A methane gas concentration on-line detection device

Abstract

The application discloses a kind of methane gas concentration on-line detection device, including display module, sound-light alarm module, CPU module, sampling head and power module, the sampling head includes sampling gas chamber, the front end of the sampling gas chamber is equipped with air inlet, rear end is equipped with laser, photoelectric detector, temperature sensor, pressure sensor and signal processor, and hollow optical fiber is equipped between laser and photoelectric detector branch, one end of the hollow optical fiber is close to laser and opposite to the emission end of laser, and the other end is close to photoelectric detector and opposite to the receiving end of photoelectric detector.The application can provide super high order of magnitude optical path, reduce the influence of light attenuation on detection accuracy, so as to improve detection accuracy.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of gas detection, in particular to a kind of methane gas concentration on-line detection device. BACKGROUND

[0002] At present, more toxic and harmful gases can be generated in resident life and industrial production, among which, gas (main component methane) becomes a major safety hazard for urban residents safety. At present, domestic methane concentration sensor mainly uses catalytic, thermal conductivity principle, uses infrared principle to detect. But the above-mentioned ways all have various shortcomings, catalytic principle measurement range is narrow (0~4%), long-term stability is poor, is easily interfered by water and other gases, short service life and the like. Thermal conductivity principle exists measurement precision is low, long-term stability is poor after being influenced by water, water vapor, short service life and the like. Infrared principle is seriously influenced by water vapor, and the sensor used for monitoring is mainly electrochemical, which will fail after being used for a period of time, and the stability is poor.

[0003] Meanwhile, the existing laser methane sensor, sampling head is mostly chamber structure, laser emitter and probe are installed in the chamber, after gas enters the chamber, laser emitter is started, light beam reaches the probe after multiple reflections in the chamber, in the process of detecting methane concentration, generally, the longer the optical path, the more sufficient the contact between laser and methane, so that the absorption effect of methane on laser is better, thereby the accuracy of laser detection is higher. At present, light beam reflection is mainly realized by reflecting mirror, if it is needed to increase the optical path length, reflecting mirror needs to be increased, which causes the structure and processing of sampling head to be complex, and due to the volume limitation of sampling head, the length of light beam is also greatly limited, when the optical path reaches a certain length, which is basically 10cm at present, it is very difficult to further increase. In addition, in the process of methane detection, temperature has a great influence, in the process of light beam reflection, light beam energy is also attenuated, which causes large error in the sampling process, so that in the process of calculating concentration according to sampling data, error compensation caused by temperature is needed, the compensation accuracy of the present calculation model is low, which also greatly affects the accuracy of gas concentration detection.

[0004] Therefore, how to provide an on-line detection device with simple structure and high detection accuracy is a technical problem continued to be solved by those skilled in the art. SUMMARY

[0005] In view of the above-mentioned deficiencies in the prior art, the purpose of the present application is to provide a kind of methane gas concentration on-line detection device, can provide super high order of magnitude optical path, reduce the influence of light attenuation on detection precision, thereby improve detection precision, and overcome the influence of temperature on detection precision by new temperature compensation model, further effectively improve the detection precision of on-line detection device.

[0006] In order to solve the above technical problems, the technical scheme adopted by the present application is as follows: a methane gas concentration on-line detection device, comprising a display module, an audible and visual alarm module, a CPU module, a sampling head and a power module, the display module, the audible and visual alarm module and the sampling head are connected with the CPU module, and the power module supplies power to each electrical element; characterized in that: the sampling head comprises a sampling gas chamber, the front end of the sampling gas chamber is provided with an air inlet, the rear end is provided with a laser, a photoelectric detector, a temperature sensor, a pressure sensor and a signal processor, a hollow optical fiber is arranged between the laser and the photoelectric detector, one end of the hollow optical fiber is close to the laser and opposite to the emitting end of the laser, the other end of the hollow optical fiber is close to the photoelectric detector and opposite to the receiving end of the photoelectric detector; the photoelectric detector, the temperature sensor and the pressure sensor are connected with the signal processor, and the signal processor is connected with the CPU module, the CPU module receives the data processed by the signal processor, calculates the methane concentration data according to the data, and sends the methane data to the display module for display; at the same time, when the methane data exceeds the safety threshold, the audible and visual alarm module is controlled to give an alarm.

[0007] Further, the CPU module has a methane concentration calculation model and a temperature compensation model, wherein the methane concentration calculation model is:

[0008] I(v) = I0(v) * exp[-S(T) * g(v-v0) * P * C * L];

[0009] The compensation process of the temperature compensation model is as follows:

[0010] 1) A plurality of temperature points are uniformly set in the working temperature range of the laser sensor, a plurality of concentration characteristic points are set at intervals in the measurement range of the detection device, and the corresponding light absorption values corresponding to each temperature point and concentration characteristic point are calibrated;

[0011] 2) The actual temperature T is obtained by the temperature sensor 测 , and the initial gas concentration value C0 is obtained by the calculation model;

[0012] 3) If C0 exceeds the set safety threshold, it is directly outputted and alarmed; otherwise, the concentration characteristic point interval where C0 is located is judged, the deviation degree of C0 and the adjacent two concentration characteristic points is analyzed, and the concentration characteristic point with smaller deviation degree is selected as the reference concentration characteristic point;

[0013] 4) According to the actual temperature T 测 , the temperature point interval where T 测 is located is judged, the light absorption value A 吸上 of the upper temperature point and the light absorption value A 吸下The mean temperature T between two adjacent temperature points was calculated using the 1 / 2 interpolation method. 中1 Corresponding absorbance value A1: A1=(A 吸上 +A 吸下 ) / 2; then calculate the corresponding concentration C1 based on the absorbance value A1;

[0014] 5) Calculate the absolute difference between C1 and C0. If it is less than the set error threshold, end the compensation and output C1; if it is greater than the set error threshold, output T1 and T0. 中1 T2 is the reference interval; T is then redefined. 测 Within the temperature range, the mean temperature T is determined again using the 1 / 2 interpolation method. 中2 The corresponding absorbance value is A2, and then the corresponding concentration C2 is calculated based on the absorbance value A2;

[0015] 6) Calculate the absolute difference between C2 and C1. If it is less than the set error threshold, end the compensation and output C2; if it is greater than the set error threshold, repeat step 5) to continue determining the mean temperature T using the 1 / 2 interpolation method within the new temperature range. 中n Corresponding absorbance value A n Then according to A n Calculate the corresponding concentration C n ;

[0016] 7) Calculate C n and C n-1 The absolute error is determined, and it is judged whether the absolute error is less than the set error threshold. If it is, the compensation ends and the concentration value obtained by the last compensation is output as the methane concentration value; otherwise, the 1 / 2 interpolation method is used to continue iterating until the iteration time or number of iterations is completed, and the concentration value obtained by the last compensation is output as the methane concentration value.

[0017] Furthermore, the fitting equation corresponding to temperature and methane absorption coefficient is: K(T)=84.99148-0.15036*T, where T=273.15+t, and t is the ambient temperature.

[0018] Furthermore, the relationship between the absorbance value and the methane absorption coefficient is: A = K(T) * C * L, where A is the absorbance value, K is the absorption coefficient, C is the gas concentration (mole fraction), and L is the optical path length.

[0019] Furthermore, the wavelength of the laser emitted by the laser is 1653.27±1nm.

[0020] Furthermore, the hollow optical fiber is wound in an S-shape within the sampling gas chamber.

[0021] Furthermore, a filter screen is installed at the air inlet of the sampling gas chamber.

[0022] Furthermore, it also includes a signal output module and a remote control receiver module.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1) A different laser system was established, using a 1653.27±1nm laser. It uses a sensitive absorption spectrum to monitor methane and measures gas concentration by utilizing the principle that laser energy is selectively absorbed by gas molecules to form a high-resolution absorption spectrum. It has outstanding advantages such as intrinsic safety, wide measurement range, high accuracy, good working stability, long service life, and fast response time.

[0025] 2) Hollow optical fiber is used in the air chamber, and its length can be set to 100cm or more to form an ultra-high optical path. The optical path length is fixed and can be accurately obtained. It is not limited by the space of the sensor head air cavity in the traditional test that relies on mirror reflection. It reduces the disadvantage of the original large volume, and the optical path increases by orders of magnitude, which can effectively improve the measurement accuracy.

[0026] 3) By utilizing a new temperature compensation model, the impact of temperature on detection accuracy is greatly reduced, thereby improving the detection accuracy of the detection device. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the sampling head. Detailed Implementation

[0028] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0029] Example: See Figure 1 An online methane gas concentration detection device includes a display module, an audible and visual alarm module, a CPU module, a sampling head, and a power supply module. The display module, the audible and visual alarm module, and the sampling head are all connected to the CPU module, and the power supply module supplies power to each electrical component. It also includes a signal output module and a remote control receiving module, which are also connected to the CPU module.

[0030] The sampling head includes a sampling gas chamber with an air inlet at its front end and a filter screen at the inlet. A laser, photodetector, temperature sensor, pressure sensor, and signal processor are located at the rear end. A hollow optical fiber is positioned between the laser and the photodetector, wound in an S-shape within the sampling gas chamber. The length of the hollow optical fiber is set as needed, such as 100 cm, to form an ultra-high optical path. One end of the hollow optical fiber is close to the laser and faces its emitting end, while the other end is close to the photodetector and faces its receiving end. The laser emits a wavelength of 1653.27±1 nm, which is compatible with the wavelength of methane gas, resulting in higher accuracy in methane detection. The wavelength, energy, and wave velocity of methane are shown in the table below.

[0031]

[0032] The photodetector, temperature sensor, and pressure sensor are all connected to a signal processor, which in turn connects to a CPU module. The CPU module receives the data processed by the signal processor, performs calculations based on the data to obtain methane concentration data, and sends the methane data to the display module for display. Simultaneously, when the methane data exceeds the safety threshold, the audible and visual alarm module is activated to issue a warning.

[0033] Among them: the display module: displays the current concentration value in real time using a digital tube.

[0034] Audible and visual alarm module: When the methane concentration is detected to exceed the set safety threshold, an alarm will be triggered by sound and light.

[0035] In implementation, it also includes a signal output module and a remote control receiving module. The signal output module converts the detected methane concentration value into a standard frequency signal, a 4-20mA / 1mA-5mA current signal, or an RS485 interface output to the connected device, such as a Wi-Fi module, to transmit real-time data to the user via a computer, smartphone, or tablet. The remote control receiving module receives control signals from the remote control and transmits them to the CPU module.

[0036] Power management module: Converts the voltage provided by the power supply equipment into DC power supplies such as 15V, 5V, and 3.3V used by each module.

[0037] CPU module: Used to receive laser attenuation value, temperature and pressure data output from the sensor head; calculate and process the three data to obtain methane concentration data, and perform operation control such as display, over-limit audible and visual alarm, signal output, data storage, and parameter setting.

[0038] Sampling head: Integrates laser detection, temperature detection and pressure detection functions. The sampling head disclosed in this solution can provide optical path length (optical path travel length) that exceeds conventional levels. It provides the CPU module with the laser attenuation data, temperature data and pressure data required for processing and analysis.

[0039] Signal processing circuit: Employs a low-noise lock-in amplifier to process the output signals of temperature, pressure, and photodetectors. It mainly includes a preamplifier circuit, a bandpass filter circuit, a lock-in amplifier, and an A / D conversion circuit.

[0040] In this solution, the CPU module has a methane concentration calculation model and a temperature compensation model, wherein:

[0041] The methane concentration model is: I(ν)=I0(ν)*exp[-S(T)*g(ν-ν0)*P*C*L];

[0042] Where: I(ν) represents the incident light intensity of a laser beam with frequency ν;

[0043] I0(ν) represents the intensity of transmitted light after a laser beam with frequency ν passes through a gas of pressure P, concentration C, and optical path L.

[0044] L represents the length of the gas being measured through which the light passes, in cm;

[0045] P represents gas pressure, with the unit being atm;

[0046] C is the molecular concentration of the gas being measured, in mol·cm⁻¹. -3 .atm -1 ;

[0047] S(T) is the spectral intensity of the gas absorption line at temperature T, which is a function of the gas temperature;

[0048] g(ν-ν0) is a line shape function describing the shape of the absorption spectral line, which is related to the gas temperature and pressure;

[0049] ν0 is the center frequency of the absorption spectral line.

[0050] The compensation process of the temperature compensation model is as follows:

[0051] 1) Set several temperature points evenly within the operating temperature range of the laser sensor, and set several concentration characteristic points at intervals within the measurement range of the detection device. Calibrate the corresponding absorbance value for each temperature point and concentration characteristic point.

[0052] Specifically, as one implementation method, the temperature points are 24 evenly spaced points, set at 5°, 6°…29° in this scheme; then, using concentrations of 1% and 3% as concentration characteristic points, the absorbance values ​​at each temperature are calibrated, as shown in the table below:

[0053]

[0054] 2) Obtain the actual temperature T through a temperature sensor. 测 Then, the initial gas concentration value C0 is obtained by solving the model.

[0055] 3) Determine if C0 exceeds the set safety threshold. For example, if the set concentration safety threshold is 4.5%, if it exceeds this threshold, output an alarm directly. Otherwise, determine the concentration characteristic point range where C0 is located, analyze the degree of deviation between C0 and two adjacent concentration characteristic points, and select the concentration characteristic point with the smaller deviation as the reference concentration characteristic point. In determining the degree of deviation, the absolute difference between C0 and two adjacent concentration characteristic points is compared. If:

[0056] If |C0-1%|≤|C0-3%|, then the absorbance value corresponding to a 1% concentration of standard methane gas is selected for calculation;

[0057] If |C0-1%|>|C0-3%|, then the absorbance value corresponding to a standard methane gas concentration of 3% is selected for calculation.

[0058] 4) Based on the actual temperature T 测 Determine T 测 Within the given temperature range, obtain the previous temperature point T1 and the next temperature point T2, and determine the absorbance value A of the temperature point preceding the reference concentration characteristic point. 吸上 and the absorbance value A at the next temperature point 吸下 The mean temperature T between two adjacent temperature points is calculated using the 1 / 2 interpolation method. 中1 Corresponding absorbance value A1: A1=(A 吸上 +A 吸下 ) / 2. Where, T 中1 = (T1+T2) / 2. Then, calculate the corresponding concentration C1 based on the absorbance value A1; where, the concentration C1 is calculated by inversely based on the relationship between the absorbance value and the methane absorption coefficient, the relationship between the absorbance value and the methane absorption coefficient is: A=K(T)*C*L, where A is the absorbance value, K is the absorption coefficient; C is the gas concentration, mole fraction; L is the optical path (length); where, the absorption coefficient K is obtained from the fitting equation of temperature and methane absorption coefficient, the corresponding fitting equation of temperature and methane absorption coefficient is: K(T)=84.99148-0.15036*T, where, T=273.15+t, t is the ambient temperature.

[0059] 5) Calculate the absolute difference between C1 and C0. If it is less than the set error threshold (e.g., 0.1%), end the compensation and output C1. If it is greater than the set error threshold of 0.1%, use T1 and T0 as the basis for further calculations. 中1 T2 is the reference interval; T is then redefined. 测 The temperature range in which it is located, i.e., determining T 测 Located at T1 and T 中1 Between, or located in T 中1 Between T1 and T2; once determined, calculate T1 and T2 using the 1 / 2 interpolation method. 中1 or T 中1 The average temperature T of T2 中2 Corresponding absorbance value A2: A2=(A 吸上 +A1) / 2 or A2=(A1+A 吸下 Then, the corresponding concentration C2 is calculated based on the absorbance value A2.

[0060] 6) Calculate the absolute difference between C2 and C1. If it is less than the set error threshold of 0.1%, end the compensation and output C2; if it is greater than the set error threshold of 0.1%, repeat step 5) to continue determining the mean temperature T using the 1 / 2 interpolation method within the new temperature range. 中n Corresponding absorbance value A n Then according to A n Calculate the corresponding concentration C n .

[0061] 7) Calculate C n and C n-1 The absolute error is determined, and it is judged whether the absolute error is less than the set error threshold of 0.1%. If it is, the compensation ends and the concentration value obtained by the last compensation is output as the methane concentration value; otherwise, the 1 / 2 interpolation method is used to iterate until the iteration time or number of iterations is completed, and the concentration value obtained by the last compensation is output as the methane concentration value.

[0062] Among them, the safety threshold, error threshold, and iteration time and number of iterations for a single compensation calculation are all set manually and can be set before use according to the actual situation.

[0063] In the specific implementation process:

[0064] Step 1: Set the online detection device disclosed in this solution 30-60cm directly above the source to be tested, turn it on and warm it up for 1-2 minutes before use.

[0065] Step 2: When methane (gas) leaks in the environment to be tested, the gas diffuses into the sampling head chamber, then permeates from the cladding of the hollow fiber to the core and fills the entire hollow fiber.

[0066] Step 3: The online measurement device uses a DFB laser as the light source. The CPU module drives the DFB laser by superimposing a sinusoidal modulation signal with a triangular wave signal. The laser emitted by the laser, at 1653.27±1nm, passes through a 1-meter-long hollow optical fiber containing the gas sample to be tested.

[0067] Step 4: The concentration of methane gas is detected using a tunable light source and harmonic absorption method. A modulation signal consisting of a superimposed sine wave and a triangular wave is added to modulate the laser wavelength. Lock-in amplification technology is used to detect the second harmonic signal caused by changes in methane gas concentration, obtaining laser attenuation data, temperature data, and pressure data.

[0068] Step 5: The CPU module calculates and processes the sampled data to obtain the initial methane concentration data, performs temperature compensation on the initial methane concentration data, and finally obtains the compensated current methane concentration. The current concentration value is displayed in real time on a digital tube, and the signal output module transmits it to the monitoring terminal via Wi-Fi, enabling online real-time monitoring.

[0069] Step 6: When the methane concentration exceeds the set value, an alarm will be triggered by sound and light to remind the user to perform monitoring and maintenance.

[0070] This invention addresses the shortcomings of current methane detection methods in China by introducing stable and highly accurate laser measurement technology. It achieves online detection through local data display and remote transmission to a monitoring terminal, offering advantages such as safety, controllability, high measurement accuracy, and reliability. The device boasts advantages including high measurement accuracy, high stability, wide measurement range, long lifespan, and good operational stability. Extensive testing has demonstrated the stable operation of this online detection device.

[0071] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A methane gas concentration on-line detection device, comprising a display module, an acousto-optic alarm module, a CPU module, a sampling head and a power module, the display module, the acousto-optic alarm module and the sampling head are connected with the CPU module, and the power module supplies power for each electrical element; characterized in that: The sampling head comprises a sampling gas chamber, the front end of the sampling gas chamber is provided with an air inlet, the rear end is provided with a laser, a photodetector, a temperature sensor, a pressure sensor and a signal processor, a hollow optical fiber is arranged between the laser and the photodetector, and the hollow optical fiber is arranged in an S shape in the sampling gas chamber; one end of the hollow optical fiber is close to the laser and opposite to the emitting end of the laser, and the other end of the hollow optical fiber is close to the photodetector and opposite to the receiving end of the photodetector; the gas diffuses into the sampling gas chamber by floating diffusion, then penetrates from the cladding layer of the hollow optical fiber to the core inside, and fills the whole hollow optical fiber; The photodetector, the temperature sensor and the pressure sensor are connected with the signal processor, and are connected with a CPU module after the signal processor; the CPU module receives the data processed by the signal processor, calculates the methane concentration data according to the data, and sends the methane data to a display module for display; at the same time, when the methane data exceeds a safety threshold, a sound-light alarm module is controlled to give an alarm; The CPU module has a methane concentration calculation model and a temperature compensation model, wherein the methane concentration calculation model is: I(v) = I0(v) * exp[-S(T) * g(v-v0) * P * C * L]; Wherein: I(v) represents the incident light intensity of the laser beam with frequency v; I0(v) represents the transmission light intensity of the laser beam with frequency v after passing through the measured gas with pressure P, concentration C and optical path L; L represents the length of the measured gas through which the light passes, in cm; P represents the gas pressure, in atm; C represents the molecular concentration of the measured gas, unit, mol.cm -3 atm -1 ; S(T) is the spectral line intensity of the gas absorption spectrum at temperature T, which is a function of the gas temperature; g(v-v0) is a linear function describing the shape of the absorption spectrum, which is related to the gas temperature and pressure; v0 is the center frequency of the absorption spectrum; The compensation process of the temperature compensation model is as follows: 1) a plurality of temperature points are uniformly set in the working temperature range of the laser sensor, a plurality of concentration characteristic points are set at intervals in the measurement range of the detection device, and the corresponding absorption value is calibrated for each temperature point and concentration characteristic point; 2) Obtain actual temperature T by temperature sensor 测 Then, obtain gas initial concentration value Co by solving model; 3) judge C0, if it exceeds the set safety threshold, directly output and alarm; otherwise, judge the concentration characteristic point interval where C0 is located, analyze the deviation degree of C0 and the adjacent two concentration characteristic points, and select the concentration characteristic point with smaller deviation degree as the reference concentration characteristic point; 4) According to the actual temperature T 测 , determine the temperature point interval where T 测 is located, obtain the previous temperature point T1 and the next temperature point T2, determine the absorbance A 吸上 of the previous temperature point of the reference concentration characteristic point and the absorbance A 吸下 of the next temperature point; use the 1 / 2 interpolation method to calculate the average temperature T 中1 corresponding to the absorbance A1: A 1= (A 吸上 +A 吸下 ) / 2; then calculate the corresponding concentration C1 according to the absorbance A1; 5) calculate the absolute value difference of C1 and C0, if less than the set error threshold, end compensation, output C1; if greater than the set error threshold, with T1, T 中1 , T2 is the reference interval, re-determine the temperature interval where T 测 is located, and determine the mean temperature T 中2 corresponding to the absorbance A2 by using the 1 / 2 insertion method again, and then calculate the corresponding concentration C2 according to the absorbance A2; 6) Calculate the absolute value difference of C2 and C1, if less than the set error threshold, end compensation, output C2; if greater than the set error threshold, repeat step 5), continue to determine the mean temperature T in the new temperature interval by 1 / 2 interpolation method 中n Corresponding to the absorbance A n Then according to A n Calculate the corresponding concentration C n ; 7) Calculate C n and the absolute error of C n-1 is determined. If the absolute error is less than a set error threshold, the compensation is ended and the concentration value obtained in the last compensation is output as the methane concentration value. Otherwise, the iteration is continued using the 1 / 2 interpolation method until the iteration time or iteration number is completed, and the concentration value obtained in the last iteration is output as the methane concentration value.

2. The online methane gas concentration detection device according to claim 1, characterized in that: The fitting equation corresponding to the temperature and methane absorption coefficient is: K(T) = 84.99148-0.15036*T, wherein T = 273.15+t, t is the ambient temperature.

3. The online methane gas concentration detection device according to claim 2, characterized in that: The relationship between the absorption value and the methane absorption coefficient is: A = K(T) * C * L, wherein A is the absorption value, K is the absorption coefficient; C is the gas concentration, molar fraction; L is the optical path.

4. The online methane gas concentration detection device according to claim 1, characterized in that: The wavelength of the laser emitted by the laser is 1653.27±1nm.

5. The online methane gas concentration detection device according to claim 1, characterized in that: A filter screen is arranged at the air inlet of the sampling gas chamber.

6. The online methane gas concentration detection device according to claim 1, characterized in that: It also includes a signal output module and a remote control receiving module.

Images