JPH02500541A - Method for measuring the concentration of a gas in a gas mixture and apparatus for carrying out this method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] The method for using the body 4's method and the method for using this method The present invention provides a method for measuring the concentration of a gas in a gas mixture and a method for implementing this method. related to the equipment.

Measurement of gas concentration by gas absorption spectroscopy involves the application of the Fourier format and Remarkable progress has been made by the conception of simple devices for carrying out these methods. came.

French Painting Patent No. 2,168,948, No. 2,216,565, No. 2,300,9 No. 98, No. 2,300,999, No. 2,340,540, No. 2,420,7 No. 54, No. 2.555,747, No. 2,555,748, No. 2,566.5 No. 32 and No. 2,581゜190 contain an approximately periodic model with a period p based on the wave number σ. The method described above is suitable for measuring the concentration of a gas that has an absorption spectrum that locally includes The main operational steps underlying the law are disclosed.

In fact, the absorption spectra of the gases mentioned above obtained by the interferometer system are The Fourier transform figure (transform6e de Fourier) is Correspondingly, the optical path difference is close to Δe=1/p. This signal is located around the spectral position of the absorption spectral line, with a detectable signal nearby. Represents periodicity.

This signal is specific to the molecule and is used to detect the molecule and measure its concentration. Can be used for. The spectral position σ of the absorption band and the interferometer signal are used simultaneously. The two-component spectral sign constant (sign ature) is obtained. Therefore, the use of the Michelson interferometer becomes unnecessary and Δ All it takes is a simple device using a birefringent plate adjusted to a constant optical path difference close to C. It will be bad.

Of course, it is advantageous to choose the value of Δ at which the signal is at its maximum.

The various patent specifications mentioned above contain details of this method and some methods for carrying out the method. The following equipment is described.

This method is suitable for many gases, especially species such as SO3, No, HCI, etc. The concentration of pollutants can be easily and reliably measured.

However, this method actually only applies to have similar or the same period P, po (and therefore have the same sign constant) A gas mixture containing two gases with absorption spectra with approximately periodic refinement. It is not suitable for inspecting compounds.

It is an object of the present invention to have the above-mentioned advantages and to avoid complicating the device too much. A parasitic molecule (m olecule para-site) to detect molecule H8 in the presence of N2. The purpose of the present invention is to provide a new measurement method that can also be used in typical applications. If used, gas molecules can be identified more reliably.

If the characteristic values σ and ΔC of the two molecules are substantially the same, then look at these molecules. It would be convenient to find a third criterion for the separation; for this purpose, within the △C region, It is proposed to use interferometers precisely.

Therefore, in the present invention, first, an absorption spectrum that locally includes a substantially periodic structure with a period p is obtained. a first gas having a period p° similar to or the same as p; Measuring the concentration of a first gas in a gas mixture containing a second gas having an absorption spectrum We propose a method for This method uses the absorption spectrum produced by an interferometer system. It consists of analyzing the Fourier transform shape, and the optical path difference is derived by an interferometer system. It is characterized by measuring the strength of the signal generated when the signal is input.

Note that the optical path difference is such that the second gas has no influence on the amplitude of the signal or The optical path difference appears to be constant.

The invention furthermore proposes a device for carrying out the method. This device is a light source and , an optical system forming a beam of light transmitted through a container containing the gas mixture to be tested, and an optical path. and an interference device including a compensating plate that changes the difference accurately.

Hereinafter, the present invention will be explained in more detail based on the accompanying drawings.

In these drawings.

- Figure 1 shows the emission spectrum from a light source and (absorption spectral lines) located at regular intervals. 3 shows the absorption spectrum of the first gas when

-Figure 2 shows the absorption spectrum of light transmitted through the sample of the first gas -Figure 3 shows the absorption spectrum of the light transmitted through the sample of the first gas The oscillating part of the interferogram of the spectrum after transmission is expressed as A comparison with the resulting spectra is shown in detail.

- FIG. 4 is an explanatory diagram of the measuring device of the present invention.

- FIG. 5 shows a first embodiment of the measuring device of the invention.

- FIG. 6 shows a second embodiment of the measuring device according to the invention.

Figure 1 shows the continuous spectrum S0 of the light emitted from the light source, and the fact that it is separated from 50 degrees. The absorption spectrum S1 to be detected of low concentration molecules is shown. This spec The absorption maxima vary in strength but are located at regular intervals, so that the It can be said to be cyclical. The interval between these maxima determines the spectral period p.

Figure 2 shows the luminous flux generated as a result of the luminous flux transmitted from the light source passing through the absorbing gas. A typical interferogram is shown (curve C), so in this second figure , the optical path difference is shown on the abscissa and the interferometer signal is shown on the ordinate. Regarding S. If the variable part that carries information is i(Δ), then ■(Δ)=Io”i(Δ). Since the absorption spectrum of the gas is almost periodic, the period of corresponding optical path difference △e=17p A signal is generated in the surrounding area.

As is well known, many molecules that are frequently scattered are approximately periodic, as shown by the curve S. It has an absorption spectrum that locally includes a structure (Fig. 1), which is particularly due to the above-mentioned It is remarkable for contaminants such as S02, Not, No, IIcI, 03, etc. .

Also, this is well known, but the Fourier transform figure of the spectral distribution of the luminous flux is determined by the interferometer. Therefore, based on the signal strength near the optical path difference Δext/p, It is possible to measure the concentration of a gas containing an approximately periodic structure with a spectral period p. Wear.

This method, of course, allows the absorption spectra to be within the same spectral range and Contains two types of molecules M1 and N2 with an almost periodic structure with a period very similar to Cannot be used for test gas mixtures containing The interferogram shows the amplitude of i(Δ) of the platform and M, (curves C1 and C2, respectively). This is because they can be similar to each other, as shown in FIG. 3, which shows an enlarged view of the motion. these The two types of molecules are different molecules (even if the values of σ and △C are the same). Therefore, the zero position and i(Δ) position of Ml and N2 cannot be the same.

Therefore, it is extremely important to accurately know the position of the curve regarding △, especially the zero position of i (mu). The marking of the Δ value that maximizes or minimizes 91 (△), which is especially important for (due to modulation), i(△)3 is fixedly The value of Δ can also be manipulated.

In the present invention, we propose to use a third criterion together with the above two criteria. this The third criterion is the exact location of the zero value of the variable part of the spectral signal. actual, The optical path difference when the variable part of the spectral signal becomes zero is for each of the two gases. They are usually not the same. Therefore, this property can be used to measure the concentration of one gas. can be used.

Testing of a mixture of two gases M1 and N2 can be carried out as follows: First, an atmosphere containing an appropriate proportion of molecules M2 that can interfere with the measurement of N1, for example. Air is introduced into the container to cancel out the N2 signal so that the device is not sensitive to the presence of N2. Make accurate adjustments to the optical path difference △C to , to calibrate the sensitivity of the instrument to molecule M1. Finally M... M, a mixture is introduced and the measurement is carried out in the presence of this mixture.

When using such an optical path difference, the sensitivity to the first gas is usually maximized. Measurements are taken at a location that is not available. Therefore, the measurement conditions for only gas M1 are not the best. However, the disadvantage of this is that there is no influence of the second gas and therefore the measurements carried out are (M, due to the fact that it depends only on the concentration) will be canceled out.

Also, so that the response curve of the device is accurately measured in the presence of M, , M, mixtures. You can also. When the concentrations C1 and C2 are low, the experimental curve changes to C3 and C3. Concentration curve of only N3 and concentration of only N2 which can be easily known by proportional coefficients It consists of a combination of curved lines in a straight line. This method can also improve accuracy. Ru.

The apparatus of the invention is for carrying out the method.

As a preferred example, the device of the invention comprises a continuous polychromatic light source (1), for example in the infrared region. tungsten filament lamp and an absorption vessel containing the gas mixture to be tested (4) an optical system (2, 3) that forms a beam of light that passes through the A non-achromatic filter (5) adjusted according to the area σ and an adjusted according to the optical path difference △C. a coherent interference system (6), a photoelectric receiver (7), and a synchronization detector (8). 0 A number of interference systems can be used in the device of the invention. This system It represents the spectral characteristics of the light flux transmitted through the container (4) containing the body mixture, and the device so that an interferometer signal depending on the optical path difference introduced into the receiver (7) level is obtained. Any system you want to use.

As one preferred example, an interferometer with the simplest structure used in the measuring device includes a birefringent crystal plate (9) placed between the polarizer (10) and the analyzer (11). nothing.

The crystal plate 9 is arranged at an angle of 45° to the axis of the light produced by the polarizer. So The thickness of The selection is made depending on the constituent material of the word root so that ΔC occurs. The synchronous detector interferes with the signal. Most of the potentially harmful noise is removed from the signal in the usual way.

In order to carry out the invention, the interferometer system (6) further comprises polarizing light in a wavelength range consisting of: It is possible to very precisely vary the optical path difference introduced between the two components of the transmitted light. It also includes a birefringence compensator (12) that can be used.

The birefringence compensating plate (12) has one side in order to refine the uniformity of the birefringent plate whose thickness changes. "Soleil compensator" consisting of two wedge-shaped birefringent plates that can slide on top of each other But that's fine.

Because of this compensating plate, the device of the present invention has a nearly periodic structure with a variable period. This compensator plate can also be used to measure the concentration of several gases. contained in one gas mixture When it is desired to measure the concentration of the first gas and the second gas contained in the It also makes it possible to vary the optical path difference determining the measurement position according to some embodiments. .

The optical path difference can also be changed by changing the temperature of the birefringent crystal plate (9).

The structure in this case is as shown in FIGS. 5 and 6. In these structures, the compensator plate (1 2) is not used, but a control device is used to regulate the temperature of the plate (9).

In another embodiment, particularly used in the ultraviolet, the interferometer system (6) comprises a birefringent plate ( 9) and placed between the two Oraston prisms (14) and (15) includes a photoelastic modulator (13). In this case as well, a compensating plate (12) is used.

A photoelastic modulator (13) modulates the optical path difference introduced by the interferometer system at high frequencies. change and thus improve the signal-to-noise ratio.

The filter (5) has a period almost or exactly the same as the period p of the first gas, but Spectral absorption spectral lines that are approximately periodic and located in a different spectral region than Prevent gas having over a portion of the torque from having any effect on the signal .

In a particularly useful embodiment, instead of the filter (5), for example two slits (17 ), (18) may be used. Ru. In this case, the spectrum can be changed by changing the orientation of the grating with respect to the axis of the incident beam. You can change the selection area of the file.

The present invention has been described above in detail using a mixture containing two types of gas as an example. The method of the invention can also be easily applied to gas mixtures consisting of n types of gases. each measurement Measurements were carried out three times, with optical path differences such that there was no influence of either gas. Then, linear equations with n unknowns are established, and by solving these equations, n concentrations are obtained. degree is detected.

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Claims (6)

[Claims] 1. a first gas having an absorption spectrum that locally includes a substantially periodic structure with period p; and has an absorption spectrum with an almost periodic structure with a period p′ similar to or the same as p. A method for measuring the concentration of a first gas in a gas mixture containing a second gas, the method comprising: Analyzing the Fourier transform shape of the absorption spectrum produced by the interpolation system and the influence of the second gas on the amplitude of the signal is zero or constant. Measures the strength of the signal that occurs when an optical path difference like this is introduced by an interferometer system A method characterized by: 2. The optical path difference during measurement can be used to analyze the interferogram of a pure second gas. 2. A method according to claim 1, characterized in that it is therefore predetermined. 3. By sequentially exchanging the roles of the first gas and the second gas, each concentration of these two gases is The method according to claim 1 or 2, characterized in that: . 4. An apparatus for carrying out the method according to any one of claims 1 to 3, comprising: optics forming a beam of light transmitted through the source (1) and the container (4) containing the gas mixture to be tested; system (2, 3) and an interferometric device including means for precisely varying the optical path difference. A device characterized by: 5. The means to accurately change the optical path difference is a compensation plate called a ``Soleil compensation plate.'' 5. A device according to claim 4, characterized in that: 6. The interference device changes the introduced optical path by changing the birefringent plate (9) and the temperature of the plate. 5. The apparatus according to claim 4, further comprising means for varying the difference.