CN211347925U - Gas concentration measuring device - Google Patents
Gas concentration measuring device Download PDFInfo
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- CN211347925U CN211347925U CN201922469214.4U CN201922469214U CN211347925U CN 211347925 U CN211347925 U CN 211347925U CN 201922469214 U CN201922469214 U CN 201922469214U CN 211347925 U CN211347925 U CN 211347925U
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Abstract
The utility model discloses a gas concentration measuring device, which comprises a signal source, a laser, an optical fiber coupler, a first gas absorption cell, a second gas absorption cell, a first semiconductor laser detector, a second semiconductor laser detector, a data acquisition card and a computer; after calibration is carried out by using gas with known concentration, a laser generates laser with a certain wave number by using a fixed offset sine wave, after the gas to be tested passes through the gas to be tested, acquired signals are subjected to FFT (fast Fourier transform) conversion to obtain a second term after corresponding conversion, namely the amplitude of second harmonic, then the amplitude of the second harmonic under different offsets is obtained by changing the offset, a relation curve of the offset and the amplitude of the second harmonic is drawn, and then the concentration of the gas to be tested is obtained by inverting the amplitude of the curve, wherein the data acquisition card is used for acquiring the signals, so that the cost of experimental equipment is reduced and the volume of an experimental device is reduced while the experimental result is confirmed.
Description
Technical Field
The utility model relates to a safety test technical field relates to gaseous measuring equipment, concretely relates to 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.
SUMMERY OF THE UTILITY MODEL
An object of the utility model is to provide a gas concentration measuring device solves the high price, bulky that exist among the prior art, is not conform to portable measuring tool's actual requirement's problem.
In order to achieve the above object, the present invention provides a gas concentration measuring device, which comprises a signal source, a laser and an optical fiber coupler, which are arranged in sequence and connected with each other; 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 gas concentration measuring apparatus, 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.
The utility model has the advantages of as follows:
the utility model discloses a gas concentration measuring device can solve the problem that the price is high, bulky among the prior art, does not conform to the actual requirement of portable measuring tool, and it reduces the quantity of signal source, avoids using the adder, reduces the noise that leads to because of the adder in the 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 schematic structural diagram of the gas concentration measuring device of the present invention.
Fig. 2 is a block diagram of the gas concentration measuring device of the present invention.
Fig. 3 is a flow chart of a measuring method of the gas concentration measuring apparatus of the present invention.
Fig. 4 is a harmonic spectrum diagram of a bias voltage of 1.03V according to an embodiment of the measurement method of the gas concentration measurement apparatus of the present invention. In fig. 4, the dashed part is a total spectrum diagram, wherein the second peak is the amplitude of the second harmonic; the solid line part is a signal obtained by detecting the laser through the gas to be detected by a detector.
Fig. 5 is a harmonic spectrum diagram of the embodiment shown in fig. 4 of the measurement method of the gas concentration measurement apparatus of the present invention, in which the bias voltage is 1.04V.
Fig. 6 is a harmonic spectrum diagram of the embodiment shown in fig. 4 of the measurement method of the gas concentration measurement apparatus of the present invention, in which the bias voltage is 1.05V.
Fig. 7 is a harmonic spectrum diagram of the embodiment shown in fig. 4 of the measurement method of the gas concentration measurement apparatus of the present invention, in which the bias voltage is 1.06V.
Fig. 8 is a harmonic spectrum diagram of the embodiment shown in fig. 4 of the measurement method of the gas concentration measurement apparatus of the present invention, in which the bias voltage is 1.07V.
Fig. 9 is a harmonic spectrum diagram of the embodiment shown in fig. 4 of the measurement method of the gas concentration measurement apparatus according to the present invention, in which the bias voltage is 1.08V.
Fig. 10 is a second harmonic scattergram of the embodiment shown in fig. 4 of the measurement method of the gas concentration measurement device according to the present invention.
In fig. 4 to 10, the dashed part is a total spectrum diagram, wherein the second peak is the amplitude of the second harmonic; in fig. 4 to 9, the solid line part is a signal detected by the detector after the laser light passes through the gas to be detected.
Fig. 11 is a graph showing the relationship between the second harmonic amplitude and the methane concentration according to the embodiment shown in fig. 4 of the measurement method of the gas concentration measurement apparatus of 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 measuring device, see figure 1, includes signal source 1, laser 2 and optical fiber coupler 3 set up and connected with the optical axis sequentially; referring to fig. 2, a first gas absorption cell 401 and a second gas absorption cell 402 are arranged at the rear side of the optical fiber coupler 3, a first semiconductor laser detector 411 and a second semiconductor laser detector 412 are respectively installed on the first gas absorption cell 401 and the second gas absorption cell 402, and a data acquisition card 5 and a computer 6 are sequentially connected at the rear sides of the first semiconductor laser detector 411 and the second semiconductor laser detector 412.
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 2
In addition to the gas concentration measuring apparatus described in embodiment 1, a first fiber collimator 421 and a second fiber collimator 422 located behind the fiber coupler 3 may be further provided on the front sides of the first gas absorption cell 401 and the second gas absorption cell 402, respectively.
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.
The measurement method of the gas concentration measurement apparatus of embodiments 1 and 2, referring to fig. 3, may include 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 (ν) cL 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 of the frequency modulation signal,νamplitude 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+Icoswt][1-α0cL(A0-A2cos2wt+A4cos4wt+...)](4)
in the formula (4), IoutIs the intensity of the emergent light, I0(v) is the light intensity input to the laser 2 by the signal source,Iamplitude 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.
The measuring method of the gas concentration measuring device can adopt the signal source 1 to generate a high-frequency sine signal with fixed bias, control the working temperature of the laser 2 through a temperature control chip in the signal source, and simultaneously coarsely tune 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 can be used for processing the acquired signals, so that the amplitude of the second harmonic obtained by FFT under a 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.
In the third step, before the emergent light of the laser 2 passes through the gas absorption cell containing the gas to be detected, the emergent light of the laser 2 is divided into two paths by using the optical fiber coupler 3, wherein one path of the emergent light passes through the first gas absorption cell 401 containing the gas to be detected and then is detected by the 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.
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 may be collimated before passing through the first gas absorption cell 401 and the second gas absorption cell 402.
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.
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 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.
In the fourth step, LabVIEW software may be 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 detected in the gas pipeline.
Taking the concentration of the measured methane gas as an example, fig. 4 to 9 are signal change graphs obtained by changing the bias voltage, and as shown in fig. 4, 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. 4 to 9, 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 10, and a graph 11 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 11, 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.
Although the invention has been described in detail with respect to the general description and the specific embodiments, it will be apparent to those skilled in the art that modifications and improvements can be made based on the invention. Therefore, such modifications and improvements are intended to be within the scope of the invention as claimed.
Claims (2)
1. A gas concentration measuring device comprises a signal source (1), a laser (2) and an optical fiber coupler (3) which are sequentially connected and have the same optical axis; the optical fiber coupler is characterized in that a first gas absorption cell (401) and a second gas absorption cell (402) are arranged on the rear side of the optical fiber coupler (3), a first semiconductor laser detector (411) and a second semiconductor laser detector (412) are respectively installed on the first gas absorption cell (401) and the second gas absorption cell (402), and a data acquisition card (5) and a computer (6) are sequentially connected on the rear sides of the first semiconductor laser detector (411) and the second semiconductor laser detector (412).
2. The gas concentration measuring apparatus according to claim 1, 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).
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112213283A (en) * | 2020-09-15 | 2021-01-12 | 江苏方天电力技术有限公司 | Gas concentration measuring method |
| CN112229805A (en) * | 2020-09-29 | 2021-01-15 | 苏州迈优医疗科技有限公司 | Medical gas detection system and method |
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2019
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112213283A (en) * | 2020-09-15 | 2021-01-12 | 江苏方天电力技术有限公司 | Gas concentration measuring method |
| CN112213283B (en) * | 2020-09-15 | 2023-12-15 | 江苏方天电力技术有限公司 | Gas concentration measuring method |
| CN112229805A (en) * | 2020-09-29 | 2021-01-15 | 苏州迈优医疗科技有限公司 | Medical gas detection system and method |
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