JP5170034B2 - Gas analyzer - Google Patents

Description

  The present invention relates to a gas analyzer that measures the concentration of a specific gas in a gas to be measured using 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 gas to be measured is introduced is irradiated with laser light of a predetermined wavelength, the transmitted laser light is analyzed, and the concentration of the gas is determined from the degree of absorption of a specific component gas in the gas to be measured. To derive. Since this apparatus is a non-contact type in which the light receiving unit as a sensor does not contact the gas to be measured, it has an advantage that measurement can be performed without disturbing the field of the sample and response time is extremely short.

Assuming that the incident light to the sample cell is I ₀ , the transmitted light is I, the molecular number density of the specific gas is c, the optical path length passing through the gas to be measured is L, and the absorption coefficient is ε, Lambert-Bale From Beer's law, the following equation (1) holds.

ここで、Ｉ₀（ν）は波長νにおける入射光強度、Ｉ（ν）は波長νにおける透過光強度、Ｓは吸収線強度、φ（ν）は吸収線のプロファイル関数であり、φ（ν）は次の(3)式のように定義される。

In recent years, a TDLAS (Tunable Diode Laser Absorption Spectroscopy) method has been established that uses a tunable semiconductor laser diode as a light source and measures the concentration of a specific gas. In the TDLAS method, since the monochromaticity of the light source is high, there is an advantage that it is difficult to be influenced by other gas components by selectively receiving light absorption of only a specific gas. In addition, since the laser diode can easily perform high-speed lighting and high-speed modulation driving, it has excellent responsiveness. When the wavelength of the laser beam is ν, the above equation (1) can be rewritten as the following equation (2).

Here, I ₀ (ν) is the incident light intensity at the wavelength ν, I (ν) is the transmitted light intensity at the wavelength ν, S is the absorption line intensity, φ (ν) is the profile function of the absorption line, and φ (ν ) Is defined as the following equation (3).

一般に、吸収線強度Ｓの値は吸収線のデータベースとして提供されているHITRANから容易に得ることができる。したがって、(4)式において、プロファイル関数φ（ν）さえ正確に決定することができれば、特定ガスの濃度を知ることができる。プロファイル関数φ（ν）は被測定ガスの圧力に応じて３種類の関数のいずれかとなることが知られている。 In the TDLAS method, since the light intensity at the center wavelength ν ₀ of the absorption spectrum is usually measured, the equation (2) becomes the following equation (4).

In general, the value of the absorption line strength S can be easily obtained from HITRAN provided as a database of absorption lines. Therefore, if only the profile function φ (ν) can be accurately determined in the equation (4), the concentration of the specific gas can be known. It is known that the profile function φ (ν) is one of three functions depending on the pressure of the gas to be measured.

特に中心波長ν₀においては次の(6)式となる。

ここで、γ_Lは大気圧における吸収スペクトルの半値半幅であり、被測定ガスの種類と温度に依存する。 [1] When the gas to be measured is atmospheric pressure In this case, the profile function φ (ν) is expressed by a Lorentz function as shown in the following equation (5).

Especially at the center wavelength ν ₀ , the following equation (6) is obtained.

Here, γ _L is the half-width of the absorption spectrum at atmospheric pressure, and depends on the type and temperature of the gas to be measured.

特に中心波長ν₀においては次の(8)式となる。

ここで、γ_EDはドップラ幅と呼ばれ、次の(9)式で表される。

ｋ_Bはボルツマン定数、Ｔはガス温度、Ｍは特定ガスの分子量である。(9)式より、ドップラ幅γ_EDは、吸収周波数の中心周波数、分子量、及び温度、に依存しており、１[Torr]よりも高真空領域では、プロファイル関数φ（ν）は被測定ガスの圧力による影響を受けないことが分かる。 [2] When the total pressure of the gas to be measured is in a vacuum region higher than 1 [Torr] In this case, the profile function φ (ν) is a Gaussian function as shown in the following equation (7).

Especially at the center wavelength ν ₀ , the following equation (8) is obtained.

Here, γ _ED is called the Doppler width and is expressed by the following equation (9).

k _B is the Boltzmann constant, T is the gas temperature, and M is the molecular weight of the specific gas. From equation (9), the Doppler width γ _ED depends on the center frequency, molecular weight, and temperature of the absorption frequency, and the profile function φ (ν) is the gas to be measured in a vacuum region higher than 1 [Torr]. It can be seen that it is not affected by the pressure.

[3] In the case of an intermediate pressure between atmospheric pressure and 1 [Torr] In this case, the profile function φ (ν) is expressed by a convolution function of the Lorentz function and Gaussian function called a Voigt function.

The above is a concentration measurement method using spectral detection by direct absorption (hereinafter referred to as “direct absorption spectroscopy”). As a method effective when the concentration is very low, detection is performed at a frequency that is an integral multiple of the laser modulation wave. Spectroscopy (hereinafter referred to as “wavelength modulation spectroscopy”) is known (see, for example, Non-Patent Document 1). When performing wavelength modulation spectroscopy, the frequency of light irradiated to the gas to be measured is modulated. 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 (10).

When the laser light frequency-modulated by the equation (10) is detected at 2ω, which is twice that frequency, the extracted signal depends on the pressure of the gas to be measured. One of three types of relational expressions.
In moisture measurement at atmospheric pressure, the second harmonic detection signal signal (ν ₀ ) at the center frequency ν ₀ is defined by the following equation (11).

大気圧と１{Torr}との間の中間圧力では、上記直接吸収分光法と同様に、(11)式の関係式と(12)式の関係式との畳み込みとして表される。これらの関係式は非特許文献２で提案されている。波長変調分光法の場合のプロファイル関数φ'（ν₀）は上記直接吸収分光法のプロファイル関数φ（ν）と異なるが、被測定ガスの圧力に応じてプロファイル関数φ'（ν₀）が変わることは同様である。 Similarly, in the moisture measurement in a vacuum region higher than 1 {Torr}, the second harmonic detection signal signal (ν ₀ ) at the center frequency ν ₀ is defined by the following equation (12).

At an intermediate pressure between the atmospheric pressure and 1 {Torr}, it is expressed as a convolution of the relational expression (11) and the relational expression (12) as in the case of the direct absorption spectroscopy. These relational expressions are proposed in Non-Patent Document 2. The profile function φ ′ (ν ₀ ) in the case of wavelength modulation spectroscopy is different from the profile function φ (ν) of the direct absorption spectroscopy, but the profile function φ ′ (ν ₀ ) changes according to the pressure of the gas to be measured. The same is true.

Japanese Patent Laid-Open No. 5-99845

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, p. 3276 (1963)

  Conventionally, specific gas concentration measurement by the TDLAS method has been used for, for example, continuous monitoring of specific gases in the atmosphere (for example, carbon monoxide, nitrogen dioxide, etc.), continuous monitoring of moisture in semiconductor processing gases, etc. Then, measurement is generally performed under the condition that the pressure of the gas to be measured is approximately constant. On the other hand, when monitoring the specific residual gas in the vacuum chamber, it is necessary to measure the concentration of the specific gas under a situation where the pressure of the gas to be measured decreases suddenly and greatly from near atmospheric pressure. . In this case, it is necessary to change the profile function φ (ν) or φ ′ (ν) in accordance with the rapid pressure fluctuation of the gas to be measured, but the timing of the pressure value measurement by the pressure gauge and the measurement of the signal intensity of the absorbed light When a time lag occurs in timing, an appropriate profile function is not used, and the calculation accuracy of the specific gas concentration decreases.

  One of the major reasons for the time difference between the pressure value measurement timing by the pressure gauge and the measurement timing of the signal intensity of the absorbed light described above is the restriction on data processing hardware / software. In other words, if the pressure value is monitored at a high speed in parallel with the measurement of the absorbed light at a short time interval along with the wavelength scanning, it is necessary to perform parallel processing of data or greatly speed up the data processing system. Occurs. Therefore, the apparatus cost is inevitably increased. Further, a pressure gauge that is generally used has a relatively slow response, and it is necessary to adjust the pressure value measurement timing to the measurement timing of the absorbed light in consideration of the delay in the response, so that the data processing becomes more complicated. . Although pressure gauges with a fast response are available, such pressure gauges are expensive and expensive.

  The present invention has been made to solve the above-described problems, and the object of the present invention is to suppress an increase in the cost of a data processing system and the like, even under a situation where the pressure of the gas to be measured changes suddenly and greatly. An object of the present invention is to provide a laser absorption type gas analyzer capable of measuring a specific gas concentration with high accuracy.

The present invention made to solve the above problems comprises a sample cell into which a gas to be measured is introduced, and a laser irradiation unit and a light receiving unit arranged outside the sample cell. In the gas analyzer that detects the detected laser beam after passing the emitted laser beam through the gas to be measured in the sample cell and calculates the concentration of the specific gas contained in the gas to be measured based on the detection signal.
a) pressure detecting means for detecting the pressure of the gas to be measured in the sample cell;
b) The pressure detection at a plurality of time points each including at least one time point before the time point when the detection signal is acquired by the light receiving unit and a time point after the time point when the detection signal is acquired. Pressure estimation means for obtaining a pressure value by means, and estimating a pressure value at the time of obtaining the detection signal based on the plurality of obtained pressure values;
c) correction calculation means for correcting the pressure dependence of light absorption when calculating the concentration of the specific gas based on the detection signal using the pressure value estimated by the pressure estimation means;
It is characterized by having.

  As one aspect of the gas analyzer according to the present invention, the frequency modulation means for modulating the laser beam emitted from the laser irradiation unit with the frequency f, and the detection signal from the light receiving unit with a frequency that is an integral multiple of the frequency f. Synchronization detecting means for detecting synchronously, and gas concentration calculating means for calculating the concentration of the specific gas based on the synchronous detection result, and the correction calculating means when calculating the specific gas concentration by the gas concentration calculating means Thus, the pressure dependency of light absorption can be corrected. That is, the gas analyzer of this aspect obtains the concentration of a specific gas by wavelength modulation spectroscopy. Of course, the gas analyzer according to the present invention can also be applied to the case where the concentration of a specific gas is obtained by direct absorption detection, but according to wavelength modulation spectroscopy, a trace amount of gas component is measured with high accuracy. be able to.

  In the gas analyzer according to the present invention, the pressure estimating means uses a predetermined calculation formula or a table corresponding thereto, and obtains a detection signal corresponding to the absorption of the laser beam by the specific gas component. The pressure value at the time when the detection signal is acquired is estimated from the pressure values obtained at the previous time point and the later time point. If the pressure detection means has a response delay, the above calculation formula or information corresponding thereto may be determined in consideration of the response delay. If the number of pressure values actually acquired by the pressure detecting means is large, the estimation accuracy of the pressure value is increased accordingly.

  Specifically, the profile function of the absorption line is used when calculating the concentration (amount) of the component from the height of the absorption peak due to the specific gas component, and this profile function has pressure dependency. Therefore, the correction calculation means can perform correction by changing the profile function according to the estimated pressure value.

  According to the gas analyzer of the present invention, the pressure value of the gas to be measured at the time when the signal intensity of the absorbed light is acquired can be known with high accuracy, whereby the pressure of the gas to be measured is rapidly and greatly increased. Even under changing circumstances, the concentration of the specific gas component can be accurately calculated. In addition, data processing such as reading of the pressure value detected by the pressure detection means and data processing such as reading of the detection signal obtained by the light receiving unit do not need to be performed in parallel. It is not necessary to make it more cost-effective and the cost increase can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS The schematic block diagram of the measurement optical system of the moisture measuring apparatus which is one Example of the gas analyzer which concerns on this invention. The schematic block diagram of the signal processing system and control system of the moisture measuring apparatus of 1st Example. The figure which shows an example of the signal waveform obtained by the direct absorption spectroscopy calculated | required by calculation. The graph which shows the relationship between the to-be-measured gas pressure and the magnitude | size of the value (phi) ((nu) ₀ ) in the absorption line center of a profile function. The figure which shows an example of the estimation method of a pressure value. The schematic block diagram of the signal processing system and control system of the moisture measuring apparatus of 2nd Example. The figure which shows an example of the signal waveform obtained by the wavelength modulation spectroscopy calculated | required by calculation.

[First embodiment]
A moisture measuring apparatus which is a first embodiment of a gas analyzer according to the present invention will be described with reference to FIGS. The apparatus of this embodiment measures the concentration of moisture in the gas to be measured by direct absorption spectroscopy, but other gas concentrations can be measured as long as the gas absorbs laser light at a specific wavelength. is there. FIG. 1 is a schematic configuration diagram of a measurement optical system of a moisture measuring apparatus according to this embodiment, and FIG. 2 is a schematic block diagram of a signal processing system and a control system in the moisture measuring apparatus.

  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 gas to be measured 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, a DFB (Distributed Feedback) type laser that oscillates at 1.3 [μm] in which water molecules have an absorption spectrum can be used. Other than this, any wavelength tunable laser that oscillates at a wavelength at which an absorption spectrum of water molecules exists can be used. Of course, when measuring the concentration of another specific gas instead of water, a wavelength tunable laser that oscillates at a wavelength at which the absorption spectrum of the specific gas exists may 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.

  A pressure gauge 12 is attached to the sample cell 1, and the pressure of the gas to be measured can be measured by the pressure gauge 12. As the pressure gauge 12, one corresponding mainly to a measurement range from approximately 1 [Torr] to approximately atmospheric pressure is suitable. Here, as the pressure gauge 12, a diaphragm type pressure transducer that outputs a voltage of 0 to 5 [V] in an absolute pressure range of 0 to 760 [Torr] is used.

  In FIG. 1, laser light L <b> 1 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 reflected between the reflecting mirrors 3 and 4. repeat. In the optical path illustrated 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 gas to be measured. The laser beam L2 after being absorbed in this way returns to the optical chamber 6 through the transparent window 5 and reaches the light detection unit 8. The electrical signal extracted by the light detection unit 8 is input to the signal processing unit 11.

  As shown in FIG. 2, the voltage signal obtained by the light detection unit 8 is amplified by an amplifier 21, and then a noise component is removed by a low-pass filter (LPF) 25 and input to an analog / digital converter (ADC) 26. Is done. The output of the ADC 26 is input to the calculation unit 28, more specifically, to the concentration calculation unit 281 for calculating the gas concentration.

  The LD wavelength scanning voltage generator 37 includes a digital / analog converter, and outputs digital data for repeating sweeping over a predetermined wavelength region near the absorption spectrum of water molecules, which is output from the controller 29, as a sweep voltage. Convert and output. This 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. Thereby, the wavelength tunable laser device 7 emits the laser light L1 whose wavelength is repeatedly changed in a predetermined wavelength range with time.

  The pressure of the gas to be measured in the sample cell 1 is converted into a voltage signal by the pressure gauge 12, and the voltage signal is converted into a digital value by the ADC 27 and input to the calculation unit 28, more specifically, the pressure estimation unit 282 during measurement. . The measurement pressure estimation unit 282 estimates the pressure of the gas to be measured at the time of detection signal acquisition for calculating the gas concentration as will be described later. The pressure correction data storage unit 283 sends a profile function value to the concentration calculation unit 281 as correction data corresponding to the estimated pressure value.

  The principle and operation of measuring the amount of moisture in the gas to be measured in the moisture measuring device of this embodiment will be described.

When the water content is measured by the TDLAS method, the incident light intensity I (ν ₀ ) and transmitted light intensity I ₀ (ν ₀ ) at the center wavelength ν ₀ of the absorption spectrum are expressed as shown in the above equation (4). Need to be measured.

FIG. 3 shows an example of a signal waveform obtained by direct absorption spectroscopy obtained by calculation (simulation). The horizontal axis represents frequency (wavelength) deviation ν−ν ₀ , and the vertical axis represents signal intensity. A peak at which the signal intensity greatly decreases due to absorption by water molecules is observed near zero frequency deviation, that is, around the center frequency ν ₀ . I (ν ₀ ) is the signal intensity at the peak top of the absorption peak obtained by the light detection unit 8, and I ₀ (ν ₀ ) is the signal intensity when it is assumed that there is no absorption by water molecules. As shown in FIG. 3, the latter can be obtained by drawing an approximate curve from the signal intensity of the non-absorbing region. That is, both I (ν ₀ ) and I ₀ (ν ₀ ) are obtained from the detection signal obtained by the light detection unit 8 when scanning is performed in a predetermined frequency range centered on the frequency ν ₀ . If I ₀ (ν ₀ ) and I (ν ₀ ) are obtained in this way, the value of S in equation (4) can be easily obtained from the HITRAN database, and L is known as the optical path length. If the profile function φ (ν ₀ ) is known, the volume concentration c of water molecules can be calculated.

4, the relationship between the magnitude of the value at the absorption line center of pressure and the profile function of the measured gas φ (ν _0), the value of φ (ν ₀₎ in 760 [Torr] in the graph shown as 1 is there. Here, the type and temperature of the gas are assumed to be constant. As can be seen from FIG. 4, φ (ν ₀ ) changes greatly when the gas pressure to be measured is in the range of atmospheric pressure to 1 [Torr]. Also, it can be seen that in a higher vacuum region than 1 [Torr], since the function does not depend on the pressure as shown in the equations (8) and (9), it is constant. In general, in a vacuum device, the pressure changes between about atmospheric pressure and about 1 [Torr] in a very short time, so even a slight difference between the pressure measurement timing and the absorbance measurement timing can be used to calculate the concentration. It can be understood that the results are greatly affected.

In the moisture measuring apparatus of this embodiment, the relationship between the gas pressure to be measured and the profile function value at the absorption center wavelength ν ₀ as shown in FIG. 4 is calculated in advance and stored in a database, and this is stored in the pressure correction data storage unit 283. Store it. Thus, when a pressure value is input to the pressure correction data storage unit 283, a profile function value φ (ν ₀ ) corresponding to the pressure value is immediately output. In addition, since the profile function value is different if the type of the gas component that is the concentration measurement target is different, the data stored in the pressure correction data storage unit 283 is also different.

FIG. 5 is a schematic diagram showing the timing of measurement. In the moisture measuring apparatus of the present embodiment, the pressure of the gas to be measured is first measured prior to the absorption measurement accompanying the wavelength scanning as described above. That is, at time T 1, the pressure value X 1 obtained by digitizing the voltage signal from the pressure gauge 12 by the ADC 27 is taken into the measurement time pressure estimation unit 282. Subsequently, the detection signal amplified by the amplifier 21 corresponding to the wavelength scanning of the laser light over a predetermined wavelength range is taken into the concentration calculation unit 281 through the ADC 26. The shape of the detection signal obtained along with the wavelength scanning at this time is as shown in FIG. 3, and this signal is digitized at a predetermined sampling time interval and taken into the density calculation unit 281. The density calculation unit 281 acquires I (ν ₀ ) and I ₀ (ν ₀ ) based on the captured data. The acquisition timing of I (ν ₀ ) and I ₀ (ν ₀ ) is a time T2 when the wavelength becomes ν ₀ during the wavelength scanning. Since the start, end, and scanning speed of the wavelength scanning are controlled by the control unit 29, the control unit 29 can easily obtain the time T2.

After the absorption measurement associated with the wavelength scanning is completed, the pressure value X2 obtained by digitizing the voltage signal from the pressure gauge 12 by the ADC 27 is taken into the measurement pressure estimation unit 282 again at time T3. Since the pressure value is not measured during the wavelength scanning, the pressure value at time T2 is unknown. Therefore, the measurement pressure estimation unit 282 estimates the measured gas pressure value Xadj at times T1, T2, and T3 and the actually measured pressure values X1 and X2. For example, if the response delay in detection by the pressure gauge 12 can be ignored, the simplest estimation method is by linear interpolation according to the following equation (13).

If the estimated pressure value Xadj can be calculated as described above, the estimated pressure value Xadj is input to the pressure correction data storage unit 283, and the profile function value φ (ν ₀ ) is acquired as an output. Density arithmetic unit 281, the profile function value φ (ν ₀₎ and I (ν ₀₎ obtained by measurement based on the estimated pressure value Xadj thus obtained and I ₀ (ν ₀₎ and used further as described above The volume concentration c of water molecules is calculated from the equation (4) using S and L obtained from the above equation. Thereby, it is possible to calculate an accurate water molecule volume concentration reflecting the pressure of the gas to be measured at the timing when the peak top of the absorption peak appears in the detection signal.

  In the above description, Xadj is estimated based only on the measured pressure values at two points (time T1, T3), but fitting to a higher-order interpolation curve using measured pressure values at three or more points. By performing the above, it is possible to improve the estimation accuracy of the pressure value.

  If the response delay time detected by the pressure gauge 12 is so long that it cannot be ignored, a correction formula may be determined in consideration of this response delay time Δt in advance. More specifically, in the example of FIG. 5, assuming that the pressure value data acquired at times T1 and T3 is the pressure of the gas to be measured at the time of ΔT from T1 and T3, respectively, the equation (13) can be rewritten. Good. Since ΔT in this case is determined by the characteristics of the pressure gauge, it can be obtained by preliminary measurement.

[Second Embodiment]
Next, a moisture measuring apparatus according to a second embodiment of the present invention will be described with reference to FIGS. The apparatus of this embodiment measures the concentration of moisture in the gas to be measured by wavelength modulation spectroscopy. FIG. 6 is a schematic block diagram of a signal processing system and a control system in the moisture measuring apparatus of the present embodiment, and FIG. 7 is an example of a signal waveform obtained by wavelength modulation spectroscopy obtained by calculation (simulation). In FIG. 6, the same components as those in the first embodiment are denoted by the same reference numerals.

In this moisture content measuring device, a voltage signal obtained by the optical detector 8 after being amplified by the amplifier 21 is input to a synchronization detector 2 2. 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 the LPF 23 and converted into a digital signal by the ADC 24. The output of the ADC 24 is input to the concentration calculation unit 284 of the calculation unit 28.

  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. Under the control of the control unit 29 ′, the 2f clock generation unit 31 generates a clock signal having a frequency 2f, and the frequency divider 32 divides the clock signal by two to thereby divide the clock signal by 2 and the duty ratio is A clock signal that is 50% is generated. The modulation amplitude control voltage generator 33 includes a digital / analog converter that converts digital data supplied from the controller 29 '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 the digital data output from the control unit 29 'to sweep data over a predetermined wavelength region near the absorption spectrum of water molecules. Convert 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 the voltage / current converter 39 and supplied to the wavelength tunable laser device 7 as a drive current. As a result, the wavelength tunable laser device 7 emits a laser beam L1 whose wavelength changes with time and frequency-modulated with a predetermined modulation amplitude.

During such wavelength scanning, the output of the synchronization detector 22, that is, the 2f synchronization detection signal has a shape as shown in FIG. The frequency deviation is zero, that is, the signal intensity at the center frequency ν ₀ indicates the strength of absorption by water molecules, but the actual synchronization detection signal is the sum of the positive direction peak and the negative direction peak. It is difficult to separate above. 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 (11) and (12). The proportional constant B that defines this proportional relationship is theoretically 1 if the signal of the frequency 2f used in the synchronous detection (the output signal of the 2f clock generator 31) is a perfect sine wave. Even if the 2f signal is not a sine wave but a rectangular wave, it can be experimentally obtained in advance. When the density calculation unit 281 receives the synchronization detection data through the ADC 24 in accordance with the wavelength scanning, the density calculation unit 281 calculates the signal intensity SIG of the peak-to-peak signal waveform and calculates the signal (ν ₀ ) using the proportionality constant B.

Although not explicitly shown in the present embodiment, it is necessary to acquire I ₀ (ν ₀ ) separately. For this purpose, the same configuration of the detection unit as in the first embodiment may be added in parallel with the synchronous detector 22, the LPF 23, and the ADC 24, and it is stable to the extent that fluctuations in I ₀ do not affect the actual measurement. In some cases, the density calculation can be performed by regarding I ₀ (ν ₀ ) as a fixed value in advance.

In the case of wavelength modulation spectroscopy, although the profile function value φ ′ (ν ₀ ) at the center wavelength ν ₀ is different from the direct absorption spectroscopy of the first embodiment, the pressure of the gas to be measured is still from atmospheric pressure to 1 [ In the range of Torr], φ ′ (ν ₀ ) varies greatly. Therefore, the relationship between the pressure and φ ′ (ν ₀ ) is calculated in advance and stored in a database, and this is stored in the pressure correction data storage unit 285. Thus, when a pressure value is input to the pressure correction data storage unit 285, a profile function value φ ′ (ν ₀ ) corresponding to the pressure value is immediately output.

As in the first embodiment, the measured pressure value is obtained at least one point before and after the absorption measurement associated with the wavelength scanning, and the measurement time pressure estimation unit 282 obtains a signal for the center wavelength ν _{0 by the} wavelength scanning. The pressure value Xadj is estimated, the estimated pressure value Xadj is input to the pressure correction data storage unit 285, and the profile function value φ ′ (ν ₀ ) is acquired as an output. The concentration calculation unit 284 uses the profile function value φ ′ (ν ₀ ) based on the estimated pressure value Xadj thus obtained and the Singnal (ν ₀ ) obtained based on the actual measurement, and further obtains S, Using L, the volume concentration c of water molecules is calculated by the equation (11). Thereby, it is possible to calculate an accurate water molecule volume concentration reflecting the pressure of the gas to be measured at the timing when the peak top appears in the synchronous detection signal.

  It should be noted that any of the above-described embodiments is an example of the present invention, and it is obvious that modifications, corrections, additions, and the like as appropriate within the scope of the present invention are included in the scope of the claims of the present application.

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 12 ... Pressure gauge 21 ... Amplifier 22 ... Synchronization detector 23, 25 ... LPF
24, 26, 27 ... ADC
28 ... calculation units 281, 284 ... concentration calculation unit 282 ... measurement pressure estimation unit 283,285 ... pressure correction data storage unit 29, 29 '... control unit 30 ... sine wave generator 31 ... 2f clock generation unit 32 ... frequency division Unit 33 ... Modulation amplitude control voltage generator 34 ... Multiplier 35 ... BPF
36 ... Phase shifter 37 ... LD wavelength scanning voltage generator 38 ... Adder 39 ... Voltage / current converter

Claims (2)

A sample cell into which a gas to be measured is introduced and a laser irradiation unit and a light receiving unit arranged outside the sample cell are provided, and laser light emitted from the laser irradiation unit passes through the gas to be measured in the sample cell. In the gas analyzer that detects the concentration of the specific gas contained in the gas to be measured based on the detection signal after the detection by the light receiving unit,
a) pressure detecting means for detecting the pressure of the gas to be measured in the sample cell;
b) The pressure detection at a plurality of time points each including at least one time point before the time point when the detection signal is acquired by the light receiving unit and a time point after the time point when the detection signal is acquired. Pressure estimation means for obtaining a pressure value by means, and estimating a pressure value at the time of obtaining the detection signal based on the plurality of obtained pressure values;
c) correction calculation means for correcting the pressure dependence of light absorption when calculating the concentration of the specific gas based on the detection signal using the pressure value estimated by the pressure estimation means;
A gas analyzer comprising: The gas analyzer according to claim 1,
Frequency modulation means for modulating the laser light emitted from the laser irradiation section at a frequency f, synchronization detection means for synchronously detecting a detection signal from the light receiving section at an integer multiple of the frequency f, and the synchronization detection result Gas concentration calculating means for calculating the concentration of the specific gas based on the above, and correcting the pressure dependence of light absorption by the correction calculating means when calculating the specific gas concentration by the gas concentration calculating means. Characteristic gas analyzer.

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