RU2558630C1 - Method to measure level of substance in tank - Google Patents

Abstract

FIELD: instrumentation.

SUBSTANCE: method to measure level of substance in tank under which in tank the cavity resonator is installed, substance level in it is equal to its level in tank, electromagnetic oscillations are excited in the cavity resonator, and their resonant frequency is measured, in the resonator cavity the substance is located with even one frequency-dependent electrophysical parameter, the which frequency range of change is selected within range of measurements of the resonance frequency of the resonator, that occurred during filling of the resonator cavity with the checked substance. As substance with even one frequency-dependent electrophysical parameter water is used, it is filled in tight flask located in the cavity of the resonator at its top end, and as water electrophysical parameter its dielectric permeability is used.

EFFECT: improved sensitivity and accuracy of measurements.

2 cl, 5 dwg

Description

The invention relates to measuring equipment and can be used for high-precision measurement of the level of a substance (liquid, granular substance) located in any container. In particular, it can be used to measure the level of oil products, liquefied gases, etc.

Known methods for measuring the level of liquids in various containers, which determine the liquid level in the tank using sensors in the form of segments of transmission lines of electromagnetic waves - segments of long lines, hollow waveguides, waveguide resonators located in containers with controlled liquids (V.A., Lunkin B.V., Sovlukov A.S. High-frequency method for measuring non-electric quantities (Moscow: Nauka, 1980, 280 p.). When measuring the level of dielectric liquids, the range of variation of the informative parameter, in particular, the resonant frequency of the electromagnetic oscillations of the resonator in the form of a long line segment or a segment of a hollow waveguide (waveguide resonator), turns out to be small, which makes it difficult to carry out measurements with the necessary high values of the sensitivity of level sensors and the accuracy of level measurements. This is typical for level measurements of liquids with a low dielectric constant, in particular, for cryogenic liquids (liquid oxygen, hydrogen, helium, etc.).

A technical solution is also known (Viktorov V.A., Lunkin B.V., Sovlukov A.S. Radio wave measurements of technological process parameters. M: Energoatomizdat. 1989. 208 pp. 86-90), which is the most technical in nature. close to the proposed method and adopted as a prototype. This prototype method consists in exciting electromagnetic waves in a metal hollow waveguide resonator placed vertically in a container with a controlled dielectric fluid. The liquid level in the tank corresponds to its level in a partially-filled waveguide resonator. By measuring the resonant (natural) frequency of the electromagnetic oscillations of the resonator, it is possible to determine the level of the dielectric fluid filling the cavity of this resonator. However, for liquids with a small dielectric constant (less than 2), the range of variation of the resonant frequency and, accordingly, the sensitivity of the level gauge with a sensitive element in the form of such a waveguide resonator is small, which makes it difficult to measure the level with high accuracy.

The technical result of the present invention is to increase the sensitivity and, as a consequence, the accuracy of measuring the level of a substance.

The technical result is achieved by the fact that in the proposed method for measuring the level of a substance in a tank, in which a volume resonator is placed in the tank, the level of the substance in which is equal to its level in the tank, electromagnetic waves are excited in the volume resonator and their resonance frequency is measured, and the substance is placed in the cavity with at least one frequency-dependent electrophysical parameter, the frequency range of which is chosen within the variation of the resonant frequency of the resonator, which occurs when ying cavity resonator controlled substance. As a substance with at least one frequency-dependent electrophysical parameter, water is used, enclosed in a sealed cuvette placed in the cavity of the resonator at its upper end, and its dielectric constant is used as the electrophysical parameter of water.

The method is illustrated by drawings.

In FIG. 1 - cavity cavity with a controlled substance and a substance with at least one frequency-dependent electrophysical parameter.

In FIG. 2 is a diagram of a measuring device for implementing a measurement method.

In FIG. 3 is a graph of the dependence of the dielectric constant of water on frequency in a wide range of its changes.

In FIG. 4 and FIG. 5 - graphs of the dependence of the resonant (natural) frequency f of the electromagnetic oscillations of the waveguide resonator on level x, respectively, by dielectric and conductive substances.

Shown here are cavity resonator 1, controlled substance 2, substance 3 with at least one frequency-dependent electrophysical parameter, coupling element 4, electromagnetic oscillation generator 5, coupling element 6, recorder 7.

The method is implemented as follows.

In the device (Fig. 1) to implement this measurement method in a volume resonator 1 with a controlled substance 2, the level x of which is to be measured, electromagnetic waves are excited on one of the selected, in particular the main (lowest) type of electromagnetic waves and their resonant frequency is measured f. Methods of excitation of various types of electromagnetic oscillations in resonators, their isolation and measurement of characteristics are known (Lebedev I.V. Technique and microwave devices. T. 1. M.: Higher school. 1970. 440 pp. 337-369).

According to this method, a substance 3 is placed in the cavity of a volume resonator, in particular a waveguide resonator, with at least one electrophysical parameter depending on the frequency f (i.e., having frequency dispersion) —the dielectric constant ε (f) or (i) the dielectric loss tangent tanδ (f) (electrical conductivity σ (f)) —the range of variation of which is chosen within the variation of the resonant frequency of the resonator, which occurs when the cavity is filled with a controlled substance.

In FIG. 2 is a diagram of a measuring device for implementing this measurement method, where a waveguide resonator placed vertically in a container with a controlled substance 2 is used as a volume resonator 1. In this case, the level of the substance in the tank corresponds to its value in the waveguide resonator.

The excitation of electromagnetic waves is carried out using the coupling element 4 from the generator of electromagnetic waves 5. The reception of electromagnetic waves is carried out using the coupling element 6 connected via a communication line to the recorder 7, which serves to determine the resonant frequency of the volume resonator 1 and, therefore, the level of the substance 2 in capacities.

As substance 3 with at least one electrophysical parameter depending on the frequency f, it is possible to use, in particular, water enclosed in a sealed cuvette placed inside a volume resonator, for example, at its upper end (Fig. 2), and as an electrophysical parameter of water - its dielectric constant ε _in (f) or dielectric loss tangent tanδ _in (f). Figure 3 shows a graph of the dependence of ε _in (f) in a wide frequency range, including frequencies (10-30 GHz) of the microwave range, where there is a pronounced dependence of ε _in on frequency (Benzar V.K. Technique of microwave moisture measurement. Minsk: Higher School. 1974. 349 p.).

This leads, as a result, to an increase in the range of variation of the resonant (natural) frequency f of the resonator when the level x changes within the same range, in particular, from its zero value (no liquid) to the maximum value l (full filling) in the cavity of the resonator ( and a container containing the substance). This is due to the redistribution of the energy of the electromagnetic field of the standing wave in the resonator volume with a change in the level of a substance in its cavity and in the presence of a frequency-dependent substance in this electromagnetic field.

Choosing the design parameters of the resonator so that its initial natural frequency f _{0 of} electromagnetic oscillations is in the microwave frequency range, for example, in the range of 10-30 GHz, i.e. in the region of the presence of frequency dispersion in water (Fig. 3), it is possible to control the sensitivity S _x = df / dx of such a resonant level sensor x of the substance.

Consider, for example, a change in f both as a function of the measured level x and of the dielectric constant ε _in (f) water (in this case, the presence of the dependence of tanδ _in (f) on water leads to a certain decrease in the quality factor of the volume resonator, without significantly interfering with the possibility of measuring its resonance frequency f). There are two mechanisms for changing the resonant frequency: 1) due to the presence of a controlled substance in the cavity of the resonator; 2) due to the presence of a substance with a frequency dispersion of the dielectric constant of water, which also changes the value of the resonant frequency f with a change in the level x. Moreover, as shown by the consideration of the action of these mechanisms, they affect f (x) in one direction: when the level x of both the dielectric substance (Fig. 4) and the conductive substance (Fig. 5) changes, the corresponding change in the resonance frequency f (x ) increases. Due to this, the dependence f (x) when filling this resonator with a dielectric substance is characterized by a higher sensitivity S _x = df / dx (see Fig. 4, curve 2) than that which takes place in the absence of a cell with water in the cavity of the resonator (Fig. . 4, curve 1). An increase in sensitivity S _x occurs when the cavity is filled with an electrically conductive substance (Fig. 5, curve 2) compared to its value in the case of a sensor in the form of a hollow resonator (Fig. 5, curve 1).

Let us analytically determine the sensitivity of a resonant level sensor containing a substance with a frequency dispersion ε _in (water) in the cavity of the cavity, using the example of filling a cavity of a volume waveguide resonator placed vertically in a container with a dielectric fluid. Moreover: V ₀ = Al, V = Ax, where V and V ₀ are the volume, respectively, of the entire cavity of the resonator and its part, filled to the level x; l is the maximum value of level x, corresponding to the full filling of the volume V ₀ ; A is the cross-sectional area of the cavity of the waveguide resonator. In this case, we have: sensitivity S _v = df / dV = (1 / A) · (df / dx) = (1 / A) · S _x .

Since when filling the volume resonator with a dielectric substance with ε = ε (V), the relation is valid (Nikolsky V.V. Variational methods for internal problems of electrodynamics. M: Nauka. 1967. 460 p.)

where f ₀ is the value of f for V = 0, then in this case we will have

where ε _in , V _in - respectively, the dielectric constant of water and the volume occupied by it.

отсюда следуетIn the zeroth approximation of perturbation theory $$(\overrightarrow{E}={\overrightarrow{E}}_0)$$

this implies

; $$\varphi (V_{в})={\displaystyle \underset{V_{в}}{\int }{E}_0^{2}dv}/{\displaystyle \underset{V_0}{\int }{E}_0^{2}dv}$$

where indicated: $$\varphi (V)={\displaystyle \underset{V}{\int }{E}_0^{2}dv}/{\displaystyle \underset{V_0}{\int }{E}_0^{2}dv}$$

; $$\varphi (V_{\mathrm{at}})={\displaystyle \underset{V_{\mathrm{at}}}{\int }{E}_0^{2}dv}/{\displaystyle \underset{V_0}{\int }{E}_0^{2}dv}$$

Taking into account (3), we find and from here the sensitivity S of the sensor as a result of the following transformations:

In the absence of a dispersion element (V _in = 0), the sensitivity S _{v0 of the} sensor is

Then, taking into account (4) and (5), we obtain after the transformations:

Since we can assume φ (V _in ) << 1, then

, то S_v>S_v0, причемThis shows that, since $$\frac{\partial {\epsilon }_{\mathrm{at}}}{\partial f}<0$$

, then S _v > S _v0 , and

At V≈V ₀ we get

With a uniform distribution of field energy along the waveguide (E ₀ = const, φ (V) = V / V ₀ , φ (V _in ) = V _in / V ₀ and, therefore,

where l is the length of the resonator, x _in is the height of the water layer in the cell having the same cross section as the resonator;

at V≈V ₀

This shows that the increase in sensitivity increases sharply with a decrease in ε, that is, it is advisable to use this path for ε <2 (oil products, cryogenic liquids, etc.).

имеет для воды определенное значение, которое легко находится при рассмотрении графика зависимости ε_в(f) на фиг. 3.Value $$\frac{\partial {\epsilon }_{\mathrm{at}}}{\partial f}$$

has a certain value for water, which is easily found when considering the graph of ε _in (f) in FIG. 3.

Thus, due to the placement of a substance with at least one frequency-dependent electrophysical parameter in the cavity of the volume resonator and the associated redistribution of the energy of the electromagnetic field of the standing wave in the resonator volume, an increase in the range of variation of the resonance frequency in the same range of change in the liquid level is provided, increasing the sensitivity and as a consequence of this, improving the accuracy of its measurement.

Claims (2)

1. A method of measuring the level of a substance in a tank at which a cavity resonator is placed in the tank, the level of the substance at which is equal to its level in the tank, electromagnetic oscillations are excited in the cavity resonator and their resonance frequency is measured, characterized in that the substance is placed in the cavity with would be one frequency-dependent electrophysical parameter, the frequency range of which is chosen within the variation of the resonant frequency of the resonator, which occurs when filling the cavity of the cavity uemym substance. 2. The method according to p. 1, characterized in that as a substance with at least one frequency-dependent electrophysical parameter, water is used, enclosed in a sealed cuvette placed in the cavity of the resonator at its upper end, and its dielectric as the electrophysical parameter of water permeability.

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