JP6624505B2 - Laser gas analyzer - Google Patents

Description

  The present invention relates to a laser gas analyzer for analyzing the presence or absence and concentration of a gas to be measured in a space to be measured.

  Each gaseous gas molecule has an absorption line spectrum representing a unique light absorption wavelength and absorption intensity. Laser light is light having a narrow spectral line width at a specific wavelength. In a laser gas analyzer, a laser element emits laser light having a light absorption wavelength that is absorbed by a gas to be measured, which is a gaseous gas molecule, and causes the gas to be measured to absorb the laser light. The presence or absence of the gas to be measured is detected based on the amount of light absorbed. In addition, the laser gas analyzer can also detect the concentration of laser light at the light absorption wavelength since the amount of absorption is proportional to the concentration of the gas to be measured.

  It is necessary to select and analyze only a specific gas to be measured from a large number of gases existing in the space to be measured. Therefore, among the light absorption wavelengths of the gas to be measured and other gases in the space to be measured, a light absorption wavelength that absorbs only the gas to be measured but does not absorb the other gases is selected.

  If the gas pressure is low, the absorption line spectrum at the light absorption wavelength of the gas to be measured becomes an ideal absorption line spectrum having a narrow spectral line width. However, in reality, the gas pressure is high, and the absorption line spectrum has a pressure spread.

  This pressure spread is caused by collision between gas molecules. In the absorption line spectrum in which the pressure spread occurs, the spectral line width increases and the absorption intensity decreases. In other words, when the pressure of the gas changes, the light amount of the laser light to be detected also changes due to the change in the absorption amount of the gas to be measured, which may cause an error in the gas concentration.

  A conventional technique of a laser gas analyzer that performs gas analysis in consideration of such pressure spread is disclosed in, for example, Japanese Patent Application Laid-Open No. 2012-233900. As shown in FIG. 1 of Patent Document 1, this prior art is capable of scanning over the spectral line width of an absorption line spectrum of a target gas and passing between a gas and a tunable laser light source 10 represented by a semiconductor laser. And a control device 16 including a lock-in amplifier 24 and a microprocessor 26.

  The laser gas analyzer of Patent Document 1 performs detection by wavelength modulation spectroscopy. A wavelength-variable laser light source 10 emits a laser light whose wavelength is swept by a drive current and modulated at a specific frequency, the laser light is detected by a photodetector 12, and a lock-in amplifier 24 multiplies a signal by a modulation frequency. To detect the lock-in, and calculate the gas concentration from the amplitude of the lock-in detection waveform. Since the signal-to-noise ratio is improved by lock-in detection, it is suitable for measurement of a trace gas.

  When the composition of a plurality of gases existing in the measurement target space is determined, the amplitude of the lock-in detection wavelength obtained by the absorption of the measurement target gas is a function of the wavelength modulation amplitude, and has a maximum value. Therefore, when calibrating the standard gas, the signal-to-noise ratio can be maximized by adjusting the wavelength modulation amplitude so that the amplitude of the lock-in detection waveform is maximized. Then, the gas concentration can be calculated based on the correspondence relationship (such as a proportional relationship) between the gas concentration of the measurement target gas and the amplitude of the lock-in detection waveform.

  However, in the actual measurement target space, the gas composition may be different from the analyzed gas composition, for example, high-temperature flue gas, and when the gas concentration (or partial pressure) also fluctuates and the pressure spread changes, Since the spectral line width fluctuates and these effects appear in the amplitude of the lock-in detection waveform, an error occurs in gas concentration measurement. In such a case, unless the influence of the fluctuation of the spectral line width is corrected, the gas concentration measurement becomes uncertain.

  FIG. 7 is a quotation from Patent Document 1 and shows the above phenomenon. The 2f signal on the vertical axis is the amplitude of the lock-in detection waveform, and the horizontal axis is the wavelength modulation amplitude. If the pressure is different, the form of the function changes due to the influence of the pressure spread. Therefore, simply calculating the gas concentration from the 2f signal results in a value including an error.

  In Patent Document 1, in order to solve the above-described problem, the wavelength modulation amplitude is set so as to be referred to as “the point of action to pressure” shown in FIG. Thereby, the pressure dependency of the gas concentration is reduced.

JP 2012-233900 A (paragraph [0030], FIG. 7)

  The "point of action on pressure" of the 2f signal in Patent Document 1 described above is not necessarily invariant with respect to pressure fluctuations, so that a measurement error of the gas concentration may remain. In addition, the signal-to-noise ratio may not be as high as the operating point where the 2f signal has the maximum value, and may be inferior to the maximum value. These issues were newly discovered.

  Therefore, the present invention has been made to solve the above-mentioned problems, and an object of the present invention is to improve the measurement accuracy while minimizing the pressure dependency of the gas concentration and increasing the signal-to-noise ratio to improve the measurement stability. Therefore, it is an object of the present invention to provide a laser gas analyzer for measuring the gas concentration of a gas to be measured with high accuracy and high stability.

In order to achieve the above object, a laser gas analyzer according to claim 1 is
A laser gas analyzer for measuring the gas concentration of the gas to be measured present in the space to be measured by the wavelength tunable laser spectroscopy and the wavelength modulation light spectroscopy,
A laser element that emits laser light in a wavelength band including the light absorption wavelength of the absorption line spectrum of the gas to be measured,
A modulated light generating unit that supplies a drive current to the laser element so that the wavelength is swept in a wavelength band including a light absorption wavelength of the absorption line spectrum of the gas to be measured and is modulated,
A light emitting unit having
A light receiving element that receives the laser light that has passed through the measurement target space,
For a detection signal output from the light receiving element, a light receiving signal processing unit that performs gas analysis based on the amplitude of a lock-in detection waveform obtained by performing lock-in detection at twice the modulation frequency,
A light receiving unit having
With
The modulated light generating unit outputs a drive current to perform wavelength sweep by at least two wavelength modulation amplitudes,
The light receiving signal processing unit,
A first amplitude of a lock-in detection waveform when a calibration gas having a known concentration and a predetermined spectral line width is irradiated with laser light having first and second wavelength modulation amplitudes, respectively. A known amplitude ratio, which is a ratio between the value and the second amplitude value, includes a gas to be measured whose concentration is unknown and has a pressure dependence of the gas concentration by another gas, and a gas having a predetermined spectral line width. On the other hand, an actually measured amplitude ratio which is a ratio between a third amplitude value and a fourth amplitude value of the lock-in detection waveform when each of the laser beams having the first and second wavelength modulation amplitudes is irradiated is obtained,
The difference between the known amplitude ratio and the measured amplitude ratio is multiplied by a correction coefficient corresponding to a spectral line width when the laser light having the second wavelength modulation amplitude is irradiated, and the multiplication result is referred to as the second. The gas concentration is measured based on the amplitude value obtained by adding to the amplitude value of No. 3 .

Laser gas analyzer is of the 請 Motomeko 2,
Te laser gas analyzer smell of claim 1,
The pre KIHA length modulation amplitude, characterized by being adjusted such that the maximum amplitude of the lock-in detection waveform before SL calibration gas.

  According to the present invention, while minimizing the pressure dependence of the gas concentration and increasing the measurement accuracy, and also increasing the signal-to-noise ratio to increase the stability of the measurement, the gas concentration of the gas to be measured can be increased with high accuracy, A laser gas analyzer that performs highly stable measurement can be provided.

FIG. 1 is a configuration diagram of a laser gas analyzer of an embodiment for carrying out the present invention. FIG. 2 (a) is a characteristic diagram of a wavelength-absorption line spectrum in an ideal state, and FIG. 2 (b) is a characteristic diagram of a wavelength-lock-in detection waveform in an ideal state. 2 (c) is a characteristic diagram of a wavelength-absorption line spectrum in a pressure fluctuation state, and FIG. 2 (d) is a characteristic diagram of a wavelength-lock-in detection waveform in a pressure fluctuation state. FIG. 9 is an explanatory diagram showing that the relationship between the amplitude of the lock-in detection waveform and the wavelength modulation amplitude changes depending on the spectral line width. FIG. 4 is an explanatory diagram illustrating a relationship between an amplitude of a lock-in detection waveform and a spectral line width. FIG. 9 is an explanatory diagram showing that the amplitude can be corrected by the amplitude and the amplitude ratio of the lock-in detection waveform. FIG. 4 is an explanatory diagram illustrating an example of a method for determining a wavelength modulation amplitude. It is a figure which shows the relationship between 2f signal and modulation amplitude of a prior art, and those pressure dependence.

  Next, a laser gas analyzer according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is an overall configuration diagram of a laser gas analyzer of the present embodiment.

  The laser gas analyzer 1 of the present embodiment measures the gas concentration of the measurement target gas contained in the gas flowing through the measurement target space inside the walls 50a and 50b. If the gas concentration is 0 or less than a predetermined value, it can be determined that there is no gas, so that the presence or absence of gas can also be detected.

  The laser gas analyzer 1 includes a light emitting unit 10, a light receiving unit 20, and a communication line 30. The communication line 30 communicates between the light emitting unit 10 and the light receiving unit 20 by an electric signal. Further, a communication unit such as wireless communication or optical communication may be employed instead of the communication line. Communication units using these communication lines, wireless communication, and optical communication can be employed.

  In such a laser gas analyzer 1, the light emitting unit 10 emits the detection light 40. Then, the detection light 40 is emitted to the measurement target space inside the walls 50a and 50b.

  At this time, a part of the detection light 40 is absorbed by the gas to be measured. The remaining light that has not been absorbed, that is, the transmitted light, enters the light receiving unit 20, and the amount of light is detected. The gas concentration of the gas to be measured is determined from a detection signal corresponding to the detected light amount.

Subsequently, details of each unit will be described.
The light emitting unit 10 includes at least a modulated light generating unit 11, a laser element 12, a collimating lens 13, a light emitting unit window plate 14, and a light emitting unit container 15.

  The light receiving unit 20 includes at least a light receiving signal processing unit 21, a light receiving element 22, a condenser lens 23, a light receiving unit window plate 24, and a light receiving unit container 25.

First, the structure will be described.
As shown in FIG. 1, holes are respectively formed in walls 50a and 50b such as pipes through which a gas containing a gas to be measured flows. The flanges 51a and 51b are fixed to those holes by welding or the like. The optical axis adjusting flanges 52a and 52b are mechanically movably attached to the flanges 51a and 51b. The position of the light emitting unit 10 and the light receiving unit 20 can be adjusted by the optical axis adjusting flanges 52a and 52b.

  Therefore, the optical axis adjustment flange 52a adjusts the output angle of the detection light 40, and the optical axis adjustment flange 52b adjusts the incident angle of the detection light 40. The detection light 40 emitted from the light emitting unit 10 is received by the light receiving unit 20 with the maximum light amount by the optical axis adjusting flanges 52a and 52b.

  Each of the light emitting unit container 15 and the light receiving unit container 25 has a laser element, an optical component, and an electric / electronic circuit built therein, isolates them from the outside air, and protects them from wind, rain, dust, dirt, and the like.

  The light emitting unit window plate 14 and the light receiving unit window plate 24 are provided so as to make holes in a part of the light emitting unit container 15 and the light receiving unit container 25 to close them. The light-emitting unit window plate 14 and the light-receiving unit window plate 24 are in the optical path of the detection light 40 and prevent the gas in the measurement target space from entering the light-emitting unit 10 and the light-receiving unit 20 while transmitting the detection light 40. I do. Thereby, the laser element, the optical component, and the electric / electronic circuit do not directly contact the gas, and the inside is protected. The mechanical structure is like this.

Next, the optical functions of the light emitting unit 10 and the light receiving unit 20 will be described. The center wavelength of a specific absorption line spectrum absorbed by the gas to be measured is defined as λ.
The laser element 12 emits light at the center wavelength λ and wavelengths around the center wavelength λ.
The collimating lens 13 is made of a material having a high transmittance at the center wavelength λ and the wavelength around the center wavelength λ. The detection light 40 is converted into substantially parallel light by the collimating lens 13 and can be transmitted to the light receiving unit 20 while suppressing loss due to diffusion.

As the light receiving element 22, a light receiving element having sensitivity at the center wavelength λ and the wavelength around the center wavelength λ can be selected.
The condenser lens 23 is made of a material having a high transmittance at the center wavelength λ and the wavelength around the center wavelength λ. Since the detection light 40 is condensed on the light receiving element 22 by the condenser lens 23, a high signal intensity can be obtained.

Next, optical system processing and signal processing by the light emitting unit 10 and the light receiving unit 20 will be described. First, the light emitting unit 10 will be described.
The modulated light generator 11 is a signal processing / current drive circuit. It is necessary to irradiate a laser beam according to the absorption characteristics of the gas to be measured. In addition, the laser light needs to be frequency-modulated modulated light. Therefore, the modulated light generation unit 11 supplies a drive current signal for emitting such a laser beam to the laser element 12. The laser element 12 is, for example, a DFB laser (Distributed Feedback Laser Diode), a VCSEL (Vertical Cavity Surface Emitting Laser Diode), or a DBR laser diode (Distributed Bragg Reflector Laser Diode).

  The laser element 12 can variably control the emission wavelength according to the drive current and the temperature. Therefore, the temperature is controlled so that the center wavelength of the laser light emitted by the laser element 12 becomes the center wavelength of the absorption line spectrum of the gas to be measured. Further, the drive current is controlled so that the wavelength around the center wavelength of the laser beam is swept over time. Further, a sine wave having an appropriate wavelength modulation amplitude and frequency is superimposed on the drive current so that measurement can be performed with high sensitivity by wavelength modulation spectroscopy.

  The light emitting point of the laser element 12 is arranged near the focal point of the collimator lens 13. The emitted light from the laser element 12 enters the collimating lens 13 while being diffused, and is converted into detection light 40 that is substantially parallel light. In the present embodiment, the collimator lens 13 will be described. However, the present invention is not limited to the collimator lens. For example, a parabolic mirror can be used instead of a collimating lens.

  The detection light 40, which is substantially parallel light, passes through the light emitting unit window plate 14, and propagates inside the walls 50a and 50b, that is, the space through which the gas containing the gas to be measured flows.

  Next, the light receiving unit 20 will be described. The light receiving unit 20 receives the detection light 40 transmitted through the light receiving unit window plate 24 and analyzes the amount of light absorbed by the measurement target gas. The detection light 40 enters the light receiving element 22 having a light receiving surface arranged near the focal point of the condenser lens 23. Although the condenser lens 23 is used in this embodiment, a parabolic mirror, a doublet lens, a diffractive lens, or the like may be used instead of the condenser lens 23.

  The light receiving signal from the light receiving element 22 is sent to the light receiving signal processing unit 21 as a detection signal. The light receiving signal processing unit 21 processes the detection signal to calculate the gas concentration.

  Next, measurement of the gas concentration of the gas to be measured will be described. First, the principle of correcting the gas concentration, which is a feature of the present invention, will be described. Here, it is assumed that the measurement target space includes the measurement target gas and other gases.

  First, the shape of the absorption line spectrum of the gas to be measured when the gas analysis is performed in an ideal state where the gas concentration of the gas to be measured is constant in the space to be measured and the gas concentrations of other gases are also constant. And the lock-in detection waveform will be described.

  FIG. 2A shows an absorption line spectrum in an ideal state, in which the horizontal axis represents wavelength and the vertical axis represents absorption intensity. The dashed line in FIG. 2A shows an absorption line spectrum whose peak height indicates A when the gas concentration of the gas to be measured is c, and the solid line in FIG. An absorption line spectrum having a peak height of 0.5 A when the concentration is 0.5 c is shown. However, it is assumed that the absorption is linear, that is, the absorption is not saturated and the Lambert-Beer law can be expressed by a first-order approximation. The same applies to the following.

  In general, such absorption line spectra have a broad spectral line width due to effects such as pressure broadening. The functional form is well approximated by the Lorentz function when the pressure spread is dominant.

  FIG. 2B shows a lock-in detection waveform signal when the absorption line spectrum shown in FIG. 2A is lock-in detected at twice the frequency of the wavelength modulation, and the horizontal axis represents the wavelength. The vertical axis indicates the signal level of the lock-in detection waveform. The lock-in detection waveform has a shape approximated by the second derivative of the absorption line spectrum.

As shown by the broken line in FIG. 2B, when the peak height when the gas concentration of the gas to be measured is c is A _2f , as shown by the solid line in FIG. At 5c, the peak height is 0.5A _2f , and its amplitude is proportional to the gas concentration. This indicates that the gas concentration can be calculated from the amplitude of the lock-in detection waveform if there is no change in pressure spread due to a gas other than the measurement target gas.

  Subsequently, in the measurement target space, the concentration of the measurement target gas is constant, but the absorption line of the measurement target gas when the gas analysis is performed in an actual measurement state in which the concentration and pressure of other gases change. The shape of the spectrum and the lock-in detection waveform will be described.

  FIG. 2C shows an absorption line spectrum in an actual measurement state as compared with the ideal state of FIG. 2A, in which the horizontal axis represents wavelength and the vertical axis represents absorption intensity. The dashed line in FIG. 2C indicates an absorption line spectrum in which the peak height indicates A when the gas concentration of the gas to be measured is c and the spectral line width is γ, and the solid line in FIG. And the absorption line spectrum having a peak height of 0.5 A when the gas concentration of the gas to be measured is c and the spectral line width is 2γ.

  The spectral line width is determined by the linear sum of the concentrations of the gas to be measured and the other gases, and the respective pressure spreading coefficients. The function form of the absorption line spectrum can be approximated to that in which the line width and the peak height change while keeping the Lorentz function. FIG. 2C shows that, even if the concentration is the same, when the spectral line width doubles (from γ to 2γ), the peak height becomes half (from A to 0.5 A).

  FIG. 2D shows a lock-in detection waveform signal when the absorption line spectrum shown in FIG. 2C is lock-in detected at twice the frequency of the wavelength modulation, and the horizontal axis represents the wavelength. The vertical axis indicates the signal level of the lock-in detection waveform.

As shown by the dashed line in FIG. 2D, the absorption line spectrum is a waveform obtained by lock-in detection at the frequency of the wavelength modulation, and the peak height when the gas concentration of the gas to be measured is c and the spectrum line width is γ. _Is A _2f , as shown by the solid line in FIG. 2 (d), the waveform obtained by lock-in detection of the absorption spectrum at the frequency of the wavelength modulation is obtained when the gas concentration of the gas to be measured is c and the spectral line width is 2γ. the peak height becomes aA _2f, the change in the height of the detection waveform is small, and the change of the detection line width hardly observed.

  That is, when the line width is doubled, the amplitude of the lock-in detection waveform behaves non-linearly. In addition, as a result, in a measurement state in which the gas concentration of the measurement target gas fluctuates, and the concentration of other gases also fluctuates, it is not possible to separate which gas concentration of the measurement target gas fluctuated, The cause of the change in the amplitude of the lock-in detection waveform cannot be specified, and an error occurs in the gas concentration measurement.

  Therefore, in order to reduce the error of the gas concentration measurement due to the fluctuation of the spectral line width, it is considered to acquire the amplitude of the lock-in detection waveform at a plurality of different wavelength modulation amplitudes. In the modulated light generation unit 11, the wavelength modulation amplitude can be changed to a plurality (a and a 'in the present embodiment).

FIG. 3 shows the amplitude of the lock-in detection waveform and the wavelength modulation amplitude when the spectral line width fluctuates in the range of γ ₀ , γ ₁ , γ ₂ depending on the gas composition or concentration of the gas to be measured or another gas. The function is shown. Both the vertical and horizontal axes in FIG. 3 have no significance in the absoluteness of the numerical values, and are represented by arbitrary units (au) arbitrarily determined for comparison by comparison of relative values.

The spectral line width γ ₀ includes a standard gas used for adjustment at the time of manufacturing at a factory or calibration at an installation site, for example, including a gas to be measured and other gases, and the other gas is composed of nitrogen. Obtained when using a suitable gas. The state in which the spectral line width γ ₀ is obtained is an ideal state in which the gas concentration of the measurement target gas is constant and the gas concentrations of other gases are constant in the measurement target space. Also, the spectral line width of the measurement field is assumed to vary the concentration and pressure of the other gases, from gamma ₁ by temperature fluctuations up to gamma _2. The state in which the spectral line width fluctuates is an actual measurement state in which the concentration of the gas to be measured is constant in the space to be measured, but the concentrations and pressures of other gases change.

  As shown in FIG. 3, it can be seen that changing the wavelength modulation amplitude can obtain a different lock-in detection waveform amplitude. Among these, the wavelength modulation amplitudes a and a 'are selected. Each of a and a 'can be selected so as to obtain a larger amplitude of the lock-in detection waveform than the "point of action to pressure" described in Patent Document 1 in order to increase the signal-to-noise ratio.

A method for selecting such a wavelength modulation amplitude will be described. FIG. 6 illustrates how to select a plurality of wavelength modulation amplitudes. The wavelength modulation amplitudes a and a 'are adjusted so that the amplitude of the lock-in detection waveform is maximized for any one of the calibration gases composed of a plurality of different gases. That is, the wavelength modulation amplitude a is the amplitude A ₀ of the lock-in detection waveform is adjusted such that the maximum A _max in a calibration gas having a spectral linewidth gamma ₀ due to pressure broadening. Further, the wavelength modulation amplitude a ′ is adjusted so that the amplitude A ₂ ′ of the lock-in detection waveform becomes the maximum A _max ′ in the calibration gas having the spectral line width γ ₂ due to the pressure spread when the pressure is high. .

  There are two advantages of selecting such wavelength modulation amplitudes a and a '. One is that if the composition of a plurality of calibration gases is determined, the wavelength modulation amplitude is uniquely determined, so that the ambiguity of setting the wavelength modulation amplitude can be eliminated.

  Second, if the composition of multiple gases in the calibration gas simulates the fluctuation range of the pressure spread of the gas in the measurement target space (a fluctuation in which the pressure increases from the standard pressure), Even if the pressure spread of the gas in the space fluctuates, the amplitude of one of the lock-in detection waveforms takes the maximum value or a value close to it, so that the deterioration of the signal-to-noise ratio of the gas concentration measurement is minimized. is there. Here, it is assumed that the gas pressure is high. However, this is an example of the wavelength modulation amplitude, and is not necessarily limited to an increase in gas pressure. By measuring with a plurality of wavelength modulation amplitudes, the purpose of pressure spread correction is achieved.

Next, the principle of density correction will be described. When the wavelength modulation amplitude is measured only by a, the amplitude of the lock-in detection waveform fluctuates between A ₁ and A ₂ , and this becomes an apparent density fluctuation, that is, an error. Therefore, it is considered that the amplitude of the lock-in detection waveform is also acquired at a different wavelength modulation amplitude a ′ to correct the apparent density fluctuation. 4 and 5 are used to explain this.

  FIG. 4 is a graph obtained by changing the view of FIG. 3. The horizontal axis represents the spectral line width, and the vertical axis represents the amplitude of the lock-in detection waveform. It can be seen that if the amplitudes of the lock-in detection waveform are acquired at different wavelength modulation amplitudes, different spectral line width dependencies can be obtained.

Therefore, when a graph is drawn with the ratio of the amplitude of the lock-in detection waveform at different wavelength modulation amplitudes to the same spectral line width being R _n = A _n ′ / A _n (where n = 0, 1, 2), FIG. Become like From the graph of FIG. 5, R _n It can be seen that changes with respect to the spectral line width. Therefore, by calculating the difference between R _n and R _0, by adding to the A _n over an appropriate coefficient, it is possible to correct the error of the apparent. ΔR ₌ R 0 -R _n, if the correction coefficient alpha and _(gamma), may be represented as _{A n "= A n + α} (γ) × ΔR. The correction coefficient alpha (gamma) is a function of gamma It is.

For example, if the spectral line width by pressure broadening at low pressure is narrow gamma _1, select _{n = 1, ΔR = R 0} -R 1 = A 0 as shown in FIG. 5 _'/ A 0 -A _1' / A ₁ and the correction coefficient α (γ ₁ ), A ₁ ″ = A ₁ + α (γ ₁ ) × ΔR. Therefore, the correction coefficient α (γ) and the spectral line width If A ₀ and A ₀ ′ at the time of γ ₀ are already known and stored in, for example, the storage unit of the light reception signal processing unit 21, if the spectral line width is γ ₁ and the wavelength modulation amplitude a A ₁ ′ at the time and A ₁ ′ at the wavelength modulation amplitude a ′ are measured, γ ₁ is substituted for the correction coefficient α (γ) to calculate α (γ ₁ ), and the value is substituted for the above-mentioned correction formula. By doing so, it is possible to calculate the density A ₁ ″ corrected for the apparent density fluctuation.

Also, if the spectral line width by pressure broadening at high pressure is a broad gamma _2, and select _{n = 2, ΔR = R 0} -R 2 = A 0 as shown in FIG. 5 _'/ A 0 -A _2' If / A ₂ and the correction coefficient are α (γ ₂ ), then A ₂ ″ = A ₂ + α (γ ₂ ) × ΔR. Therefore, the correction coefficient α (γ) and the spectral line width are γ. _If A ₀ and A ₀ ′ at the time of ₀ are already known and stored in, for example, the storage unit of the received light signal processing unit 21, the case where the spectral line width is γ ₂ and the wavelength modulation amplitude is a measured 'a ₂ when _the' in a ₂ and the wavelength modulation amplitude a, by substituting gamma ₂ the correction coefficient alpha (gamma) is calculated alpha a (gamma _2), to assign a value to the correction formula Thus, the density A ₂ ″ in which the apparent density fluctuation is corrected can be calculated.

  The correction coefficient α (γ) can be obtained in advance by calibration using a plurality of types of calibration gases having different compositions and concentrations of other gases. Alternatively, if the function form of the absorption spectrum is approximated by a Lorentz function, an approximate value can be obtained even by simulation using a computer. By using the correction coefficients obtained by these means, the amplitude of the lock-in detection waveform may be measured while switching the wavelength modulation amplitude at the time of measurement. This is the principle of detection.

  Subsequently, the analysis processing by the laser gas analyzer will be described. First, the light receiving signal processing unit 21 performs gas analysis of the calibration gas and registers correction information in advance.

  Details of registration of the correction information will be described. A plurality of calibration gases containing a gas to be measured having a known concentration in advance and having different compositions and concentrations of other gases are allowed to flow in the flue, and the above gas analysis is performed on the calibration gas. This is the obtained correction information.

First, using a calibration gas comprising a spectral linewidth gamma _0. The light emitting unit 10 irradiates a laser beam having a light absorption wavelength λ of the gas to be measured and a wavelength modulation amplitude a. Light-receiving signal processing section 21 obtains the height A ₀ from the lock-in detection waveform in a state of the spectral linewidth gamma _0. Subsequently, a laser beam having a light absorption wavelength λ of the gas to be measured and a wavelength modulation amplitude of a ′ is applied. The light reception signal processing unit 21 acquires the height A ₀ ′ from the lock-in detection waveform in the state of the spectral line width γ ₀ .

Then, using a calibration gas comprising a spectral linewidth gamma ₁ by adjusting the concentration, lock-in detection height A ₁ from the waveform in the form of spectral line width gamma ₁ wavelength modulation amplitude is irradiated with laser light is a To get. Similarly, a height A ₁ ′ is acquired from the lock-in detection waveform in the state of the spectral line width γ ₁ by irradiating a laser beam having a wavelength modulation amplitude of a ′.

Then, using a calibration gas having a spectral line width γ ₂ by adjusting the concentration, a laser beam having a wavelength modulation amplitude a is irradiated to obtain a height A ₂ in the state of the spectral line width γ ₂ from the lock-in detection waveform. To get. Similarly wavelength modulation amplitude to obtain the 'height A ₂ in the state of the spectral line width gamma ₂ from the lock-in detection waveform by irradiating a laser beam which _is' a.

A correction coefficient α (γ) by actual measurement obtained by changing the spectral line width γ from these values and the above equation is obtained. The correction coefficient α (γ) may be correction information previously obtained by a computer such as a computer. The correction information A ₀ , A ₀ ′ and α (γ) when the calculated spectral line width is γ ₀ is acquired at the time of factory shipment or on-site installation and registered in advance. The light reception signal processing unit 21 stores such correction information.

That is, γ _n and the correction coefficient α (γ _n ) corresponding to γ _n are registered in the memory (not shown) of the light reception signal processing unit 21 in association with each other. Many combinations are registered by changing the value of γ _n . Once γ _n has been calculated from the lock-in detection waveform, the corresponding correction coefficient α (γ _n ) can be read.
It is also assumed that A ₀ and A ₀ ′ are measured at the time of factory shipment or on-site installation and registered in a memory (not shown).

Subsequently, the analysis is started. Light-receiving signal processing section 21 corrects the pressure dependence of the gas concentration on the basis of the lock-in detected plurality of amplitude Best and correction information of the waveform were respectively corresponding to a plurality of wavelength modulation amplitude.

Specifically, it is as follows. Here, it is assumed that the gas containing the gas to be measured flows in the flue and the pressure is high due to the influence of other gases. First, the laser element 12 of the light emitting unit 10 irradiates a laser beam having the light absorption wavelength λ of the gas to be measured and the wavelength modulation amplitude a. First, light-receiving signal processing section 21 obtains the spectral line width gamma ₂ and height A ₂ from the lock-in detection waveform.

Subsequently, the light receiving signal processing unit 21 sends a switching signal for switching the wavelength modulation amplitude from a to a ′ via the communication line 30 to the modulated light generating unit 11 on the light emitting unit side, and transmits the laser element through the modulated light generating unit 11. Reference numeral 12 denotes a laser beam having a light absorption wavelength λ of the gas to be measured and a wavelength modulation amplitude of a ′. Light-receiving signal processing section 21 obtains a lock spectral linewidth gamma ₂ and height from in detection waveform A _{2 '.}

The light reception signal processing unit 21 reads out the coefficient α (γ ₂ ) from the line width γ ₂ .

The light reception signal processing unit 21 calculates ΔR = R ₀ −R ₂ from the known R ₀ = A ₀ ′ / A ₀ and the actually measured R ₂ = A ₂ ′ / A ₂ .

The light reception signal processing unit 21 calculates A ₂ ″ by calculating A ₂ ″ = A ₂ + α (γ ₂ ) × ΔR. This A ₂ ″ is set as the amplitude of the lock-in detection waveform.

The light reception signal processing unit 21 calculates the gas concentration using the amplitude A ₂ ″ of the lock-in detection waveform. Further, if the value of the gas concentration is equal to or less than the predetermined value, it is possible to detect that there is no gas. The concentration and presence or absence of gas are detected, and the same analysis is performed when the pressure is low.

  The laser gas analyzer 1 of the present invention has been described above. In such a laser gas analyzer, accurate gas concentration can be calculated because gas analysis is performed in consideration of pressure dependency.

  The laser gas analyzer of the present invention is used for analyzing exhaust gas (excluding testers) from internal combustion engines of vehicles and ships, disaster prevention (explosive gas detection, toxic gas detection, new building material combustion gas analysis), plant growth, and chemical analysis. It is also useful as an analyzer for petroleum refining plants, petrochemical plants, gas generating plants, for environmental purposes [landing concentration, concentration in tunnels, parking lots, building management], and for various physical and chemical experiments.

1: Laser gas analyzer 10: Light emitting unit 11: Modulated light generating unit 12: Laser element 13: Collimating lens 14: Light emitting unit window plate 15: Light emitting unit container 20: Light receiving unit 21: Light receiving signal processing unit 22: Light receiving element 23: condenser lens 24: light receiving unit window plate 30: communication line 40: detection light 50a, 50b: wall 51a, 51b: flange 52a, 52b: optical axis adjustment flange

Claims (2)

A laser gas analyzer for measuring the gas concentration of the gas to be measured present in the space to be measured by the wavelength tunable laser spectroscopy and the wavelength modulation light spectroscopy,
A laser element that emits laser light in a wavelength band including the light absorption wavelength of the absorption line spectrum of the gas to be measured,
A modulated light generating unit that supplies a drive current to the laser element so that the wavelength is swept in a wavelength band including a light absorption wavelength of the absorption line spectrum of the gas to be measured and is modulated,
A light emitting unit having
A light receiving element that receives the laser light that has passed through the measurement target space,
For a detection signal output from the light receiving element, a light receiving signal processing unit that performs gas analysis based on the amplitude of a lock-in detection waveform obtained by performing lock-in detection at twice the modulation frequency,
A light receiving unit having
With
The modulated light generating unit outputs a drive current to perform wavelength sweep by at least two wavelength modulation amplitudes,
The light receiving signal processing unit,
A first amplitude of a lock-in detection waveform when a calibration gas having a known concentration and a predetermined spectral line width is irradiated with laser light having first and second wavelength modulation amplitudes, respectively. A known amplitude ratio, which is a ratio between the value and the second amplitude value, includes a gas to be measured whose concentration is unknown and has a pressure dependence of the gas concentration by another gas, and a gas having a predetermined spectral line width. On the other hand, an actually measured amplitude ratio which is a ratio between a third amplitude value and a fourth amplitude value of the lock-in detection waveform when each of the laser beams having the first and second wavelength modulation amplitudes is irradiated is obtained,
The difference between the known amplitude ratio and the measured amplitude ratio is multiplied by a correction coefficient corresponding to a spectral line width when the laser light having the second wavelength modulation amplitude is irradiated, and the multiplication result is referred to as the second. 3. A laser gas analyzer which measures a gas concentration based on an amplitude value obtained by adding to the amplitude value of No. 3 . Te laser gas analyzer smell of claim 1,
Before KIHA length modulation amplitude, pre SL laser gas analyzer being characterized in that adjusted to maximize the amplitude of the lock-in detection waveform in the calibration gas.

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