Gas measurement device and gas measurement method

Abstract

According to an aspect of the present invention, a gas measurement apparatus includes a measurement controller (6), a spectrum generator (72), a processing unit (73), and a result obtaining unit (74). The measurement controller (6) controls the apparatus so that a laser-light source (1) causes laser light to be incident to an optical resonator (4) and a wavelength of the laser light is scanned within a predetermined wavelength range, the range including an absorption peak of a target component, thereby performing a CRDS measurement. The spectrum generator (72) generates an absorption spectrum based on data obtained at each wavelength within a predetermined wavelength range. The processing unit (73) approximates a waveform shape of the absorption peak of the target component in the absorption spectrum with a polynomial and acquires a coefficient of a term of a predetermined degree in the polynomial. The result obtaining unit (74) obtains absorption intensity from the coefficient, based on predetermined reference information indicating a correspondence relation between a coefficient of the term of the predetermined degree and the absorption intensity.

Claims

1. A gas measurement apparatus for obtaining concentration of a target component in a gas to be measured by cavity ring-down absorption spectroscopy, the apparatus comprising a laser-light source configured to have a variable wavelength; an optical resonator which includes a pair of highly-reflective mirrors and a measurement cell in which the gas to be measured is to be contained, and resonates laser light which is emitted from the laser-light source and is introduced into the measurement cell; an optical detection unit configured to detect laser light output from the optical resonator; a measurement controller configured to control the apparatus so that the laser-light source causes laser light to be incident to the optical resonator, and a wavelength of the laser light is scanned within a predetermined wavelength range, the range including an absorption peak of the target component, thereby performing the cavity ring-down absorption spectroscopy measurement through an output of the optical detection unit; a spectrum generator configured to generate an absorption spectrum based on data obtained at each wavelength within the predetermined wavelength range under the control of the measurement controller; a processing unit configured to approximate a waveform shape of the absorption peak of the target component in the absorption spectrum with a polynomial and configured to acquire a coefficient of a term of a predetermined degree in the polynomial; and a result obtaining unit configured to obtain absorption intensity from the coefficient obtained by the processing unit based on predetermined reference information indicating a correspondence relation between a coefficient of the term of the predetermined degree and the absorption intensity.

2. The gas measurement apparatus according to claim 1, wherein the polynomial is a quadratic polynomial, and the term of the predetermined degree is a quadratic term.

3. The gas measurement apparatus according to claim 2, wherein the measurement controller performs wavelength scanning in a wavelength range necessary and sufficient for calculating a coefficient of the quadratic term of the polynomial.

4. The gas measurement apparatus according to claim 3, wherein the necessary and sufficient wavelength range is a wavelength range corresponding to about ½ of a peak width of the absorption peak of the target component.

5. A gas measurement method for obtaining concentration of a target component in a gas to be measured by cavity ring-down absorption spectroscopy, the method comprising: a measurement step of performing the cavity ring-down absorption spectroscopy measurement through optical detection while laser light is incident to an optical resonator and a wavelength of the laser light is scanned within a predetermined wavelength range including an absorption peak of the target component, the optical resonator including a pair of high-reflective mirrors and a measurement cell in which the gas to be measured is contained; a spectrum generation step of generating an absorption spectrum based on data obtained at each wavelength within the predetermined wavelength range in the measurement step; a processing step of approximating a waveform shape of the absorption peak of the target component in the absorption spectrum with a polynomial and acquiring a coefficient of a term of a predetermined degree in the polynomial; and a result obtaining step of obtaining absorption intensity from the coefficient based on predetermined reference information indicating a correspondence relation between a coefficient of the term of the predetermined degree and the absorption intensity.

6. The gas measurement method according to claim 5, wherein the polynomial is a quadratic polynomial, and the term of the predetermined degree is a quadratic term.

7. The gas measurement method according to claim 6, wherein the measurement step performs wavelength scanning in a wavelength range necessary and sufficient for calculating a coefficient of the quadratic term of the polynomial.

8. The gas measurement method according to claim 7, wherein the necessary and sufficient wavelength range is a wavelength range corresponding to about ½ of a peak width of the absorption peak of the target component.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a configuration diagram of a main part of a CRDS apparatus according to an embodiment of the present invention.

(2) FIG. 2 is a flowchart illustrating an example of a procedure of measurement and processing when the concentration of a target component is obtained in the CRDS apparatus in the present embodiment.

(3) FIGS. 3A to 3D are diagrams for explaining the principle of a method for calculating the concentration of the target component in the CRDS apparatus in the present embodiment.

(4) FIG. 4 is a schematic configuration diagram of a general CRDS apparatus.

(5) FIG. 5 is a schematic diagram illustrating a relation between a mode frequency of an optical resonator and an oscillation frequency of laser light.

(6) FIG. 6 is a diagram illustrating an example of a change in an absorption spectrum when the effective reflectance of a mirror in the optical resonator decreases during a period from when the ring-down time τ.sub.0 is measured until the ring-down time τ is measured.

DESCRIPTION OF EMBODIMENTS

(7) Hereinafter, a gas measurement apparatus and a gas measurement method according to an embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic configuration diagram of the gas measurement apparatus in the present embodiment.

(8) In the gas measurement apparatus in the present embodiment, the configuration of a measurement system is the same as the configuration of a general CRDS apparatus illustrated in FIG. 4. The gas measurement apparatus includes a laser-light source 1, a laser driving unit 2, an optical switch 3, an optical resonator 4, and a photodetector 5. The optical resonator 4 includes a substantially cylindrical measurement cell 40 that contains a sample gas being a measured gas, and a pair of high-reflectance mirrors 47 and 48 arranged at both ends of the measurement cell 40 to face each other. A gas introduction tube 41 and a gas discharge tube 43 are connected to the measurement cell 40. An introduction valve 42 is provided in the gas introduction tube 41. A discharge valve 44 is provided in the gas discharge tube 43.

(9) The measurement controller 6 to which an input unit 8 is connected controls the units such as the laser driving unit 2 in order to perform measurement and data processing described later. In a memory (not illustrated) in the measurement controller 6, information such as an absorption peak position (wavelength) and a wavelength scanning range corresponding to the type of the component to be measured is stored in advance. Further, the data processing unit 7 to which a detection signal by the photodetector 5 is input includes a ring-down time calculation unit 71, a spectrum generator 72, a fitting processing unit 73, a concentration calculation unit 74, a reference information storage unit 75, and the like as functional blocks. Further, an output unit 9 connected to the data processing unit 7 is, for example, a display monitor or the like.

(10) A measurement operation and a processing operation when the concentration of the target component in the measured gas is obtained in the gas measurement apparatus in the present embodiment, that is, the gas measurement method performed by the present apparatus will be described with reference to FIGS. 1 and 2. FIG. 2 is a flowchart illustrating an example of a procedure of measurement and processing when the concentration of the target component is obtained using the gas measurement apparatus.

(11) A user inputs the type of the target component and the like from the input unit 8 in advance.

(12) The measurement controller 6 opens the introduction valve 42 in a state where the discharge valve 44 is closed, and introduces the measured gas into the measurement cell 40. When the pressure detected by a pressure sensor (not illustrated) reaches a predetermined value, the introduction valve 42 is closed and the measurement cell 40 is filled with the measured gas. Then, the measurement controller 6 acquires information on a wavelength range corresponding to the target component designated in advance, and performs measurement at each wavelength with the CRDS and measure a ring-down time while sequentially scanning the wavelength of laser light generated by the laser-light source 1 through the laser driving unit 2 within that wavelength range (Step S1: measurement step).

(13) That is, at each wavelength within the predetermined wavelength range, the laser-light source 1 casts laser light into the measured gas in the measurement cell 40, and the optical switch 3 blocks the laser light at a predetermined timing. The ring-down time calculation unit 71 collects data detected by the photodetector 5 until a predetermined time elapses from immediately before the laser light is blocked. Then, the ring-down time τ is calculated for each wavelength based on the data.

(14) Then, the spectrum generator 72 calculates the absorption coefficient at each wavelength, based on the ring-down time τ based on the actually-measured data and the ring-down time τ.sub.0 in a reference state, which is stored in the reference information storage unit 75. Such a method of calculating the absorption coefficient is the same as the method in the related art. For example, the above equations (1) and (2) may be used. Then, by collecting the values of the absorption coefficient calculated at each wavelength, the absorption spectrum within the predetermined wavelength range is obtained (Step S2: spectrum generation step).

(15) The absorption spectrum obtained at this time may include the influence of drift of the measured value due to the various factors described above. Therefore, the accurate concentration is calculated by removing fluctuations in the measured value including such drift in the following procedure. Firstly, the principle of removing the influence of fluctuations in the measured value will be described.

(16) A case where the reflectance of the mirror of the optical resonator 4 is different (that is, drifting occurs) between when the ring-down time τ.sub.0 in the reference state is measured and when the ring-down time τ is measured for the measured gas is assumed. When the reflectance of the mirror in measurement of the ring-down time τ.sub.0 is set as R and the reflectance of the mirror in measurement of the ring-down time τ is set as R′, a CRD signal S(ν) calculated from the measurement result is an equation (3) as follows.
S(ν)=(1/c){(1/τ)−(1/τ.sub.0)}=(1/c){(c[(1−R′)+α(ν)L]/L)−(c[1−R]/L)}=α(ν)+(R−R′)/L  (3)

(17) Here, α(ν) is the absorption coefficient when the reflectance of the mirror does not fluctuate.

(18) The absorption spectrum is obtained by drawing the CRD signal S(ν) drawn with a wavenumber axis as the horizontal axis. The application of the method disclosed in Patent Literature 1 to this absorption spectrum will be examined. FIGS. 3B, 3C, and 3D illustrate that processing of polynomial approximation disclosed in Patent Literature 1 is applied to an absorption spectrum illustrated in FIG. 3A (here, the horizontal axis is the wavenumber difference Δν axis from the central wavenumber) and obtaining curves corresponding to profiles of the 2f signal (second derivative), the 1f signal (first derivative), and the constant term (0th derivative) of a modulation frequency in laser-wavelength-modulation absorption spectroscopy.

(19) In the method disclosed in Patent Literature 1, polynomial approximation is performed within each range of a wavelength width corresponding to the wavelength modulation width in the absorption spectrum. However, in the method in the present embodiment, it is not necessary to obtain the curve itself as illustrated in FIG. 3B and the like, and it is necessary to obtain only the value of the 2f signal at Δν=0. The reason is as follows.

(20) When S(ν) and α(ν) in the above equation (3) can be approximated by polynomials, S(ν) and α(ν) can be expressed by the following equations, respectively.
S(ν)=b.sub.0′+b.sub.1′(ν−<ν>)+b.sub.2′(ν−<ν>).sup.2+b.sub.3′(ν−<ν>).sup.3+ ⋅ ⋅ ⋅  (4)
α(ν)=b.sub.0+b.sub.1(ν−<ν>)+b.sub.2(ν−<ν>).sup.2+b.sub.3(ν−<ν>).sup.3+ ⋅ ⋅ ⋅  (5)

(21) Here, <ν> is the wavelength at the center of the wavelength range for acquiring the absorption spectrum, that is, the position of Δν=0 in the present embodiment.

(22) Results of comparing coefficients of terms in the equations (4) and (5) are as follows.
b.sub.0′≈b.sub.0+(R−R′)/L  (6)
b.sub.1′≈b.sub.1  (7)
b.sub.2′≈b.sub.2  (8)

(23) The equation (8) means that the coefficient b.sub.2 of the quadratic term in the polynomial does not depend on the variation of the reflectance of the mirror. Note that, according to the equation (7), the coefficient b.sub.1 of the first-order term does not depend on the fluctuation in the reflectance of the mirror in the mathematical formula. However, as is clear from FIG. 3C, the values of the odd-order terms including the first-order term are 0 at the peak top wavelength of the absorption peak of the target component. Therefore, when the absorption intensity by the target component is obtained, it is not possible to use the odd-order terms of the approximate polynomial, and it is possible to use only the coefficients of the even-order terms of the second order or higher.

(24) Further, the method disclosed in Patent Literature 1 targets the absorption spectrum acquired by direct laser absorption spectroscopy. In the direct laser absorption spectroscopy, when the intensity of the laser light cast into the measured gas fluctuates, the fluctuation appears in the absorption spectrum. In order to avoid the fluctuation appearing, a value depending on only the transmission characteristics without depending on the fluctuations in light intensity is obtained by performing normalization processing of dividing the the b.sub.2 signal obtained by polynomial approximation by the b.sub.1 signal and b.sub.0 signal. On the other hand, in the CRDS, in principle, the fluctuations in the light intensity of light incident to the measured gas do not influence the measured value. Therefore, it is possible to use the coefficient itself of the quadratic term obtained by polynomial approximation without performing the normalization processing described above.

(25) In the above description, a case where the reflectance of the mirror differs between the ring-down time τ.sub.0 measurement in the reference state and the ring-down time measurement for the measured gas is assumed. However, as described above, it is possible to use the coefficient of the even-ordered terms of the second order or higher in the approximate polynomial for the absorption spectrum to obtain a signal that is less influenced by the above fluctuation factors, in a similar manner, when the effective optical resonator length changes, or when both the optical resonator length and the reflectance of the mirror change.

(26) When the drift occurs in the resonator length, and the optical resonator length in measurement of the ring-down time τ.sub.0 is set as L and the optical resonator length in measurement of the ring-down time τ is set as L′, the CRD signal S(ν) calculated from the measurement result is the following equation (9).
S(ν)=(1/c){(1/τ)−(1/τ.sub.0)}=(1/c){(c[(1−R)+α(ν)L′]/L′)−(c[1−R]/L)}=α(ν)+{(1−R)(L−L′)/L.Math.L′}  (9)

(27) Further, when the drift occurs in both the reflectance of the mirrors 47 and 48 and the resonator length in the optical resonator 4, the CRD signal S(ν) calculated from the measurement result is the following equation (10).
S(ν)=(1/c){(1/τ)−(1/τ.sub.0)}=(1/c){(c[(1−R′)+α(ν)L′]/L′)−(c[1−R]/L)}=α(ν)+{[(1−R′)L−(1−R)L′]/L.Math.L′}  (10)

(28) Since the equations are similar to the equation (3), it is understood that, in this case as well, it means that the coefficient b.sub.2 of the quadratic term of the approximate polynomial of S(ν) does not depend on the fluctuations.

(29) Note that, assuming that the drift in the reflectance of the mirror and the optical resonator length has wavelength dependency of the laser light, an error also occurs in the 2f signal calculated from the CRD signal S(ν) obtained by the measurement, and the equation (8) may not be established. However, since there is no factor depending on the wavelength of the laser light for the fluctuation in the resonator length, it is possible to ignore the wavelength dependency. Meanwhile, regarding the decrease in the reflectance due to the adhesion of substances in the measured gas to the mirror, the wavelength dependency can be considered, but the wavelength dependency can be considered as being ignored in the wavenumber range of 1 cm.sup.−1 or less used here. Thus, here, in order to obtain a signal that is less influenced by the above fluctuation factors, the coefficients of even-ordered terms of the second order or higher in the approximate polynomial for the absorption spectrum may be used.

(30) Returning to the flowchart illustrated in FIG. 2, the processing procedure will be described. In the gas measurement apparatus in the present embodiment, the fitting processing unit 73 determines the coefficient of each term in the polynomial to fit the waveform of the absorption peak on the absorption spectrum with a quadratic polynomial (Step S3: processing step). In the fitting processing, for example, known methods such as the least square method can be used. Since the shape in the vicinity of the peak top of the absorption peak may be able to appropriately approximated, a quadratic polynomial may be used as the polynomial, and fitting may be performed only in the wavelength range of about ½ of the peak width of the absorption peak. Thus, for the absorption peak illustrated in FIG. 3A, fitting may be performed for the peak waveform in the range surrounded by the dotted line in FIG. 3A. That is, it is sufficient to perform the measurement in Step S1 only in this wavelength range. Then, the fitting processing unit 73 acquires the coefficient of the quadratic term in the approximate polynomial (Step S4: processing step).

(31) As described above, the coefficient of the quadratic term directly corresponds to the absorption intensity (absorption coefficient). Thus, the relation between the coefficient of the quadratic term and the absorption intensity for the target component (and other components that may be measured) is obtained by preliminary experiments, and the like, and such a relation is stored in the reference information storage unit 75 in a table format, for example. Note that, information stored in the reference information storage unit 75 can be generated by the user himself or herself, but can also be set to be generated by the manufacturer of the present apparatus. Further, since the absorption intensity depends on the temperature and the pressure of the measured gas, the information stored in the reference information storage unit 75 is the relation between the coefficient of the quadratic term and the absorption intensity under a predetermined temperature and predetermined pressure. It is assumed that the measurement of the measured gas is performed under the same predetermined temperature and predetermined pressure.

(32) The concentration calculation unit 74 collates the coefficient of the quadratic term obtained based on the actual measurement with the information stored in the reference information storage unit 75, and acquires the corresponding absorption intensity (Step S5: result obtaining step). Then, the absolute concentration of the target component in the measured gas is calculated from the absorption intensity (Step S6), and the result is output to the output unit 9 and displayed.

(33) As described above, in the gas measurement apparatus in the present embodiment, it is possible to reduce the influence of the drift including short-term fluctuation in the measured value, and to calculate the concentration with high accuracy. Further, in the gas measurement apparatus in the present embodiment, since the measurement may be performed only in a wavelength range that centers on the peak top of the absorption peak and is relatively narrow, it is possible to reduce the measurement time. It is also possible to improve the measurement accuracy by improving the measurement throughput by the amount of reducing the measurement time, or instead by increasing the number of times of repeating the measurement for the same measured gas and integrating the measurement results.

(34) It should be noted that the above embodiment is an example of the present invention, and it is clear that appropriate changes, modifications, additions, and the like of the present invention are included in the claims within the scope of the present invention.

REFERENCE SIGNS LIST

(35) 1 . . . Laser-Light Source 2 . . . Laser Driving Unit 3 . . . Optical Switch 4 . . . Optical Resonator 40 . . . Measurement Cell 41 . . . Gas Introduction Tube 42 . . . Introduction Valve 43 . . . Gas Discharge Tube 44 . . . Discharge Valve 47, 48 . . . Mirror 5 . . . Photodetector 6 . . . Measurement Controller 7 . . . Data Processing Unit 71 . . . Ring-Down Time Calculation Unit 72 . . . Spectrum Generator 73 . . . Fitting Processing Unit 74 . . . Concentration Calculation Unit 75 . . . Reference Information Storage Unit 8 . . . Input Unit 9 . . . Output Unit