CN116242790A - Long and short double-light-path measuring system and method based on non-spectroscopic infrared principle - Google Patents
Long and short double-light-path measuring system and method based on non-spectroscopic infrared principle Download PDFInfo
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Abstract
The invention relates to a long and short double-light-path measuring system and method based on a non-spectroscopic infrared principle, and belongs to the field of gas concentration detection. The invention aims to solve the problem of inaccurate measurement caused by drift in the prior art, and provides a long and short double-light-path measuring system and method based on a non-spectroscopic infrared principle; the method is based on a non-spectroscopic infrared absorption type gas sensor, discloses a short and short double-light-path measuring system and a specific design method of the system, wherein the system adopts two air chamber channels with different light path lengths for measurement, can compensate sensor errors caused by infrared light source spectrum drift, improves the accuracy of the sensor, and simultaneously discloses a specific calibration method of the system.
Description
Technical Field
The invention relates to a long and short double-light-path measuring system and method based on a non-spectroscopic infrared principle, and belongs to the field of gas concentration detection.
Background
In the field of environmental monitoring, gas detection has a wide range of requirements. At present, the sensor based on the non-spectroscopic infrared absorption principle is widely applied to the field of gas measurement due to the simple structure, small volume and low power consumption. The non-spectral infrared absorption sensor is mainly based on lambert-beer law, meaning that when a beam of light is injected into a gas chamber filled with absorption gas, the light intensity of the beam of light is reduced due to the absorption effect of the gas, and the emergent light intensity I and the incident light intensity I 0 The following relationship exists:
wherein k is υ The absorption coefficient of the gas, C the concentration of the gas, and L the optical path length of the light beam propagating in the gas cell. When the structure of the system is determined and the gas to be measured fills the whole gas chamber to reach balance, the absorption coefficient k of the gas υ Optical transmission path L and incident light intensity I 0 The emitted light intensity I is determined and known only in relation to the concentration C of the measured gas, so that the concentration of the measured gas can be calculated by measuring the emitted light intensity I.
Based on lambert-beer law, researchers designed a non-spectroscopic gas sensor, which does not need to perform spectroscopic treatment on a light source, but screens a light beam through a filter after the light beam passes through the gas to be measured to obtain the light beam in the wave number range in which the gas to be measured can have an absorption effect, for example, the carbon dioxide gas measurement generally selects 2350cm -1 And measuring the light intensity of the light beams by a detector, and further calculating the concentration of the carbon dioxide. Considering that the light intensity emitted by the light source is difficult to directly measure, the non-spectroscopic sensor generally adopts a dual-channel structure, and the influence of the detector on the gas concentration caused by the environment is reduced by selecting optical filters with different wavelength ranges for the measurement channel and the reference channel, so that the measurement of the gas concentration is realized.
The technical reasons for determining the existing non-spectroscopic sensor technology and generating these disadvantages are generally described as follows: when error compensation is carried out, the compensation is carried out only through the double-channel measurement scheme of the reference channel and the measurement channel, the individual difference of the detector caused by the manufacturing process and the drift caused by the environmental factors are well compensated, but because the light passing ranges of the optical filters adopted by the two channels of the detector are different, the output result of the detector is highly related to the light intensity of the light source in the corresponding spectrum range, if the light emitting spectrum of the light source changes, the light intensity measured by the two channels also fluctuates, so that the sensor indication drift occurs. The drift of the light source spectrum is very common in the actual use condition, the drift of the light source spectrum can be caused by the aging of the light source and the fluctuation of the light source voltage, through actual analysis, the fluctuation of the light source voltage is 1%, 40ppm of indication drift can be caused to the indication of the sensor due to the spectrum drift, and the drift can not be tolerated in the measurement environment requiring higher precision such as carbon emission.
Disclosure of Invention
The invention aims to solve the problem of inaccurate measurement caused by drift in the prior art, and provides a long and short double-light-path measuring system and method based on a non-spectroscopic infrared principle; the method is based on a non-spectroscopic infrared absorption type gas sensor, discloses a short and short double-light-path measuring system and a specific design method of the system, wherein the system adopts two air chamber channels with different light path lengths for measurement, can compensate sensor errors caused by infrared light source spectrum drift, improves the accuracy of the sensor, and simultaneously discloses a specific calibration method of the system.
The invention aims at realizing the following technical scheme:
long and short double-light-path measuring system based on non-spectroscopic infrared principle includes: the device comprises a light source, a measuring air chamber, a spherical mirror, an optical filter and a detector; the measuring air chamber consists of a first air chamber, a second air chamber and an interlayer; the optical path of the first air chamber is larger than that of the second air chamber; the light emitted by the light source irradiates the end part of the interlayer, and the reflected light enters the detector with the optical filter after being focused by the spherical mirrors of the first air chamber and the second air chamber.
The short and short double-light path measuring method based on the non-spectroscopic infrared principle comprises the following steps:
step one, calculating the optical path length of a first air chamber according to a light source spectrum;
the intensity I of the light actually measured by the detector is shown as follows
I＝∫S(υ)F(υ)T(υ)dυ (2)
Wherein, the spectral density function S (v) is a constant S; f (v) represents the function of the transmission coefficient of the filter with respect to the wave number of light, and T (v) represents the absorption coefficient spectrum of the measured gas;
the transmission function of the filter is
In the formula, v L Representing the minimum light wave number of the filter, v H Representing the maximum light-transmitting wave number of the filter, i.e. [ v ] L ,υ H ]Representing the light transmission range of the optical filter;
t (v) is expressed as
T(υ)＝e -k(υ)CL (4)
Wherein k (v) represents a gas absorption coefficient of a corresponding wave number, C represents a measured gas concentration, and L represents an optical path length of an optical path;
according to the formulas (1), (2) and (3), the transmittance of the light beam in the light passing range of the filter is calculated as shown in the following formula
According to the measurement precision required by the measurement system, the value of the transmissivity can be determined, and the optical path length of the first air chamber is calculated;
step two, determining the optical path length difference value of the first air chamber and the second air chamber, and further obtaining the optical path length of the second air chamber;
the measurement result of the measurement system is expressed as the ratio of the intensities of the two detectors, which is obtained according to the formula (3):
wherein T is L (v) and T S (v) represents the absorption coefficient spectra of the first and second air chamber channels, t L And t S The transmittance of the first air chamber channel and the second air chamber channel are respectively represented, and DeltaL is the length difference of the first air chamber and the second air chamber; determining a required detector intensity ratio according to the required measurement precision of the measurement system, calculating to obtain a gas chamber length difference delta L, and calculating to obtain the optical path length of the second gas chamber;
thirdly, reflecting light emitted by the light source, converging the light to the detector, and calculating the light intensity according to the Langmuir-beer law to obtain the gas concentration;
the optical path length of the first air chamber is denoted as L L The optical path length of the second air chamber is denoted as L S The signal intensity measured by the first air chamber is I L The signal intensity measured by the second air chamber is I S Then according to the lambert-beer law, get
Wherein k is L Representing the proportionality coefficient, k, of the first plenum channel S Representing the proportionality coefficient of the second air chamber channel, I 0 Representing the light intensity of the light source, and C represents the concentration of the detected gas; dividing the two types to obtain
The concentration C of the measured gas is obtained by solving the formula (9), and the constant a and the constant b are recorded as the constant which does not change along with the measuring process
The values of the constants a and b can be determined through the calibration in advance, so that the concentration of the measured gas can be calculated.
The calibration method comprises the following steps:
the lambert-beer law adds a constant term d' in the actual calibration;
f＝a'e b ' C +d' (11)
wherein f represents sensor indication, namely the ratio of the light intensity of the first air chamber to the light intensity of the second air chamber, a ', b ', d ' are constants to be calibrated, and the concentration is required to be calculated according to the sensor indication after calibration, so that the independent variable and the dependent variable in the formula (11) are converted, and the sensor indication is taken as the independent variable to obtain the formula (12)
Redefining the constant in formula (12) for calibration convenience to obtain
C＝a”ln(f-b”)+d” (13)
Knowing from equation (13) that the sensor readings need to be subtracted by an offset before taking the logarithm, the calibration formula changes to:
wherein f long For the output of the first air chamber channel, f short B for the output of the second air chamber channel long For the value b, b in the first gas chamber channel calibration curve short For the value b, a in the second chamber channel calibration curve double 、b double And d double Are all constants;
the calibration flow is as follows: firstly, calibrating according to the formula (13) to obtain respective calibration curves of a first air chamber detector and a second air chamber detector to obtain b long And b short Is a value of (2); then further calibrating and resolving by the formula (14) to obtain a double And d double And (3) calibrating the measuring system.
When the interlayer is of a structure with end points of the inclination angle being vertically symmetrical, the method for determining the thickness and the inclination angle of the middle interlayer comprises the following steps:
for symmetry, consider only the barrier layer located in the first plenum, abstract the first plenum into a polygon enclosed by six points ABCDEF in fig. 3; EG represents the inclined reflecting surface of the air chamber interlayer, a plane mirror can be arranged on the inclined reflecting surface, the inclination angle of the reflecting surface is marked as alpha, and the size of EF can be determined as constant x according to the size of the plane mirror; the light source is arranged at the point A, is abstracted into a point light source for processing, and the luminous cone angle of the light source is recorded as beta; according to geometrical optics, the symmetrical point J of A relative to EG is made, and then the light rays reflected by the EG plane from A can be regarded as the light rays emitted from the point J; connecting JG and extending the cross AB to H; as the intersection AB to I of the +.HJI, the +.HJI=beta is satisfied; to facilitate later computational presentation, extending BA to K such that KJ is parallel to AF;
the design of the barrier layer requires that the following two conditions are met: 1. cone angle condition of light source: the light source generally has a certain collimation characteristic, so that light beams emitted by the light source can reach the spherical mirror after being reflected by the interlayer; 2. detecting the surface opening angle condition: the light reflected by the spherical mirror can reach the detection surface and is not blocked by the interlayer;
the size of the spherical mirror is square with the side length of D, and the limiting condition is converted into a geometric limiting condition;
the area of the wall of the chamber that the light beam reaches after reflecting from the barrier EG is the area HI in fig. 3, so the cone angle condition of the light source can be written as: the length of HI is smaller than or equal to the side length D of the spherical mirror, namely HI is smaller than or equal to D, the EF length is noted as x, and the AF length is noted as y, and the following steps are included:
KJ＝AG×[1+tan(2α)]＝[y-x·tan(α)][1+tan(2α)] (15)
HI＝KI-KH＝KJ×[tan(2α)-tan(2α+β)] (16)
therefore, the cone angle condition of the light source is expressed as
[y-x·tan(α)][1+tan(2α)][tan(2α)-tan(2α+β)]≤D (17)
The condition of the angle of the detection surface needs to calculate the position of the light beam reaching the detection surface, namely the position of the image of the light source on the detector surface, according to the imaging principle of a spherical mirror, the imaging method comprises the following steps of
Wherein, I Article (B) And l Image forming apparatus Representing the horizontal distance r of the object plane or image plane, respectively, from the centre of the mirror Article (B) And r Image forming apparatus Representing the perpendicular distance of the object plane or image plane from the optical axis of the mirror, the spherical mirror is mounted on AB with the center not just taken as the midpoint of HI to maximize the utilization of the mirror, therefore there is l Article (B) ＝KJ，r Article (B) = (kh+ki)/2; for the first air chamber, there is l Article (B) ＝L L The method comprises the steps of carrying out a first treatment on the surface of the For the second air chamber, there is l Article (B) ＝L S Thereby, the height r of the image from the center of the spherical mirror can be calculated according to the formula (31) Image forming apparatus ；
The geometric observation that the connection line between the H point and the image point does not pass through the line segment EG is that
In the formula, HI is expressed as formula (2.29), and AH is expressed as follows
AH＝AG×tan(2α)＝[y-xtan(α)]tan(2α) (20)
Solving a feasible region determined by a light source cone angle condition and a detection surface opening angle condition; finally, the spacing and the inclination angle of the interlayer can be determined according to the result.
Determining the focal length of the spherical mirror;
according to the imaging rule of the spherical reflector, there are
Wherein r is Ball with ball body Representing the focal length of the sphere mirror, then according to the determination intervalGeometrical calculation of layer parameters, there are
And solving and calculating to obtain the focal length of the spherical mirror.
The beneficial effects are that:
1. according to the short and short double-light-path measuring method based on the non-spectroscopic infrared principle, two air chambers with different optical path lengths are adopted for measurement, so that the influence of light source spectrum drift on a system measuring result can be eliminated, and the measuring accuracy is higher compared with the existing scheme;
2. compared with the existing calibration scheme, the calibration method of the long and short double-light-path measurement system based on the non-spectroscopic infrared principle can obtain higher calibration precision and reduce measurement errors caused by calibration curve fitting;
3. the invention discloses a length double-light-path measuring method based on a non-spectroscopic infrared principle, which gives a generalized system design flow, can adopt different design indexes according to different required measuring precision and requirements, and is designed based on a design scheme given in the measuring method to obtain a measuring system meeting different measuring precision.
Drawings
FIG. 1 is a schematic diagram of the basic structure of the system of the present invention;
FIG. 2 is a schematic diagram of the system of the present invention;
FIG. 3 is a geometric abstract of the spacer design of the present invention;
FIG. 4 shows the transmittance change curves for different length air cells;
FIG. 5 is a graph showing the variation of the detector intensity ratio versus the difference in the length of the chamber for different gas concentrations;
FIG. 6 is a plot of the difference in detector intensity ratio as a function of the difference in plenum length;
FIG. 7 is a schematic view of an embodiment of the same length air chamber but different optical paths;
FIG. 8 is a schematic diagram of an embodiment of a measurement scheme using a probe;
FIG. 9 is a schematic view of an embodiment of the same length air chamber but different optical paths;
fig. 10 is a schematic structural diagram of an embodiment of a measurement scheme using a probe.
Detailed Description
For a better description of the objects and advantages of the present invention, the following description will be given with reference to the accompanying drawings and examples.
Example 1:
the embodiment discloses a length double light path measurement system based on non-spectroscopic infrared principle, including: the device comprises a light source, a measuring air chamber, a spherical mirror, an optical filter and a detector; the measuring air chamber consists of a first air chamber, a second air chamber and an interlayer; the optical path of the first air chamber is larger than that of the second air chamber; the light emitted by the light source irradiates the end part of the interlayer, and the reflected light enters the detector with the optical filter after being focused by the spherical mirrors of the first air chamber and the second air chamber. The system structure diagram is shown in fig. 2, the light source and the spherical mirror are both arranged on the plane of the left side of the air chamber, the two detectors are respectively arranged on the right sides of the two air chamber channels, and the right sides of the air chamber channels are provided with air holes for guiding the detected air to enter and fill the whole air chamber.
Step one: calculating the optical path length of the first air chamber according to the light source spectrum;
the intensity I of the light actually measured by the detector is shown as follows
I＝∫S(υ)F(υ)T(υ)dυ (23)
Wherein, the spectral density function S (v) is a constant S; f (v) represents the function of the transmission coefficient of the filter with respect to the wave number of light, T (v) represents the absorption coefficient spectrum of the measured gas,
the transmission function of the filter is
In the formula, v L Representing the minimum light wave number of the filter, v H Representing the maximum light-transmitting wave number of the filter, i.e. [ v ] L ,υ H ]Indicating the light transmission range of the filter, considering the application to carbon dioxide in practical useThe center wavelength of the light transmission range of the optical filter is about 4.26 mu m, the half-pass bandwidth is generally 90nm or 180nm, and the smaller half-pass bandwidth (90 nm) is adopted in calculation to consider the worst case of weak system light intensity. Calculating the light transmission range of the optical filter according to the center wavelength and the half-pass bandwidth to obtain upsilon L ＝2300cm -1 ，υ H ＝2400cm -1 ；
T (v) is expressed as
T(υ)＝e -k(υ)CL (25)
Wherein k (v) represents a gas absorption coefficient of a corresponding wave number, C represents a measured gas concentration, and L represents an optical path length of an optical path;
according to the formulas (1), (2) and (3), the transmittance of the light beam in the light passing range of the filter is calculated as shown in the following formula
According to the formula (4), calculating to obtain the transmittance corresponding to the optical paths with different lengths, drawing a change curve of the transmittance along with the length of the air chamber, wherein the calculated result is shown in figure 4, and the maximum length of the air chamber is only 20cm in consideration of the miniaturization of the sensor, so that the size is too large to be beneficial to the development of experiments;
as can be seen from FIG. 4, the transmittance varies significantly with the length of the air chamber, and when the length of the air chamber is 10cm, the transmittance is basically at the position with the maximum slope of the variation curve, the benefit caused by further increasing the length of the air chamber is gradually reduced, and the transmittance of the air chamber of 10cm is about 0.92, so that the measurement requirement of the detector can be basically met, and meanwhile, the volume of the air chamber is not excessively large, so that 10cm is selected as the length of the first air chamber.
Step two: determining the optical path length difference of the first air chamber and the second air chamber, and further obtaining the optical path length of the second air chamber;
the measurement result of the measurement system is expressed as the ratio of the intensities of the two detectors, which is obtained according to the formula (3):
wherein T is L (v) and T S (v) represents the absorption coefficient spectra of the first and second air chamber channels, t L And t S The transmittance of the first air chamber channel and the second air chamber channel are respectively represented, and DeltaL is the length difference of the first air chamber and the second air chamber;
as can be seen from equation (5), the ratio of the intensities of the detectors is the ratio of the transmittance of the corresponding channels, and the ratio of the detectors is related to the optical path difference of the air chamber only, but is independent of the specific length of the air chamber, and the first air chamber is determined to be 10cm, so that the length difference of the air chamber is changed, and a change curve of the intensity ratio of the detectors along with the length difference of the air chamber can be calculated, as shown in fig. 5, in order to judge whether the system can distinguish the gas concentration with a difference of 40ppm, two transmittance ratio curves with a difference of 40ppm of carbon dioxide concentration are given in the figure, and in order to more intuitively show, the absolute value of the difference of the two curves is taken as the ordinate, and the change curve of the transmittance ratio difference along with the length difference of the air chamber is drawn, as shown in fig. 6. It can be found that the difference in the intensity of the detector gradually increases with the increase in the length difference of the air chamber, but the increasing amplitude gradually becomes gentle after 4cm, i.e. the first derivative of the curve increases and decreases, and the maximum value is obtained around 4cm, and at this time, the benefit caused by further increasing the length difference of the air chamber gradually decreases. And considering that the length of the first air chamber is already determined to be 10cm, the too large difference of the length of the air chamber can cause the length of the second air chamber to be too small, so that the absorption effect of carbon dioxide is not obvious, and the sensor signal is more sensitive to external noise and is not reimbursed. Therefore, the air cell length difference Δl was selected to be 4cm, i.e., the length of the second air cell was selected to be 4cm.
Step three: the thickness and tilt angle of the middle spacer layer of fig. 2 were determined.
For symmetry, consider only the barrier layer located in the first plenum, abstract the first plenum into a polygon enclosed by six points ABCDEF in fig. 3; EG represents the inclined reflecting surface of the air chamber interlayer, a plane mirror can be arranged on the inclined reflecting surface, the inclination angle of the reflecting surface is marked as alpha, and the size of EF can be determined as constant x according to the size of the plane mirror; the light source is arranged at the point A, is abstracted into a point light source for processing, and the luminous cone angle of the light source is recorded as beta; according to geometrical optics, the symmetrical point J of A relative to EG is made, and then the light rays reflected by the EG plane from A can be regarded as the light rays emitted from the point J; connecting JG and extending the cross AB to H; as the intersection AB to I of the +.HJI, the +.HJI=beta is satisfied; to facilitate later computational presentation, extending BA to K such that KJ is parallel to AF;
the design of the barrier layer requires that the following two conditions are met: 1. cone angle condition of light source: the light source generally has a certain collimation characteristic, so that light beams emitted by the light source can reach the spherical mirror after being reflected by the interlayer; 2. detecting the surface opening angle condition: the light reflected by the spherical mirror can reach the detection surface and is not blocked by the interlayer;
before determining the specific constraints, it is first necessary to determine the dimensions of the spherical mirror. The spherical mirror also selects a square mirror surface to ensure that the light beam is totally reflected, otherwise, the reflection proportion of the upper boundary and the lower boundary of the mirror is small and even the mirror cannot reflect. Considering that the size of the air cell is not too large and that the size of the spacer mirror is 12.5X12.5 mm, determining EF to be 12.5mm facilitates mounting the mirror. The size of the spherical reflector is 25.00×25.00mm, and the width of the air chamber is 30mm to ensure the installation of the spherical reflector. After the spherical mirror is sized, the constraints described above can be converted into geometric constraints. According to the previous analysis of geometrical optics, the area of the wall of the chamber where the light beam reaches after being reflected from the light source by the plane mirror is the area HI in FIG. 3, so the condition of the cone angle of the light source is equivalent to HI being less than or equal to the size of the spherical mirror, i.e. HI is less than or equal to 25mm. Since EF is determined to be 12.5mm, for convenience of representation, it is not necessary to record it as a constant x, and there are
KJ＝AG×[1+tan(2α)]＝[y-x·tan(α)][1+tan(2α)] (28)
HI＝KI-KH＝KJ×[tan(2α)-tan(2α+β)] (29)
Therefore, the cone angle condition of the light source is expressed as
[y-x·tan(α)][1+tan(2α)][tan(2α)-tan(2α+β)]≤D (30)
The condition of the angle of the detection surface needs to calculate the position of the light beam reaching the detection surface, namely the position of the image of the light source on the detector surface, according to the imaging principle of a spherical mirror, the imaging method comprises the following steps of
Wherein, I Article (B) And l Image forming apparatus Representing the horizontal distance r of the object plane or image plane, respectively, from the centre of the mirror Article (B) And r Image forming apparatus Representing the perpendicular distance of the object plane or image plane from the optical axis of the mirror, the spherical mirror is mounted on AB with the center not just taken as the midpoint of HI to maximize the utilization of the mirror, therefore there is l Article (B) ＝KJ，r Article (B) = (kh+ki)/2; for the first air chamber, there is l Article (B) ＝L L The method comprises the steps of carrying out a first treatment on the surface of the For the second air chamber, there is l Article (B) ＝L S Thereby, the height r of the image from the center of the spherical mirror can be calculated according to the formula (31) Image forming apparatus ；
The geometric observation that the connection line between the H point and the image point does not pass through the line segment EG is that
In the formula, HI is expressed as formula (2.29), and AH is expressed as follows
AH＝AG×tan(2α)＝[y-xtan(α)]tan(2α) (33)
Solving a feasible region determined by a light source cone angle condition and a detection surface opening angle condition; finally, the interval and the inclination angle of the interlayer are respectively 15mm, and the inclination angle is 8 degrees.
Step four: determining optical parameters of the spherical mirror:
further, the optical parameters of the spherical mirror are determined, and only the focal length is related to the subject, and the calculation is performed below. According to the imaging rule of the spherical reflector, there are
Wherein r represents the focal length of the spherical mirror, there are
Solving for r L ＝49.834mm，r S ＝42.736mm。
Finally, the height of the air chamber is adjusted through the simulation of COMSOL so as to select the air chamber height with the best focusing effect of the detector surface. The plenum height refers to the furthest vertical distance of the light source from the channel face, i.e., the length of section AB in fig. 3. In order to avoid attenuation of light intensity caused by excessive reflection times of light on the air chamber wall and to prevent the sensor from being excessively large, simulation by COMSOL can be considered, and finally, the height of the first air chamber channel is selected to be 38mm, and the height of the second air chamber channel is selected to be 44mm.
Processing the sensor according to the designed scheme to obtain a sensor physical diagram as shown in figure 7, calibrating the sensor according to the method proposed by the method to obtain a calibration curve function as shown below
C＝1239×ln(f-0.2526)+3365 (36)
The test is carried out on the traditional measurement scheme and the double-air-chamber channel measurement scheme, the drift of the light source spectrum is simulated by changing the voltage at the two ends of the light source, the experiment is carried out in the 300ppm standard air environment, the obtained test result is shown in fig. 8, the error of the long and short double-light-path measurement scheme is far lower than that of the traditional measurement scheme, the error reduction rate reaches more than 50%, and the effectiveness of the long and short double-light-path measurement scheme is fully proved.
Example 2:
the scheme disclosed by the invention does not need to have difference in physical length of the two air chamber channels, and only needs to have different optical path lengths, for example, a structural schematic diagram shown in fig. 9 can be designed, a broken line represents a light propagation path, and light emitted by a light source reaches a detector after being refracted for many times on the wall of the air chamber. The incident angle of the light of the first air chamber and the second air chamber is limited by processing the air chamber interlayer, so that the refraction times of the light in the air chamber propagation process are controlled, the refraction times of the first air chamber are far greater than those of the light in the second air chamber, the optical path length of the light in the first air chamber is far greater than that of the light in the second air chamber, and the concentration of the measured gas in the air chamber can be calculated according to the formula (10), so that the measuring effect of the patent is achieved. Meanwhile, the embodiment can also adopt the calibration scheme in the method for calibration to obtain a calibration curve.
Example 3:
the number of detectors in the air chamber is not necessarily two, and one detector can be used if the light rays emitted by the light source can be converged on two different signal channels of the same detector. The two paths of signal channels of the detector are not reference channels and measuring channels, but are measuring channels, optical filters adopted on the channels are identical, and light waves in a light transmission range can generate absorption effects on the detected gas. At this time, the first air chamber and the second air chamber still need to have different optical path lengths, but the specific method for generating the optical path difference can be the method in embodiment 1, the method in embodiment 2, or other methods, and only the optical path lengths of the two paths are required to be ensured to be different, so that the intensities of the absorption effects are different, and the sensitivity to the gas concentration can be realized, wherein a specific embodiment is given according to the scheme of the optical path difference described in embodiment 2, as shown in fig. 9; this solution allows a better miniaturization of the space and requires only one detector, which further saves the costs of sensor manufacture compared to previous solutions.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
Claims (5)
1. Long and short double-light-path measuring system based on non-spectroscopic infrared principle, its characterized in that: comprising the following steps: the device comprises a light source, a measuring air chamber, a spherical mirror, an optical filter and a detector; the measuring air chamber consists of a first air chamber, a second air chamber and an interlayer; the optical path of the first air chamber is larger than that of the second air chamber; the light emitted by the light source irradiates the end part of the interlayer, and the reflected light enters the detector with the optical filter after being focused by the spherical mirrors of the first air chamber and the second air chamber.
2. A method of gas concentration measurement using the system of claim 1, wherein: the method comprises the following steps:
step one, calculating the optical path length of a first air chamber according to a light source spectrum;
the intensity I of the light actually measured by the detector is shown as follows
I＝∫S(υ)F(υ)T(υ)dυ (1)
Wherein, the spectral density function S (v) is a constant S; f (v) represents the function of the transmission coefficient of the filter with respect to the wave number of light, and T (v) represents the absorption coefficient spectrum of the measured gas;
the transmission function of the filter is
In the formula, v L Representing the minimum light wave number of the filter, v H Representing the maximum light-transmitting wave number of the filter, i.e. [ v ] L ,υ H ]Representing the light transmission range of the optical filter;
t (v) is expressed as
T(υ)＝e -k(υ)CL (3)
Wherein k (v) represents a gas absorption coefficient of a corresponding wave number, C represents a measured gas concentration, and L represents an optical path length of an optical path;
according to the formulas (1), (2) and (3), the transmittance of the light beam in the light passing range of the filter is calculated as shown in the following formula
According to the measurement precision required by the measurement system, the value of the transmissivity can be determined, and the optical path length of the first air chamber is calculated;
step two, determining the optical path length difference value of the first air chamber and the second air chamber, and further obtaining the optical path length of the second air chamber;
the measurement result of the measurement system is expressed as the ratio of the intensities of the two detectors, which is obtained according to the formula (3):
wherein T is L (v) and T S (v) represents the absorption coefficient spectra of the first and second air chamber channels, t L And t S The transmittance of the first air chamber channel and the second air chamber channel are respectively represented, and DeltaL is the length difference of the first air chamber and the second air chamber; determining a required detector intensity ratio according to the required measurement precision of the measurement system, calculating to obtain a gas chamber length difference delta L, and calculating to obtain the optical path length of the second gas chamber;
thirdly, reflecting light emitted by the light source, converging the light to the detector, and calculating the light intensity according to the Langmuir-beer law to obtain the gas concentration;
the optical path length of the first air chamber is denoted as L L The optical path length of the second air chamber is denoted as L S The signal intensity measured by the first air chamber is I L The signal intensity measured by the second air chamber is I S Then according to the lambert-beer law, get
Wherein k is L Representing the proportionality coefficient, k, of the first air chamber optical path S Indicating the proportionality coefficient of the second air chamber light path, I 0 Representing the light intensity of the light source, and C represents the concentration of the detected gas; dividing the two types to obtain
The concentration C of the measured gas is obtained by solving the formula (8), and the constant a and the constant b are recorded as the constant which does not change along with the measuring process
The values of the constants a and b can be determined through the calibration in advance, so that the concentration of the measured gas can be calculated.
3. The method of claim 2, wherein: the calibration method comprises the following steps:
the lambert-beer law adds a constant term d' in the actual calibration;
f＝a'e b ' C +d' (10)
wherein f represents sensor indication, namely the ratio of the light intensity of the first air chamber to the light intensity of the second air chamber, a ', b ', d ' are constants to be calibrated, and the concentration is required to be calculated according to the sensor indication after calibration, so that the independent variable and the dependent variable in the formula (10) are converted, and the sensor indication is taken as the independent variable to obtain the formula (11)
Redefining the constant in formula (11) for calibration convenience to obtain
C＝a”ln(f-b”)+d” (12)
Knowing that the sensor reading requires subtracting an offset before taking the logarithm according to equation (12), the calibration equation changes to:
wherein f long For the output of the first air chamber channel, f short B for the output of the second air chamber channel long For the value b, b in the first gas chamber channel calibration curve short For the value b, a in the second chamber channel calibration curve double 、b double And d double Are all constants;
the calibration flow is as follows: firstly, calibrating according to (12) to obtain respective calibration curves of a first air chamber detector and a second air chamber detector to obtain b long And b short Is a value of (2); then further calibrating and resolving by the formula (13) to obtain a double And d double And (3) calibrating the measuring system.
4. The method of claim 2, wherein: when the interlayer is of a structure with end points of the inclination angle being vertically symmetrical, the method for determining the thickness and the inclination angle of the middle interlayer comprises the following steps:
for symmetry, consider only the barrier layer located in the first plenum, abstract the first plenum into a polygon enclosed by six points ABCDEF in fig. 3; EG represents the inclined reflecting surface of the air chamber interlayer, a plane mirror can be arranged on the inclined reflecting surface, the inclination angle of the reflecting surface is marked as alpha, and the size of EF can be determined as constant x according to the size of the plane mirror; the light source is arranged at the point A, is abstracted into a point light source for processing, and the luminous cone angle of the light source is recorded as beta; according to geometrical optics, the symmetrical point J of A relative to EG is made, and then the light rays reflected by the EG plane from A can be regarded as the light rays emitted from the point J; connecting JG and extending the cross AB to H; as the intersection AB to I of the +.HJI, the +.HJI=beta is satisfied; to facilitate later computational presentation, extending BA to K such that KJ is parallel to AF;
the design of the barrier layer requires that the following two conditions are met: 1. cone angle condition of light source: the light source generally has a certain collimation characteristic, so that light beams emitted by the light source can reach the spherical mirror after being reflected by the interlayer; 2. detecting the surface opening angle condition: the light reflected by the spherical mirror can reach the detection surface and is not blocked by the interlayer;
the size of the spherical mirror is square with the side length of D, and the limiting condition is converted into a geometric limiting condition;
the area of the wall of the chamber that the light beam reaches after reflecting from the barrier EG is the area HI in fig. 3, so the cone angle condition of the light source can be written as: the length of HI is smaller than or equal to the side length D of the spherical mirror, namely HI is smaller than or equal to D, the EF length is noted as x, and the AF length is noted as y, and the following steps are included:
KJ＝AG×[1+tan(2α)]＝[y-x·tan(α)][1+tan(2α)] (14)
HI＝KI-KH＝KJ×[tan(2α)-tan(2α+β)] (15)
therefore, the cone angle condition of the light source is expressed as
[y-x·tan(α)][1+tan(2α)][tan(2α)-tan(2α+β)]≤D (16)
The condition of the angle of the detection surface needs to calculate the position of the light beam reaching the detection surface, namely the position of the image of the light source on the detector surface, according to the imaging principle of a spherical mirror, the imaging method comprises the following steps of
Wherein, I Article (B) And l Image forming apparatus Representing the horizontal distance r of the object plane or image plane, respectively, from the centre of the mirror Article (B) And r Image forming apparatus Representing the perpendicular distance of the object plane or image plane from the optical axis of the mirror, the spherical mirror is mounted on AB with the center not just taken as the midpoint of HI to maximize the utilization of the mirror, therefore there is l Article (B) ＝KJ，r Article (B) = (kh+ki)/2; for the first air chamber, there is l Article (B) ＝L L The method comprises the steps of carrying out a first treatment on the surface of the For the second air chamber, there is l Article (B) ＝L S Thereby, the height r of the image from the center of the spherical mirror can be calculated according to the formula (17) Image forming apparatus ；
The geometric observation that the connection line between the H point and the image point does not pass through the line segment EG is that
In the formula, HI is expressed as formula (2.29), and AH is expressed as follows
AH＝AG×tan(2α)＝[y-xtan(α)]tan(2α) (19)
Solving a feasible region determined by a light source cone angle condition and a detection surface opening angle condition; finally, the spacing and the inclination angle of the interlayer can be determined according to the result.
5. The method of claim 2, wherein: determining the focal length of the spherical mirror;
according to the imaging rule of the spherical reflector, there are
Wherein r is Ball with ball body Representing the focal length of the sphere, there are, based on geometrical calculations in determining the interlayer parameters
And solving and calculating to obtain the focal length of the spherical mirror.
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