A Trace Hydrogen Concentration Detection Device and Method

Abstract

The present invention discloses a trace hydrogen concentration detection device, which includes a coaxial double-light-path light source, a first photoacoustic cell, a second photoacoustic cell, a first acoustic-electric conversion unit, a second acoustic-electric conversion unit, a differential operational amplifier and an output module. The coaxial double-light-path light source provides a first collimated light beam and a second collimated light beam with the same optical path, wavelength and frequency for the first photoacoustic cell and the second photoacoustic cell. A conduction path is provided between the first photoacoustic cell and the second photoacoustic cell, and a catalyst for catalyzing the oxidation of hydrogen to water is provided in the conduction path. The differential operational amplifier is used to output the difference of the voltage signals obtained by the first photoacoustic cell and the second photoacoustic cell, and the output module is used to obtain the hydrogen concentration. The present invention also discloses a corresponding trace hydrogen concentration detection method. The present invention can achieve the detection of trace hydrogen concentration at the ppb level.

Description

Technical Field

[0001] The present invention relates to a trace hydrogen concentration detection device and method, belonging to the technical field of substance concentration detection. Background Art

[0002] Hydrogen sensors are mainly divided into two categories: electro-chemical type and catalytic combustion type. Due to the limitations of the detection principle, the existing hydrogen sensors have significant deficiencies in trace hydrogen monitoring, and their accuracy is difficult to meet the requirements of the ppb level. When the hydrogen concentration is low, the heat released by oxidation combustion is less, resulting in the inability of the catalytic combustion sensor to perceive normally and low sensitivity. The measurement accuracy of the electro-chemical hydrogen sensor is at the ppm level. At the same time, since hydrogen is a symmetric diatomic gas and has no absorption characteristics for infrared spectra, it is difficult for traditional optical sensors to detect hydrogen.

[0003] Photoacoustic spectroscopy technology is one of the important methods for trace concentration detection of gases at present. The hydrogen-induced photoacoustic frequency shift technology based on photoacoustic spectroscopy technology utilizes the modulation effect of hydrogen concentration on the resonant frequency of the photoacoustic cell, and uses other gases with strong absorption spectral lines as pump gases to provide sound pressure for the system to achieve the detection of hydrogen concentration. However, the detection accuracy of this technology is still at the ppm level and does not meet the requirements of ppb-level trace hydrogen concentration detection. Summary of the Invention

[0004] Aiming at the above-mentioned defects of the prior art, the task of the present invention is to provide a trace hydrogen concentration detection device and method to achieve ppb-level trace hydrogen concentration detection.

[0005] The technical solution of the present invention is as follows: A trace hydrogen concentration detection device includes a coaxial double-light-path light source, a first photoacoustic cell, a second photoacoustic cell, a first acoustic-electric conversion unit, a second acoustic-electric conversion unit, a differential operational amplifier, and an output module. The coaxial double-light-path light source provides a first collimated light beam and a second collimated light beam with the same optical path, wavelength, and frequency for the first photoacoustic cell and the second photoacoustic cell. The wavelengths of the first collimated light beam and the second collimated light beam correspond to the absorption band of water molecules. A conduction path is provided between the first photoacoustic cell and the second photoacoustic cell, and a catalyst is provided in the conduction path. The catalyst is used to catalyze the oxidation of hydrogen to water. The first photoacoustic cell is provided with a test sample inlet, and the second photoacoustic cell is connected to a sampling pump. The first acoustic-electric conversion unit is used to convert the vibration signal of the resonant tube of the first photoacoustic cell into a first voltage signal and send it to the differential operational amplifier. The second acoustic-electric conversion unit is used to convert the vibration signal of the resonant tube of the second photoacoustic cell into a second voltage signal and send it to the differential operational amplifier. The differential operational amplifier is used to output the difference between the first voltage signal and the second voltage signal. The output module is used to output the concentration result according to the difference based on the linear relationship between voltage and concentration.

[0006] Furthermore, the wavelengths of the first collimated light beam and the second collimated light beam are 1450 - 1950 nm.

[0007] Furthermore, in order to facilitate the confirmation that the first collimated light beam and the second collimated light beam meet the requirements of the same optical path, wavelength, and frequency, photoelectric conversion units are provided at the relative ends of the first photoacoustic cell and the second photoacoustic cell where the first collimated light beam and the second collimated light beam are incident.

[0008] Furthermore, a one - way valve that conducts unidirectionally from the first photoacoustic cell to the second photoacoustic cell is provided in the conduction path to prevent the backflow of water vapor in the second photoacoustic cell.

[0009] Furthermore, a honeycomb desiccant is provided at the inlet of the sample to be measured. The water vapor in the sample to be measured is removed by the honeycomb desiccant and a steady flow is achieved.

[0010] Furthermore, the first photoacoustic cell and the second photoacoustic cell are arranged in a photoacoustic cell chamber, and a heater is provided in the photoacoustic cell chamber. The environmental temperature where the first photoacoustic cell and the second photoacoustic cell are located is maintained by heating with the heater to prevent water vapor condensation.

[0011] Furthermore, it includes a calibration device. The calibration device includes a hydrogen gas source, an air gas source, a gas mixer, a pressure reducing valve, and a needle valve. The hydrogen gas source and the air gas source are connected to the inlet of the gas mixer. The outlet of the gas mixer is connected to the pressure reducing valve. The outlet of the pressure reducing valve is connected to the needle valve. The outlet of the needle valve is connected to the inlet of the sample to be measured.

[0012] Furthermore, a gas buffer is provided between the outlet of the gas mixer and the pressure reducing valve.

[0013] Another technical solution of the present invention is a method for detecting trace hydrogen concentration, which is carried out based on the aforementioned trace hydrogen concentration detection. The detection method includes the steps: the sampling pump operates to pump the sample to be measured from the inlet of the sample to be measured into the first photoacoustic cell and sequentially passes through the conduction path and the second photoacoustic cell. The same - optical - path double - optical - path light source emits the first collimated light beam and the second collimated light beam into the first photoacoustic cell and the second photoacoustic cell. The differential operational amplifier outputs the difference between the first voltage signal and the second voltage signal obtained by the first acoustic - electric conversion unit and the second acoustic - electric conversion unit. The output module outputs the concentration result according to the corresponding linear relationship between the voltage and the concentration based on the difference.

[0014] Further, the detection method includes a calibration step, which is: mixing hydrogen and air in a set ratio to obtain a plurality of premixed standard samples with different hydrogen concentrations. The premixed standard samples enter the first photoacoustic cell from the sample inlet to be measured and sequentially pass through the conduction path and the second photoacoustic cell. The same optical path double-light path light source emits the first collimated light and the second collimated light into the first photoacoustic cell and the second photoacoustic cell. The differential operational amplifier outputs the difference between the first voltage signal obtained by the first acoustic-electric conversion unit and the second voltage signal to obtain the corresponding linear relationship between voltage and concentration.

[0015] The advantages of the present invention compared with the prior art are as follows:

[0016] Through the designed double-path photoacoustic cell, trace hydrogen is catalytically oxidized into water vapor, and the photoacoustic spectroscopy technology is used to detect trace water vapor, and then the corresponding hydrogen concentration is calculated. The design strategy of the same optical path double-light path and the processing by the differential operation of the voltage signal ensure an excellent linear relationship between the output electrical signal and the hydrogen concentration to be measured, significantly improving the accuracy and reliability of the measurement results and realizing the detection of trace hydrogen concentration at the ppb level. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic structural diagram of a trace hydrogen concentration detection device.

[0018] Figure 2 It is a schematic structural diagram of a calibration device of a trace hydrogen concentration detection device.

[0019] Figure 3 It is a schematic diagram of the principle of trace hydrogen concentration detection.

[0020] Figure 4 It is a graph of the corresponding linear relationship between the fitted voltage and concentration. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] The present invention will be further described below in conjunction with embodiments, but it is not intended to limit the present invention.

[0022] Please refer to Figures 1 to 3As shown in the figure, the trace hydrogen concentration detection device of the present invention includes a same optical path double optical path light source, a first photoacoustic cell 1, a second photoacoustic cell 2, a first acoustic-electric conversion unit 3, a second acoustic-electric conversion unit 4, a differential operational amplifier 5 and an output module 6. The same optical path double optical path light source provides a first collimated light ray 7 and a second collimated light ray 8 with the same optical path, wavelength and frequency for the first photoacoustic cell 1 and the second photoacoustic cell 2. In order to ensure the consistency of the first collimated light ray 7 and the second collimated light ray 8, the same optical path double optical path light source includes a laser driver 9, a laser diode 10, a beam splitter 11, a first reflector 12, a second reflector 13, a third reflector 14 and a compensating mirror 15. The laser wavelength of the laser diode 10 is selected between 1450 nm and 1950 nm, and this wavelength corresponds to the absorption band of water molecules. The beam splitter 11 divides the light source into two paths and ensures that its optical path satisfies L 4 =L 5 . The compensating mirror 15 makes up for the optical path difference of the beam splitter 11. Cooperating with the fine adjustment of the second reflector 13 and the third reflector 14, the size of L 3 is changed so that L 3 + L 7 =L 1 +L 2+ L 6 , ensuring that the first collimated light ray 7 and the second collimated light ray 8 have the same optical path.

[0023] The same optical path double optical path light source is arranged in the optical path chamber 16. Adjacent to the optical path chamber 16, a photoacoustic cell chamber 17 is arranged. The optical path chamber 16 and the photoacoustic cell chamber 17 are separated by a heat-insulating and flame-retardant material 18. The first photoacoustic cell 1 and the second photoacoustic cell 2 with the same internal structure are arranged in parallel in the photoacoustic cell chamber 17. Resonant tubes are arranged at the first ends of the first photoacoustic cell 1 and the second photoacoustic cell 2. The first ends of the first photoacoustic cell 1 and the second photoacoustic cell 2 are connected through a conduction path 19. The conduction path 19 is provided with a one-way valve that conducts unidirectionally from the first photoacoustic cell 1 to the second photoacoustic cell 2 and a honeycomb oxidation catalyst 20. This catalyst material is platinum wire, etc. This kind of oxidation catalyst can ensure the catalytic oxidation reaction of trace combustible gases at room temperature, so that trace hydrogen and oxygen are catalytically oxidized to generate water vapor.

[0024] A first photoacoustic cell 1 is provided with a first acoustic-electric conversion unit 3 for converting the vibration signal of the resonance tube of the first photoacoustic cell 1 into a first voltage signal, and a second photoacoustic cell 2 is provided with a second acoustic-electric conversion unit 4 for converting the vibration signal of the resonance tube of the second photoacoustic cell into a second voltage signal. In this embodiment, the acoustic wave acquisition parts of the first acoustic-electric conversion unit 3 and the second acoustic-electric conversion unit 4 are quartz tuning forks. A sample to be measured inlet 21 is arranged at the second end of the first photoacoustic cell 1, and a honeycomb desiccant 22 is arranged at the sample to be measured inlet 21. The honeycomb desiccant 22 is activated carbon for removing water vapor in the gas sample and stabilizing the air flow flowing into the first photoacoustic cell 1. The second end of the second photoacoustic cell 2 is connected to a sampling pump 24 through a buffer chamber 23.

[0025] A first photosensitive sensor 25 is further arranged at the second end of the first photoacoustic cell 1 for receiving a first collimated light beam 7 after passing through the resonance tube of the first photoacoustic cell 1, and a second photosensitive sensor 26 is further arranged at the second end of the second photoacoustic cell 2 for receiving a second collimated light beam 8 after passing through the resonance tube of the second photoacoustic cell 2. The signals measured by the first photosensitive sensor 25 and the second photosensitive sensor 26 are used for optical path calibration.

[0026] During the catalytic oxidation reaction process, trace hydrogen is converted to generate water vapor slightly higher than room temperature. If these water vapors directly enter the second photoacoustic cell 2, they will partially condense into water droplets. To avoid this phenomenon, an electric heating wire 27 is used to heat the photoacoustic cell chamber 17. A thermocouple 28 is arranged in the photoacoustic cell chamber 17 for temperature detection to control the working state of the electric heating wire, ensuring that the temperature of the photoacoustic cell chamber 17 is stably within the range of 45 °C to 50 °C, which can effectively prevent the occurrence of water vapor condensation phenomenon. Since the gas in the photoacoustic cell chamber 17 will expand after being heated, to ensure the stable operation of the system, a tiny air hole 29 is designed on one side of the photoacoustic cell chamber 17.

[0027] The differential operational amplifier 5 and the output module 6 are signal processing units. The differential operational amplifier 5 receives the first voltage signal and the second voltage signal and outputs the difference between the two. The output module 6 is used to output the concentration result according to the linear relationship between the voltage and the concentration obtained after calibration based on the difference.

[0028] During calibration, a premixed standard sample with a determined hydrogen concentration is provided to the inlet of the sample to be measured through a calibration device. The calibration device includes a hydrogen gas source 30, an air gas source 31, a gas mixer 32, a gas buffer 33, a pressure reducing valve 34, and a needle valve 35. The hydrogen gas source 30 is connected to the gas mixer 32 after passing through a first regulating valve 36 and a first mass flowmeter 37. The air gas source 31 is connected to the gas mixer 32 after passing through a second regulating valve 38 and a second mass flowmeter 39. The outlet of the gas mixer 32 is connected to the gas buffer 33. The outlet of the gas buffer 33 is connected to the pressure reducing valve 34. The outlet of the pressure reducing valve 34 is connected to the needle valve 35 through a pressure gauge 40. The outlet of the needle valve 35 is connected to the inlet 21 of the sample to be measured.

[0029] The principle of detection by the trace hydrogen concentration detection device is as follows: When the first collimated light 7 and the second collimated light 8 enter the first photoacoustic cell 1 and the second photoacoustic cell 2, water molecules will absorb light energy and cause periodic expansion, thereby generating sound waves. The weak sound waves resonate with the quartz tuning fork under the action of the resonance tube, converting the sound signal into a voltage signal. By analyzing the voltage signal of the quartz tuning fork, the corresponding water vapor concentration information can be obtained. According to the photoacoustic spectroscopy principle of the quartz tuning fork, the signal intensity is proportional to the gas absorption coefficient:

[0030] (1)

[0031] δ is a constant, S is the signal intensity, α is the absorption coefficient, P is the laser power, Q is the quality factor of the quartz tuning fork, f 0 is the resonance frequency of the quartz tuning fork. The main function of the first photoacoustic cell 1 is to measure the residual water vapor in the dried gas. The hydrogen in the sample gas reacts with oxygen through catalytic oxidation to generate water vapor after passing through the honeycomb oxidation catalyst 22. The main function of the second photoacoustic cell 2 is to measure the total amount of the residual water vapor in the dried gas entering the first photoacoustic cell 1 and the water vapor generated after catalytic oxidation.

[0032] The concentration of the residual trace water vapor in the first photoacoustic cell 1 is proportional to the signal intensity. At this time, the signal intensity output by the first sound-electric conversion unit 3 is S 1 . The voltage signal intensity output by the second sound-electric conversion unit 4 of the second photoacoustic cell 2 is S 2 . After being processed by the differential operational amplifier 5, the output signal S t is S 2 the difference between S 1 and

[0033] S t = S 2 - S 1 (2)

[0034] As can be seen from Equation (1), there is a linear relationship between the voltage signal output by the quartz tuning fork and the directly measured gas concentration. The voltage signal S t , after being processed by differential operation, also forms a clear linear correlation with the water vapor concentration generated during the catalytic oxidation process. Trace hydrogen is completely oxidized to water vapor under the action of the catalyst. Therefore, the voltage signal S t also shows a linear relationship with the hydrogen concentration. By configuring a standard concentration of hydrogen and detecting it, the output signal S t is fitted with the hydrogen concentration, and the corresponding fitting result gives the linear relationship between voltage and concentration, which can be used for accurate and reliable measurement of the actual hydrogen concentration.

[0035] The method of detecting based on the trace hydrogen concentration detection device of this embodiment is as follows:

[0036] First, perform optical path calibration. The sampling pump starts to work. First, check whether the signal values of the first photosensitive sensor 25 and the second photosensitive sensor 26 are normal. If the signal of the second photosensitive sensor 26 is weak, the third reflector 14 needs to be finely adjusted forward and backward. Then, observe whether the absolute value of the voltage signal S t is less than 10 -6 V. If not, the second reflector 13 and the third reflector 14 need to be adjusted. The second reflector 13 and the third reflector 14 always remain parallel. When the voltage signal value is greater than 0, it is necessary to reduce the optical path size of the second collimated light 8 corresponding to the second photoacoustic cell 2. The specific operation steps are as follows: finely adjust the second reflector 13 and the third reflector 14 counterclockwise, and at the same time, along the horizontal direction, finely adjust the third reflector 14 in the direction approaching the second photoacoustic cell 2. Conversely, if the absolute value of the voltage signal is less than 0, the second reflector 13 and the third reflector 14 should be finely adjusted clockwise, and at the same time, along the horizontal direction, finely adjust the third reflector 14 in the direction away from the second photoacoustic cell 2.

[0037] Then, calibration is carried out to obtain the corresponding linear relationship between voltage and concentration. An air pump is used to sample air standardly to obtain an air gas source 31, and the hydrogen provided by a hydrogen cylinder is a hydrogen gas source 30. The flow rates of air and hydrogen are controlled by a first regulating valve 36, a first mass flowmeter 37, a second regulating valve 38, and a second mass flowmeter 39, and are introduced into a gas mixer 32 for uniform mixing. Finally, the fully mixed gas is transported to a gas buffer 33 to provide a stable and reliable gas source for subsequent experiments or tests. The pressure range of the gas buffer 33 is controlled between 4 and 5 MPa, and the volume of the gas buffer 33 is 1 m 3 . Taking the configuration of a 100 ppb H 2 gas sample as an example: ① Roughly calculate the gas mole number according to the pressure of the gas buffer 33. Assuming that the pressure of the gas buffer 33 is 4.5 MPa and the room temperature is 298.15 K, the corresponding gas mole number is calculated to be 1613.67 mol by the ideal gas state equation. ② Determine the final mass ratio of hydrogen and air according to the rough calculation result. The mole number of hydrogen is taken as 1.60×10 -5 mol, and the mass of hydrogen is 0.000032 g; the mole number of air is taken as 1.60×10 -5 mol, and the corresponding mass of air is 4640 g. ③ According to the mass configuration situation, control the intake air to configure a 100 ppb H 2 gas sample.

[0038] During the calibration process, by adjusting the pressure reducing valve 34, ensure that the pressure at the inlet 21 of the sample to be measured is stable at 0.15 MPa to meet the stable operation requirements of the system. By testing different hydrogen mixed gas samples, the corresponding linear relationship between voltage and concentration can be obtained, as Figure 4 shown.

[0039] During specific testing, start the sampling pump 24 to pump the sample to be measured from the inlet 21 of the sample to be measured into the first photoacoustic cell 1 and sequentially pass through the conduction path 19 and the second photoacoustic cell 2. The same optical path double-light-path light source emits the first collimated light 7 and the second collimated light 8 into the first photoacoustic cell 1 and the second photoacoustic cell 2. The differential operational amplifier 5 outputs the difference between the first voltage signal and the second voltage signal obtained by the first acoustic-electric conversion unit 3 and the second acoustic-electric conversion unit 4, and the output module 6 outputs the concentration result according to the difference based on the corresponding linear relationship between voltage and concentration. To verify the reliability of the trace hydrogen concentration detection device, the calibration device is still used to configure different hydrogen concentrations for measurement. The following table records the comparison data between different hydrogen detection results and theoretical results,

[0040]

[0041] The above results further verify the reliability of the present invention in trace hydrogen detection.

Claims

1. A trace hydrogen concentration detection device, characterized in that: The invention comprises a same-optical-path dual-path light source, a first photoacoustic cell, a second photoacoustic cell, a first acoustic-to-electric conversion unit, a second acoustic-to-electric conversion unit, a differential operational amplifier and an output module. The same-optical-path dual-path light source is arranged in an optical path chamber. The same-optical-path dual-path light source provides the first photoacoustic cell and the second photoacoustic cell with a first collimated light and a second collimated light having the same optical path, wavelength and frequency. The same-optical-path dual-path light source comprises a laser driver, a laser diode, a beam splitter, a first reflector, a second reflector, a third reflector and a compensating mirror. The beam splitter divides the light emitted by the laser diode into two paths of light. One path of light passes through the first reflector and the compensating mirror in sequence to form the first collimated light. The other path of light passes through the second reflector and the third reflector arranged in parallel to each other in sequence to form the second collimated light. The wavelengths of the first collimated light and the second collimated light correspond to the absorption band of water molecules. The first photoacoustic cell and the second photoacoustic cell are arranged in parallel with each other in a photoacoustic cell chamber adjacent to the optical path chamber, a conduction path is provided between the first photoacoustic cell and the second photoacoustic cell, a catalyst is provided in the conduction path, the catalyst is used to catalyze the oxidation of hydrogen into water, the first photoacoustic cell is provided with a sample inlet to be tested, one end of the second photoacoustic cell is connected to a sampling pump through a buffer chamber, the first acoustic-to-electric conversion unit is used to convert the resonance tube vibration signal of the first photoacoustic cell into a first voltage signal and send it to the differential operational amplifier, the second acoustic-to-electric conversion unit is used to convert the resonance tube vibration signal of the second photoacoustic cell into a second voltage signal and send it to the differential operational amplifier, the differential operational amplifier is used to output the difference between the first voltage signal and the second voltage signal, and the output module is used to output the concentration result according to the difference according to the corresponding linear relationship between voltage and concentration.

2. The trace hydrogen concentration detection device according to claim 1, characterized in that: The wavelengths of the first collimated light and the second collimated light are 1450-1950 nm.

3. The trace hydrogen concentration detection device according to claim 1, characterized in that: The first photoacoustic cell and the second photoacoustic cell are provided with photosensitive sensors at opposite ends where the first collimated light and the second collimated light are incident.

4. The trace hydrogen concentration detection device according to claim 1, characterized in that: The conduction passage is provided with a one-way valve for one-way conduction from the first photoacoustic cell to the second photoacoustic cell.

5. The trace hydrogen concentration detection device according to claim 1, characterized in that: The inlet of the sample to be tested is provided with a honeycomb desiccant.

6. The trace hydrogen concentration detection device according to claim 1, characterized in that: The first photoacoustic cell and the second photoacoustic cell are arranged in a photoacoustic cell chamber, and a heater is arranged in the photoacoustic cell chamber.

7. The trace hydrogen concentration detection device according to claim 1, characterized in that: The calibration device comprises a hydrogen gas source, an air gas source, a gas mixer, a pressure reducing valve and a needle valve. The hydrogen gas source and the air gas source are connected to the inlet of the gas mixer, the outlet of the gas mixer is connected to the pressure reducing valve, the outlet of the pressure reducing valve is connected to the needle valve, and the outlet of the needle valve is connected to the inlet of the sample to be tested.

8. The trace hydrogen concentration detection device according to claim 7, characterized in that: A gas buffer is provided between the outlet of the gas mixer and the pressure reducing valve.

9. A method for detecting trace hydrogen concentration, characterized in that: Based on the trace hydrogen concentration detection device described in any one of claims 1 to 6, the detection method includes a detection step: a sampling pump operates to draw the sample to be tested from the sample inlet into the first photoacoustic cell and passes through the conduction path and the second photoacoustic cell in sequence, a dual-light path light source with the same optical path emits a first collimated light and a second collimated light into the first photoacoustic cell and the second photoacoustic cell, a differential operational amplifier outputs the difference between the first voltage signal obtained by the first acoustic-to-electric conversion unit and the second acoustic-to-electric conversion unit and the second voltage signal, and the output module outputs the concentration result according to the difference based on the corresponding linear relationship between voltage and concentration.

10. A method for detecting trace hydrogen concentration, characterized in that: Based on the trace hydrogen concentration detection device according to claim 7 or 8, the detection method includes a calibration step and a detection step, and the calibration step is: hydrogen and air are mixed in a set ratio to obtain a plurality of premixed standard samples with different hydrogen concentrations, the premixed standard samples enter the first photoacoustic cell from the sample inlet to be tested and pass through the conduction path and the second photoacoustic cell in sequence, the same optical path dual light path light source emits the first collimated light and the second collimated light into the first photoacoustic cell and the second photoacoustic cell, and the differential operational amplifier outputs the first voltage signal and the second voltage signal obtained by the first acoustic-to-electric conversion unit and the second acoustic-to-electric conversion unit The difference of the second voltage signal obtains a linear relationship between voltage and concentration; the detection step is: the sampling pump operates to draw the sample to be tested from the sample inlet into the first photoacoustic cell and passes through the conduction path and the second photoacoustic cell in sequence, and the same optical path dual-path light source emits the first collimated light and the second collimated light into the first photoacoustic cell and the second photoacoustic cell, the differential operational amplifier outputs the difference between the first voltage signal obtained by the first acoustic-to-electric conversion unit and the second acoustic-to-electric conversion unit and the second voltage signal, and the output module outputs the concentration result according to the difference based on the linear relationship between voltage and concentration.

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