A method for calibrating an optical sensor

Abstract

The present invention relates to a calibration method for an optical sensor, comprising the following steps: when the optical sensor meets the accuracy requirements, inserting a calibration filter into a measuring slot of the optical sensor in a natural air environment, reading and recording the initial absorbance; when the optical sensor needs to be calibrated, placing the measuring slot or the entire optical sensor in distilled water for zeroing to obtain a first absorbance; taking the optical sensor out of the water, drying the optical sensor, inserting the calibration filter into the measuring slot of the optical sensor in a natural air environment to obtain the absorbance of the current calibration filter, and using the absorbance as a second absorbance; determining a linear relationship between the current absorbance and the absorbance after calibration; substituting the linear relationship into the relationship between the absorbance and the concentration to calculate the concentration value after calibration. The present invention has the advantages of improving the measurement accuracy of the optical sensor calibrated by the calibration filter.

Description

Technical Field

[0001] The invention relates to the technical field of water quality analysis, and in particular to a calibration method for an optical sensor. Background Art

[0002] Chemical oxygen demand (COD) is an important indicator for evaluating the degree of water pollution. At the same time, the nitrate and nitrite content in water is also a conventional indicator for water quality testing. Currently, COD measurement sensors and nitrate nitrogen measurement sensors on the market basically require multiple calibration points, that is, they require multiple standard solutions of different concentrations. However, standard solutions have the disadvantages of being inconvenient to carry, easily contaminated, and producing contaminated waste liquid.

[0003] In order to reduce the number of standard solutions used by on-site water quality detection sensors (water quality detection sensors are optical sensors), or even to use no standard solutions, so as to improve the convenience of on-site detection equipment calibration, patent CN110057759A discloses a method for calibrating optical sensors using filters, which uses a neutral attenuation filter to replace the standard solution to calibrate the optical sensor. Since the optical sensor tests the content of a certain substance in the water sample based on the different absorbance of the measured water sample or standard solution to the light signal (i.e., different absorbance values), and the filter can also absorb the light signal by coating a specific material on the glass substrate. Therefore, in theory, a filter with light absorption ability can be used to replace the measured water sample or standard solution. According to patent CN110057759A, the calibration module reported therein uses two calibration filters, which can be combined to achieve three calibration points, that is, two calibration filters are used alone to obtain two points, and two calibration filters are superimposed to obtain the third calibration point. At the same time, the calibration filter reported in the patent can be used for the same model of optical sensors. However, considering the processing errors of the internal components of the optical sensor, such as the light source, lens, light detector, structural mounting parts, etc., and the influence of various factors such as differences in human techniques during assembly, the optical axis of the optical sensor and the geometric axis of the structural mounting parts are not parallel, and the angle between the optical axis and the geometric axis of the structural mounting parts is also different in each optical sensor. Therefore, if the same type of optical sensors share the same calibration filter for calibration, due to the angle between the two axes mentioned above, the measurement light beam will not be incident vertically on the surface of the calibration filter. Then, according to the law of refraction of light, after the light beam passes through the calibration filter, the light energy reaching the internal light detector of the optical sensor will be very different, which will affect the calibration accuracy of the calibration filter.

[0004] In addition, when the on-site water quality detection sensor is used, the object to be measured is water, and when calibrated with a calibration filter, the objects to be measured are air and a calibration filter (the calibration filter is a glass filter). The refractive index of air and the calibration filter is different from that of water. According to the law of refraction of light, the light signal intensity measured by the on-site water quality detection sensor in the use state and the calibration state will be different, and the eccentricity of the optical path is different (that is, the path of light will be different). The light intensity values ​​received by the water quality detection sensor in the two states are also different. Therefore, the absorbance value of the calibration filter cannot be equivalent to the absorbance value of the calibration standard solution, which has a great impact on the measurement accuracy of the water quality detection sensor. Summary of the invention

[0005] In order to solve the above problems, an object of the present invention is to provide a calibration method for an optical sensor, which improves the measurement accuracy of the optical sensor calibrated by a calibration filter.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A calibration method for an optical sensor comprises the following steps: when the optical sensor meets the accuracy requirement, inserting a calibration filter into a measuring slot of the optical sensor in a natural air environment, reading and recording the absorbance of the calibration filter, and taking the absorbance as the initial absorbance; the optical sensor has a built-in relationship between absorbance and concentration; when the optical sensor needs to be calibrated, placing the measuring slot or the entire optical sensor in distilled water for zeroing to obtain a first absorbance; taking the optical sensor out of the water, drying the optical sensor, and then inserting the calibration filter into the measuring slot of the optical sensor in a natural air environment to obtain the absorbance of the current calibration filter, and taking the absorbance as the second absorbance; determining a linear relationship between the current absorbance and the absorbance after calibration based on the first absorbance, the second absorbance and the initial absorbance; substituting the linear relationship into the relationship between the absorbance and the concentration to calculate the concentration value after calibration.

[0008] More preferably, each optical sensor corresponds to a calibration filter, and the two are used in a one-to-one pair.

[0009] More preferably, the optical sensor meets the accuracy requirement when the optical sensor is calibrated with a plurality of standard solutions of different concentrations before leaving the factory, and the accuracy of the optical sensor meets the requirement.

[0010] More preferably, the calibration concentration corresponding to the absorbance of the calibration filter is a value within a range of 30% to 60% of the measurement range of the optical sensor.

[0011] More preferably, the optical sensor is a COD measurement sensor or a nitrate nitrogen measurement sensor.

[0012] The present invention has the following beneficial effects:

[0013] The calibration method of the present invention only requires distilled water and a calibration filter, and the absorbance of the calibration filter at two different time points can be used to calibrate the absorbance of the optical sensor, thereby achieving calibration of the whole measurement value of the optical sensor. Since the calibration scheme of the present invention only requires one calibration filter, an indispensable prerequisite is provided for the one-to-one matching of the optical sensor and the calibration filter. At the same time, the one-to-one matching solves the calibration error caused by the difference in the optical paths of different optical sensors, and improves the accuracy of the optical sensor after calibration with the calibration filter. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a calibration flow chart of the present invention;

[0015] Figure 2 Schematic diagram of the optical path during COD sensor calibration;

[0016] Figure 3 This is the main view of the COD sensor;

[0017] Figure 4 for Figure 3 CC view (calibration filter is in the light path);

[0018] Figure 5 Schematic diagram of the calibration filter not being in the light path.

[0019] Description of reference numerals:

[0020] 1. COD sensor; 2. Calibration filter; 1-1. Light source; 1-2. Collimating lens; 1-3. First window; 1-4. Second window; 1-5. Focusing lens; 1-6. Beam splitter; 1-7. 254 filter; 1-8. Detector; 1-9. 550 filter. DETAILED DESCRIPTION

[0021] See also Figure 1 , a calibration method for an optical sensor, comprising the following steps:

[0022] Step 1: When the optical sensor meets the accuracy requirements, insert the calibration filter 2 into the measuring slot of the optical sensor in a natural air environment, read and record the absorbance of the calibration filter 2, and use the absorbance as the initial absorbance. Before the optical sensor leaves the factory, a plurality of standard solutions of different concentrations are used to calibrate the optical sensor so that its accuracy meets the equipment requirements. Therefore, the present invention chooses to record the absorbance of the calibration filter 2 at this moment as the initial absorbance, and the initial absorbance is used as the target value of the later calibration process. The present invention replaces the calibration standard solution of the optical sensor by using a calibration filter 2 whose absorbance is close to the absorbance of a standard solution of a certain concentration. For the selection of concentration, preferably: considering that the initial absorbance of the calibration filter 2 is measured by the optical sensor, the accuracy of the optical sensor itself will affect the accuracy of the initial absorbance of the calibration filter 2, and when the optical sensor needs to be recalibrated again, the initial absorbance of the calibration filter 2 is used as the target value of the calibration process, therefore, the accuracy of the initial absorbance measurement of the calibration filter 2 will affect the accuracy of the optical sensor after recalibration. Therefore, the concentration of the standard solution corresponding to the calibration filter 2 needs to be determined according to the range and accuracy of the optical sensor. The selection principle is: select a position where the optical sensor has higher accuracy and is near the middle of the range. At the same time, the middle is located near the front to make the results of low concentration measurements more accurate, because the accuracy is higher near the calibration point of the optical sensor. Therefore, the calibration concentration corresponding to the absorbance of the calibration filter 2 is a value within the range of 30% to 60% of the measurement range of the optical sensor. In the field of water pollution monitoring, commonly used optical sensors include COD measurement sensors, nitrate nitrogen measurement sensors, etc. For optical sensors that measure concentration, the relationship between absorbance and concentration is built-in. For example, N=a3A 3 +a2A 2 +a1A+a0, where N is the concentration, {a3,a2,a1,a0} are coefficients, and A is the absorbance.

[0023] Step 2: After the optical sensor has been used for a period of time, the measurement accuracy becomes poor and needs to be calibrated. After the optical sensor is cleaned with clean water, the measuring slot of the optical sensor or the entire optical sensor is placed in distilled water for zeroing. At this time, the measuring slot of the optical sensor is not placed in the calibration filter 2, and the first absorbance is measured.

[0024] Step 3, taking out the optical sensor from the water, drying the optical sensor, and then inserting the calibration filter 2 into the measuring slot of the optical sensor in a natural air environment to obtain the absorbance of the current calibration filter 2, and using the absorbance as the second absorbance;

[0025] Step 4: Determine the linear relationship between the absorbance A1 measured during calibration and the absorbance A after calibration, i.e., the absorbance A when the accuracy meets the requirements at the time of leaving the factory, according to the first absorbance, the second absorbance, and the initial absorbance, A=KA1+b, wherein K is the slope of the linear relationship and b is the intercept. When the optical sensor is assembled, its internal parts and the relative positions between the parts are fixed, so the propagation path of the light and the incident angle of the light on the surface of each component will also remain unchanged. Therefore, according to Beer's law, when the optical sensor's accuracy deteriorates due to factors such as aging of internal circuit components or attenuation of light energy of the light source, the absorbance values ​​corresponding to each concentration point within the entire range of the optical sensor change linearly. Therefore, as long as the coefficient K and the intercept b are calculated, the linear relationship between the absorbance A1 measured during calibration and the absorbance A when the accuracy meets the requirements at the time of leaving the factory can be determined. When zeroing, since distilled water has no impurities and its absorbance is 0, the first absorbance measured is the intercept b, and the second absorbance of the calibration filter 2 measured in the air is combined with the known initial absorbance of the calibration filter 2 to determine the coefficient K. Since the optical sensor is placed in the air when reading the initial absorbance of the initial calibration filter 2 and the current absorbance of the calibration filter 2 when recalibrating later, i.e., the second absorbance, the medium for propagating light is the same, and the error can be offset.

[0026] Step 6: Substitute the linear relationship A=KA1+b into the relationship between absorbance and concentration, and calculate the calibrated concentration value. It should be noted that in this embodiment, the concentration of a certain object in water measured by an optical sensor is obtained by processing the measured absorbance through a software algorithm. Therefore, calibrating the absorbance of the optical sensor can achieve the concentration calibration of the optical sensor. For example, N=a3A 3 +a2A 2 +a1A+a0=a3(KA1+b) 3 +a2(KA1+b) 2 +a1(KA1+b)+a0

[0027] Since even if there are multiple optical sensors of the same model, the internal part errors and assembly errors of each optical sensor may affect the calibration effect of the calibration filter 2, in order to eliminate the influence, this patent proposes a method of improving the accuracy of the calibrated optical sensor by pairing the calibration filter 2 with the optical sensor one-to-one, that is, each optical sensor has a corresponding calibration filter 2. When the optical sensor needs to be recalibrated, the corresponding calibration filter 2 is used to calibrate the optical sensor. This ensures that the measured object of the optical sensor is the same each time it is calibrated, which can eliminate the influence of the optical path differences between the optical sensors and improve the measurement accuracy of the optical sensor after calibration through the calibration filter 2.

[0028] The measurement process and effect of the present invention are now described using the COD sensor 1.

[0029] like Figures 2 to 5 As shown, there are two paths of light inside the COD sensor 1, one at 254nm and the other at 550nm. The absorbance measured by the COD sensor 1 is the combined absorbance of the two paths at 254nm and 550nm, so the calibration filter 2 needs to consider the absorbance of the two wavelengths. The calibration filter 2 is a quartz glass material with a layer of specific chemical material, such as hafnium oxide, coated on the surface to achieve a specific absorbance in the 254±10nm and 550±30nm bands (this specific absorbance depends on the absorbance of the standard solution to be replaced by the calibration filter 2), so that the absorbance value measured on the COD sensor 1 can be close to the absorbance of the standard solution to be replaced, that is, the calibration filter 2 involved in this patent can be used to replace the COD standard solution with a similar corresponding absorbance value. When the COD sensor 1 is calibrated using the calibration filter 2, the optical path diagram of the optical signal propagation is as shown in the figure. Figure 2 As shown in the direction of the middle arrow, the optical signal output by the light source 1-1 is collimated by the collimating lens 1-2 to become parallel light, passes through the first window 1-3, the calibration filter 2, and the second window 1-4 in sequence, is focused by the focusing lens 1-5, and is divided into two beams by the beam splitter 1-6. After the stray light is filtered out by the 254 filter 1-7 and the 550 filter 1-9 respectively, they are received by their respective corresponding detectors 1-8, and finally the final measurement result is obtained by circuit conversion and software calculation. Figure 2In the embodiment, the area between the first window 1-3 and the second window 1-4 is the measuring slot of the COD sensor 1. When using the standard liquid on site, the measured water sample or the standard liquid needs to immerse the measuring slot, and when using the calibration filter 2 for calibration, the installation position of the calibration filter 2 is also in the measuring slot. Due to the processing and assembly errors of the internal components of the COD sensor 1, such as the light source 1-1 and the collimating lens 1-2, the light beam collimated by the collimating lens 1-2 is no longer vertically incident on the calibration filter 2, but will have a deflection angle with the geometric center axis of the calibration filter 2. Then, according to the law of refraction of light, the intensity of the light signal received by the detector 1-8 will eventually become smaller, and for each COD sensor 1 of the same model, the light beam will have a different deflection angle after passing through the calibration filter 2, which will also cause differences in the light signals received between the various COD sensors 1. Therefore, theoretically speaking, using a calibration filter 2 to calibrate multiple COD sensors 1 of the same model will result in a large difference in accuracy after calibration.

[0030] The present embodiment is a COD sensor 1 with a measuring slot distance of 5mm, i.e., an optical path of 5mm, and a range of its COD parameter of 0-1000mg / L. With reference to the selection principle of the calibration filter 2 of the foregoing text, the absorbance value of the selected calibration filter 2 is close to the absorbance value of the COD standard solution with a concentration of 250mg / L. Because it can be seen from Table 1 that the error percentage of the reverse measurement of the COD sensor 1 at the 250mg / L standard solution is about -0.76%, then, the COD sensor 1 of the present embodiment has a higher measurement accuracy near 250mg / L, and then can ensure that the COD sensor 1 of the present embodiment can more accurately measure the initial absorbance of the calibration filter 2. Experiments were carried out according to the calibration method described above, and its measurement results are shown in the following table.

[0031] Table 1

[0032]

[0033] Table 2

[0034]

[0035] Table 3

[0036]

[0037]

[0038] From the above results, it can be seen that according to the calibration scheme above, after calibration with calibration filter 2, the accuracy of UVCOD can basically reach the result of calibration with standard solution.

[0039] Since the calibration scheme of the present invention is to compare the differences of the same calibration filter at different times of the optical sensor - when the accuracy meets the requirements when leaving the factory and when calibration is required, the measured object is the same, the optical sensor being calibrated is the same, and the absorbance value of the calibration filter itself does not change. Therefore, this calibration scheme can avoid the calibration error caused by the difference in optical paths between the various optical sensors, and achieve high measurement accuracy for each optical sensor after calibration.

[0040] The calibration method of the present invention only requires distilled water and a calibration filter, and the absorbance of the calibration filter at two different time points can be used to calibrate the absorbance of the optical sensor, thereby achieving calibration of the whole measurement value of the optical sensor. Since the calibration scheme of the present invention only requires one calibration filter, an indispensable prerequisite is provided for the one-to-one matching of the optical sensor and the calibration filter. At the same time, the one-to-one matching solves the calibration error caused by the difference in the optical paths of different optical sensors, and improves the accuracy of the optical sensor after calibration with the calibration filter.

[0041] The above description is only a specific implementation mode of the present invention, and does not limit the patent scope of the present invention. Any equivalent structural transformation made by using the contents of the present invention description and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for calibrating an optical sensor, characterized in that: The method comprises the following steps: when the optical sensor meets the accuracy requirement, inserting a calibration filter into a measuring slot of the optical sensor in a natural air environment, reading and recording the absorbance of the calibration filter, and taking the absorbance as the initial absorbance; the optical sensor has a built-in relationship between absorbance and concentration; When the optical sensor needs to be calibrated, the measuring tank or the entire optical sensor is placed in distilled water for zeroing to obtain the first absorbance; The optical sensor is taken out of the water, dried, and then in a natural air environment, the calibration filter is inserted into a measuring slot of the optical sensor to obtain the absorbance of the current calibration filter, which is used as the second absorbance; Determine the linear relationship between the current absorbance and the absorbance after calibration according to the first absorbance, the second absorbance and the initial absorbance, specifically: determine the linear relationship between the absorbance A1 measured during calibration and the absorbance A after calibration, that is, the absorbance A when the accuracy meets the requirements when leaving the factory, according to the first absorbance, the second absorbance and the initial absorbance, A=KA1+b, wherein K is the slope of the linear relationship and b is the intercept; when performing zero adjustment, since distilled water has no impurities and the absorbance is 0, the measured first absorbance is the intercept b, and then the second absorbance of the calibration filter measured in the air is used in combination with the known initial absorbance of the calibration filter to determine the coefficient K; Substitute the linear relationship into the relationship between absorbance and concentration to calculate the calibrated concentration value.

2. The optical sensor calibration method according to claim 1, characterized in that: Each optical sensor corresponds to a calibration filter, and the two are used in a one-to-one pair.

3. The optical sensor calibration method according to claim 1, characterized in that: The optical sensor meets the accuracy requirement when the optical sensor is calibrated with a plurality of standard solutions of different concentrations before leaving the factory, and the accuracy of the optical sensor meets the requirement.

4. The optical sensor calibration method according to claim 1, characterized in that: The calibration concentration corresponding to the absorbance of the calibration filter is a value within a range of 30% to 60% of the measurement range of the optical sensor.

5. The optical sensor calibration method according to claim 1, characterized in that: The optical sensor is a COD measurement sensor or a nitrate nitrogen measurement sensor.

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