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Analysis of a new four electrode conductivity probe

1 introduction

at present, the domestic electrode conductivity probe is generally two electrode type, and its disadvantage is that once the probe surface is polluted, it will directly affect the measurement results. Moreover, this kind of conductivity meter relies on the change of the measured solution resistance between the two electrodes to measure its conductivity. When the measured solution resistance changes, the terminal voltage changes due to the load change between the two poles, which also changes the electric field characteristics between the two electrodes, so that the measured conductivity is disturbed and the measurement accuracy is reduced

in water environment monitoring, the probe of conductivity meter needs to be placed in the water environment (river, river, lake, sea) for a long time, and some plankton or dirt in the water may adhere to the surface of the probe. In addition to regular cleaning, anti pollution measures must also be taken in the structural design and circuit design of the probe. Therefore, we have developed a four electrode conductivity probe, which has the advantages of strong anti pollution and high precision

2 probe structure and measurement method

probe structure is shown in Figure 1. It consists of two pairs of electrodes. C1 and C2 are hemispherical, oppositely fixed on the two walls of the scaffold, called current electrodes, which are used to establish an electric field in the solution. In order to meet the normal operation of all experiments, they are driven by 10kHz sinusoidal AC current to form the structural components of the cells, so as to eliminate the influence of polarization. At this time, the measured solution can be equivalent to a pure resistance; V1 and V2 are circular rings, which are exactly coaxial with C1 and C2 respectively. They are called voltage electrodes, which are used to measure the electric field formed by C1 and C2, and control the alternating current I between C1 and C2 through the negative feedback loop (see Figure 2). When the conductivity K of the solution changes, the voltage EV between V1 and V2 is always maintained constant. At this time, the current I of the electrode circuit should have a corresponding relationship with the conductivity K of the measured solution. According to this relationship, the conductivity K can be obtained by measuring I. The electrode material is platinum with strong corrosion resistance and good conductivity. The probe support adopts engineering plastic polycarbonate with small temperature coefficient and good chemical stability. Next, we will study the probe electric field to theoretically analyze the performance characteristics of the probe

3 theoretical analysis of probe

3.1 approximate solution of probe electric field

Figure 1

Figure 2

Figure 3

because the boundary conditions of the probe are complex, it is difficult to accurately solve its electric field. However, the sizes of V1, V2, C1 and C2 are much smaller than their support sizes. Therefore, we can use the following simplified model to analyze the probe electric field. The simplified model assumes that C1, V1, C2 and V2 pairs of electrodes are fixed opposite to each other between two infinite insulating walls, which are filled with uniform conductive media with conductivity K. Voltage electrodes V1 and V2 do not participate in the formation of electric field. There is 10kHz AC between C1 and C2. The generated electric field belongs to the category of quasi stable field. Its instantaneous electric field can be analyzed according to the stable current field, so its potential function Φ The Laplace equation should be satisfied, and the boundary condition of zero current density flux should be satisfied on the two insulation walls. The potential function can be obtained by solving the Laplace equation with the second kind of boundary conditions Φ， Then use the stream function Ψ And potential function Φ The corresponding stream function can be obtained by the relationship of orthogonality everywhere in space Ψ。 Figure 3 shows a series of mutually orthogonal equipotential lines and tensile tests obtained after solving the instantaneous electric field, which are the most basic experimental methods of mechanical properties of metal materials. The isocurrent function lines (current lines) in which the lines with arrows are current lines, and Φ、Ψ The values are the dimensionless potential function and flow function values respectively, and the spatial scale of the probe has been dimensionless with the spacing w of C1 and C2. The ratio of the spherical radius RC of electrodes C1 and C2 to W is 0.10, and the ratio of the circular radius RV of electrodes V1 and V2 to w is 0.177

3.2 equivalent circuit of the probe

according to the analysis of the instantaneous electric field of the probe, the potentials of C1, V1, V2 and C2 are 1.0, 0.5, -0.5 and -1.0 respectively, which are located on the four equipotential surfaces with successively reduced potentials. Therefore, some places of the measured solution between the two adjacent equipotential surfaces can become stress concentration areas and can be replaced by equivalent resistance. Assuming that R1 represents the equivalent resistance of the solution between C1 and V1, R3 represents the equivalent resistance of the solution between V1 and V2, and R2 represents the equivalent resistance of the solution between V2 and C2, the equivalent circuit of the probe is shown in Figure 4. It should be noted that this refers to the equivalent circuit after eliminating the influence of polarization

3.3 the spatial resolution of the probe

can be seen from the physical meaning of the flow function, in the flow surface shown in Figure 3 Ψ= In 0, 0.2, 0.4, 0.6, 0.8, 1.0, the current contained between each adjacent two flow surfaces is 1/5 of the total current flow. It can also be seen from Figure 3 that on the equipotential line at x=0, different Y values correspond to different Ψ Value, draw this relationship in Figure 5. It can be seen that when y =1.0, Ψ When the value is about 0.86 and Y =2.0 Ψ The value exceeds 0.99, that is to say, 86% of the current is included in the range of Y ≤ 1.0, and 99% of the current is included in the range of Y ≤ 2.0. Therefore, the effective sensing scale of this probe on the plane perpendicular to the X axis is about 2W; The effective sensing scale in the x-axis is w, which is the spatial resolution of this probe

3.4 probe performance test

we used the method in Figure 6 to test the performance of the probe. HP hp3300a function generator generates high-precision 10kHz sine wave current to drive C1 and C2. The circuit is connected in series with a high-precision resistance with a resistance value of 50.00 Ω, which is used to detect the electrode circuit current. The gain of the precision differential instrument op amp is 1.000, the input impedance is 20m Ω, and the common mode rejection ratio is 80dB. It measures the voltage at both ends of the resistance or between V1 and V2 through a double pole double throw switch. The solution used in the experiment is mixed with 500cc deionized water and an appropriate amount of potassium chloride, and then put into the thermostat as a solution with known conductivity, and the constant temperature is controlled at 25.00 ℃± 0.005 ℃. After immersing the probe in the solution, adjust the sine wave generator so that the voltage between V1 and V2 is an accurate 0.1000vrms, then read the voltage (V) at both ends of the resistance, divide this voltage by 50 (Ω), and then the current I (a) of the electrode circuit can be obtained. The following is the test data of the probe we equipped for the ocean salinometer (see Table 1)

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