JPH04328423A - Ultrasonic wave gas flowmeter - Google Patents

Abstract

PURPOSE:To measure a flow rate which in corrected by the specified temperature by using the ultrasonic wave in the gas flowing through a pipe. CONSTITUTION:A pair of ultrasonic wave probes 2 and 3 are arranged so as to face each other through fluid to be measured on the straight line which is inclined with respect to the central axis of a pipe 1 wherein gas flows. An ultrasonic wave is propagated in the forward direction and in the reverse direction with respect to the flowing direction of the fluid. The flow rate of the gas in the standard state is measured by using the time difference between the propagating times of the ultrasonic wave. Since the difference between the propagating times of the ultrasonic wave in the forward direction and in the reverse direction in the measurement is used, the effect of the common fluctuation in the fixed delay time for both directions can be removed. Since the flow rate which is corrected for temperature is obtained by using the relationship among the sound speed, the absolute temperature and the volume, mounting of a temperature transmitter is not required, and the flow rate which is corrected for the temperature can be accurately measured.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic gas flow meter that uses, for example, ultrasonic waves to measure the temperature-corrected flow rate of gas flowing in a pipe.

0002

[Prior Art] FIG. 5 is a block diagram showing an example of a conventional ultrasonic gas flowmeter, in which 1 is a pipe line, 2 is a first ultrasonic probe, 3 is a second ultrasonic probe, and 27 is a temperature transmitter. ,2
8 is a propagation time measurement circuit, 29 is a flow rate calculation circuit, 30 is a temperature correction circuit for temperature correction of the measured flow rate, D is the inner diameter of the pipe 1, L is the length of the ultrasonic propagation path in the pipe 1, and V is the flow velocity. be. The conventional ultrasonic gas flow meter is constructed as described above, and the first ultrasonic probe 2 and the second ultrasonic probe 3 are arranged on an axis tilted by θ from the center of the pipe line 1 with an inner diameter D, and each A propagation time measurement circuit 28, a flow rate calculation circuit 29, and a temperature correction circuit 30 are connected to the ultrasonic probe.

[0003] When gas with a sonic velocity C is flowing in the direction of the arrow at a velocity V in the conduit 1, the time required for the ultrasound to propagate from the first ultrasonic probe 2 to the second ultrasonic probe 3 is time tu is

[Equation 1] The ultrasonic propagation time td from the second ultrasonic probe 3 to the first ultrasonic probe 2 is

[Equation 2] τ represents a fixed delay time, that is, the total time delay from the first wave of the ultrasound signal to the trigger wave or caused by circuits, cables, etc.

A flow rate calculation circuit 29 calculates the flow rate using the propagation times tu and td. The flow velocity V from the difference in the reciprocal of the propagation time is

[Math 3] Therefore, if the cross-sectional area of pipe 1 is S, then the flow rate Q is

[Math 4] (3) Substituting Eq.

[Math 5] Obtain the flow rate Q according to the above formula.

However, in general, the volume of gas expands and contracts depending on the temperature.

[Equation 6] Here, Q1 is the volume at absolute temperature T1 (°k), Q2
is the volume at absolute temperature T2 (°k). Therefore, if the absolute temperature in the standard state is Tn, the flow rate Qn in the standard state is

[Equation 7] Here, T is the absolute temperature when the flow rate is measured from the temperature transmitter 27.

[0006]

[Problems to be Solved by the Invention] In the above-mentioned ultrasonic gas flowmeter, the gas flow rate is measured using the difference in the reciprocal of the ultrasonic propagation time in the forward and reverse directions of the gas flow in the pipe 1 and temperature correction. This is done using the sound velocity C. The ultrasonic propagation time in the above measurement includes a fixed delay time τ. The ultrasonic propagation time is related to the ultrasonic propagation path formed from the transmitting circuit to the ultrasonic probe, the ultrasonic propagation path length L, and the ultrasonic probe to the receiving circuit. For example, for φ100 pipe 1, the above value is about 400 μs, but The propagation time difference Δt between the forward direction and the reverse direction involved in flow rate measurement is approximately 30 to 40 μs, although it depends on the dimensions of the pipe line 1 and the flow rate. The fixed delay time τ that occurs in the ultrasonic propagation path cannot be measured after the ultrasonic probe is installed in conduit 1, and its value differs for each ultrasonic probe, so it must be measured before installation in conduit 1 and used as the set value. Must be remembered.

[0007] Also, the fixed delay time τ changes as the elastic constant of the vibrator changes, resulting in a change in the resonant frequency, which causes the fixed delay time τ in time difference measurement to fluctuate, resulting in a deviation from the above set value and a change in the scale factor. Therefore, the accuracy of flow rate measurement decreases. Furthermore, the calculated gas flow rate is subjected to temperature correction. Therefore, there was a problem in that it was necessary to provide a temperature transmitter 27 used to measure the temperature of the gas inside the pipe 1, a fitting for mounting it on the pipe 1, and a temperature correction circuit 30.

The present invention was made to solve these problems, and it is possible to measure the fixed delay time τ of the ultrasonic propagation path and to install a temperature transmitter in the pipe 1 to measure the gas temperature. The purpose of this invention is to obtain an ultrasonic gas flow meter that can obtain temperature-corrected gas flow rates.

[0009]

[Means for Solving the Problems] The ultrasonic gas flowmeter according to the present invention includes a propagation time measurement circuit that measures the ultrasonic propagation time in the forward and reverse directions with respect to the flow of gas in the pipe, and the pipe dimensions and A coefficient setter that sets the constants related to the installation position of the ultrasonic probe in the pipeline and the temperature specification in the standard state, and the propagation time difference obtained from the forward and reverse propagation times measured by the propagation time measurement circuit. A flow rate calculation circuit is provided that performs conversion into a temperature-corrected gas flow rate based on a constant set by a coefficient setter.

0010

[Operation] In this invention, a pair of ultrasonic probes are placed opposite each other on a straight line inclined with respect to the central axis of the pipe through which the gas flows, and the ultrasonic probes are arranged oppositely in the direction of the gas flowing between them. Measures the ultrasonic propagation time in the forward and reverse directions, and measures the gas flow rate, which is temperature-corrected in the flow rate calculation circuit based on the propagation time difference obtained from these and coefficients such as pipe dimensions and how the probe is attached to the pipe. do.

[0011] In the above measurement, since the propagation time difference between the forward and reverse ultrasonic propagation times is used, the fixed delay time of the ultrasonic propagation path can eliminate the influence of fluctuations caused by temperature and other factors that are common to both. . Further, changes in gas volume due to temperature can be corrected without using a temperature transmitter.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a block diagram showing an embodiment of the present invention, in which 1, 2, 3, C, V, D, θ, and τ are the same as those in the conventional flowmeter, and the same symbols indicate the same or corresponding parts. 5 is a propagation time measurement circuit that measures ultrasonic propagation time;
6 is a coefficient setting device for setting various constants indicating the dimensions of the pipe 1, the attachment state of the ultrasonic probe to the pipe 1, and the relationship between temperature and sound speed; 7 is a propagation time difference △ in the forward and reverse directions of gas flow; Flow rate calculation circuit that calculates the flow rate based on t, 8
9 indicates a subtracter, and 9 indicates a multiplier.

In the ultrasonic gas flowmeter configured as described above, the first sensor is mounted so as to face the fluid to be measured on a straight line inclined with respect to the central axis of the pipe 1 through which the gas flows. The moving average value tu of the time required for the ultrasound to propagate from the first ultrasound probe 2 to the second ultrasound probe 3, and conversely for the ultrasound to propagate from the second ultrasound probe 3 to the first ultrasound probe 2. The moving average value td of the actual time is determined as follows.

[0014] When a gas with a sonic speed of C is flowing with a flow velocity of V,
The time tu required for the ultrasound to propagate from the first ultrasound probe 2 to the second ultrasound probe 3 is

[Equation 8] The propagation time td of the ultrasound from the second ultrasound probe 3 to the first ultrasound probe 2 is

[Equation 9] In the flow rate calculation circuit 7, each ultrasonic propagation time tu
, calculate the flow rate from td, that is, the propagation time difference △t is

[Math. 10]

In measuring the air flow rate, C=340(
m/s), V=30(m/s), θ=(60°),

[Equation 11] Since V2cos2θ is about 0.19% of C2, it will be omitted. therefore

[Math. 12] In addition, the flow rate Q is calculated by assuming that the cross-sectional area of pipe 1 is s.

[Math. 13] Substituting the equation for flow velocity

[Math. 14]

[0016] Here, the sound speed C of air is expressed by the following equation.

[Formula 15] a is the proportionality coefficient, T is the absolute temperature of the fluid (°k), and therefore

[
Equation 16] The flow rate Qn in the standard state is

[Equation 17] If the absolute temperature in the standard state is defined, a gas flow rate that does not depend on the gas temperature and is proportional only to the propagation time difference Δt can be obtained.

The ultrasonic propagation times tu and td in the forward and reverse directions of the gas flow are measured by the propagation time measuring circuit 5 and are averaged by a moving average, etc., and the supersonic waves of the two are passed through the subtractor 8 of the flow rate calculation circuit 7. Obtain the sound wave propagation time difference Δt. Conduit 1
It is applied to the multiplier 9 together with the output from the coefficient setter 6 which sets constants such as dimensions, the manner in which the probe is attached to the conduit 1, and the designation of the temperature in the standard state. A temperature-corrected gas flow rate Qn is obtained.

FIG. 2 is an explanatory diagram showing an example of a fixed delay time, and shows an example of the rise characteristic of a transducer used in a probe during sound-to-electrical conversion, where s1 is the rise characteristic of the transducer under normal conditions, and s2 indicates a threshold value, and the circuit is operated by a signal exceeding the threshold value, and the rise time until the signal intersects the base line s3 is defined as a fixed delay time τ. s4 indicates another rise characteristic of the vibrator.

The rise characteristic of the vibrator in the characteristic s4 has a delay of Δτ1 with respect to the fixed delay time τ due to differences in the vibrator characteristics, bonding structure, etc. In addition, the resonator resonates when its thickness is equal to half the wavelength λ/2 of the operating frequency, and is normally used in this highly sensitive state.However, since the sound speed changes depending on the gas temperature, the thickness remains constant. The wavelength, or resonant frequency, of the oscillator changes. In this state as well, the fixed delay time τ changes and a delay of Δτ2 similarly occurs. Therefore, a delay of △τ = △τ1 + △τ2 occurs, but
By calculating the time difference between the forward and reverse directions of the ultrasonic propagation path, the delay is canceled out and does not affect measurement accuracy.

FIG. 3 is a block diagram showing an example of an ultrasonic gas flowmeter, and FIG. 4 is a time chart of the operation of the ultrasonic gas flowmeter. Same as the above embodiment, the first switch 17 is operated by the timer command s4 from the timer 16 driven by the clock pulse generator 15.
switches the propagation direction of the ultrasonic waves in the conduit 1.

For example, the first ultrasonic probe 2 is energized by the transmitted wave s5 from the transmitting circuit 18, and the ultrasonic waves propagated in the opposite direction of the fluid flow in the conduit 1 are received by the second ultrasonic probe 3. The signal then passes through the first switch 17 and enters the receiving circuit 19 again to obtain a received wave s6. The above timer command s4 and received wave s
6 are both applied to a flip-flop circuit 20 to generate a square wave s7 whose duration is the ultrasound propagation time. The counter 21 performs counting using the clock pulse of the square wave s7, and its output is applied to the first measuring circuit 23 via the second switch 22 which operates in synchronization with the first switch 17.

[0022] The first switch 1 is activated by the next timer command s4.
7 and the second switching device 22 reverse their operations, and when the second ultrasonic probe 4 is energized by the transmission wave s5 from the transmission circuit 18, the ultrasonic waves propagate in the forward direction of the fluid flow in the pipe line 1. do. Similarly, clock pulses corresponding to the ultrasound propagation time are counted and applied to the second measurement circuit 24.

The clock pulses obtained from the ultrasonic propagation time in the forward and reverse directions in the conduit 1 by the above-described timer command s4 are sequentially measured in the forward direction by the first measuring circuit 23.
8 and the reverse direction measurement s9 in the second measurement circuit 24,
A moving average is performed and the average value of the propagation time is output. The flow measurement circuit 7 performs temperature-corrected flow measurement using the time difference of ultrasonic propagation in the forward and reverse directions of the gas flow in the pipe line 1 and the constant from the coefficient setter 6.

By defining the absolute temperature in the standard state using the coefficient setter 6, the flow rate Qn in the standard state can be obtained. When measuring the temperature-corrected flow rate Qn as described above, it is not necessary to measure the gas temperature, and the influence of the fixed delay time τ can be avoided. Note that the above calculation may be performed by software.

[0025]

[Effects of the Invention] As explained above, the present invention is based on a coefficient setting device that sets the propagation time difference obtained from the ultrasonic propagation time in the forward and reverse directions of the gas flow in the pipe and various constants related to measurement. With a simple structure that includes a flow rate calculation circuit that calculates the gas flow rate, temperature-corrected flow rate measurements can be performed without using a temperature transmitter. There is no need to actually measure the fixed delay time, which differs for each device, at the time of shipment and set it as a fixed value. This has the effect of preventing flow rate calculation errors from occurring due to fluctuations in the fixed delay time after setting, and improving flow rate measurement accuracy.

[Brief explanation of drawings]

[Fig. 1] Block diagram showing one embodiment of this invention

[Figure 2]
Explanatory diagram showing an example of fixed delay time

[Figure 3] Block diagram showing an example of an ultrasonic gas flowmeter

[Figure 4] Time chart of ultrasonic gas flowmeter operation

[Figure 5] Block diagram showing an example of a conventional ultrasonic gas flowmeter

[Explanation of symbols]

1 Pipeline 2 First ultrasonic probe 3 Second ultrasonic probe 5 Propagation time measurement circuit 6 Coefficient setter 7 Flow rate calculation circuit

Claims (1)

[Claims] [Claim 1] A pair of ultrasonic probes are mounted opposite to each other with the fluid to be measured interposed on a straight line inclined with respect to the central axis of a conduit through which gas flows, and the direction of the fluid flowing between them is In an ultrasonic gas flowmeter that transmits ultrasonic waves in the forward and reverse directions and measures the gas flow rate from the ultrasonic propagation time in each direction, the forward and reverse directions of the gas flow in the pipe are measured. A propagation time measurement circuit that measures the ultrasonic propagation time, a coefficient setting device that sets constants related to the pipe dimensions, the way the ultrasonic probe is attached to the pipe, and the temperature specification in the standard state, and the propagation time measurement circuit described above. Equipped with a flow rate calculation circuit that converts the propagation time difference obtained from the measured forward and reverse propagation times into a temperature-corrected gas flow rate based on the constant set by the coefficient setting device. A gas flowmeter featuring: