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E-mail: michael@shboqu.com   |   Expert in Water Quality Measurement and Water Treatment Project

What types of conductivity electrodes are there? What are the different uses?

by:BOQU     2023-04-11
Conductivity electrodes are generally divided into two types: two-electrode type and multi-electrode type. The two-electrode conductance electrode is currently the most widely used conductance electrode type in China. The structure of the experimental two-electrode conductance electrode is to sinter two platinum sheets on two parallel glass sheets, or on the inner wall of a circular glass tube, and adjust the platinum sheet The area and distance can be made into conductance electrodes with different constant values. Usually there are K=1, K=5, K=10 and other types. The two-electrode conductivity electrode used on the online conductivity meter is often made into a cylindrical symmetrical electrode. When K=1, graphite is often used, and when K=0.1, 0.01, the material can be stainless steel or titanium alloy. Multi-electrode conductance electrodes generally have several ring-shaped electrodes on the support, and conductance electrodes with different constants can be made through different combinations of ring-shaped electrodes in series and parallel. The material of the ring electrode can be graphite, stainless steel, titanium alloy and platinum. Conductivity electrodes are also available in four-electrode type and electromagnetic type. The advantage of the four-electrode conductivity electrode is that it can avoid the measurement error caused by electrode polarization, and it is widely used in foreign experimental and online conductivity meters. The characteristic of the electromagnetic conductivity electrode is that it is suitable for measuring high-conductivity solutions. It is generally used in industrial conductivity meters, or it can be used to make single-component concentration meters, such as hydrochloric acid concentration meters and nitric acid concentration meters. The verification method and problems of the temperature compensation of the conductivity measuring instrument The verification method and problems of the temperature compensation of the conductivity measuring instrument. Users who use conductivity meters know this. The conductivity of the solution is closely related to the temperature, because when the temperature changes, the ionization degree, solubility, ion migration speed, solution viscosity, etc. of the electrolyte will change, and the conductivity will also change. As the temperature increases, the conductivity increases. At this moment, the temperature compensation function of the conductivity meter is to overcome the influence of temperature. 1. What is the temperature compensation function of the conductivity measuring instrument: the conductivity value of the solution at the actual temperature is converted into the conductivity value at the reference temperature (generally 25°C), so that the conductivity of the solution at different temperatures is comparable The conductivity meter used in the market now has a temperature compensation function to meet the needs of comparison or control indicators in various industries. This paper briefly analyzes and discusses the temperature compensation function required in the verification process when using a conductivity meter. It is also necessary to add this verification item in the verification process. There are two ways to verify the temperature compensation of the conductivity meter, one is KMR before temperature compensation as a fixed value, and the other is KMV after temperature compensation as a fixed value, the two methods are based on the same principle, and the specific verification steps are based on The difference in instrument design can also be divided into two methods. During the verification process, we also found that the temperature setting will affect the conductivity cell constant. The analysis shows that the temperature compensation of the conductivity meter is essentially the same as the conductivity cell constant compensation. When the temperature compensation of the instrument is missing or faulty, the conductivity cell constant can be used. Compensation to achieve conductivity temperature compensation. 2. Calibration methods and problems of temperature compensation For strong electrolytes with conductivity greater than 1×10-4S cm-1, there is a linear relationship between conductivity and temperature: KT=K0〔1+α(T-T0)〕(1 ); in the verification process, as long as the conductivity values ​​at different temperatures are measured, the temperature coefficient α of the instrument can be obtained through the formula (5) in JJG376-2007, so as to realize the verification of the temperature compensation coefficient of the conductivity meter. Set the constant Kcell of the conductivity meter to 1.00cm-1, input the conductivity value of a certain signal (such as 50μS cm-1), adjust the analog resistance of the temperature sensor, and make the temperature display value 25°C and 15°C (35°C) , and then read the corresponding conductivity meter measurement values ​​KMR and KMV respectively. According to formula (1), there are: (2) (3) Problems: 1) Domestic conductivity meters are manually temperature compensated, and the temperature coefficient cannot be set, and its default value is 2.00%/℃. For this type of instrument, when the temperature is set to 25°C, it is in the non-compensated state, and the measured conductivity is KMR, while the measured conductivity value at other temperatures is the compensated conductivity value KMV, which can realize temperature compensation. test. 2) For different conductivity meters, the verification steps of temperature compensation are also different. For Type I instruments, the error of temperature coefficient can be measured according to the method described in JJG376-2007. First set the constant of the conductivity cell, and then adjust the temperature display. value. For type II instruments, the temperature indication has no effect on the conductivity value, and it does not mean that the analog resistor of the temperature sensor has failed, because if the instrument is adjusted to the 'check' state, it is found that when the temperature indication is adjusted, the conductivity cell constant also occurs. When the temperature display is adjusted to 15°C and 35°C, the conductivity cell constant changes to about 1.200cm-1 and 0.800cm-1 respectively. For the verification of temperature compensation of this type of instrument, the temperature should be adjusted to the target temperature (15°C or 35°C), and then the conductivity cell constant should be adjusted to 1.00cm-1, and then the corresponding conductivity values ​​should be read, according to the formula (3 ) to find the temperature coefficient of the instrument. However, for the data obtained by this type of instrument, the value calculated according to formula (3) deviates greatly from the standard value (α=2.0%), and the temperature compensation coefficient values ​​obtained at different temperatures vary greatly. This type of instrument has similar deviations and is not caused by a substandard instrument. This issue requires further study. The relationship between temperature compensation and conductivity cell constant compensation For some domestic conductivity meters mentioned above, the temperature setting has a significant impact on the conductivity cell constant. For instrument I (such as the new DDS-307), when the temperature indication is adjusted to At ℃, the conductivity cell constants change to about 1.250cm-1 and 0.833cm-1 respectively. This phenomenon can also be observed in type II instruments (the numerical value is different from that of instrument I, which needs to be further studied). This shows that temperature compensation and conductivity cell constant compensation are interlinked, and the two have an intrinsic relationship.
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