A temperature compensation method and device for online monitoring of COD in water using ultraviolet spectroscopy

Abstract

A temperature compensation method and apparatus for online monitoring of COD in water using ultraviolet spectroscopy is disclosed. The apparatus includes a light source, an optical absorption cell, and a spectrometer. Ultraviolet light is split into two paths by a spectrometer in the optical absorption cell; one path is incident on the online monitoring cell, and the other on the reference cell. The transmitted ultraviolet light is received by two miniature spectrometers, and the absorbance value is calculated. The online monitoring cell and the reference cell are always in the same temperature field. The liquid to be tested flows through the online monitoring cell, while the reference cell is filled with a fixed concentration of dimethyl silicone oil as a reference solution. The absorbance of the solution is affected by both COD concentration and temperature, while the absorbance of the dimethyl silicone oil is only affected by temperature. The two do not interfere with each other, providing a basic mathematical basis for temperature compensation. Through the all-light temperature compensation method and apparatus for online COD monitoring using ultraviolet spectroscopy of this invention, the influence of temperature on online COD monitoring can be effectively compensated, enabling accurate online monitoring of COD concentration in water.

Description

Technical Field

[0001] This invention belongs to the field of online water quality monitoring technology, and relates to a temperature compensation method and device for online monitoring of COD in water using ultraviolet spectroscopy. Background Technology

[0002] Online monitoring methods for COD content in water include chemical and physical methods. Chemical methods require manual sampling and offline testing, making them time-consuming and difficult to implement online. Physical methods, however, utilize the characteristic absorption peak of COD at 254 nm in the ultraviolet spectrum, thus making them widely used for online monitoring.

[0003] This field is characterized by online COD monitoring instruments offered by companies such as Hach Corporation in the United States and Yantai Kemis in China. While Hach's instruments offer stable performance, they are expensive, and Kemis's products have significant room for improvement in temperature stability. This invention addresses the online COD temperature compensation problem by proposing a temperature compensation method for a liquid reference cell.

[0004] This invention is based on Beer-Lambert law, which governs the absorption of COD in the ultraviolet band, and designs an absorption cell to establish the relationship between absorbance, absorption coefficient, optical path length, and concentration: Once the optical path length and the analyte are determined, a linear regression model between absorbance and concentration is established. The concentration of the corresponding substance can be measured by online monitoring of the ultraviolet absorbance of the water sample. However, temperature affects actual online monitoring of water samples. Temperature influences molecular motion and chemical reactions, especially the outer electrons of atoms, which are more susceptible to temperature-induced absorption spectrum shifts. The absorbance of a COD solution of the same concentration differs at different temperatures. Therefore, studying and compensating for the impact of temperature changes on COD measurement is essential for online monitoring. Currently, the main method is to incorporate an electrical temperature sensor and achieve compensation through temperature calibration, but this method has significant drawbacks.

[0005] 1. The liquid being tested has an absorption range, which cannot be compensated for by measuring the temperature at a single point, resulting in a significant loss of accuracy.

[0006] 2. In underwater measurement environments, electronic temperature sensors are easily affected by factors such as humidity and water flow vibration, which can affect measurement accuracy and stability.

[0007] 3. Electronic temperature sensors must be immersed in liquid, therefore the wiring must also be submerged in water, making sealing and maintenance quite difficult. Therefore, the encapsulation typically needs to be re-inspected every six months, which is time-consuming and labor-intensive.

[0008] 4. Electronic temperature sensors need to heat themselves to the temperature of the measured object before they can reflect temperature changes, which has a certain relaxation time and affects measurement accuracy.

[0009] 5. Electronic temperature sensors are susceptible to electromagnetic interference, which can easily lead to errors in industrial areas with dense wiring, such as water purification plants. Summary of the Invention

[0010] To address the problems existing in the prior art, this invention provides a method and apparatus for online COD monitoring using ultraviolet spectroscopy with temperature compensation, eliminating the need for a built-in electronic temperature sensor. This invention provides an all-optical, electricity-free, rapid, accurate, and pollution-free temperature compensation method for online COD monitoring using ultraviolet spectroscopy.

[0011] The technical solution of the present invention is as follows:

[0012] An online COD monitoring device for water quality using ultraviolet spectroscopy is disclosed. The device includes a light source, an optical absorption cell, and spectrometers. Before monitoring, after the light source has stabilized, two spectrometers are used to measure the light intensity of the light source. The light source is selected in the 200nm~400nm ultraviolet band. The ultraviolet light is split into two paths by a spectrometer in the optical absorption cell; one path is incident on the online monitoring cell, and the other is incident on the reference cell. The transmitted ultraviolet light is received by two miniature spectrometers. During monitoring, both spectrometers receive and measure the intensity of the transmitted light. The absorbance values ​​of the online monitoring cell and the reference cell are calculated based on the previously measured light intensity information. The online monitoring cell and the reference cell are always in the same temperature field. The liquid to be tested flows through the online monitoring cell, while the reference cell is filled with dimethyl silicone oil as a reference solution. The absorbance of the solution is affected by both COD concentration and temperature, but the two do not interfere with each other, providing a basic mathematical basis for temperature compensation.

[0013] The optical absorption cell includes a partition 1, right-angle prisms 2, side plates 3, an online monitoring cell 4, a reference cell 5, and a base plate 6. A cavity is formed in the center of the base plate 6, and two pairs of channels are symmetrically formed on both sides of the cavity. The outer pair of channels are the first fiber optic inlet A2 and the first fiber optic outlet A1, and the inner pair of channels are the second fiber optic inlet B2 and the second fiber optic outlet B1. A set of right-angle prisms 2 are symmetrically arranged directly below the two pairs of channels. The distance between the two right-angle prisms is equal to the width of the cavity in the center of the base plate 6. A partition 1 is horizontally arranged in the middle between the two right-angle prisms. The reference cell 5 is above the partition 1, and the online monitoring cell 4 is below it. The side plates 3 are vertically arranged on the base plate 6, located on both sides of the two right-angle prisms.

[0014] A method for online monitoring of COD in water quality using ultraviolet spectroscopy with all-optical temperature compensation, the method specifically comprising:

[0015] Step 1, Standard Solution COD Concentration Absorbance Calibration Process: Based on the required COD concentration range for the measurement, prepare standard solutions of different concentrations within this range. Measure the absorbance of the standard solutions at 254 nm for different COD concentrations and temperature variations, establishing a calibration model for absorbance at 254 nm in relation to concentration and temperature. Note that absorbance calibration is equipment-dependent and must be performed individually for each device. In laboratory calibration, potassium hydrogen phthalate solution is preferably used to prepare the COD equivalent standard solution.

[0016] Step 2, Temperature and Absorbance Calibration of Dimethyl Silicone Oil: Using a water bath, heat 99% pure encapsulating dimethyl silicone oil from room temperature (25°C) to 50°C, then slowly allow it to return to room temperature. During this process, measure the UV absorption spectrum at 273nm at fixed temperature intervals as the temperature decreases, establishing a fitting function between absorbance at 273nm and temperature. If a larger temperature calibration range is required, a temperature-controlled chamber or similar device can be used to expand the temperature calibration range.

[0017] Step 3, Actual water sample testing: First, based on the absorbance data of dimethyl silicone oil at 273nm, and according to the fitting function between absorbance at 273nm and temperature in Step 2, the temperature T is deduced.

[0018] Step 4: Substitute the actual absorbance of the water sample and the temperature T obtained in Step 3 into the calibration model of absorbance at 254 nm, concentration, and temperature obtained in Step 1 to obtain the COD concentration of the water sample.

[0019] In step 1, the coefficient forms in the calibration model for absorbance, concentration, and temperature at 254 nm may differ depending on the type of sensor used. Let the COD absorbance of the standard solution be A, the temperature be T, and the COD concentration be C; the general formula for the calibration model is:

[0020] ,in, is a coefficient.

[0021] For example, the spectrometer used is the Haiguang Optoelectronics XS116399 miniature laser spectrometer, along with its accompanying 1m long ultraviolet special optical fiber. The light source is a Japanese Hamamatsu L6301-50 deuterium lamp. With this setup, the polynomial fitting of the COD concentration C yields the following calibration model for absorbance at 254nm in relation to concentration and temperature:

[0022] Note that the coefficients in the formula above are obtained from our actual measurements and are related to each device. Therefore, specific measurements and calibrations should be performed for each device.

[0023] In step 2, the fitting function between absorbance and temperature at 273 nm is: In the formula This indicates the absorbance of pure dimethyl silicone oil. is a coefficient.

[0024] In step 1, the optical absorption cell is first cleaned with deionized water, and then deionized water is poured into both cells for baseline correction to improve measurement accuracy.

[0025] In step 2, considering the good temperature effect, characteristic absorption in the ultraviolet band, easy storage, and safety of dimethyl silicone oil, it was selected as the reference material for temperature compensation. By screening for characteristic wavelengths with good linearity of dimethyl silicone oil, a wavelength of 273 nm was ultimately chosen as the characteristic wavelength.

[0026] In step 3, the intermediate temperature value is eliminated, leaving only the nonlinear relationship between the changes in absorbance. At this point, there is no need for online temperature monitoring; the temperature information can be directly deduced from the absorbance to subtract the influence of temperature on COD absorbance.

[0027] In step 4, the compensated COD concentration can be calculated based on the relationship between the measuring cell and the reference cell obtained in step 3.

[0028] The effects and benefits of this invention are as follows: The temperature compensation method and apparatus for online COD monitoring using ultraviolet spectroscopy of this invention can effectively compensate for the impact of temperature on online COD monitoring, enabling accurate online monitoring of water COD concentration. The liquid spectral flow cell of this invention can directly establish the correlation between the absorbance changes of dimethyl silicone oil and COD at different temperatures without directly measuring the temperature value, thus compensating for the absorbance shift caused by temperature. This results in more accurate measurements, unaffected by temperature instability, and also reduces instrument costs and extends its lifespan. Attached Figure Description

[0029] The accompanying drawings are provided to fully illustrate the visibility mechanism of the invention; however, the drawings are for illustrative purposes only and do not constitute a limitation on the scope of the invention.

[0030] Figure 1 This is a design diagram of the liquid spectral flow cell of the present invention.

[0031] Figure 2 The linear fitting curve of absorbance at 273 nm versus temperature for the dimethyl silicone oil of the present invention is shown.

[0032] Figure 3 This invention illustrates the nonlinear relationship between the absorbance of dimethyl silicone oil and the change in absorbance of COD solution at different temperatures.

[0033] In the diagram: 1-partition; 2-right-angle prism; 3-side plate; 4-online monitoring cell; 5-reference cell; 6-bottom plate; A1-first fiber optic outlet; B1-second fiber optic outlet; A2-first fiber optic inlet; B2-second fiber optic inlet;

[0034] The change in absorbance with absorbance at a fixed concentration of COD. The absorbance of dimethyl silicone oil varies with absorbance. Detailed Implementation

[0035] As shown in the figure, the novel temperature compensation method and device for online COD monitoring using ultraviolet spectroscopy of the present invention are used to improve the accuracy of online monitoring under actual water sample temperature changes. Figure 1 The ultraviolet spectroscopy online COD temperature compensation device shown is a liquid optical absorption cell with strong resistance to temperature interference. The overall dimensions are 72×40×32.62mm, and the outer shell is made of stainless steel. It consists of a base plate 6, a partition plate 1, a side plate 3, and a right-angle prism 2.

[0036] The base plate 6 and side plate 3 are integrated, serving to fix and protect the right-angle prism 2. The base plate 6 has two SMA905 fiber optic interfaces on each side of its center. One side is the incident light port (first fiber optic inlet A2, second fiber optic inlet B2), and the other side is the optical coupling output port (first fiber optic outlet A1, second fiber optic outlet B1). The port closer to the center is a 273nm optical port (second fiber optic outlet B1, second fiber optic inlet B2), used for 273nm light transmission into the dimethyl silicone oil in the reference cell 5; the port further from the center is a 254nm optical port (first fiber optic outlet A1, second fiber optic inlet A2), used for 254nm light transmission into the COD solution to be measured in the measurement cell.

[0037] During calibration, we used a wide-range ultraviolet spectrum for band selection and a spectrometer for measurement. When subsequently manufacturing or using the product, we can use specific wavelength light sources and photodiodes (PDs) to reduce costs. The 254nm light emitted from the light source is split in two: 50% is directly connected to PD1, and the other 50% enters the online detection cell and is transmitted to PD2. The absorbance can be directly obtained by comparing the two. Similarly, the 273nm light emitted from the light source is split in two: 50% is directly connected to PD3, and the other 50% enters the reference cell and is transmitted to PD4. The absorbance can also be directly obtained by comparing the two. This fiber optic splitting method, where one fiber branches directly from the light source to the liquid cell, allows for more real-time absorbance calculation, reduces the requirement for a highly stable light source, and lowers costs.

[0038] The partition 1 is placed on both sides of the optical fiber port at the central axis position, and the reference cell 5 is located at the center of the partition, with dimensions of 10×10×10mm, thus fixing the optical path of the dimethyl silicone oil at 10mm. Below the partition is the online monitoring cell 4, which also has an optical path of 10mm.

[0039] The right-angle prism 2 is located at the lower end of the fiber optic interfaces at both ends. To facilitate fixing it between the base plate 6 and the side plate 3, two right-angle prisms 2 are used on each side to form a reflecting prism. This is used to change the direction of the incident light, because bending the fiber will cause light loss, and in severe cases, fiber breakage. Light on the side closer to the center is reflected by the right-angle prism into the reference cell 5, while light on the side farther from the center is reflected by the right-angle prism into the online monitoring cell 4.

[0040] The base plate 6, side plates 3, partition 1, and right-angle prism 2 are all sealed with adhesive, and the dimethyl silicone oil contained in the reference cell 5 must be tightly sealed to prevent water stains from entering the device and affecting the absorbance measurement.

[0041] Therefore, we first performed a calibration operation. During calibration, the water was in a pure water environment, with no COD concentration affecting the absorbance. The absorbance was only introduced by temperature. Under the same temperature field, the absorbance of the test water was measured at 254 nm in the measurement cell, and the absorbance of dimethyl silicone oil in the reference cell was measured at 273 nm. The choice of 254 nm is common in the industry, mainly considering the influence of COD in actual working conditions, with the principle of exclusivity being the primary consideration. Since pure dimethyl silicone oil is sealed and its concentration does not change, we chose 273 nm, which has the highest corresponding intensity, as the characteristic wavelength. This is a pioneering approach we proposed.

[0042] Theoretically, the absorbance of a liquid is directly proportional to its temperature within a certain temperature range, such as... Figure 2 The linear model is shown, but it's insufficient for more precise linear calibration. In practice, for more accurate calibration, lookup tables or polynomial fitting are used. Considering the absorbance at different concentrations and temperatures, a lookup table is established and a high-order fit is performed. A compensation model for temperature, COD concentration, and absorbance is established as follows: Figure 3 As shown, for example, our calibration equipment can obtain a polynomial fitting relationship between the COD absorbance A, temperature T, and COD concentration C of a standard solution, such as the following format (the coefficients may not be the same for different sensors).

[0043]

[0044] like Figure 3 As shown.

[0045] The absorbance of pure dimethyl silicone oil shows a non-linear positive correlation with temperature, such as... Figure 2 For the curve, we use quadratic fitting. In the formula This indicates the absorbance of pure dimethyl silicone oil. , , The coefficients are given. Using our equipment, the calibration equation is: Similar to the calibration above, the coefficients of different sensors may not have the same form, but the trend is consistent, and they can all be obtained by fitting a quadratic polynomial.

[0046] After calibration, the water temperature can be calculated by observing the absorbance of a fixed concentration of dimethyl silicone oil stored during calibration and its variation with temperature. Combined with the measured absorbance, the COD concentration can be found in a compensation model of temperature, COD concentration, and absorbance. This serves as the basic basis for measurement calculations and compensates for the drift in the measurement cell based on the absorbance of the reference cell.

[0047] The online absorbance monitoring and correction method shown is as follows:

[0048] Step 1: After determining the COD concentration range to be monitored online, prepare potassium hydrogen phthalate standard solutions of different concentrations within this range. Add deionized water to reference cell 5, and pour the potassium hydrogen phthalate solution into online monitoring cell 4 as the standard COD solution. Measure the UV absorption spectra of COD at different concentrations at 254 nm. Take multiple measurements and calculate the average value to reduce measurement error. Establish a linear regression curve between absorbance and concentration at this temperature (254 nm). Finally, we establish a calibration model for COD concentration, temperature, and absorbance at 254 nm.

[0049] Step 2: Measure the UV absorption spectra of 99% dimethyl silicone oil at different temperatures: Heat the liquid spectral flow cell in a water bath to 50°C and then stop heating. Slowly decrease the temperature to room temperature (25°C) at 273 nm. Measure one UV absorption spectrum every 5°C during this process, obtaining the UV absorption spectra at different temperatures. Establish a linear fitting curve between the absorbance of dimethyl silicone oil at 273 nm and temperature.

[0050] Step 3, Usage Procedure. First, based on the absorbance data of dimethyl silicone oil at 273 nm, according to the calibration equation... From this, the temperature T is deduced. The temperature information T is then input into the COD temperature-concentration model, as follows: Figure 3 .

[0051] Step 4: Based on the actual liquid absorbance measured and the temperature obtained in the previous step, determine the COD concentration C.

[0052] In this embodiment, temperature and absorbance data are input into Origin for fitting to obtain a fitting curve, and then the temperature is subtracted to directly establish a nonlinear relationship between COD and dimethyl silicone oil absorbance.

[0053] This invention enables online monitoring of COD concentration using ultraviolet spectroscopy, and incorporates temperature compensation to improve measurement accuracy, avoiding COD measurement errors caused by inaccurate temperature sensors. It features low cost and long lifespan, and can monitor water COD concentration online in real time.

Claims

1. A method for online monitoring of COD in water quality using ultraviolet spectroscopy with all-optical temperature compensation, characterized in that, The method is specifically as follows: Step 1, Standard solution COD concentration absorbance calibration process: According to the required COD concentration range for measurement, prepare standard solutions of different concentrations within this range. Measure the absorbance of the standard solutions with different COD concentrations and temperature changes at 254nm, and establish a calibration model of absorbance at 254nm versus concentration and temperature. Step 2, temperature absorbance calibration process of dimethyl silicone oil: Use a water bath to heat the 99% pure encapsulated dimethyl silicone oil from room temperature (25°C) to 50°C and stop. As the temperature slowly returns to room temperature, at 273nm, a UV absorption spectrum is measured at a fixed temperature interval to establish a fitting function between absorbance at 273nm and temperature. Step 3, Actual water sample testing: Based on the absorbance data of dimethyl silicone oil at 273nm, and based on the fitting function between absorbance at 273nm and temperature in Step 2, the temperature T is deduced. An online device for monitoring COD in water quality using ultraviolet spectroscopy was employed. The device includes a light source, an optical absorption cell, and a spectrometer. The light source is selected in the ultraviolet band of 200nm~400nm. The ultraviolet light is split into two paths by the spectrometer of the optical absorption cell. One path is incident on the online monitoring cell, and the other path is incident on the reference cell. The transmitted ultraviolet light is received by the two spectrometers and the absorbance value is calculated. The online monitoring cell and the reference cell are always in the same temperature field. The liquid to be tested flows through the online monitoring cell, while the reference cell is filled with dimethyl silicone oil as a reference liquid. The dimethyl silicone oil sealed in the reference cell (5) needs to be sealed tightly. The optical absorption cell includes a partition (1), a right-angle prism (2), a side plate (3), an online monitoring cell (4), a reference cell (5), and a base plate (6). The base plate (6) has a cavity in the center, and two pairs of channels are symmetrically opened on both sides of the cavity. The outer pair of channels are the first optical fiber inlet (A2) and the first optical fiber outlet (A1), and the inner pair of channels are the second optical fiber inlet (B2) and the second optical fiber outlet (B1). A set of right-angle prisms (2) are symmetrically arranged directly below the two pairs of channels. The distance between the two right-angle prisms (2) is equal to the width of the cavity in the center of the base plate (6). The partition (1) is horizontally arranged in the middle between the two right-angle prisms (2). The reference cell (5) is above the partition (1), and the online monitoring cell (4) is below it. The side plate (3) is vertically arranged on the base plate (6) and located on both sides of the two right-angle prisms. The incident light entering from the first optical fiber inlet (A2) is reflected by a right-angle prism and enters the reference cell (5); the incident light entering from the second optical fiber inlet (B2) is reflected by a right-angle prism and enters the online monitoring cell (4). Step 4: Substitute the actual absorbance of the water sample and the temperature T obtained in Step 3 into the calibration model of absorbance at 254 nm, concentration, and temperature obtained in Step 1 to obtain the COD concentration of the water sample.

2. The all-optical temperature compensation method for online monitoring of COD in water quality using ultraviolet spectroscopy according to claim 1, characterized in that, In step 1, the coefficient forms in the calibration model for absorbance, concentration, and temperature at 254 nm differ depending on the type of sensor used. Let the COD absorbance of the standard solution be A, the temperature be T, and the COD concentration be C; the general formula for the calibration model is: ,in, is a coefficient.

3. The all-optical temperature compensation method for online monitoring of COD in water quality using ultraviolet spectroscopy according to claim 1, characterized in that, In step 2, the fitting function between absorbance and temperature at 273 nm is a quadratic function. In the formula This indicates the absorbance of pure dimethyl silicone oil. is a coefficient.

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