CN110907398A - Gas concentration measuring method and measuring device - Google Patents
Gas concentration measuring method and measuring device Download PDFInfo
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- CN110907398A CN110907398A CN201911412196.4A CN201911412196A CN110907398A CN 110907398 A CN110907398 A CN 110907398A CN 201911412196 A CN201911412196 A CN 201911412196A CN 110907398 A CN110907398 A CN 110907398A
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- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
Abstract
The invention discloses a gas concentration measuring method and a measuring device, the method comprises the steps of calibrating gas with known concentration, firstly utilizing fixed offset sine wave to enable a laser to generate laser with a certain wave number, after the gas to be measured passes through, acquiring signals, then carrying out FFT (fast Fourier transform) conversion to obtain a second term after corresponding conversion, namely the amplitude of second harmonic, then changing the offset to further obtain the amplitudes of the second harmonic under different offsets, drawing a relation curve of the offset and the amplitude of the second harmonic, and then carrying out inversion according to the amplitude of the curve to obtain the concentration of the gas to be measured, wherein a data acquisition card is used for acquiring the signals, then the acquired signals can be processed through LabVIEW to obtain the concentration of the gas to be measured, the cost of experimental equipment is reduced while the result is verified, and the volume of the experimental device is reduced.
Description
Technical Field
The invention relates to the technical field of safety testing, relates to a gas measuring method and equipment, and particularly relates to a gas concentration measuring method and a gas concentration measuring device.
Background
In recent years, with the rapid development of Spectroscopy and Laser technologies, Laser Spectroscopy technologies such as Tunable Diode Laser Absorption Spectroscopy (TDLAS) and the like are applied to the field of detection of various gas concentrations, and TDLAS detection has many advantages, such as high stability, good selectivity, long service life and the like. The technology of adopting wavelength modulation is applied to TDLAS, can be used for detecting gas with low concentration, obtains the concentration of unknown gas by extracting the direct proportion relation between the amplitude of second harmonic and the concentration, and is widely applied due to high detection precision. The tunable semiconductor laser absorption spectrum Technology (TDLAS) is one of spectrum measurement, a tunable semiconductor laser is used as a detection light source, a beer-Lambert law and a wavelength modulation technology are utilized, a concentration detection limit with high precision is obtained by detecting the amplitude change of second harmonic, and the wavelength corresponding to the gas absorption peak of the output wavelength of the laser is found by a fixed offset method. The number of signal sources is reduced, the use of an adder is eliminated, the noise caused by the adder in a light path is reduced, the device is suitable for various on-line gas detection occasions, the high-precision on-line measurement of pollutant emission concentration is realized, the safety problem of a working environment is solved, and the harmful gas concentration range alarm device can effectively solve the problem that the existing measurement method and instrument cannot realize on-line measurement. However, the existing wavelength modulation trace gas concentration measuring device based on the TDLAS mostly adopts the lock-in amplifier to realize the amplitude of the second harmonic, which causes the following problems that firstly, the price of the lock-in amplifier is higher, and secondly, the volume of the lock-in amplifier is larger, which does not accord with the actual requirement of the portable measuring tool.
Disclosure of Invention
The invention aims to provide a gas concentration measuring method and a gas concentration measuring device, which solve the problems of high price, large volume and incapability of meeting the actual requirements of portable measuring tools in the prior art.
To achieve the above object, an aspect of the present invention provides a gas concentration measuring method including the steps of,
firstly, calibrating gas to be measured with known concentration to obtain a relation curve between second harmonic amplitude after FFT and gas concentration under fixed bias, and taking the relation curve as a reference baseline for solving unknown concentration;
secondly, controlling a laser to generate laser in a preset wave number range;
thirdly, enabling emergent light of the laser to pass through a gas absorption cell containing gas to be detected, and then detecting the emergent light through a laser detector;
fourthly, collecting an output signal of the laser detector when the emergent light of the laser passes through a gas absorption cell containing gas to be detected, calculating to obtain the concentration of the gas to be detected by the formula (1),
Iout＝Iin(v)exp[-α(v)cL](1)
in the formula (1), Iin(v) is the original intensity, i.e. the intensity of the laser light without absorption in a gaseous medium, IoutThe method is characterized in that the method is emergent light intensity, namely the light intensity of laser after medium treatment, c is the concentration of gas to be measured, L is the effective length of the laser passing through the gas to be measured, α (v) is the absorption line type of the gas to be measured, and the expression of α (v) is as shown in formula (2):
in formula (2), α0Is the gas absorption coefficient, gamma is the half line width of the gas absorption line; when the center frequency of the laser output laser light of the laser is equal to the center frequency of the gas absorption line type, the gas absorption line type is as follows (3):
in the formula (3), w is the frequency at which the signal is frequency-modulated, δνAmplitude when frequency-modulating the signal, vDIs the center frequency of the laser output frequency; when the laser passes through the control signal source to make the output frequency of the laser equal to the central frequency of the gas line type, vD＝ν0To give formula (4):
Iout＝I0(ν)[1+δIcoswt][1-α0cL(A0-A2cos2wt+A4cos4wt+...)](4)
in the formula (4), IoutIs the intensity of the emergent light, I0(v) is the intensity of light input to the laser from the signal source, deltaIAmplitude when the signal is power modulated, A0,A2,A4.. when vD＝ν0When is to IoutPerforming Fourier transform to obtain coefficients of each subharmonic, wherein the second harmonic obtained by performing Fourier transform according to the formula (4) is the formula (5):
I2f＝I0(ν)α0A2cL (5)
in the formula (5), I2fFor the intensity of emergent light IoutThe value of L is obtained by measuring the effective length of the laser passing through the gas to be measured, so as to calculate the unknown concentration c.
In the third step, before the emergent light of the laser passes through the gas absorption pool containing the gas to be detected, the emergent light of the laser is divided into two paths by adopting an optical fiber coupler, wherein one path of the emergent light passes through the first gas absorption pool containing the gas to be detected and then is detected by the first semiconductor laser detector; and the other path of emergent light passes through a second gas absorption cell without gas and then is detected by a second semiconductor laser detector.
Preferably, in the fourth step, the output signals of the two first semiconductor laser detectors and the second semiconductor laser detector are collected, and the concentration of the gas to be measured is calculated by the formula (1).
It is also preferable that the laser light is collimated before passing through the first gas absorption cell and the second gas absorption cell in the third step.
Preferably, in the fourth step, the output signals of the first semiconductor laser detector and the second semiconductor laser detector are input into a data acquisition card, and then the concentration value of the gas to be detected is obtained through computer processing.
It is also preferable that, in the first step, the range of absorption of the gas to be measured is locked by the direct absorption method, then the step value is reduced to lock the range of the offset value, then the second harmonic value under each offset value is obtained by fixing at least two offset values, and then the second harmonic curve is drawn according to the second harmonic value.
Preferably, in the fourth step, LabVIEW software is installed in the computer, and a relationship curve between the second harmonic amplitude and the gas concentration under a fixed bias and after FFT is drawn by the LabVIEW software, so as to obtain the concentration value of the gas to be measured in the gas pipeline.
The invention provides a measuring device of a gas concentration measuring method, which comprises a signal source, a laser and an optical fiber coupler which are sequentially connected and have the same optical axis; the rear side of the optical fiber coupler is provided with a first gas absorption pool and a second gas absorption pool, the first gas absorption pool and the second gas absorption pool are respectively provided with a first semiconductor laser detector and a second semiconductor laser detector, and the rear sides of the first semiconductor laser detector and the second semiconductor laser detector are sequentially connected with a data acquisition card and a computer.
In the above aspect of the measurement apparatus of the gas concentration measurement method, it is preferable that the first gas absorption cell and the second gas absorption cell are respectively provided with a first fiber collimator and a second fiber collimator on the rear side of the fiber coupler on the front sides thereof.
Preferably, LabVIEW software for drawing a relationship curve between the second harmonic amplitude and the gas concentration after FFT under a fixed bias and obtaining the concentration value of the gas to be detected in the gas pipeline is installed in the computer.
The invention has the following advantages:
the gas concentration measuring method and the gas concentration measuring device can solve the problems of high price, large volume and non-conformity with the actual requirements of portable measuring tools in the prior art, reduce the number of signal sources, avoid the use of an adder and reduce the noise caused by the adder in a light path; the device is suitable for various on-line gas detection occasions, realizes high-precision on-line measurement of pollutant emission concentration, can effectively solve the problem that the existing measurement method and instrument cannot realize on-line measurement, and solves the safety problem of working environment; specifically, the traditional analog signals are converted into digital signals, the digital signals have strong anti-interference capability, and the signals are convenient to store, process and exchange in the subsequent inversion concentration process; LabVIEW software can be adopted to process data, an FFT algorithm is adopted in a program to process a known signal, and then the amplitude of a second harmonic under fixed bias is obtained; the concentration of the gas to be measured is measured through double optical paths, and the variable method is strictly controlled, so that the noise influence in the experimental process is greatly reduced, and the accuracy of the experiment is improved; the output wavelength of the temperature-controlled coarse tuning laser can be used, the output wavelength of the laser is finely tuned by adopting a fixed offset high-frequency modulation signal, and only the amplitude of the high-frequency modulation signal in the fixed offset is used, so that the transmission of useless data in an experiment can be greatly reduced, and the data transmission speed is improved; the amplitude of the second harmonic is used, but the amplitude is not extracted by a phase-locked amplifier, but is directly obtained by FFT (fast Fourier transform), so that the use of the phase-locked amplifier is reduced on the premise of keeping the test result unchanged, the cost of experimental equipment is reduced, and the volume of an experimental device is reduced.
Drawings
Fig. 1 is a flow chart of a gas concentration measurement method of the present invention.
Fig. 2 is a harmonic spectrum diagram of a bias voltage of 1.03V according to an embodiment of the gas concentration measuring method of the present invention.
FIG. 3 is a harmonic spectrum diagram of the bias voltage of 1.04V of the embodiment shown in FIG. 2 of the gas concentration measuring method of the present invention.
FIG. 4 is a harmonic spectrum diagram of the bias voltage of 1.05V of the embodiment shown in FIG. 2 of the gas concentration measuring method of the present invention.
Fig. 5 is a harmonic spectrum diagram of the bias voltage of 1.06V of the embodiment shown in fig. 2 of the gas concentration measuring method of the present invention.
Fig. 6 is a harmonic spectrum diagram of the bias voltage of 1.07V of the embodiment shown in fig. 2 of the gas concentration measuring method of the present invention.
Fig. 7 is a harmonic spectrum diagram of the bias voltage of 1.08V of the embodiment shown in fig. 2 of the gas concentration measuring method of the present invention.
Fig. 8 is a second harmonic scattergram of the embodiment shown in fig. 2 of the gas concentration measuring method of the present invention.
In fig. 2 to 8, the dashed part is a total spectrum diagram, wherein the second peak is the amplitude of the second harmonic; in fig. 2 to 7, the solid line part is a signal detected by the detector after the laser light passes through the gas to be detected.
FIG. 9 is a graph of second harmonic amplitude versus methane concentration for the embodiment of FIG. 2 of the gas concentration measurement method of the present invention.
Fig. 10 is a schematic structural view of the gas concentration measuring apparatus of the present invention.
Fig. 11 is a block diagram showing the structure of the gas concentration measuring apparatus according to the present invention.
In the figure, 1 is a signal source, 2 is a laser, 3 is a fiber coupler, 401 is a first gas absorption cell, 402 is a second gas absorption cell, 403 is a gas inlet of the first gas absorption cell or the first gas absorption cell, 404 is a gas outlet of the first gas absorption cell or the first gas absorption cell, 411 is a first semiconductor laser detector, 412 is a second semiconductor laser detector, 421 is a first fiber collimator, 422 is a second fiber collimator, 5 is a data acquisition card, and 6 is a computer.
Detailed Description
The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.
Example 1
A gas concentration measurement method, see fig. 1, comprising the steps of,
firstly, calibrating gas to be measured with known concentration to obtain a relation curve between second harmonic amplitude after FFT and gas concentration under fixed bias, and taking the relation curve as a reference baseline for solving unknown concentration;
secondly, controlling a laser 2 to enable the laser 2 to generate laser in a preset wave number range;
thirdly, enabling emergent light of the laser 2 to pass through a gas absorption cell containing gas to be detected, and then detecting the emergent light through a laser detector;
fourthly, collecting an output signal of the laser detector when the emergent light of the laser 2 passes through a gas absorption cell containing gas to be detected, calculating by the formula (1) to obtain the concentration of the gas to be detected,
Iout＝Iin(v)exp[-α(v)cL](1)
when α (v) c L approaches zero, as shown in the following formula (1.1),
Iout＝Iin(ν)[1-α(ν)cL](1.1)
in the formula (1), Iin(v) is the original intensity, i.e. the intensity of the laser light without absorption in a gaseous medium, IoutThe method is characterized in that the method is emergent light intensity, namely the light intensity of laser after medium treatment, c is the concentration of gas to be measured, L is the effective length of the laser passing through the gas to be measured, α (v) is the absorption line type of the gas to be measured, and the expression of α (v) is as shown in formula (2):
in formula (2), α0Is the gas absorption coefficient, gamma is the half line width of the gas absorption line; when the center frequency of the laser output by the laser (2) is equal to the center frequency of the gas absorption line type, the gas absorption line type is as follows (3):
in the formula (3), w is the frequency at which the signal is frequency-modulated, δνAmplitude when frequency-modulating the signal, vDIs the center frequency of the output frequency of the laser 2; when the laser 2 passes through the control signal source 1 to make the output frequency of the laser 2 equal to the central frequency of the gas line type, vD＝ν0To give formula (4):
Iout＝I0(ν)[1+δIcoswt][1-α0cL(A0-A2cos2wt+A4cos4wt+...)](4)
in the formula (4), IoutIs the intensity of the emergent light, I0(v) is the intensity of light input to the laser 2 from the signal source, deltaIAmplitude when the signal is power modulated, A0,A2,A4.. when vD＝ν0When is to IoutFourier transform is carried out to obtainThe coefficient of each subharmonic is a second harmonic obtained by fourier transform of equation (4) and is equation (5):
I2f＝I0(ν)α0A2cL (5)
in the formula (5), I2fFor the intensity of emergent light IoutThe value of L is obtained by measuring the effective length of the laser passing through the gas to be measured, so as to calculate the unknown concentration c.
The gas concentration measuring method of the above embodiment may use the signal source 1 to generate a high-frequency sinusoidal signal with a fixed bias, and control the operating temperature of the laser 2 through a temperature control chip therein, while coarsely tuning the output wavelength of the laser 2. At this time, the laser 2 emits a single parallel laser beam with a certain wave number, and the laser beam is divided into two beams of same light after passing through the fiber coupler 3, wherein one beam of the same light can be directly transmitted to the first fiber collimator 421 through the fiber, then enters the first gas absorption cell 401 containing gas, and then is received by the first semiconductor laser detector 411, and the other beam of the same light can be directly transmitted to the second fiber collimator 422 through the fiber, then enters the second gas absorption cell 402 containing gas, and then is received by the second semiconductor laser detector 412, but the corresponding second gas absorption cell 402 does not contain the gas to be detected; data obtained by the two first semiconductor laser detectors 411 and the second semiconductor laser detector 412 are acquired by the data acquisition card 5 and then transmitted to the computer 6, and then LabVIEW software is used for processing the acquired signals, so that the amplitude of the second harmonic obtained by FFT under fixed bias can be obtained, and further the concentration of the gas to be detected can be obtained. Wherein formula (1) is based on beer-Lambert's law; the formula (2) is suitable for normal temperature and pressure, namely the temperature is 25 ℃, and the pressure is 1 standard atmospheric pressure. In addition, in the third step, when the emergent light of the laser 2 passes through the gas absorption cell containing the gas to be detected and then is detected by the laser detector, the number of the gas absorption cells may be one, that is, the emergent light of the laser 2 only passes through one gas absorption cell containing the gas to be detected, and the number of the gas absorption cells may also be two, at this time, refer to the contents of embodiment 2 to embodiment 4 specifically.
Example 2
A gas concentration measuring method, similar to that in embodiment 1, except that in the third step, before the emergent light of a laser 2 passes through a gas absorption cell containing gas to be measured, an optical fiber coupler 3 is adopted to divide the emergent light of the laser 2 into two paths, wherein one path of the emergent light passes through a first gas absorption cell 401 containing gas to be measured and then is detected by a first semiconductor laser detector 411; the other path of the emergent light passes through the second gas absorption cell 402 without gas and is detected by a second semiconductor laser detector 421.
Further, in the fourth step, the output signals of the two first semiconductor laser detectors 411 and the two second semiconductor laser detectors 421 are collected, and the concentration of the gas to be measured is calculated by formula 1.
In the third step, the laser is collimated before passing through the first gas absorption cell 401 and the second gas absorption cell 402.
Further, in the fourth step, the output signals of the first semiconductor laser detector 411 and the second semiconductor laser detector 421 are input into the data acquisition card 5, and then processed by the computer 6 to obtain the concentration value of the gas to be measured.
Furthermore, in the first step, the absorption range of the gas to be measured is locked by a direct absorption method, then the step value is reduced to lock the range of the offset value, then the second harmonic value under each offset value is obtained by fixing at least two offset values, and then a second harmonic curve is drawn according to the second harmonic value. In the first step, the absorption range of the gas to be detected is locked by a direct absorption method, then the step value is reduced to lock the range of the offset value, then the second harmonic value under each offset value is obtained by fixing different offset values, and then a second harmonic curve is drawn according to the value of the second harmonic.
Furthermore, in the fourth step, LabVIEW software is installed in the computer 6, and a relationship curve between the second harmonic amplitude and the gas concentration under a fixed bias and after FFT is drawn by the LabVIEW software, so as to obtain the concentration value of the gas to be measured in the gas pipeline.
Taking the concentration of the measured methane gas as an example, fig. 2 to 7 are signal change graphs obtained by changing the bias voltage, and as shown in fig. 2, the solid line curve is a curve displayed on the oscilloscope 7 by the signal acquired by the first semiconductor laser detector 411 after corresponding to the absorbed gas, and the dashed line curve is a curve obtained after FFT change corresponding to the solid line curve, as can be seen from fig. 2 to 7, when the bias voltage of the signal changes, the corresponding change when the signal sweeps across the gas absorption peak can be obviously found. The amplitude of the second harmonic is collected each time, then a curve obtained by taking the bias voltage as an abscissa and the amplitude of the collected second harmonic as an ordinate is taken as a graph 8, and a graph 9 is a corresponding fitted graph of the relationship between the concentration of the gas to be measured and the amplitude of the second harmonic, wherein in the graph 9, the abscissa is the methane concentration, and the unit is% and the ordinate is the amplitude of the second harmonic, and the unit is mV.
Example 3
A gas concentration measuring apparatus of the gas concentration measuring method according to embodiment 1 or embodiment 2, referring to fig. 10 and fig. 11, includes a signal source 1, a laser 2, and a fiber coupler 3, which are connected to each other and arranged in sequence; the rear side of the optical fiber coupler 3 is provided with a first gas absorption cell 401 and a second gas absorption cell 402, the first gas absorption cell 401 and the second gas absorption cell 402 are respectively provided with a first semiconductor laser detector 411 and a second semiconductor laser detector 412, and the rear sides of the first semiconductor laser detector 411 and the second semiconductor laser detector 412 are sequentially provided with a data acquisition card 5 and a computer 6.
In the gas concentration measuring device of the embodiment, a signal source 1 is used for generating a suitable fixed-bias high-frequency sine wave signal, so that a laser 2 generates laser in a certain wave number range; the laser light generated by the laser 2 is divided into two paths by the optical fiber coupler 3: one path passes through a first gas absorption cell 401 containing gas to be detected, and then is detected by a first semiconductor laser detector 411; the other path is detected by a second semiconductor laser detector 412 through a second gas absorption cell 402 which does not contain gas; the data acquisition card 5 is used for acquiring signals output by the first semiconductor laser detector 411 and the second semiconductor laser detector 412 and sending the signals to the computer 6; the computer 6 is used for calculating according to the signals sent by the data acquisition card 5 to obtain the amplitude of second harmonic obtained by FFT under fixed bias so as to obtain the concentration of the gas to be detected; a first optical fiber collimator 421 and a second optical fiber collimator 422 may be further included for collimating the light entering the first gas absorption cell 401 and the second gas absorption cell 402, respectively; the computer 6 is used for editing a computer processing module at the later stage to realize the extraction process of the second harmonic amplitude, meanwhile, the function of the part can be realized by a single chip microcomputer or FFT software on an oscilloscope, if the oscilloscope is used, the function of subtracting two paths of signals cannot be realized, but the amplitude of the second harmonic of a single path can be extracted.
Example 4
On the basis of the measuring apparatus of the gas concentration measuring method in embodiment 3, it is further possible that the first gas absorption cell 401 and the second gas absorption cell 402 are respectively provided with a first fiber collimator 421 and a second fiber collimator 422 on the rear side of the fiber coupler 3.
Furthermore, LabVIEW software for drawing a relationship curve between the second harmonic amplitude and the gas concentration after FFT under a fixed bias and obtaining the concentration value of the gas to be detected in the gas pipeline is installed in the computer 6.
In the gas concentration measuring device of the embodiment, the direct-current voltage controls the chip working temperature of the laser 2 so as to coarsely adjust the output wavelength of the laser 2, and the signal source 1 fixes, biases and superposes a high-frequency signal to be used for finely adjusting the wavelength of the laser 2. In fig. 10 and 11, the gas inlets 403 and the gas outlets 404 of the first gas absorption cell 401 and the second gas absorption cell 402 are used as channel ports for inputting and outputting the gas to be detected, when the gas is filled, the two gas absorption cells may be filled with nitrogen gas for five minutes, respectively, after the gas tightness is checked and the influence of other gases is eliminated, the gas outlets 404 are tightly covered and sealed, then the gas to be detected is filled into the first gas absorption cell 401, and then the gas inlets 403 thereof are tightly covered and sealed. The second gas absorption cell 402 remains unchanged as a noise rejection reference. As shown in fig. 11, the laser output from the laser 2 is divided into two parts by the fiber coupler 3, wherein one part enters the first gas absorption cell 401 containing the gas to be detected and then enters the first semiconductor laser detector 411, and the other part enters the second gas absorption cell 402 not containing the gas to be detected and then enters the second semiconductor laser detector 412; the semiconductor detector transmits detected signals into the computer 6 through two channel ports of the data acquisition card 5, then uses LabVIEW to perform programming processing, firstly performs difference processing on the two collected signals, then performs FFT conversion on the obtained data, collects the second term of the corresponding conversion result, namely the amplitude of the second harmonic under fixed bias, and then obtains the concentration of the gas to be measured according to the amplitude inversion.
Although the invention has been described in detail above with reference to a general description and specific examples, it will be apparent to one skilled in the art that modifications or improvements may be made thereto based on the invention. Accordingly, such modifications and improvements are intended to be within the scope of the invention as claimed.
Claims (10)
1. A gas concentration measuring method is characterized by comprising the following steps,
firstly, calibrating gas to be measured with known concentration to obtain a relation curve between second harmonic amplitude after FFT and gas concentration under fixed bias, and taking the relation curve as a reference baseline for solving unknown concentration;
secondly, controlling a laser (2) to enable the laser (2) to generate laser in a preset wave number range;
thirdly, enabling emergent light of the laser (2) to pass through a gas absorption cell containing gas to be detected, and then detecting the emergent light through a laser detector;
fourthly, collecting an output signal of the laser detector when the emergent light of the laser (2) passes through a gas absorption cell containing gas to be detected, and calculating to obtain the concentration of the gas to be detected through a formula (1),
Iout＝Iin(v)exp[-α(v)cL](1)
in the formula (1), Iin(v) is the original intensity, i.e. the intensity of the laser light without absorption in a gaseous medium, IoutThe method is characterized in that the method is emergent light intensity, namely the light intensity of laser after medium treatment, c is the concentration of gas to be measured, L is the effective length of the laser passing through the gas to be measured, α (v) is the absorption line type of the gas to be measured, and the expression of α (v) is as shown in formula (2):
in formula (2), α0Is the gas absorption coefficient, gamma is the half line width of the gas absorption line; when the center frequency of the laser output by the laser (2) is equal to the center frequency of the gas absorption line type, the gas absorption line type is as follows (3):
in the formula (3), w is the frequency at which the signal is frequency-modulated, δνAmplitude when frequency-modulating the signal, vDIs the center frequency of the output frequency of the laser (2); when the laser (2) passes through the control signal source (1) to enable the output frequency of the laser (2) to be equal to the central frequency of the gas line type, vD＝ν0To give formula (4):
Iout＝I0(ν)[1+δIcoswt][1-α0cL(A0-A2cos2wt+A4cos4wt+...)](4)
in the formula (4), IoutIs the intensity of the emergent light, I0(v) is the intensity of light input to the laser (2) from the signal source, deltaIAmplitude when the signal is power modulated, A0,A2,A4.. when vD＝ν0When is to IoutPerforming Fourier transform to obtain coefficients of each subharmonic, wherein the second harmonic obtained by performing Fourier transform according to the formula (4) is the formula (5):
I2f＝I0(ν)α0A2cL (5)
in the formula (5), I2fFor the intensity of emergent light IoutThe value of L is obtained by measuring the effective length of the laser passing through the gas to be measured, so as to calculate the unknown concentration c.
2. The gas concentration measuring method according to claim 1, wherein in the third step, before the outgoing light of the laser (2) passes through the gas absorption cell containing the gas to be measured, the outgoing light of the laser (2) is divided into two paths by using the optical fiber coupler (3), wherein one path of the outgoing light passes through the first gas absorption cell (401) containing the gas to be measured and then is detected by the first semiconductor laser detector (411); and the other path of the emergent light passes through a second gas absorption cell (402) without gas and then is detected by a second semiconductor laser detector (421).
3. The gas concentration measuring method according to claim 2, wherein in the fourth step, output signals of two first semiconductor laser detectors (411) and two second semiconductor laser detectors (421) are collected, and the concentration of the gas to be measured is calculated by equation (1).
4. A gas concentration measuring method according to claim 3, wherein in the third step, the laser light is collimated before passing through the first gas absorption cell (401) and the second gas absorption cell (402).
5. The method of claim 4, wherein in the fourth step, the output signals of the first semiconductor laser detector (411) and the second semiconductor laser detector (421) are input into a data acquisition card (5), and then processed by a computer (6) to obtain the concentration value of the gas to be measured.
6. A gas concentration measuring method according to any one of claims 1 to 5, wherein in the first step, the range of the absorption of the gas to be measured is locked by a direct absorption method, then the step value is reduced to lock the range of the offset value, then the second harmonic value at each offset value is obtained by fixing at least two offset values, and then the second harmonic curve is drawn based on the values of the second harmonic.
7. The gas concentration measurement method according to claim 6, wherein in the fourth step, LabVIEW software is installed in the computer (6), and a relationship curve between the second harmonic amplitude after FFT and the gas concentration under a fixed bias is drawn through the LabVIEW software to obtain the concentration value of the gas to be measured in the gas pipeline.
8. The measuring apparatus of the gas concentration measuring method according to any one of claims 1 to 7, comprising a signal source (1), a laser (2) and a fiber coupler (3) which are connected to each other in this order; the rear side of the optical fiber coupler (3) is provided with a first gas absorption cell (401) and a second gas absorption cell (402), the first gas absorption cell (401) and the second gas absorption cell (402) are respectively provided with a first semiconductor laser detector (411) and a second semiconductor laser detector (412), and the rear sides of the first semiconductor laser detector (411) and the second semiconductor laser detector (412) are sequentially connected with a data acquisition card (5) and a computer (6).
9. The measuring apparatus of the gas concentration measuring method according to claim 8, wherein the first gas absorption cell (401) and the second gas absorption cell (402) are provided at front sides thereof with a first fiber collimator (421) and a second fiber collimator (422) respectively, which are located at rear sides of the fiber couplers (3).
10. The measuring apparatus according to claim 9, wherein LabVIEW software for plotting a relationship curve between the second harmonic amplitude after FFT and the gas concentration under a fixed bias and obtaining the value of the concentration of the gas to be measured in the gas line is installed in the computer (6).
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN111537470A (en) * | 2020-05-25 | 2020-08-14 | 应急管理部沈阳消防研究所 | TDLAS gas concentration detection method based on digital modulation |
| CN113390825A (en) * | 2021-05-17 | 2021-09-14 | 西安理工大学 | TDLAS-based time-frequency domain combined gas concentration inversion method and device |
| CN114295581A (en) * | 2021-12-31 | 2022-04-08 | 厦门大学 | Gas concentration detection method and device insensitive to DFB laser wavelength characteristic |
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2019
- 2019-12-31 CN CN201911412196.4A patent/CN110907398A/en active Pending
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN111537470A (en) * | 2020-05-25 | 2020-08-14 | 应急管理部沈阳消防研究所 | TDLAS gas concentration detection method based on digital modulation |
| CN113390825A (en) * | 2021-05-17 | 2021-09-14 | 西安理工大学 | TDLAS-based time-frequency domain combined gas concentration inversion method and device |
| CN113390825B (en) * | 2021-05-17 | 2023-04-07 | 西安理工大学 | TDLAS-based time-frequency domain combined gas concentration inversion method and device |
| CN114295581A (en) * | 2021-12-31 | 2022-04-08 | 厦门大学 | Gas concentration detection method and device insensitive to DFB laser wavelength characteristic |
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