JP2844506B2 - Gas concentration measuring method and device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas concentration measuring method and apparatus suitable for accurately measuring the concentration of a mixed gas composed of a plurality of component gases, particularly a mixed gas of stable isotopes.

[0002]

BACKGROUND OF THE INVENTION Numerous stable isotopes have been conventionally known, C12 methane ⁽¹² C, a recently stable isotopes of carbon
H ₄ ) and C13 methane ( ¹³ CH ₄ ) have unique uses, respectively. In particular, ¹³ CH ₄ has attracted attention because it can be separated and concentrated from LNG (liquefied natural gas) by low-temperature precision distillation. I have.

[0003] Diamond films of ¹² CH ₄ of high purity as a raw material applications as about 50% than the conventional diamond thin film in various fields as a new material for thermal conductivity is excellent is expected, whereas the mass number 13 Since carbon performs nuclear magnetic resonance, it is expected to be applied to the life science field such as nondestructive examination and diagnosis of living bodies by tracking its behavior and distribution as a labeled compound.

Since such stable isotopes have the same chemical reaction, chemical methods cannot be used to separate a plurality of stable isotopes or to measure their component ratios or concentrations. A method utilizing the difference in properties is used.

[0005] One example is a stable isotope of carbon.
^A description will be given by taking ¹² CH ₄ and ¹³ CH ₄ as examples.

[0006]

Conventionally, ¹² CH ₄ and ¹³ C
GC-MS (GasChr for quantitatively analyzing a gaseous mixture of H ₄
(omatography and Mass Spectrometry) method and a laser method.

In the GC-MS method, a gaseous sample is ionized by electron bombardment or the like, the ions are separated according to m / e (m is ion mass, e is ion charge), the amount of each ion is recorded, and these ions are recorded. Are arranged in the order of m / e to produce a mass spectrum. Since the intensity of each spectral line and its m / e are unique to each substance, when a spectrum of a single component substance is obtained, it is identified by collating it with a known data collection, a spectrum library, or the like. When measuring high purity methane gas in this GC-MS method, it may be contaminated with air during methane-on and increasing the charging amount detector at low concentrations reasons from ¹³ CH _4, such as may saturate Lack of accuracy (at most 0.9-1.3%). In addition, the dispersion of the measured values is large. There is also a method of converting methane to CO or CO ₂ having a small number of atoms for measurement in order to increase measurement accuracy. However, a converter is required for that, and the measurement device becomes expensive.

On the other hand, in the laser method, a laser beam is passed through a gas whose concentration is to be measured, and the concentration is measured by observing the absorbance at an absorption line peculiar to the gas.
Since the oscillation wavelength of a G laser or the like is limited, a plurality of lasers are required when targeting a plurality of gases.
Therefore, there are semiconductor lasers and dye lasers as wavelength tunable lasers. However, if the wavelength is swept and the degree of absorption is checked, accuracy cannot be obtained, and if the wavelength is adjusted to the absorption line, the semiconductor laser will change with time. In the case of a dye laser, alignment of a birefringent filter or the like is troublesome.

The present invention has been made in view of the above points, and is intended to accurately measure the concentration of a mixed gas of stable isotopes such as ¹² CH ₄ and ¹³ CH ₄ using a semiconductor laser with an inexpensive facility. Aim.

[0010]

In order to achieve the above object, in the concentration measuring method of the present invention, a laser beam is transmitted through a reference gas while sequentially changing the oscillation wavelength of a semiconductor laser prior to concentration measurement, The operating conditions of the semiconductor laser when the absorption of the laser light by the component gas of the reference gas becomes a peak, and the signal intensity obtained by fundamental phase sensitive detection of the signal obtained by photoelectrically converting the transmitted laser light are stored,
Thereafter, the semiconductor laser is driven based on the operating conditions and the signal intensity.

Further, in the concentration measuring apparatus of the present invention, prior to the concentration measurement, the laser beam is transmitted through the reference gas while sequentially changing the oscillation wavelength of the semiconductor laser within a predetermined range. There is provided storage means for storing at least the operating conditions of the semiconductor laser when light absorption reaches a peak and the intensity of a signal output from the second phase-sensitive detector.

[0012]

According to the gas concentration measuring method of the present invention, the oscillation wavelength of the semiconductor laser at the time of measurement is measured by using the terminal voltage of the semiconductor laser at the peak and the fundamental wave sensitive detection signal of the phase sensitive detector stored before the measurement. Is controlled.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of an apparatus for performing a gas concentration measuring method according to the present invention. In this example, the concentration of a mixed gas of ¹² CH ₄ and ¹³ CH ₄ , which are stable isotopes, is measured.

Reference numeral 1 denotes a laser oscillation unit including a semiconductor laser and a reference cell. The laser oscillation unit emits a 1.66 μm laser beam for concentration measurement from the semiconductor laser. The detailed structure will be described later with reference to FIG.

Reference numeral 2 denotes a detection unit provided with a test cell 2a into which a reference gas having a known concentration and a gas having an unknown concentration (referred to as a test gas) whose concentration is to be measured are described in detail later with reference to FIG.

Reference numeral 3 denotes a solenoid valve unit for adjusting the pressure of the reference gas or the test gas in the test cell 2a, the details of which will be described later with reference to FIG.

Reference numeral 4 denotes a laser driver for driving the semiconductor laser 1a by the output of a parallel output board 15 to be described later, starting and stopping the output of a bias current, and a laser driver for externally controlling a lower Peltier element 1f to be described later. A sine wave voltage for a current is applied to a current mixer 6 described later, and a phase-sensitive detector 8 described later
A programmable oscillator transmitting a synchronization signal to 9 and 11;
Reference numeral 6 denotes a current mixer which mixes a bias current from the laser driver 4 and a sine wave voltage from the programmable oscillator 5 and supplies the mixed voltage to the semiconductor laser 1a. Reference numeral 7 denotes a terminal voltage of the semiconductor laser 1a. This is a digital multimeter that outputs a measured value to the GP-IB board 12 by a signal from the board 12, and roughly adjusts the measured value at the peak of the gas absorption line when stabilizing the oscillation wavelength of the semiconductor laser 1a. It is used when performing various controls.

Reference numeral 8 denotes phase-sensitive detection of the intensity of the laser beam transmitted through the reference cell 1c, and converts the fundamental-phase detection signal to A.
A phase-sensitive detector for outputting to the / D converter 13;
This is used when precise control is performed to stabilize the oscillation wavelength of the semiconductor laser 1a so as to obtain a measured value at the peak of the gas absorption line.

Reference numeral 9 denotes phase-sensitive detection of the intensity of the laser beam transmitted through the reference cell 1c, and outputs the second-order phase-sensitive detection signal to A
A phase-sensitive detector for outputting to the / D converter 13;
The second-harmonic phase-sensitive detection signal output from the phase-sensitive detector 9 is used to detect the peak of the gas absorption line during the wavelength sweep. The fundamental phase phase sensitive detection signal at the peak and the semiconductor laser 1a measured by the digital multimeter 7
And the set current of the lower Peltier element 1f are stored in the memory 17.

Reference numeral 10 denotes a D / A in which the difference between the fundamental phase sensitive detection signal of the phase sensitive detector 8 and the fundamental phase sensitive signal at the peak is used as a control signal when stabilizing the oscillation wavelength of the semiconductor laser 1a. A programmable power supply 11 sends out a current to be supplied to the upper Peltier element 1e according to a set voltage from the converter 14. A phase-sensitive detection unit 11 detects the intensity of the laser beam transmitted through the test cell 2a, and outputs the second harmonic phase-sensitive detection signal. A phase-sensitive detector for outputting to the A / D converter 13 is a GP-IB board 12 for performing communication between the CPU 16 and a programmable oscillator 5, a digital meter 7, and phase-sensitive detectors 8, 9, and 11 using ASCII codes. , 13 convert analog outputs from the phase-sensitive detectors 8, 9, 11 and a pressure gauge 2c of the detection unit 2 described later into CPs.
An A / D converter 14 converts a digital signal from the U16 into a digital signal according to an instruction from the U16 and sends it to the CPU 16, and converts a digital signal from the CPU 16 into an analog signal to set a bias current for the semiconductor laser 1a. A D / A converter that sends a set voltage to the laser driver 4 that sends the control signal, a D / A converter that sends a set voltage to the programmable power supply 10 that sends the control current supplied to the upper Peltier element 1e, and 15 is a solenoid valve unit 3 that receives a signal from the CPU 16 A parallel output board for opening / closing of the semiconductor laser 1a, external control of the bias current of the semiconductor laser 1a, external setting of the current of the lower Peltier element 1f, and the like.

Reference numeral 17 denotes a peak obtained as a result of a wavelength sweep described later.
Terminal voltage of semiconductor laser 1a at the time of leakage, phase sensitive detector
8, the current flowing to the lower Peltier element 1f at the peak time
The output of the phase sensitive detector 11 at the time of measuring the value and the concentration is stored.
Memory, 18¹² C_H4When¹³ CH₄About each of
Modulation amplitude value and temperature range in which wavelength sweep is performed (2 ° C. to 1
8 ° C), the number N of data to be sampled during the wavelength sweep
= 500, a memo that stores the calculation formula for calculating the concentration
And 19, the pressure in the test cell 2a detected by the pressure gauge 2c.
And a display such as a CRT for displaying the calculated concentration.
This is a printer for printing density.

Next, the laser oscillation unit 1, the detection unit 2, and the solenoid valve unit 3 will be described in detail with reference to FIG.

The laser oscillation unit 1 uses the former absorption line 2ν ₃ band R (0) and the latter absorption line 2ν for ¹² CH ₄ and ¹³ CH ₄ whose concentration is to be measured in this embodiment.
1.66 μm selected to oscillate _3- band R (2)
Semiconductor laser 1 that oscillates forward and backward laser beams
a, the laser light is reflected at the end face of the test cell 2a, and the reflected light is transmitted to the semiconductor laser 1a to prevent the oscillation wavelength from fluctuating. The isolator 1b and a high-purity gas (referred to as a reference gas) having the same components as the gas to be measured
A reference cell 1c which is sealed at 70 Torr, makes the laser light emitted backward from the semiconductor laser 1a parallel at the incident end face, transmits the reference gas encapsulated therein, and then condenses it on the photodetector 1d at the emission end face; The photodetector 1 includes a pin photodiode that receives the laser beam transmitted through the reference cell 1c and converts the current, and a preamplifier that performs amplification and voltage conversion.
d, an upper Peltier element 1e provided so as to be capable of transferring heat to the semiconductor laser 1a and precisely controlling the temperature of the semiconductor laser 1a by supplying current from the programmable power supply 10 when the oscillation wavelength of the semiconductor laser 1a is stabilized.
The lower Peltier element 1f, which is thermally coupled to the upper Peltier element 1e and performs temperature control for sweeping the wavelength or roughly stabilizing the wavelength of the semiconductor laser 1a, and the approximate temperature of the semiconductor laser 1a. 1g of thermistor to be measured
It is composed of

The detection unit 2 controls a solenoid valve of a solenoid valve unit 3 to be described later, a test cell 2a for introducing a reference gas or a test gas, and a pin for receiving a laser beam transmitted through the test cell 2a and converting the current to a current. A photodetector 2b composed of a photodiode and a preamplifier for amplification and voltage conversion, and a pressure in the test cell 2a of 0 to 1000
And a pressure gauge 2c that outputs Torr at 0 to 1V.

The solenoid valve unit 3 is a parallel output board 1
The reference gas is supplied from a reference gas cylinder 3a.
In the figure, a solenoid valve V1 is connected to a test gas source,
5, V6 and V7 are connected to a vacuum pump.

Next, using the gas concentration measuring device having the above-described structure,
The operation for measuring the concentration of the mixed gas of ¹² CH ₄ and ¹³ CH ₄ will be described with reference to the flowcharts shown in FIGS.

FIG. 3 is a flowchart showing the entire density measuring operation.

At the start of the test, the currents of the GP-IB board 12, the upper Peltier element 1e and the lower Peltier element 1f, the programmable oscillator 5, the phase sensitive detectors 8, 9, and 11, and the semiconductor laser 1a are controlled by a command from the CPU 16.
(F-1), as a result, the programmable power supply 1 is connected to the upper Peltier element 1e.
A set current is supplied from 0, and a drive current is supplied from the laser driver 4 to the lower Peltier element 1f so as to reach 18 ° C. at which a later-described sweep starts. The phase-sensitive detectors 8, 9, and 11 are set to a predetermined frequency and a predetermined phase, and a bias current is supplied from the laser driver 4 to the semiconductor laser 1a.

When the oscillation of the laser beam by the semiconductor laser 1a is stabilized, the CPU 16 determines whether or not the terminal voltage of the semiconductor laser 1a measured by the digital multimeter 7 is 1 V or more (F-2). This is to know whether or not the semiconductor laser 1a has started normally.
If the terminal voltage is less than 1 V, it is determined that the measurement operation is abnormal, and the measurement operation is terminated (for example, the supply of the modulation current is stopped, or the upper Peltier device 1a or the lower Peltier device 1 is used).
f supply current to zero, etc.)
3) Check the terminal voltage of the semiconductor laser 1a again (F-
4) If less than 1 V, display an error. If more than 1 V, after the measurement is completed, print out the measurement results up to that time (F-6). Then, supply of bias current from laser driver 4 is stopped. The GP-IB end processing is performed and the measurement operation ends (F-7).

If it is determined in step (F-2) that the terminal voltage of the semiconductor laser 1a is 1 V or more, the GP-IB board 12 receives a command from the CPU 16 on the assumption that the semiconductor laser 1a has started normally. The sine voltage oscillated by the programmable oscillator 5 based on the command is superimposed on the bias current from the laser driver 4 in the current mixer 6 so that the semiconductor laser 1 a
(F-8).

Next, a command from the CPU 16 is received by the parallel output board 15, and the solenoid valves V4, V
5, V6 and V7 are released to remove residual gas and the like in the test cell 2a (test cell purge) (F-9).

Here, it is determined whether or not the concentration measurement has been completed by checking whether or not a measurement end button provided on the apparatus has been pressed (F-10). If the measurement is completed, the process proceeds to the above-described step (F-3). If the measurement is completed, the wavelength sweep operation is started (F-11).

The wavelength sweep is to gradually lower the temperature of the lower Peltier device 1f prior to actual measurement of the reference gas sealed in the reference cell 1c in order to correct the influence of the aging of the semiconductor laser 1a. Sweeps the oscillation wavelength of the semiconductor laser 1a, and detects the terminal voltage of the semiconductor laser 1a at the peak when the maximum absorption is obtained for the two types of gases ¹² CH ₄ and ¹³ CH ₄ to be measured, and the phase sensitive detection. Output of the device 8 and the lower Peltier element 1f
Is stored in the memory 17.

Next, the measurement of ¹³ CH ₄ (F-12) and the measurement of ¹² CH ₄ (F-13) are performed using the results of the wavelength sweep, and finally the concentration is calculated using the results of both measurements (F-13). F-14).

The above is the outline of the concentration measuring operation according to the present invention. Here, the wavelength sweeping operation which is important in the method of the present invention will be described with reference to FIGS.

FIG. 4 shows the outputs of the phase sensitive detectors 8 and 9 for ¹² CH ₄ and ¹³ CH ₄ , where A is the output of the phase sensitive detector 8, ie, the fundamental phase sensitive signal, and B is the phase sensitive detector. 4 shows an output of the sensitive detector 9, that is, a second-harmonic phase sensitive detection signal.

The CPU 16 sets the sweep temperature range to a sweep temperature range (2 ° C. to 18 ° C.) stored in the memory 18 in advance, and the number of samples N (= 500) within this range.
Is set (T-1). However, the initial value is 18 ° C.

First, the initial value of N is set to 1, the maximum output value MAX of the phase sensitive detector 9 is set to 0 (T-2), and the laser driver 4 controls the current supplied to the lower Peltier element 1f to control its temperature. Is gradually lowered from 18 ° C. (T-3). First, the lower Peltier element 1f at N = 1
, The outputs of the phase-sensitive detectors 8 and 9, and the terminal voltage of the semiconductor laser 1a are temporarily stored in a register in the CPU 16 (T-4).

Now, paying attention to the output of the phase sensitive detector 9, N
The output when N = 1 is compared with the maximum output value MAX determined in step (T-2) (T-5). When the output when N = 1 is larger, the output of the phase sensitive detector 9 is compared. Is temporarily stored in a register in the CPU 16 as MAX, and the current value flowing through the lower Peltier element 1f, the output of the phase sensitive detector 8, and the terminal voltage of the semiconductor laser 1a at that time are stored in the memory 17 ( T-7). If the output when N = 1 is smaller, MAX = 0 is not changed and N = 2 (T−
9) Steps (T-3) and (T) until N becomes 250
-4), (T-5), (T-6), (T-7), (T-
9) is repeated, and only when the output of the phase sensitive detector 9 is larger than the maximum output value MAX up to that point, the current value flowing through the lower Peltier element 1f, the output of the phase sensitive detector 8, and the terminal voltage of the semiconductor laser 1a. Is stored in the memory 17. Therefore, the terminal voltage of the semiconductor laser 1a when the output of the phase sensitive detector 9 has become the maximum, the current value flowing through the lower Peltier element 1f, and the output of the phase sensitive detector 8 are always stored in the memory 17. It will be.

When N = 251 (T-1)
0), the modulation amplitude value for ¹³ CH ₄ stored in the memory 18 is read out, and the output of the current mixer 6 is controlled via the programmable oscillator 5 to change the amplitude of the modulation current and MAX = 0 again. (T-11). N
Is greater than 251 until 500 (T-12)
The operations from steps (T-3) to (T-9) are repeated with the current value of which the modulation amplitude value has been changed, and the maximum output value of the phase sensitive detector 9 is always the maximum output voltage of the semiconductor laser 1a with respect to MAX, and the lower Peltier. The current flowing through the element 1f and the output of the phase sensitive detector 8 are stored in the memory 17 (T-
8).

This completes the sweep operation.

Next, measurement of ¹² CH ₄ and ¹³ CH ₄ will be described. Since the procedure is exactly the same, only measurement of ¹³ CH ₄ will be described.

First, the modulation amplitude value of ¹³ CH ₄ stored in the memory 18 is read, and the amplitude of the modulation current to be superimposed on the drive current of the semiconductor laser 1a flowing from the current mixer 6 is set (S-1).

On the other hand, the value of the current flowing through the lower Peltier element 1f at the peak of ¹³ CH ₄ obtained as a result of the wavelength sweep is stored in the memory 17, and the value is read out. Is controlled so as to have the current value so that the temperature becomes near the peak (S-2). Thus, the semiconductor laser 1a
Wait until the device temperature becomes such that the oscillation wavelength of the device becomes near the R (2) line of ¹³ CH ₄ .

Next, by controlling the current supplied from the laser driver 4 to the lower Peltier element 1f on the basis of the terminal voltage of the semiconductor laser 1a measured by the digital multimeter 7, the terminal voltage becomes ± 0. 00
Control is performed so as to be within 15 V (S-3).

After that, the current supplied from the programmable power supply 10 to the upper Peltier element 1e is controlled based on the output of the photodetector 1d, and the output value of the phase sensitive detector 8 is stored in the memory 17 at the peak phase. The terminal voltage of the semiconductor laser 1a is controlled to be equal to the output of the sensitive detector 8 and the current supplied from the laser driver 4 to the lower Peltier element 1f so that the terminal voltage of the
(S-4). These controls are continued until the measurement operation ends. However, the control by the terminal voltage in the step (S-4) is performed in the step (S-) so that the control performed by the terminal voltage and the control by the output of the phase sensitive detector 8 do not interfere with each other.
The control is performed more loosely than the control of 3).

Here, the gas in the pipe between the valve of the reference gas cylinder 3a and the solenoid valve V2 may leak air. V4, V6 and V7 are closed and the solenoid valve V2
, And gas flows from the reference gas cylinder 3a, and the solenoid valve V
2 is renewed, the solenoid valve V2 is closed, and after a while, the solenoid valve V5 is closed.

Then, the solenoid valves V2 and V4 of the solenoid valve unit 3 are opened to introduce the reference gas into the test cell 2a, and the solenoid valve V3 is turned on. All other solenoid valves are closed. Then, the reference gas enters the test cell 2a from the reference gas cylinder 3a (gas pressure of about 100 atm) through the solenoid valves V2, V3, and V4. At this time, the valve on the output side of the cylinder 3a is adjusted so that the gas pressure in the test cell 2a becomes 790 to 800 Torr. Thereafter, the solenoid valve V2 is closed, and the solenoid valve V3 is turned off.

Next, when the solenoid valve V8 is turned on, the test cell 2
A part of the reference gas in a flows into the buffer 3b already in a vacuum state, and the gas pressure in the test cell 2a decreases slightly. The CPU 16 receives the output of the pressure gauge 2c, and if the pressure is 780 Torr or more, turns off the solenoid valve 8, turns on the solenoid valve 7, turns it off after a while, and turns on the solenoid valve 8. By repeating the above, the gas pressure is adjusted so as to be approximately 760 to 780 Torr of 1 atm. Thus, the measurement of the reference cell is started.

Here, the laser beam is transmitted through the reference gas sealed at a specified pressure in the test cell 2a, the transmitted light is received by the photodetector 2b, and the output of the photodetector 2b is detected by the phase sensitive detector 11b. And its output is measured as the signal intensity S (13) _REF of the reference gas. Next, the solenoid valve V
4, V5, V6, and V7 are opened to release the reference gas in the test cell 2a. Next, a test gas source is connected to the solenoid valve V1, the solenoid valves V6 and V7 are closed, the solenoid valve V1 is opened, the residual gas between the test gas source and the solenoid valve V1 is purged, the solenoid valve V5 is closed, and the solenoid valve V5 is closed. With V3 turned on, the test gas is introduced into the test cell 2a by opening the cock of the test gas source. After adjusting the gas pressure in the test cell 2a to 760 to 780 Torr, the solenoid valves V1 and V4 are closed, and the solenoid valve V3 is turned off. The signal intensity S of the test gas is the same as that for the reference gas.
₁ (13) Measure _TEST . S ₁ CPU 16 is thus measured (13) _REF and S ₁ (13) The ratio S ₁ between the _TEST (13)
_TEST / S ₁ (13) _REF = R ₁ (13) is calculated (S−
6).

When the first measurement is completed, the solenoid valves V4, V5, V6, and V7 are opened to release the test gas in the test cell 2a.

Next, a second measurement is performed in the same procedure as described above to obtain the signal intensity S ₂ (13) _{REF of the} reference gas and the signal intensity S ₂ (13) _TEST of the test gas, and the ratio S ₂ ( 13) _TEST
/ S ₂ (13) _REF = R ₂ (13)

Thus, the ratio R obtained in the first measurement
_The ratio of the difference between ₁ (13) and the ratio R ₂ (13) obtained in the second measurement is determined by Equation 1.

[0055]

(Equation 1) It is determined whether the ratio is 5% or more (S-7). As a result, if it is 5% or more, the same measurement is performed once again (S-8), and the same operation as in step (S-7) is performed by using the obtained ratio R ₃ (13) according to equation (2). (S-
9).

[0056]

(Equation 2) As a result, when the rate of change is 5% or more, it is determined that the measurement is abnormal, and the step (F-1) in the overall flowchart shown in FIG.
Return to 0). If the ratio obtained by the equation (1) or (2) in step (S-7) or (S-9) is 5% or less, the measurement is considered to have been performed normally, that is, the measured value, ie, S
₂ (13) _REF , S ₂ (13) _TEST or S ₃ (13) _REF , S
₃ (13) _TEST is stored in the memory 17 as S (13) _REF and S (13) _TEST .

Thus, the measurement operation of ¹³ CH ₄ is completed.

Next, the measurement of ¹² CH ₄ is performed in exactly the same way as the measurement of ¹³ CH ₄ described above (F-13). The operation procedure is the same as that of the flowchart shown in FIG. As a result of the measurement of ¹² CH ₄ , the signal strength S (1
2) _REF and S (12) _TEST are stored in the memory 17.

After the measurement of ¹³ CH ₄ and ¹² CH ₄ is completed, the concentration is calculated (F-14). The concentration is calculated as follows.

In general, when calculating the concentration, if the ratio の of the phase-sensitive detection outputs S _REF and S _TEST of the reference gas and the test gas is set as in _Equation 3, the concentration C is obtained by a function F (χ) having χ as a variable. It is obtained as in Equation 4.

[0061]

数 = S _TEST / S _REF

[0062]

C = F (χ) According to the trial calculation of the inventor, the function F (χ) can be represented by a quartic function as shown in Expression 5.

[0063]

Equation 5] ^{C = aχ 4 + bχ 3 +} cχ 2 + dχ where a, b, c, d are constants.

However, in general, the absorption line of one gas may overlap the absorption line of another gas, and in many cases, the influence of another gas cannot be ignored. Therefore, when obtaining the concentration of the target gas, it is necessary to correct so as not to be affected by other gases.

When obtaining the concentrations of the N kinds of component gases, a reference gas used for measuring a certain “i” gas,
Phase sensitive detection output S (i) _REF , S of the second harmonic of the test gas
(I) _TEST, and output of other gases is S (j) _REF , S
(J) If _TEST , it is not affected by other gases. Χ _i can be expressed as in Equation 6.

[0066]

(Equation 6) Here, p _j can be considered as an influence component of the other gas “j” in S (i) _REF when trying to measure the gas “i”, and the numerator and denominator of Equation 6 are both “j”. The effect is eliminated.

[0067] Since the other weak absorption lines in each of the absorption lines in the case of this ¹² CH ₄ and of ¹³ CH ₄ concentration measurement overlap, number 5, is represented on the number 6 ¹² CH _4, Number 7 , Equation 8

[0068]

Equation 7] ^{_{C 12 = a 12 χ 12 4}} + b 12 χ 12 3 + c 12 χ 12 2 + d 12 χ 12

[0069]

(Equation 8) ^{13 In} CH ₄

[0070]

Equation 9] ^{_{C 13 = a 13 χ 13 4}} + b 13 χ 13 3 + c 13 χ 13 2 + d 13 χ 13

[0071]

(Equation 10) Becomes

Here, a ₁₂ , b ₁₂ , c ₁₂ , d ₁₂ , p ₁₂ , a
_{13, b 13, c 13,} d 13, p 13 are constants.

Based on the above grounds, the memory 18
To, ¹² because arithmetic expression represented by the number 9 and number 10 for the density arithmetic calculation formula and ¹³ CH ₄ represented by the number 7 preliminary number 8 for concentration calculation of CH ₄ are previously stored, ¹²
The output signal intensities S (12) _REF and S (12) _{TEST of} the phase sensitive detector 11 obtained by the second or third measurement for each of CH ₄ and ¹³ CH ₄ and S (1
3) By substituting _REF and S (13) _TEST into these equations, the concentrations C ₁₂ and C ₁₃ of ¹² CH ₄ and ¹³ CH ₄ can be obtained.
Can be requested.

In the above example, the stable isotope ¹² CH ₄
If has been illustrated for the concentration measurement of ¹³ CH _4, the present invention is of course can be applied for the concentration measurement of a gas mixture of two or more common ingredients gas.

[0075]

As described above, according to the present invention, a laser beam oscillated from a semiconductor laser driven by a current in which a modulation current of a predetermined frequency is superimposed is transmitted through a mixed gas comprising a plurality of component gases. A method for measuring the concentration of a mixed gas, which measures the phase-sensitive detection output of a signal obtained by photoelectrically converting a transmitted laser beam generated at the peak when light is absorbed by a component gas and calculates the concentration of the component gas based on the output. Prior to the measurement, the oscillation wavelength of the semiconductor laser is variably swept to a reference gas composed of the same component gas as the mixed gas to be measured, and at least the peak of the semiconductor laser caused by absorption of the laser light by each component gas Terminal voltage and the fundamental wave phase sensitive detection output of the transmitted laser beam are stored, and the stored terminal voltage and fundamental wave phase sensitive Since so as to control the oscillation wavelength of the semiconductor laser with a wave output, always accurate concentration measurement component gases independently of the change over time in the characteristics of the semiconductor laser.

Since the method of the present invention can be applied to the measurement of the concentration of a gas mixture of stable isotopes, it is relatively easy and inexpensive to measure the concentration of stable isotopes without using large-scale and expensive equipment such as conventional GC-MS. Can be.

[Brief description of the drawings]

FIG. 1 is a block diagram of an embodiment of a gas concentration measuring device according to the present invention.

FIG. 2 is a block diagram showing details of a part of the gas concentration measuring device shown in FIG.

FIG. 3 is a flowchart showing the entire gas concentration measurement operation according to the present invention.

FIG. 4 is a waveform diagram showing an output signal intensity of a phase sensitive detector for explaining a wavelength sweep in gas concentration measurement according to the present invention.

FIG. 5 is a flowchart of a wavelength sweep operation.

FIG. 6 is a flowchart of the measurement of ¹³ CH ₄ in the present invention.

[Explanation of symbols]

 Reference Signs List 1 laser oscillation unit 1a semiconductor laser 1c reference cell 1d photodetector 1e upper Peltier element 1f lower Peltier element 1g thermistor 2 detection unit 2a test cell 2b photodetector 2c pressure gauge 3 solenoid valve unit 3a reference gas cylinder 3b buffer 4 laser driver 5 Programmable oscillator 6 Current mixer 7 Digital multimeter 8, 9, 11 Phase sensitive detector 10 Programmable power supply 12 GP-IB board 13 A / D converter 14 D / A converter 15 Parallel output board 16 CPU 17, 18 Memory 19 Display 20 Printer

Claims (9)

(57) [Claims] 1. A laser beam oscillated from a semiconductor laser driven by a current obtained by superimposing a modulation current of a predetermined frequency on a reference gas having a known concentration comprising a test gas comprising a plurality of component gases and the same concentration as a test gas comprising an unknown concentration. Concentration measuring method for measuring the concentration of the component gas of the test gas based on the intensity of the signal obtained by phase-sensitive detection of the second harmonic of the signal obtained by photoelectrically converting the transmitted laser light separately In, the laser beam is transmitted through the reference gas while sequentially changing the oscillation wavelength of the semiconductor laser prior to the concentration measurement, the operating conditions of the semiconductor laser when the absorption of the laser beam by the component gas of the reference gas peaks, The signal strength obtained by the fundamental wave phase sensitive detection of the signal obtained by photoelectrically converting the transmitted laser light is stored, and then the semiconductor laser is stored based on the operating conditions and the signal strength. A gas concentration measuring method characterized in that the concentration is measured by driving a user. 2. The gas concentration measuring method according to claim 1, wherein the operating condition of the semiconductor laser is a terminal voltage of the semiconductor laser. 3. The method according to claim 2, wherein the operating conditions of the semiconductor laser include a temperature of the semiconductor laser. 4. The method according to claim 1, wherein the component gases of the test gas and the reference gas are stable isotopes. 5. The method according to claim 1, wherein said stable isotopes are ¹² CH ₄ and ¹³ CH
_The method for measuring gas concentration according to claim 4, wherein 6. A semiconductor laser driven by a current having a modulation current of a predetermined frequency superimposed thereon, a test gas having an unknown concentration comprising a plurality of component gases, and a reference gas having a known concentration comprising the same component gas as the test gas. Test cell, a reference cell in which a reference gas is sealed, a first photodetector for photoelectrically converting a test gas in the test cell or a laser beam transmitted through the reference gas, and a reference gas in the reference cell. A second photodetector for photoelectrically converting the laser light transmitted through the second photodetector;
A first phase-sensitive detector for phase-sensitive detection of a second harmonic of a signal output from the first photodetector, and a second phase-sensitive detection for a fundamental wave of a signal output from the second photodetector. A phase-sensitive detector, a third phase-sensitive detector for phase-sensitive detection of a second harmonic of a signal output from the second photodetector, a heating element for variably adjusting the temperature of the semiconductor laser, Prior to measurement, the laser light is transmitted through the reference gas while sequentially changing the oscillation wavelength of the semiconductor laser within a predetermined range, and at least the operation of the semiconductor laser when the absorption of the laser light by the component gas of the reference gas reaches a peak. Storage means for storing the conditions and the intensity of the signal output from the second phase-sensitive detector; and controlling the oscillation wavelength of the semiconductor laser based on the operating conditions and the signal intensity stored in the storage means when measuring the concentration. And a concentration calculating means for calculating the concentration of the component gas of the test gas by a predetermined arithmetic expression based on the signal intensity output from the first phase-sensitive detector. apparatus. 7. The gas concentration measuring device according to claim 6, wherein said concentration calculating means stores an arithmetic expression for calculating the concentration for each component gas. 8. The gas concentration measuring device according to claim 1, further comprising an electromagnetic valve unit for switching and introducing a test gas and a reference gas into the test cell. 9. The heating element comprises a first Peltier element thermally coupled to the semiconductor laser, and a second Peltier element arranged so as to be able to transfer heat to the first Peltier element. Item 2. The gas concentration measurement device according to Item 1.