CN114235741A - TDLAS-based gas concentration measurement method and system - Google Patents
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Abstract
The invention discloses a TDLAS-based gas concentration measurement method and a TDLAS-based gas concentration measurement system, which comprise the following steps: acquiring environmental parameters and spectral parameters; collecting spectral absorption data of laser, and calculating the maximum value of logarithmic absorbance; calculating the maximum FWHM in a lorentz line type; calculating the approximate concentration of the gas by using a concentration calculation formula as an initial iteration value and assigning the initial iteration value to X1; calculate FWHM in Lorentz line form with X1; calculating phi under Voigt profile using approximate concentration valuesv(v 0); will phiv(v0) substituting into a concentration calculation formula to calculate the concentration X2; judging whether the absolute value of the difference value between the X1 and the X2 is smaller than a preset value or not, and if so, outputting X1 as the calculated concentration; otherwise, the FWHM in the lorentz line style is recalculated by changing the value of X1. The invention does not need to use an additional auxiliary measuring tool, greatly reduces the computation complexity and improves the concentration detection efficiency.
Description
Technical Field
The invention relates to the technical field of gas concentration measurement, in particular to a TDLAS-based gas concentration measurement method and system.
Background
With the development of semiconductor lasers, a Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology has been developed, which measures gas concentration by using absorption of light by gas molecules, has advantages of non-contact measurement, response speed block, good selectivity, high measurement accuracy, and the like, and can satisfy measurement of gas concentration in different environments, and has been favored by many researchers in recent years. At present, more than 1000 TDLAS instruments are applied to the fields of continuous emission monitoring, industrial process control and the like, the TDLAS detection instruments sold worldwide every year occupy 5% -10% of the total number of the infrared gas sensing detection instruments, high-precision detection of parameters such as gas concentration, temperature, flow rate and the like in different fields is achieved, and important technical support is provided for development of various fields.
The traditional TDLAS direct absorption spectrum technology measures the spectrum signal or the transmission laser intensity signal I when no gas is absorbedtObtaining initial laser intensity I by polynomial fitting in the non-absorption region0Then go right again
And fitting and integrating to obtain integral absorbance, and finally solving the gas concentration by using the beer Lambert law. In calculating integrated absorbance, an etalon or wavemeter is often required to convert the absorption spectrum signal from the time domain to the frequency domain, which increases the complexity and cost of the device.
In the prior art, a TDLAS gas concentration detection method (with publication number CN108181266B) includes the following steps: D. calculating 5 absorption peak areas in the step C as concentration characteristic quantities of the target gas, and performing linear fitting by taking the absorption peak areas as x and 5 actual concentrations of the target gas as y to obtain an actual concentration-absorption peak area calibration model; E. b, introducing gas to be detected with unknown concentration into the gas pool, measuring a spectrum to be detected after the spectrum is stable, carrying out spectrum scaling and alignment on the spectrum to be detected and the standard zero spectrum obtained in the step A according to the falling edge of the sawtooth wave of the spectrum, carrying out subtraction on the aligned spectrum, and extracting a rising edge difference curve of the sawtooth wave of the spectrum to obtain an absorption peak of the spectrum to be detected; F. and D, calculating the absorption peak area of the spectrum to be detected, substituting the absorption peak area into the actual concentration-absorption peak area calibration model in the step D, and calculating to obtain the concentration of the gas to be detected. And determining the absorption peak area of the gas to be detected with unknown concentration, and further inverting the actual concentration of the gas to be detected. The scheme is that a concentration-absorption peak area calibration model is used for calculating the gas concentration, and the calculation complexity is high.
Disclosure of Invention
The invention provides a TDLAS-based gas concentration measurement method and system for overcoming the defects of more additional auxiliary tools and complex calculation in the prior art.
The invention is realized by at least one of the following technical schemes.
A TDLAS-based gas concentration measurement method comprises the following steps:
s1, acquiring environmental parameters and spectral parameters;
s2, collecting spectral absorption data of laser, and calculating logarithmic absorbance to obtain the maximum value of the logarithmic absorbance;
s3, calculating the maximum full width at half maximum FWHM under a lorentz line type according to the environmental parameters and the spectral parameters;
s4, calculating the approximate concentration of the gas by using a concentration calculation formula to serve as an initial iteration value X0, and assigning X0 to X1;
s5 FWHM in lorentz line, denoted as Δ v, calculated using X1c1；
S6, calculating the linear function value phi (v) at the central wavelength under the Voigt linear by using the concentration value of the step S4, and recording as phiv(v0)；
S7, will phiv(v0) substituting into a concentration calculation formula to calculate the concentration, and marking as X2;
s8, judging whether the absolute value of the difference value between the X1 and the X2 is smaller than a preset value or not, and if the absolute value of the difference value is smaller than the preset value, outputting X1 as calculated concentration; if the X1 value is larger than or equal to the preset value, the value of X1 is changed back to the step S5 to continue the calculation.
Preferably, theThe environmental parameters comprise pressure P, temperature T and optical path length L; the spectral parameters include: y-shape with air spreading widthairAnd a target gas self-expanding aspectself。
Preferably, the approximate concentration X0 of the gas is calculated by the formula:
wherein ,
is the maximum value I of the emergent signalt(v0) And incident signal I0(v0) At a central wavelength v0The ratio of (A) to (B), L represents the optical path length, P is the total pressure of the gas medium,
is the maximum value of the logarithmic absorbance, φ (v)0) V is ═ v0Peak of the calculated lorentzian profile:
wherein S (T) is the linear intensity of the gas absorption line, Δ νc0Is the maximum full width at half maximum FWHM in the lorentz line.
Preferably, the specific formula for calculating FWHM under lorentz line type using X1 is:
Δvc1＝P*[((1-X1)*2*Υair)+X1*2*Υself]
wherein P is the pressure of gas medium, upsilonairIs wide for air and upsilonselfIs the self-broadening of the target gas.
Preferably, phi in step S6v(v0) The calculation formula of (2) is as follows:
wherein ,
is a function of the gaussian function and is,
a. y is a self-defined parameter,
ΔνDat a concentration of X1, the FWHM (. DELTA.v) is in the Gaussian line formcIs half-width, v is the laser wavelength, v0The three parameter units are cm for the central wavelength of spectral absorption-1。
Preferably, the concentration calculation formula in step S7 is:
wherein ,
is the maximum value I of the emergent signalt(v0) And incident signal I0(v0) The ratio at the center wavelength, L represents the optical path length,
is the maximum value of the logarithmic absorbance, v0Is the spectral absorption center wavelength, and S (T) is the line intensity of the gas absorption line.
Preferably, the preset value in step S8 is determined according to the iteration step.
The system for realizing the TDLAS-based gas concentration measurement method comprises the following steps: the gas detection device comprises a first gas cylinder to be detected, a second gas cylinder to be detected, a first flow controller, a second flow controller, a gas mixer, a gas absorption cell, a detector, a collimator, a laser, a preamplifier, a laser controller and a PC (personal computer); the first gas cylinder to be detected is communicated to a first input port of the gas mixer through a first flow controller, the second gas cylinder to be detected is communicated to a second input port of the gas mixer through a second flow controller, an output port of the gas mixer is communicated to a first input port of a gas absorption pool, a second input port of the gas absorption pool is connected with an optical output end of the laser through a collimator, a control input end of the laser is connected with a first control output end of a laser controller, an output port of the gas absorption pool is connected with an input end of a detector, an output end of the detector is connected to an input end of a preamplifier, an output end of the preamplifier is connected with an input end of the laser controller, and a second control output end of the laser controller is connected with a PC.
Preferably, the PC includes a memory and a processor for storing and executing the steps of claim 1, respectively.
Preferably, the system further comprises: the gas absorption device comprises a flow display instrument and a temperature controller, wherein a first input end and a second input end of the flow display instrument are respectively and electrically connected with a first flow controller and a second flow controller, and the temperature controller is electrically connected with a gas absorption cell.
A third aspect of the present invention provides a computer-readable storage medium having embodied therein a TDLAS-based gas concentration measurement method program, which when executed by a processor, implements the steps of a TDLAS-based gas concentration measurement method as described above.
Compared with the prior art, the technical scheme of the invention has the beneficial effects that:
according to the invention, the gas concentration is obtained by acquiring the environmental parameters, the spectral parameters and the spectral absorption data of the laser and performing inversion calculation based on the TDLAS, an additional auxiliary measuring tool is not needed, the calculation complexity is extremely reduced, and the concentration detection efficiency is improved.
Drawings
Fig. 1 is a flow chart of a TDLAS-based gas concentration measurement method according to the present invention.
Fig. 2 is a block diagram of a TDLAS-based gas concentration measurement system according to the present invention.
Detailed Description
In order that the above objects, features and advantages of the present invention can be more clearly understood, a more particular description of the invention will be rendered by reference to the appended drawings. It should be noted that the embodiments and features of the embodiments of the present application may be combined with each other without conflict.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, however, the present invention may be practiced in other ways than those specifically described herein, and therefore the scope of the present invention is not limited by the specific embodiments disclosed below.
The noun explains:
TDLAS technique
The basic principle of TDLAS technology is the Beer-Lambert law, which describes the relationship between the transmitted spectral intensity and the incident spectral intensity of a laser beam passing through a homogeneous gaseous medium, and can be expressed as follows:
wherein ,τvIs the transmittance of the laser beam; i is0(v) is the incident laser intensity; i ist(v) is the intensity of the emergent laser after passing through the medium; p is the total pressure of the gas medium, and the unit is atm; s (T) is the linear intensity of the gas absorption line, and the unit is cm-2atm-1(ii) a Phi (v) is a linear function with the unit of cm, and meets the normalization condition
X is the volume concentration of the gas to be detected; and L is the distance of the laser in the gas medium and is expressed in cm.
In general, the concentration of the target gas can be obtained by performing a logarithmic operation on equation (1) and integrating the logarithmic operation in the entire frequency domain based on the Beer-Lambert law:
the above is the most commonly used integration area method in the TDLAS technique. There is some error in measuring concentration due to inaccuracy of baseline fitting and inaccuracy of fitting of the integrated area in the integrated area method.
Example 1
According to the invention, the environmental parameters and the spectral parameters are obtained, and the concentration of the target gas can be inverted by an iteration method through measuring the peak value of the absorbance at the central wavelength, so that the frequency domain and time domain conversion is avoided.
As shown in fig. 1, a first aspect of the present invention provides a TDLAS-based gas concentration measuring method, including the following steps:
s1, acquiring environmental parameters and spectral parameters;
in the scheme, the environmental parameters comprise pressure P, temperature T and optical path length L; the spectral parameters include: upsilon-y-shaped air widening deviceairAnd a target gas self-expanding aspectself。
S2, collecting spectral absorption data of laser, and calculating logarithmic absorbance to obtain the maximum value of the logarithmic absorbance;
it should be noted that the maximum value of the outgoing signal and the central wavelength v of the incoming signal need to be calculated before calculating the logarithmic absorbance0Ratio of (A) to (B)
The maximum value of the logarithmic absorbance was taken as
S3, calculating the maximum FWHM under the lorentz line type according to the environmental parameters and the spectral parameters, and recording as delta vc0；
In addition, Δ νc0The maximum half-height width is expressed and is calculated according to the environmental parameters and the spectral parameters to obtain the half-height width delta vcMinimum, not of other physical significance, only to assume a value closer to the true concentration in advance, to reduce the number of iterations, where Δ νc0＝P*2*min[γair,γself],min[γair,γself]Is a small value between γ air and γ self, where P represents the total pressure of the medium in atm.
S4, calculating the approximate concentration of the gas by using a concentration calculation formula, taking the approximate concentration as an initial iteration value, marking as X0, and assigning the value to X1;
the concentration calculation formula is as follows:
wherein ,
the ratio of the maximum of the outgoing signal to the incoming signal at the center wavelength,
is the maximum value of logarithmic absorbance, v is the laser wavelength, v0Is a spectral absorption center wavelength, phi (v)0) V is ═ v0Peak value of lorentzian line type calculated:
s5 FWHM in lorentz line, denoted as Δ v, calculated using X1c1(ii) a The specific calculation formula is as follows:
Δvc1＝P*[((1-X1)*2*Υair)+X1*2*Υself]。
s6, calculating V (v) under Voigt linear by using the density value of step S4, and recording as Vv(v0)；
φv(v0) The calculation formula of (2) is as follows:
wherein ,
is a function of the gaussian function and is,
a. y is a self-defined parameter; Δ νDAt a concentration of X1, the FWHM (. DELTA.v) is in the Gaussian line formDThe calculation formula of (2) is as follows:
wherein M is mass, M is gas relative molecular mass, k is Boltzmann constant, v is laser wavelength, v is0The three parameter units are cm for the central wavelength of spectral absorption-1；
S7, will phiv(v0) substituting into a concentration calculation formula to calculate the concentration, and marking as X2;
the formula for the concentration X2 is:
s8, judging whether the absolute value of the difference value between the X1 and the X2 is smaller than a preset value or not, and if the absolute value of the difference value is smaller than the preset value, outputting X1 as calculated concentration; if the X1 value is larger than or equal to the preset value, the value of X1 is changed back to the step S5 to continue the calculation.
It should be noted that the preset value in step S8 is determined according to an iteration step, in this embodiment, the iteration step is 0.0001, and the step depends on the order of magnitude of the actual concentration, so that the relative error is small. For example, for a target gas concentration of the order of 10%, a step size of 0.0001 may be chosen, and the relative error at the end of the iteration may be only of the order of 0.1% at the maximum.
Fig. 2 shows a block diagram of a TDLAS-based gas concentration measurement system.
A second aspect of the present invention provides a TDLAS-based gas concentration measurement system, the system comprising: the gas detection device comprises a first gas cylinder to be detected 1, a second gas cylinder to be detected 2, a first flow controller 3, a second flow controller 4, a gas mixer 5, a gas absorption pool 6, a detector 7, a collimator 8, a laser 9, a preamplifier 10, a laser controller 11 and a PC 12, wherein the first gas cylinder to be detected 1 is communicated with a first input port of the gas mixer 5 through the first flow controller 3, the second gas cylinder to be detected 2 is communicated with a second input port of the gas mixer 5 through the second flow controller 4, an output port of the gas mixer 5 is communicated with a first input port of the gas absorption pool 6, the second input port of the gas absorption pool 6 is connected with an optical output end of the laser 9 through the collimator 8, a control input end of the laser 9 is connected with a first control output end of the laser controller 11, an output port of the gas absorption pool 6 is connected with an input end of the detector 7, the output end of the detector 7 is connected to the input end of a preamplifier 10, the output end of the preamplifier 10 is connected to the input end of a laser controller 11, a second control output end of the laser controller 11 is connected to a PC 12, the PC includes a memory and a processor, the memory includes a TDLAS-based gas concentration measurement method program, and the TDLAS-based gas concentration measurement method program when executed by the processor implements the following steps:
s1, acquiring environmental parameters and spectral parameters;
s2, collecting spectral absorption data of laser, and calculating logarithmic absorbance to obtain the maximum value of the logarithmic absorbance;
s3, calculating the maximum FWHM of lorentz line type according to the environmental parameters and the spectral parameters, and naming the FWHM as delta vc0；
S4, calculating the approximate concentration of the gas by using a concentration calculation formula, taking the approximate concentration as an initial iteration value, marking as X0, and assigning the value to X1;
s5, calculating FWHM in lorentz line form by X1, and recording as delta vc1；
S6, calculating V (v) under Voigt linear by using the density value of step S4, and recording as Vv(ν0)；
S7, will phiv(v0) substituting into a concentration calculation formula to calculate the concentration, and marking as X2;
s8, judging whether the absolute value of the difference value between the X1 and the X2 is smaller than a preset value or not, and if the absolute value of the difference value is smaller than the preset value, outputting X1 as calculated concentration; if the X1 value is larger than or equal to the preset value, the value of X1 is changed back to the step S5 to continue the calculation.
It should be noted that, during gas measurement, a first gas to be measured and a second gas to be measured enter the gas mixer 5 through the first flow controller 3 and the second flow controller 4 respectively to be mixed, then the mixed gas enters the gas absorption cell 6, in the gas absorption cell 6, the gas has a spectrum absorption effect on laser light emitted from the laser 9, a spectrum signal detected by the detector 7 is transmitted to the meter PC 12 through the preamplifier 10 and the laser controller 11 to obtain corresponding spectrum absorption data, and the processor in the PC 12 executes the TDLAS gas concentration measurement method program to implement the above steps.
In this scheme, the system further includes: the gas absorption device comprises a flow display instrument 13 and a temperature controller 14, wherein a first input end and a second input end of the flow display instrument 13 are respectively and electrically connected with a first flow controller 3 and a second flow controller 4, and the temperature controller 14 is electrically connected with a gas absorption cell 6.
A third aspect of the present invention provides a computer-readable storage medium having embodied therein a TDLAS-based gas concentration measurement method program, which when executed by a processor, implements the steps of a TDLAS-based gas concentration measurement method as described above.
Example 2
Verification analysis
In this embodiment, the method of the present invention is verified by taking a near infrared CO2 infrared absorption spectroscopy system as an example, and as shown in fig. 2, the system for measuring gas concentration can be divided into a gas preparation part and a laser detection part. The gas preparation part mainly comprises two mass flow valves, a matched flow display instrument and a Herriott absorption cell with the optical path of 2000 cm. During the verification process, high-purity nitrogen (99.999%) and high-purity carbon dioxide (99.999%) pass through a mass flow valve and a flow display instrument to configure CO2 with six concentrations of 10%, 12%, 14%, 16%, 18% and 20%.
Under the working conditions that the temperature is 298K and the pressure is 1atm, C is collectedO2Spectral absorption signal around 1580 nm. The peak iteration method provided by the invention is adopted for the signal to carry out the inversion of the concentration, and the comparison is carried out with the traditional integral area method, and the results are shown in the following table 1:
TABLE 1 results comparison Table
As can be seen from the table, the accuracy of the inversion concentration of the method provided by the invention is superior to that of the traditional integral area method.
It should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.
Claims (10)
1. A TDLAS-based gas concentration measurement method is characterized by comprising the following steps:
s1, acquiring environmental parameters and spectral parameters;
s2, collecting spectral absorption data of laser, and calculating logarithmic absorbance to obtain the maximum value of the logarithmic absorbance;
s3, calculating the maximum full width at half maximum FWHM under a lorentz line type according to the environmental parameters and the spectral parameters;
s4, calculating the approximate concentration of the gas by using a concentration calculation formula to serve as an initial iteration value X0, and assigning X0 to X1;
s5 FWHM in lorentz line, denoted as Δ v, calculated using X1c1；
S6, calculating the linear function value phi (v) at the central wavelength under the Voigt linear by using the concentration value of the step S4, and recording the linear function value phi (v) as phiv(v0)；
S7, will phiv(v0) substituting into a concentration calculation formula to calculate the concentration, which is marked as X2;
s8, judging whether the absolute value of the difference value between the X1 and the X2 is smaller than a preset value or not, and if the absolute value of the difference value is smaller than the preset value, outputting X1 as the calculated concentration; if the X1 value is larger than or equal to the preset value, the value of X1 is changed back to the step S5 to continue the calculation.
2. The TDLAS-based gas concentration measuring method of claim 1, wherein the environmental parameters comprise: pressure P, temperature T, optical path length L; the spectral parameters include: air broadening of gammaairAnd self-broadening of target gas gammaself。
3. The TDLAS-based gas concentration measurement method of claim 1, wherein the approximate concentration X0 of the gas is calculated as:
wherein ,
is the maximum value I of the emergent signalt(v0) And incident signal I0(v0) At a central wavelength v0The ratio of (A) to (B), L represents the optical path length, P is the total pressure of the gas medium,
is the maximum value of the logarithmic absorbance, φ (v)0) Is v ═ v0Peak of the calculated lorentzian profile:
where S (T) is the linear intensity of the gas absorption line, Δ νc0Is the maximum full width at half maximum FWHM in the lorentz line.
4. The TDLAS-based gas concentration measuring method as claimed in claim 1, wherein the FWHM in Lorentz is calculated using X1 with specific formula:
vc1＝P*[((1-X1)*2*γair)+x1*2*γself]
wherein P is the pressure of the gaseous medium, gammaairFor air broadening, gammaselfIs the self-broadening of the target gas.
5. The TDLAS-based gas concentration measuring method as claimed in claim 1, wherein φ in step S6v(v0) The calculation formula of (2) is as follows:
wherein ,
is a function of the gaussian function and is,
a. y are all self-defined parameters, Δ vDIs the FWHM, Δ v, in a Gaussian line at a concentration of X1cIs half-width, v is the wavelength of the laser, v0The three parameter units are cm-1。
6. The TDLAS-based gas concentration measuring method as set forth in claim 1, wherein the concentration calculation formula in step S7 is:
wherein ,
is the maximum value I of the emergent signalt(v0) And incident signal I0(v0) The ratio at the center wavelength, L represents the optical path length,
is the maximum value of the logarithmic absorbance, v0Is the spectral absorption center wavelength, and S (T) is the line intensity of the gas absorption line.
7. The TDLAS-based gas concentration measuring method as claimed in claim 1, wherein the preset value in step S8 is determined according to iteration step.
8. The system for implementing the TDLAS-based gas concentration measurement method of claim 1, comprising: the gas detection device comprises a first gas cylinder to be detected, a second gas cylinder to be detected, a first flow controller, a second flow controller, a gas mixer, a gas absorption cell, a detector, a collimator, a laser, a preamplifier, a laser controller and a PC (personal computer); the gas cylinder to be detected is communicated with a first input port of the gas mixer through a first flow controller, the second gas cylinder to be detected is communicated with a second input port of the gas mixer through a second flow controller, an output port of the gas mixer is communicated with a first input port of the gas absorption pool, the second input port of the gas absorption pool is connected with an optical output end of the laser through a collimator, a control input end of the laser is connected with a first control output end of the laser controller, an output port of the gas absorption pool is connected with an input end of the detector, an output end of the detector is connected with an input end of the preamplifier, an output end of the preamplifier is connected with an input end of the laser controller, and a second control output end of the laser controller is connected with the PC.
9. The TDLAS-based gas concentration measurement system as set forth in claim 8, wherein the PC comprises a memory and a processor for storing and executing the steps of claim 1 respectively.
10. The TDLAS-based gas concentration measurement system of claim 8, further comprising: the gas absorption device comprises a flow display instrument and a temperature controller, wherein a first input end and a second input end of the flow display instrument are respectively and electrically connected with a first flow controller and a second flow controller, and the temperature controller is electrically connected with a gas absorption cell.
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| CN102798610A (en) * | 2012-06-05 | 2012-11-28 | 清华大学 | Gas concentration on-line measurement method based on full-width-at-half-maximum integral method |
| CN108181266A (en) * | 2017-12-14 | 2018-06-19 | 合肥金星机电科技发展有限公司 | TDLAS gas concentration detection methods |
| CN109100325A (en) * | 2018-06-14 | 2018-12-28 | 东南大学 | A kind of gas concentration measuring method based on spectral absorption second harmonic feature extraction |
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| CN102798610A (en) * | 2012-06-05 | 2012-11-28 | 清华大学 | Gas concentration on-line measurement method based on full-width-at-half-maximum integral method |
| CN108181266A (en) * | 2017-12-14 | 2018-06-19 | 合肥金星机电科技发展有限公司 | TDLAS gas concentration detection methods |
| CN109100325A (en) * | 2018-06-14 | 2018-12-28 | 东南大学 | A kind of gas concentration measuring method based on spectral absorption second harmonic feature extraction |
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| CN116879227A (en) * | 2023-07-07 | 2023-10-13 | 安徽农业大学 | Rapid gas inversion method based on laser spectrum |
| CN116879227B (en) * | 2023-07-07 | 2024-06-11 | 安徽农业大学 | Rapid gas inversion method based on laser spectrum |
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