CN103528952B - A kind of open light path type gas analyzer flux correction measurement apparatus and measuring method - Google Patents

Abstract

An open optical path gas analyzer flux correction measurement device and method, including: a temperature detector, used to detect the surface temperature of the gas measuring instrument; a Peltier refrigeration element, in contact with the bottom shell of the optical path, used to adjust the temperature of the instrument shell ; A code disc equipped with an infrared filter, the perimeter edge of the code disc is engraved with slits arranged at equal intervals, and the code disc generates a chopping signal through the slits during the rotation of the gas measurement for synchronization Trigger the A/D module to collect the signal on the temperature detector; the temperature control circuit based on DSP and CPLD collects the temperature data of the temperature detector according to the synchronization signal of the code wheel rotation, and generates a pulse width modulation signal (PMW) Drive the Peltier element for refrigeration; at the same time, perform calculation and processing according to the temperature data to generate WPL flux calibration items, and perform real-time calibration of the flux. The invention calculates the WPL flux calibration item through the real-time temperature difference data, which can further improve the measurement accuracy and stability of the gas analyzer.

Description

An open optical path gas analyzer flux calibration measurement device and measurement method

technical field

The invention relates to a method for improving the flux measurement accuracy of an open optical path type non-dispersive infrared (NDIR) gas analyzer, in particular to a correction method and a circuit for flux measurement errors caused by heating on the surface of the gas measuring instrument.

Background technique

Understanding the carbon-water cycle process of terrestrial ecosystems is an important way to understand global climate change, and can provide scientific basis for alleviating the shortage of fresh water resources, regulating the process of global warming, and agricultural production and life. International flux sites usually use open optical path NDIR gas analyzers as equipment for long-term field research on the exchange of gases such as CO2, H2O and CH4 in terrestrial and marine ecosystems. However, since the surface heating of the open-path gas analyzer will cause gas flux measurement errors, the instrument company and researchers at each flux site use different methods to calibrate the measurement errors.

At present, the LI-7500 of LI-COR Company is the mainstream gas analyzer used in various gas flux measurement sites in the world. The instrument uses internal Peltier cooling to control the surface heating of the instrument and reduce the impact of instrument heating on gas flux measurement. influences. However, the instrument only sets a fixed temperature control threshold of +30°C, which cannot be adjusted with the ambient temperature. When the temperature difference between the ambient temperature and the temperature control threshold is large, the phenomenon of increased flux caused by surface heating is still serious. , especially when the ambient temperature is lower than 0°C, the flux measurement error caused is more serious. At the same time, some site researchers use post-data processing to eliminate data errors caused by thermal phenomena on the gas analyzer surface. The general method is to add a temperature difference correction item in the Webb-Pearman-Leuning (WPL) function, but this method has the following disadvantages: ( 1) The correction items of instruments with different shapes are different and cannot be used universally; (2) The correction item coefficients are all generated by prior data, and cannot be used for different gas analyzers to perform error correction by matching real-time environmental data and gas analyzer surface shape data .

The surface thermal effect of the open-path gas analyzer is not only related to the heating of the internal electronic circuit, but also has a lot to do with the three-dimensional shape of the instrument's optical path, the installation position of the instrument, the angle of the sun, and the long-wave radiation of the instrument shell. Fixed temperature settings or past empirical model matching cannot be accomplished to accurately correct errors in gas flux measurements.

Contents of the invention

Technical solution of the present invention: In order to solve the problems existing in the above-mentioned background technology, the present invention proposes a correction method and circuit for flux measurement errors caused by heating on the surface of an open optical path gas measuring instrument, which can be installed according to the optical path structure of the gas analyzer The temperature detector improves the accuracy of data acquisition; the temperature detector supports up to 8 channels, which meets the three-dimensional temperature data acquisition requirements of complex gas analyzers, and provides a basis for accurate inversion of the surface heating phenomenon of open-circuit gas analyzers; through real-time temperature difference The data calculates the WPL flux calibration item, which can further improve the measurement accuracy and stability of the gas analyzer.

The technical solution of the present invention includes: a device for improving the flux measurement error caused by the surface heating of an open optical path gas measuring instrument, which includes a group of temperature detectors, and these temperature detectors are installed on the casing of the gas analyzer according to the surface shape of the optical path Different positions are used to detect the surface temperature of the instrument; a set of Peltier cooling elements are installed inside the instrument and are in contact with the bottom shell of the optical path to adjust the temperature of the instrument shell; a code disc equipped with an infrared filter, the Slits arranged at equal intervals are engraved on the perimeter of the code disc. During the rotation of the measured gas, the code disc can generate a chopping signal through the slits, which is used to synchronously trigger the A/D module to collect the signal on the temperature detector;

The device also includes a set of temperature control circuit based on DSP and CPLD. According to the synchronous signal of code disc rotation, the circuit collects the temperature data of the above-mentioned temperature detector, and generates a pulse width modulation signal (PMW) to drive the Peltier element to cool; at the same time, according to the temperature data, DSP performs calculation processing to generate WPL flux calibration item for real-time calibration of flux.

The number of temperature detectors installed in the above device can have up to 8 channels working at the same time; the number of temperature detectors can be determined according to the shape of the heating surface of the gas analyzer to be installed.

Of the temperature detectors installed in the above device, one of them must be installed outside the optical path of the gas analyzer for use as ambient temperature detection.

The temperature detector installed in the above-mentioned device adopts a thermistor with wide temperature range and small thermal inertia.

The number of Peltier refrigeration elements installed in the above device is 2 pieces.

The cooling temperature to be achieved by the Peltier refrigeration element installed in the above device is determined jointly by the temperature in the optical path of the gas analyzer obtained by the temperature detector, the ambient temperature and the threshold temperature set by the software.

The Peltier refrigeration element installed in the above-mentioned device is connected with a drive circuit behind it, and the drive circuit receives CPLD to generate PMW pulses for refrigeration.

The above-mentioned device is equipped with a code wheel with an infrared filter, and 127 slits are equally spaced around the code wheel, and the first slit is missing. During the rotation of the code disc, the optical signal passes through the slit to generate a continuous chopping signal, and the CPLD processing circuit generates a synchronous trigger signal according to the chopping signal, which triggers the A/D circuit to collect the value of the temperature detector.

In the above device, the number of infrared filters on the code disc is set to 4, 2 reference filters, and 2 filters for the gas to be measured, and the temperature measurement only needs to occur when the gas is measured.

Two counters are set in the CPLD in the above device, counter 1 uses chopping signal as the counting clock, when the count value is equal to the set value of the corresponding gas channel, a synchronous trigger signal is generated to the A/D acquisition module; counter 2 is used for chopping Each pulse width of the square wave is counted, and when the count value is the width of the first missing slit above, the circle synchronization signal of the code wheel is generated.

The generation of the chopping signal of the code disc in the above-mentioned device uses a light-emitting diode and a PIN photodetector.

An amplifier is connected behind the PIN photodetector in the above device, and the output signal of the amplifier enters the CPLD through the comparator.

The DSP in the above device performs the following processing on the received A/D signal to obtain the calibration item of the WPL flux:

A. Obtain the temperature in the optical path of the gas analyzer and the ambient temperature, and calculate the temperature difference.

B. Compare the temperature difference with the threshold value set by the program, and judge whether it is necessary to drive the CPLD to generate a cooling signal.

C. When the temperature difference is within the range set by the program, regain the temperature in the optical path of the gas analyzer and the ambient temperature, calculate the temperature difference again, and obtain the average wind speed variable from the gas analyzer.

D. Calculate the WPL flux calibration term.

E. Bring the calibration item into the flux formula to calculate the calibrated flux.

The method, wherein the temperature difference data in step A generally includes the bottom temperature difference, the top temperature difference and the support rod temperature difference.

Said method, wherein in step B, when judging whether refrigeration is required, the temperature difference at the bottom is selected as the judgment basis for the required temperature difference value.

In the above method, the temperature difference set by the program in step B is the temperature threshold value solidified in the program, and the threshold value can be changed by the user as required.

The method, wherein the WPL flux calibration items in step D mainly include bottom calibration items, top calibration items and support bar calibration items, and the calibration items should not be limited to these three items, and can be increased or increased according to the surface shape of the gas analyzer. reduce.

Compared with the prior art, the present invention has the advantages that temperature detectors can be installed according to the optical path structure of the gas analyzer to improve the accuracy of data acquisition; at most 8 temperature detectors can be collected at the same time to meet the three-dimensional temperature data of complex gas analyzers Obtain the requirements to provide a basis for accurate inversion of the surface heating phenomenon of the open-path gas analyzer; calculate the WPL flux calibration item through the real-time temperature difference data, which can further improve the measurement accuracy and stability of the gas analyzer.

Description of drawings

Fig. 1 is a structural diagram of a gas analyzer installed with a thermal compensation circuit of the present invention.

Fig. 2 is the structural diagram of the signal processing circuit based on DSP and CPLD of the present invention.

Fig. 3 is a schematic diagram of the signal processing flow of the present invention.

Among them, the labels in Figure 1 are as follows: 1. The top shell of the gas analyzer; 2. The thermal detector installed on the top; 3. The receiving window of the optical path on the top of the gas analyzer; 4. The optical path; 5. The support rod of the gas analyzer; 6 . The thermal detector installed on the support rod of the gas analyzer; 7. The thermal detector installed at the bottom of the gas analyzer; 8. The Peltier 1 in the bottom shell of the gas analyzer; 9. Used to detect the chopping signal PIN tube; 10. Optical emission window at the bottom of the gas analyzer; 11. Peltier 2 in the bottom shell of the gas analyzer; 12. Code disc; 13. Motor; 14. Thermal detector for detecting ambient temperature; 15. DSP data processing board.

detailed description

The present invention will be described in more detail through examples below in conjunction with the accompanying drawings.

As shown in Figure 1, the present invention is a fast-response thermal detector that can be installed on different parts of the surface of the gas analyzer to detect the difference between the temperature of the instrument shell and the ambient temperature in real time, and generate calibration data through signal processor calculation, which can be quickly adjusted. Gas flux measurement data were corrected.

The rapidity and real-time performance of temperature data collection in the present invention is ensured by generating chopper signals through the rotation of the code disc inside the gas analyzer, and synchronously triggering the A/D to collect temperature data when collecting gas signals.

In the present invention, after the temperature data is obtained, the temperature difference data is first calculated and compared with the threshold value set inside the software, and the Peltier refrigeration is started if it is greater than the threshold value to achieve the purpose of reducing the temperature difference. After the temperature difference is less than the threshold value, the temperature is obtained. data, calculate the WPL calibration item, and perform flux calibration; if it is less than the threshold, you don’t need to start Peltier, and you can directly calculate the WPL calibration item.

In this embodiment, the basic structure of the gas analyzer surface heating thermal compensation device of the present invention includes: four temperature detectors are used, 2, 6, 7, and 14, which are respectively installed on the top and lower surface of the instrument of the gas analyzer ( Top), on the support rod (Spar), on the bottom surface (Bottom) and around the gas analyzer (Ambient), connected to the DSP signal acquisition board 15 through wires; two cooling Peltiers 8, 11, installed at the bottom to emit Both sides of the light path are connected on the DSP signal acquisition board 15 by wires; a motor 13 is driven by a power supply; a code disc 12 is driven by the motor 13 to rotate at a set frequency; four optical filters are housed on the code disc 12, Two measuring filters and two reference filters, 127 slots are equally spaced around the code wheel 12, and one slot is missing between the two reference filters; a light-emitting diode and a PIN tube 9; When the code wheel 12 rotates, the PIN tube 9 receives the chopping signal, which is input to the signal processing board 15 through the signal amplifier, and the signal processing board 15 triggers the A/ D circuit collects temperature detectors 2, 6, 7, 14; signal processing board 15 obtains temperature data, calculates temperature difference data, drives Peltier 8, 11 to refrigerate, and calculates WPL calibration items.

As shown in Figure 2, after the above-mentioned chopping signal is generated, the signal processing and circuit action process of the present invention are as follows: the code disc rotates to generate the chopping signal, and the square wave signal is generated by the amplification and comparison shaping circuit, which is input to the CPLD synchronous output module, and the CPLD Through counting, a trigger pulse is generated, and this pulse is input to the multi-channel A/D acquisition circuit, and the A/D acquisition circuit completes the thermistor signal acquisition, generates an interrupt, and DSP enters the interrupt program to complete the data reading; DSP according to the threshold set by the program, Generate a driving signal to control the driving module to make Peltier refrigeration.

In the CPLD synchronous output module of the present invention, the circle synchronization of the code disc and the synchronization of gas signal and temperature collection are realized by setting two counters. The counter count1 counts the width of each square wave generated by the code disk chopping to generate a circle synchronization signal; the counter count2 counts the number of square waves generated by the code disk chopping to generate a temperature acquisition synchronization signal.

In the CPLD synchronous output module of the present invention, in this embodiment, the working clock of count1 is 40MHz, and the working clock of count2 is the chopping frequency of the code disc, which is about 26KHz.

In the CPLD synchronous output module of the present invention, when the square wave signal generated by the chopping is at a high level, count1 counts, and when it is at a low level, count1 is cleared.

In the CPLD synchronous output module of the present invention, the square wave signal that above-mentioned chopping produces triggers count2 to count, and program setting count2 counter value is 48 and 80, produces A/D trigger pulse; Counter count1 when counting value is greater than 1540, count2 cleared.

In this embodiment, the above count2 counter values are 48 and 80, which are determined according to the position of the gas filter corresponding to the slit number, and the count value of the counter count1 is greater than 1540, which is determined according to the width of the missing groove of the code disc. In the present invention, count1 And count2 counter count value should not be limited to this value.

After the CPLD synchronous output module of the present invention produces A/D trigger pulse, in the present embodiment, DSP reads the value of 4 temperature sensors in A/D successively, and the value of setting thermistor 14 is above-mentioned Parr The threshold that needs to be judged when the paste starts. If the difference between the temperature value of the thermistor 7 and the threshold is greater than 5°C, a driving signal is generated to start Peltier refrigeration, and when the temperature difference is less than 5°C, the CPLD drive module is turned off.

In the above-mentioned threshold stage where Peltier startup needs to be determined, in this embodiment, the temperature difference is set to 5° C., and the temperature difference value can be adjusted according to the power consumption requirement, and the present invention should not be limited to this value.

In the Peltier refrigeration stage driven by the above-mentioned CPLD drive module, the CPLD generates a 78.12KHz square wave signal, which is input to one end of the integral amplifier to form a triangular wave signal; the other end of the integral amplifier is input to the DSP set level, which is the cooling target value. The output of the amplifier passes through the comparator to generate a PWM signal with adjustable width, which is driven by the H bridge to make the Peltier cool.

In the Peltier refrigeration start-up determination stage of the present invention, the DSP collects the temperature signal, and performs signal processing in the DSP, and the signal processing flow is as shown in Figure 3:

1. In this embodiment, the temperature values of 4 parts of the gas analyzer are obtained, the temperature of the connecting rod of the optical path is 6: T _spar , the temperature of the bottom of the optical path is 7: T _bottom , the temperature of the top of the optical path is 2: T _top , and the peripheral temperature of the optical path is 14: T _a ; and calculate the temperature difference item, the temperature difference of the connecting rod: T _spar -T _a , the temperature difference at the bottom of the optical path: T _bottom -T _a , the temperature difference at the top of the optical path: T _top -T _a ;

2. In this embodiment, T _bottom -T _a is used as the temperature difference item. When the temperature difference is greater than 5°C, the DSP generates a driving signal to drive the Peltier refrigeration; when the Peltier is in the off state, when the temperature difference is less than 5°C, When the disk rotates and enters the next round of measurement, the temperature values and temperature difference values of the four parts of the gas analyzer are obtained again, and the wind speed variable U is obtained at the same time:

3. According to the structure of the gas analyzer, the following parameters are obtained through the following expressions: is the air boundary layer thickness of the optical path connecting rod, is the thickness of the air boundary layer at the bottom of the optical path, is the thickness of the air boundary layer at the top of the optical path, and the meanings of other parameters are as follows: k ^air is the thermal conductivity coefficient of the air, r ^spar is the radius of the connecting rod of the optical path, l _spar is the diameter of the connecting rod of the optical path, r ^bottom is the radius of the cylinder at the bottom of the optical path, l _bottom is the diameter of the cylinder at the bottom of the optical path, r ^top is the radius of the cylinder at the top of the optical path, and l _top is the diameter of the cylinder at the top of the optical path.

4. According to the calculated values of different parts of the above-mentioned gas analyzer, respectively bring in the WPL calibration item of the corresponding gas analyzer surface heat contribution, where the flux calibration item of the optical path connecting rod is expressed as: The flux calibration term at the bottom of the optical path is expressed as: The flux calibration term at the top of the optical path is expressed as: $S_{\mathrm{top}}=k^{\mathrm{the\; air}}\frac{(r^{\mathrm{top}}+{\mathrm{\δ}}^{\mathrm{top}})(T_{\mathrm{top}}-T_a)}{r^{\mathrm{top}}{\mathrm{\δ}}^{\mathrm{top}}}.$

5. The above three flux calibration items are brought into the flux formula calculation:

$f_{\mathrm{cnew}}=f_c+\frac{S_{\mathrm{spar}}+S_{\mathrm{bottom}}+S_{\mathrm{top}}}{\mathrm{\ρ}{C}_p}\frac{q_c}{T_a}(1.6077\frac{{\mathrm{\ρ}}_v}{{\mathrm{\ρ}}_d}+1);$ Among them, ρ _d is the density of dry air, ρ _v is the density of water vapor, ρ is the density of mixed air, q _c is the density of carbon dioxide around the optical path, and C _p is the specific heat of air. Here, F _c is the standard flux formula of the gas before calibration, which is well known in the prior art and will not be repeated here.

The invention organically combines the above-mentioned software and hardware methods, solves the real-time nature of data, reduces the complexity of later data processing, and improves the accuracy of gas flux measurement. Moreover, the invention has strong applicability and can be used in various gas analyzer structures.

Parts not described in detail in the present invention belong to the well-known technology in the art.

The above are only some specific implementations of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be covered within the protection scope of the present invention.

Claims (17)

1. An open optical path type gas analyzer flux correction measurement device, characterized in that it comprises: A group of temperature detectors are installed at different positions of the gas analyzer casing according to the surface shape of the optical path, and are used to detect the surface temperature of the gas analyzer; A set of Peltier cooling elements, installed inside the gas analyzer, in contact with the bottom shell of the optical path, used to adjust the temperature of the instrument shell; A code wheel equipped with an infrared filter, the perimeter of the code wheel is engraved with slits arranged at equal intervals, and the code wheel generates a chopping signal through the slits during the rotation of the measured gas for synchronization Trigger the A/D module to collect the signal on the temperature detector; A set of temperature control circuits based on DSP and CPLD collects the temperature data of the temperature detector according to the synchronous signal of the rotation of the code disc, and generates a pulse width modulation signal (PMW) to drive the Peltier element to refrigerate; at the same time, according to the The temperature data is processed to generate WPL flux calibration items, and the flux is calibrated in real time; Wherein, the temperature control circuit based on DSP and CPLD includes multi-channel A/D acquisition module, DSP data processing and WPL calibration module, CPLD drive module, CPLD synchronous output module, shaping circuit; code disc rotation makes PIN photodetector generate The chopping signal is amplified and compared by the shaping circuit to generate a square wave signal, which is input to the CPLD synchronous output module. The CPLD synchronous output module generates a trigger pulse through counting. This pulse is input to the multi-channel A/D acquisition module, and the multi-channel A/D acquisition module completes The thermistor signal acquisition generates an interrupt, the DSP data processing and WPL calibration module enters the interrupt program to complete the temperature data reading, judge the temperature difference data, if the temperature difference is greater than the set threshold, generate a driving signal, control the CPLD driver module to make the Peltier refrigeration element Refrigeration, the temperature difference is less than the set threshold, calculate the optical path flux calibration item, and obtain the gas flux value. 2. A kind of open optical path gas analyzer flux correction measuring device according to claim 1, characterized in that: the number of said temperature detectors is determined according to the shape of the heating surface of the gas analyzer to be installed, and the number is at most 8 ways work at the same time. 3. An open optical path gas analyzer flux correction measurement device according to claim 1, characterized in that: one of the group of temperature detectors must be installed outside the optical path of the gas analyzer as an ambient temperature detection use. 4. An open optical path gas analyzer flux correction measuring device according to claim 1, characterized in that: the temperature detector is a thermistor with a measuring temperature range of -40°C to +150°C. 5 . The open optical path gas analyzer flux correction measuring device according to claim 1 , wherein the number of said Peltier refrigeration elements is 2 pieces. 5 . 6. A kind of open optical path type gas analyzer flux correction measurement device according to claim 1, characterized in that: the cooling temperature to be reached by the Peltier refrigeration element, according to the gas analyzer optical path obtained by the temperature detector The internal temperature, the ambient temperature and the threshold temperature set by the software are jointly determined. 7. A kind of open optical path type gas analyzer flux correction measurement device according to claim 1, it is characterized in that: said Peltier refrigeration element is connected with drive circuit before, and this drive circuit receives CPLD drive module and produces PMW pulse Refrigerate. 8. A kind of open optical path type gas analyzer flux correction measuring device according to claim 1, characterized in that: 127 slits are equally spaced around the code disk periphery of the infrared filter, and the first The slit is missing; during the rotation of the code disc, the optical signal passes through the slit to generate a continuous chopping signal, and the CPLD synchronous output module generates a synchronous trigger signal according to the chopping signal, triggering the multi-channel A/D acquisition module to collect the value of the temperature detector . 9. A kind of open optical path type gas analyzer flux correction measuring device according to claim 1, characterized in that: the number of infrared filters on the code disc is set to 4, of which 2 are reference filters, 2 filters for the gas to be measured, the temperature measurement only takes place during the gas measurement. 10. A kind of open optical path type gas analyzer flux correction measurement device according to claim 8, it is characterized in that: two counters are set in the CPLD synchronous output module, and the first counter uses chopping signal as counting clock , when the count value is equal to the set value of the corresponding gas channel, a synchronous trigger signal is generated to the multi-channel A/D acquisition module; the second counter counts each pulse width of the chopping square wave, and when the count value is the above-mentioned first When the width of the slit is missing, the circle synchronization signal of the code wheel is generated. 11. An open light path gas analyzer flux correction measurement device according to claim 1, characterized in that: the code disc chopping signal is generated using a light emitting diode and a PIN photodetector. 12. A flux correction measurement device for an open optical path gas analyzer according to claim 1, wherein an amplifier is connected behind the PIN photodetector, and the output signal of the amplifier enters the CPLD synchronous output module through a comparator. 13. An open optical path type gas analyzer flux correction measurement method, characterized in that the implementation steps are as follows: A. The rotation of the code disc makes the PIN photodetector generate a chopping signal, which is amplified by the shaping circuit and compared to generate a square wave signal, which is input to the CPLD synchronous output module, and the CPLD synchronous output module generates a trigger pulse through counting, and this pulse is input to the multi-channel A/D Acquisition module, multi-channel A/D acquisition module completes the thermistor signal acquisition, generates an interrupt, DSP data processing and WPL calibration module enters the interrupt program to complete temperature data reading, obtains the temperature in the optical path of the gas analyzer and the ambient temperature, and calculates the temperature difference; B. Compare the temperature difference and the threshold value set by the program to judge whether it is necessary to drive the CPLD driver module to generate a cooling signal; C. When the temperature difference is within the program setting range, regain the temperature in the optical path of the gas analyzer and the ambient temperature, calculate the temperature difference again, and obtain the average wind speed variable from the gas analyzer; D. Calculate the WPL flux calibration item; E. Bring the calibration item into the flux formula to calculate the calibrated flux. 14. The method according to claim 13, characterized in that: the temperature difference data in the step A includes the bottom temperature difference, the top temperature difference and the support rod temperature difference. 15. The method according to claim 13, characterized in that: in step B, when judging whether refrigeration is required, the required temperature difference value is selected as the basis for judging by the temperature difference at the bottom. 16. The method according to claim 13, characterized in that: the temperature difference set by the program in the step B is a temperature threshold value solidified in the program, and the threshold value can be changed by the user as required. 17. The method according to claim 13, characterized in that: the WPL flux calibration items in the step D mainly include bottom calibration items, top calibration items and support bar calibration items, and the calibration items should not be based on these three items. limit, which increases or decreases depending on the shape of the gas analyzer surface.