Gas absolute concentration calibration-free measuring device and method based on photoinduced thermoelastic spectrum

Abstract

The invention discloses a gas absolute concentration calibration-free measuring device and method based on a photoinduced thermoelastic spectrum. The device comprises a signal generator, a laser, a laser collimation system, a laser intensity modulator controller, a laser intensity modulator, a gas chamber, a lens, a quartz tuning fork, a lock-in amplifier and a computer. The invention provides a gas absolute concentration calibration-free measurement method based on a photoinduced thermoelastic spectrum by utilizing an intensity modulation technology on the basis of the photoinduced thermoelastic characteristic of a quartz tuning fork, and the method avoids the unavoidable calibration technical process in the photoinduced thermoelastic spectrum technology all the time. And a new technical scheme is provided for solving the problems of weak anti-interference capability and poor long-term stability of a sensor system caused by the method. The detection device disclosed by the invention has the advantages of high sensitivity, strong disturbance immunity, low cost and the like.

Description

Technical Field

[0001] The present invention relates to a gas concentration measuring device and method, and particularly to a gas absolute concentration calibration-free measuring device and method based on photoacoustic spectroscopy. Background Art

[0002] Photoacoustic spectroscopy is a trace gas sensing technology that uses a quartz tuning fork as a light intensity detection element. It has the advantages of high detection sensitivity, fast response speed, good gas selectivity, and non-contact measurement, and has important application values in many fields such as environmental monitoring, power industry safety, and medical diagnosis. For a sensor, having good disturbance immunity and realizing a calibration-free measurement method for gas concentration are the keys to ensuring the long-term, stable, and accurate measurement of the system.

[0003] Photoacoustic spectroscopy is a typical representative of laser absorption spectroscopy sensing technology. When a modulated laser beam is incident on the target gas to be measured, the gas will absorb part of the laser energy and be excited to a high-energy state. The outgoing laser beam is focused on the surface of the quartz tuning fork, and the tuning fork absorbs the remaining energy of the laser and generates thermal expansion, resulting in elastic deformation. When the modulation frequency of the laser is equal to the resonance frequency of the quartz tuning fork, the periodic change of the light intensity will cause the quartz tuning fork to resonate. According to the piezoelectric characteristics of the quartz tuning fork, a current signal is finally generated, and the concentration information of the gas can be inversely calculated by demodulating the amplitude of the signal.

[0004] In photoacoustic spectroscopy technology, the system measures the laser energy absorbed by the gas to inversely calculate the concentration information of the gas. In this technology, a quartz tuning fork is used as a light intensity detection element, and by converting light energy into electrical energy, a current signal containing gas concentration information is obtained. However, due to the unknown photo-electric conversion coefficient of the tuning fork, the absolute concentration value of the gas cannot be directly inversely calculated from this signal, which means that this technology needs to rely on known standard concentration gases for pre-calibration to establish the corresponding relationship between concentration and signal before it can accurately detect the target gas with unknown concentration. However, during the long-term operation of the sensor system, the laser power, the optical path state of the system, and the performance and parameters of the quartz tuning fork will inevitably change, which will all cause the original calibration function to fail, ultimately resulting in inaccurate detection of gas concentration. Therefore, the calibration process makes the anti-interference ability of this technology weak and the long-term stability poor. As an inherent technical bottleneck that has existed since the birth of photoacoustic spectroscopy technology, it is difficult to achieve long-term stable and accurate measurement of gas concentration, seriously restricting the application and popularization of this technology in practical engineering. Summary of the Invention

[0005] Aiming at the problems of weak anti-interference ability and poor long-term stability in photoacoustic spectroscopy, the present invention provides a device and method for measuring the absolute concentration of gas based on photoacoustic spectroscopy without calibration.

[0006] The object of the present invention is achieved by the following technical solutions:

[0007] A device for measuring the absolute concentration of gas based on photoacoustic spectroscopy without calibration, comprising a signal generator, a laser, a laser collimation system, a laser intensity modulator controller, a laser intensity modulator, a gas cell, a lens, a quartz tuning fork, a lock-in amplifier, and a computer. The signal generator generates a low-frequency sawtooth signal acting on the laser to scan the laser wavelength. The laser outputs a laser beam, which is intensity-modulated by the laser intensity modulator after passing through the laser collimation system. The laser intensity modulator controller generates an intensity modulation signal and controls the modulation frequency of the laser intensity modulator to match the resonance frequency of the quartz tuning fork. The modulated laser is incident into the gas cell and absorbed by the target gas to be measured. The absorbed laser beam is focused on the surface of the quartz tuning fork through the lens. Under the modulation of the laser, the quartz tuning fork will undergo periodic elastic deformation and then generate vibrations. Based on the piezoelectric effect of quartz, the quartz tuning fork generates a current signal containing gas concentration information. This current signal is input into the lock-in amplifier for demodulation, and finally transmitted to the computer for processing to obtain the absolute concentration value of the gas.

[0008] A method for measuring the absolute concentration of gas based on photoacoustic spectroscopy without calibration, comprising the following steps:

[0009] Step 1: The signal generator generates a low-frequency sawtooth wave to control the output wavelength of the laser to scan, so that the wavelength scanning range covers the complete gas absorption line;

[0010] Step 2: After the laser beam output by the laser passes through the laser collimation system, intensity modulation is achieved through the laser intensity modulator. The laser intensity modulator controller generates a modulation signal to control the light intensity modulation frequency to be the same as the resonance frequency of the quartz tuning fork;

[0011] Step 3: The modulated laser beam passes through the gas cell, and the target gas with the concentration to be measured is introduced into the gas cell. After absorption by the gas, it is focused by the lens and irradiated on the surface of the quartz tuning fork;

[0012] Step 4: The detection signal generated by the quartz tuning fork is collected and demodulated by the lock-in amplifier;

[0013] Step 5: The demodulated signal is calculated and processed by the computer according to the following formula, and finally the absolute concentration value of the gas to be measured in the gas cell is obtained:

[0014]

[0015] Where C is the gas concentration; p is the gas pressure; S(T) is the gas absorption line intensity at temperature T; L is the gas absorption length; S0 is the photo-thermal elastic signal detected by the tuning fork when the concentration of the gas to be measured in the gas chamber is zero; S LITES is the demodulated photo-thermal elastic signal.

[0016] Compared with the prior art, the present invention has the following advantages:

[0017] Based on the photo-thermal elastic characteristics of a quartz tuning fork and using intensity modulation technology, the present invention proposes a method for measuring the absolute concentration of a gas without calibration based on photo-thermal elastic spectroscopy. This method avoids the inevitable calibration process in photo-thermal elastic spectroscopy, and provides a new technical solution to solve the problems of weak anti-interference ability and poor long-term stability of the sensor system caused by it. The detection device of the present invention has the advantages of high sensitivity, strong disturbance immunity, and low cost. Brief Description of the Drawings

[0018] Figure 1 is a schematic structural diagram of a device for measuring the absolute concentration of a gas without calibration based on photo-thermal elastic spectroscopy;

[0019] Figure 2 are the signals detected by the quartz tuning fork and the results obtained after processing. Detailed Embodiments

[0020] The technical solution of the present invention will be further described below with reference to the drawings, but it is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention shall be covered by the protection scope of the present invention.

[0021] The present invention provides a device for measuring the absolute concentration of a gas without calibration based on photo-thermal elastic spectroscopy, as shown in Figure 1As shown in the figure, the device includes a signal generator 1, a laser 2, a laser collimation system 3, a laser intensity modulator controller 4, a laser intensity modulator 5, a gas chamber 6, a lens 7, a quartz tuning fork 8, a lock-in amplifier 9, and a computer 10. The signal generator 1 generates a low-frequency sawtooth signal that acts on the laser 2 to scan the laser wavelength. The laser 2 outputs a laser beam that is intensity-modulated by the laser intensity modulator 5 after passing through the laser collimation system 3. The laser intensity modulator controller 4 generates an optical intensity modulation signal and controls the modulation frequency of the laser intensity modulator 5 to match the resonance frequency of the quartz tuning fork 8. The modulated laser is incident on the gas chamber 6 and absorbed by the target gas to be measured. The absorbed laser beam is focused on the surface of the quartz tuning fork 8 through the lens 7. Under the modulation of the laser, the quartz tuning fork 8 will undergo periodic elastic deformation and then generate vibrations. Based on the piezoelectric effect of quartz, the quartz tuning fork 8 generates a current signal containing gas concentration information. This current signal is input into the lock-in amplifier 9 for demodulation and finally transmitted to the computer 10 for subsequent processing to obtain the absolute concentration value of the gas. The specific measurement principle is as follows:

[0022] The outgoing laser is first intensity-modulated by the laser intensity modulator 5 and then incident into the gas chamber 6 filled with the target gas to be measured. When the laser wavelength satisfies the gas absorption condition, the gas absorption of the laser energy satisfies the Lambert-Beer law:

[0023] I(v) = I0(v)exp[-S(T)g(v,v0)LpC] (1)

[0024] where, I(v) is the outgoing light intensity at frequency v; I0(v) is the incident light intensity at frequency v; S(T) is the gas absorption line intensity at temperature T; g(v,v0) is the gas absorption line intensity at the temperature; L is the gas absorption length; p is the gas pressure; C is the gas concentration.

[0025] The outgoing laser is focused on the surface of the quartz tuning fork through the lens. The tuning fork absorbs part of the laser energy and undergoes thermal expansion, resulting in elastic deformation. The optical intensity modulation frequency of the laser intensity modulator 5 is set to be consistent with the resonance frequency of the tuning fork. Therefore, the periodic change in optical intensity will cause the tuning fork to resonate, causing the tuning fork to generate a piezoelectric signal. The intensity of this signal is proportional to the amplitude of the change in laser intensity. Since the laser intensity modulator 5 is used for modulation, the laser intensity changes from 0 to 1. Therefore, the amplitude of the change in optical intensity is the optical intensity after gas absorption. Based on the above analysis, the piezoelectric signal generated by the tuning fork can be expressed as:

[0026] S QTF = βI(v)sin(2πft) = βI0(v)sin(2πft)exp[-S(T)g(v,v0)LpC] (2)

[0027] Among them, β is the piezoelectric conversion coefficient of the quartz tuning fork; f is the modulation frequency of the laser equal to the resonance frequency of the quartz tuning fork.

[0028] The signal generated by the tuning fork is demodulated by the lock-in amplifier 9 using the correlation demodulation technique. The frequency of the reference signal is equal to the frequency of the tuning fork signal, and the phase is also set to be the same. The demodulated signal will pass through a low-pass filter in the phase-locked amplifier 9 to filter out the high-frequency signal. Therefore, the finally demodulated photoacoustic signal is expressed as:

[0029]

[0030] When the concentration of the gas to be measured in the gas chamber is zero, the photoacoustic signal detected by the tuning fork is:

[0031]

[0032] Dividing Equation (3) by Equation (4) gives the detection signal of the photoacoustic spectroscopy calibration-free measurement method as:

[0033]

[0034] It can be seen from Equation (5) that the processed tuning fork detection signal is independent of the incident light intensity I0(v) and the piezoelectric conversion coefficient β of the quartz tuning fork. This means that when the above parameters change, they will not affect the detection signal of the system, proving that the sensor system can achieve immunity to perturbations of relevant parameters. By scanning the laser wavelength using a signal generator, the gas absorption coefficient line shape function can be obtained, that is:

[0035]

[0036] This formula describes the variation relationship of the detection signal with the laser wavelength, representing the absorption of the laser by the gas at different wavelengths. By further processing the signal after wavelength scanning, the calculation expression for the absolute concentration of the gas using the calibration-free photoacoustic spectroscopy technique can be obtained:

[0037]

[0038] Among them, the numerator represents the integral area of the processed signal, and the parameters in the denominator are known quantities or can be obtained through simple measurements. Therefore, this method can obtain the absolute concentration of the gas, and can accurately detect the target gas with unknown concentration without pre-calibrating the sensor system using a gas with a standard concentration.

[0039] The present invention also provides a calibration-free measurement method for the absolute concentration of a gas based on photoacoustic spectroscopy. The specific implementation process of the method is as follows:

[0040] Step 1: The signal generator 1 generates a low-frequency sawtooth wave to control the output wavelength of the laser 2 to perform a scan, so that the wavelength scan range covers the complete gas absorption line.

[0041] Step 2: After the laser beam output by the laser 2 passes through the laser collimation system 3, intensity modulation is achieved through the laser intensity modulator 5. The laser intensity modulator controller 4 generates a modulation signal to control the optical intensity modulation frequency to be the same as the resonance frequency of the quartz tuning fork 8.

[0042] Step 3: The modulated laser beam passes through the gas cell 6. The target gas with the concentration to be measured is introduced into the gas cell. After being absorbed by the gas, it is focused by the lens 7 and irradiated on the surface of the quartz tuning fork 8.

[0043] Step 4: The detection signal generated by the quartz tuning fork 8 is collected and demodulated by the lock-in amplifier 9.

[0044] Step 5: The demodulated signal is calculated and processed by the computer according to formula (7), and finally the absolute concentration value of the gas to be measured in the gas cell is obtained.

[0045] In the present invention, the signal generator 1 provides a sawtooth wave signal to realize the scan of the output wavelength of the laser 2. The sawtooth wave scanning amplitude is required to enable the laser wavelength scanning range to completely cover the selected gas absorption spectrum line, and the scanning frequency is not higher than 10 Hz.

[0046] In the present invention, the laser 2 is a tunable single-longitudinal-mode distributed feedback semiconductor laser with continuous wave output or a tunable single-longitudinal-mode pulsed laser in any wavelength band.

[0047] In the present invention, the laser intensity modulator 4 can be a chopper, an acousto-optic modulator, an electro-optic modulator or other devices that can realize laser intensity modulation.

[0048] In the present invention, the laser intensity modulator controller 5 generates a modulation signal to control the laser intensity modulator 4 to realize the intensity modulation of the laser. To make the quartz tuning fork 8 generate the strongest piezoelectric signal, the frequency of the modulation signal needs to be set to be the same as the resonance frequency of the quartz tuning fork 8.

[0049] In the present invention, the gas cell 6 is a gas absorption cell and can also be replaced by a multi-pass cell to increase the absorption length of the gas and realize more sensitive detection of the gas.

[0050] In the present invention, the focus of the laser beam after being focused by the lens 7 is adjusted to the exposed quartz position at the finger joint of the quartz tuning fork 8, so that the tuning fork absorbs more laser energy and generates a larger vibration amplitude.

[0051] In the present invention, the resonance frequency range of the quartz tuning fork 8 is 2 kHz to 100 kHz, and the quality factor is not less than 1000.

[0052] In the present invention, the detection signal generated by the quartz tuning fork 8 is transmitted to a lock-in amplifier 9 for correlation demodulation. During the demodulation process, the reference signal is provided by the lock-in amplifier 9, whose frequency is the same as that of the detection signal, and the phase is set to be consistent with the detection signal.

[0053] In the present invention, the demodulated signal passes through a low-pass filter in the lock-in amplifier 9 to filter out high-frequency components, thereby obtaining a DC signal representing the absorption of the gas to the laser.

[0054] In the present invention, the computer 10 is connected to the lock-in amplifier 9, and real-time signal demodulation and acquisition are performed through software.

[0055] In the present invention, the computer 10 processes and calculates the demodulated signal according to formula (7), and finally obtains the absolute concentration value of the target gas in the gas chamber 6.

[0056] Embodiment:

[0057] In this embodiment, acetylene (C2H2) with a concentration of 19800 ppm is used as the detection object. The laser intensity modulator is a chopper, and the gas chamber is a gas absorption cell with a length of 20 cm. The technical solution of the present invention is verified. The signals detected by the quartz tuning fork and the results obtained after processing are as Figure 2 shown. The signals measured by the tuning fork with and without gas are S LITES and S0 respectively, and the linear function of the gas absorption coefficient obtained after processing is ln(S0 / S LITES ). According to formula (7), it can be calculated that the concentration of the gas to be measured in the gas absorption cell is 19700 ppm. Its measurement result is close to the concentration value of the standard gas used, and the relative error is only 0.5%. Therefore, this method can realize the inversion of the absolute concentration value of the unknown gas without calibration.

Claims

1. An absolute concentration calibration-free measurement device for gas based on photoacoustic spectroscopy, characterized in that The device includes a signal generator, a laser, a laser collimation system, a laser intensity modulator controller, a laser intensity modulator, a gas cell, a lens, a quartz tuning fork, a lock-in amplifier, and a computer. The signal generator generates a low-frequency sawtooth signal that acts on the laser to scan the laser wavelength. The laser outputs a laser beam that is intensity-modulated by the laser intensity modulator after passing through the laser collimation system. The laser intensity modulator controller generates an optical intensity modulation signal and controls the modulation frequency of the laser intensity modulator to match the resonance frequency of the quartz tuning fork. The modulated laser is incident on the gas cell and absorbed by the target gas to be measured. The absorbed laser beam is focused on the surface of the quartz tuning fork through the lens. Under the modulation of the laser, the quartz tuning fork will undergo periodic elastic deformation and thus generate vibrations. Based on the piezoelectric effect of quartz, the quartz tuning fork generates a current signal containing gas concentration information. This current signal is input into the lock-in amplifier for demodulation and finally transmitted to the computer for processing to obtain the absolute concentration value of the gas.

2. The gas absolute concentration calibration-free measurement device based on photo-thermoelastic spectroscopy according to claim 1, wherein The sawtooth scanning amplitude can enable the laser wavelength scanning range to completely cover the selected gas absorption line, and the scanning frequency is not higher than 10 Hz.

3. The gas absolute concentration calibration-free measurement device based on photo-thermoelastic spectroscopy according to claim 1, wherein The laser is a tunable single-longitudinal-mode distributed feedback semiconductor laser with continuous-wave output or a tunable single-longitudinal-mode pulsed laser in any wavelength band.

4. The gas absolute concentration calibration-free measurement device based on photoacoustic spectroscopy according to claim 1, wherein The laser intensity modulator is a chopper, an acousto-optic modulator, or an electro-optic modulator.

5. The gas absolute concentration calibration-free measurement device based on photoacoustic spectroscopy according to claim 1, wherein The gas cell is a gas absorption cell or a multi-pass cell.

6. The gas absolute concentration calibration-free measurement device based on photo-thermoelastic spectroscopy according to claim 1, wherein The focus of the laser beam after being focused by the lens is adjusted to the exposed quartz position at the finger joint of the quartz tuning fork.

7. The gas absolute concentration calibration-free measurement device based on photo-thermoelastic spectroscopy according to claim 1, wherein The resonance frequency range of the quartz tuning fork is 2 kHz to 100 kHz, and the quality factor is not less than 1000.

8. The gas absolute concentration calibration-free measurement device based on photo-thermoelastic spectroscopy according to claim 1, characterized in that The detection signal generated by the quartz tuning fork is transmitted into the lock-in amplifier for correlation demodulation. During the demodulation process, the reference signal is provided by the lock-in amplifier, its frequency is the same as the detection signal, and the phase is set to be consistent with the detection signal.

9. The gas absolute concentration calibration-free measurement device based on photo-thermoelastic spectroscopy according to claim 1, wherein The current signal is input into the lock-in amplifier for demodulation. The demodulated signal passes through a low-pass filter in the lock-in amplifier to filter out high-frequency components, thereby obtaining a direct current signal representing the gas absorption of the laser.

10. A method for measuring the absolute concentration of a gas based on optothermal spectroscopy without calibration using the device according to any one of claims 1-9, characterized in that The method includes the following steps: Step 1: The signal generator generates a low-frequency sawtooth wave to control the output wavelength of the laser to scan, so that the wavelength scanning range covers the complete gas absorption line; Step 2: After the laser beam output by the laser passes through the laser collimation system, intensity modulation is achieved through the laser intensity modulator. The laser intensity modulator controller generates a modulation signal to control the optical intensity modulation frequency to be the same as the resonance frequency of the quartz tuning fork; Step 3: The modulated laser beam passes through the gas cell, and the target gas with the concentration to be measured is introduced into the gas cell. After being absorbed by the gas, it is focused by the lens and irradiated on the surface of the quartz tuning fork; Step 4: The detection signal generated by the quartz tuning fork is collected and demodulated by the lock-in amplifier; Step 5: The demodulated signal is calculated and processed by the computer according to the following formula, and finally the absolute concentration value of the gas to be measured in the gas cell is obtained: In the formula, C is the gas concentration; p is the gas pressure; S(T) is the gas absorption line intensity at temperature T; L is the gas absorption length; S0 is the optothermal elastic signal detected by the tuning fork when the concentration of the gas to be measured in the gas chamber is zero; S LITES is the demodulated optothermal elastic signal.

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