RU2221993C1 - Acoustic-impedance method to measure levels of liquid media - Google Patents

Abstract

FIELD: automatics, measurement technology. SUBSTANCE: proposed method can be utilized in heat power industry, in chemical, oil and other industries to measure levels of liquid media in tanks. Method includes placement of metal waveguide in liquid medium along height of tank, excitation of longitudinal elastic wave of zero order in one end of metal waveguide, reading of forward wave and wave reflected from the other end of waveguide of elastic waves of zero order, determination of intensity levels of forward and reflected elastic waves of zero order. Voltages of forward and reflected signals corresponding to intensities of forward and reflected elastic waves of zero order are memorized in sequence by first and second storage locations with capacitors C. Resistor is connected to first storage location with capacitor C. Voltages of forward and reflected signals are compared. Time interval from moment of connection of resistor to first storage location with capacitor C to moment of equality of voltage across resistor R and voltage across capacitor C of second storage location is measured. EFFECT: secured constancy of sensitivity of acoustic-impedance method measuring levels of liquid media in range of conversion. 1 cl, 2 dwg

Description

 The invention relates to automation and measuring equipment and can be used to measure the level of liquid media in tanks in the heat power, oil, chemical and other industries.

 A known method of measuring liquid media in tanks, based on measuring the energy of an ultrasonic wave passing from one medium to another [O. Babikov Level control using ultrasound. M: Energy, 1970, 80 p. (Automation book. Issue 459, pp. 18-20)].

 The disadvantage of this method is the inconstancy of sensitivity over the conversion range due to the logarithmic dependence of the measured level on the intensity of the ultrasonic wave, due to diffraction divergence of the ultrasonic beam, the dependence of the quality factor of electro-acoustic transducers and the speed of the ultrasonic wave on the parameters of the controlled medium.

 The closest technical solution to the invention is the acousto-impedance method of measuring the level of coolant in the tank, when a metal waveguide is placed in the latter, which is in contact with the liquid along its height, a zero-order longitudinal elastic wave is excited at one end of the waveguide, the elastic wave reflected from the other end of the waveguide is read and determined intensity level of the reflected elastic wave [V. Melnikov, G. Usynin. Acoustic methods for the diagnosis of two-phase coolants of nuclear power plants. -M.: Energoatomizdat, 1987, 168 p.].

 The disadvantage of this method is the inconstancy of sensitivity over the conversion range due to the logarithmic dependence of the level on the intensity of the ultrasound wave reflected from the end of the waveguide.

 The technical problem is the creation of an acousto-impedance method for measuring the level of liquid media, which provides a linear dependence of the level on the intensity of the ultrasound wave reflected from the end of the waveguide.

 EFFECT: constant sensitivity of the acousto-impedance method for measuring the level of liquid media in the conversion range.

 To achieve a technical result, a metal waveguide is placed in a liquid medium along the height of the tank, a zero-order longitudinal elastic wave is excited in the waveguide at one end of the metal waveguide, zero-order elastic waves are reflected and reflected from the other end of the waveguide, and intensity levels of direct and reflected zero-elastic waves are determined order, sequentially stored on the first and second memory elements with capacitors C voltage of the direct and reflected signals, proportional to The corresponding intensities of direct and reflected elastic waves of zero order are connected to the first memory element with a capacitor C resistor, the voltage of the direct and reflected signals is compared and the time interval from the moment the resistor is connected to the first memory element with capacitor C is measured until the voltage across the resistor R and the voltage are equal on capacitor C of the second memory element.

 In addition, the excitation and reading of elastic waves of zero order is carried out at points of a metal waveguide located at a distance greater than the "dead zone" of the reading channel of a direct elastic wave of zero order.

When zero-order longitudinal elastic waves propagate through a metal waveguide in the latter, in addition to attenuation in the material of the metal waveguide itself, there is an additional attenuation of zero-order elastic waves caused by their radiation from the surface of the waveguide into a liquid medium. The relationship between the intensities of the direct and reflected waves is described by the equation
J = J ₀ exp (-2 [α _w H + α _g (H _m -H) + α _m H _m ]). (1)
The intensities of the direct J ₀ and reflected J zero-order elastic waves in the metal waveguide are associated, respectively, with the stresses U ₀ and U of the direct and reflected zero-order elastic waves on the read element by the quadratic dependence
U ₀ ² = kJ ₀ and U ² = kJ. (2)
In view of (2), equation (1) takes the form
U = U ₀ exp (- [α _w H + α _g (H _m -H) + α _m H _m ]), (3)
where α _m is the absorption coefficient of elastic waves of the material of the waveguide;
α _W and α _g - damping coefficients of longitudinal elastic waves of zero order, caused by the emission of elastic waves of zero order by the surface of the waveguide, respectively, in liquid and gas environments;
H _m and H - the maximum and current levels of the liquid medium;
k is the coefficient of proportionality.

On the first and second memory elements with capacitors C, voltages U ₀ and U are respectively stored.

When connected to the capacitor C of the first memory element of the resistor R, the voltage at the last will change over time t according to the law
U * = U ₀ exp (-t / RC). (4)
After a period of time Δt, the voltage U * on the resistor R is equal to the voltage U on the capacitor C of the second memory element (3)
U * = U. (5)
Then, taking into account (3) and (4), the output informative time interval Δt is related to the level H by a linear dependence
Δt = kH + b, (6)
where k = (α _W -α _g ) RC is the sensitivity of the method; (7)
b = (α _g + α _m ) H _m RC. (8)
Under normal operating conditions
α _w = const,
α _g = const, (9)
α _m = const
and, therefore, the sensitivity of the method for measuring the level k over the conversion range is constant, which ensures the achievement of the technical result.

When an elastic wave of zero order is excited due to the nonzero duration of the excitation pulse and transients in the input circuit of the read channel of a direct elastic wave of zero order, a transient occurs with a duration of t _ms (dead zone). Reading the signal at this time is not possible. Spatial separation of the excitation and read transducers by a distance exceeding the path traveled by a zero-order elastic wave in time t _mz with a speed c _min
d = t _ms * c _min , (10)
allows, firstly, to exclude the influence of the transition process on the operation of the circuit and, secondly, to form identical reading channels for direct and reflected elastic waves, which additionally ensures the invariance of the sensitivity of the measurement method.

 In FIG. Figures 1 and 2 show a block diagram and a time diagram of an acousto-impedance level gauge.

The acoustic impedance level gauge contains a metal waveguide 1 of length H _m , which is in contact with a liquid medium located in the tank 2, the level N of which is measured by the transducers of excitation 3 and read 4 longitudinal elastic waves of zero order, the excitation generator 6, connected to the excitation transducer 3, the first and the second memory elements 9 and 10 with capacitors C, the input connected to the read converter 4 through the keys 7 and 8, respectively, the resistor R connected to the first input of the comparator 12 and through the key 11 to With ndensatoru first memory element 9. The end of the waveguide from the excitation generator is placed in the damper 5, and the other end is left free.

 The output of the second memory element 10 with capacitor C is connected to the second input of the comparator 12, the output of which is connected to the second input of the control unit 14 and to the first input of the shaper time intervals 13, the second input of which is connected to the fourth output of the control unit 14.

 The synchronization of the generator of the keys 7, 8, 11 and the block for the formation of time intervals 13 in accordance with the algorithm of operation is carried out from the control unit 14, respectively, from the first, second, third and fourth outputs; the inclusion of the level meter is carried out when the signal "start" to the first input of the control unit 14.

 The distance between the field transducers 3 and the read 4 is d.

 Acousto-impedance level gauge works as follows.

In the initial state, the keys 7, 8 and 11 are open. On the command "start", received at the first input of the control unit 14 at time t ₁ (figure 2) at the output 1 of the control unit 14, there is a voltage pulse U _14.1 , which starts the excitation generator 6; at the output of the latter, an excitation pulse of U _{6 of} duration τ is formed.
After a period of time
(t ₂ - t ₁ ) = d / c _min ≥ t _ms (11)
at the second output of the control unit 14 there is a voltage pulse U _14.2 duration
t ₂ * - t ₂ ≥ d (c _max - c _min ) / (c _max • c _min ) = τ, (12)
under the action of which the key 7 is activated and the input of the first memory element 9 with a capacitor C is connected to the read converter 4.

In equations (11) and (12)
c _max and c _min are the maximum and minimum possible velocities of longitudinal elastic waves of zero order in the waveguide. (The velocity of longitudinal elastic waves in a waveguide depends, for example, on a change in the temperature of the controlled medium).

After a while (t ₂ * -t ₂ ), key 7 will open. During this time, the capacitor C of the first memory element 9 is charged to a voltage U ₀ corresponding to the intensity J _{0 of the} zero-order direct elastic wave.

Through time
(t ₃ - t ₁ ) = 2H _m / s _min (13)
from the moment of starting the excitation generator 6 at the output 3 of the control unit 14 there is a voltage pulse U _14.3 duration
t ₃ * - t ₃ = 2H _m (c _max - c _min ) / (c _max • c _min ) + τ, (14)
under the action of which the key 8 is activated and connects the input of the second memory element 10 with capacitor C to the read converter 4.

After time (t ₃ * -t ₃ ) key 8 returns to its original state. During this time, the capacitor C of the second memory element 10 is charged to a voltage U corresponding to the intensity J of the reflected elastic wave.

After a time (t ₄ -t ₁ ), a voltage pulse U _14.4 arises at the output 4 of the control unit 14, under the action of which the key 11 is activated and connects the resistor R to the capacitor C of the first memory element 9, which is discharged according to equation (4). Simultaneously with this pulse from the input 4 of the control unit 14 starts the shaper time intervals 13.

When the voltage U * on the resistor R is equal to the voltage U, at the output of the second memory element 10 with capacitor C, the comparator 12 is activated and an pulse of duration (6) will be generated at the output of the time interval shaper 13
t ₅ - t ₄ = Δt = kH + b.

 At the same time, a signal is returned from the output of the comparator 12 to the first input of the control unit 14, which returns the circuit to its original state.

After a time interval
T ≥ H _m / c _min + t _ms + Δt _max (15)
the level gauge cycle is repeated.

Here
Δt _max = kH _m + b (16)
- the period of time during which the voltage is discharged on the capacitor C of the first memory element 9 at a maximum H _m liquid level.

Sources of information
1. Babikov O.I. Level control using ultrasound. M .: Energy, 1970, 80 p. (Automation book. Issue. 459, pp. 18-20).

 2. Melnikov V.I., Usynin G.B. Acoustic methods for the diagnosis of two-phase coolants for nuclear power plants. - M.: Energoatomizdat, 1987, 168 p.

Claims (2)

1. An acoustic-impedance method for measuring the level of liquid media, including placing a metal waveguide in a liquid along the height of the tank, exciting a zero-order longitudinal elastic wave at one end of the metal waveguide, reading direct and reflected zero-order elastic waves from the other end of the waveguide, determining intensity levels direct and reflected elastic waves of zero order, characterized in that they are sequentially stored on the first and second memory element with voltage capacitors C direct and reflected signals corresponding to the intensities of direct and reflected elastic waves of zero order, connect a resistor to the first memory element with a capacitor C, compare the voltage of the direct and reflected signals and measure the time interval from the moment the resistor is connected to the first memory element with capacitor C until the voltage is equal on the resistor R and the voltage across the capacitor C of the second memory element. 2. The acoustic impedance method for measuring the level of liquid media according to claim 1, characterized in that the excitation and reading of elastic waves of zero order is carried out at points of a metal waveguide located at a distance greater than the "dead zone" of the reading channel of a direct elastic wave of zero order.

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