Method for reducing interference of temperature and pressure on TDLAS (Tunable Diode Laser Absorption Spectroscopy) technology for detecting gas concentration

Abstract

The invention relates to a method for reducing interference of temperature and pressure on TDLAS (Tunable Diode Laser Absorption Spectroscopy) technology detection of gas concentration, belonging to the technical field of tunable diode laser absorption spectrum. By changing the modulation amplitude of wavelength modulation, the influence of gas temperature change on a detection signal is greatly reduced, and gas pressure measurement and a gas pressure compensation algorithm are introduced, so that gas pressure interference is eliminated. The method has the advantages that the influence of the gas temperature on the gas concentration detection is greatly reduced by changing the modulation amplitude in the wavelength modulation method, and meanwhile, the influence of the gas pressure on the detection result is considered through the measurement and compensation algorithm of the gas pressure. Through the combination of the two methods, the error of the gas temperature and pressure on the detection of the gas concentration by using a wavelength modulation method in the TDLAS technology is greatly reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of tunable diode laser absorption spectroscopy and relates to a method for reducing the interference of temperature and pressure on TDLAS technology in detecting gas concentration. Background Art

[0002] As a gas concentration detection technology, Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology has the advantages of non-contact online measurement, high sensitivity, and strong selectivity. At present, TDLAS technology is mainly divided into direct absorption method and wavelength modulation method. The detection signal is not only affected by the peak height of the gas absorption peak, but also by the peak width of the gas absorption peak in the wavelength modulation method. In the actual detection process, the peak height and peak width of the gas absorption peak are affected by the temperature and pressure of the gas. The gas temperature and pressure are both changing and to be measured during the measurement process, which will have a great interference on the detection results. Summary of the invention

[0003] The present invention aims at solving the problems in the prior art and provides a method for reducing the interference of temperature and pressure on TDLAS technology in detecting gas concentration.

[0004] A method for reducing the interference of temperature and pressure on TDLAS technology for detecting gas concentration includes the following steps: by changing the modulation amplitude of wavelength modulation, the influence of gas temperature change on the detection signal is greatly reduced, and the measurement of gas pressure and gas pressure compensation algorithm are introduced to suppress the interference of gas pressure.

[0005] The advantages of the present invention are: by changing the modulation amplitude in the wavelength modulation method, the influence of gas temperature on gas concentration detection is greatly reduced, and at the same time, by measuring the gas pressure and compensating the algorithm, the influence of gas pressure on the detection result is fully considered. By combining the above two methods, the error of gas temperature and pressure on gas concentration detection using wavelength modulation method in TDLAS technology is fully reduced.

[0006] When the wavelength modulation amplitude is modified to the optimal modulation amplitude defined in the present invention, the effect of gas temperature changes on detection is greatly reduced. When the gas temperature changes between -30 and 70°C, the relative error of the detection signal change is only 0.6%. When the modulation amplitude is 2.2 times the half-height and half-width of the absorption peak that maximizes the second harmonic signal, the relative error of the detection signal reaches 9.2%. The method used in the present invention can reduce the signal change caused by gas temperature changes by 93%. The interference caused by the change of gas pressure in the range of 0.8atm to 1.2atm can be suppressed to less than 0.1% through the pressure compensation algorithm. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the prior art descriptions. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work. As shown in the figure:

[0008] Figure 1 It is a curve diagram of measuring the modulation amplitude current and the ratio of the second harmonic to the first harmonic at different temperatures according to the present invention.

[0009] Figure 2 This is a diagram showing the effect of temperature on the ratio of the second harmonic to the first harmonic after the modulation amplitude is modified to an approximate intersection point according to the present invention.

[0010] Figure 3 This is a diagram showing the influence of pressure on the ratio of the second harmonic to the first harmonic after the modulation amplitude is modified to the approximate intersection point according to the present invention. DETAILED DESCRIPTION

[0011] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0012] Example 1: Figure 1 , Figure 2 and Figure 3 As shown in the figure, a method for reducing the interference of temperature and pressure on TDLAS technology for detecting gas concentration is used for high-precision detection of gas concentration in complex environments. By changing the modulation amplitude of wavelength modulation, the influence of gas temperature change on the detection signal is greatly reduced, and the measurement of gas pressure and gas pressure compensation algorithm are introduced to suppress the interference of gas pressure.

[0013] By combining the two methods, the interference of temperature and pressure on the detection of TDLAS technology is reduced, and accurate measurement of gas concentration in complex environments is achieved.

[0014] A method for reducing the interference of temperature and pressure on TDLAS technology for detecting gas concentration, comprising the following steps:

[0015] Step 1. Measure the modulation amplitude value to reduce the gas temperature interference: introduce a constant concentration of the test gas into the gas detection cavity of the sensor. After stabilization, change the gas temperature and keep the gas concentration and pressure unchanged. Measure multiple curves of the ratio of the second to first harmonic and the modulation amplitude at different gas temperatures, plot them on the same graph, and obtain an approximate intersection. Set the horizontal coordinate value of the approximate intersection as the working modulation amplitude of the sensor.

[0016] Step 2: Measure the curve of high-concentration gas signal changing with pressure: introduce high-concentration and constant-concentration gas to be tested into the gas detection chamber of the sensor. The concentration at this time is recorded as Cmax. After stabilization, change the gas pressure and keep the gas concentration and gas temperature unchanged. Measure the curve of the ratio of the second and first harmonics and pressure, and get S(P) by fitting. Cmax .

[0017] Step 3: Measure the curve of low-concentration gas signal changing with pressure: introduce background gas (such as nitrogen which is not the detection gas) into the gas detection chamber of the sensor. The concentration at this time is recorded as Cmin. After stabilization, change the gas pressure and keep the gas concentration and gas temperature unchanged. Measure the curve of the ratio of the second harmonic to the first harmonic and the pressure, and get S(P) by fitting. Cmin .

[0018] Step 4: The actual measurement process requires measuring the ratio of the second harmonic to the first harmonic S0 and the gas pressure P0. The gas concentration is calculated as follows:

[0019]

[0020] Cmax: gas concentration under high concentration gas,

[0021] S(P0) Cmax : The fitting curve of signal and gas pressure under high concentration gas,

[0022] Cmin: gas concentration under low concentration gas,

[0023] S(P0) Cmin : Fitting curve of signal and gas pressure under low concentration gas,

[0024] P0: The pressure of the gas whose concentration is to be measured,

[0025] S0: Detection signal of the gas whose concentration is to be measured.

[0026] Example 2: Figure 1 , Figure 2 and Figure 3As shown, a method for reducing the interference of temperature and pressure on the gas concentration detection of TDLAS technology is used for high-precision detection of gas concentration in complex environments. It solves the interference of gas temperature and pressure when using wavelength modulation technology in TDLAS detection technology, and can reduce the detection error caused by gas temperature and pressure in the gas concentration detection process.

[0027] A method for reducing the interference of temperature and pressure on TDLAS technology for detecting gas concentration, comprising the following steps:

[0028] Step 1. Measure the modulation amplitude value to reduce the gas temperature interference: introduce a constant concentration of the test gas into the gas detection cavity of the sensor. After stabilization, change the gas temperature and keep the gas concentration and pressure unchanged. Measure multiple curves of the ratio of the second to first harmonic and the modulation amplitude at different gas temperatures, plot them on the same graph, and obtain an approximate intersection. Set the horizontal coordinate value of the approximate intersection as the working modulation amplitude of the sensor.

[0029] Step 2: Measure the curve of high-concentration gas signal changing with pressure: introduce high-concentration and constant-concentration gas to be tested into the gas detection chamber of the sensor. The concentration at this time is recorded as Cmax. After stabilization, change the gas pressure and keep the gas concentration and gas temperature unchanged. Measure the curve of the ratio of the second and first harmonics and pressure, and get S(P) by fitting. Cmax .

[0030] Step 3: Measure the curve of low-concentration gas signal changing with pressure: introduce background gas (such as nitrogen which is not the detection gas) into the gas detection chamber of the sensor. The concentration at this time is recorded as Cmin. After stabilization, change the gas pressure and keep the gas concentration and gas temperature unchanged. Measure the curve of the ratio of the second harmonic to the first harmonic and the pressure, and get S(P) by fitting. Cmin .

[0031] Step 4: The actual measurement process requires measuring the ratio of the second harmonic to the first harmonic S0 and the gas pressure P0. The gas concentration is calculated as follows:

[0032]

[0033] Embodiment 3: A method for reducing the interference of temperature and pressure on the TDLAS technology for detecting gas concentration. First, it is necessary to determine a suitable modulation amplitude to reduce the interference of gas temperature on the detection signal.

[0034] The steps to determine the appropriate modulation amplitude are as follows:

[0035] like Figure 1 As shown, by actual measurement or mathematical model simulation, multiple curves of modulation amplitude current and ratio of second harmonic to first harmonic are drawn at different temperatures.

[0036] from Figure 1 An approximate intersection point is obtained, and its coordinates are (262μA, 0.87). The modulation amplitude of the abscissa is about 1.18 times the half-height and half-width of the absorption peak.

[0037] like Figure 2 As shown in the figure, when the wavelength modulation amplitude is modified to the approximate intersection horizontal coordinate, the influence of gas temperature change on detection is greatly reduced. When the temperature changes between -30 and 70°C, the relative error of the detection signal change is only 0.6%. When the modulation amplitude is 2.2 times the half-height half-width of the absorption peak that maximizes the 2f signal, the relative error of the detection signal reaches 9.2%. Therefore, by optimizing the modulation amplitude in the wavelength modulation method, the interference of gas temperature on the detection signal can be effectively reduced.

[0038] like Figure 3 As shown in FIG. 1 , when the wavelength modulation amplitude is modified to the approximate intersection abscissa, the ratio of the second harmonic to the first harmonic of the detection signal is still affected by the pressure, but is basically linear. The pressure can be measured by a pressure sensor to compensate the signal.

[0039] The steps of the compensation method are as follows:

[0040] 1) Measure the curve of high concentration gas signal changing with pressure: introduce high concentration and constant concentration of the gas to be tested into the gas detection chamber of the sensor. The concentration at this time is recorded as Cmax. After stabilization, change the gas pressure and keep the gas concentration and gas temperature unchanged. Measure the curve of the ratio of the second harmonic to the first harmonic and the pressure, and get S(P) by fitting. Cmax .

[0041] 2) Measure the curve of low-concentration gas signal changing with pressure: introduce background gas (such as nitrogen which is not the detection gas) into the gas detection chamber of the sensor. The concentration at this time is recorded as Cmin. After stabilization, change the gas pressure and keep the gas concentration and gas temperature unchanged. Measure the curve of the ratio of the second harmonic to the first harmonic and the pressure, and get S(P) by fitting. Cmin .

[0042] 3) The actual measurement process requires measuring the ratio of the second harmonic to the first harmonic S0 and the gas pressure P0. The gas concentration is calculated as follows:

[0043]

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

Claims

1. A method for reducing the interference of temperature and pressure on TDLAS technology in detecting gas concentration, characterized in that: The following steps are involved: By changing the modulation amplitude of wavelength modulation, the influence of gas temperature change on the detection signal is greatly reduced. The measurement of gas pressure and gas pressure compensation algorithm are introduced to eliminate the interference of gas pressure.

2. A method for reducing the interference of temperature and pressure on TDLAS technology detection of gas concentration according to claim 1, characterized in that: The following steps are also included: Step 1, measure the modulation amplitude value to reduce the gas temperature interference: introduce the gas to be tested with constant concentration into the gas detection cavity of the sensor, change the gas temperature after stabilization and keep the gas concentration and pressure unchanged, measure multiple curves of the ratio of the second harmonic to the first harmonic and the modulation amplitude at different gas temperatures, plot them on the same graph, and obtain an approximate intersection point, and set the abscissa value of the approximate intersection point as the working modulation amplitude of the sensor; Step 2: Measure the curve of high-concentration gas signal changing with pressure: introduce high-concentration and constant-concentration gas to be tested into the gas detection chamber of the sensor. The concentration at this time is recorded as Cmax. After stabilization, change the gas pressure and keep the gas concentration and gas temperature unchanged. Measure the curve of the ratio of the second and first harmonics and pressure, and get S(P) by fitting. Cmax ; Step 3: Measure the curve of low-concentration gas signal changing with pressure: introduce background gas (nitrogen, not the detection gas) into the gas detection chamber of the sensor. The concentration at this time is recorded as Cmin. After stabilization, change the gas pressure and keep the gas concentration and gas temperature unchanged. Measure the curve of the ratio of the second harmonic to the first harmonic and the pressure, and get S(P) by fitting. Cmin ; Step 4: The actual measurement process requires measuring the ratio of the second harmonic to the first harmonic S0 and the gas pressure P0. The gas concentration is calculated as follows:

3. The method of reducing the interference of temperature and pressure on TDLAS technology detection of gas concentration according to claim 1, characterized in that: Determine the appropriate modulation amplitude to reduce the effect of gas temperature on the detection signal.

4. A method for reducing the interference of temperature and pressure on TDLAS technology detection of gas concentration according to claim 3, characterized in that: Through actual measurement or mathematical model simulation, multiple modulation amplitude current and second-to-first harmonic ratio curves at different temperatures are plotted to obtain an approximate intersection point. By optimizing the modulation amplitude value in the wavelength modulation method, the influence of gas temperature on the detection signal is effectively reduced, and the signal is compensated by measuring the pressure through a pressure sensor.

5. A method for reducing the interference of temperature and pressure on TDLAS technology detection of gas concentration according to claim 4, characterized in that: The steps of the compensation method are as follows: Step A, measure the curve of high concentration gas signal changing with pressure: introduce high concentration and constant concentration of the gas to be tested into the gas detection chamber of the sensor. The concentration at this time is recorded as Cmax. After stabilization, change the gas pressure and keep the gas concentration and gas temperature unchanged. Measure the curve of the ratio of the second harmonic to the first harmonic and the pressure, and get S(P) by fitting. Cmax ; Step B, measure the curve of low concentration gas signal changing with pressure: introduce background gas (such as nitrogen which is not the detection gas) into the gas detection chamber of the sensor, the concentration at this time is recorded as Cmin, after stabilization, change the gas pressure and keep the gas concentration and gas temperature unchanged, measure the curve of the ratio of the second harmonic to the first harmonic and the pressure, and get S(P) by fitting. Cmin ; Step C: The actual measurement process requires measuring the ratio of the second harmonic to the first harmonic S0 and the gas pressure P0. The gas concentration is calculated as follows:

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