JPS6098313A - Ultrasonic flowmeter - Google Patents

Abstract

PURPOSE:To enable measurement of the exact velocity and eventually the rate of flow of a fluid by providing a correction circuit which uses the change in the average time of propagation because said time changes when the velocity of sound in the fluid changes as a result of a change in the temp. and pressure of the fluid. CONSTITUTION:A circuit 12 which calculates the average propagation time T0 of propagation time T1, T2 and an arithmetic part 13 which corrects and calculates the velocity of sound according to said time T0 are provided to a general ultrasonic flowmeter consisting of an ultrasonic oscillator 1 and a time measuring circuit 7. If the propagation time at a wedge member or pipe wall except the fluid is designated as tau and the incident angle of an ultrasonic wave as theta, the velocity V of flow is V=D/sin2thetaXDELTAT/(T0-tau)<2>. If the velocity of the sound on the outside part is determined here, T0, tau, 1/sin2theta can be respectively calculated and the measurement of the velocity of flow is made possible.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to an ultrasonic flowmeter. The propagation time T between sending and receiving waves
l v T2 and the average propagation time To of both, the T
The present invention relates to an ultrasonic flowmeter that removes flow rate measurement errors caused by changes in sound speed from .

[Prior art and its problems]

FIG. 1 is a block diagram showing a conventional ultrasonic flowmeter.

In the figure, ultrasonic transducers 1 and 1' are attached to wedge members 2 and 2, respectively, as shown, and the wedge members 2 and 2 connect a tube 4 through which a fluid 3 flows, which is generated from one of the transducers. They are mounted on the outside of the tube opposite each other so that the ultrasonic waves are received diagonally across the other transducer.

Moreover, the ultrasonic transducer 1, the wedge member 2, the wall of the tube 4,
The fluid 3, the wall of g4, the wedge member 2, and the ultrasonic transducer 1 are acoustically coupled to each other.

The ultrasonic transducer 1 converts an electric signal inputted from the transmitter 5 via the changeover switch 6a located at the solid line position into an ultrasonic wave, and transmits it to the other opposing ultrasonic transducer 1. The ultrasonic transducer 1 converts the received ultrasonic waves into electrical signals and outputs them.

In the time measurement circuit 7, a memory circuit 8 measures the time T1 from the transmission to the reception of the ultrasonic wave using the transmission signal from the transmission unit 5 and the signal received from the transducer 1 via the changeover switch 6b located at the solid line position. Now, the measured time TI is received via the changeover switch 6C located at the solid line position, and the switches 6a, 6b, and 6c are switched to the dashed line position to store this time, and the transducer 1 is placed on the transmitting side and the transducer 1 is placed on the receiving side. By doing so,
Ultrasonic waves are transmitted and received in the opposite direction to the above, and the time T2 from transmission to reception is measured and stored in the memory circuit 9.

The time difference ΔT between the time T1 stored in the memory circuit 8 and the time T2 stored in the memory circuit 9 is calculated by the calculation unit 10, and the time difference ΔT is multiplied by a predetermined scale factor in the scale 7 actor calculation unit 11 to determine the flow rate. Obtain and output a signal.

By the way, the ultrasonic propagation time TI from the transducers 1 to 1' in the forward direction with respect to the flow and the ultrasonic propagation time T2 from the transducers 1' to 1 in the opposite direction are respectively the propagation time t in the fluid.
Letting l and t2 be the propagation time in the wedge member and the pipe wall other than the fluid, it can be expressed by the following equation.

Tl == tl +r (1) T2=t
2+τ (2) Here, the propagation time difference ΔT can be calculated as follows.

ΔT=T2-Ti=(t2+τ)-(tz+τ)=t
2- tl ・・・・・・ (3) monk; the inner diameter of 4 is D1
It is known that the propagation time 'i p '2 is generally expressed by the following equation, where Cw is the sound velocity in the fluid (ultrasonic propagation velocity), θ is the incident angle of the ultrasonic wave, and V is the flow velocity in the pipe. There is.

Therefore, ΔT becomes as follows.

Cw''-Vsi.θ' Here, comparing the sound velocity Cw in the fluid and the flow velocity V in the pipe, when the fluid is water, Cw is generally 1,000 to 1,600.
m/s, while V is l Q m/s
It is as follows. Therefore Cv? )) V"sinθ2, and ΔT can be expressed by the following approximate formula. 0 If the sound speed Cw in the fluid is constant, (sinθ/Cw'c
osθ) is constant, and ΔT is proportional to the flow velocity V. That is, by measuring ΔT, the flow iV can be obtained and the flow rate can be measured.

Incidentally, in FIG. 2, the above-mentioned quantities are illustrated for easy understanding.

However, there is one problem here. In equation (7) above, when the sound velocity Cw of the fluid does not change, the relationship ΔTOcV holds true, but the sound velocity Cw in the fluid changes as the temperature or pressure of the fluid changes.

Further, as the sound speed Cw changes, the angle 0 also changes according to Snell's law regarding reflection and refraction. Therefore, when the sound velocity Cw changes, an error occurs in the measured value of the flow velocity (■). In particular, when the fluid temperature changes from room temperature to high temperature (approximately 300°C) or when the pipe is thick, the influence of changes in the sound velocity Cw tends to be large and measurement errors tend to increase. This can be said to be a major drawback in ultrasonic flowmeters.

[Purpose of the invention]

The present invention provides an ultrasonic flow meter that is capable of correcting the change in the sound velocity Cw in the fluid due to changes in the temperature and pressure of the fluid and measuring the accurate flow velocity V and thus the flow rate. The purpose is to provide a

[Key points of the invention]

When the sound velocity Cw in the fluid changes due to a change in the temperature and pressure of the fluid, the average propagation time To of the propagation times T1 and T2 also changes, so by providing a correction circuit using this, the temperature of the fluid, The key point of the present invention is that the accurate flow 31v of fluid, and thus the flow rate, can be measured without the need to directly measure pressure.

[Embodiments of the invention]

FIG. 3 is a block diagram showing one embodiment of the present invention. In the same figure, parts common to those in FIG. 1 are given the same numbers.

The difference from the borrowing shown in FIG. 1 is that the propagation time TI,
Circuit 12 that calculates the average propagation time To of T2 and its To
The present invention is also provided with a calculation section 13 that performs a sound velocity correction calculation based on the speed of sound.

Here, when the fluid is stationary, that is, when ■ = O, then from the above equation (4) and equation (5), tl : t2 0 Therefore, T1 = T2 = T.

Then, from the above equation (1) or (2), if we substitute the α0 equation into the above equation (7) from the above equation (from equation 10), we get Tl + T2 Here, when the fluid is flowing, To=- and approximately T
o can be set, and the above zero-order formula also holds true when V~0. In other words, in the above equation, in addition to ΔT, angles θ and To
If τ and τ can be measured, the flow velocity ■ can be calculated using an electronic calculator.

However, angle θ, if; propagation time τ in parts other than the body
First of all, it is difficult to measure. If gtffIr
Even if J is possible, calculations using these at the level of industrial instruments are difficult as a practical matter.

Incidentally, the dimensions of the wedge member, the mounting position of the wedge member, the inner diameter of the pipe, and the wall thickness of the pipe are determined by measuring or determining the dimensions in advance during manufacturing.

Furthermore, if the temperature and pressure are known, the speed of sound in the fluid, wedge member, and pipe can be determined and known. Therefore,
In this way, it is possible to determine the speed of sound at each part.

Therefore, if we summarize the range in which the speed of sound in each part can change, we can see that there is a fairly constant relationship between To and τ, as shown in Figure 4, no matter how the temperature and pressure change. It turns out.

It was found that there is an almost constant relationship. Kokote 0
Point is 0℃, 0 point g-111370'C, 6 points is about 300
It shows the relationship at °C. This relationship holds true even if conditions such as pipe inner diameter, thickness, temperature, and pressure change.

If only To is measured, the above α4 equation 111] (T
o-τ) In FIGS. 4 and 5, a slight deviation occurs, but this does not pose a problem in the correction calculation of the ultrasonic flowmeter.

In other words, let To be the long sword signal, and its generator is the it in Figure 3.
By using it in the speed correction calculation section, changes in sound speed can be corrected relatively accurately without performing trigonometric function calculations.

Incidentally, in the actual correction, the correction may be performed by using one function generator having the characteristics shown in FIG. 6 obtained by multiplying the respective characteristics shown in FIGS. 4 and 5.

As a function generator here, it is of course possible to use a ROM that stores these calculated values by using addresses for each part of To.

〔Effect of the invention〕

When the temperature and pressure of the fluid to be measured and the tube through which the fluid passes change, the speed of sound propagating through the fluid and the tube changes.

According to the present invention, changes in the speed of sound caused by changes in temperature and pressure can be corrected without directly measuring these temperatures and pressures. There is also a method of measuring the temperature and pressure of the fluid and pipes using other sensors and correcting the speed of sound, but in this case, errors occur due to the time difference between the time of measurement (detection) and the time of correcting the speed of sound. Particularly when rapid temperature changes occur, the error becomes large.

In this regard, according to the present invention, the propagation time difference signal, which is the basis of the flow rate, and the average propagation time, which is the basis of the correction, are always measured simultaneously, so that no error occurs due to a difference in detection time.

The present invention can be applied to ultrasonic flowmeters that measure any fluid by changing the function of the function generator used.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing a conventional ultrasonic flowmeter, Fig. 2 is an explanatory diagram of the relationship between various quantities necessary for flow measurement, Fig. 3 is a block diagram showing an embodiment of the present invention, and Fig. 4 is a block diagram showing a conventional ultrasonic flowmeter. Average propagation time (
Fig. 5 is a graph showing the relationship between the average propagation time (To) and the propagation time in the fluid (To - τ), and Fig. 5 is a graph showing the relationship between the average propagation time (To) and the correction coefficient. Code explanation 1.1... Ultrasonic transducer, 2, 2...
wedge member, 3... fluid, 4... pipe,
5... Transmission section, 6a. 6b, 6c... Changeover switch section, 7...
...Reception and time measurement circuit, 8...Memory circuit (
T1), 9... Memory circuit (T2), 10...
... Time difference (ΔT) calculation section, 11 ... Scale 7 actor calculation section, Engineering 2 ... Average propagation time (T
o) Calculation unit, 13...Sound velocity correction calculation unit Representative Patent attorney Akio Namiki Patent attorney Kiyohui Matsuzaki Figure 1 Figure 2 Groove 4 Figure J'r, -

Claims (1)

[Claims] 1) Transmitting and receiving ultrasonic waves in the forward and reverse directions across the fluid flow, and measuring the propagation time Tl between them.
# 'r2 and the average propagation time TO of both, the To
An ultrasonic flowmeter that measures the flow rate of the C body by calculating a predetermined calculation formula including: A correction amount depending on the sound speed during ultrasonic propagation in the calculation formula is stored as a function of the TO. An ultrasonic flowmeter characterized in that it is equipped with a function generator that can be used to generate data, and is designed to remove the effects of changes in sound speed from flow rate measurements.