RU2659569C1 - Dielectric fluid moisture content measurement method - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention relates to the field of electrical engineering and can be used in the moisture content high-precision determination of various dielectric liquids located in tanks (process vessels, measuring cells, etc.) or transferred through the pipelines. In the dielectric liquid moisture content measurement method, in the first measurements step, in the waveguide with controlled liquid at one of its ends exciting the first fixed frequency electromagnetic waves, at that, the of the excited electromagnetic waves frequency is selected below the waveguide critical frequency, receiving the electromagnetic waves after their propagation along the waveguide at its other end and measuring the electric field strength amplitude; in addition, in the second measurement cycle, exciting the second fixed frequency electromagnetic waves in the waveguide, at that, the excited electromagnetic waves frequency is selecting below the waveguide critical frequency, receiving the electromagnetic waves after their propagation along the waveguide at its other end and measuring the electric field strength amplitude, at that, at least one of the frequencies is selected in the region of the water dielectric constant frequency dispersion presence and at this frequency performing the joint functional conversion of amplitudes measured in the first and second measurement steps, by which result judging on the liquid moisture content.

EFFECT: expansion of the measurement method functionality due to increase in the measurement accuracy.

1 cl, 1 dwg

Description

The invention relates to measuring technique and can be used for high-precision determination of the moisture content of various dielectric fluids in containers (process tanks, measuring cells, etc.) or transported through pipelines.

There are various methods and devices for measuring the moisture content of liquids based on the measurement of electrophysical parameters (dielectric constant or (and) dielectric loss tangent) of liquids using radio wave RF and microwave resonators containing a controlled liquid (monographs: Brandt A.A. Investigation of dielectrics on superhigh frequencies.M.: Fizmatgiz. 1963. 403 pp. 37-144; Viktorov V.A., Lunkin B.V., Sovlukov A.S. Radio wave measurements of process parameters.M .: Nauka. 1989. 208 S.P. 168-177). The disadvantage of such methods and measuring devices that implement these methods is their limited scope, due to the inability to control small changes in the moisture content of liquids due to the low accuracy of measurement of the corresponding small changes in informative parameters (resonant frequency, quality factor of the resonator, etc.). To ensure the possibility of such measurements, two-channel measuring circuits with independent measuring and reference channels are used. In the reference channel, the sensing element contains a liquid with known physical properties (monograph: A. Brandt. Investigation of dielectrics at microwave frequencies. M: Fizmatgiz. 1963. 403 pp. 258-268).

Also known is a technical solution (RU No. 2626409 C1, 07.27.2017), which contains a description of the method, the technical essence of which is closest to the proposed method and adopted as a prototype. According to this prototype method, the moisture content of a liquid is measured when electromagnetic waves of a fixed frequency are excited in a waveguide with a controlled liquid at one of its ends, while the frequency of the excited electromagnetic waves is chosen below the critical frequency of the waveguide, electromagnetic waves are received after their propagation along the waveguide at its other end and measure the amplitude of the electric field. The disadvantage of this method of measurement is the limited functionality, due to the low accuracy of the measurements when the dielectric constant of the moisture-containing liquid changes, which occurs, for example, when changing the grade of liquid fuel (oil product).

The technical result of the present invention is to expand the functionality of the measurement method by increasing the accuracy of the measurement.

The technical result is achieved by the fact that in the proposed method for measuring the moisture content of a dielectric fluid, in which in the first measurement step, electromagnetic waves of a first fixed frequency are excited in a waveguide with a controlled liquid at one of its ends, while the frequency of the excited electromagnetic waves is chosen below the critical frequency of the waveguide, take electromagnetic waves after their propagation along the waveguide at its other end and measure the amplitude of the electric field, in addition Finally, in the second measurement step, electromagnetic waves of a second fixed frequency are excited in the waveguide, while the frequency of the excited electromagnetic waves is chosen below the critical frequency of the waveguide, electromagnetic waves are received after their propagation along the waveguide at its other end, and the amplitude of the electric field is measured, at least one of the frequencies selected in the region of the presence of a frequency dispersion of the dielectric constant of water at this frequency, a joint functional transformation is performed the amplitudes measured in the first and second measurement steps, which are used to judge the moisture content of the liquid.

The proposed method is illustrated in the drawing. In FIG. 1 is a diagram of a device explaining the principle of measurement using the method.

Shown here are waveguide 1, generators 2 and 3, switch 4, communication elements 5 and 6, detector 7, functional converter 8, recorder 9.

The method is implemented as follows.

This method implements a structural approach to achieving the invariance of the dielectric constant ε _{n of the} controlled fluid, in particular, its grade, the changes of which take place, in particular, when controlling oil and oil products in any tank or during their transportation through the pipeline. This approach is associated with the organization of two measuring channels (two consecutive measurement clocks at two different fixed frequencies ƒ ₁ and ƒ ₂ ) and the joint functional transformation of their output quantities in order to obtain the result of this transformation, which does not depend on the disturbing factor, in this case, on the dielectric constant ε _{n of the} controlled fluid and changes ε _n .

The proposed method consists in the excitation of electromagnetic waves in a waveguide used as a measuring cell (for measurements under stationary conditions) or as a segment of a measuring waveguide integrated in a pipeline with pumped liquid (for measurements in pipelines). Electromagnetic waves in a waveguide are excited sequentially at two different fixed frequencies ƒ ₁ and ƒ ₂ , each of which is below the critical frequency ƒ _kr for a lower type wave. Moreover, at each of these frequencies along the waveguide there is only a weakening reactive field, decreasing with distance from the exciting element at one of the ends of the capacitance. After their propagation along the given waveguide at its other end, electromagnetic waves are received and the amplitude of the electric field strength is measured at each of the frequencies ƒ ₁ and поля ₂ . By the result of the joint conversion of the measured amplitudes, the moisture content of the liquid is judged.

The condition for the propagation of electromagnetic waves along any waveguide is the following inequality: ƒ> ƒ _cr , which must be satisfied by the operating frequency ƒ and the critical frequency ƒ _cr for a lower type wave, for example, for wave H ₁₁ in a circular waveguide. For ƒ <ƒ _{cr, there} is a regime in which the propagation of waves along the waveguide does not occur, and there is only a weakening reactive field, decreasing with distance from the exciting element (Lebedev IV Technique and microwave devices. T.1. M .: Higher School. 1970.440 p. S. 132-136).

In this case, the electric field (like the magnetic field) changes along the z coordinate (axis of the waveguide) according to the law:

and the attenuation constant α is

In these formulas, E ₀ is the amplitude of the electric field at z = 0; ε is the dielectric constant of the dielectric fluid in the waveguide, c is the speed of light. Choosing the ratio between ƒ and ƒ _cr , we can control the amount of attenuation α.

If the generator frequency ƒ is less than the critical frequency ƒ _{cr of a} given waveguide, then the amplitude of the electric field strength E, which is an informative parameter, is

, Е₀ - амплитуда напряженности поля в области возбуждения электромагнитных волн в рассматриваемом волноводе (т.е. в области расположения связи 3), λ_кр - критическая длина волны для данного волновода,

- длина измерительного участка, т.е. расстояние вдоль волновода между элементами возбуждения и съема электромагнитных колебаний. Напряженность электрического поля Е при удалении от элемента связи, служащего для возбуждения и приема электромагнитных колебаний, спадает в соответствии с соотношением (3). При этом значение Е зависит от влагосодержания контролируемой жидкости в волноводе.Where

, E ₀ is the amplitude of the field strength in the region of excitation of electromagnetic waves in the waveguide in question (i.e., in the region of the coupling 3), λ _cr is the critical wavelength for this waveguide,

is the length of the measuring section, i.e. the distance along the waveguide between the elements of excitation and removal of electromagnetic waves. The electric field strength E decreases with distance from the communication element, which serves to excite and receive electromagnetic waves, in accordance with relation (3). The value of E depends on the moisture content of the controlled fluid in the waveguide.

, где d - внутренний диаметр волновода (Лебедев И.В. Техника и приборы СВЧ. Т.1. М.: Высшая школа. 1970. 440 с. С. 132-136). Например, при d=30 мм для волн типа Н₁₁ будем иметь λ_кр=3,41d/2=5,115 см; тогда ƒ_кр=5,865/√ε ГГц. Если, например, ε=2,0, то должно быть ƒ<ƒ_кр=5,865/√ε ГГц=4,148 ГГц.For waves of the lower type H ₁₁ we have λ _cr = 3,41d / 2 and, accordingly,

, where d is the internal diameter of the waveguide (Lebedev I.V. Technique and microwave devices. T.1. M: Higher school. 1970. 440 p. S. 132-136). For example, when d = 30 mm for waves of type H ₁₁ we will have λ _cr = 3.41d / 2 = 5.115 cm; then ƒ _cr = 5.865 / √ε GHz. If, for example, ε = 2.0, then there should be ƒ <ƒ _cr = 5.865 / √ε GHz = 4.148 GHz.

измерительного участка, частота ƒ генератора выбираются с учетом диаметра волновода, электрофизических параметров контролируемой жидкости и диапазона их изменения.Length

of the measuring section, the frequency ƒ of the generator is selected taking into account the diameter of the waveguide, the electrophysical parameters of the controlled fluid and the range of their change.

According to this method, the determination of moisture content is carried out during two consecutive measurement cycles at fixed frequencies ƒ ₁ and ƒ ₂ , each of which is less than the critical frequency ƒ _{cr of the} waveguide.

In the first measurement step, when electromagnetic waves are excited in one of the ends of the waveguide at a fixed frequency ƒ ₁ , less _{than the} critical frequency ода _{cr of the} waveguide, electromagnetic waves are received after their propagation along the waveguide at its other end and the electric field amplitude E _{1 is} measured:

, E₁₀ - амплитуда напряженности поля в области возбуждения электромагнитных волн в рассматриваемом волноводе,

- длина измерительного участка, т.е. расстояние вдоль волновода между элементами возбуждения и съема электромагнитных колебаний, λ_кр - критическая длина волны в волноводе, ε(ƒ₁) - диэлектрическая проницаемость жидкости на частоте ƒ₁.Where

, E ₁₀ - the amplitude of the field strength in the field of excitation of electromagnetic waves in the waveguide under consideration,

is the length of the measuring section, i.e. the distance along the waveguide between the elements of excitation and removal of electromagnetic waves, λ _cr is the critical wavelength in the waveguide, ε (ƒ ₁ ) is the dielectric constant of the liquid at a frequency of ƒ ₁ .

In the second measurement step, when electromagnetic waves are excited at one of its ends at a fixed frequency ƒ ₂ , less than the same critical frequency ƒ _{cr of the} waveguide (or another critical frequency for some other working type of waves, if it is chosen for measurements) receive electromagnetic waves after their propagation along the waveguide at its other end and measure the amplitude E ₂ of the electric field:

, Е₂₀ - амплитуда напряженности поля в области возбуждения электромагнитных волн в рассматриваемом волноводе,

- длина измерительного участка, т.е. расстояние вдоль волновода между элементами возбуждения и съема электромагнитных колебаний, λ_кр - критическая длина волны в волноводе, ε(ƒ₂) - диэлектрическая проницаемость жидкости на частоте ƒ₂.Where

, E ₂₀ - the amplitude of the field strength in the field of excitation of electromagnetic waves in the waveguide under consideration,

is the length of the measuring section, i.e. the distance along the waveguide between the elements of excitation and removal of electromagnetic waves, λ _kr is the critical wavelength in the waveguide, ε (ƒ ₂ ) is the dielectric constant of the liquid at a frequency of ƒ ₂ .

The dielectric constant ε of a moisture-containing dielectric having a dielectric constant ε _n and a moisture content of W is described for small W by the Wiener formula (Theory and practice of express control of humidity of solid and liquid materials / Krichevsky ES, Benzar VK, Venediktov M.V. Under the general editorship of Krichevsky ES M .: Energy. 1980. 240 p. S. 55-66):

, ε_в(ƒ) - диэлектрическую проницаемость воды, являющаяся функцией частоты ƒ в СВЧ-диапазоне частот.Where

, ε _in (ƒ) is the dielectric constant of water, which is a function of frequency ƒ in the microwave frequency range.

For example, if measurements are taken at a frequency of ƒ ₁ = 10 GHz, then D (ƒ ₁ ) = 1.095, and at a frequency of ƒ ₂ = 37.5 GHz, then D (ƒ ₂ ) = 1.383, if we take ε _n = 2.

The expressions for D (ƒ ₁ ), D (ƒ ₂ ) in formulas (4) and (5) can be simplified if we put D (ƒ ₁ ) -W≈D (ƒ ₁ ), D (ƒ ₂ ) -W≈D (ƒ ₂ ) and do not depend on ε _n , which is permissible for small values of moisture content (up to ~ 5%) and real limits of variation of ε _n .

The constancy of the quantities D (ƒ ₁ ) and D (ƒ ₂ ) for the corresponding frequencies ƒ ₁ and ƒ ₂ follows from the constancy of the quantities ε _н and ε _в , which are included in the formula for D (ƒ ₁ ) and D (ƒ ₂ ). The value of ε _{n is} constant over a wide range of changes in the frequency ƒ, the value of ε _is constant in the non-dispersive section of the ε curve _in (ƒ) and is assumed to be constant in the dispersion section of this curve. This is true when making changes using measuring devices operating at fixed frequencies.

We now show by a real example that D (ƒ ₁ ) -W and D (ƒ ₂ ) -W, or rather, the quantities 3 / (D (ƒ ₁ ) -W) and 3 / (D (ƒ ₂ ) -W ), entering the general form (for an arbitrary value of the frequency ƒ) into formula (1), do not depend (with some permissible error) on ε _n . So, with a real change in ε _in by 10% compared with the initial value of ε _n = 2, i.e. to a value of 2.2, for frequencies ƒ ₁ = 10 GHz and ƒ ₂ = 37.5 GHz we will have:

when ε _n = 2: D (ƒ ₁ ) = 1.095, D (ƒ ₂ ) = 1.383;

for ε _n = 2.2: D (ƒ ₁ ) = 1.105, D (ƒ ₂ ) = 1.337.

It follows that the relative change in D (ƒ ₁ ) is ~ 0.9%, and the relative change in D (ƒ ₂ ) is ~ 2.6%.

Let us evaluate how these changes D (ƒ ₁ ) and D (ƒ ₂ ) affect the coefficients of W in formula (1) for ƒ = ƒ ₁ and ƒ = ƒ ₂ , namely, the coefficient k ₁ = 3 / (D (ƒ ₁ ) -W) and the coefficient k ₂ = 3 / (D (ƒ ₂ ) -W):

when ε _n = 2 we have: k ₁ ≈2.752, k ₂ ≈2.31;

when ε _n = 2.2 we have: k ₁ ≈2.727, k ₂ ≈2.33.

- номинальное значение диэлектрической проницаемости обезвоженной диэлектрической жидкости.It follows that the relative change in both k ₁ and k ₂ is ~ 0.9%, which is ~ 100 times less than the relative change in ε _n , i.e. the real change in ε _n does not practically affect k ₁ and k ₂ . In the formula for D (ƒ ₁₎ and D (ƒ ₂₎ can be used to express the value of ε _n

- the nominal value of the dielectric constant of the dehydrated dielectric fluid.

Note that for the implementation of this method of measurement, it is sufficient if the considered frequency dispersion has water at only one of two operating frequencies (ƒ ₁ or ƒ ₂ ).

When making measurements in the first cycle at a frequency of ƒ _1, we will have:

, ε_в(ƒ₁) - диэлектрическая проницаемость воды, являющаяся функцией частоты ƒ₁ в СВЧ-диапазоне частот,

- номинальное значение диэлектрической проницаемости обезвоженной диэлектрической жидкости.Where

, ε _in (ƒ ₁ ) is the dielectric constant of water, which is a function of frequency ƒ ₁ in the microwave frequency range,

- the nominal value of the dielectric constant of the dehydrated dielectric fluid.

When taking measurements in the second cycle at a frequency of ƒ _2, we will have:

, ε_в(ƒ₂) - диэлектрическая проницаемость воды, являющаяся функцией частоты ƒ₂ в СВЧ-диапазоне частот,

- номинальное значение диэлектрической проницаемости обезвоженной диэлектрической жидкости.Where

, ε _in (ƒ ₂ ) is the dielectric constant of water, which is a function of the frequency ƒ ₂ in the microwave frequency range,

- the nominal value of the dielectric constant of the dehydrated dielectric fluid.

For sufficiently large values of W, other well-known expressions for ε should be used (Theory and practice of express control of humidity of solid and liquid materials / Krichevsky E.S., Benzar V.K., Venediktov M.V. Ed. M.: Energy. 1980.240 s.).

When considering (7) and (8) as a system of equations and its solution with respect to the desired moisture content W, we obtain

Taking into account formulas (4) and (5), expression (9) takes the following form:

We write formula (10) as follows:

where k ₁ , k ₂ , k ₃ , k ₄ , k ₅ , k ₆ are constant values, and

;

;

;

;

;

.

;

;

;

;

;

.

Thus, carrying out a joint conversion of the measured values of the amplitudes E ₁ and E ₂ according to relation (11), we obtain the moisture content value W, which is independent of the dielectric constant ε _{n of the} controlled fluid and its possible changes.

In FIG. 1 shows a diagram of a device for implementing this method.

вдоль волновода 1. Принимаемые элементом связи 6 волны поступают на детектор 7 и затем продетектированные сигналы подаются в функциональный преобразователь 8, подсоединенный выходом к регистратору 9 для определения искомого влагосодержания, которое не зависит от диэлектрической проницаемости ε_н контролируемой жидкости.The excitation of electromagnetic waves in waveguide 1 is carried out sequentially, in the first and second measurement steps, at frequencies ƒ ₁ and ƒ ₂ lower than the critical frequency ƒ _cr for this waveguide using, respectively, generator 2 and generator 3 through switch 4 and communication element 5. Another element of communication (reception) of 6 electromagnetic waves is located at a distance

along the waveguide 1. The waves received by the coupling element 6 are fed to the detector 7 and then the detected signals are fed to a functional converter 8, connected by an output to the recorder 9 to determine the desired moisture content, which is independent of the dielectric constant ε _{n of the} controlled fluid.

For waveguides of specific sizes, the choice of frequencies ƒ ₁ and ƒ _{2 of} generators 2 and 3 can optimize the sensitivity of such a device for measuring the moisture content of a liquid in the working range of its change. In this case, there is a monotonic dependence of the informative parameters — the amplitudes E ₁ and E ₁ of the electric field — on the values of ε ₁ and ε ₂ , respectively, functionally related to the measured moisture content of the liquid.

Thus, this method allows quite simply and with high accuracy to measure the moisture content of various dielectric fluids with high accuracy, regardless of the dielectric constant of the controlled fluid. The proposed method can be implemented both when working with samples of a controlled moisture-containing fluid in stationary conditions, and when it moves - when moving the fluid through the pipeline.

Claims (1)

A method for measuring the moisture content of a dielectric fluid, in which, in the first measurement step, electromagnetic waves of a first fixed frequency are excited in a waveguide with a controlled liquid at one of its ends, the frequency of the excited electromagnetic waves being chosen below the critical frequency of the waveguide, electromagnetic waves are received after they are propagated along the waveguide onto its other end and measure the amplitude of the electric field, characterized in that in addition to the second measurement step electromagnetic waves of a second fixed frequency are choked in the waveguide, while the frequency of the excited electromagnetic waves is chosen below the critical frequency of the waveguide, electromagnetic waves are received after their propagation along the waveguide at its other end, and the amplitude of the electric field is measured, while at least one of the frequencies is selected in the region the presence of a frequency dispersion of the dielectric constant of water at this frequency, produce a joint functional transformation of the amplitudes measured in the first and the second measuring cycles, the results of which are judged on the moisture content of the liquid.

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