JP5359832B2 - Gas analyzer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To extend a measurement concentration range in a gas analysis device for measuring a concentration in a particular gas within a gas to be measured by using an absorption of laser light. <P>SOLUTION: When the concentration in the particular gas is relatively high, a control unit 28 sets a modulation amplitude for frequency-modulating the laser light in a modulation amplitude controlling voltage generation unit 33 to 0 and controls a switch 26 so as to select an output from a second ADC 25. A calculation unit 27 calculates a volume concentration of a water molecule by implementing a calculation processing according to a direct absorption detection method. When the concentration in the particular gas is relatively low, the modulation amplitude is set to A other than 0. The switch 26 is controlled so as to select an output from a first ADC 24 for digitalizing a synchronization detection signal. The calculation unit 27 calculates the volume concentration of the water molecule by implementing a calculation processing according to a harmonic synchronization detection method. The concentration calculated by one of the methods is determined, and compared with a threshold. When it determines that correct results can not be obtained, a measurement is continuously implemented as the methods are switched. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a gas analyzer that measures the concentration of a specific gas in a measurement target gas by utilizing absorption of laser light.

  In recent years, as a method for measuring the concentration of a specific gas in a gas, laser absorption spectroscopy using absorption with respect to a wavelength tunable laser beam has been proposed (see, for example, Patent Document 1). In this method, a sample cell into which a measurement target gas is introduced is irradiated with laser light of a predetermined wavelength, the transmitted laser light is analyzed, and the gas concentration is derived from the degree of absorption of a specific gas in the gas. . This device has the advantage that measurement is possible without disturbing the field of the sample and the response time is extremely short because the light receiving unit that is a sensor does not come into contact with the gas to be measured. .

  Among infrared absorption spectroscopy using laser light as described above, in particular, spectrum spectroscopy by harmonic detection (Harmonic Detection) using second harmonics (hereinafter referred to as “harmonic synchronous detection method”), In particular, it is known as a highly sensitive detection method (see, for example, Non-Patent Document 1). The theory of the detection method based on this non-patent document 1 will be briefly described. Here, a case where a trace amount of water vapor concentration in nitrogen gas is measured is taken as an example.

ここで、Ｉ₀（ν）は周波数νにおける入射光強度、Ｉ（ν）は周波数νにおける透過光強度である。また、ｃは水分子の体積濃度、Ｌは測定対象ガスを通過する光路の長さ、Ｓは所定の吸収特性の線強度、である。さらに、γ_Lは吸収特性の半値幅であり、サンプルガスの種類と温度、及び圧力により決まる。ν₀は周波数変調の中心周波数である。 When the sample gas is at atmospheric pressure, the shape of the light absorption characteristic is represented by a Lorentz profile, and the relationship between the detected laser beam received intensity and the water vapor concentration is expressed by the following equation (1).

Here, I ₀ (ν) is the incident light intensity at the frequency ν, and I (ν) is the transmitted light intensity at the frequency ν. Further, c is the volume concentration of water molecules, L is the length of the optical path that passes through the measurement target gas, and S is the line intensity of a predetermined absorption characteristic. Further, γ _L is the half width of the absorption characteristic, and is determined by the type, temperature, and pressure of the sample gas. ν ₀ is the center frequency of frequency modulation.

The absorption intensity I (ν ₀ ) at the center frequency ν ₀ is expressed by the following equation (2).

On the other hand, in the infrared absorption by water molecules under a very low total pressure region (total pressure of the gas to be measured is higher than 1 [Torr]), the absorption characteristic width is larger than the above-mentioned Lorentz profile spread. It is narrowed to a fraction of a few to a few tenths. In this total pressure region, the absorption characteristic width is mainly determined by the Doppler effect. The shape of the absorption characteristic is expressed by a Gaussian line, and the relationship between the detected light intensity of the laser beam and the water vapor concentration is expressed by the following equation (3).

高真空状態で室温約２５℃の条件の下では、一般的な近赤外半導体レーザを利用可能な、比較的強い吸収がみられる領域に存在する吸収スペクトルの場合、γ_EDはほぼ０．０１[cm^-1]に等しくなる。一方、１気圧の空気又は窒素のマトリックス内の水分子においては、γの一般的な値は０．１[cm^-1]である。 In this equation (3), γ _ED is called the Doppler width and depends on the center frequency, molecular weight, and temperature of the absorption frequency. In this case, the absorption intensity I (ν ₀ ) at the center frequency ν ₀ is expressed by the following equation (4).

Under conditions of high vacuum and room temperature of about 25 ° C., in the case of an absorption spectrum that exists in a region where a relatively near absorption is observed, in which a general near infrared semiconductor laser can be used, γ _ED is approximately 0.01. equal to [cm ^-1 ]. On the other hand, for water molecules in a 1 atm air or nitrogen matrix, a typical value of γ is 0.1 [cm ⁻¹ ].

In order to perform harmonic detection, it is necessary to modulate the frequency of light applied to the measurement target gas. Now, assuming that the modulation amplitude of a sine wave signal for frequency modulation is a and the frequency is ω, the frequency of light at time t is defined by the following equation (5).

In the second harmonic detection, a signal component corresponding to the double frequency 2ω is extracted. For water molecules in air or nitrogen at 1 atm, the second harmonic detection signal at the center frequency ν ₀ is defined by the following equation (6).

これら関係式は非特許文献２で提案されており、該文献では、上記(6)式及び(7)式においてａ／γ（又はａ／γ_ED）＝２．２になるような変調振幅ａを選択したときに、最も感度の高い信号signal(ν₀）が得られることが証明されている。 Similarly, for the water molecules in the vacuum atmosphere, the second harmonic detection signal at the center frequency ν ₀ is defined by the following equation (7).

These relational expressions are proposed in Non-Patent Document 2, in which the modulation amplitude a is such that a / γ (or a / γ _ED ) = 2.2 in the above equations (6) and (7). It has been proved that the most sensitive signal signal (ν ₀ ) can be obtained when is selected.

  The above harmonic synchronous detection method has an advantage of high sensitivity, but has a problem that the dynamic range of sensitivity is narrow. That is, an accurate detection result can be obtained when the measurement target gas has a low concentration, but if the measurement target gas has a high concentration, the signal intensity is saturated and an accurate result cannot be obtained. For this reason, when the concentration of the specific gas in the measurement target gas is continuously measured, if the amount of change in the concentration of the specific gas is large, the measurement range may be out of range.

Japanese Patent Laid-Open No. 5-99845 Japanese Patent Laid-Open No. 11-83665

Webster, “Infrared Laser Absorption: Theory and Applications in Laser Remote Chemical Analysis,” “Infrared Laser Absorption: Theory and Applications in Laser Remote Chemical Analysis”, Wiley, New York, 1988 Wilson (GVHWilson), “Modulation Broadening of NMR and ESR Line Shapes”, Journal Applied Physics (J. Appl. Phys. ), Vol.34, No.11, pp.3276 (1963)

  The present invention has been made in view of the above problems, and its object is to perform gas analysis using a laser absorption method that can measure the concentration of a specific gas in a measurement target gas in a wide dynamic range. To provide an apparatus.

The present invention made to solve the above problems comprises a sample cell into which a gas to be measured is introduced, a laser irradiation unit and a light receiving unit arranged outside the sample cell, and the laser irradiation unit In the gas analyzer for detecting the laser beam emitted from the gas to be measured in the sample cell and then detecting it by the light receiving unit, and calculating the concentration of the specific gas contained in the gas to be measured based on the detection signal,
a) modulation switching means for switching the laser light emitted from the laser irradiation unit between a state modulated with a frequency f and an unmodulated state;
b) In a state in which modulation is set by the modulation switching means, the detection signal from the light receiving unit is synchronously detected at a frequency that is an integral multiple of the frequency, and the concentration of the specific gas is calculated based on the detection result. 1 measuring means;
c) a second state in which the detection signal from the light receiving unit is directly detected without synchronous detection under the state where no modulation is set by the modulation switching means, and the concentration of the specific gas is calculated based on the detection result; Measuring means;
d) Determine the concentration obtained by the first or second measuring means in the presence or absence of modulation, and based on the determination result , the first gas when the concentration of the specific gas is relatively low The modulation switching unit and the first and second measurement units are controlled so that the concentration measurement is performed by the second measurement unit when the concentration of the specific gas is relatively high. Control means to
It is characterized by having.

  In the gas analyzer according to the present invention, the concentration measurement performed by the first measuring unit under the condition that the modulation is set by the modulation switching unit is based on the harmonic synchronous detection method described above. On the other hand, the concentration measurement performed by the second measuring unit under the state in which no modulation is set by the modulation switching unit utilizes the direct absorption phenomenon of light of a predetermined wavelength by a specific gas. . While the harmonic synchronous detection method is highly sensitive, signal saturation occurs when the concentration is high. On the other hand, the detection method using direct absorption has relatively low sensitivity, but signal saturation does not occur even at high concentrations. In the gas analyzer according to the present invention, the two detection methods are appropriately switched according to the concentration of the specific gas, thereby compensating for the disadvantages of both detection methods and expanding the measurement concentration range.

Criterion, i.e. the threshold concentration due to the prior SL control means may be determined in advance experimentally according to the type of the specific gas.

  As a result, when the concentration of the specific gas in the measurement target gas is continuously measured, even if the concentration fluctuates greatly, a more appropriate detection method is sequentially selected according to the concentration variation. Therefore, an accurate density value can be continuously monitored.

  In the present invention, the type of the specific gas that is the measurement target is not particularly limited, but is effective for measuring a concentration of water in the measurement target gas because it is effective for a gas having a large concentration change.

  The gas analyzer according to the present invention can measure the concentration of the specific gas in the measurement target gas in a wide dynamic range and in real time. Thereby, for example, when the moisture concentration in the measurement target gas is continuously measured, even if the moisture concentration greatly changes, it is possible to promptly notify the user of an accurate measurement result.

  In addition, in order to calculate the concentration in the harmonic synchronous detection method, calibration using a standard gas whose concentration is known in advance is necessary. However, in the gas analyzer according to the present invention, a specific gas has a high concentration. In some cases, the absorption spectrum is directly measured, and calibration can be performed by checking the result against a standard database (typically HITRAN). Therefore, calibration using the standard gas is not necessary, and an additional effect that the measurement work can be saved can be obtained.

The schematic block diagram of the measurement optical system of the moisture measuring apparatus which is one Example of this invention. The schematic block diagram of the signal processing system and control system of the moisture measuring apparatus of a present Example. The flowchart of the measurement sequence in the moisture measuring apparatus of a present Example. The flowchart of the other measurement sequence in the moisture measuring apparatus of a present Example. The figure which shows an example of the signal waveform obtained by the harmonic synchronous detection method and the direct absorption detection method. The schematic block diagram of the signal processing system which is another Example of this invention, and a control system.

  An embodiment of a gas analyzer according to the present invention will be described with reference to the accompanying drawings. The apparatus of this embodiment is a moisture measuring apparatus that measures the concentration of moisture in the measurement target gas. FIG. 1 is a schematic configuration diagram of a measurement optical system of a moisture measuring device according to the present embodiment, and FIG. 2 is a schematic configuration diagram of a signal processing system and a control system in the moisture measuring device.

  The moisture measuring apparatus of the present embodiment includes a sample cell 1 in a substantially horizontal direction in the middle of a gas flow path 2 through which a measurement target gas flows downward. Reflecting mirrors 3 and 4 are provided at the left and right opening ends of the sample cell 1 to face each other. One reflecting mirror 3 is provided with a transparent window 5 through which only light can pass, and an optical chamber 6 having a substantially sealed structure and a substantially atmospheric pressure is installed outside the sample cell 1 across the transparent window 5. Has been. In the optical chamber 6, a wavelength tunable laser device 7 as a laser irradiation unit and a light detection unit 8 as a light receiving unit are accommodated. As the wavelength tunable laser device 7, for example, a DFB (Distributed Feedback) type laser having a wavelength in the near-infrared region to the mid-infrared region can be used. The light detection unit 8 includes a photoelectric conversion element such as a photodiode, and an I / V conversion amplifier that converts a current signal obtained by the photoelectric conversion element into a voltage signal. The moisture (interfering moisture) in the optical chamber 6 is removed by a dehumidifying agent, a purge gas, or the like, and its concentration is assumed to be negligibly small.

  The laser beam L1 emitted from the wavelength tunable laser device 7 under the control of the laser control unit 10 passes through the transparent window 5 and enters the sample cell 1, and is repeatedly reflected between the reflecting mirrors 3 and 4. In the example of the optical path shown in FIG. 1, the laser light travels back and forth between the reflecting mirrors 3 and 4 across the gas flow path 2, but an optical system that further increases the number of reciprocations may be used. When passing through the gas flow path 2, the laser light is absorbed by various components in the measurement target gas. The laser beam L2 after being absorbed in this way returns to the optical chamber 6 through the transparent window 5, reaches the light detection unit 8, is detected, is taken out as an electric signal, and is input to the signal processing unit 11. In the example of FIG. 1, the transparent window 5 is also used for the incidence and emission of the laser beam to the sample cell 1, but a configuration in which a transparent window is separately provided may be employed.

  As shown in FIG. 2, the voltage signal obtained by the light detection unit 8 is amplified by the amplifier 21 and then input to the synchronization detector 22 and the second analog / digital converter (ADC) 25. A clock signal having a frequency of 2f generated by a 2f clock generation unit 31 (to be described later) is input to the synchronization detector 22 as a reference signal. The synchronization detector 22 receives a reference signal from the detection signal input through the amplifier 21. A signal synchronized with the phase and frequency is extracted. A high-frequency component is removed from the synchronization detection signal by a low-pass filter (LPF) 23 and converted into a digital signal by a first analog / digital converter (ADC) 24. The output of the first ADC 24 and the output of the second ADC 25 not via the synchronization detector 22 are selected by the switching unit 26 and input to the arithmetic unit 27.

  The 2f clock generation unit 31, the frequency divider 32, the modulation amplitude control voltage generation unit 33, the multiplier 34, and the band pass filter (BPF) 35 are sine wave generators having a frequency f capable of setting an arbitrary modulation amplitude. 30 is configured. In other words, under the control of the control unit 28, the 2f clock generation unit 31 generates a clock signal having a frequency 2f, and the frequency divider 32 divides this clock signal by ½, so that the frequency is f and the duty ratio. Produces a clock signal with 50%. The modulation amplitude control voltage generator 33 includes a digital / analog converter that converts the digital data supplied from the controller 28 into an analog value, and outputs a DC voltage corresponding to the modulation amplitude. The DC voltage and the clock signal having the frequency f are multiplied by the multiplier 34. The clock signal after multiplication has a frequency f and an amplitude determined by a DC voltage. The BPF 35 has a predetermined passband having a center frequency f, and converts a rectangular wave clock signal having a center frequency f into a sine wave signal having a center frequency f. This sine wave signal is a modulation signal for frequency modulation. Instead of such a configuration, a sine wave having a frequency f may be directly generated by conversion by a digital / analog converter.

  The LD wavelength scanning voltage generation unit 37 includes a digital / analog converter, and converts digital data output from the control unit 28 for sweeping over a predetermined wavelength region near the absorption spectrum of water molecules into a sweep voltage. And output. The phase of the sine wave signal from the BPF 35 described above is shifted by the phase shifter 36 so as to be synchronized with the detection signal, and then added to the sweep voltage by the adder 38. A voltage obtained by superimposing a sine wave signal on the sweep voltage is converted into a current signal by a voltage / current converter (V / I) 39 and supplied to the wavelength tunable laser device 7 as a drive current. The wavelength tunable laser device 7 emits laser light L1 whose wavelength changes with time and is frequency-modulated with a predetermined modulation amplitude.

  In the moisture measuring apparatus of the present embodiment, the control unit 28 is a digital unit that switches the amplitude of the sine wave signal by the sine wave generator 30 to a predetermined value other than 0 (this value is A) or 0. The data is output to the modulation amplitude control voltage generator 33, and when the amplitude of the sine wave signal is set to A, the output of the first ADC 24 is selected, and when the amplitude of the sine wave signal is set to 0, the output of the second ADC 25 is selected. Thus, the switching unit 26 is controlled, and the processing algorithm in the calculation unit 27 is switched. That is, when the amplitude of the sine wave signal is set to A, the concentration is calculated based on the harmonic synchronous detection method, and when the amplitude of the sine wave signal is set to 0, spectrum detection by direct absorption is performed. (Hereinafter referred to as “direct absorption detection method”), the concentration is calculated.

FIGS. 5A and 5B are examples of signal waveforms obtained by calculation (simulation) and obtained by the harmonic synchronous detection method and the direct absorption detection method. The horizontal axis represents the frequency deviation ν−ν ₀ and the vertical axis. The axis is signal strength.

In the 2f synchronization detection signal shown in FIG. 5A, the frequency deviation is zero, that is, the signal intensity at the center frequency ν ₀ indicates the strength of absorption by water molecules. However, the synchronization detection signal obtained by an actual apparatus is obtained by adding the positive direction peak and the negative direction peak shown in FIG. 5A, and it is difficult to separate them in calculation. Therefore, the peak-to-peak signal intensity SIG of the signal waveform is proportional to the signal (ν ₀ ) in the left side of the above-described equations (6) and (7). By obtaining in advance a proportionality constant B that defines the proportional relationship, the volume concentration c of water molecules can be calculated from the signal intensity of the obtained synchronous detection signal.

On the other hand, in the signal obtained by the direct absorption detection method shown in FIG. 5B, a peak at which the signal intensity decreases due to absorption by water molecules is observed near zero frequency deviation, that is, around the center frequency ν ₀ . The signal strength when the assumed absorption is no I ₀ ([nu ₀₎ and the signal intensity at the absorption peak I ([nu _0), water molecules by applying the aforementioned (2) or (4) Can be calculated.

  The proportional constant B is theoretically 1 if the signal of the frequency 2f used in the synchronous detection (the output signal of the 2f clock generation unit 31) is a perfect sine wave. On the other hand, even if the 2f signal is not a sine wave but a rectangular wave, the proportionality constant B can be obtained by the following method. That is, first, a relatively high concentration of moisture (specific gas) that can be applied to both the harmonic synchronous detection method and the direct absorption detection method is selected as a measurement target, and then the volume of water molecules is determined by the direct absorption detection method. The density c is calculated. Then, the signal intensity is obtained by the harmonic synchronous detection method, and the proportionality constant B is determined so that the volume concentration of water molecules based on the signal intensity matches the volume concentration c by the direct absorption detection method. Of course, when the volume concentration of water molecules can be obtained by another method instead of the direct absorption detection method, or when a gas having a known volume concentration of water molecules can be obtained, it can be obtained by the harmonic synchronous detection method. The proportionality constant B may be determined so that the volume concentration of the water molecule matches the known volume concentration.

  In general, the harmonic synchronous detection method is a highly sensitive detection method compared to the direct absorption detection method and is very useful for detection of trace amounts, but under conditions where the gas concentration to be measured changes greatly. It is not necessarily suitable for the measurement. For example, in the case of measuring the gas present in the atmosphere, such as oxygen, in addition to the moisture to be measured here even in a vacuum, the measured concentration range is more than 1000 times, exceeding the measured concentration range by the harmonic synchronous detection method. End up. The moisture measuring apparatus according to the present embodiment performs measurement in the sequence as shown in FIG. 3 in order to continuously and accurately measure the volume concentration of moisture over such a wide measurement concentration range.

  When the start of measurement is instructed, the control unit 28 sets the modulation amplitude to 0 and starts measurement (step S1). Then, the amplitude of the sine wave signal by the sine wave generator 30 becomes 0, and the voltage / current converter 39 is applied with a sweep voltage over a predetermined wavelength region in the vicinity of the absorption spectrum of water molecules. A drive current is supplied to the wavelength tunable laser device 7. The control unit 28 switches the switching unit 26 so as to select the output of the second ADC 25. The arithmetic unit 27 receives data obtained by digitizing a detection signal corresponding to wavelength scanning (a signal having a shape as shown in FIG. 5B), and uses a processing algorithm based on the direct absorption detection method as described above to detect water molecules. The volume concentration Ca is calculated (step S2). Upon receiving this result, the control unit 28 determines whether or not the volume concentration Ca is equal to or less than a preset threshold value α (step S3). If the volume concentration Ca is greater than the threshold value α, it is determined that the direct absorption detection method is suitable. Then, after adopting the volume concentration Ca as a result (step S4), the process returns to step S2.

On the other hand, when the volume concentration Ca is equal to or less than the threshold value α, the direct absorption detection method determines that the sensitivity is insufficient, and the control unit 28 sets the modulation amplitude to A without adopting the volume concentration Ca as a result. Data set in [cm ⁻¹ ] is sent to the modulation amplitude control voltage generator 33 (step S5). The control unit 28 switches the switching unit 26 so as to select the output of the first ADC 24. That is, switching from the direct absorption detection method to the harmonic synchronous detection method is performed. As a result, a sine wave signal having a frequency f and a modulation amplitude A is output from the sine wave generator 30, and a voltage obtained by superimposing the modulation signal on a sweep voltage over a predetermined wavelength region near the absorption spectrum of water molecules is obtained. A voltage / current converter 39 is applied, and a drive current corresponding to the voltage / current converter 39 is supplied to the wavelength tunable laser device 7. The computing unit 27 receives data obtained by digitizing a harmonic synchronization detection signal (a signal having a shape as shown in FIG. 5A) corresponding to wavelength scanning, and uses a processing algorithm based on the harmonic synchronization detection method as described above. Then, the volume concentration Cb of water molecules is calculated (step S6).

  Upon receiving this result, the control unit 28 determines whether or not the volume concentration Cb is equal to or higher than a preset threshold value β (step S7). If the volume concentration Cb is smaller than the threshold value β, the harmonic synchronization detection method is suitable. After determining and adopting the volume concentration Cb as a result (step S8), the process returns to step S6. On the other hand, if the volume concentration Cb is equal to or greater than the threshold value β, the harmonic synchronization detection method determines that the concentration is too high and signal saturation occurs, and returns to step S1 without adopting the volume concentration Cb as a result. In this case, switching from the harmonic synchronous detection method to the direct absorption detection method is performed.

  The threshold value α and the threshold value β are appropriately determined in advance according to the S / N of the detection signal by the harmonic synchronous detection method and the detection signal by the direct absorption detection method, the signal saturation level by the harmonic synchronous detection method, and the like. As a result, when continuously measuring the volume concentration of water molecules in the gas to be measured, even if the concentration changes greatly during the measurement, either the harmonic synchronous detection method or the direct absorption detection method is appropriate. It is possible to quickly switch to the correct concentration.

  Note that the sequence shown in FIG. 3 may be changed like the sequence shown in FIG. That is, in this sequence, contrary to the above sequence, when measurement start is instructed, first, concentration measurement by the harmonic synchronous detection method is performed (steps S11 and S12), and the concentration Cb is set to a preset threshold value γ. If not (NO in step S13), the concentration Cb is adopted (step S14) and the harmonic synchronous detection method is continued. On the other hand, if the concentration Cb is greater than or equal to the preset threshold value γ (YES in step S13), the concentration measurement is performed by switching to the direct absorption detection method (steps S15 and S16). If the concentration Ca is less than or equal to the preset threshold value ε (YES in step S17), the process returns to step S11. If the concentration Ca is greater than the threshold value ε, the concentration Ca is adopted (step S18) and the direct absorption detection method is continued.

  In the moisture measuring apparatus according to the above embodiment, the first ADC 24 that digitizes the synchronous detection signal and the second ADC 25 that digitizes the original detection signal that is not synchronously detected are provided together. However, the ADC may be shared. That is, as shown in the block diagram of FIG. 6, the switching unit 41 is provided in the preceding stage of the ADC 42, and the switching unit 41 selects the synchronization detection signal that has passed through the synchronization detector 22 and the LPF 23 and the detection signal that does not perform synchronization detection. It may be introduced into a common ADC 42 and digitized. Even in this configuration, the basic operation is the same as that of the above-described embodiment.

  Moreover, although the said Example applies the gas analyzer which concerns on this invention to the measurement of the moisture concentration in measurement object gas, this invention is applicable to the measurement of arbitrary gas concentrations other than a water | moisture content. Of course, the values of the modulation amplitude A, the threshold values α, β, γ, ε and the like described above need to be appropriately changed according to the type of the specific gas.

  Further, the above embodiment is an example of the present invention, and it is obvious that any modifications, corrections, additions, etc. as appropriate within the scope of the present invention other than the above description are included in the scope of the claims of the present application. It is.

DESCRIPTION OF SYMBOLS 1 ... Sample cell 2 ... Gas flow path 3, 4 ... Reflector 5 ... Transparent window 6 ... Optical chamber 7 ... Wavelength variable laser apparatus 8 ... Light detection part 10 ... Laser control part 11 ... Signal processing part 21 ... Amplifier 22 ... Synchronization Detector 23 ... Low-pass filter (LPF)
24, 25, 42 ... Analog / digital converter (ADC)
26, 41 ... switching unit 27 ... arithmetic unit 28 ... control unit 30 ... sine wave generator 31 ... 2f clock generation unit 32 ... frequency divider 33 ... voltage generator 34 for modulation amplitude control ... multiplier 35 ... band pass filter ( BPF)
36 ... Phase shifter 37 ... LD wavelength scanning voltage generator 38 ... Adder 39 ... Voltage / current converter (V / I)

Claims (2)

A sample cell into which a measurement target gas is introduced, and a laser irradiation unit and a light receiving unit arranged outside the sample cell, and the laser beam emitted from the laser irradiation unit is used as the measurement target gas in the sample cell In the gas analyzer that detects by the light receiving unit after passing and calculates the concentration of the specific gas contained in the measurement target gas based on the detection signal,
a) modulation switching means for switching the laser light emitted from the laser irradiation unit between a state modulated with a frequency f and an unmodulated state;
b) In a state in which modulation is set by the modulation switching means, the detection signal from the light receiving unit is synchronously detected at a frequency that is an integral multiple of the frequency, and the concentration of the specific gas is calculated based on the detection result. 1 measuring means;
c) a second state in which the detection signal from the light receiving unit is directly detected without synchronous detection under the state where no modulation is set by the modulation switching means, and the concentration of the specific gas is calculated based on the detection result; Measuring means;
d) Determine the concentration obtained by the first or second measuring means in the presence or absence of modulation, and based on the determination result , the first gas when the concentration of the specific gas is relatively low The modulation switching unit and the first and second measurement units are controlled so that the concentration measurement is performed by the second measurement unit when the concentration of the specific gas is relatively high. Control means to
A gas analyzer comprising: The gas analyzer according to claim 1 ,
The gas analyzer according to claim 1, wherein the specific gas is moisture in the measurement target gas.

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