KR20240052242A - Method and apparatus for measuring concentration of gas by generating exponential decay of cavity - Google Patents

Abstract

A method for measuring the concentration value of a target gas using a laser, comprising the steps of opening a shutter located at the entrance of the cavity to allow light to enter the cavity, and allowing the light incident on the cavity to enter the cavity. Reflecting between at least two reflectors located inside; measuring whether a preset resonance state occurs with respect to the optical signal reflected between the reflectors; When the preset resonance state is measured, fixing the position of one of the reflectors; measuring the optical intensity of the optical signal output from the cavity and incident on a photodiode; and measuring a concentration value for the target gas based on the optical signal output from the cavity, wherein the step of measuring the light intensity includes identifying whether the measured light intensity is greater than an elevation reference value. It may include: 1 identification step, and storing data based on the results of the first identification step.

Description

Method and device for measuring gas concentration by generating exponential decay of the cavity {METHOD AND APPARATUS FOR MEASURING CONCENTRATION OF GAS BY GENERATING EXPONENTIAL DECAY OF CAVITY}

The present invention relates to a method and device for measuring concentration values. More specifically, the present invention relates to a method and device for measuring concentration values. More specifically, the intensity of light emitted from a cavity is compared in real time to cause light attenuation to start at the same intensity of light, thereby creating an exponential decay time of the emitted light. It is about technology that can measure accurately.

A representative technology for measuring gas concentration is laser spectroscopic analysis technology. Gases with asymmetric molecular bonds have the characteristic of changing dipole moment. Laser spectroscopic analysis technology is a method of injecting gas that has the characteristic of being absorbed at a unique wavelength corresponding to changes in dipole moment into a closed space and measuring the amount of light absorption by injecting a laser with a wavelength that is absorbed into the gas and converting the concentration of the gas. am.

Laser spectroscopic analysis technology has an optical path of several centimeters to several meters, and measurement sensitivity is improved in proportion to the optical path length, and can measure gas concentrations from hundreds of ppb to thousands of ppm.

Korean Patent Publication No. 10-2007-0047358 (published May 4, 2007)

Based on the above-described discussion, the present disclosure provides a method and device for comparing the intensity of light emitted from a cavity in real time so that light attenuation starts at the same intensity of light.

According to one embodiment of the present disclosure, a method for measuring the concentration value of a target gas using a laser includes the steps of opening a shutter located at the entrance of the cavity to allow light to enter the cavity; reflecting light incident on the cavity between at least two reflectors located inside the cavity; measuring whether a preset resonance state occurs with respect to the optical signal reflected between the reflectors; When the preset resonance state is measured, fixing the position of one of the reflectors; measuring the optical intensity of the optical signal output from the cavity and incident on a photodiode; and measuring a concentration value for the target gas based on the optical signal output from the cavity, wherein the step of measuring the light intensity includes identifying whether the measured light intensity is greater than an elevation reference value. It may include: 1 identification step, and storing data based on the results of the first identification step.

According to other embodiments of the present disclosure, an apparatus for measuring the concentration value of a target gas using a laser includes a shutter control unit that opens a shutter located at the entrance of the cavity to allow light to enter the cavity; The cavity into which an optical signal emitted from a laser diode is incident; at least two reflectors located inside the cavity and through which the incident optical signal is reflected; a resonance state measurement unit that measures whether a preset resonance state occurs in the optical signal reflected between the reflectors; a moving member that fixes the position of one of the reflectors when the preset resonance state is measured; a light quantity measurement comparison unit that measures the light intensity of the light signal output from the cavity and incident on a photodiode; and a concentration measuring unit that measures a concentration value for the target gas based on the optical signal output from the cavity, wherein the light quantity measurement comparison unit measures the light intensity of the signal incident on the photodiode. It may be configured to identify whether the light intensity is greater than the rising reference value and store data based on the result of the identification.

The apparatus and method according to various embodiments of the present disclosure continuously create a resonance state by irradiating a laser whose wavelength is finely adjusted into a cavity, and compare the intensity of light emitted from the cavity in real time. By making the light attenuation start at the same light intensity, it is possible to accurately measure the exponential decay time of the emitted light. By converting the measured exponential decay time, the exact gas concentration can be measured.

The effects that can be obtained from the present disclosure are not limited to the effects mentioned above, and other effects not mentioned can be clearly understood by those skilled in the art from the description below. will be.

Figure 1 shows an example of a device for measuring the concentration of a target gas using intrinsic wavelength absorption according to an embodiment of the present disclosure.
Figure 2 shows an example of a method for measuring the concentration of a target gas using a cavity attenuation spectrometer according to an embodiment of the present disclosure.
Figure 3 shows a graph to explain the principle of a method for measuring the concentration of a target gas using a cavity attenuation spectrometer according to an embodiment of the present disclosure.
Figure 4 shows a block diagram of a device for measuring the concentration of a target gas according to an embodiment of the present disclosure.
Figure 5 shows a flowchart of an operation method for measuring the concentration value of a target gas according to an embodiment of the present disclosure.
Figure 6 shows a flowchart of an operation method for measuring the concentration value of a target gas according to an embodiment of the present disclosure.
Figure 7 shows a conceptual diagram explaining the cavity attenuation phenomenon according to an embodiment of the present disclosure.
Figure 8 shows a graph explaining a case where resonance occurs in a cavity according to an embodiment of the present disclosure.
Figure 9 shows a graph of existing light attenuation data according to an embodiment of the present disclosure.
Figure 10 shows a graph of light attenuation data of the present invention according to an embodiment of the present disclosure.

Below, with reference to the attached drawings, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention may be implemented in many different forms and is not limited to the embodiments described herein. In order to clearly explain the present invention in the drawings, parts unrelated to the description are omitted, and similar parts are given similar reference numerals throughout the specification.

Throughout the specification, when a part is said to be "connected" to another part, this includes not only the case where it is "directly connected," but also the case where it is "electrically connected" with another element in between. . In addition, when a part is said to "include" a certain component, this means that it does not exclude other components, but may further include other components, unless specifically stated to the contrary, and one or more other features. It should be understood that it does not exclude in advance the presence or addition of numbers, steps, operations, components, parts, or combinations thereof.

In this specification, 'part' includes a unit realized by hardware, a unit realized by software, and a unit realized using both. Additionally, one unit may be realized using two or more pieces of hardware, and two or more units may be realized using one piece of hardware.

In this specification, some of the operations or functions described as being performed by a terminal or device may instead be performed on a server connected to the terminal or device. Likewise, some of the operations or functions described as being performed by the server may also be performed on a terminal or device connected to the server.

A representative method used to measure the concentration of a target gas is a laser spectroscopic analysis technique using intrinsic wavelength absorption shown in FIG. 1.

Laser spectroscopic analysis technology is a method of injecting gas that has the characteristic of being absorbed at a unique wavelength corresponding to changes in dipole moment into a closed space and measuring the amount of light absorption by injecting a laser with a wavelength that is absorbed into the gas and converting the concentration of the gas. It can be.

This laser spectroscopic analysis technology uses the Beer-Lambert principle.

The Beer-Lambert principle defines the signal relationship before and after the amount of light passing through the material to be measured. If the length of time the material to be measured is long, the amount of light decreases. If the concentration of the target gas is high, the amount of light decreases. If the absorption of the target gas is high, the amount of light decreases. This decreases principle is used.

This can be expressed in a formula as follows.

Here, I: measured light amount, I _o : initial light amount, , l: measurement length, c: concentration.

Here, I: measured light amount, I _o : initial light amount, , l: measurement length, c: concentration, A: absorbance.

Here, I: measured light amount, I _o : initial light amount, , l: measurement length, c: concentration, T: transmittance.

A device for measuring the concentration of a target gas using the Beer-Lambert principle is shown in FIG. 1. Figure 1 shows an example of a device for measuring the concentration of a target gas using intrinsic wavelength absorption according to an embodiment of the present disclosure.

Referring to FIG. 1, a device for measuring the concentration of a target gas using intrinsic wavelength absorption includes a laser (101), a shutter (103), a reflector (105), and a gas cell (107). , and may include a detector 109.

The laser 101 may serve to emit light to make light incident on a gas cell.

The shutter 103 may serve to make light emitted by the laser 101 incident. Specifically, the shutter 103 may be opened to allow light emitted by the laser 101 to enter the gas cell 107, and the shutter 103 may be closed to prevent light from entering the gas cell 107. It may be possible.

The gas cell 107 may include a plurality of reflectors 105. Additionally, the gas cell 107 may include an inlet or outlet for the target gas.

The windows 105 of the light incident and exit parts may transmit the light incident on the gas cell 107 and allow the gas of the gas cell to pass through. Accordingly, the optical path can be formed from several centimeters to tens of centimeters depending on the length of the gas cell.

The detector 109 may detect the intensity of light emitted from the gas cell 107.

Through this structure, the concentration of the target gas can be adjusted using the Beer-Lambert principle, in which the absorption amount increases when the length through which the substance to be measured is long, the absorption amount increases when the concentration of the target gas is high, and the absorption amount increases when the absorption rate of the target gas is high. It can be measured.

Figure 2 shows an example of a device for measuring the concentration of a target gas using a cavity attenuation spectrometer according to an embodiment of the present disclosure.

Referring to Figure 2, compared to Figure 1, Figure 2 additionally includes a reflector 202, an analog-to-digital converter (A/D) 211, a moving member (e.g. PZT) 207, and a PC 213. It can be provided. According to one embodiment, the reflector 202 may be a high reflectivity mirror having a reflectance of 99.99% or more.

The laser 201, cavity 205, and detector 209 are the laser 101, gas cell 207, and detector 109 of FIG. 1. It may have a functionally identical configuration.

The concentration of the target gas can be measured by feeding back the result value output from the analog-to-digital converter 211 to the shutter 203 and the moving member 215 and controlling the shutter 203 and the moving member 207 to measure resonance data. You can.

The detailed process for this will be explained in detail in FIGS. 3 to 10 below.

Figure 3 shows a graph to explain the principle of a method for measuring the concentration of a target gas using a cavity attenuation spectrometer according to an embodiment of the present disclosure.

Referring to FIG. 3, the red line may mean a case where the target gas is present (with sample), and the black line may mean a case where the target gas is not present (without sample).

When light is blocked in a resonance state using a device having the block diagram of FIG. 2, the light in the cavity 205 travels around the reflector tens of thousands of times and decreases in an exponential decay (ring-down) form as shown in the graph of FIG. 3. . At this time, the cavity 205 can create a round-trip distance of light of several kilometers.

In this exponential decay (ring-down), there is a difference in exponential decay time between the case where the target gas is present (red line) (with sample) and the case where the target gas is not (black line) (without sample), and this difference occurs. It can be converted to gas concentration based on the extinction coefficient depending on the value and wavelength.

Figure 4 shows a block diagram of a device for measuring the concentration of a target gas according to an embodiment of the present disclosure. FIG. 4 is a block diagram embodying the device of FIG. 2 using the principle of FIG. 3.

Referring to FIG. 4, a device for measuring the concentration of a target gas includes a laser diode module 410, a laser diode control unit 420, a shutter 430, a cavity 440, and a photodiode. ) (450), TI AMP (trans-impedance Amp) (460), analog-to-digital converter (ADC) (470), light quantity measurement comparison unit (480), CPU (491), and PC (495). .

The laser diode module 410 may include a temperature sensor 401, a laser diode 403, a thermoelectric element 405, and a heat sink 407.

The laser diode control unit 420 can control the thermoelectric element 405 of the laser diode module 410 via cable 1, and can control the thermoelectric element 405 of the laser diode module 410 via cable 2. Current can be controlled, and temperature information of the temperature sensor 401 of the laser diode module 410 can be collected via cable 3.

The shutter 430 may serve to make light emitted by a laser incident. Specifically, the shutter 430 may be opened to allow light emitted by the laser to enter the cavity 440, or the shutter 430 may be closed to prevent light from entering the cavity 440. According to one embodiment, the shutter 203 may be an optical modulator AOM (Acousto-Optic Modulator).

The cavity 440 may include a plurality of reflectors 441, a moving member (eg, piezo) 443, an inlet 445, and an outlet 447. The light that enters when the shutter 430 is opened is reflected by the plurality of reflectors 441, and the moving member 443 can move the plurality of reflectors 441 to find the maximum resonance position.

The light receiving element 45 may detect an optical signal by converting the light emitted from the cavity 440 into electric current.

The TI Amp (460) can amplify the detected optical signal and convert it into voltage.

The analog-to-digital converter 470 can convert the output (current or voltage) of the TI Amp into a digital signal, and the converted output can be input to the light quantity measurement comparison unit 480.

The light quantity measurement comparison unit 480 may include a light quantity comparator 481 that compares a rising reference value 485 and a falling reference value 487, and a block memory 484. The block memory 484 may store the measurement value completed by the light quantity comparator 481. The light quantity measurement comparison unit 480 is connected to the shutter 430 via cable 4 and can control the shutter.

CPU 491 can process data stored in block memory 483. Additionally, the CPU 491 can process and control the operation of the laser diode module 410.

The PC 495 is connected to the CPU 491 through cable 6 and can collect and analyze data stored in the block memory 483 to calculate the concentration value of the target gas.

Figure 5 shows a flowchart of an operation method for measuring the concentration value of a target gas according to an embodiment of the present disclosure.

Referring to FIG. 5 , an operating method for measuring the concentration value for a target gas may include the step 501 of opening a shutter located at the entrance of the cavity to allow light to enter the cavity.

The method for measuring the concentration value for the target gas in response to step 501 may include a step 503 in which the incident light is reflected between at least two reflectors located inside the cavity. According to one embodiment, the reflectors may be moved by a moving member (eg, piezo) and the reflectors may be moved to achieve maximum resonance.

The method for measuring the concentration value for the target gas in response to step 503 may include a step 505 of measuring whether a preset resonance state occurs in light being reflected between reflectors.

The method for measuring the concentration value for the target gas in response to step 505 may include a step 507 of fixing the position of one of the reflectors when a preset resonance state is measured.

The method for measuring the concentration value of the target gas in response to step 507 may include a step 509 of measuring the concentration of the gas by measuring the exponential decay time of light emitted from the cavity.

Each step 501 to 509 will be described in detail with reference to FIG. 6 below.

Figure 6 shows a flowchart of an operation method for measuring the concentration value of a target gas according to an embodiment of the present disclosure.

Referring to FIG. 6, an operating method for measuring the concentration value for a target gas may include a step 601 of fixing the laser diode temperature. Specifically, the temperature of the laser diode 403 can be measured by reading the temperature with the temperature sensor 401 of the laser diode module 410, and the temperature of the laser diode can be maintained at a preset temperature by supplying current to the thermoelectric element 405. there is.

For example, in the case of NH ₃ , since the absorption area is 1512.2 nm, the temperature may need to be adjusted to set the laser wavelength to 1512.2 nm.

Corresponding to step 601, the operating method for measuring the concentration value for the target gas may include step 603 of opening a shutter. Step 603 may correspond to step 501 in FIG. 5 . When the shutter is opened, the light emitted from the laser diode enters the cavity.

Corresponding to step 603, the operating method for measuring the concentration value for the target gas may include step 605 of adjusting the position of a moving member (eg, piezo) and measuring the number of resonances. Specifically, the reflector 441 is moved at regular intervals using the moving member 443, and the light receiving element 450 is used at each interval so that the length of the cavity is equal to the length of the wavelength of the laser incident on the cavity multiplied by a specific integer. You can measure the number of resonances that appear when doing this. This step can be repeated several times to move the reflector 441 to a position where the number of laser resonances is maximum. The laser resonance state may mean a state in which the intensity of light passing through the reflector 441 and reaching the light receiving element 450 increases rapidly when the distance between the laser wavelength and the reflector 441 is equal to an integer multiple.

Corresponding to step 605, the operating method for measuring the concentration value for the target gas may include maintaining the temperature of the cavity and fixing the position of the moving member (step 607). Specifically, step 607 may be a step of fixing the moving member 443 at the maximum resonance position and maintaining the temperature of the cavity to maintain the maximum resonance state for several minutes.

In response to step 607, the operating method for measuring the concentration value for the target gas may include step 609 of repeatedly fine-tuning the laser wavelength by modulating the current of the thermoelectric element. Specifically, step 609 may be a step in which the laser wavelength is quickly modulated to efficiently generate laser resonance while stably maintaining the cavity 440. Here, the current of the thermoelectric element can be modulated from tens to several kHz in frequency to quickly fine-tune the laser wavelength.

Corresponding to step 609, the operating method for measuring the concentration value for the target gas may include step 611 of comparing the light quantity measurement value with an elevation reference value. Specifically, step 611 may be an operation in which the signal of light incident on the light receiving element 450 is amplified by the TI Amp (460), converted by the ADC (470), and compared in the light quantity comparator (481). If the light amount is greater than the rising reference value 485, step 609 can be stopped and step 613 can be performed.

Corresponding to step 611, the operating method for measuring the concentration value for the target gas may include step 613 of comparing the light quantity measurement value with a falling reference value. Specifically, step 613 may be an operation in which the signal of light incident on the light receiving element 450 is amplified by the TI Amp (460), converted by the ADC (470), and compared in the light quantity comparator (481). If the amount of light is less than the falling reference value 487, the shutter 430 may be closed through the cable 4, blocking the light incident on the cavity 440, and data may be stored in the block memory 483.

Corresponding to step 613, the operating method for measuring the concentration value for the target gas may include step 615 of acquiring light attenuation data. Specifically, step 615 may be an operation to store thousands of pieces of data in block memory at intervals of several nanoseconds. This may be an operation of storing the light signal incident on the light receiving element 450 via the TI Amp 460 and the ADC 470 and storing the light attenuation signal input in the block memory 483. The optical attenuation signal depends on the reflectivity of the reflector. From hundreds During this period, it can decrease exponentially to 0.

In response to step 615, the operating method for measuring the concentration value for the target gas may include a step 617 of measuring the decay time by fitting the data obtained in step 615 with an exponential function. Specifically, step 617 may be a step of transmitting data in the block memory 483 to the PC 495 through the CPU 491. The PC 495 is a gas concentration analysis program that fits the light attenuation signal with an exponential function to calculate the light attenuation time and converts it to the concentration of the target gas based on this.

Corresponding to step 617, the operating method for measuring the concentration value for the target gas may include step 619 of comparing the number of measurements. Specifically, if step 619 is smaller than the number of measurements, the number moves to step 609 and step 617 can be repeatedly executed. If the number of measurements is large, step 621 can be performed.

In response to step 619, the operation method for measuring the concentration value for the target gas may measure the final concentration of the target gas. Specifically, the measured concentration can be averaged and output as the final concentration of the target gas.

Figure 7 shows a conceptual diagram explaining the cavity attenuation phenomenon according to an embodiment of the present disclosure.

Referring to FIG. 7, it can be seen that the reflector changes into a resonance state or a non-resonance state depending on the position of the reflector. Specifically, the distance between the reflectors in the cavity is made of a metal material of 20 cm or more, and the cavity can be converted to a non-resonant state even if the distance between the reflectors is changed by just a few nm. In other words, the cavity cannot maintain resonance in a normal environment. Therefore, in order to maintain the resonance state, the concentration value of the target gas according to the present invention can be measured by changing the wavelength of the laser diode and injecting a laser with a wavelength matching the resonance condition into the cavity at every moment to cause cavity attenuation. there is.

Figure 8 shows a graph explaining a case where resonance occurs in a cavity according to an embodiment of the present disclosure.

FIG. 8 explains a case where resonance occurs in the cavity by step 609 of FIG. 6. If the temperature of the laser diode is rapidly changed through thermoelectric element modulation in step 609, the wavelength of the laser diode is rapidly changed as shown in (b) of FIG. 8. Resonance occurs whenever the cavity resonance conditions and wavelength match, and situations in which resonance does not occur without modulation, as shown in (b) of FIG. 8, frequently occur, making it impossible to measure several times in a short period of time.

Figure 9 shows a graph of light attenuation data according to an embodiment of the present disclosure. FIG. 9 is a graph of optical attenuation data for a conventional laser spectroscopic analysis technique using intrinsic wavelength absorption corresponding to FIG. 1.

Referring to FIG. 9, it can be seen that data is acquired through exponential attenuation (ring-down) after reaching the maximum light amount.

Figure 10 shows a graph of light attenuation data according to an embodiment of the present disclosure.

Figure 10 is a graph of light attenuation data for the method of measuring target gas concentration corresponding to the present invention corresponding to Figures 2 to 8.

Referring to FIG. 10, it can be seen that data is obtained by comparing the light quantity with a rising reference value and a falling reference value and performing an exponential attenuation (ring-down) when the amount of light is below the falling reference value.

The description of the present invention described above is for illustrative purposes, and those skilled in the art will understand that the present invention can be easily modified into other specific forms without changing the technical idea or essential features of the present invention. will be. Therefore, the embodiments described above should be understood in all respects as illustrative and not restrictive. For example, each component described as single may be implemented in a distributed manner, and similarly, components described as distributed may also be implemented in a combined form.

The scope of the present invention is indicated by the claims described below rather than the detailed description above, and all changes or modified forms derived from the meaning and scope of the claims and their equivalent concepts should be construed as being included in the scope of the present invention. do.

410: Laser diode module
420: Laser diode control unit
430: shutter
440: cavity
480: Light quantity measurement comparison unit

Claims (14)

In a method for measuring the concentration value of a target gas using a laser,
opening a shutter located at the entrance of the cavity to allow light to enter the cavity;
reflecting light incident on the cavity between at least two reflectors located inside the cavity;
measuring whether a preset resonance state occurs with respect to the optical signal reflected between the reflectors;
When the preset resonance state is measured, fixing the position of one of the reflectors;
measuring the optical intensity of the optical signal output from the cavity and incident on a photodiode; and
Measuring the concentration value for the target gas based on the optical signal output from the cavity
Including,
The step of measuring the light intensity is,
A first identification step of identifying whether the measured light intensity is greater than a rising reference value,
A method for measuring a concentration value, comprising the step of storing data based on the result of the first identification step. According to claim 1,
Further comprising a second identification step of identifying whether the measured light intensity is less than a fall reference value,
Wherein the step of storing the data stores the data further based on the result of the second identification step. According to claim 1,
Maintaining the temperature of the laser diode located outside the cavity into which the target gas is injected.
A method for measuring concentration values, further comprising: According to claim 1,
The step of reflecting the optical signal between at least two reflectors located inside the cavity includes: moving the position of one of the at least two reflectors using a moving member connected to one of the reflectors; A method for measuring concentration values, including: According to claim 1,
A method for measuring a concentration value, further comprising measuring an optical decay time based on the optical intensity of the measured optical signal. According to clause 5,
The step of measuring the light decay time is,
A method of measuring a concentration value, comprising the step of storing data on the measured light decay time based on a preset reference value. According to clause 6,
The step of measuring the concentration value for the target gas is,
A method for measuring a concentration value, which measures the concentration value for the target gas based on the light decay time. In a device for measuring the concentration value of a target gas using a laser,
a shutter control unit that opens a shutter located at the entrance of the cavity to allow light to enter the cavity;
The cavity into which an optical signal emitted from a laser diode is incident;
at least two reflectors located inside the cavity and reflecting the incident optical signal;
a resonance state measurement unit that measures whether a preset resonance state occurs in the optical signal reflected between the reflectors;
a moving member that fixes the position of one of the reflectors when the preset resonance state is measured;
a light quantity measurement comparison unit that measures the light intensity of the light signal output from the cavity and incident on a photodiode; and
Concentration measurement unit that measures the concentration value of the target gas based on the optical signal output from the cavity
Including,
The light quantity measurement comparison unit is configured to measure the light intensity of a signal incident on the photodiode, identify whether the measured light intensity is greater than a rise reference value, and store data based on the result of the identification. , a device for measuring gas concentration. According to clause 8,
The light quantity measurement comparison unit is further configured to measure the light intensity of a signal incident on the photodiode, and to identify whether the measured light intensity is less than a fall reference value,
The step of storing the data is to store the data further based on a result of identifying whether the drop is less than the reference value. According to clause 9,
The gas concentration measuring device,
A gas concentration measuring device further comprising a laser diode control unit configured to maintain the temperature of a laser diode located outside the cavity into which the target gas is injected. According to clause 8,
The gas concentration measuring device uses a moving member connected to one of the at least two reflectors to reflect the optical signal between at least two reflectors located inside the cavity. A device for measuring gas concentration. According to clause 8,
The light quantity measurement comparison unit,
A gas concentration measuring device further configured to measure light decay time based on the light intensity of the measured light signal. According to claim 12,
The light quantity measurement comparison unit is further configured to store data on the measured light decay time based on a preset reference value in order to measure the light decay time. According to claim 13,
The gas concentration measuring device further includes a computer device for basic concentration analysis,
The computer device is configured to measure the concentration value for the target gas based on the light decay time.

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