CN204631135U - A kind of aqueous solution conductivity detector - Google Patents

Abstract

The utility model discloses an aqueous solution conductivity detection device, which comprises a detection body with a detection head and an excitation module. The detection head is provided with four measurement electrodes, and the excitation module is loaded between two adjacent measurement electrodes. During the period, the excitation module includes an AC voltage source and an adjustable resistor that converts the excitation voltage into an excitation current. The adjustable resistor is a plurality of resistors with different resistance values, and each resistor is connected in parallel to the AC through a multi-channel analog switch. Voltage source; the utility model adopts a structure in which the excited measuring electrode is separated from the responding measuring electrode, which reduces the influence of the polarization effect of the measuring electrode and the double layer effect on the measurement, and uses an AC voltage source instead of an AC current source to effectively overcome It solves the problem of complex design and high cost of AC current source, and can realize the conductivity measurement of aqueous solution in a large range, and has a wider application range.

Description

An aqueous solution conductivity detection device

technical field

The utility model relates to a solution conductivity detection technology, in particular to a solution conductivity detection device.

Background technique

Conductivity is a basic parameter of water bodies and an important criterion for measuring water quality. In the food hygiene industry, various physical and chemical testing laboratories, chemical industry and semiconductor industry and other fields, it is necessary to measure the conductivity to check whether the pure water used meets the production and experimental requirements (the conductivity of the first-grade pure water is 0.055μs/cm ). In the field of water quality monitoring, conductivity, as an important water quality parameter, can reflect the quality status of water bodies, predict the development trend of water quality, and provide an indispensable scientific basis for water quality monitoring. In the field of marine science and marine applications, important information such as seawater salinity, seawater density, and seawater sound velocity can be obtained by measuring electrical conductivity, and these information are useful for marine scientific research, marine economy and national defense construction, marine environmental protection and detection. extremely important value.

The conductivity measurement of aqueous solution usually chooses laboratory conductivity meter or online conductivity meter according to actual needs. Laboratory conductivity meter (such as JONES conductivity cell designed by NIST in the United States) has high measurement accuracy, but it has strict requirements on the measurement environment. Requirements, it is necessary to ensure that it is carried out under sealed and constant temperature conditions, so the measurement is not very practical. The online conductivity meters currently on the market usually use the traditional two-electrode measurement system, but factors such as the polarization of the electrodes and the external temperature will affect the measurement accuracy, and the accuracy of the measurement cannot be guaranteed. In addition, most of the above-mentioned conductivity meters have the problem of a small measurement range, which cannot meet the needs of large-scale measurement in actual use.

The pure water conductivity measurement system shown in the patent number CN 103412192 A introduces the van der Pauw (VDP) method into the absolute measurement of the conductivity of the aqueous solution, and proposes a closed conductivity cell measurement method, with four probes on the probe head The measuring electrodes are arranged symmetrically at the same height and the center, and the excitation signal is applied to the two measuring electrodes on the same side with a micro-current source, and the conductivity cell is placed on the constant temperature water bath, and then the two measuring electrodes on the response end are measured. The voltage was used to calculate the conductivity. This measurement method can obtain accurate measurement results, but it puts forward higher requirements on the measurement conditions. At the same time, the development of micro-current sources is difficult and costly, and it is not suitable for the measurement and large-scale promotion of open aqueous solutions.

Through research, there is currently no relevant research on the conductivity measurement of aqueous solutions suitable for a wide range in China; while foreign research is mainly aimed at the measurement of high-value aqueous solutions above 1000 μS/cm, and has not carried out research on the conductivity measurement of the full range. . Therefore, it is of great significance to study the measurement method of the conductivity of the solution and develop a set of detection devices applicable to the measurement of the conductivity of aqueous solutions in various ranges.

Utility model content

The utility model provides an aqueous solution conductivity detection device, which has a simple structure and is convenient for maintenance, and can realize immersion measurement of the aqueous solution conductivity in multiple measuring ranges.

An aqueous solution conductivity detection device includes a detection body with a probe head and an excitation module, the probe head is provided with four measurement electrodes, the excitation module is loaded between two adjacent measurement electrodes, and the excitation The module includes an AC voltage source and an adjustable resistor that converts the excitation voltage into an excitation current. The adjustable resistor is a plurality of resistors with different resistance values, and each resistor is connected in parallel to the AC voltage source through multiple analog switches.

The multi-channel analog switch controls which resistor is connected according to the order of magnitude of the conductivity of the solution to be measured, so that the utility model has different measuring ranges and is suitable for measuring aqueous solutions in multiple measuring ranges.

The utility model adopts an AC voltage source as an excitation source, which is converted into an excitation current by an adjustable resistance and applied between two adjacent measuring electrodes. When measuring the aqueous solution in an open water area, the voltage at both ends of the adjustable resistance is U _R , get the excitation current as I=U _R /R, measure the response voltage of the solution to be tested between the measuring electrodes of the two response terminals as U _out ; at this time, the equivalent resistance of the solution Will Substituting into the Vanderbilt principle formula, the calculation formula of solution conductivity can be obtained as follows:

$\mathrm{<span\; class="notranslate">\; \&sigma;</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; a\&kappa;</span>}}{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; a\&kappa;</span>}\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; R</span>}}}{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; out</span>}}\mathrm{<span\; class="notranslate">\; R</span>}}$

In the formula, a is the calibration coefficient. Multiple sets of U _out and _UR obtained by measuring various standard solutions, combined with the conductivity cell constant κ of the electrode and the resistance value of the adjustable resistor R, can complete the calibration of a through fitting.

After the calibration is completed, the utility model can be used to measure the conductivity of different solutions, measure U _out , U _R , combine the conductivity cell constant κ of the electrode, the resistance value of the adjustable resistor R and the calibration value a, the conductivity of the solution can be obtained . The above calibration and measurement process can use instruments that directly measure U _out and _UR , such as a voltmeter, and then the staff can calculate according to the formula, or use the sampling circuit to collect U _out and _UR , and input the results into the calculation module for calculation.

The utility model changes the intensity and frequency of the excitation current by controlling the resistance value of the adjustable resistor and the frequency of the excitation voltage, and realizes the measurement of conductivity in different ranges by using the same device, which has a wide application range, and at the same time uses an AC voltage source instead The AC current source in the prior art effectively overcomes the problems of complex design and high cost of the AC current source, greatly reduces the development cost of the measuring device, and improves the practicability of the device.

For ease of use, preferably, the detection body is a measuring rod with a cavity, an excitation module and a measuring module are installed in the cavity, and the detection head is located at the end of the measuring rod. The measuring rod can extend into a relatively narrow container inlet, which improves the practicability of the utility model.

In order to further improve detection accuracy, preferably, the manufacturing material of the measuring rod is polytetrafluoroethylene. Tetrafluoroethylene materials have good chemical stability and will not be corroded by general corrosive solutions to prevent the test results from being affected.

Further preferably, the manufacturing material of the measuring electrode is titanium metal. Metal titanium also has good chemical stability and will not dissolve in general corrosive solutions.

In order to realize the multi-channel analog switch to adjust the adjustable resistance according to the order of magnitude of the conductivity of the solution to be measured, preferably, it also includes:

a first sampling circuit, configured to collect the voltage of the adjustable resistor;

The second sampling circuit is used to collect the voltage between the two measuring electrodes at the response end; the two measuring electrodes at the responding end are two measuring electrodes that are not loaded with excitation current.

The control unit is used to receive the sampling signals from the first sampling circuit and the second sampling circuit, and correspondingly control the conduction states of the multi-channel analog switches.

The conduction state of the multi-channel analog switch controls the connected resistance. When the voltage U _R of the adjustable resistance or the voltage U _out between the measuring electrodes of the two response terminals exceeds the measurable range, the control unit adjusts the connected resistance, thereby Adjust the magnitude of the excitation current so that the U _out and U _R to be tested are adjusted within the measurable range.

In order to simplify the circuit, the control unit uses the same interface to connect the two sampling circuits. Preferably, it also includes a switching circuit connected between the two sampling circuits and the control unit, which is used to alternately switch the signals from the first sampling circuit and the second sampling circuit. The sampling signal is input to the control unit.

In order to simplify the circuit, the two sampling circuits use existing amplifiers, preferably, the first sampling circuit is a first amplifier, and its output end is connected to the first input end of the switching circuit;

The second sampling circuit is a second amplifier whose output terminal is connected to the second input terminal of the switching circuit. .

Further preferably, the switching circuit is a relay, and the first input end and the second input end respectively correspond to one of normally open and normally closed contacts of the relay. The two amplifiers are switched by the relay, the circuit structure is simple, and the operation is stable.

There are many ways to collect voltage control resistance, the control unit can be realized by pure hardware circuit, also can be realized by software programming, adopt software to realize, the circuit structure is simpler, preferably, the output end of described switching circuit is through A/D converter Access control unit. The control unit uses the converter to obtain the digital signal of the voltage, calculates through the program, obtains the judgment result, and controls and adjusts the multi-channel analog switch according to the result to realize the adjustment of the resistance. For example, references: Hu Jisheng. A range automatic conversion technology [J]. Journal of Anhui Textile Vocational and Technical College, 2003, 2(3): 15-16. Provides a method to realize the control of the acquisition voltage control load resistance through software system.

In order to facilitate voltage collection, preferably, the two input terminals of the first amplifier are connected to both ends of the adjustable resistor, and the two input terminals of the second amplifier are respectively connected to a measuring electrode of a response terminal.

Beneficial effects of the utility model:

The aqueous solution conductivity detection device of the utility model adopts an AC voltage source instead of an AC current source, effectively overcomes the problems of complex design and high cost of the AC current source, greatly reduces the development cost of the measuring device, and improves the practicability of the device. And it can realize the conductivity measurement of aqueous solution in a large range, and the application range is wider.

Description of drawings

Fig. 1 is the structural representation of the utility model.

Fig. 2 is a schematic diagram of the circuit structure of the utility model.

Detailed ways

As shown in Figures 1 and 2, the aqueous solution conductivity detection device of the present embodiment includes: a measuring rod 1 with a cavity 11, one end of the measuring rod 1 is provided with a probe 2, and four probes, etc. High and centrally symmetrically arranged measuring electrodes 21, measuring electrodes 22, measuring electrodes 23 and measuring electrodes 24, an excitation module and a measurement module (not shown) are installed in the cavity 11, and the excitation module includes an AC voltage source and an excitation voltage An adjustable resistance converted into an excitation current, the excitation current is loaded between the measurement electrode 21 and the measurement electrode 22; the adjustable resistance is three resistances R1, R2 and R3 with different resistance values, and the three resistances are passed through a multi-channel analog switch Connect to an AC voltage source.

It also includes the first amplifier for collecting the voltage signal on the adjustable resistance, the second amplifier for collecting the voltage signal between the measuring electrode 23 and the measuring electrode 24, and the control unit for receiving the collected voltage signal, connected between the two amplifiers and the control unit There is a relay for intermittently inputting the first amplifier or the second amplifier into the control unit, and the output end of the relay is connected to the control unit through the A/D converter.

In this embodiment, the AC voltage signal is provided by AD 9833, the multi-channel analog switch adopts ADG1404, the single-chip microcomputer adopts 68013, the interfaces P1.0 and P1.1 of the single-chip microcomputer are connected to the multi-channel analog switch, and both the first amplifier and the second amplifier adopt AD620.

The control unit calculates through the program according to the digital signal converted from the collected voltage, and controls the multi-channel analog switch to adjust the access resistance according to the judgment result to complete the adjustment of the measuring range.

Each measuring electrode is radially adjustable with the center of symmetry as the center of the circle. The end face of the probe head is provided with four adjustment slots respectively accommodating each measuring electrode and extending radially. Each measuring electrode has a locking nut 3 for positioning.

The manufacturing material of the measuring rod 1 is polytetrafluoroethylene, the manufacturing material of each measuring electrode is metal titanium, and the length is 4.4 mm.

In this embodiment, a calibration is first performed, and the calibration process is as follows:

According to the configuration standard, use sodium chloride and deionized water to prepare a series of standard solutions with different conductivity values. During the calibration process, four measuring electrodes are inserted into the standard solution. When measuring, apply the excitation voltage and wait for 1 to 2 minutes. By measuring U _out and U _R , combined with the cell constant κ of the electrode and the resistance value of the adjustable resistor R, the detection data of this group can be calculated, and the standard solutions with different conductivity values can be replaced, and the above steps can be repeated to obtain multiple sets of detection data , establish the relational model of standard conductivity and use the detection data of this embodiment, the data of calibration are as shown in the following table:

Standard conductivity (μS/cm) The detection data of this embodiment 18.45 0.302 100.21 2.123 499.80 10.497 998.60 21.108 4995 105.289 10010 211.081 22270 466.479 52080 1092.792

Through fitting, the linear relationship between the conductivity of the solution and the detection data of this embodiment can be established: calibration coefficient a=47.61±0.12, correlation coefficient r ² =0.99998.

After the calibration is completed, the conductivity of different solutions can be detected, and the actual water samples such as purified water, bottled drinking water, tap water, West Lake water, and sea water can be measured with the device of this embodiment that has been calibrated. Place the probe head in the water sample to be tested so that the measuring electrodes are completely submerged. Under the excitation of the AC voltage source, the data is collected and analyzed. During the measurement process, due to different solutions, the order of magnitude of the conductivity is different. The order of magnitude of the resistance value on the circuit is different, so that the order of magnitude of the excitation current is different. In order to make U _out and U _R within the measurement range, the control module will automatically adjust the resistance value of the adjustable resistance according to the collected voltage, thereby adjusting the excitation current. , so that U _out and U _R are within the measurement range, so as to realize the measurement of solutions with different orders of magnitude of conductivity.

In the present embodiment, the conductivity value of the sample was measured by a high-precision Mettler Toledo conductivity meter for comparison, and the measurement results are shown in the table below:

It can be seen from the results that, in the actual water samples in different ranges, except for the measurement error of less than 10μS/cm pure water, the relative error of other water samples is less than 1.0%. It shows that the device of this embodiment can meet the measurement requirements of most water bodies in real life.

Claims (10)

1. An aqueous solution conductivity detection device comprises a detection body and an excitation module with a probe head, said probe head is provided with four measurement electrodes, and the excitation module is loaded between two adjacent measurement electrodes wherein It is characterized in that the excitation module includes an AC voltage source and an adjustable resistor that converts the excitation voltage into an excitation current. The adjustable resistor is a plurality of resistors with different resistance values, and each resistor is connected in parallel to the AC voltage source. 2. The aqueous solution conductivity detection device as claimed in claim 1, wherein the detection body is a measuring rod with a cavity, an excitation module and a measuring module are installed in the cavity, and the probe is positioned at the measuring rod. Ends. 3. The aqueous solution conductivity detection device according to claim 2, characterized in that, the manufacturing material of the measuring rod is polytetrafluoroethylene. 4. The aqueous solution conductivity detection device according to claim 1, characterized in that, the manufacturing material of the measuring electrode is titanium metal. 5. aqueous solution conductivity detection device as claimed in claim 1, is characterized in that, also comprises: a first sampling circuit, configured to collect the voltage of the adjustable resistor; The second sampling circuit is used to collect the voltage between the two measuring electrodes at the response end; The control unit is used to receive the sampling signals from the first sampling circuit and the second sampling circuit, and correspondingly control the conduction states of the multi-channel analog switches. 6. aqueous solution conductivity detection device as claimed in claim 5, is characterized in that, also comprises the switching circuit that is connected between two sampling circuits and control unit, is used for alternately from the first sampling circuit and the second sampling circuit The sampling signal of the circuit is input to the control unit. 7. aqueous solution conductivity detection device as claimed in claim 6, is characterized in that, the first sampling circuit is the first amplifier, and its output end accesses the first input end of switching circuit; The second sampling circuit is a second amplifier whose output terminal is connected to the second input terminal of the switching circuit. 8. The aqueous solution conductivity detection device as claimed in claim 7, wherein the switching circuit is a relay, and the first input end and the second input end correspond to one of the normally open and normally closed contacts of the relay respectively . 9. The aqueous solution conductivity detection device according to claim 8, wherein the output end of the switching circuit is connected to the control unit through an A/D converter. 10. aqueous solution conductivity detection device as claimed in claim 7, is characterized in that, two input ends of the first amplifier are connected at adjustable resistance two ends, and two input ends of the second amplifier are respectively connected the measuring electrode of a response end .