RU2661349C1 - Dielectric fluid moisture content determination 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 fixed frequency electromagnetic waves in the waveguide, which is selected above the waveguide critical frequency, receiving the electromagnetic waves after their propagation along the waveguide at its other end and measuring the excited and received electromagnetic waves phase shift, at that, at least one of the frequencies is selected in the region of the frequency dispersion presence of the water dielectric constant at this frequency, performing the measured amplitude and phase shift joint functional conversion, by which result judging on the liquid moisture content.

EFFECT: technical result of the invention is extension of the measurement method functionality by 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 determining the moisture content of liquids based on the measurement of electrophysical parameters (dielectric constant or (and) the dielectric loss tangent) of liquids using radio wave RF and microwave waveguides and resonators containing a controlled liquid (monographs: A. Brandt, Research dielectrics at microwave frequencies. - M.: Fizmatgiz. 1963. 403 pp. S. 37-144; Viktorov V.A., Lunkin B.V., Sovlukov A.S. Radio wave measurements of process parameters.- M .: Nauka 1989. 208 pp. 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).

There is also a method of determining the moisture content of a liquid, which consists in measuring the phase shift of the probe waves that interacted with the controlled substance and electromagnetic waves at two fixed frequencies (RU 2614054 C1, 03/22/2017). This method ensures that the moisture content measurement results are independent of the dielectric constant of the controlled dielectric substance. The disadvantage of this method is its limited functionality, characterized by the complexity of implementation and the need to use antennas for sensing and receiving electromagnetic waves in the direction transverse to the walls of the vessel or pipeline through special dielectric "windows" in the walls of the vessel (measuring cell) with the substance (when measured in stationary conditions) or a pipeline with a moving substance (when measuring in pipelines). This may encounter difficulties in implementation, in particular, when measuring in containers (measuring cells) or pipelines of small diameters.

A technical solution is also known (RU 2626409 C1, 07.27.2017), which contains a description of the method, which, by technical essence, is closest to the proposed method and adopted as a prototype. According to this prototype method, the moisture content of a liquid is determined by excitation of electromagnetic waves of a fixed frequency 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, according but the invention additionally in the second measurement step excites in the waveguide electromagnetic waves of a fixed frequency, which is selected above the critical frequency of the waveguide, receives electromagnetic waves after their propagation along the waveguide at its other end and measures the phase shift of the excited and received electromagnetic waves, while at least one of frequencies are selected in the field of the presence of a frequency dispersion of the dielectric constant of water at this frequency; a joint functional transformation is performed amplitude and phase shift, the result of which is 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, first switch 4, communication elements 5 and 6, second switch 7, detector 8, functional converter 9, phase meter 10, recorder 11.

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 excite sequentially at two different fixed frequencies ƒ ₁ and ƒ ₂ , one of which, for definiteness, is ƒ ₁ below the critical frequency ƒ _kr for a lower type wave, and the other frequency, for definiteness, is ƒ ₂ above this critical frequency ƒ _cr . In this case, at a frequency of вдоль ₁ along the waveguide, there is only a weakening reactive field that decreases with distance from the exciting element at one of the ends of the capacitance. At a frequency of ƒ _2, electromagnetic waves propagate in the usual mode on the "working" type of waves, in particular on the lower type H ₁₁ in a circular waveguide (Lebedev I.V. Technique and microwave devices. T. 1. - M .: Higher school. 1970 440 p. S. 80-89).

After their propagation along a given waveguide at its other end, electromagnetic waves are received and measured at a frequency ƒ ₁ , below the critical waveguide frequency, the amplitude E of the electric field strength and at a frequency ƒ ₂ , above the critical waveguide frequency, the phase shift Δϕ of electromagnetic waves excited at one the end of the waveguide and taken at its other end. According to the result of the joint conversion of the measured amplitude E and the phase shift Δϕ judge the moisture content of the liquid. Moreover, at least one of the frequencies ƒ ₁ or ƒ _{2 is} selected in the region of the presence of a frequency dispersion of the dielectric constant of water at this frequency.

The condition for the propagation of electromagnetic waves along any waveguide is the following inequality: ƒ> ƒ _kp , which must be satisfied by the operating frequency ƒ and the critical frequency ƒ _kr for a lower type wave, for example, for wave H ₁₁ in a circular waveguide. When ƒ <ƒ _{kp, there} is a regime in which the propagation of waves along the waveguide does not occur, but there is only a weakening reactive field that decreases with distance from the exciting element (Lebedev I.V. 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, l - 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 under consideration (i.e., in the region of coupling location 3), λ _cr is the critical wavelength for a given waveguide, l 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_кp=5,865/√ε ГГц. Если, например, ε=2,0, то должно быть ƒ<ƒ_кp = 5,865/√ε ГГц = 4,148 ГГц.For waves of the lower type H ₁₁ we have λ _cr = 3.4d / 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, with d = 30 mm, for waves of type H ₁₁ we will have λ _cr = 3.41 d / 2 = 5.115 cm; then ƒ _kp = 5.865 / √ε GHz. If, for example, ε = 2.0, then there should be ƒ <ƒ _кp = 5.865 / √ε GHz = 4.148 GHz.

The length l of the measuring section, the frequency ƒ of the generator are selected taking into account the diameter of the waveguide, the electrophysical parameters of the controlled fluid and the range of their variation.

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.

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 ƒ _{cr of the} waveguide, electromagnetic waves are received after their propagation along the waveguide at its other end and the amplitude E of the electric field is measured:

Where

E ₀ is the amplitude of the field strength in the field of excitation of electromagnetic waves in the waveguide under consideration, l 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 at a fixed frequency ƒ _{2 are} excited in a waveguide at one of its ends, greater than the same critical frequency ƒ _{kp of the} waveguide (or another critical frequency for some other working wave type, if it is chosen for measurements) receive electromagnetic waves after their propagation along the waveguide at its other end and measure the phase shift Δϕ of electromagnetic waves excited at one end of the waveguide and received at its other end:

where 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 V. M .V. Under the general editorship of Krichevsky E.S. 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 (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 ₁ ) -W≈D (ƒ ₂ ), and do not depend on ε _n , then it 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 enter into the formulas 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) , appearing in general form for an arbitrary value of the frequency ƒ in formula (1), do not depend (with some permissible error) on ε _in . 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 k ₁ 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. При проведении измерений во втором такте на частоте ƒ₂ будем иметь: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:

Where

- номинальное значение диэлектрической проницаемости обезвоженной диэлектрической жидкости.ε _in (ƒ ₂ ) is the dielectric constant of water, which is a function of 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 V.V. M. Under the general editorship of Krichevsky E .S. - 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, performing a joint conversion of the measured values of the amplitude E and the phase shift Δϕ 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.

The excitation of electromagnetic waves in waveguide 1 is carried out sequentially, in the first and second measurement steps, at a fixed frequency ƒ ₁ , less _{than the} critical frequency ƒ _cr for this waveguide, and at a fixed frequency ƒ ₂ , greater than the critical frequency ƒ _cr , using, respectively, a generator 2 and the generator 3 through the first switch 4 and the coupling element 5. The other coupling element (receiving) 6 of electromagnetic waves located at a distance l along the waveguide 1. The first measure taken by the measurement wave at the frequency ƒ ₁ fed through a second com utator 7 to the detector 8, and then the detected signals are amplitude E supplied to the first input of the functional converter 9. In the second cycle of measurements taken on the wave frequency ƒ ₂ receives the output from the switch 7 to the first input of the phase meter 10, at the second input of which is supplied with the wave generator 2 determining the phase shift Δϕ. From the output of the phase meter 10, the signals are fed to the second input of the functional transducer 9, in which they jointly convert the values of the amplitude E and the phase shift Δϕ according to the above relation. The output of the functional Converter 9 is connected to the registrar 11 to determine the desired moisture content W, which does not depend on the value of ε _n .

For waveguides of specific sizes, the choice of frequencies ƒ ₁ and ƒ _{2 of} generators 2 and 3 and the sizes of the waveguide (measuring cell) can optimize the sensitivity of the device for measuring the moisture content of a liquid in the operating range of its change. In this case, there is a monotonic dependence of the informative parameters — the values of the amplitude E and the phase shift Δϕ — on the values, respectively, of ε (ƒ ₁ ) and ε (ƒ ₂ ), functionally related to the measured moisture content of the liquid.

Thus, this method allows a sufficiently simple and high accuracy to determine 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 determining 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 their propagation 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 in electromagnetic waves of a fixed frequency, which are selected above the critical frequency of the waveguide, excite in the waveguide, receive electromagnetic waves after they propagate along the waveguide at its other end, and measure the phase shift of the excited and received electromagnetic waves, while at least one of the frequencies is selected in the region of frequency dispersion dielectric constant of water at this frequency, produce a joint functional transformation of the measured amplitude and phase shift, as a result of which judge the moisture content of the liquid.

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