阅读说明：本技术 一种基于tdlas直接吸收光谱的气体浓度标定及测量方法 (gas concentration calibration and measurement method based on TDLAS direct absorption spectrum ) 是由 陈昊 鞠昱 赵慧斌 韩立 常洋 于 2019-11-27 设计创作，主要内容包括：本发明涉及一种基于TDLAS直接吸收光谱的气体浓度标定及测量方法,通过比尔-朗伯定律推导出适用于大量程范围下的气体浓度与光强透射率对数的关系式并进行了简化处理,提出了用气体浓度倒数与光强透射率对数倒数的关系来进行拟合标定的算法,标定中通过一次函数关系对气体浓度倒数与光强透射率对数倒数进行拟合,实测时只需将光强透射率对数倒数代入拟合的一次函数关系式,求得气体浓度倒数,再进行一次求倒数运算即可得到实测浓度。本发明推导了完整的气体浓度与光强透射率对数之间的关系,并对关系式进行了简化处理,极大地简化了标定的复杂程度,提高标定精度,扩大了标定量程范围。(The invention relates to gas concentration calibration and measurement methods based on TDLAS direct absorption spectrum, which derive a relational expression of gas concentration and light intensity transmittance logarithm suitable for a wide range and simplify the process by the beer-Lambert law, and provide an algorithm for fitting and calibrating by the relationship of the gas concentration reciprocal and the light intensity transmittance logarithm reciprocal, wherein the gas concentration reciprocal and the light intensity transmittance logarithm reciprocal are fitted by times of function relationship in the calibration, and the actual measurement only needs to substitute the light intensity transmittance logarithm reciprocal into the times of function relationship to obtain the gas concentration reciprocal, and then times of reciprocal calculation are carried out to obtain the actual measurement concentration.)

1, kind of TDLAS direct absorption spectrum-based gas concentration calibration method, its characterized in that:

step , setting a plurality of groups of calibration environments, and measuring a gas concentration value C and a light intensity transmittance logarithm Ratio value under each calibration environment;

step two, performing reciprocal processing on the gas concentration C and the logarithm Ratio of the light intensity transmissivity to obtain reciprocal 1/C of the gas concentration and reciprocal 1/Ratio of the logarithm of the light intensity transmissivity;

and thirdly, fitting times by using a least square method to the reciprocal 1/C of the gas concentration and the reciprocal 1/Ratio of the logarithm of the light intensity transmittance to obtain corresponding fitting relational expressions and fitting coefficients, and completing calibration.

2. The TDLAS direct absorption spectroscopy-based gas concentration calibration method as claimed in claim 1, wherein:

the setting of multiple groups of calibration environments in the step 1 specifically includes:

calibrating environment generating equipment, namely a constant temperature and humidity box;

calibrating reference equipment, namely an online humidity detector for measuring the concentration of the current calibration environment gas;

the device to be calibrated, namely the device to be calibrated, and the TDLAS water vapor online monitoring device;

calibrating the stability time of the environment, and calibrating the stability time when the environment reaches a preset concentration;

number of test sets.

3. The TDLAS direct absorption spectroscopy-based gas concentration calibration method as claimed in claim 1, wherein:

in the step 1, the relationship between the full width at half maximum of the gas absorption spectrum line and the gas concentration is as follows:

n is the temperature coefficient, p₀,T₀Is standard air pressure and standard temperature, p, T is ambient air pressure and ambient temperature, gamma_airIs the full width at half maximum of the air absorption line, gamma_selfThe full width at half maximum of the absorption spectrum line of the gas to be measured; gamma ray_LThe line width of the gas to be measured and the line width of the background air are jointly determined; detecting the content of the trace gas, namely the concentration C of the gas to be detected is less than 1, omitting the latter term and simplifying the detection to

4. The TDLAS direct absorption Spectroscopy-based gas concentration calibration method of claim 3, wherein:

for the environment with the concentration of the gas to be measured higher than the preset value, the condition that C is less than 1 is no longer satisfied, and the influence of C on the line width must be considered;

when v ═ v₀I.e. at the central position of the absorption peak, then:

K_s(T) is the correction factor of the absorption line intensity with respect to temperature, S₀Is the absorption line intensity at standard atmospheric temperature, N₀Is the number of molecules per unit volume at standard atmospheric temperature;

the complete relationship between the gas concentration and the logarithm of the light intensity transmittance is as follows, taking into account the influence of the gas concentration on the absorption coefficient:

wherein k is₀＝S₀·N₀/π，K_s(T) is the correction factor of the absorption line intensity with respect to temperature, S₀Is the absorption line intensity at standard atmospheric temperature, N₀Is the number of molecules per unit volume at standard atmospheric temperature, n is the temperature coefficient, p₀,T₀At standard pressure and standard temperature, gamma_airIs the full width at half maximum of the air absorption line, gamma_selfThe full width at half maximum of the absorption spectrum line of the gas to be measured, C is the gas concentration, and L is the length of the gas chamber.

5. The TDLAS direct absorption Spectroscopy-based gas concentration calibration method of claim 4, wherein:

the reciprocal relationship between the simplified reciprocal of the gas concentration and the logarithm of the light intensity transmittance is as follows:

make the gas concentration reciprocal

6, gas concentration measurement method based on TDLAS direct absorption spectrum, its characteristic lies in:

step , setting a plurality of groups of calibration environments, and measuring a gas concentration value C and a light intensity transmittance logarithm Ratio value under each calibration environment;

step two, performing reciprocal processing on the gas concentration C and the logarithm Ratio of the light intensity transmissivity to obtain reciprocal 1/C of the gas concentration and reciprocal 1/Ratio of the logarithm of the light intensity transmissivity;

thirdly, times of fitting are carried out on the reciprocal 1/C of the gas concentration and the reciprocal 1/Ratio of the logarithm of the light intensity transmittance by using a least square method, times of fitting relational expressions and fitting coefficients corresponding to the gas concentration and the reciprocal 1/Ratio are obtained, and calibration is completed;

and step four, substituting the reciprocal 1/Ratio of the logarithm of the light intensity transmissivity under the current environment into a calibration fitting relational expression to obtain a reciprocal value 1/C of the gas concentration, and obtaining the gas concentration under the current environment after solving the reciprocal value.

Technical Field

The invention relates to wide-range gas concentration calibration methods and measurement methods based on TDLAS direct absorption spectrum, and belongs to the technical field of optical fiber sensing.

Background

In recent years, with the rapid development of spectroscopy, Tunable laser spectroscopy (TDLAS) has been developed and matured, and has the advantages of high sensitivity, fast response speed, real-time monitoring, excellent portability and the like, which has become , an important technology for gas detection.

The direct absorption spectrum calibration algorithm based on the TDLAS technology mainly utilizes light intensity and transmittance logarithm of measured light intensity to fit with standard concentration after laser with specific wavelength passes through a gas sample, and substitutes the actually measured light intensity and transmittance logarithm into a fitting relational expression to calculate the gas concentration under the current environment.

According to the traditional gas calibration method, according to the Beer-Lambert law, the logarithm of the gas concentration and the light intensity transmissivity is times of function relationship, times of functions are usually adopted for fitting during calibration, however, the influence of the gas concentration on the absorption coefficient is ignored in the condition, particularly, when the gas concentration is high, the relation between the gas concentration and the logarithm of the light intensity transmissivity is not strict times of function relationship, and the original calibration algorithm brings serious errors and influences the measurement accuracy.

Disclosure of Invention

The invention provides methods for calibrating the gas concentration in a wide-range based on TDLAS direct absorption spectroscopy, which can greatly reduce the calibration difficulty, improve the calibration precision and enlarge the calibration range by performing fitting calibration through the simplified reciprocal relation between the reciprocal of the gas concentration and the reciprocal of the logarithm of the luminous intensity transmittance.

The invention provides gas concentration calibration methods based on TDLAS direct absorption spectrum, which comprises the following steps:

step , setting a plurality of groups of calibration environments, and measuring a gas concentration value C and a light intensity transmittance logarithm Ratio value under each calibration environment;

step two, performing reciprocal processing on the gas concentration C and the logarithm Ratio of the light intensity transmissivity to obtain reciprocal 1/C of the gas concentration and reciprocal 1/Ratio of the logarithm of the light intensity transmissivity;

and thirdly, fitting times by using a least square method to the reciprocal 1/C of the gas concentration and the reciprocal 1/Ratio of the logarithm of the light intensity transmittance to obtain corresponding fitting relational expressions and fitting coefficients, and completing calibration.

The invention also provides gas concentration measuring methods based on TDLAS direct absorption spectrum, which comprises the following steps:

step , setting a plurality of groups of calibration environments, and measuring a gas concentration value C and a light intensity transmittance logarithm Ratio value under each calibration environment;

step two, performing reciprocal processing on the gas concentration C and the logarithm Ratio of the light intensity transmissivity to obtain reciprocal 1/C of the gas concentration and reciprocal 1/Ratio of the logarithm of the light intensity transmissivity;

thirdly, times of fitting are carried out on the reciprocal 1/C of the gas concentration and the reciprocal 1/Ratio of the logarithm of the light intensity transmittance by using a least square method, times of fitting relational expressions and fitting coefficients corresponding to the gas concentration and the reciprocal 1/Ratio are obtained, and calibration is completed;

and step four, substituting the reciprocal 1/Ratio of the logarithm of the light intensity transmissivity under the current environment into a calibration fitting relational expression to obtain a reciprocal value 1/C of the gas concentration, and obtaining the gas concentration under the current environment after solving the reciprocal value.

The technical scheme of the invention is as follows: the influence of the gas concentration on the absorption coefficient is considered, a complete relational expression of the gas concentration and the logarithm of the light intensity transmissivity is deduced, an algorithm for fitting and calibrating a relational curve of the reciprocal of the gas concentration and the reciprocal of the logarithm of the light intensity transmissivity is provided, and the original relational expression is simplified.

Compared with the prior art, the invention has the advantages that:

(1) the invention discovers that the relation between the gas concentration and the light intensity transmissivity is not linear in the wide-range calibration process in the TDLAS direct spectrometry, and the method is not in accordance with the original theory, so that the traditional calibration method can not be applied to the wide-range gas calibration process.

(2) The invention considers the influence of gas concentration on the absorption coefficient, deduces a complete relational expression of the gas concentration and the logarithm of the light intensity transmissivity, provides a method for fitting and calibrating by using the relation between the reciprocal of the gas concentration and the reciprocal of the logarithm of the transmissivity, greatly simplifies the calibration complexity, and compared with the traditional direct fitting method, the calibration method of the invention improves the calibration precision, enlarges the calibration range and reduces the calibration difficulty.

Drawings

FIG. 1 is a graph showing a simulation relationship between a gas concentration C and a logarithmic Ratio of light intensity transmittance according to the present invention;

FIG. 2 is a graph of the log curves of gas concentration and luminous intensity transmission, the corresponding fitted curves, and the relative errors of the fitted curves in an embodiment of the present invention;

FIG. 3 is a graph of reciprocal gas concentration and logarithmic reciprocal light transmittance and a back-extrapolated fitted graph according to an embodiment of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only some embodiments of the present invention, rather than all embodiments, and all other embodiments obtained by a person skilled in the art without creative efforts belong to the scope of the present invention.

The invention provides gas concentration calibration and measurement methods based on TDLAS direct absorption spectrum, the calibration process comprises the steps to the third step, the measurement step is the fourth step, and the method specifically comprises the following steps:

step , setting a plurality of calibration environments, and measuring a gas concentration value C and a light intensity transmittance logarithm Ratio value under each calibration environment.

And step two, performing reciprocal processing on the gas concentration C and the logarithm Ratio of the light intensity transmittance to obtain reciprocal 1/C of the gas concentration and reciprocal 1/Ratio of the logarithm of the light intensity transmittance.

And thirdly, fitting times by using a least square method to the reciprocal 1/C of the gas concentration and the reciprocal 1/Ratio of the logarithm of the light intensity transmittance to obtain corresponding fitting relational expressions and fitting coefficients, and completing calibration.

And step four, in actual measurement, substituting the reciprocal 1/Ratio of the logarithm of the light intensity transmittance in the current environment into a calibration fitting curve to obtain a reciprocal value 1/C of the gas concentration, and obtaining the gas concentration in the current environment after solving the reciprocal value.

The logarithmic relation of the gas concentration and the light intensity transmittance is as follows: according to Beer-Lambert's law, there are:

I_t＝I₀·exp[-α(v)CL](1)

wherein I_tThe transmitted light intensity after passing through the gas to be measured; i is₀Which is the intensity of incident light upon entering the gas to be measured, α (v) is the absorption coefficient, which is related to the type of gas and the frequency (wavelength) of light passing through the gas, C is the concentration of the gas to be measured, and L is the absorption path length of the gas to be measured through which the light passes.

The relation between the concentration of the gas to be measured and the logarithm Ratio of the light intensity transmittance can be obtained from the formula (1):

from the equation (2), the logarithm of the light transmittance is times as a function of the gas concentration, but the absorption coefficient α (v) is affected when the gas concentration is high.

The absorption coefficient expression α (v):

wherein, g_L(v) Is a Lorentzian linear function, S is the intensity of the absorption line, N is the total number of gas molecules per unit volume, α₀＝NS/πγ_LIs the central position of the gas absorption peak (v ═ v)₀) Absorption coefficient of (a), γ_LIs the half-width height of the gas absorption line, v is the light-emitting frequency, v₀Is the center frequency.

The relation between the full width at half maximum of the gas absorption line and the gas concentration is as follows:

n is the temperature coefficient, p₀,T₀Is standard air pressure and standard temperature, p, T is ambient air pressure and ambient temperature, gamma_airIs the full width at half maximum of the air absorption line, gamma_selfIs the full width at half maximum of the absorption spectrum of the gas to be measured.

As shown in formula (5),. gamma._LThe method is characterized in that the line width of the gas to be detected and the line width of background air are jointly determined, -type content detection of the trace gas is carried out, namely the concentration C of the gas to be detected is less than 1, the latter item is usually omitted, and the detection is simplified into

n_airIs the temperature coefficient of air. For the environment with high concentration of the gas to be measured, the condition of C < 1 is no longer satisfied, so the influence of C on the line width must be considered.

According to formula (3), when v ═ v₀I.e. at the central position of the absorption peak, then:

K_s(T) is the correction factor of the absorption line intensity with respect to temperature, S₀Is the absorption line intensity at standard atmospheric temperature, N₀Is the number of molecules per unit volume at the standard gas pressure temperature, and the complete relationship between the gas concentration and the logarithm of the light intensity transmittance is shown as (7) by taking the influence of the gas concentration on the absorption coefficient into consideration:

wherein k is₀＝S₀·N₀And/pi, and FIG. 1 is a graph of a simulation of the gas concentration C and the logarithm of the light intensity transmittance Ratio.

The simplified logarithmic relation of the gas concentration and the light intensity transmittance is as follows:

order to

Equation (7) reduces to:

as shown in the formula (8), the reciprocal of the gas concentration and the reciprocal of the logarithm of the light intensity transmittance are linear functions, so that the calibration process can be simplified.

According to embodiments of the present invention, as shown in fig. 2, (fig. 1 is a simulation result, fig. 2 and 3 are experimental results) the present invention selects water vapor as a measurement gas, sets 18 different water vapor concentration test points with a concentration range of 0.7% -50% VOL, extracts the direct absorption spectrum of water vapor, and compares the direct calibration method and the calibration method provided by the present invention, the calibration environment of the embodiments of the present invention is set as follows:

calibrating the environment generating equipment: a constant temperature and humidity box of DHS-100 of Beijing Yashilin;

calibration reference device (for measuring current calibration ambient gas concentration): a visala HMT337 online humidity detector;

equipment to be calibrated (equipment to be calibrated): a self-grinding TDLAS water vapor online monitoring device;

calibration environment stabilization duration (stabilization duration when the calibration environment reaches a preset concentration): 30 min;

number of test groups: and (4) 18 groups.

Specifically, in step , the direct absorption spectroscopy is adopted in this embodiment, 18 different water vapor concentration test points are set, the concentration range is 0.7% -50% VOL, the direct absorption spectrum of water vapor is extracted, and 18 sets of light intensity transmittance logarithm results under different water vapor concentrations are obtained, as shown in table 1.

TABLE 1 logarithmic results of light intensity transmittance of water vapor absorption spectrum at different concentrations

And step two, directly fitting the data in the table 1, selecting -time functions, quadratic functions and exponential functions as fitting functions, and analyzing correlation coefficients and root-mean-square errors of the results of the three fitting functions, wherein the results are shown in table 2, and a logarithmic curve of gas concentration and light intensity transmittance and a corresponding fitting curve are shown in fig. 2.

TABLE 2 correlation coefficient and root mean square error for three fitted curves

As shown in FIG. 2, under the condition of large-range concentration (gas concentration is 1-99%), an actually measured curve of logarithm of gas concentration and light intensity transmittance is not times of function relationship and accords with theoretical analysis, the whole curve trend of the actually measured curve is similar to the simulation result in FIG. 1. the fitting result in Table 2 also verifies that the fitting result of the function of times is poor, the correlation coefficient is minimum and the root mean square error is maximum, the fitting results of the quadratic function and the exponential function are close, the correlation coefficients are all larger than 0.99, the fitting result of the quadratic function is slightly better than that of the exponential function, and from the relative error curve in FIG. 2, the error of the three fitting curves under low concentration is larger and exceeds 100%, and the measurement is completely inaccurate.

Step three, performing reciprocal processing on the data in the table 1, wherein the result is shown in the table 2:

the reciprocal gas concentration and logarithmic reciprocal light transmittance curve and the inversely fitted logarithmic gas concentration and logarithmic light transmittance curve obtained according to table 2 are shown in fig. 3.

The reciprocal of the gas concentration and the logarithmic reciprocal curve of the light intensity transmittance in fig. 3 accord with the theoretically derived formula (8), the result is times of function line type, the times of function is adopted to fit the curve, the fitting coefficient is 0.9996, the root mean square error is 0.4901, the fitted curve is reversely pushed to obtain the logarithmic curve of the gas concentration and the light intensity transmittance, the result is shown in fig. 3, the fitting coefficient is 0.9999, the root mean square error is 0.0015, and the relative error in the whole concentration range is not more than 4 percent, which shows that the calibration method provided by the invention is effective, improves the calibration precision and enlarges the calibration range.

The above examples are provided only for the purpose of describing the present invention, and are not intended to limit the scope of the present invention. The scope of the invention is defined by the appended claims. Various equivalent substitutions and modifications can be made without departing from the spirit and principles of the invention, and are intended to be within the scope of the invention.