JP4287539B2 - Ultrasonic flow meter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an improvement in an ultrasonic flow meter.
[0002]
[Prior art]
As an example of the measurement principle, as shown in FIG. 2, a transmitter / receiver 1 from one transmitter / receiver 1 of a set of ultrasonic transmitters / receivers arranged at upstream and downstream of the flow tube 3 at a distance L in the fluid. The forward propagation time t ₁ to the waver 2 is as follows: C is the sound velocity of the ultrasonic wave in the static fluid, and V is the flow velocity of the fluid.
t ₁ = L / (C + V) (1)
It becomes.
[0003]
The backward propagation time t ₂ from the transducer 2 to the transducer 1 is
t ₂ = L / (C−V) (2)
It becomes.
[0004]
From the propagation times t ₁ and t ₂ , the flow velocity V is
V = (L / 2) {(1 / t ₁ ) − (1 / t ₂ )}
Was asking.
[0005]
In the measurement principle described above, the zero cross point of a specific wave is detected as a reception detection method for specifying the time when the ultrasonic wave from the transmitter / receiver receives the transmitter / receiver on the receiving side, that is, the arrival point. There is what I did.
[0006]
FIG. 3 shows a transmission drive signal and a reception wave indicating the timing of transmission. The actual received wave is very small and is first amplified. The received wave in the figure shows the waveform after amplification.
[0007]
a is the arrival point, and the amplitude gradually increases. After that, it becomes maximum amplitude and gradually decreases.
However, the arrival point a is hidden behind noise and cannot be detected. Therefore, the following method is performed.
[0008]
A threshold V _TH as a reference voltage level that is sufficiently larger than noise is determined, and a zero cross point that passes through the zero level after the wave that first reaches this level, for example, the third wave in FIG. This is a method of detecting c and setting it as reception detection.
[0009]
The threshold value V _TH is determined so as to always detect the zero cross point of some specific wave (for example, the third wave), and the actual propagation time t is the time τ from point a to point c. Obtained in advance and stored, and obtained by subtracting the time τ from the measured arrival time t + τ.
[0010]
[Problems to be solved by the invention]
In the conventional technique, when the temperature of the flow tube 3 and the temperature of the fluid are different, the temperature of the fluid, particularly the temperature of the fluid near the inner wall of the flow tube 3 approaches the temperature of the flow tube 3. This tendency becomes particularly remarkable at the time of zero flow rate or small flow rate, and the temperature of the fluid has a temperature gradient in the radial direction of the flow tube.
[0011]
For this reason, the propagation state of the ultrasonic wave changes, and the received wave may become small due to interference, making it difficult to detect the zero cross point. This is particularly problematic when the flow path is gap-like and the temperature difference between the opposing wall surfaces sandwiching the flow path is large.
[0012]
For example, when a fluid flows through a narrow channel having an annular cross section, the propagation of ultrasonic waves is considered to be a plane wave, and if the opposing wall surfaces A and B are at the same temperature as shown in FIG. The ultrasonic wave transmitted from the ultrasonic transducer 1 propagates rightward in the fluid in the narrow flow path 4 sandwiched between both wall surfaces A and B, and obediently the other ultrasonic transducer 2. Will be received.
[0013]
However, when the temperature of one wall surface A is high and the temperature of the other wall surface B is low and the temperature difference between both wall surfaces A and B is large, the temperature of the fluid, for example, the gas in the flow path 4 is a temperature gradient in the vertical direction in the figure. 4B, the wavefront of the ultrasonic wave becomes as shown in FIG. 4B, and interference occurs in the ultrasonic wave received by the ultrasonic transducer 2.
[0014]
In FIG. 5, (a) is a normal received waveform when there is no temperature difference. In such a case, the target zero-cross point can be detected with high accuracy. (B), (c), and (d) in the figure are cases where there is a temperature difference, and (b) is slightly affected, and the zero cross point is deviated from the normal time, and an error occurs in the measured value of the propagation time. FIG. 6C is considerably interfered, and it is difficult to detect the received wave. In FIG. 4D, interference waves shifted by a half cycle overlap, the amplitude of the received wave becomes extremely small, and almost disappears.
[0015]
Therefore, an object of the present invention is to provide an ultrasonic flowmeter that can improve the stability of measurement by reducing the temperature gradient in the propagation path of ultrasonic waves and reducing interference of received waves.
[0016]
[Means for Solving the Problems]
In order to achieve the above object, the invention of claim 1 includes a metal outer cylinder (5) integrally formed with flange portions at both ends of a substantially cylindrical portion;
A cylindrical inner cylinder (6) made of a heat insulating material that is attached to the inside of the outer cylinder (5) through two bases (7) and (8) made of a heat insulating material, and guides the fluid to be measured,
An air layer (9) having an annular cross section formed between a cylindrical portion of the metal outer cylinder (5) and the inner cylinder (6) made of a heat insulating material;
A boss-like member (10) provided concentrically with the outer cylinder (5) and having a large-diameter portion at the center;
A channel (40) having an annular cross section formed between the inner wall (A ₁ ) of the inner cylinder (6) made of a heat insulating material and the outer wall (B ₁ ) of the large-diameter portion of the boss-like member (10 ). When,
The cross section is disposed away from the upstream and downstream of the annular flow path (40), and includes a pair of ultrasonic transducers for transmitting and receiving ultrasonic waves for flow velocity measurement in the flow path (40),
The two bases (7) and (8) include a first base (7) for attaching one end of the inner cylinder (6) to the outer cylinder (5), and the other end of the inner cylinder (6) as the outer cylinder (5). ), And the bases (7) and (8) have an inner diameter that increases as they move away from the ends of the inner cylinder (6), that is, toward the inlet / outlet of the flow meter. It is formed in a tapered shape that expands in diameter,
The ultrasonic flowmeter is characterized in that the fluid to be measured flowing through the flow path (40) is a gas .
[0017]
The invention according to claim 2 is the ultrasonic flowmeter according to claim 1, wherein the inner cylinder (6) is made of a plastic material.
The invention according to claim 3 is the ultrasonic flowmeter according to claim 1 or 2, wherein the bases (7) and (8) are made of a plastic material .
[0018]
The invention according to claim 4 is the ultrasonic flowmeter according to claim 3, characterized in that the inner cylinder (6) and the base (7) (8) are integrally formed of the same plastic material. It is.
According to a fifth aspect of the present invention, in the ultrasonic flowmeter according to any one of the first to fourth aspects, the fluid to be measured is a gas.
[0019]
According to a sixth aspect of the present invention, in the ultrasonic flowmeter according to any one of the first to fifth aspects, the boss-like member (10) has a small diameter at both ends, and the tip of the boss-shaped member (5) has an outer cylinder (5). ) Is fixed to the arm fixed to the flange.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Next, a preferred embodiment of the present invention will be described with reference to the embodiment of FIG.
The metal outer cylinder 5 having a high strength has flange portions integrally formed at the left and right ends of the substantially cylindrical portion in the figure. The plastic cylindrical inner cylinder 6 is attached to the inner side of the outer cylinder 5 via plastic bases 7 and 8, and an air layer 9 is formed between the cylindrical portion of the outer cylinder 5 and the inner cylinder 6. Heat conduction between the outer cylinder 5 and the inner cylinder 6 is prevented. In addition, the bases 7 and 8 are made of a plastic material as a heat insulating material, thereby preventing heat conduction from the outer cylinder 5 to the inner cylinder 6 through the bases 7 and 8. The production cost may be reduced by integrally forming the same plastic material as the cylinder 6.
[0021]
A boss-like member 10 having a large diameter portion with a diameter φ1 at the center is provided concentrically on the outer cylinder 5. The left and right ends of the boss-like member 10 have a small diameter, and the tip of the boss-like member 10 is screwed to an arm fixed to the flange.
[0022]
An annular channel 40 is formed between the inner wall A ₁ of the inner cylinder 6 and the outer wall B ₁ of the boss-like member 10. Ultrasonic wave transmission / reception for generating and receiving ultrasonic waves for measuring flow velocity in this channel 40 is formed. Wavers 1 and 2 are arranged at a distance L.
[0023]
The ultrasonic transducers 1 and 2 are connected to the electronic circuit board 11, and a microcomputer of the electronic circuit mounted on the board displays the flow rate on the liquid crystal display 12. Reference numeral 13 denotes a power supply battery.
[0024]
The annular cross-sectional flow path 40 is a narrow flow path sandwiched between the boss-like member 10 having an outer diameter of φ1 and the inner cylinder 6 having an inner diameter of φ2. Propagate to.
[0025]
Even if the temperature of the outer cylinder 5 is different from that of the fluid in which the temperature of the outer cylinder 5 flows, for example, gas, a radial temperature gradient is applied to the fluid in the flow path 40 by the heat insulating action of the air layer 9 and the plastic inner cylinder 6 and the bases 7 and 8. It rarely occurs. Therefore, since the zero cross point targeted by the received wave can be detected reliably, the measurement accuracy of the flow meter is improved.
[0026]
【The invention's effect】
Since the ultrasonic flowmeter of the present invention is configured as described above, the influence of the temperature of the outer cylinder on the temperature of the fluid is reduced. Therefore, the temperature gradient of the fluid can be reduced and interference of the received wave of the ultrasonic wave can be prevented, so that the zero cross point of the received wave can be reliably detected with high accuracy. As a result, the flow rate measurement accuracy is improved and the instrumental error characteristic is stabilized.
[0027]
In addition, since the inner wall of the outer cylinder that serves as a case is not directly used as the wall surface of the flow path, precision machining is not necessary, contributing to cost reduction.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view of an embodiment of the present invention.
FIG. 2 is a schematic diagram illustrating the principle of an ultrasonic flow meter.
FIG. 3 is a diagram showing an electrical signal waveform for explaining the operation of the received wave detection unit of the ultrasonic flowmeter.
FIG. 4 is a schematic diagram showing a wavefront of an ultrasonic wave propagating through a narrow flow path between opposing wall surfaces.
FIG. 5 is a diagram showing various aspects of a received electrical signal waveform.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1, 2 Ultrasonic transmitter / receiver 5 Outer cylinder 6 Inner cylinder 7, 8 Base 9 Air layer 10 Boss-like member 40 Flow path

Claims (6)

A metal outer tube (5) integrally formed with flange portions at both ends of a substantially cylindrical portion;
A cylindrical inner cylinder (6) made of a heat insulating material that is attached to the inside of the outer cylinder (5) through two bases (7) and (8) made of a heat insulating material, and guides the fluid to be measured,
An air layer (9) having an annular cross section formed between a cylindrical portion of the metal outer cylinder (5) and the inner cylinder (6) made of a heat insulating material;
A boss-like member (10) provided concentrically with the outer cylinder (5) and having a large-diameter portion at the center;
A channel (40) having an annular cross section formed between the inner wall (A ₁ ) of the inner cylinder (6) made of a heat insulating material and the outer wall (B ₁ ) of the large-diameter portion of the boss-like member (10 ). When,
The cross section is disposed away from the upstream and downstream of the annular flow path (40), and includes a pair of ultrasonic transducers for transmitting and receiving ultrasonic waves for flow velocity measurement in the flow path (40),
The two bases (7) and (8) include a first base (7) for attaching one end of the inner cylinder (6) to the outer cylinder (5), and the other end of the inner cylinder (6) as the outer cylinder (5). ), And the bases (7) and (8) have an inner diameter that increases as they move away from the ends of the inner cylinder (6), that is, toward the inlet / outlet of the flow meter. It is formed in a tapered shape that expands in diameter,
The ultrasonic flowmeter, wherein the fluid to be measured flowing through the flow path (40) is a gas . The ultrasonic flowmeter according to claim 1, wherein the inner cylinder (6) is made of a plastic material . 3. The ultrasonic flowmeter according to claim 1, wherein the base (7) (8) is made of a plastic material . The ultrasonic flowmeter according to claim 3, wherein the inner cylinder (6) and the base (7) (8) are integrally formed of the same plastic material . The ultrasonic flowmeter according to any one of claims 1 to 4, wherein the fluid to be measured is a gas . The boss-like member (10) has a small diameter at both ends, and a tip thereof is screwed to an arm fixed to the flange of the outer cylinder (5). The ultrasonic flowmeter according to claim 1 .

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