JP2792782B2 - Gas concentration measuring method and its measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method and a device for measuring gas concentration.

[0002]

2. Description of the Related Art Methane gas is a major component of city gas.
Leakage of city gas can be detected by detecting methane gas.
For this reason, in specific areas such as underground shopping malls and high-rise buildings, it is necessary to detect the presence or absence of methane gas safely, reliably, and quickly. However, conventional gas sensors of the semiconductor type and the combustion type are inferior in reliability. Therefore, in recent years, optical gas sensors have been developed.

An optical gas sensor utilizes the principle that laser light of a specific wavelength is easily absorbed by a certain kind of gas. Sensing technology applying this principle is widely used in industrial measurement, pollution monitoring, and the like. If the transmission path of the laser light is an optical fiber, remote monitoring becomes possible.

Therefore, the present inventors have developed a new remote gas (methane gas) detecting device using an optical fiber as a transmission line. In a method using this principle, a laser beam is modulated at a high frequency around a driving current of a semiconductor laser, and a laser beam having a modulated wavelength and intensity is oscillated. Further, a laser beam emitted backward of the semiconductor laser is used for monitoring so that the center wavelength of the oscillation is at the center of the methane absorption line by controlling the current and the temperature. The stabilized laser light emitted forward is transmitted through the optical fiber to the cell for the measurement gas containing the unknown concentration, and the transmitted light is guided to the light receiving section by another optical fiber facing the laser light. The gas concentration can be detected at a higher S / N ratio than the double detection signal or the basic detection signal of the light.

However, focusing on one isolated absorption line of methane gas, the absorption coefficient α has a value depending on the total pressure of the atmosphere. Therefore, when measuring the concentration in a place where the atmospheric pressure changes rapidly, such as a coal mine or a plant, accurate pressure measurement cannot be performed unless a pressure sensor is separately provided to measure the pressure and correction is performed based on the value.

Therefore, the present inventors oscillate a laser beam having a wavelength and intensity corresponding to the drive current and temperature, sweep the center wavelength of the laser beam, and bring the laser beam to a constant temperature to be measured. We propose a method to detect the intensity of transmitted light after passing through a kept gas atmosphere, phase-sensitive detection of specific components in this detection signal, and measure the specific gas concentration under the above atmospheric pressure from this detection signal. did.

[0007]

In the above-mentioned conventional method, monitoring of the center frequency of the laser utilizes the fact that the relationship between the oscillation frequency of the laser and the amount of change in the forward resistance is reproducible.

First, the difference between the laser terminal voltage and a predetermined constant voltage is obtained, and the difference voltage is amplified to monitor the amount of change in the laser terminal voltage.

FIG. 3 shows the center frequency and the second harmonic phase of the laser .
FIG. 4 is a diagram showing a relationship with a sensitive detection signal, obtained by recording a signal obtained by a gas concentration detector with an XY recorder.
You . At this time, the horizontal axis represents the differential voltage, and the vertical axis represents the second-harmonic phase-sensitive detection signal obtained by detecting light transmitted through the methane gas and performing phase-sensitive detection.

In FIG. 1, the width of the two minimum values corresponds to the full width at half maximum of the gas absorption line, and varies depending on the pressure of the measurement atmosphere. If the signal processing unit (not shown) grasps the relationship between the voltage difference between these two points and the pressure, the pressure can be converted.

However, since the amount of change in the voltage between the laser terminals is as small as about 1/1000, an amplifier which has a high gain and can operate stably is required.

It is an object of the present invention to solve the above-mentioned problems and to provide a gas concentration measuring method and a measuring apparatus capable of performing pressure correction without using a high gain and high stability amplifier.

In order to achieve the above object, a laser which oscillates a laser beam having a wavelength and intensity corresponding to the drive current and temperature is used, and the wavelength and intensity are modulated by changing the drive current or temperature of the laser. Oscillates the laser light and sweeps the center wavelength of the laser light, detects the intensity of the transmitted light obtained by passing the laser light through the gas atmosphere to be measured, and detects a specific component in the detection signal by phase-sensitive detection. Then, in the gas concentration measuring method for measuring the concentration of the specific gas in the gas to be measured under the atmospheric pressure from the detection signal, the laser light before being incident on the gas atmosphere to be measured is passed through a branching device. branched, second harmonic component from within the branch is not a laser beam a constant temperature, constant pressure and through a reference gas atmosphere of known concentration detecting the intensity of the transmitted light detection signal Te Along with the phase-sensitive detection, the rate
Monitor the center frequency of the light with the amplified output of laser DC voltage
And the second harmonic detected from the detection signal of the transmitted light of the reference gas.
Monitor voltage and reference gas pressure that give the minimum value to the detection signal
And the ratio representing the relationship between
Gives the minimum value to the double detection signal detected from the detection signal
Monitor voltage is corrected by the above ratio, and the corrected voltage value is used.
Te is intended to correct the concentration of a specific gas in the gas as the object of measurement obtained by the measurement at the pressure with obtaining the pressure of the measurement target gas.

Further, the gas concentration measuring apparatus of the present invention accommodates a laser that oscillates a laser beam having a wavelength and intensity corresponding to a drive current and a temperature, a specific gas to be measured, and keeps the temperature of the gas constant. A cell for measuring gas to be kept at
A measuring gas-side photodetector for detecting the intensity of transmitted light obtained by passing the laser light through the measuring gas cell; and a phase-sensitive detection of a specific component in a detection signal from the detection gas. A gas concentration measuring device comprising: a measuring means for measuring the concentration of the specific gas, an optical branching device for branching a laser beam incident on the measurement gas cell, and a laser beam branched by the optical branching device. Let it penetrate,
A reference gas cell containing a gas of a known concentration maintained at a constant temperature and a constant pressure, and detecting the intensity of the laser beam transmitted through the reference gas cell and detecting the second harmonic component from the detection signal by phase-sensitive detection. Reference gas side photodetector and laser light
The center frequency of the laser with the amplified output of the laser DC voltage
Amplifier and second harmonic obtained from the reference gas side photodetector
Monitor-to-monitor voltage and reference gas pressure that give the minimum value to the detection signal
Determine the ratio that expresses the relationship with the force and determine the transmitted light
Gives the minimum value to the 2nd harmonic detection signal detected from the detection signal of
The monitor voltage is corrected by the above ratio and the corrected voltage value is used.
The pressure of the gas to be measured
Correct the concentration of the specified gas in the measured gas
Correction means .

[0015]

First, in spectral measurement, there is a frequency modulation method as a method for improving measurement sensitivity. This is because when the frequency-modulated light is transmitted through the atmosphere containing the gas to be detected, the detection signal of the transmitted light includes a direct current component, a fundamental wave component having the same frequency as the modulation frequency, and its harmonic components. can get. When the fundamental component and the second harmonic component are phase-sensitive detected respectively, the fundamental component corresponds to the first derivative with respect to the absorption line, and the second harmonic component corresponds to the second derivative with respect to the absorption line. Accordingly, when the laser light whose drive current has been modulated is transmitted through an atmosphere containing a specific gas, and a specific component in a detection signal of the transmitted light is phase-sensitive detected, information on the gas concentration can be obtained from the detection signal. This mo
The laser concentration is affected by changes in pressure,
Sweep the center wavelength (center frequency) before and after the absorption line.
Pressure is obtained from the change of the second harmonic component with respect to the heart frequency, and
The gas concentration obtained above was corrected at the pressure of
The gas concentration at the can be accurately measured.

To determine the atmospheric pressure, the center circumference of the laser beam
It is necessary to know the wave number, and the center frequency is the laser DC voltage.
Can be monitored, but the
It is necessary to correct the amplification factor fluctuation of the loop. Therefore, the laser beam transmitted through the reference gas cell containing a gas of a constant temperature, a constant pressure, and a known concentration is detected, and the detection signal is obtained.
A double-wave detection signal is detected from the inside. This 2nd harmonic detection signal
Between the monitor voltage which gives the minimum value to the pressure and the pressure of the reference gas
Is determined. Next, it passes through the cell for measuring gas.
From the detection signal of the laser beam
The monitor voltage that gives the minimum value is corrected by this ratio and corrected.
The pressure of the gas to be measured is determined using the obtained value. Correcting the concentration of the specific gas as the measurement target gas with this pressure enables accurate concentration measurement. Here, the phase-sensitive detection is to extract only a component having a specific frequency and phase and measure its amplitude.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the accompanying drawings. Here, an example in which methane gas is measured using a semiconductor laser as a light source will be described.

When the oscillation current Ω of the laser light is modulated by modulating the drive current of the semiconductor laser, not only the oscillation frequency but also the oscillation intensity is modulated. Now, when the laser light whose frequency and intensity have been modulated as described above is transmitted through an atmosphere containing methane gas, a detection signal P of the transmitted light is represented by Expression 1.

[0019]

[Number 1] _{P = A [I 0 + ΔIcos} (ωt + φ)] × [C 0 + ΔΩ · T 01 cosωt + ((ΔΩ) 2/4) T 02 cos2ωt] However,

[0020]

[Number 2 is a _{C 0 = T + ((ΔΩ} ) 2/4) · T 02. The detection signal P includes a DC component and cos ωt
Component and a cos2ωt component. Here, A is a constant depending on the reflection conditions and the like, I ₀ is the central intensity of the laser output,
ΔI is intensity amplitude modulation, ω is the modulation frequency of the drive current, φ is the phase difference between ω and Ω, and ΔΩ is the frequency modulation amplitude.
T, T ₀₁ , and T ₀₂ are values of transmittance, Ω = Ω _{0 of} the first derivative dT / dΩ, and the second derivative d ² T / dΩ ² (where ω ₀ is the center frequency of the laser). The shape is shown in FIG.

FIG. 4 shows the transmittance T with respect to the frequency.
FIG. 4 shows a first derivative T ₀₁ and a second derivative T ₀₂ . In each waveform, the horizontal axis represents frequency, and the vertical axis represents transmittance T (a), first derivative T ₀₁ (b), and second derivative T ₀₂ (c).

When the frequency and phase component φ of cos ωt in Equation 1 are phase-sensitive detected,

[0023]

Equation 3] P (2ω) = A [I 0 ((ΔΩ) 2/4) T 02 + ΔI · ΔΩcosφ · T 01] is obtained, the change detection signal P (2 [omega) is based on the T ₀₁ and T ₀₂ You can see that

When detecting the absorption of methane gas by the detection signal P (2ω), the center frequency Ω ₀ of the laser beam is
The fact that the maximum sensitivity is obtained when the center coincides with the center ω ₀ of the absorption line of methane gas is used (see FIG. 4). At this time, since _T01 is "0" and _T02 is the maximum, the second term of Equation 3 is deleted, and only the first term remains. That is, Ω ₀ = ω
_{When 0} , T ₀₂ is

[0025]

[Number 4] _{T 02 (Ω 0 = ω 0} ) = 2 · α becomes _{(ω 0) · c · L} / γ 2. Therefore, substituting this into the first term of Equation 3 gives

[0026]

P (2ω) = A · I ₀ (ΔΩ) ² · α (ω ₀ ) · c · L / 2γ ² = K ₁ α ((ω ₀ ) / γ ² ) c · L Here, K ₁ is a constant, and α (ω ₀ ) is Ω ₀ = ω
The absorption coefficient of methane gas at ₀ , 2γ is the full width at half maximum of the gas absorption line, and c · L is the product of the gas concentration c and the optical path length L.

As described above, the detection signal P (2ω) is proportional to the product of the gas concentration c and the optical path length L, from which the concentration c of the methane gas can be detected with extremely high sensitivity.

By the way, α (ω ₀ ) and γ ^{2 in} Expression ⁵
Varies according to the pressure of the gas atmosphere, as shown in FIG.

FIG. 2 is a diagram showing the relationship between the absorption coefficient α (ω) for the pressure in the gas cell, the detection signal P (2ω), and the full width at half maximum 2γ described later. In the figure, the horizontal axis represents pressure (torr), the vertical axis represents absorption coefficient α (ω), detection signal P (2ω), and full width at half maximum 2γ.

In order to accurately measure the gas concentration according to the above formula 5, the values of α (ω ₀ ) and γ ² under the atmospheric pressure must be obtained. These accurate values can be obtained from the output waveform of the detection signal P (2ω) when the center frequency Ω ₀ of the laser beam is swept before and after the methane gas absorption line.

Now, when the center frequency Ω ₀ of the laser beam is changed, the first term of the equation (3) becomes a waveform in which a certain coefficient is added to T ₀₂ , and the second term becomes a waveform in which a coefficient is added to T ₀₁ . The coefficient is
I ₀ , ΔΩ, etc. If the oscillation conditions of the semiconductor laser are set, there is no problem even if they are treated as constants. Therefore, the waveform of the detection signal P (2ω) has a shape in which (b) and (c) in FIG. 4 are integrated with a certain coefficient, respectively, and these are added to each other (see FIG. 3). However, in practice, the first term in Equation 3 is superior to the second term,
The width of the center frequency Omega ₀ between the minimum value of the minimum value and the higher wavelength side of the low-wavelength side shown in FIG. 4 (c) corresponds to the full width at half maximum 2γ in a gas atmosphere pressure. When the full width at half maximum 2γ is obtained in this manner, a pressure can be obtained based on FIG. 2, and an absorption coefficient α (ω ₀ ) under the pressure can be obtained. It should be noted that P (2ω) shown in FIG.
₀ ) / γ ^{2 due} to pressure change, total pressure 100 torr
The maximum value is shown near r.

In order to determine the atmospheric pressure from FIG.
If either (ω ₀ ) or γ is known, the pressure can be known, and the pressure-corrected concentration can be detected.

FIG. 1 is a schematic configuration diagram of a gas concentration measuring device as one embodiment of the present invention.

In FIG. 1, reference numeral 1 denotes a semiconductor laser, which uses a distributed feedback laser because it is necessary to oscillate laser light of a single wavelength. Reference numeral 2 denotes an optical system for coupling the laser light from the semiconductor laser 1 to the quartz optical fiber 3a, and includes a condenser lens and an optical isolator for cutting return light. The end face of the optical system 2 is further subjected to an anti-reflection coating process to minimize return light to the semiconductor laser 1. Reference numeral 4 denotes a Peltier element for mounting the semiconductor laser 1 and controlling its temperature by an external current source (not shown). The laser module 5 is constituted as described above.

Reference numeral 6 denotes an optical splitter for splitting a laser beam from the optical fiber 3a, which is connected to the optical fibers 3a and 3b. Reference numeral 7 denotes a measuring gas cell through which light from the optical fiber 3b passes, which contains methane gas of an unknown concentration at a constant temperature. Reference numeral 8 denotes a reference gas cell through which light from the optical fiber 3c passes, and contains methane gas of a known concentration at a constant temperature and a constant pressure. Since the reference gas cell 8 needs to be set to a constant temperature and a constant pressure, it is necessary to have a structure that is not easily affected by disturbance. Therefore, the reference gas cell 8 is covered with a heat insulating material, is temperature-controlled, and is kept at a constant temperature.

Reference numeral 3d denotes a return optical fiber for transmitting the laser beam transmitted through the measuring gas cell 7, and reference numeral 9 denotes a photodetector comprising a pin photodiode for detecting the intensity of the laser beam from the optical fiber 3d. is there. Reference numeral 3c denotes an optical fiber through which the laser beam split by the splitter 6 passes through the reference gas cell 8, and reference numeral 10 denotes a light detection comprising a pin photodiode for detecting the intensity of the laser beam passing through the reference gas cell 8. It is a vessel.

Here, the end faces of the optical fibers 3a to 3d are:
The oblique cut anti-reflection coating or the like is processed so as not to generate an interference system inside.

On the other hand, 11 is an oscillator for outputting a sine wave signal of frequency ω, and 12 is an oscillator for outputting a sine wave signal of frequency ω.
A frequency multiplier 13 for generating a second harmonic signal of ω is a constant current power supply for applying a bias current to the semiconductor laser 1, and the laser drive circuit 14 is configured as described above.

The laser drive circuit 14 drives the semiconductor laser 1 by superimposing a sine wave signal of the frequency ω from the oscillator 11 on the output from the constant current power supply 13. In this example,
The modulation frequency was ω = 50 KHz. In addition, an inductance L is connected to the output side of the constant current power supply 13 to prevent the influence of the output of the oscillator 11, and a capacitor C is connected to the output side of the oscillator 11.

Reference numeral 15 denotes a lock-in amplifier that performs phase-sensitive detection of the output of the photodetector 9 in synchronization with the frequency ω of the sine wave signal from the oscillator 11, and 16 synchronizes with the frequency 2ω of the sine wave signal of the frequency multiplier 12. And a lock-in amplifier 17 for performing phase-sensitive detection of the output of the photodetector 9.
This is a divider for calculating the output ratio of 5 and 16. Reference numeral 18 denotes a density calculator for correcting the density from the output of the divider 17. Reference numeral 19 denotes a lock-in amplifier that performs phase-sensitive detection of the output of the photodetector 10 in synchronization with the frequency 2ω of the sine wave signal of the frequency multiplier 12.

On the other hand, reference numeral 20 denotes a low-pass filter for cutting the modulation frequency ω component, reference numeral 21 denotes a reference power supply for generating a predetermined reference voltage, and reference numeral 22 denotes a low-pass filter for obtaining a change in the DC component of the forward voltage of the semiconductor laser 1. A subtractor 23 calculates the difference between the value of the output voltage and the value of the reference voltage, and an amplifier 23 amplifies the output from the subtractor 22. In addition, 24
The output of the amplifier 23 to the X-axis, the output of the lock-in amplifier 19 to Y ₁ axis, an XY recorder for recording by inputting the outputs of the lock-in amplifier 16 to the Y ₂ axis, is the measuring means 25 by the above It is configured. The measuring means 25 is further provided with a signal processing device (not shown) connected to the XY recorder 24.

Next, the operation of the embodiment will be described.

In FIG. 1, the measurement is performed by first supplying a current having a magnitude equal to or larger than a certain oscillation threshold to the semiconductor laser 1 from the constant current power supply 13. A sine wave current having a modulation frequency ω is superimposed on this current to modulate the frequency and intensity of the laser light. Then, by adjusting the applied current of the Peltier element 4,
The center frequency of the semiconductor laser 1 is changed.

At this time, the center frequency of the semiconductor laser 1 is
The output of the subtracter 22 is monitored based on the output value of the amplifier 23 that has amplified the output. This is because the relationship between the variation in the oscillation frequency of each semiconductor laser and the variation in the forward resistance component has reproducibility. Therefore, the output of the amplifier 23 is put into the X-axis component of the XY recorder 24 as a center frequency monitor.

On the other hand, a part of the laser light oscillated by the semiconductor laser 1 is transmitted through the optical fiber 3a by the optical branching device 6 through the methane gas atmosphere of unknown concentration and constant temperature in the measuring gas cell 7, The light is guided to the photodetector 9 via the optical fiber 3d, where the intensity is detected.

On the other hand, after the remaining light passes through the optical splitter 6 and passes through the optical fiber 3c to the reference gas cell 8 containing a gas of the same type as the measurement gas of known concentration, constant temperature and constant pressure, The light is guided to the photodetector 10 where the intensity is detected.

The detection signal from the measuring gas cell 7 is phase-sensitive detected by the lock-in amplifiers 15 and 16, and the fundamental wave signal P _S (ω) and the second harmonic detection signal P _S (2ω) are detected.
Is input to the divider 17 and P _S (2
ω) / P _s (ω) gives the concentration before pressure conversion.

[0048] then enter the second harmonic detection signal _P R _(2ω) obtained from the light receiver 9, 10 _{Y 1} axis XY recorder 24, the second harmonic detection signal _P S a (2 [omega) _{Y 2} Input to the axis.

FIG . 3 shows the XY recorder 2 as described above .
4 shows a part of the output waveform obtained from FIG . Figure 3 shows a laser
Shows the relationship between the center frequency of the signal and the second-harmonic phase-sensitive detection signal
In the figure, the horizontal axis represents frequency, and the vertical axis represents absorption coefficient α, concentration c, product k · α · c · L of optical path length L and constant k.
It is. The waveform obtained by the XY recorder 24 is shown in FIG.
The horizontal axis is the output voltage of the amplifier 23.

The difference between the frequency giving the local maximum value and the frequency giving the local minimum value of the waveform shown in FIG.
The pressure in the measurement gas cell 7 is determined from the difference (γ).
The minimum value on the low wavelength side and the minimum value on the long wavelength side at the center of the absorption line
Pressure may be determined from the frequency difference 2γ of In the signal processing unit, the relationship between the voltage difference between the two points and the pressure may be stored in advance, and the density may be corrected from this relationship.
3, the amplification factor varies depending on the ambient temperature, the temperature of the element used, and the like. Therefore, it is necessary to correct the output voltage corresponding to the variation .

Therefore, first, the second harmonic voltage of the lock-in amplifier 19 obtained from the transmitted light of the reference gas cell 8 has a minimum value.
Is stored in advance, the ratio representing the relationship between the monitor voltage giving the pressure and the pressure in the reference gas cell 8. Next, the second harmonic voltage of the lock-in amplifier 16 obtained from the transmitted light of the measurement gas cell 7
The monitor voltage that gives the minimum value is corrected by this ratio, and the pressure of the measurement gas is determined from the corrected value and the value in the table in the signal processing unit. An accurate density is obtained from the obtained pressure and the density before correction.

As described above, according to the present embodiment, the modulated laser light is branched by the branching device 6, and one of the branched lights is transmitted through the methane gas in the measuring gas cell 7 of unknown concentration and unknown pressure. Lock-in amplifiers 15 and 16 via detector 9
In phase-sensitive detection-obtained secondary differential signal T ₀₂ the pressure accessory in divided to 1 Rukoto in divider 17 in a first-order derivative signal T ₀₁
The gas concentration at the front is calculated, and the gas is transmitted through the reference gas cell 8 containing methane gas at a constant temperature, a constant pressure, and a known concentration, and phase-sensitive detection is performed by the lock-in amplifier 19 via the photodetector 10 to obtain a signal. Minimum value of the obtained second derivative signal
Find the relationship between the monitor voltage that gives
Secondary fine phase obtained by phase-sensitive detection
The monitor voltage that gives the minimum value to the minute signal is corrected, and this correction
A gas concentration measuring method and a measuring apparatus capable of performing pressure correction without using a high-gain and high-stability amplifier because the measured gas pressure is obtained using the obtained voltage value and the calculated concentration is corrected. Can be realized. Therefore, it is possible to accurately measure the methane gas concentration without installing a new pressure sensor even in a place or environment where the atmospheric pressure changes rapidly, such as a coal mine or a plant, which was not possible without using a pressure sensor in the conventional method. it can.

Further, in this embodiment, the phase sensitive detection signal for pressure correction is applied to the reference gas cell and the measurement gas cell.
Although the harmonic signal is used, the present invention is not limited to this, and a fundamental signal may be used. Further, in this embodiment, the methane gas is stored in the reference gas cell and the methane gas is measured. However, another gas may be stored in the reference gas cell and the concentration of the other gas may be measured.

[0054]

In summary, according to the present invention, a laser beam before being incident on a gas atmosphere to be measured is branched via a branching device, and the branched laser beam is defined as a reference for constant temperature, pressure and concentration. Detection of transmitted light through gas atmosphere
A mode that gives a minimum value to the double detection signal detected from the signal.
The ratio indicating the relationship between the monitor voltage and the pressure of the reference gas is detected, and the monitor voltage is corrected by this ratio to reduce the measured gas pressure.
Since finding, it is possible to realize a gas concentration measuring method and measuring apparatus capable of pressure compensation without using a high stability of the amplifier with high gain.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a gas concentration measuring device as one embodiment of the present invention.

FIG. 2 shows an absorption coefficient α (ω) and a detection signal P (2) with respect to pressure in a cell used in the gas concentration measuring device shown in FIG.
FIG. 6 is a diagram illustrating a relationship between ω) and a half width 2γ.

FIG. 3 shows a laser center frequency and a second-harmonic phase-sensitive detection signal.
FIG. 3 is a diagram showing a relationship between the output waveform and a part of an output waveform obtained by an XY recorder used in the gas concentration measuring device shown in FIG.

FIG. 4 shows the transmittance T with respect to frequency and its first derivative T
₀₁ and the second derivative _T02 .

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser 6 Branch device 7 Cell for measurement gas 8 Cell for reference gas 9, 10 Photodetector 11 Oscillator 12 Doubler 13 Constant current power supply 15, 16, 19 Lock-in amplifier 17 Divider 24 XY recorder

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Masahiko Uchida 5-1-1 Hidaka-cho, Hitachi City, Ibaraki Prefecture Nippon Electric Cable Co., Ltd. Opto-System Laboratories (56) References JP-A-4-326604 ( JP, A) JP-A-3-277945 (JP, A) JP-A-62-88943 (JP, A) JP-A-1-100437 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , (DB name) G01N 21/00-21/01 G01N 21/17-21/61

Claims (2)

(57) [Claims] 1. A laser which oscillates a laser beam having a wavelength and intensity corresponding to a drive current and a temperature, and oscillates a laser beam whose wavelength and intensity are modulated by changing the drive current or the temperature of the laser. The central wavelength of the laser light is swept, the intensity of the transmitted light obtained by passing the laser light through the gas atmosphere to be measured is detected, a specific component in the detection signal is phase-sensitive detected, and the detection signal is detected. In the gas concentration measuring method for measuring the concentration of a specific gas in the gas to be measured under the above-mentioned atmospheric pressure, the laser light before being incident on the gas atmosphere to be measured is branched via a branching device, and the branching is performed. The laser beam that has been
It passes through a reference gas atmosphere of constant pressure and known concentration, detects the intensity of the transmitted light, and detects the second harmonic component from the detection signal.
Detect and detect the center frequency of the laser light
Monitor with the amplified output of DC voltage and detect the transmitted light of the reference gas.
Gives a minimum value to the double-wave detection signal detected from the output signal
Find the ratio that represents the relationship between the monitor voltage and the pressure of the reference gas,
2 detected from the detection signal of the transmitted light of the gas to be measured
The monitor voltage that gives the minimum value to the harmonic detection signal is compensated by the above ratio.
Correct, the gas concentration measuring method and correcting the concentration of a specific gas in the gas as the object of measurement obtained by the measurement at the pressure with obtaining the pressure of the measurement target gas by using the correction voltage value. 2. A laser for oscillating a laser beam having a wavelength and an intensity corresponding to a drive current and a temperature, a measurement gas cell containing a specific gas to be measured and keeping the temperature of the gas constant, A measuring gas side photodetector that detects the intensity of transmitted light obtained by passing the laser beam through the measuring gas cell, and a phase-sensitive detection of a specific component in a detection signal from the detector, and from the detection signal, A gas concentration measuring device having a measuring means for measuring the concentration of the specific gas, wherein an optical splitter for splitting a laser beam incident on the cell for measuring gas and a laser beam split by the optical splitter are transmitted; At the same time, a reference gas cell containing a gas of a known concentration kept at a constant temperature and a constant pressure, and the intensity of laser light transmitted through the reference gas cell are detected, and a second harmonic component is detected from the detection signal. A reference gas side photodetector for phase sensitive detection, and the center frequency of the laser beam is increased by increasing the laser DC voltage.
Amplifier to monitor with width output and photo detector on the reference gas side
Monitor that gives a minimum value to the second harmonic detection signal obtained from
Obtains a ratio representing the relationship between the pressure of the pressure and the reference gas, the measurement
Second-harmonic detection detected from the detection signal of the transmitted light of the target gas
The monitor voltage that gives the minimum value to the signal is corrected by the above ratio,
Using the corrected voltage value, find the pressure of the gas to be measured
And the specified gas in the gas to be measured
A gas concentration measuring device comprising: a correcting unit that corrects a gas concentration.