JP2017106742A - Laser gas analyzer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser gas analyzer which minimizes pressure dependence of gas concentration to enhance measurement accuracy and increases a signal-to-noise ratio to enhance measurement stability, thereby enabling highly accurate and highly stable measurement of target gas concentration.SOLUTION: A laser gas analyzer capable of computing accurate gas concentration is realized by pre-registering correction information, and correcting gas concentration for pressure dependence thereof on the basis of the correction information and amplitudes of a plurality of lock-in detected waveforms corresponding to wavelength modulation amplitudes.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a laser gas analyzer that analyzes the presence and concentration of a measurement target gas in a measurement target space.

  Each gaseous gas molecule has an absorption line spectrum that represents a specific 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 a laser beam having a light absorption wavelength that is absorbed by a gas to be measured, which is a gaseous gas molecule, and the laser beam is absorbed by the gas to be measured, and a laser at the light absorption wavelength. The presence or absence of the measurement target gas is detected based on the amount of light absorption. In addition, the laser gas analyzer can detect the concentration because the amount of absorption of laser light at the light absorption wavelength is proportional to the concentration of the gas to be measured.

  Note that it is necessary to select and analyze only a specific measurement target gas from among many gases present in the measurement target space. Therefore, among the light absorption wavelengths of the measurement target gas and other gases in the measurement target space, a light absorption wavelength that absorbs only the measurement target gas but does not absorb other gases is selected.

  The absorption line spectrum at the light absorption wavelength of the measurement target gas becomes an ideal absorption line spectrum having a narrow spectral line width if the gas pressure is low. However, in practice, the gas pressure is high, resulting in an absorption line spectrum in which pressure spread occurs.

  This pressure broadening is caused by collisions between gas molecules, and the absorption line spectrum in which the pressure broadening occurs has a wide spectral line width and a low absorption intensity. In other words, when the gas pressure changes, the amount of laser light to be detected also varies due to a change in the absorption amount of the measurement target gas, which may cause an error in the gas concentration.

  For example, Patent Document 1 (Japanese Patent Laid-Open No. 2012-233900) discloses a conventional technique of a laser gas analyzer that performs gas analysis in consideration of such pressure spread. As shown in FIG. 1 of Patent Document 1, this conventional technique is capable of scanning over a spectral line width of an absorption line spectrum of a target gas and passes between a tunable laser light source 10 typified by a semiconductor laser and a gas. And a control device 16 including a lock-in amplifier 24 and a microprocessor 26. The photodetector 12 detects the intensity of the laser beam to be detected by DC and a multiple of the modulation frequency.

  The laser gas analyzer of Patent Document 1 performs detection by wavelength modulation spectroscopy. The wavelength tunable laser light source 10 emits a laser beam whose wavelength is swept by a drive current and is modulated at a specific frequency, the photodetector 12 detects the laser beam, and the lock-in amplifier 24 multiplies the modulation frequency by the signal. And lock-in detection, and the gas concentration is calculated 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 measuring trace gases.

  When the composition of a plurality of gases existing in the measurement target space is fixed, the amplitude of the lock-in detection wavelength obtained by absorption of the measurement target gas is a function of the wavelength modulation amplitude, and there is 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 (proportional relationship or the like) between the gas concentration of the measurement target gas and the amplitude of the lock-in detection waveform.

  However, the actual measurement target space may be different from the analyzed gas composition, such as high-temperature combustion exhaust gas, and if the gas concentration (or partial pressure) also fluctuates and the pressure spread also 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, the gas concentration measurement becomes uncertain unless the influence of the fluctuation of the spectral line width is corrected.

  FIG. 7 is cited 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. When the pressure is different, the shape of the function changes due to the influence of the pressure spread. Therefore, if the gas concentration is simply calculated from the 2f signal, the value includes an error.

  In Patent Document 1, in order to solve the above-described problem, the wavelength modulation amplitude is set so that the fluctuation of the 2f signal is minimized with respect to the fluctuation of the pressure, which is referred to as an “action point for pressure” shown in FIG. This reduces the pressure dependence of the gas concentration.

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

  Now, since the “action point for pressure” of the 2f signal in the above-mentioned Patent Document 1 is not necessarily invariant to pressure fluctuation, there is a possibility that a measurement error of gas concentration may remain. In addition, the signal noise ratio may be inferior to the maximum value instead of the operating point at which the 2f signal has the maximum value. These issues were newly discovered.

  Therefore, the present invention has been made to solve the above-mentioned problems, and its purpose is to minimize the pressure dependence of the gas concentration to improve the measurement accuracy and to increase the signal-to-noise ratio to improve the measurement stability. It is an object of the present invention to provide a laser type gas analyzer that measures the gas concentration of the measurement target gas with high accuracy and high stability.

A laser type gas analyzer according to claim 1 of the present invention comprises:
A laser type gas analyzer that measures a gas concentration of a measurement target gas existing in a measurement target space by means of wavelength tunable laser spectroscopy and wavelength modulated light spectroscopy,
A laser element that emits a laser beam in a wavelength band including a light absorption wavelength of an absorption line spectrum of the measurement target gas;
A modulated light generation unit that supplies a drive current to the laser element so that a wavelength is swept and modulated in a wavelength band including a light absorption wavelength of an absorption line spectrum of the measurement target gas;
A light emitting unit having
A light receiving element that receives the laser light that has passed through the measurement target space;
A light receiving signal processing unit that performs gas analysis based on the amplitude of a waveform obtained by lock-in detection at a frequency twice the modulation frequency with respect to the detection signal output from the light receiving element;
A light receiving unit having
With
The modulated light generation unit outputs a drive current so as to perform wavelength sweeping with a plurality of wavelength modulation amplitudes,
The light reception signal processing unit registers correction information in advance, and determines the pressure dependence of the gas concentration based on the amplitude of a plurality of waveforms obtained by lock-in detection corresponding to each of the plurality of wavelength modulation amplitudes and the correction information. It was set as the laser type gas analyzer characterized by correcting.

A laser gas analyzer according to claim 2 of the present invention is
The laser gas analyzer according to claim 1,
The pressure dependence of the amplitudes of the plurality of waveforms corresponds to correction information obtained by a plurality of calibration gases that include the gas to be measured having a known concentration in advance and are configured so that the composition and concentration of other gases are different. The laser gas analyzer is characterized in that each is corrected based on the above.

A laser gas analyzer according to claim 3 of the present invention is
The laser gas analyzer according to claim 1,
The laser gas analyzer is characterized in that the pressure dependence of the amplitude of the plurality of waveforms is corrected based on correction information obtained in advance by a computer.

A laser gas analyzer according to claim 4 of the present invention is
The laser gas analyzer according to any one of claims 1 to 3,
The laser-type gas analyzer is characterized in that the plurality of wavelength modulation amplitudes are adjusted so as to be a maximum amplitude of a lock-in detection detection waveform in the plurality of calibration gases.

  According to the present invention, the gas concentration of the measurement target gas can be increased with high accuracy by minimizing the pressure dependence of the gas concentration and increasing the measurement accuracy, and also increasing the signal-to-noise ratio and the measurement stability. It is possible to provide a laser type gas analyzer that performs highly stable measurement.

It is a block diagram of the laser type gas analyzer of the form for implementing this invention. FIG. 2A is an explanatory diagram of pressure dependence of gas analysis, FIG. 2A is a characteristic diagram of an ideal state wavelength-absorption line spectrum, FIG. 2B is a characteristic diagram of an ideal state wavelength-lock-in detection waveform, and FIG. 2 (c) is a characteristic diagram of the wavelength-absorption line spectrum in the pressure fluctuation state, and FIG. 2 (d) is a characteristic diagram of the wavelength-lock-in detection waveform in the pressure fluctuation state. It is explanatory drawing which shows that the relationship between the amplitude of a lock-in detection waveform and a wavelength modulation amplitude changes with spectrum line widths. It is explanatory drawing which shows the relationship between the amplitude of a lock-in detection waveform, and a spectrum line width. It is explanatory drawing which shows that an amplitude can be correct | amended with the amplitude and amplitude ratio of a lock-in detection waveform. It is explanatory drawing which shows an example of the determination method of a wavelength modulation amplitude. It is a figure which shows the 2f signal of a prior art, the relationship of a modulation amplitude, and those pressure dependence.

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

  The laser type gas analyzer 1 of this embodiment measures the gas concentration of the measurement target gas contained in the gas flowing in the measurement target space inside the walls 50a and 50b. Further, if the gas concentration is 0 or less than a predetermined value, it can be determined that there is no gas, so 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 electrical signal. Further, a communication unit such as wireless or optical communication may be employed instead of the communication line. A communication unit using these communication lines, wireless communication, or optical communication can be employed.

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

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

Next, 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 50a and 50b such as pipes and the like through which a gas containing a measurement target gas flows are made with holes. The flanges 51a and 51b are fixed to those holes by welding or the like. The optical axis adjusting flanges 52a and 52b are attached to the flanges 51a and 51b so as to be mechanically movable. The positions 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 emission 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 incorporates a laser element, an optical component, and an electric / electronic circuit, and isolates them from the outside air to protect them from wind and rain, dust, and dirt.

  The light emitting unit window plate 14 and the light receiving unit window plate 24 are provided so as to make a hole in a part of the light emitting unit container 15 and the light receiving unit container 25 and close the holes. 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, so that the gas in the measurement target space does not enter the light-emitting unit 10 or the light-receiving unit 20 while transmitting the detection light 40. To do. As a result, the laser element, the optical component, and the electric / electronic circuit do not directly touch 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. Let λ be the center wavelength of a specific absorption line spectrum absorbed by the measurement target gas.
The laser element 12 emits light at the center wavelength λ and wavelengths around it.
The collimating lens 13 is made of a material having a high transmittance at the center wavelength λ and its peripheral wavelengths. 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 its peripheral wavelengths can be selected.
The condensing lens 23 is made of a material having a high transmittance at the center wavelength λ and its peripheral wavelengths. 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 driving circuit. It is necessary to irradiate a laser beam corresponding to the light absorption characteristics of the measurement target gas. 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 laser light 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 temperature. Therefore, the temperature is controlled so that the center wavelength of the laser light emitted from the laser element 12 becomes the center wavelength of the absorption line spectrum of the measurement target gas. Further, the drive current is controlled so that the wavelength around the center wavelength of the laser light is swept in time. Furthermore, a sine wave having an appropriate wavelength modulation amplitude and frequency is superimposed on the drive current so that it can be measured with high sensitivity by wavelength modulation spectroscopy.

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

  The detection light 40, which is substantially parallel light, passes through the light-emitting unit window plate 14 and propagates into the walls 50a and 50b, that is, the space where the gas including the measurement target gas 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 is incident on the light receiving element 22 having a light receiving surface disposed near the focal point of the condenser lens 23. Although the condensing lens 23 is used in this embodiment, a parabolic mirror, a doublet lens, a diffractive lens, or the like may be employed instead of the condensing lens 23.

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

  Next, measurement of the gas concentration of the measurement target gas will be described. First, the correction principle of the gas concentration that characterizes the present invention will be described. Here, it is assumed that the measurement target space contains the measurement target gas and other gases.

  First, the shape of the absorption line spectrum of the measurement target gas when performing gas analysis in an ideal state where the gas concentration of the measurement target gas is constant in the measurement target space and the gas concentration of other gases is also constant. The lock-in detection waveform will be described.

  FIG. 2A shows an absorption line spectrum in an ideal state, where the horizontal axis indicates the wavelength and the vertical axis indicates the absorption intensity. The broken line in FIG. 2A indicates an absorption line spectrum in which the peak height is A when the gas concentration of the measurement target gas is c, and the solid line in FIG. 2A indicates the gas of the measurement target gas. An absorption line spectrum in which the peak height is 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 an absorption line spectrum has a broad spectrum line width due to effects such as pressure broadening. The function form is well approximated by the Lorentz function when the pressure spread is dominant.

  FIG. 2B shows a lock-in detection waveform signal when lock-in detection is performed on the absorption line spectrum shown in FIG. 2A at twice the frequency of the wavelength modulation, and the horizontal axis indicates 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 measurement target gas is c is _A2f , the gas concentration is 0. As shown by the solid line in FIG. next 0.5A _2f height of the peak at the time of 5c, the amplitude is proportional to the gas concentration. From this, it can be seen that the gas concentration can be calculated from the amplitude of the lock-in detection waveform if there is no change in the pressure spread due to the 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 spectrum shape and the lock-in detection waveform will be described.

  FIG. 2 (c) shows an absorption line spectrum in an actual measurement state as compared with the ideal state of FIG. 2 (a). The horizontal axis indicates the wavelength and the vertical axis indicates the absorption intensity. The broken line in FIG. 2C shows an absorption line spectrum in which the peak height is A when the gas concentration of the measurement target gas is c and the spectral line width is γ, and the solid line in FIG. The absorption line spectrum shows a peak height of 0.5 A when the gas concentration of the measurement target gas is also c and the spectral line width is 2γ.

  The spectral line width is determined by the linear sum of the concentration of the gas to be measured and the concentrations of the other gases and the respective pressure spread coefficients. The function form of the absorption line spectrum remains the Lorentz function and can be approximated as a change in line width and peak height. In the case of FIG. 2 (c), even when the concentration is the same, when the spectral line width is doubled (from γ to 2γ), the peak height is halved (from A to 0.5A).

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

As shown by the broken line in FIG. 2 (d), it is a waveform obtained by lock-in detection of the absorption line spectrum at the frequency of frequency modulation, and the peak height when the gas concentration of the measurement target gas is c and the spectral line width is γ. the when a _2f, as shown by the solid line in FIG. 2 (d), the lock-in detection waveform absorption line spectrum at the frequency of the wavelength modulation the gas concentration in the measurement target gas when the spectral line width is 2γ in c The height of the peak is aA _2f , the change in the height of the detection waveform is slight, and the change in the detection line width is hardly seen.

  In other words, when the line width is doubled, the amplitude of the lock-in detection waveform behaves nonlinearly. As a result, in the measurement state in which the gas concentration of the measurement target gas fluctuates and the concentration of other gases also fluctuates, in the measurement target gas, it is impossible to separate which gas concentration has fluctuated. The cause of the amplitude change of the lock-in detection waveform cannot be specified, and an error occurs in the gas concentration measurement.

  Therefore, in order to reduce the gas concentration measurement error 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 (in this embodiment, a and a ').

FIG. 3 shows a function of the amplitude of the lock-in detection waveform and the wavelength modulation amplitude when the spectrum amplitude fluctuates in the range of γ ₀ , γ ₁ , γ ₂ depending on the gas composition and concentration of the measurement target gas and other gases. It is shown. The absolute value of the numerical values on both the vertical axis and the horizontal axis in FIG. 3 is meaningless, and is expressed by an arbitrary unit (au) that is arbitrarily determined in order to discuss relative value comparison.

The spectral line width γ ₀ includes a standard gas used for adjustment at the time of manufacturing at the factory or calibration at the installation location, for example, a measurement target gas and other gases, and the other gas is constituted by nitrogen. It is obtained when using various gases. 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 also constant in the measurement target space. Further, it is assumed that the spectral line width at the measurement site varies from γ ₁ to γ ₂ due to the concentration, pressure, and temperature variation of other gases. The state in which the spectral line width fluctuates is an actual measurement state in which the concentration of the measurement target gas is constant in the measurement target space, but the concentration and pressure of other gases change.

  As shown in FIG. 3, it can be seen that when the wavelength modulation amplitude is changed, different amplitudes of the lock-in detection waveform are obtained. Of these, a and a 'are selected as the wavelength modulation amplitudes. Each of a and a ′ can be selected so as to obtain a larger amplitude of the lock-in detection waveform than the “action point for 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 explains 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 with respect to any of calibration gases composed of a plurality of different gases. That is, the wavelength modulation amplitude a is adjusted so that the amplitude A ₀ of the lock-in detection waveform becomes the maximum A _max in the calibration gas having the spectral line width γ ₀ due to the pressure spread. 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 ambiguity in setting the wavelength modulation amplitude can be eliminated.

  The other is the measurement target if the composition of multiple gases of the calibration gas is designed to simulate the fluctuation range of the pressure spread of the gas in the measurement target space (the 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 degradation of the signal-to-noise ratio in gas concentration measurement can be minimized. is there. Here, it is assumed that the gas pressure becomes high. However, this shows an example of the wavelength modulation amplitude, and the gas pressure is not necessarily increased. The purpose of pressure spread correction is achieved by measurement with multiple wavelength modulation amplitudes.

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

  FIG. 4 is a graph in which the way of viewing FIG. 3 is changed. 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 amplitude of the lock-in detection waveform is acquired with different wavelength modulation amplitudes, different spectral line width dependencies can be obtained.

Therefore, if the ratio of the amplitude of the lock-in detection waveform at different wavelength modulation amplitudes with respect to the same spectral line width is R _n = A _n ′ / A _n (where n = 0, 1, 2), a graph is drawn as shown in FIG. It becomes like this. From the graph of FIG. 5, it can be seen that R _n varies 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. If ΔR = R ₀ −R _n and the correction coefficient is α (γ), then A _n ″ = A _n + α (γ) × ΔR can be expressed. Here, the correction coefficient α (γ) is a function of γ. It is.

For example, when γ ₁ has a narrow spectral line width due to pressure spread at low pressure, n = 1 is selected, and ΔR = R ₀ −R ₁ = A ₀ ′ / A ₀ −A ₁ ′ as shown in FIG. / A ₁ , if the correction coefficient is α (γ ₁ ), it can be expressed as A ₁ ″ = A ₁ + α (γ ₁ ) × ΔR. Therefore, the correction coefficient α (γ) and the spectral line width are If A ₀ and A ₀ ′ at γ ₀ are already known and stored in the storage unit of the received light signal processing unit 21, for example, 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, α (γ ₁ ) is calculated by substituting γ ₁ into the correction coefficient α (γ), and the value is substituted into the correction equation. By doing so, it is possible to calculate the density A ₁ ″ in which the apparent density fluctuation is corrected.

Further, when γ ₂ has a wide spectral line width due to pressure spread at high pressure, n = 2 is selected, and ΔR = R ₀ −R ₂ = A ₀ ′ / A ₀ −A ₂ ′ as shown in FIG. / A ₂ , if the correction coefficient is α (γ ₂ ), it can be expressed as 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 ₀ are already known and stored in the storage unit of the received light signal processing unit 21, for example, when 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, it is possible to calculate the density A ₂ ″ in which the apparent density fluctuation is corrected.

  The correction coefficient α (γ) can be acquired 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 line spectrum is approximated by a Lorentz function, an approximate value can be obtained even by a computer simulation. 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 during measurement. The detection principle is like this.

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

  Details of the 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 flowed in the flue, and the above gas analysis is performed on the calibration gas. It is the correction information obtained.

First, a calibration gas having a spectral line width γ ₀ is used. The light emitting unit 10 emits laser light having a light absorption wavelength λ of the measurement target gas and a wavelength modulation amplitude of a. The received light signal processing unit 21 acquires the height A ₀ in the state of the spectral line width γ ₀ from the lock-in detection waveform. Subsequently, a laser beam having a light absorption wavelength λ of the measurement target gas and a wavelength modulation amplitude a ′ is irradiated. The received light 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 having a spectral line width γ ₁ by adjusting the concentration, a laser beam having a wavelength modulation amplitude of a is applied to the height A ₁ in the state of the spectral line width γ ₁ from the lock-in detection waveform. To get. Similarly, a laser beam having a wavelength modulation amplitude of a ′ is irradiated to obtain a height A ₁ ′ in a state of a spectral line width γ ₁ from a lock-in detection waveform.

Then, a calibration gas having a spectral line width γ ₂ by adjusting the concentration is irradiated with laser light having a wavelength modulation amplitude of a, and the height A ₂ in the state of the spectral line width γ ₂ from the lock-in detection waveform. To get. Similarly, a laser beam having a wavelength modulation amplitude of a ′ is irradiated to obtain a height A ₂ ′ from the lock-in detection waveform in the state of the spectral line width γ ₂ .

From these numerical values and the above formula, a correction coefficient α (γ) by actual measurement obtained by changing the spectral line width γ is acquired. Further, the correction coefficient α (γ) may be correction information obtained in advance 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 is registered in advance. The received light signal processing unit 21 stores such correction information.

That is, the memory (not shown) of the light-receiving signal processing section 21, and r _n, the correction coefficient corresponding to the r _n α (γ _n), are registered in the associated. a large number of combinations by changing the value of r _n has been registered. If the lock-in detection waveform r _n is calculated, it is possible to read the corresponding correction factor α a (γ _n).
In addition, A ₀ and A _{0 'is} assumed to be registered in the memory (not shown) are measured at the time of installation the factory or in the field.

  The analysis is then started. The received light signal processing unit 21 corrects the pressure dependence of the gas concentration based on the amplitudes of the plurality of waveforms and the correction information based on the amplitudes of the plurality of waveforms detected in lock-in corresponding to the plurality of wavelength modulation amplitudes.

Specifically: Here, it is assumed that the gas containing the measurement target gas 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 laser light having the light absorption wavelength λ of the measurement target gas and the wavelength modulation amplitude a. First, the received light signal processing unit 21 acquires the spectral line width γ ₂ and the height A ₂ from the lock-in detection waveform.

Subsequently, the received light 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 the laser element is transmitted via the modulated light generating unit 11. A laser beam 12 is irradiated with the light absorption wavelength λ of the measurement target gas and the wavelength modulation amplitude is a ′. The received light signal processing unit 21 acquires the spectral line width γ ₂ and the height A ₂ ′ from the lock-in detection waveform.

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

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

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

The received light signal processing unit 21 calculates the gas concentration using the amplitude A ₂ ″ of the lock-in detection waveform. Further, if the gas concentration value is equal to or less than a predetermined value, it can be detected that there is no gas. Concentration and presence / 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 type gas analyzer, since gas analysis is performed in consideration of pressure dependency, an accurate gas concentration can be calculated.

  The laser gas analyzer of the present invention is an exhaust gas (removal tester) for internal combustion engines such as cars and ships, disaster prevention [explosive gas detection, toxic gas detection, new building material combustion gas analysis], plant growth, chemical analysis It is also useful as an analyzer for [oil refinery plant, petrochemical plant, gas generation plant], environmental [landing concentration, concentration in tunnel, parking lot, building management], and various experiments for physics and chemistry.

1: Laser gas analyzer 10: Light emitting part 11: Modulated light generating part 12: Laser element 13: Collimating lens 14: Light emitting part window plate 15: Light emitting part container 20: Light receiving part 21: Light receiving signal processing part 22: Light receiving element 23: Condensing lens 24: Light receiving part window plate 30: Communication line 40: Detection light 50a, 50b: Walls 51a, 51b: Flange 52a, 52b: Optical axis adjustment flange

Claims (4)

A laser type gas analyzer that measures a gas concentration of a measurement target gas existing in a measurement target space by means of wavelength tunable laser spectroscopy and wavelength modulated light spectroscopy,
A laser element that emits a laser beam in a wavelength band including a light absorption wavelength of an absorption line spectrum of the measurement target gas;
A modulated light generation unit that supplies a drive current to the laser element so that a wavelength is swept and modulated in a wavelength band including a light absorption wavelength of an absorption line spectrum of the measurement target gas;
A light emitting unit having
A light receiving element that receives the laser light that has passed through the measurement target space;
A light receiving signal processing unit that performs gas analysis based on the amplitude of a waveform obtained by lock-in detection at a frequency twice the modulation frequency with respect to the detection signal output from the light receiving element;
A light receiving unit having
With
The modulated light generation unit outputs a drive current so as to perform wavelength sweeping with a plurality of wavelength modulation amplitudes,
The light reception signal processing unit registers correction information in advance, and determines the pressure dependence of the gas concentration based on the amplitude of a plurality of waveforms obtained by lock-in detection corresponding to each of the plurality of wavelength modulation amplitudes and the correction information. A laser gas analyzer characterized by correcting. The laser gas analyzer according to claim 1,
The pressure dependence of the amplitudes of the plurality of waveforms corresponds to correction information obtained by a plurality of calibration gases that include the gas to be measured having a known concentration in advance and are configured so that the composition and concentration of other gases are different. A laser type gas analyzer which is corrected based on each. The laser gas analyzer according to claim 1,
The pressure dependency of the amplitude of the plurality of waveforms is corrected based on correction information obtained in advance by a computer, respectively. The laser gas analyzer according to any one of claims 1 to 3,
The laser gas analyzer according to claim 1, wherein the plurality of wavelength modulation amplitudes are adjusted to be a maximum amplitude of a lock-in detection detection waveform in the plurality of calibration gases.

Images