RU2011971C1 - Method of analyzing gases by means of microwave energy - Google Patents

Abstract

FIELD: analysis of matters. SUBSTANCE: microwave energy is generated at least at single frequency which has to be monotonously changed according to harmonic law. Central frequency of scanning range is matched with center of absorption line of the gas to be analyzed. Gas concentration is judged from amplitude of the second harmonic of alternating signal of absorption detector. Value of contribution of absorption lines of the disturbing components is judged from amplitude of the first harmonic. EFFECT: improved precision. 5 dwg

Description

 The invention relates to radio measurements and molecular spectroscopy and can be used in absorption molecular and isotopic analysis of substances in the gas and vapor phases.

 A known measurement method in which the radiation source with amplitude modulation is tuned to a characteristic absorption wavelength of the analyzed gas. An alternating signal from the detector enters the processing device, which generates a constant signal proportional to the concentration of the analyzed gas.

 The disadvantage of this method is the low selectivity, since any line or strip located in the vicinity of the analytical line and belonging to the interfering gas contained in the analyzed mixture will distort the result of the analysis.

 Closest to the invention is a method in which microwave energy is generated at least at one frequency, which is continuously changed so that it passes through the absorption frequency of the analyzed gas and the absorbed microwave energy is integrated.

 However, with this method for analysis, an isolated absorption line or absorption band is required, since the integral absorption in the range of the generator frequency is influenced by the wings of closely spaced lines and bands of other gases present in the mixture being analyzed, as well as continuous absorption; in addition, any changes in the propagation conditions of microwave radiation on the way from the generator to the detector also lead to additional errors in determining the concentration of the analyzed gas.

 The aim of the invention is to increase the selectivity and increase the accuracy of determining the concentration of gases.

 This is achieved by the fact that in the method of analysis using microwave energy, which consists in the generation of microwave energy at one frequency, which is continuously changed so that it passes through the absorption frequency of the analyzed gas, the generation frequency is changed periodically according to the harmonic law. The center frequency of the frequency scanning range is aligned with the center of the absorption line of the analyzed gas.

 The concentration of the analyzed gas is recorded by the amplitude of the second harmonic of the alternating signal, while the contribution of the absorption lines of external gases in the mixture is controlled by the amplitude of the first harmonic of the alternating signal from the detector.

 The proposed method differs from the prototype in that the integration of the intensity of the absorbed energy is replaced by recording the amplitudes of the harmonics (first and second). In this case, the contribution of the absorption lines of external gases decreases, i.e., the selectivity increases and it becomes possible to simultaneously control the value of this contribution. In addition, non-selective interference is completely eliminated. Thus, the proposed method meets the criterion of "novelty."

 A known method of recording gas concentrations by the difference between the maximum and minimum values of the derivatives of the contour of the absorption line when modulating the radiation frequency of the tunable laser.

 However, with this method, the depth of the frequency modulation is small compared to the width of the absorption line (by the definition of the derivative), which leads to a significant decrease in sensitivity, and registering the difference between the maximum and minimum values of the derivatives requires a very complex counting-solving block.

 With the proposed method, there is no frequency limitation on the depth of modulation (deviation), which allows for maximum sensitivity with parallel control of the noise level, and registration of harmonics of an alternating signal does not present technical difficulties.

 Thus, the proposed method exhibits new properties and meets the criterion of "significant differences".

, (2) где N - концентрация поглощающего газа;
γ- полуширина линии поглощения;
ν_о ^л - центр линии поглощения;
σ_о - сечение поглощения при ν= ν_о ^л, то в электрическом сигнале на детекторе появятся гармонические составляющие.The essence of the method is as follows. The radiation frequency of the generator varies in harmonic law with a frequency of Ω. For instance,
ν = ν $\genfrac{}{}{0ex}{}{\text{g}}{\text{o}}$ + m cosΩt, (1) where m is the frequency deviation,
ν _о ^g is the central frequency of the spectral region in which the scan is performed;
t is the current time,
If an absorption line appears in the range [ν _о ^g - m, ν _о ^g + m], the contour of which in practical gas analysis is described by the Lorentz curve
f _ν =

, (2) where N is the concentration of the absorbing gas;
γ is the half-width of the absorption line;
ν _about ^l - the center of the absorption line;
σ _о is the absorption cross section for ν = ν _о ^l , then harmonic components will appear in the electric signal at the detector.

+

a_lcoslΩt, (3) где
a_l=

f(t)cos(lΩt)dt, (4) где l = 0, 1, 2, 3, . . . ,
Т - период функции f(t).This process is mathematically described by expanding the function in a Fourier series:
f (t) =

+

a _l coslΩt, (3) where
a _l =

f (t) cos (lΩt) dt, (4) where l = 0, 1, 2, 3,. . . ,
T is the period of the function f (t).

 Since f (t) is even, all other members of the Fourier series vanish.

An analysis of harmonics with l = 1.2 shows that when adjusting the center frequency of the region in which scanning is performed at the center of the absorption line (ν _о ^r = ν _о ^l ), the harmonic with l = 1 vanishes.

 A plot of the amplitude of the second harmonic with l = 2 on the magnitude of the frequency deviation m is shown in FIG. 1. As can be seen from FIG. 1, the maximum amplitude of the second harmonic is achieved with a frequency deviation of m = 2.2γ. In this case, the maximum sensitivity of the method is realized.

(5) где С - частотное расстояние между линиями поглощения анализируемого и стороннего газов.To consider the influence of spectral absorption lines of external gases, we take the function f (ν) in the following form:
f (ν) =

(5) where C is the frequency distance between the absorption lines of the analyzed and external gases.

+ N₂σ₀₂γ $\genfrac{}{}{0ex}{}{\text{2}}{\text{2}}$

(6)
Пределы интегрирования приняты равными величине девиации частоты при регистрации максимальной амплитуды второй гармоники, чтобы провести сравнение методов в равных условиях.In the integral method (prototype), the signal will be proportional
F = N ₁ σ ₀₁ γ $\genfrac{}{}{0ex}{}{\text{2}}{\text{1}}$

+ N ₂ σ ₀₂ γ $\genfrac{}{}{0ex}{}{\text{2}}{\text{2}}$

(6)
The integration limits are taken equal to the value of the frequency deviation when registering the maximum amplitude of the second harmonic in order to compare the methods under equal conditions.

For the same absorption coefficients in the lines of the analyzed and external gases (N ₁ σ ₀₁ γ ₁ ² = N ₂ σ ₀₂ γ ₂ ² ), the dependence of F on the spectral interval between the lines for the integral method has the form represented by curve 1 in FIG. 2. The course of curve 1 in FIG. 2. shows that a contribution to F of 10% of the line of the external gas occurs when it approaches 5γ to the analytical line (marked with a vertical line).

 The contribution of the external gas line to the amplitude of the second harmonic from the analytical line is similarly estimated by integrating the expression obtained by substituting expressions (5) and (4). A graph of the dependence of the amplitude of the second harmonic on the frequency interval between the lines of the analyzed and external gases is shown in FIG. 2, curve 2. As can be seen from the behavior of curve 2 in FIG. 2, the contribution of 10% to the amplitude of the second harmonic takes place when the line of the external gas is removed by 2.5γ.

 Thus, ceteris paribus, the method based on the registration of the second harmonic amplitude requires a half-time spectral interval, free from lines of external gases, in comparison with the integral method, which is very important when analyzing multicomponent gas mixtures. In addition, registration along with the second harmonic of the amplitude of the first harmonic makes it possible to estimate the contribution of the absorption lines of external gases. In FIG. 2 curve 3 shows a graph of the dependence of the amplitude of the first harmonic on the frequency removal of the absorption line of an external gas from the analytical line. As can be seen from the behavior of curve 3 in FIG. 2, the amplitude of the first harmonic is very sensitive to the influence of the wing of the absorption line of an external gas, the phase of the first harmonic gives information about which side of the analytical absorption line is the noise. In the absence of interference from the absorption lines of other gases, the amplitude of the first harmonic in the center of the analytical line vanishes.

 Consider the effect of non-selective losses of probe radiation — attenuation of the absorption band in the continuum, contribution of aerosol attenuation, change in the parameters of the elements of the radiation propagation path, on the signal in the integrated method (prototype) and on the amplitude of the second harmonic.

In this case, the attenuation coefficient can be represented as
k = f (ν) + b, where b are non-selective losses.

f(ν)dν+b

dν. (7) Таким образом, вклад неселективности потерь излучения пропорционален величине интервала сканирования частоты зондирующего излучения и уровню неселективных потерь Δ νи b соответственно.Then for the integral method
F ₁ = N

f (ν) dν + b

dν. (7) Thus, the contribution of non-selectivity of radiation losses is proportional to the size of the scanning interval of the frequency of the probe radiation and the level of non-selective losses Δ ν and b, respectively.

f(t)cos(2Ωt)dt+

cos(2Ωt)dt (8) В результате того, что интеграл от cos(2Ω t) в указанных пределах интегрирования обращается в нуль, то вклада неселективных потерь в амплитуду второй гармоники нет.For the amplitude of the second harmonic, the following expression holds:
a ₂ =

f (t) cos (2Ωt) dt +

cos (2Ωt) dt (8) As a result of the fact that the integral of cos (2Ω t) vanishes within the indicated integration limits, there is no contribution of non-selective losses to the amplitude of the second harmonic.

 In practice, the method is as follows.

 Microwave radiation is generated, the frequency of which is modulated according to a harmonic law. The central frequency of the scanning range is combined with the center of the absorption line of the analyzed gas. At the absorption detector, for example, radio-acoustic, harmonics of the alternating signal arise, caused by the absorption of the frequency-modulated radiation in the spectral line of the analyzed gas. Using an electronic filter, the first and second harmonics of the alternating signal are extracted and their amplitudes are measured. By the magnitude of the amplitude of the second harmonic, the content of the analyzed gas is judged, and by the amplitude of the first harmonic, the contribution of adjacent spectral lines of the external gases in the analyzed mixture is judged.

 Then, the center frequency of the scanning section is moved to the center of the spectral line of another gas and measurements are carried out in an order similar to that described.

 The proposed method was implemented on a microwave spectrometer. We analyzed a mixture of gases: hydrogen sulfide with methanol. In FIG. 3 presents a portion of the spectrum of the specified gas mixture in the range 168.70 GHZ. . . 168.90 GHz. The lines of hydrogen sulfide (curve 1) and methanol (curve 2) are superimposed on each other, which will cause a significant error in determining the concentration by the method claimed in the prototype.

 In FIG. 5 shows the spectral dependence of the amplitude of the second harmonic in the same frequency range as in FIG. 3. The symmetry of the curves in the Central part of the spectral lines with the proposed method provides undistorted data on the concentration of both hydrogen sulfide and methanol.

 In FIG. 4 shows the spectral dependence of the amplitude of the first harmonic on the same absorption lines as in FIG. 3. In the region of the centers of absorption lines, the amplitude of the first harmonic vanishes, which confirms the smallness of the distortion from neighboring absorption lines superimposed on the amplitude of the second harmonic in the center of the analytical absorption lines.

 Thus, the proposed method of gas analysis allows to increase the selectivity of the method for measuring gas concentration and reduce the requirements for the isolation of analytical absorption lines (external gas lines can be at a distance half that of the analytical line than by the prototype method; eliminate the influence of the contribution of non-selective path loss radiation propagation and increase the accuracy of measuring gas concentrations; to simultaneously monitor the contribution of absorption by third-party gases present in mixes up.

Claims (1)

 METHOD FOR GAS ANALYSIS USING MICROWAVE ENERGY, which consists in generating microwave energy at one frequency, which is continuously changed so that it passes through the absorption frequency of the analyzed gas, characterized in that, in order to increase the accuracy of determining the concentration of gases and increase the selectivity of gas analysis, the generation frequency is changed periodically according to the harmonic law, the central frequency of the scanning range is combined with the center of the absorption line of the analyzed gas, and the amplitude of the second harmonic of the signal from the ystvovavshey microwave energy - 0 concentration of the sample gas, the absorption contribution of control lines outside of gases present in the mixture being analyzed, is carried in the amplitude of the first harmonic.

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