End-tidal carbon dioxide signal self-adaptive processing method

Abstract

The invention provides an end-expiratory carbon dioxide signal self-adaptive processing method, which comprises the following steps: S1, carrying out carbon dioxide concentration detection by utilizing an end-expiratory carbon dioxide concentration detection module, setting a light source modulation frequency, and carrying out startup calibration on the carbon dioxide concentration detection module; s2, carbon dioxide concentration detection is connected to a detection device for multiple times of signal acquisition, peak-valley value calculation is utilized to obtain a peak-to-peak value, the step S2 is repeated for multiple times, and multiple peak-to-peak values are obtained; s3, according to the current temperature, based on the carbon dioxide concentrations in different temperature intervals and the peak-to-peak value curve collected by the detector, the carbon dioxide concentration of the breathing gas is obtained in a self-adaptive mode through the peak-to-peak value obtained in the step S2. According to the invention, during power-on self-test, peak value sampling time calibration and periodic detection sampling, the accuracy and resolution of end-breath carbon dioxide concentration measurement are improved by using the carbon dioxide concentration based on different temperature intervals and the peak-to-peak value curve acquired by the detector.

Description

Technical Field

[0001] This invention relates to the technical field of gas detection, and more particularly to an adaptive processing method for end-tidal carbon dioxide signals. Background Technology

[0002] Exhaled carbon dioxide (ETCO2) is one of the main indicators used by doctors to diagnose patients' respiratory system. Currently, most ETCO2 detection technologies are based on non-dispersive infrared (NDIR) technology. An infrared light source emits infrared light of 1-5 μm, which is absorbed by a gas cell of a certain length and then passes through a narrow-band filter with a wavelength of 4.26 μm. An infrared detector monitors the intensity of the transmitted 4.26 μm wavelength infrared light to calculate the concentration of CO2 gas.

[0003] Clinically, there are two methods for measuring ETCO2: bypass and mainstream. The bypass method uses an air pump to draw the gas to be measured into the air chamber, which results in waveform distortion and measurement delay. The mainstream method, on the other hand, uses an adapter that is directly embedded in the airway. The breathing gas flows through the airway into the adapter, and the carbon dioxide concentration of the breathing gas can be measured. Therefore, the response time is faster than that of the bypass method.

[0004] Mainstream end-tidal carbon dioxide sensors based on NDIR technology generally use two types of detectors: thermopile detectors and pyroelectric detectors. Thermopile detectors are susceptible to airflow interference, leading to significant fluctuations in measurement data, and their drift problem is also severe during long-term continuous monitoring. Therefore, pyroelectric detectors are mostly chosen as the sensor for acquiring carbon dioxide concentration signals. Because pyroelectric detectors cannot measure static signals, light source modulation is necessary; in this case, the modulated signal output by the pyroelectric detector carries carbon dioxide concentration information.

[0005] Despite the significant advantages of pyroelectric detectors, such as their fast response time and excellent dynamic performance, which make them crucial in gas detection, particularly in carbon dioxide concentration monitoring, they also face considerable challenges in practical applications. These challenges primarily stem from interference from various internal and external factors, posing a potential threat to the stability and accuracy of measurement data.

[0006] First, circuit noise is a major challenge for pyroelectric detectors. Due to unavoidable electromagnetic interference from the electronic components themselves and during signal transmission, circuit noise can infiltrate the detector's output signal in various forms. This noise can mask or distort the true changes in carbon dioxide concentration, causing unnecessary fluctuations in the measurement data and affecting subsequent data analysis and processing, such as the reconstruction of carbon dioxide concentration waveforms and the accurate calculation of respiratory rate.

[0007] Secondly, the modulation frequency and modulation depth of the light source also have a significant impact on the performance of the pyroelectric detector. Modulation of the light source is intended to improve the detector's sensitivity and signal-to-noise ratio, but improper modulation frequency settings or inaccurate modulation depth control can introduce additional interference. Excessively high modulation frequencies may prevent the detector from responding adequately, while insufficient modulation depth may reduce signal strength; both increase measurement errors and affect the accuracy of carbon dioxide concentration waveforms and the reliability of respiratory rate calculations.

[0008] Furthermore, the differences in dynamic response between individual pyroelectric detectors are a significant issue. Due to variations in manufacturing processes, material properties, and operating environments, different pyroelectric detectors may differ in response speed and sensitivity. This difference is particularly pronounced in mass production and practical applications, leading to deviations in measurement data from different detectors under the same conditions, further increasing the complexity of data processing and the risk of errors.

[0009] For example, invention patent application number 202210240200.9 discloses an adaptive carbon dioxide control system and method for goaf areas, including: collecting carbon dioxide concentration in the goaf area, constructing an inerting parameter inversion model, receiving carbon dioxide concentration in the goaf area, using goaf carbon dioxide monitoring data to perform feedback correction on the inerting parameter inversion model, constructing an optimized inerting parameter inversion model for carbon dioxide injection parameters, calculating the threshold of the carbon dioxide flow valve based on the optimized inerting parameter inversion model, and adaptively controlling the carbon dioxide flow rate of the carbon dioxide flow valve according to the threshold. This method achieves automatic detection of the safety performance of carbon dioxide injection in goaf areas and automatic control of the injection volume by constructing a goaf coal spontaneous combustion monitoring and early warning network, data transmission mode, and data processing mode. This monitoring and early warning method and device are highly practical and have wide application value. However, this method requires data feedback correction, which involves a large amount of computation. Summary of the Invention

[0010] To address the technical problem of errors in the calculation of carbon dioxide concentration waveform and respiratory rate in existing technologies, this invention proposes a mainstream end-tidal carbon dioxide signal adaptive processing method based on a concentration detection module. This method overcomes the problems of inaccurate data acquisition and offset caused by differences in the dynamic response characteristics between individual detectors and light source modulation, thereby improving the system's measurement accuracy and resolution.

[0011] To achieve the above objectives, the technical solution of the present invention is implemented as follows: an adaptive processing method for end-tidal carbon dioxide signals, characterized in that the steps include: S1: Set the light source modulation frequency and perform power-on calibration on the end-tidal carbon dioxide concentration detection module; S2: After completing the power-on calibration, connect the end-tidal carbon dioxide concentration detection module to the detection device to acquire signals multiple times, obtain peak and trough data and perform filtering processing, use peak-to-trough value to calculate peak-to-peak value, repeat step S2 multiple times to obtain multiple peak-to-peak values; S3: Based on the current temperature, select the relationship curve between carbon dioxide concentration and peak value in the corresponding temperature range. By comparing the peak value obtained in the previous step with the peak value on the relationship curve between carbon dioxide concentration and peak value, the corresponding carbon dioxide concentration is obtained, thus achieving adaptive acquisition of carbon dioxide concentration of respiratory gas.

[0012] The end-tidal carbon dioxide concentration detection module includes a breathing circuit adapter, which includes an adapter airway. A fixing buckle is movably installed on the outside of the adapter airway, and two slots are correspondingly provided on the fixing buckle. Lenses are installed in the slots respectively, and the center points of the two lenses are set on the same straight line. An infrared light source is installed on the outside of one lens, and a pyroelectric detector is installed on the outside of the other lens. The infrared light source and the pyroelectric detector are corresponding to each other, and both the infrared light source and the pyroelectric detector are connected to the host computer.

[0013] The pyroelectric detector includes a signal channel and a reference channel, and the detectors in both the signal channel and the reference channel are connected to a host computer.

[0014] The fixing buckle is a U-shaped fixing buckle with a notch. The adapter air passage is located in the center of the U-shaped fixing buckle, and the two slots are respectively located on both sides of the side where the notch is located.

[0015] The method for calibrating the end-tidal carbon dioxide concentration detection module in step S1 is as follows: S11: Place the end-tidal carbon dioxide concentration detection module in a clean air environment, power it on and perform a self-test. Input a modulation signal to control the modulation frequency of the infrared light source to obtain a modulation period time t. S12: The host computer controls the pyroelectric detector to perform N signal acquisitions within the modulation period time t, and transmits the acquired reference signal to the host computer through the reference channel; S13: The host computer analyzes the acquired reference signal to obtain the time when the peak signal was acquired and the time when the valley signal was acquired; S14: Calculate the maximum and minimum sampling delay times based on the modulation frequency of the infrared light source, set the time for acquiring the peak signal and the time for acquiring the peak signal, so that the pyroelectric detector can acquire signals at the peak and valley values ​​of the light source modulation frequency, and complete the power-on calibration.

[0016] The modulation signal in step S11 is a sine wave signal.

[0017] The method for obtaining the time of peak signal acquisition and valley signal acquisition in step S13 is as follows: The host computer analyzes the acquired reference signal to obtain the maximum and minimum values ​​of the signal in N acquisitions, and obtains the acquisition number 'a' corresponding to the maximum value and the acquisition number 'b' corresponding to the minimum value. The maximum value is the peak value of the period, and the minimum value is the trough value of the period. The time T for acquiring the peak signal is then given. Peak = (a-1) t / N, where the time for acquiring the valley signal is T. Trough = (b-1) t / N.

[0018] The steps, including step S14, involve calculating the maximum and minimum sampling delay times as follows: Based on the light source modulation frequency, calculate the time t / 4 corresponding to the maximum value and the time 3t / 4 corresponding to the minimum value of the light source modulation frequency. The sampling delay time for the maximum value is T1 = t / 4 - T. Peak The minimum sampling delay time T2 = 3t / 4 - T Trough The settings are configured via a host computer to ensure that the pyroelectric detector acquires signals at the peak and valley values ​​of the light source modulation frequency, thus completing the power-on calibration.

[0019] The method for obtaining the relationship curve between carbon dioxide concentration and peak value in step S3 is as follows: The temperature was divided into three temperature ranges: [0, 15)℃, [15, 30)℃, and [30, 45)℃. Within each of these three temperature ranges, carbon dioxide of different concentrations was repeatedly introduced into the end-tidal carbon dioxide concentration detection module. The peak-to-peak value of the carbon dioxide concentration in each temperature range was then obtained. The peak-to-peak values ​​corresponding to different carbon dioxide concentrations in each of the three temperature ranges were collected. The relationship curves between carbon dioxide concentration and peak-to-peak value in each temperature range were then fitted to obtain the curves of carbon dioxide concentration and peak-to-peak value collected by the detector in each temperature range.

[0020] The method for adaptively obtaining the carbon dioxide concentration of respiratory gas described in step S3 is as follows: select the relationship curve between carbon dioxide concentration and peak value in the corresponding temperature range according to the temperature range to which the current temperature belongs, and obtain the corresponding carbon dioxide concentration on the relationship curve based on the peak value obtained in step S2.

[0021] The beneficial effects of this invention are as follows: In the specific embodiments of this invention, by utilizing peak-to-peak curves of carbon dioxide concentration and detector acquisition at different temperature ranges during power-on self-test, calibration of peak sampling time, and periodic detection sampling points, the accuracy and resolution of measuring end-tidal carbon dioxide concentration are improved, and drift during long-term detection is reduced. Compared with existing technologies, this patent can overcome problems such as inaccurate data acquisition and offset caused by differences in the dynamic response characteristics between individual detectors and light source modulation without changing the existing hardware scheme, thereby improving the system's measurement accuracy and resolution. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of the structure of the present invention.

[0024] Figure 2 This is a cross-sectional view of the present invention.

[0025] Figure 3 This is a flowchart of the method of the present invention.

[0026] In the diagram, 1 is the infrared light source, 2 is the pyroelectric detector, 3 is the lens, 4 is the adapter gas path, and 5 is the fixing buckle. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] Existing detectors are susceptible to interference from circuit noise. Factors such as the modulation frequency and depth of the light source, as well as differences in the dynamic response between individual detectors, can cause fluctuations in the raw data, leading to errors in the calculation of the detected carbon dioxide concentration waveform and respiration rate. This invention utilizes an adaptive peak-to-peak processing method to obtain accurate acquired data, reduce data fluctuations, and improve measurement resolution.

[0029] like Figure 1As shown, the mainstream end-tidal carbon dioxide concentration detection module includes a breathing tubing adapter, which includes an adapter airway 4. A fixing buckle 5 is movably mounted on the outside of the adapter airway 4. The fixing buckle 5 has two slots, each containing a lens 3, with the center points of the two lenses 3 aligned on the same straight line. An infrared light source 1 is mounted on the outside of one lens 3, and a pyroelectric detector 2 is mounted on the outside of the other lens 3. The infrared light source 1 and the pyroelectric detector 2 correspond to each other and are both connected to a host computer. The pyroelectric detector 2 has two measurement channels: a signal channel and a reference channel, both of which are connected to the host computer.

[0030] Specifically, such as Figure 2 As shown, the fixing buckle 5 is a square-shaped fixing buckle with a notch. The two lenses 3 are respectively set on both sides of the side where the notch is located. The two lenses 3 are the same size and their centers are set on the same straight line to ensure that the infrared light emitted by the infrared light source 1 can pass through the two lenses 3 and the adapter gas passage 4 between the lenses 3 and be successfully received by the pyroelectric detector 2.

[0031] like Figure 3 As shown, a mainstream adaptive processing method for end-tidal carbon dioxide signals is presented using an end-tidal carbon dioxide concentration detection module. The method is as follows: S1: Set the light source modulation frequency and perform power-on calibration on the end-tidal carbon dioxide concentration detection module.

[0032] Place the carbon oxide concentration detection module in a clean air environment and power it on for self-test. Then, modulate the infrared light source according to the preset light source modulation frequency. At this time, the detector will generate a modulation signal of the corresponding frequency. After the waveform stabilizes, collect a complete light source modulation cycle and obtain the dual-channel sampling data of the detector within the entire modulation cycle. Once obtained, the power-on calibration is complete.

[0033] S11: Place the carbon oxide concentration detection module in a clean air environment, power it on, and perform a self-test. Set the modulation frequency of infrared light source 1. f The modulation signal is a sine wave signal, and a modulation period time t is obtained.

[0034] Set the modulation frequency of infrared light source 1. f The modulation signal is a sine wave signal. The host computer controls the lighting and extinguishing frequency of the infrared light source 1 according to the modulation signal. When the infrared light source 1 is lit, the pyroelectric detector 2 receives the peak signal. When the infrared light source 1 is extinguished, the pyroelectric detector 2 receives the valley signal. The peak-to-peak value is obtained by subtracting the valley value from the peak value.

[0035] S12: The host computer controls the pyroelectric detector 2 to perform N signal acquisitions within the modulation period time t, and transmits the acquired reference signal to the host computer through the reference channel.

[0036] S13: The host computer analyzes the acquired reference signal to obtain the maximum and minimum values ​​of the signal in N acquisitions, and obtains the acquisition number 'a' corresponding to the maximum value and the acquisition number 'b' corresponding to the minimum value. The maximum value is the peak value of the period, and the minimum value is the trough value of the period. The time for acquiring the peak signal is T. Peak =t(a-1) / N, where T is the time it takes to acquire the valley signal. Trough =t(b-1) / N.

[0037] S14: Calculate the sampling delay time of the maximum value and the sampling delay time of the minimum value based on the light source modulation frequency, and set the time for acquiring the peak signal and the time for acquiring the peak signal.

[0038] Based on the light source modulation frequency, calculate the time t / 4 corresponding to the maximum value and the time 3t / 4 corresponding to the minimum value of the light source modulation frequency. The maximum value sampling delay time T1 = t / 4 - T Peak The minimum sampling delay time T2 = 3t / 4 - T Trough The time for acquiring the peak signal is set to T via the host computer. Peak =t(a-1) / N+T1, where T is the time when the peak signal is acquired. Peak =t(a-1) / N+T2, to ensure that the pyroelectric detector 2 acquires signals at the peak and valley values ​​of the light source modulation frequency and completes the power-on calibration.

[0039] S2: Perform reference signal acquisition multiple times to obtain accurate peak and valley data. Then, filter the acquired peak and valley data respectively to complete adaptive peak-to-peak acquisition.

[0040] S3: Based on the current temperature, select the relationship curve between carbon dioxide concentration and peak value in the corresponding temperature range. By comparing the peak value obtained in the previous step with the peak value on the relationship curve between carbon dioxide concentration and peak value, the corresponding carbon dioxide concentration is obtained, thus achieving adaptive acquisition of carbon dioxide concentration of respiratory gas.

[0041] The method for obtaining the carbon dioxide concentration and the peak-to-peak value curves collected by the detector is as follows: The temperature is divided into three temperature ranges: [0, 15)℃, [15, 30)℃, and [30, 45)℃. Within each of these three temperature ranges, different concentrations of carbon dioxide are repeatedly introduced into the end-tidal carbon dioxide concentration detection module, and the peak-to-peak value of the carbon dioxide concentration within each temperature range is obtained. The peak-to-peak values ​​corresponding to different carbon dioxide concentrations within each of the three temperature ranges are collected, and the relationship curves between carbon dioxide concentration and peak-to-peak value within each temperature range are fitted to obtain the peak-to-peak value curves of carbon dioxide concentration and detector collection for each temperature range.

[0042] The method for adaptively acquiring carbon dioxide concentration of respiratory gas is as follows: select the corresponding relationship curve between carbon dioxide concentration and peak value according to the temperature range to which the current temperature belongs, which improves the accuracy and resolution of measuring carbon dioxide concentration at the end of respiration and reduces drift under long-term detection. The corresponding carbon dioxide concentration is obtained from the peak value obtained in step S2 on the relationship curve.

[0043] In use, first install the breathing tubing adapter onto the airway, place the airway in a clean air environment, and power on for a self-test. Based on the preset light source modulation frequency, the infrared light source is modulated. At this time, the pyroelectric detector receives the light signal emitted by the infrared light source and generates a modulation signal of the corresponding frequency. Subsequently, after the modulation signal stabilizes, a complete light source modulation cycle is acquired, obtaining the dual-channel sampling data of the pyroelectric detector within the entire modulation cycle. The sequence numbers corresponding to the peak and trough values ​​within this complete cycle are identified, and the time T for reaching the peak and trough values ​​is calculated respectively. Peak and T trough This completes the power-on calibration. After power-on calibration, perform a respiratory test. Set the sampling delay T for the peak analog-to-digital conversion. Peak This yields the peak sequence of the two channels. The sampling delay T for the valley analog-to-digital conversion is then set. trough This yields a dual-channel valley sequence. Moving average filtering is then applied to both the peak and valley values ​​to complete peak-to-peak sampling for both channels. Based on the current temperature, a curve showing the relationship between carbon dioxide concentration and peak-to-peak values ​​within the corresponding temperature range is selected. The corresponding carbon dioxide concentration C is obtained by comparing the peak-to-peak values ​​obtained in the previous step with the peak-to-peak values ​​on the curve showing the relationship between carbon dioxide concentration and peak-to-peak values. measured .

[0044] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An adaptive processing method for end-tidal carbon dioxide signals, characterized in that the steps include: The method comprises the following steps: S1: set the light source modulation frequency, and calibrate the end-tidal carbon dioxide concentration detection module; S2: after the start-up calibration is completed, the end-tidal carbon dioxide concentration detection module is connected to the detection device for multiple signal acquisitions, peak and valley data are obtained and filtered, the peak-to-peak value is calculated by using the peak-to-valley value, and the step S2 is repeated multiple times to obtain multiple peak-to-peak values; S3: according to the current temperature, the relationship curve of the carbon dioxide concentration and the peak-to-peak value in the corresponding temperature interval is selected, the corresponding carbon dioxide concentration is obtained by comparing the peak-to-peak value obtained in the previous step with the peak-to-peak value on the carbon dioxide concentration and peak-to-peak value relationship curve, and the carbon dioxide concentration of the respiratory gas is adaptively obtained.

2. The end-tidal carbon dioxide signal adaptive processing method of claim 1, wherein, The end-tidal carbon dioxide concentration detection module comprises a breathing pipeline adapter, the breathing pipeline adapter comprises an adapter air path (4), a fixed buckle (5) is movably arranged outside the adapter air path (4), two notches are correspondingly arranged on the fixed buckle (5), a lens (3) is arranged in each notch, the center points of the two lenses (3) are arranged on the same straight line, an infrared light source (1) is arranged outside one lens (3), a pyroelectric detector (2) is arranged outside the other lens (3), the infrared light source (1) corresponds to the pyroelectric detector (2), and the infrared light source (1) and the pyroelectric detector (2) are connected with the upper computer.

3. The end-tidal carbon dioxide signal adaptive processing method of claim 2, wherein, The pyroelectric detector (2) comprises a signal channel and a reference channel, and the detectors of the signal channel and the reference channel are connected with the upper computer.

4. The end-tidal carbon dioxide signal adaptive processing method of claim 3, wherein, The fixed buckle (5) is a square buckle provided with a notch, the adapter air path (4) is arranged at the center part of the square buckle, and the two notches are arranged on the two sides of the edges of the notch.

5. The end-tidal carbon dioxide signal adaptive processing method according to any one of claims 2-4, characterized in that, The method for calibrating the end-tidal carbon dioxide concentration detection module in the step S1 is as follows: S11: the end-tidal carbon dioxide concentration detection module is placed in a clean air environment for power-on self-checking, a modulation signal is input to control the modulation frequency of the infrared light source (1), and a modulation period time t is obtained; S12: the pyroelectric detector (2) is controlled by the upper computer to perform N times of signal acquisition within the modulation period time t, and the collected reference signal is transmitted to the upper computer through the reference channel; S13: the collected reference signal is analyzed by the upper computer to obtain the time of collecting the peak signal and the time of collecting the valley signal; S14: the maximum sampling delay time and the minimum sampling delay time are calculated according to the modulation frequency of the infrared light source (1), and the time of collecting the peak signal and the time of collecting the peak signal are set, so that the pyroelectric detector (2) collects signals at the peak and valley of the light source modulation frequency, and the start-up calibration is completed.

6. The end-tidal carbon dioxide signal adaptive processing method of claim 5, wherein, The modulation signal in the step S11 is a sine wave signal.

7. The method of adaptive processing of end-tidal carbon dioxide signals according to claim 6, wherein, The method for obtaining the time of collecting the peak signal and the time of collecting the valley signal in the step S13 is as follows: The maximum value and the minimum value of the signal in the N times of collection are obtained by analyzing the collected reference signal through the host computer, and the collection times a corresponding to the maximum value and the collection times b corresponding to the minimum value are obtained respectively; the maximum value is the peak value of the period, and the minimum value is the valley value of the period, so that the time of collecting the peak signal is T Peak = (a-1) t / N, and the time of collecting the valley signal is T Trough = (b-1) t / N.

8. The method of adaptive processing of end-tidal carbon dioxide signal according to claim 7, characterized in that, The method for calculating the maximum sampling delay time and the minimum sampling delay time in the step S14 is as follows: According to the light source modulation frequency, the maximum value of the light source modulation frequency corresponding time t / 4 and the minimum value corresponding time 3t / 4 are calculated, the maximum value sampling delay time T1=t / 4-T Peak , the minimum value sampling delay time T2=3t / 4-T Trough , set by the host computer to ensure that the pyroelectric detector (2) in the light source modulation frequency peak and valley signal acquisition, complete the start-up calibration.

9. The end-tidal carbon dioxide signal adaptive processing method of claim 8, wherein, The method for obtaining the carbon dioxide concentration and peak-to-peak value relationship curve in the step S3 is as follows: The temperature is divided into [0, 15) ℃, [15, 30) ℃, [30, 45) ℃ three temperature intervals, in three temperature intervals, to the end of the carbon dioxide concentration detection module is respectively multiple into different concentrations of carbon dioxide, obtain the peak-to-peak value of the carbon dioxide concentration under the temperature interval, respectively collect the corresponding peak-to-peak value under different carbon dioxide concentrations in three temperature intervals, respectively fit the relationship curve of carbon dioxide concentration and peak-to-peak value in different temperature intervals, obtain the carbon dioxide concentration and the peak-to-peak value curve collected by the detector under different temperature intervals.

10. The end-tidal carbon dioxide signal adaptive processing method according to claim 8 or 9, characterized by, The method for adaptively obtaining the carbon dioxide concentration of the respiratory gas in step S3 is: selecting the relationship curve of carbon dioxide concentration and peak-to-peak value under the corresponding temperature interval according to the temperature interval to which the current temperature belongs, and obtaining the corresponding carbon dioxide concentration on the relationship curve according to the peak-to-peak value obtained in step S2.

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