram of an embodiment of the present invention.
This shows a specific configuration of the transducer. Reference numeral 11 denotes a matching layer, which is bonded to the piezoelectric element 12 by an adhesive 18 and fixed to the 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 with a screw 19 to transmit a voltage signal.
Although such a configuration itself is not different from a general one, here, the thickness of the matching layer 11 is optimized from the above viewpoint.
[0010]
Therefore, in the present invention, while changing the matching layer thickness, the ratio between the first wave and the second wave (V2 / V1) and the ratio between Vmax and the third wave V3 (V3 / Vmax) shown in FIG. When plotting each with attention, a relational curve as shown in FIG. 2 was obtained. Therefore, paying attention to the intersection of these two curves, the ratio of V2 / V1 indicating the stability of the trigger and the ratio of V3 / Vmax indicating the sensitivity are relatively large at this intersection. The basic concept of the present invention is to adopt the matching layer thickness 0.2λ at this time as an 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 0.2λ and the range is set to 0.18λ to 0.22λ. FIG. 6B shows an example of a received waveform when the matching layer thickness is in this range. It can be seen that both the rise and fall have good waveforms.
[0011]
Thus, the thickness of the matching layer is 0.18λ or more of the wavelength λ (m) of the matching layer material obtained from the sound velocity C (m / s) of the matching layer material and the ultrasonic frequency f (Hz). By setting it to 0.22λ or less, more preferably 0.2λ, it is possible to achieve both the S / N ratio indicating the sensitivity and the stability of the trigger. As a result, the generation timing of the pulse generated from the received waveform is improved. It is possible to stabilize and improve the measurement accuracy of the propagation time.
[0012]
【The invention's effect】
According to the present invention, since the thickness of the matching layer of the ultrasonic transmitter / receiver is optimized, there is an advantage that the measurement accuracy of the propagation time is improved and measurement with less error is possible.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of an embodiment of the present invention.
FIG. 2 is a diagram illustrating the principle of the present invention.
FIG. 3 is a configuration diagram showing a conventional ultrasonic flowmeter.
FIG. 4 is a diagram for explaining the measurement principle in FIG. 3;
FIG. 5 is an explanatory diagram of a relationship between a matching layer thickness and an amplitude value of a received waveform.
FIG. 6 is a diagram of received waveforms when a matching layer thickness is specified.
FIG. 7 is a waveform diagram showing an example of a received waveform.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Measuring pipe, 2 ... Piping, 2a ... Flange, 3a, 3b ... Ultrasonic transmitter / receiver (transceiver), 4 ... Converter, 5 ... Cable, 6 ... Sensor fixing part, 11 ... Matching layer, 12 ... Piezoelectric Element, 13 ... Lid, 14 ... O-ring, 15 ... Lead wire, 16 ... Case, 17 ... Electrode, 18 ... Adhesive, 19 ... Screw.

Claims (1)

A pipe having at least two openings and passing a fluid from one open end to the other open end, an ultrasonic transceiver, and an upstream or downstream side of the fluid flow by exciting one ultrasonic transceiver The ultrasonic signal propagating in the fluid is detected by the transmission means for switching the ultrasonic signal from the transmitter and the other ultrasonic transmitter / receiver, and the propagation time of the ultrasonic signal propagating in the fluid is received and processed by the receiver. An ultrasonic sensor comprising a time measuring means for detecting and measuring the propagation time of the ultrasonic signal in the downstream direction or the upstream direction, and a transducer having a flow rate calculating means for calculating the flow velocity or flow rate of the fluid from these propagation times. In sonic flow meter,
The thickness of the matching layer of the ultrasonic transmitter / receiver is set to 0.18λ of the wavelength λ (m) of the matching layer material obtained from the sound velocity C (m / s) of the matching layer material and the frequency f (Hz) of the ultrasonic wave. An ultrasonic flowmeter characterized by being 0.22λ or less, preferably 0.2λ.

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